Heat and Energy

Heat is not just a sensation but a form of energy known as thermal energy, which flows from a region of higher temperature to one at a lower temperature. This chapter fundamentally distinguishes between these two often-confused concepts: temperature, which measures the hotness or coldness of a body and is a function of the average kinetic energy of its molecules, and heat, which is the total internal energy possessed by the body due to the combined kinetic and potential energy of all its molecules. We explore how this flow of heat is meticulously measured in joules or calories, and how different substances require varying amounts of heat to change their temperature, a property defined as specific heat capacity. This concept explains everyday phenomena, like why land heats up faster than the sea.

The journey of heat transfer is explored through its three primary mechanisms. Conduction is the process where heat travels through a material without the atoms or molecules moving from their positions, like a spoon getting hot in a soup bowl; metals are excellent conductors while wood and air are poor ones, making them good insulators. Convection involves the actual movement of the heated molecules themselves, which is why warm air rises and cool air sinks, forming cycles that are responsible for sea breezes and even weather patterns. Lastly, radiation is the transfer of heat without any medium at all, through electromagnetic waves, which is precisely how the sun’s energy warms the Earth. We learn that dark, dull surfaces are efficient absorbers and emitters of radiant heat, while shiny, light surfaces are poor ones.

The chapter then applies these principles to the real-world functioning of various thermometers, explaining how the predictable expansion of liquids like mercury is calibrated to measure temperature accurately. Furthermore, it delves into the dramatic effects of heat, such as the change of state. When a solid melts into a liquid or a liquid vaporizes into a gas, the heat energy absorbed is used to break the molecular bonds rather than raise the temperature; this ‘hidden’ energy is called latent heat. Understanding this concept clarifies why steam at 100°C causes a more severe burn than boiling water at the same temperature, as it releases a large amount of latent heat upon condensation. Ultimately, the chapter weaves these ideas together to present a cohesive picture of heat as a dynamic and transformative agent in the physical world.

Exercise 6 (A)

Question 1. 

What is heat ? Write its S.I. unit. 

Ans:

What is Heat?

In our daily lives, we often use the word “heat” to mean something that feels warm. However, in physics, it has a more precise and fundamental meaning.

Heat is a form of energy. Specifically, it is the energy that is transferred between two objects or systems due to a difference in their temperatures. The key thing to remember is that heat refers to the journey of this energy, not the energy stored in an object.

Think of it like this: An object itself doesn’t “contain” heat; it contains internal energy (the total energy of all its vibrating and moving molecules). Heat is the flow of this internal energy from a hotter object to a colder one.

For example, when you place an ice cube in your warm hand, your hand feels cooler because energy, which we call heat, is flowing from your hand to the ice cube. This transfer of energy continues until both your skin and the melted water reach the same temperature—a state known as thermal equilibrium.

The S.I. Unit of Heat

Since heat is a form of energy, its SI (Système International) unit is the same as that for energy and work.

The SI unit of heat is the Joule (symbol: J).

This unit is named after the English physicist James Prescott Joule, whose experiments were crucial in establishing that heat is a form of energy.

While the Joule is the standard SI unit, you might still encounter the calorie (cal) in some contexts, particularly when discussing the energy content of food. One calorie is historically defined as the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius. The relationship between them is:

1 calorie = 4.184 Joules

Question 2. 

Two bodies at different temperatures are placed in contact. State the direction in which the heat will flow. 

Ans:

Heat is a form of energy that naturally moves from a region of higher thermal intensity to a region of lower thermal intensity. It seeks a balance, much like water flowing downhill.

When two bodies at different temperatures are brought into contact, their particles possess different levels of vibrational and kinetic energy. The body with the higher temperature consists of more energetic particles, while the body with the lower temperature has less energetic particles.

Through direct contact, the more energetic particles transfer a portion of their energy to the less energetic particles via collisions and interactions. This transfer of energy is what we call heat flow.

Therefore, the direction of heat flow is always one-way: from the body at the higher temperature to the body at the lower temperature.

This one-directional flow will continue until the particles in both bodies reach the same average energy level, meaning the two bodies have achieved the same final temperature. At that point, a thermal equilibrium is established, and the net transfer of heat ceases.

Question 3. 

Name the S.I. unit of heat and how is it related to the unit calorie?  

Ans:

The S.I. Unit of Heat and Its Link to the Calorie

The S.I. (Système International) unit for heat, as well as for all other forms of energy, is the joule, symbolized as J.

This naming honors James Prescott Joule, a pioneering physicist whose experiments in the 19th century were crucial in establishing the principle of conservation of energy. He demonstrated that mechanical work could be directly converted into a specific, measurable amount of heat, proving that heat was a form of energy transfer. Using the joule for heat reinforces a fundamental principle of science: energy, whether it appears as motion, light, electricity, or heat, is measured using the same standard unit.

Relationship to the Calorie

The calorie (cal) is a older unit of heat that predates the modern S.I. system. Its definition is rooted in a specific, observable physical property: it was historically defined as the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius (specifically from 14.5°C to 15.5°C at standard atmospheric pressure).

The relationship between the joule and the calorie was determined through careful experiments, like those performed by Joule himself. The established conversion is:

1 calorie = 4.184 joules

This means that the energy required to warm one gram of water by 1°C is equivalent to 4.184 joules of work or any other form of energy.

It’s also common to encounter the kilocalorie (kcal), often referred to as the “Calorie” (with a capital C) on food packaging.

1 kilocalorie (or 1 Food Calorie) = 1000 calories = 4184 joules

In summary, while the calorie is a convenient unit based on the thermal behavior of water, the joule is the universal S.I. unit for energy. They are fundamentally related by the fixed conversion factor of 4.184 J/cal, a value cemented by meticulous experimental physics.

Question 4. 

Define temperature and write its S.I. unit. 

Ans:

We often think of temperature simply as how hot or cold something feels. However, in physics, it has a more precise meaning. Temperature is a objective measure that indicates the direction in which heat energy will spontaneously flow.

Let’s break that down. Imagine placing a hot piece of metal into a cup of cool water. Heat energy will naturally move from the metal into the water. We describe this by saying the metal started at a higher temperature. The water started at a lower temperature. Heat always moves on its own from a region of higher temperature to a region of lower temperature until they become equal.

At a deeper level, for ordinary matter, temperature is a direct reflection of the average random motion of its molecules. A higher temperature means the molecules are, on average, vibrating, rotating, or moving more intensely.

The standard unit for measuring this quantity in the International System of Units (S.I.) is the kelvin (K). On the kelvin scale, zero (0 K) represents the theoretical point where all molecular motion ceases, known as absolute zero.

Question 5. 

Why does a piece of ice, when touched with a hand, appear cool? Explain.

Ans:

The sensation of an ice piece feeling cool is a direct result of energy seeking balance. Your hand and the ice exist at different energy levels, specifically thermal energy. Your hand, at normal body temperature, is a source of considerable heat, while the ice is a region of very little heat.

When your skin makes contact with the ice, this energy difference creates a one-way highway for heat to travel. Heat naturally flows from the warmer object (your hand) to the colder one (the ice). The ice acts like a sponge, actively drawing thermal energy out of your skin.

This rapid loss of heat from your skin is what your nervous system detects and interprets as a “cool” or “cold” feeling. The ice itself doesn’t transfer “cold” into you; rather, it is the swift departure of your own body’s heat that creates the distinct sensation of coolness. Essentially, you are feeling your own energy being drained away to fuel the melting process of the ice.

Question 6. 

Distinguish between heat and temperature. 

Ans:

Feature	Heat (Q)	Temperature (T)
Definition	The transfer of thermal energy between two objects or systems due to a temperature difference.	A measure of the average kinetic energy of the atoms or molecules within a substance.
Nature	A form of energy; it is a process quantity (cannot be stored).	A measurable property of a system; it is a state variable (a measure of its “hotness” or “coldness”).
SI Unit	Joule (J) or Calorie (cal).	Kelvin (K) or degrees Celsius (∘C).
Measurement	Measured using a calorimeter.	Measured using a thermometer.
Dependence	Depends on the mass, specific heat, and temperature change of the object (Q=mcΔT).	Does not depend on the size or mass of the object (it is an intensive property).
Flow	Flows spontaneously from a region of higher temperature to one of lower temperature.	Determines the direction of heat flow.

Question 7. 

What do you understand by thermal expansion of a substance?

Ans:

Understanding Thermal Expansion: When Things Get Bigger with the Heat

Imagine a metal lid on a glass jar that’s stuck tight. A common trick is to run hot water over the lid. After a few seconds, it twists off easily. This everyday miracle is a perfect demonstration of thermal expansion.

In simple terms, thermal expansion is the tendency of matter to change its volume when its temperature changes. For almost all common substances, this means they expand when heated and contract when cooled.

Why Does This Happen? The Particle Perspective

To really understand it, we need to think about what’s happening at the microscopic level:

1. The Constant Dance of Particles: All matter is made of atoms or molecules that are constantly vibrating. They’re not static; they jiggle and move.
2. Heat Adds Energy: When you heat a substance, you are essentially pumping energy into it. This extra energy makes the particles vibrate more vigorously and rapidly.
3. Increased Motion Needs More Space: As the particles vibrate with more force and over a wider range, they push against their neighboring particles more strongly. This increased push effectively increases the average distance between the particles.
4. The Net Result: While the particles themselves don’t get bigger, the spaces between them do. When you add up this tiny increase in spacing across trillions of particles, the entire object gets measurably larger in all directions—its length, area, and volume all increase.

Different Types, Real-World Examples

Thermal expansion isn’t just one phenomenon; it shows up in a few key ways:

• Linear Expansion: This is the expansion in one dimension (length). It’s most critical in long, solid structures.
  • Example: Railway tracks have small gaps between them. On a hot summer day, the steel rails expand. Without these “expansion joints,” the tracks would buckle and warp, causing derailments.
• Area Expansion: This is the expansion of a surface.
  • Example: Metal bridges have expansion joints at their ends. As the temperature changes through the seasons, the entire bridge deck expands and contracts, and these joints allow for that movement without causing structural damage.
• Volume Expansion: This is the expansion in all three dimensions, which is most relevant for liquids and gases.
  • Example: The liquid in a thermometer (like mercury or colored alcohol) expands much more than the glass that contains it. When the temperature rises, the liquid expands up the narrow tube, giving us a temperature reading.

The Important Exception: Water

Most substances follow the rule of expand-when-heated, but water has a famous and life-saving exception. Water contracts as it cools until it reaches about 4°C (39°F). However, between 4°C and 0°C, it actually expands as it freezes. This is why ice is less dense than liquid water and floats. If it didn’t, lakes and oceans would freeze from the bottom up, making aquatic life in colder climates impossible.

In summary, thermal expansion is a fundamental property of matter driven by increased particle motion. It’s a force that engineers, architects, and designers must constantly account for, and its effects are woven into the very fabric of our built environment and natural world.

Question 8. 

Name two substances which expand on heating. 

Ans:

Most materials grow larger when their temperature rises, and this behavior is very common. Two clear examples are:

1. Metals: A common metal like the copper used in electrical wires visibly expands. This is why long stretches of electrical lines are installed with some slack, especially between utility poles. If they were pulled taut, the expansion on a hot summer day would cause them to sag excessively or even snap.
2. Gases: The air around us is a mixture of gases. When air inside a closed container, like a balloon, is warmed, the gas particles inside move more vigorously and push outward. This force causes the balloon to inflate and occupy a larger volume, demonstrating a very noticeable expansion.

Question 9. 

Name two substances which contract on heating. 

Ans:

While the vast majority of materials expand when they absorb heat, a few fascinating exceptions do the opposite within specific temperature ranges. Their unique internal structure causes them to contract.

Two well-known substances that exhibit this unusual behavior are:

1. Liquid Water between 0°C and 4°C: This is the most common and important example. As you warm solid ice at 0°C and it melts into liquid water, the water particles do not immediately spread out. Instead, up to a temperature of 4°C, the lingering crystalline structure continues to collapse, causing the water to become denser and occupy less space. This is why water is most dense at 4°C, not at its freezing point.
2. Molten Silica (Silicon Dioxide): The raw material for making glass, molten silica, has a complex, tangled network of molecules. When heated, the molecules gain just enough energy to relax and settle into a slightly more compact, ordered arrangement before the general expansion from higher temperatures takes over. This results in a temporary contraction.

Question 10. 

What do you mean by the anomalous expansion of water?

Ans:

The Anomalous Expansion of Water

Most liquids follow a simple rule: they get smaller and denser as they cool down. Water is strange because it disobeys this rule within a specific temperature range.

This “anomaly” happens between 4°C and 0°C. As you cool water within this range, it does the opposite of what you’d expect—it slightly expands and becomes less dense.

Why is this important?

This quirky behavior is why ice floats. By the time water freezes solid at 0°C, it has expanded significantly. This makes ice less dense than the liquid water beneath it, causing it to form a floating layer on top of ponds and lakes.

This floating ice layer acts as an insulating blanket. It prevents the entire body of water from freezing solid from the bottom up, protecting aquatic life and making Earth’s winters very different from what they would otherwise be. It’s a fascinating exception in nature that has profound consequences.

Question 11. 

At what temperature is the density of water maximum? State its value. 

Ans:

Water behaves in a counterintuitive way as it cools down. Unlike most substances, which become progressively denser as they solidify, water reaches its peak density at a temperature above its freezing point.

This unique property occurs at 4 degrees Celsius (which is approximately 39.2 degrees Fahrenheit).

At this specific temperature, a given mass of water occupies the smallest possible volume, making it the heaviest and most compact it can be in its liquid form. The widely accepted value for this maximum density is 1 gram per cubic centimeter (1000 kilograms per cubic meter).

This is why, in a lake during winter, the colder water at 4°C sinks to the bottom, creating a layer that allows life to continue beneath the surface ice.

Question 12. 

State the volume changes that you will observe when a given mass of water is heated from 0°C to 10°C. Sketch a temperature-volume graph to show the behaviour. 

Ans:

Observed Volume Changes in Water (0°C to 10°C)

When a given mass of water is heated from 0°C to 10°C, its volume does not change uniformly. Here is what you will observe:

1. From 0°C to 4°C: The volume of the water decreases.
2. At 4°C: The water reaches its minimum volume and maximum density.
3. From 4°C to 10°C: The volume of the water increases normally, as expected with heating.

This unusual behaviour occurs because of the unique way water molecules rearrange themselves as they break free from the open, crystalline structure of ice.

Temperature-Volume Graph

The graph illustrating this behaviour is a curve, not a straight line.

(A hand-sketched style graph would be here)

• The vertical (Y) axis is labeled Volume.
• The horizontal (X) axis is labeled Temperature (°C).
• The line starts at 0°C, curves downward to a clear minimum point at 4°C, and then curves upward as it extends to 10°C and beyond.
• The point at 4°C is often highlighted and labeled “Maximum Density / Minimum Volume”.

This distinctive curve visually confirms that water contracts on heating from 0°C to 4°C and expands afterwards.

Question 13. 

Draw a graph to show the variation in density of water with temperature in the range from 0°C to 10°C. 

Ans:

To sketch this graph, you are illustrating one of water’s most peculiar behaviors. Most liquids become steadily denser as they cool down. Water defies this pattern. It grows denser only until it reaches 4°C; cooling it further makes it expand and become less dense until it freezes at 0°C.

Follow these steps to capture this anomaly on paper:

1. Lay the Foundation with Your Axes

• Draw a vertical line for the Y-axis. Label this line “Density in g/cm³”.
• Draw a horizontal line for the X-axis. Label this line “Temperature in °C”.
• Along the X-axis, create marks from 0°C on the far left to 10°C on the far right.

2. Create a Focused Scale for Density

• The changes in water’s density are subtle but critical. To make them visible, we need a “zoomed-in” scale.
• At the very bottom of your Y-axis, write the value 0.9997 g/cm³.
• At the very top of your Y-axis, write the value 1.0000 g/cm³.

3. Identify and Mark the Critical Points

• Point A: The Freezing Point. At 0°C, just before turning to ice, water has a density of about 0.99987 g/cm³. Find 0°C on your X-axis, and go up to just a tiny bit above your bottom mark (0.9997) to place your first point.
• Point B: The Peak Density. This is the most important part of the graph. At 4°C, water is at its heaviest and most compact, with a density of exactly 1.0000 g/cm³. Place a point directly above the 4°C mark at the very top of your Y-axis.
• Point C: The Warm Water. At 10°C, water is warmer and has expanded again, with a density of about 0.99975 g/cm³. Place this point slightly below your 0°C point, near the 0.9997 baseline.

4. Connect the Dots to Reveal the Curve

• Using a smooth, flowing line, connect your three points. You will not be drawing a straight line or a simple curve.
• The line should start at Point A (0°C), sweep upwards to its highest peak at Point B (4°C), and then curve downwards to finish at Point C (10°C).
• The final shape should resemble a gentle, lopsided hill or a soft arch that is steepest on its left side.

5. Finalize with Clear Labels

• Write a title at the top: “Water’s Unique Density Behavior”.
• Crucially, draw a small arrow pointing directly to the peak of your curve at 4°C. Write next to it: “Heaviest Water (1.0000 g/cm³)”.

Visual Takeaway:

Your completed graph tells a clear story. It shows that a jar of water cooling from 10°C down to 0°C will first sink until it hits 4°C, and then mysteriously begin to rise as it gets colder, finally freezing from the top down. This single, non-linear curve is the reason ice floats and life can exist in frozen lakes.

Question 14. 

A given mass of water is cooled from 10°C to 0°C. State the volume changes you will observe. Represent these changes on a temperature-volume graph. 

Ans:

Volume Changes in Cooling Water (10°C to 0°C)

When a given mass of water is cooled from 10°C to 0°C, its volume does not change uniformly. Here is what you will observe:

1. From 10°C to 4°C: The volume of water gradually decreases as it cools and becomes denser.
2. At 4°C: Water reaches its maximum density and, therefore, its minimum volume.
3. From 4°C to 0°C: A unique phenomenon occurs. Instead of continuing to contract, the volume increases as the water begins to form its crystalline ice structure.

Temperature-Volume Graph

The graph below represents this non-linear relationship.

text

   Volume

      ^

      |                          •

      |                        /

      |                      /

      |                    /

      |                  •

      |                  |

      |                  |

      +——————+——————> Temperature (°C)

      0                  4                  10

      (Ice)           (Liquid Water)

Explanation of the Graph:

• The curve is not a straight line.
• It shows a distinct dip or minimum at 4°C, confirming that water occupies the least space at this temperature.
• The line slopes downward from 10°C to 4°C (showing contraction) and then slopes upward from 4°C to 0°C (showing expansion). This anomalous expansion is a critical property of water.

Question 15. 

Describe an experiment to show that water has maximum density at 4°C. What important consequences follow this peculiar property of water? Discuss the importance of this phenomenon in nature. 

Ans:

Experiment to Show Maximum Density at 4°C

A simple experiment can demonstrate this property using a tall, narrow glass jar, a thermometer, and a special tool.

1. Setup: Fill the jar with water and insert a high-precision thermometer. Then, carefully lower a density indicator into the water. This isn’t a single object, but two small, weighted bulbs connected by a narrow glass tube, forming an “H” shape. The top bulb is less dense and the bottom bulb is more dense than water.
2. Cooling: Place the entire setup in a refrigerator or an ice bath and start cooling it slowly towards 0°C.
3. Observation: As the water cools, watch the indicator.
   • As the water temperature drops from room temperature, the indicator will initially sink slightly because the water’s density is increasing as it contracts.
   • You will observe that the indicator sinks to its lowest point when the thermometer reads exactly 4°C. This is the moment the water is at its densest, providing the most buoyancy.
   • If you continue cooling below 4°C, the indicator will begin to rise again. This is counterintuitive but proves that the water is now expanding and becoming less dense as it approaches its freezing point of 0°C.

Important Consequences

This peculiar property has two major consequences:

1. Ice Forms on the Surface: Because water below 4°C is lighter, it floats. So, when a lake cools, the 4°C water sinks to the bottom, and the colder water (0-4°C) stays on top, where it freezes first. This creates an insulating ice layer.
2. Lakes Freeze from the Top Down: This is the direct result of the first point. Water bodies do not freeze solid from the bottom up.

Importance in Nature

This phenomenon is critically important for aquatic life:

• Winter Survival: The surface ice layer insulates the water below, preventing the entire lake from freezing solid. The densest water (at 4°C) settles at the bottom, creating a stable, liquid environment where fish, plants, and other organisms can survive the winter.
• Seasonal Mixing: In spring and fall, the uniform 4°C temperature of the entire water column allows for a full mixing of water (called turnover). This circulates oxygen from the surface and nutrients from the bottom, which is essential for a healthy ecosystem.

Question 16. 

A deep pond of water has its top layer frozen during winter. State the expected temperature of the water layer (i) Just in contact with ice and (ii) At the bottom of the pond. 

Ans:

The expected temperatures within the frozen pond are a direct result of a unique and vital property of water.

Water is most dense at 4°C, not at its freezing point. This unusual trait dictates the temperature layout in the pond.

(i) Temperature of the water just in contact with the ice:

This layer is in the process of freezing. For liquid water to turn into solid ice, it must release its latent heat of fusion. The point where this phase change happens is precisely 0°C. Therefore, the water immediately touching the underside of the ice will be at a temperature of 0°C.

(ii) Temperature of the water at the bottom of the pond:

As the surface water cools to 4°C, it becomes the densest water in the pond. This denser water sinks, displacing the slightly warmer and less dense water below. This process continues until the entire pond has cooled to 4°C from the bottom up. Any further cooling at the surface produces 0°C water, which is less dense than 4°C water, causing it to float and form ice. This creates a stable, layered effect, trapping the densest 4°C water at the very bottom.

Consequently, the water at the bottom of the pond will remain at a constant 4°C.

In summary:

• At the ice-water interface: 0°C (the point of freezing)
• At the pond’s bottom: 4°C (the temperature of maximum density for water)

Question 17. 

Draw a diagram showing the temperature of various layers of water in an ice covered pond. 

Ans:

Diagram of Water Temperature in an Ice-Covered Pond

Imagine a vertical cross-section of a pond in winter. From top to bottom, the temperature layers are as follows:

1. AIR: Above the pond is the cold winter air, at a temperature well below freezing (e.g., -10°C).
2. ICE LAYER: At the very top is the solid ice cover. Its temperature is coldest at the top (near the air temperature, e.g., -5°C) and warms to 0°C at its bottom surface, where it meets the water.
3. TOP WATER LAYER: Just beneath the ice is a thin layer of water. This water is at its maximum density temperature, 4°C.
4. BOTTOM WATER & SEDIMENT: The water at the very bottom of the pond is slightly warmer, typically 4°C. This is the densest water, and it sinks, creating a stable environment. The pond bed (sediment) is also at this temperature.

Key Takeaway: The water is warmest at the bottom (a constant 4°C), providing a refuge for fish and other aquatic life, while the ice layer on top acts as an insulating blanket.

Question 18.

1. Explain the following: Water pipes in colder countries often burst in winter.

2. Explain the following: In winter, water tanks (or ocean) start freezing from the surface and not from the bottom.

3. Explain the following: Fishes survive in ponds even when the atmospheric temperature is well below 0°C.

4.Explain the following: A hollow glass sphere which floats with its entire volume submerged in water at 4°C, sinks when water is heated above 4°C.

5.Explain the following: A glass bottle completely filled with water and tightly closed at room temperature is likely to burst when kept in the freezer of a refrigerator.

Ans:

1. 

This common winter mishap is a direct result of water’s unique behavior as it freezes. Unlike most substances that shrink and become denser as they solidify, water performs an unusual expansion when it turns to ice. As the temperature drops, the water inside the pipes begins to freeze. This transformation into ice causes the water molecules to arrange into a crystalline structure that occupies more space than the liquid water did. The pipe, typically made of metal or plastic, acts as a rigid container. The immense pressure created by the expanding ice has nowhere to go. Eventually, this internal force exceeds the strength of the pipe material, causing it to crack or burst open.

2. In winter, water tanks (or oceans) start freezing from the surface and not from the bottom.

This phenomenon is a lifesaving anomaly of water, linked to its density. Water is at its heaviest and most dense at 4°C. As the surface of a body of water cools, the chilled water becomes denser and sinks, pushing the slightly warmer water from below up to the surface. This process continues until the entire water body reaches 4°C. Now, when the surface cools further to 0°C, it becomes less dense than the 4°C water beneath it. This lighter ice-cold water, and eventually the ice itself, stays floating on the surface, acting as an insulating layer. This prevents the deeper water from losing more heat, so freezing only happens from the top down.

3. Fishes survive in ponds even when the atmospheric temperature is well below 0°C.

Fish survival in a frozen pond is entirely dependent on the fact explained above. Because water freezes from the surface downward and the ice itself is a poor conductor of heat, a layer of ice on the pond’s surface effectively traps the heat in the water below. The water at the bottom of the pond remains in its liquid state, typically at a stable temperature of 4°C, which is its densest point. This creates a liquid refuge at the pond’s bottom, allowing aquatic life, including fish, to survive the harsh winter above.

4. A hollow glass sphere which floats with its entire volume submerged in water at 4°C, sinks when water is heated above 4°C.

This occurs due to a critical change in the density of water. At 4°C, water possesses its maximum density. The sphere is carefully weighted so that its overall density (glass plus trapped air) is exactly equal to the density of water at this temperature, allowing it to float completely submerged. However, when the water is heated above 4°C, it expands, meaning the same amount of water now occupies more space. This causes its density to decrease. The density of the glass sphere, however, remains unchanged. Now, the sphere becomes denser than the warmer, lighter water surrounding it. Since it is denser, it can no longer float and sinks.

5. A glass bottle completely filled with water and tightly closed at room temperature is likely to burst when kept in the freezer of a refrigerator.

This is another consequence of water’s expansion upon freezing. A completely full and sealed bottle leaves no empty space—no room for anything to change. As the water inside the bottle cools and its temperature approaches 0°C

Exercise 6 (A)

Question 1. 

Calorie is the unit of :

1. Heat
2. Work
3. Temperature
4. food

Question 2. 

1 J equals to :

1. 0.24 cal
2. 4.18 cal
3. 1 cal
4. 1 kcal

Question 3. 

S.I. unit of temperature is :

1. Cal
2. Joule
3. Celsius
4. kelvin

Question 4. 

Water is cooled from 4 °C to 0 °C. It will :

1. contract
2. expand
3. first contract, then expand
4. first expand, then contract

Question 5. 

Density of water is maximum at :

1.  0°C
2. 100°C
3. 4°C
4. 15°C

Exercise 6 (B)

Question 1. 

What is an ecosystem ? Name its two components .

Ans:

An ecosystem is a community where living organisms and their physical environment interact as a single, functional unit. Think of it as a complex, interconnected neighborhood of nature.

Its two main components are:

1. Biotic Components: These are all the living and once-living parts, such as plants, animals, insects, and microorganisms.
2. Abiotic Components: These are all the non-living, physical and chemical parts, such as sunlight, water, air, soil, and minerals.

Question 2. 

What is the source of energy for all ecosystems?

Ans:

The single, universal source of energy that powers the intricate web of life in every ecosystem on Earth is the Sun.

Think of the Sun as a giant, distant power plant that continuously beams energy our way. This stream of sunlight is captured by green plants, algae, and certain bacteria through a remarkable process called photosynthesis. These organisms act like nature’s solar panels, using the sun’s energy to fuse water and carbon dioxide into sugar—a rich, stored form of chemical energy.

This sugar, produced by these sunlight-capturing organisms (known as producers), forms the foundational fuel for the entire ecosystem. When a grasshopper eats the plant, it is consuming this stored solar energy. The frog that eats the grasshopper, and the snake that eats the frog, are all, in essence, passing along that same initial packet of solar power.

Even in the deepest, darkest ocean trenches where sunlight never reaches, the energy still originates from the Sun. There, specialized ecosystems rely on chemicals for energy, a process called chemosynthesis. However, the organisms that form the base of this food web still depend on oxygen dissolved in the water—oxygen that was produced by photosynthesis in the sunlit upper layers of the ocean.

Therefore, from a sun-drenched meadow to a pitch-black abyss, the Sun’s radiant energy is the ultimate driver of all life’s processes.

Question 3. 

State the importance of green plants in an ecosystem.

Ans:

Green plants are the indispensable foundation of nearly every ecosystem on land and in water. Their unique role begins with photosynthesis, a process where they use sunlight to create their own food. This single action makes them primary producers, forming the base of the food web and supplying energy to all other life, from the smallest insect to the largest predator.

Beyond being the ultimate food source, plants are the lungs of our planet. They absorb carbon dioxide and release oxygen through photosynthesis, maintaining the delicate balance of gases in the atmosphere that animals need to breathe. Furthermore, their vast root systems act like anchors, holding soil firmly in place to prevent erosion and creating a stable habitat for countless soil organisms. In essence, they are the silent, steadfast engineers that build and sustain the living world.

Question 4. 

Differentiate between the producers and consumers.

Ans:

Feature	Producers (Autotrophs)	Consumers (Heterotrophs)
Nutritional Mode	Autotrophic (Self-feeding).	Heterotrophic (Feeds on others).
Food Source	Manufacture their own food from inorganic substances.	Obtain food by consuming other organisms (producers or other consumers).
Energy Source	Primarily use solar energy (photosynthesis) or chemical energy (chemosynthesis).	Use the chemical energy stored in the organic molecules of the food they eat.
Role in Ecosystem	Base of the Food Chain; they introduce energy into the ecosystem.	Depend on producers (directly or indirectly) for energy.
Trophic Level	Occupy the First Trophic Level.	Occupy the Second (primary), Third (secondary), or Fourth (tertiary) Trophic Levels.
Examples	Green plants, algae, and some bacteria.	All animals (e.g., cow, human, lion) and fungi.

Question 5. 

State the functions of decomposers in an ecosystem.

Ans:

Think of decomposers as nature’s ultimate cleanup and recycling team. Without these vital organisms, our world would be buried in waste and life would eventually grind to a halt. Their work is fundamental and happens mostly out of sight.

Here is a breakdown of their critical functions:

1. They Break the Lock on Nutrients: When plants and animals die, or when animals leave behind waste, the valuable nutrients and minerals they contain—like nitrogen, phosphorus, and carbon—are trapped inside their bodies. Decomposers, primarily fungi and bacteria, possess special enzymes that can dismantle tough materials like wood, bones, and even feathers. They break down this complex organic matter, releasing the locked-up nutrients back into the soil, water, and air.
2. They Feed the Producers: The nutrients released by decomposers are not lost. They become a nourishing soup for plants (the producers). Plants absorb these simple, inorganic nutrients through their roots to fuel their own growth. In this way, a molecule that was once part of a fallen leaf can become part of a new tree. This process closes the loop in the food chain, ensuring the circle of life continues.
3. They Detoxify and Clean the Environment: By consuming dead and rotting material, decomposers perform a natural sanitation service. They remove what would otherwise be a mounting pile of carcasses, fallen trees, and dung. This cleaning action helps to prevent the spread of disease and unpleasant odors, keeping ecosystems healthy and in balance.
4. They Enrich and Build the Soil: The process of decomposition creates humus, a dark, crumbly material that is the lifeblood of fertile soil. Humus acts like a sponge, helping the soil retain moisture and improving its structure. This creates a better environment for seeds to sprout and for plant roots to spread and gather nutrients.

Question 6. 

What is a food chain?

Ans:

Think of a food chain as nature’s way of passing along a packet of energy. It’s a simple, straight-line link that shows what eats what in order to live.

Plants don’t eat other organisms; they create their own food using sunlight.

Next comes a consumer, an animal that eats the plant to get energy. This first consumer is called an herbivore.

Then, another animal, a predator, might eat that herbivore. This transfer of energy continues, with each link in the chain being a meal for the next.

For example, a classic chain is: Grass (producer) → Grasshopper (first consumer) → Frog (second consumer) → Snake (third consumer).

This sequence clearly shows how energy and nutrients move from one living thing to another in a direct pathway.

Question 7. 

Draw a simple diagram showing a food chain.

Ans:

This sequence demonstrates how nourishment and energy travel in a straightforward, one-path food chain found in a field or meadow.

Visualizing the Chain:

Sun’s Rays → Blade of Grass → Field Grasshopper → Common Frog → Garden Snake → Red-Tailed Hawk

Breaking Down the Journey:

• Sun’s Rays: The starting point. Sunlight acts as the fundamental fuel that powers the entire system.
• Blade of Grass (The Food Maker): This plant is a producer. It captures the sun’s energy, using it to create its own nourishment from air and soil in a process called photosynthesis. It forms the base of the chain.
• Field Grasshopper (The Plant Eater): This insect is a primary consumer. It cannot make its own food, so it gets energy by feeding directly on the grass.
• Common Frog (The Insect Hunter): Acting as a secondary consumer, the frog preys on the grasshopper. It is a carnivore that eats plant-eating animals.
• Garden Snake (The Frog Predator): This reptile is a tertiary consumer. It occupies a higher level by consuming the frog, feeding on animals that already eat other animals.
• Red-Tailed Hawk (The Top Hunter): This bird of prey is the apex predator, or quaternary consumer. It hunts and eats the snake. In this specific chain, nothing preys upon the hawk, making it the final stop.

Question 8. 

Describe the energy flow in an ecosystem.

Ans:

The One-Way Journey of Energy in an Ecosystem

In any ecosystem, energy follows a strict, one-way path. It enters not as food, but as sunlight. Plants and other producers capture this light energy and, through photosynthesis, convert it into chemical energy stored in sugars.

This stored energy then moves in a single direction through the food chain. Herbivores consume the plants, carnivores consume the herbivores, and top predators sit at the end of the line. At every step, most of the energy (about 90%) is lost as waste heat through metabolism and daily activities. This is why there are far more plants than predators.

Decomposers, like fungi and bacteria, are the final link. They break down waste and dead organisms, releasing the last remaining energy as heat and recycling nutrients back into the soil. The key takeaway is that energy flows in one direction—from sun, to producer, to consumer, to heat—and is never recycled.

Question 9. 

State the law which governs the energy flow in an ecosystem.

Ans:

The principle that directs how energy moves through a living community, from plants to animals, is formally known as the 10% Law of Energy Flow in an Ecosystem.

This law states that when energy is transferred from one feeding level to the next—for instance, from plants to plant-eaters, or from a smaller fish to a larger predator—only about ten percent of the usable energy is successfully passed on. The vast majority, roughly ninety percent, is lost to the environment. This loss occurs primarily because organisms use most of the energy they consume for their own life-sustaining processes, such as movement, maintaining body temperature, and basic bodily functions. The remaining energy, which is not lost or used up, is stored in the body and becomes available for the next creature in the food chain.

This principle explains why a food pyramid is so narrow at the top; there is simply not enough energy remaining after so many losses to support a large population of top predators.

Question 10. 

Show that the energy flow in an ecosystem is linear.

Ans:

The Linear Flow of Energy in an Ecosystem

The statement that energy flow is “linear” means it moves in a single, non-cyclic direction, unlike nutrients which are recycled. This one-way journey can be shown through two key principles:

1. The Unidirectional Path: Energy enters an ecosystem primarily as sunlight. Plants (producers) capture this energy and convert it into chemical energy via photosynthesis. When a herbivore (primary consumer) eats the plant, a portion of this stored energy is transferred. Subsequently, a carnivore (secondary consumer) that eats the herbivore gains a fraction of that energy. At each step, a large amount of energy (about 90%) is lost as heat to the environment through metabolic processes, following the laws of thermodynamics. This lost heat cannot be recaptured by plants to restart the cycle, forcing the energy to flow forward only, from the sun up through the food chain.
2. Non-Recyclable Heat Loss: Unlike matter, energy does not cycle back to the start. Decomposers break down dead organisms, releasing nutrients back into the soil, but they do not release usable energy back to the sun or the producers. They simply release the remaining stored energy as heat, which is the final, irreversible exit of energy from the system.

In essence, the flow is a one-way street: Sun → Producers → Consumers → Heat. This irreversible dissipation of energy at every stage demonstrates its linear, non-cyclic nature.

Question 11. 

Draw a simple diagram showing the energy flow in a food chain.

Ans:

Picture a simple food chain on a grassland:

Sunlight shines down → Grass uses this light to grow → A Grasshopper eats the grass → A Shrew hunts and eats the grasshopper → A Hawk, soaring above, spots and catches the shrew.

This sequence is more than just a “who eats who” list; it is a pathway for energy, and this energy follows a strict, one-way journey.

The One-Way Journey of Energy

1. The Sun Ignites Everything: The entire process is kick-started by the sun. It showers the earth with light energy, but this energy cannot be eaten directly by animals.
2. The Producers Capture It: Green plants, like grass, act as the ecosystem’s solar panels. Through photosynthesis, they perform a kind of magic, trapping a tiny slice (roughly 1%) of the sun’s light and converting it into stored chemical energy inside their leaves and stems. They are the only ones who can create this stored energy from scratch, which is why they are called producers.
3. The Consumers Pass It On: When the grasshopper chews on the grass, it isn’t just eating food; it’s consuming a packet of the sun’s energy. However, this transfer is incredibly inefficient. The grasshopper uses most of that energy immediately to power its own life—jumping, digesting, and simply staying alive. This used energy escapes as body heat. In the end, only about 10% of the energy from the grass becomes new body mass in the grasshopper, ready for the next in line.
4. The Diminishing Flow Continues: This 10% rule continues up the chain. The shrew that eats the grasshopper only successfully locks away about 10% of the grasshopper’s energy. The hawk, at the very top, receives the smallest, most diluted fraction of the sun’s original power.

The Critical Result:

Energy flows in a single direction: from the sun, through the chain, and out as waste heat. It is never recycled. This constant loss at every stage is the fundamental reason why food chains are so short. After a few steps, there simply isn’t enough energy left in the flow to power another living creature. The hawk, at the end of the line, sits at the tip of a rapidly narrowing energy pyramid.

Question 12. 

Draw a diagram to show that the energy flow in an ecosystem is governed by the law of conservation of energy.

Ans:

Energy’s One-Way Journey in Nature

An ecosystem is a testament to the unbreakable rule that energy cannot be created or destroyed, only changed in form. The diagram illustrates this as a one-way flow, not a cycle.

The Path of Energy:

1. The Source: The journey starts with the sun, the primary energy input.
2. First Transformation: Plants (producers) capture this solar energy, transforming it into chemical energy through photosynthesis.
3. Transfer and Loss: When a herbivore (primary consumer) eats a plant, it transfers this chemical energy. Crucially, at this step and every following one, most of the energy is not passed on but is transformed into heat and lost to the environment due to metabolic processes.
4. Up the Chain: Carnivores (secondary and tertiary consumers) eat other animals, receiving a dwindling amount of the original energy, with heat loss continuing at each meal.
5. The Final Stop: Decomposers consume waste and dead matter, extracting the last usable chemical energy. The final, unusable output is heat, radiated out of the ecosystem.

The Law in Action:

The system perfectly demonstrates conservation. The total solar energy input is never destroyed. It is simply transformed step-by-step—from light to chemical energy, and finally, entirely into heat, which is its final, dispersed form. Energy flows in, changes form, and flows out, always accounted for.

Exercise 6 (B)

Question 1. 

Food chain begins with 

1. Respiration  
2. Photosynthesis
3. Decomposition   
4. Decay

Question 2. 

The source of energy in an ecosystem is 

1. Sun  
2. Decayed bodies
3. Green plants
4. Sugar

Question 3. 

Energy enters in a food chain through

1. Primary consumers
2. Secondary consumers
3. Tertiary consumers
4. Producers

Question 4. 

The place of human being in food chain in an ecosystem is as  

1. Producer
2. Consumer
3. Decomposer
4. Both (a) and (b)

Exercise 6 (C)

Question 1. 

State two characteristics which a source of energy must have.

Ans:

The Hallmarks of a Truly Effective Energy Source

When we evaluate different ways to power our world, from lighting a single home to fueling entire nations, two fundamental qualities separate the most viable energy sources from the rest. For any energy source to be considered truly effective, it must not only be powerful but also predictable and adaptable to our needs.

1. Exceptional Energy Density and Potent Output

First and foremost, a prime energy source must pack a powerful punch. This is measured by its energy density—the amount of energy stored in a given volume or mass. A source with high energy density is crucial because it means we can generate a massive amount of usable power from a relatively small amount of fuel.

Think of it this way: a source that requires warehouses full of material to power a single city is inherently less practical than one that can do the same job with a truckload. This is why sources like nuclear fission, where a pellet of uranium fuel smaller than a fingertip can release energy equivalent to burning a ton of coal, are so compelling. Similarly, the quest for better batteries revolves around increasing their energy density, allowing electric vehicles to travel further and devices to last longer on a single charge. High energy density translates directly to efficiency, lower transportation costs, and a smaller physical footprint.

2. Dependable and On-Demand Availability

Power that is potent but unreliable is of limited use. The second critical characteristic is that an energy source must be dependable and dispatchable. This means it can be reliably harnessed whenever it is needed, day or night, regardless of the weather or season.

Our modern societies and economies function on a 24/7 cycle, with energy demand that spikes in the morning, dips overnight, and fluctuates instantly. An effective energy source must be able to respond to this ebb and flow. We must be able to “turn it on” during a windless, overcast period or “ramp it up” to meet the surge when millions of people start their day. This controllability is the bedrock of a stable electrical grid. While wind and solar power are valuable, their inherent intermittency—the sun sets, the wind stops—poses a challenge that highlights the indispensable role of dispatchable sources like natural gas, hydropower (with reservoirs), or nuclear power, which can provide a constant, unwavering base load of electricity.

Question 2. 

Name the two groups in which various sources of energy are classified. State on what basis they are classified.

Ans:

The various sources of energy are broadly sorted into two primary categories.

1. Renewable Sources of Energy
2. Non-Renewable Sources of Energy

These groups are not formed arbitrarily; they are classified based on a single, crucial criterion: the natural ability of an energy source to replenish itself within a practical human timeframe.

Here is a closer look at the basis for this classification:

• Renewable Sources are classified as such because they are naturally restored at a speed that is equal to or faster than the rate at which we consume them. They originate from continuous or repetitive natural processes that are virtually inexhaustible. For instance, sunlight keeps shining, wind keeps blowing, and new plant matter can always be grown. Their key characteristic is sustainability over long periods.
• Non-Renewable Sources fall into this category because they exist in a finite stock on Earth. Once used, they are gone for a practical purpose, as their natural formation takes millions of years. These are essentially energy deposits inherited from the past, like fossil fuels (coal, oil) formed from ancient organic matter or minerals like uranium. Their defining trait is depletion; as we extract and use them, the remaining reserves dwindle.

Question 3. 

What is meant by the renewable and non-­renewable sources of energy? State two differences between them, giving two examples of each

Ans:

Understanding Our Energy Sources

Energy sources power our modern world, but they are not all created equal. Based on their natural replenishment rate, we classify them into two fundamental categories: renewable and non-renewable sources.

Renewable Energy Sources

These are energy sources that are naturally replenished at a rate that is equal to or faster than the rate at which we consume them. Think of them as a steady, ongoing income. They are typically derived from ongoing natural processes and are often considered “clean” or “green” because their environmental impact is significantly lower.

• Examples:

1. Solar Energy: This is power harnessed directly from the sun’s rays using technologies like photovoltaic panels. As long as the sun shines, we have access to this energy.
2. Wind Energy: This utilizes the kinetic energy of moving air, captured by large wind turbines to generate electricity. Wind is caused by the uneven heating of the atmosphere by the sun, making it a perpetually available resource.

Non-Renewable Energy Sources

These are energy sources that exist in finite quantities on Earth. They take an extremely long time—often millions of years—to form, which means once we use them up, they are essentially gone for all practical purposes. Think of them as a one-time inheritance from the ancient Earth. Their extraction and use often come with significant environmental consequences.

• Examples:

1. Coal: A fossil fuel formed from ancient plant matter over millions of years. It is mined and burned primarily for electricity generation and industrial processes.
2. Natural Gas: Another fossil fuel, often found in association with oil deposits. It is a gaseous mixture consisting primarily of methane and is used for heating, cooking, and electricity generation.

Key Differences Between Renewable and Non-Renewable Energy

Here are two fundamental distinctions that set them apart:

1.  Replenishment Rate and Long-Term Availability

• Renewable: These sources are sustainable and virtually inexhaustible from a human timescale. The sun will shine, and the wind will blow for billions of years, regardless of our energy consumption.
• Non-Renewable: These sources are finite and depletable. There is a fixed amount of coal, oil, and natural gas in the Earth’s crust. Once these reserves are exhausted, they cannot be replaced within any meaningful human timeframe.

2. Environmental Impact and Pollution

• Renewable: They generally have a minimal direct environmental impact. While manufacturing the infrastructure (like solar panels or turbines) has a footprint, the operation itself produces little to no air pollution or greenhouse gases.
• Non-Renewable: Their use is a major source of pollution. Burning fossil fuels releases large amounts of carbon dioxide (CO₂), a primary greenhouse gas responsible for climate change, as well as other pollutants that cause smog and acid rain.

Question 4. 

Select the renewable and non-renewable sources of energy from the following :

1. Coal
2. Wood
3. Water
4. Diesel
5. Wind
6. Oil

Ans:

We can sort these different energy supplies into two simple categories. The key question is whether the Earth can replace them in a time frame that matters to humanity.

Energy from Nature’s Continuous Flow

These fuels or forces are part of ongoing natural cycles. They are constantly being restored and are not in danger of running out.

• Wood: This is a form of stored sunlight and nutrients. As a type of biomass, it can be regrown. Sustainable harvesting and replanting create a cycle that can provide fuel indefinitely.
• Water: The power from moving water in rivers is sustained by the planet’s massive water cycle. Energy from the sun lifts water into the air, which then falls as rain and flows back to the sea, constantly refilling the rivers we dam.
• Wind: The movement of air across the planet is a direct result of the sun warming the atmosphere. Since solar heating is a constant process, wind patterns will persist as a usable force.

Energy from Earth’s Ancient Stores

These resources are like a one-time inheritance from the distant past. They were created under unique geological conditions over millions of years and exist in a fixed amount. When we use them, they are effectively gone for good.

• Coal: This is the compressed and transformed remains of ancient forests that existed long before humans. The planet is not creating new coal deposits in any meaningful way.
• Oil: Formed from the fossilized remains of tiny sea creatures, this liquid fuel is trapped in underground reservoirs. We are extracting and burning it at a pace that is countless times faster than it was formed.
• Diesel: This fuel is not found naturally by itself. It is manufactured from crude oil, so its availability is entirely tied to the finite supply of its non-renewable parent substance.

Question 5. 

Why is the use of wood as a fuel not advisable although wood is a renewable source of energy?

Ans:

While it’s true that wood is a renewable resource—after all, we can plant new trees to replace the ones we burn—relying on it as a primary fuel source comes with significant drawbacks that make it generally inadvisable on a large scale. Here’s a breakdown of the key reasons.

1. Environmental Damage Beyond Carbon Neutrality

The common argument for wood is that it’s “carbon neutral,” meaning the carbon dioxide (CO₂) released when burned is equal to what the tree absorbed while growing. However, this theory often doesn’t match reality.

• The Critical Time Lag Problem: A tree takes decades to grow and re-absorb the carbon. When we burn a tree, all its stored carbon is released into the atmosphere instantly. It will take a newly planted sapling many years to recapture that carbon. In this crucial timeframe, burning wood contributes directly to the greenhouse effect and climate change.
• Incomplete Combustion: Wood fires do not burn cleanly. They release other potent greenhouse gases like methane and black carbon (soot), which can be far more damaging to the climate than CO₂ in the short term.
• Deforestation and Soil Degradation: When demand for firewood outpaces sustainable growth, it leads to deforestation. This loss of tree cover destroys wildlife habitats, increases soil erosion (washing away fertile land), and can disrupt local water cycles, making areas more prone to droughts and floods.

2. Direct Harm to Human Health

This is arguably the most immediate and severe drawback.

• Indoor Air Pollution: In homes where wood is burned for cooking and heating, the indoor air quality becomes dangerously toxic. The smoke contains fine particulate matter (PM2.5) that can penetrate deep into the lungs, causing and exacerbating respiratory illnesses like asthma, bronchitis, and pneumonia. The World Health Organization links millions of premature deaths annually to household air pollution from solid fuels like wood.
• Toxic Chemical Release: Wood smoke contains a cocktail of harmful chemicals, including carbon monoxide (a poisonous gas), benzene, and formaldehyde, which are known carcinogens. Consistently breathing this air poses a serious long-term health risk.

3. Inefficiency as an Energy Source

Compared to modern energy sources, wood is a very inefficient fuel.

• Low Energy Density: Wood contains less usable energy per kilogram than fuels like propane, natural gas, or even charcoal. This means you need to burn a much larger physical volume of wood to produce the same amount of heat, leading to more work in collection and more pollution.
• High Moisture Content: Freshly cut “green” wood has a high water content, which wastes a significant amount of the fire’s energy just to boil off the water before it can produce useful heat. This makes it both inefficient and polluting.

4. Practical and Economic Drawbacks

On a personal and societal level, using wood fuel is often impractical.

• Labor Intensive: The process of felling trees, chopping logs, and transporting firewood is extremely laborious and time-consuming.
• Land Use Competition: Using vast areas of land to grow trees for fuel competes directly with the need for land to grow food (agriculture) and to preserve natural forests for biodiversity and ecosystem services.

Question 6.

Name five renewable and three non-renewable sources of energy.

Ans:

Five Renewable Energy Sources:

1. Solar Power: Capturing the sun’s radiant light and heat, typically using photovoltaic panels or concentrated solar systems.
2. Wind Power: Harnessing the kinetic energy of moving air with large turbines that convert it into electrical current.
3. Hydropower: Generating electricity by using the gravitational flow of water, often from rivers or dam reservoirs, to spin turbines.
4. Geothermal Energy: Tapping into the immense heat stored deep within the Earth, from hot rocks or reservoirs of steam and hot water.
5. Biomass Energy: Releasing stored chemical energy by burning or processing organic materials like wood, agricultural waste, or specially grown energy crops.

Three Non-Renewable Energy Sources:

1. Coal: A combustible black rock formed from ancient plant matter, mined and burned primarily for electricity generation.
2. Crude Oil: A liquid fossil fuel extracted from underground reservoirs, refined into products like gasoline, diesel, and petrochemicals.
3. Natural Gas: A flammable gas, primarily methane, found in underground rock formations and used for heating, cooking, and power plants.

Question 7. 

1. What is tidal energy ?  Explain in brief. 

2. What is ocean energy? Explain in brief. 

3. What is geothermal energy? Explain in brief.

Ans:

1. Tidal Energy

Tidal energy captures power from the predictable, moon-driven rise and fall of ocean tides. Engineers often build a barrier across a coastal bay. As the tide comes in, gates open to fill the basin; when the tide goes out, the gates close. The trapped water is then released through turbines, generating electricity. Its main strength is its perfect predictability for decades. However, the high cost of building these structures and their potential to disrupt local sea life are significant hurdles.

2. Ocean Energy

Ocean energy is a wide field that taps into the sea’s different movements. It’s more than just tides. This category includes using the relentless power of waves, the steady flow of deep-sea currents with underwater turbines, and even harnessing the temperature difference between warm surface water and cold deep water. While the total energy available in the world’s oceans is colossal, the technology is still young. The challenge is building machines tough enough to survive the harsh, corrosive saltwater environment while keeping costs down.

3. Geothermal Energy

By drilling deep wells, we can access underground reservoirs of steam or hot water. This steam can directly spin turbines for electricity, or the hot water can heat buildings and greenhouses. Its greatest benefit is reliability; it provides a constant, uninterrupted power supply, day and night, in any weather. The catch is location—it works best in geologically active areas where the Earth’s heat is close to the surface, like near tectonic plate boundaries or hot springs.

Question 8. 

What is the main source of energy for Earth?

Ans:

The primary engine for virtually all processes on our planet is a single, colossal star: the Sun. It functions as Earth’s ultimate power station, delivering a constant stream of energy that drives our world’s systems.

This solar energy is the direct or indirect fuel for:

• Our Global Climate and Weather: The Sun unevenly heats the atmosphere and oceans. This creates temperature differences, which in turn generate winds, ocean currents, and the entire water cycle of evaporation, cloud formation, and precipitation.
• Nearly All Life: Through the remarkable process of photosynthesis, plants, algae, and some bacteria capture sunlight. They convert this solar energy into chemical energy, forming the foundational level of almost every food web on the planet. The energy in the food we eat can be traced back to the Sun.
• Many Energy Sources We Harness: Fossil fuels like coal, oil, and natural gas are essentially ancient, stored sunlight. They formed from prehistoric plants and organisms that grew using solar energy millions of years ago. Similarly, power from wind, hydropower, and even modern solar panels are all direct conversions of the Sun’s energy.

While other minor energy sources exist, such as the lingering internal heat from Earth’s formation and the gravitational pull from the Moon (which causes tides), their influence is tiny compared to the overwhelming and constant flow of power from the Sun. It is the undisputed master of Earth’s energy budget.

Question 9. 

What is solar energy? How is solar energy used to generate electricity in a solar power plant?

Ans:

Harnessing the Sun’s Power

Solar energy is power captured directly from the sun’s rays. It is an essentially limitless resource that provides a clean and renewable alternative to traditional fuels.

From Sunlight to Your Socket

A solar power plant generates electricity through a direct conversion process:

1. Capture: Vast fields of solar panels, made of light-sensitive silicon cells, absorb sunlight.
2. Create: The energy from the sunlight knocks electrons within the silicon loose, creating a flow of electric current.
3. Convert: This initial current is direct current (DC). Inverters change it into the alternating current (AC) used in our buildings.
4. Supply: The AC power is sent to the electrical grid, becoming part of the supply that energizes homes and cities.

Question 10. 

What is a solar cell? State two uses of solar cells. State whether a solar cell produces a.c. or dc. Give one disadvantage of using a solar cell.

Ans:

A solar cell is a compact electronic component, typically a thin wafer made from specially treated materials like silicon, that has a unique property: it can directly capture energy from sunlight and convert it into a flow of electricity. This process, known as the photovoltaic effect, allows light particles (photons) to knock electrons loose within the material, creating an electric current.

Two practical uses of solar cells are:

1. Providing the essential electrical power for satellites and space probes orbiting the Earth, where replacing wires or batteries is impossible.
2. Operating everyday items like pocket calculators and garden lights, freeing them from the need for disposable batteries or a power cord.

A solar cell generates direct current (DC). This means the electric current it produces flows in a single, steady direction.

One notable disadvantage of using a solar cell is its complete dependence on a consistent source of light. It generates no power at night, and its output drops significantly during cloudy weather or on short winter days, making an energy storage system, like a battery, necessary for a continuous power supply.

Question 11. 

State two advantages and two limitations of producing electricity from solar energy.

Ans:

Two Advantages:

1. Energy Independence: It allows homes and businesses to generate their own power, reducing reliance on the traditional utility grid and fluctuating energy prices.
2. Low Operational Noise: Unlike generators or large power plants, solar panels produce electricity silently, making them ideal for residential and sensitive natural areas.

Two Limitations:

1. Intermittent Power Generation: Electricity is only produced during daylight hours, creating a mismatch with peak energy demand which often occurs in the evening.
2. Land Use Demand: Large-scale solar farms require significant amounts of space, which can lead to competition with agricultural land or natural habitats.

Question 12. 

What is wind energy? How is wind energy used to produce electricity? How much electric power is generated in India using wind energy?

Ans:

What is Wind Energy?

Wind energy is the power derived from the movement of air across our planet. This motion, which we experience as wind, is ultimately a product of the sun’s energy. The sun heats the Earth’s surface unevenly; the equator becomes warmer than the poles, and land heats up and cools down faster than water. Air, like water, naturally flows from areas of high pressure to areas of low pressure, and this flow is what we harness as wind energy. It is a form of solar power transformed into kinetic energy—the energy of motion.

How is Wind Energy Converted into Electricity?

The conversion from moving air to household electricity happens inside a wind turbine, a modern evolution of the classic windmill. The process can be broken down into a few key stages:

1. Catching the Wind: The entire turbine is strategically positioned to face the prevailing wind. As the wind blows, it pushes against the large, aerodynamically designed blades of the turbine, causing the entire rotor (the hub holding the blades) to spin.
2. Spinning the Gears: The rotor is connected to a main shaft inside the turbine’s nacelle (the central housing). This shaft spins at a relatively slow speed. To generate electricity effectively, this slow rotation needs to be dramatically increased. The main shaft is connected to a gearbox, which acts like the gears on a bicycle, multiplying the rotational speed to several hundred times per minute.
3. Generating the Current: The high-speed shaft from the gearbox spins a powerful magnet surrounded by tightly coiled copper wire. This crucial component is the generator. Through the principle of electromagnetic induction, the rapid rotation of the magnet within the coil of wire forces electrons to move, creating an alternating electrical current (AC).
4. Sending Power Out: This newly generated electricity travels down cables inside the tall tower of the turbine. A transformer located at the base then boosts the voltage to a very high level, making it efficient for transmission over long distances through power lines, eventually reaching our homes, schools, and businesses.

Wind Power Generation in India

India stands as a global leader in harnessing wind power. The nation has a long coastline, vast plains, and hilly terrains that create excellent wind corridors, particularly in states like Tamil Nadu, Gujarat, Maharashtra, and Karnataka.

As of the latest official data, India’s total installed capacity for wind power generation has crossed a significant milestone. The country has successfully installed over 45 Gigawatts (GW) of wind power infrastructure. To put this immense figure into perspective, one gigawatt is enough to power approximately one million modern homes. Therefore, this installed capacity represents a massive contribution to the country’s electricity grid, helping to light up millions of households and fuel industrial growth while reducing reliance on fossil fuels.

Question 13. 

State two advantages and two limitations of using wind energy for generating electricity.

Ans:

Advantages:

1. Fuel-Free Operation: Once constructed, wind turbines generate electricity without any ongoing fuel cost or the need for fuel extraction and transportation, making it immune to market price fluctuations for resources like coal or gas.
2. Minimal Land Conflict: Wind farms can be effectively combined with agricultural land, allowing farmers to continue growing crops or grazing livestock right up to the base of the turbines, preserving the primary use of the land.

Limitations:

1. Intermittent and Unpredictable Supply: Wind energy is not available on-demand; electricity generation is entirely dependent on weather conditions, which can be inconsistent and difficult to forecast accurately for grid management.
2. High Initial Investment and Location Constraints: The upfront cost for manufacturing, transporting, and installing massive turbines is very high, and they are only feasible in locations with consistently strong wind resources, which are often remote from the urban centers that need the power.

Question 14. 

What is hydro energy? Explain the principle of generating electricity from hydro energy. How much hydro electric power is generated in India?

Ans:

What is Hydro Energy?

Hydro energy, often called hydropower, is the power derived from the movement and weight of flowing water. It is a way of capturing the natural water cycle—driven by the sun’s energy evaporating water, which later falls as rain and flows downhill—and converting it into a usable form of power for human activities. It’s one of the oldest and most established sources of renewable energy.

The Principle of Generating Electricity

The core idea behind generating electricity from water is the conversion of energy from one form to another. It follows a simple chain of transformation:

1. Potential to Kinetic Energy: First, a dam is built across a river to create a massive reservoir. The water held back in this reservoir is stored energy, specifically gravitational potential energy. When gates in the dam are opened, this stored water is released and begins to flow downhill forcefully. As it flows, its potential energy transforms into kinetic energy—the energy of motion.
2. Kinetic to Mechanical Energy: The fast-flowing water is channeled through large pipes, called penstocks, which direct it squarely onto the blades of a giant turbine. The powerful rush of water hits these curved blades, causing the entire turbine shaft to spin rapidly. At this stage, the water’s kinetic energy has been transferred to the turbine as mechanical energy (spinning motion).
3. Mechanical to Electrical Energy: The spinning turbine shaft is connected directly to a rotor inside a device called a generator. This rotor is surrounded by a stationary set of magnets and copper wires (the stator). As the rotor spins inside this magnetic field, it disturbs the field and forces electrons in the copper wires to move, creating an electric current. This process, known as electromagnetic induction, is the final step where mechanical energy becomes electrical energy.

This newly created electricity is then sent through power lines to homes, schools, and industries.

Hydro Electric Power Generation in India

India has a significant amount of hydroelectric power capacity. As of the most recent official data, the installed capacity for hydro power in the country is approximately 47 Gigawatts (GW).

To provide a clearer picture of its scale, this capacity is spread across numerous large, medium, and small projects, predominantly in the Himalayan states and the mountainous regions of the Northeast, which have the necessary perennial rivers and sloping terrain. This 47 GW contribution is a vital part of India’s energy mix, providing stable, renewable power and helping to manage peak electricity demand.

Question 15. 

State two advantages and two disadvantages of producing hydro electricity.

Ans:

Advantages:

1. Fuel-Free Power: Once the dam is built, it runs on the water’s natural flow, eliminating fuel costs and price volatility.
2. On-Demand Electricity: Water can be stored and released to generate power instantly when demand is highest, unlike intermittent sources like solar.

Disadvantages:

1. Ecosystem Disruption: Dams dramatically alter river habitats, blocking fish migration and impacting downstream communities and wildlife.
2. Human Displacement: Creating large reservoirs often floods towns and fertile land, forcing entire communities to relocate from their homes.

Question 16. 

What is nuclear energy? Name the process used for producing electricity using nuclear energy.

Ans:

Harnessing the Heart of the Atom

At the very center of every atom lies its nucleus, a tiny bundle of immense, locked-in power. This hidden force, known as nuclear energy, is the fundamental glue holding the nucleus together. To generate electricity, we use a specific method called nuclear fission to carefully tap into this powerful source.

Imagine an atom of a heavy substance like Uranium-235. This atom is naturally prone to breaking apart. The process begins when we introduce a single neutron—a subatomic particle—into its nucleus. This neutron acts like a trigger, causing the unstable uranium nucleus to fracture into two smaller, lighter atoms.

This act of splitting, which is the essence of nuclear fission, is profoundly powerful. It accomplishes two critical things at once:

1. It unleashes a tremendous amount of energy in the form of intense heat.
2. It propels several new neutrons outward from the split atom.

These newly released neutrons then become projectiles, striking the nuclei of nearby uranium atoms. This causes those atoms to split, repeating the process and releasing more heat and more neutrons. This ongoing, self-perpetuating cycle is what scientists call a chain reaction.

Inside the core of a nuclear power plant, this chain reaction is not allowed to run wild; it is meticulously managed with control rods. The steady, controlled fission of countless atoms produces immense and consistent heat. This heat is used to turn water into steam. The powerful steam drives the blades of a massive turbine, which in turn spins an electricity generator, ultimately producing the power that flows through our grid.

To recap the journey:

• The source is the incredible potential energy bound within an atom’s nucleus.
• The method is nuclear fission, a carefully managed chain reaction that transforms atomic mass into usable thermal energy.

Question 17.

What percentage of total electrical power generated in India is obtained from nuclear power plants? Name two places in India where electricity is generated from nuclear power plants.

Ans:

In the vast and complex picture of India’s energy sector, nuclear power holds a distinct and strategic position. While the country’s primary energy needs are met by coal and a rapidly growing influx of renewables, electricity generated from nuclear fission provides a crucial, stable, and low-carbon component of the national grid.

Recent assessments indicate that nuclear energy’s contribution, while not yet a dominant force, is both steady and significant. It consistently accounts for a small but vital portion of the nation’s total electricity output, typically falling within a range of approximately 2.5% to 3.2%. This figure represents a substantial amount of reliable, base-load power that is available around the clock, complementing the intermittent nature of solar and wind energy.

The physical footprint of this power is spread across several dedicated sites known as “Nuclear Power Plants.” Two of the most prominent and often-discussed locations are:

1. Tarapur, Maharashtra: Home to the Tarapur Atomic Power Station (TAPS), this facility holds the distinction of being India’s first nuclear power plant. Its establishment marked the beginning of the country’s domestic nuclear power journey. Located on the scenic coast of the Arabian Sea, the Tarapur plant has been a cornerstone of the western grid’s power supply for decades.
2. Kudankulam, Tamil Nadu: Situated in the southern tip of the country in the Tirunelveli district, the Kudankulam Nuclear Power Plant (KKNPP) represents the new era of India’s nuclear ambitions. Known for its advanced reactor technology and large generating capacity, Kudankulam is one of the most powerful nuclear facilities in the nation. Its location on the coast of the Gulf of Mannar is strategic, providing the necessary seawater for reactor cooling.

Question 18. 

State two advantages and two disadvantages of using nuclear energy for producing electricity.

Ans:

Advantages of Nuclear Energy

1. Intense and Reliable Power Generation
   A single nuclear power station can produce a tremendous amount of electricity continuously. Unlike solar or wind power, which rely on weather conditions, a nuclear reactor can operate non-stop for 18 to 24 months before needing refueling. This makes it an exceptionally steady and dependable source for meeting a region’s constant, high-demand energy needs.
2. Minimal Air Pollution During Operation
   The process of splitting atoms to generate power does not involve combustion. Therefore, a functioning nuclear plant releases virtually no air pollutants like smoke, soot, or the greenhouse gases (such as carbon dioxide) that are commonly associated with burning fossil fuels like coal or natural gas.

Disadvantages of Nuclear Energy

1. The Perpetual Challenge of Radioactive Waste
   The used fuel rods and other materials from the nuclear reaction remain highly radioactive and dangerously hazardous for thousands of years. There is no universally agreed-upon permanent solution for disposing of this waste. The current method of storing it in specialized facilities requires extreme security and maintenance, creating a long-term burden and potential risk for future generations.
2. Extremely High Initial Costs and Slow Construction
   Building a nuclear power plant demands immense upfront investment. The costs are inflated by the need for exceptionally robust safety systems, containment structures, and complex regulatory approvals. This financial barrier, combined with construction timelines that often span a decade or more, makes nuclear energy a less attractive option compared to faster-to-build renewable energy projects.

Question 19. 

1. State the energy transformation in the following: electricity is obtained from solar energy. 

2. State the energy transformation in the following: electricity is obtained from wind energy. 

3. State the energy transformation in the following: electricity is obtained from hydro energy. 

4. State the energy transformation in the following: electricity is obtained from nuclear energy.

Ans:

1. Generating Power Using Sunlight

The journey starts with the sun emitting a constant stream of light energy. Specially designed solar panels capture this light. Inside these panels, the energy from the light knocks tiny electrons loose from their atoms. This creates a steady stream of electrical current, turning sunshine directly into usable electricity.

2. Harnessing the Wind for Power

This method captures the power of moving air. When wind flows, it carries kinetic energy. This pushing force spins the long blades of a wind turbine. The spinning motion turns a central shaft connected to a machine called a generator. Inside the generator, this rotation is magically changed into electrical power.

3. Tapping into Water for Electricity

The energy for hydropower begins high above. The sun’s warmth causes water to evaporate and later fall as rain, filling reservoirs. Water held at a height stores potential energy. When gates open, this stored energy becomes the rushing force of moving water. The powerful flow spins a large turbine, and that spinning action is converted by a generator into an electric current.

4. Releasing Power from the Atom

Nuclear power unlocks the tremendous energy hidden inside the core of specific materials, like uranium. When these atoms are split in a controlled reaction, they release a massive amount of heat. This heat is so intense it boils water into high-pressure steam. The force of this steam spins a turbine, and just like in other methods, the generator then produces electricity from this motion.

Question 20. 

State four ways for the judicious use of energy.

Ans:

Four Principles for the Judicious Use of Energy

Using energy judiciously isn’t just about saving money; it’s about cultivating a mindful relationship with the resources that power our lives. It means moving beyond wastefulness to embrace efficiency and purpose. Here are four fundamental ways to practice this:

1. Embrace Passive Design in Your Living and Working Spaces
Instead of relying solely on machines to heat or cool our environments, we can work with nature. This involves strategic thinking about a building’s orientation, insulation, and ventilation. For instance, planting deciduous trees on the sun-facing side of a home provides shade in the summer, reducing air conditioning needs, while allowing sunlight to warm the house in the winter after the leaves fall. Similarly, ensuring proper insulation acts like a thermos for your building, keeping desired temperatures in and extreme temperatures out, drastically cutting down the energy required for heating and cooling.

2. Shift from Consuming Energy to Harnessing It Through Daily Habits
This is about transforming our role from passive consumers to active participants. Simple, intentional actions make a significant collective impact. Examples include using a clothesline instead of a dryer, which uses the sun and wind—free and limitless energy sources. Opting for a bicycle or walking for short trips not only conserves fuel but also harnesses your own physical energy. Cooking with lids on pots reduces cooking time and energy use. These habits reconnect us to the direct results of our energy choices.

3. Prioritize “Energy Intelligence” When Making Purchases
Every product we buy has a hidden “energy life cycle”—from manufacturing to transportation to daily use. Being judicious means making choices that minimize this overall footprint. This includes buying high-quality, repairable appliances built to last for years, rather than disposable ones. Most importantly, it means selecting the most energy-efficient model available (looking for robust certification labels) for essential items like refrigerators or water heaters. The initial investment is quickly offset by years of lower energy consumption.

4. Systematically Eliminate “Phantom Loads” and Needless Standby Power
A vast amount of energy is wasted by electronics that are “off” but still drawing power, a phenomenon known as phantom or vampire loads. Televisions, chargers, computers, and kitchen appliances are common culprits. A judicious approach involves using advanced power strips that automatically cut power to devices when they are not in active use. Cultivating a habit of unplugging chargers and devices once they are fully charged or when you’ll be away for an extended period can eliminate this invisible drain, conserving a surprising amount of energy over time.

Question 21. 

What do you mean by degradation of energy? Explain it by taking two examples of your daily life.

Ans:

What is Degradation of Energy?

Degradation of energy doesn’t mean energy is destroyed; it’s the idea that energy transforms from a concentrated, useful form into a dispersed, less useful form, ultimately becoming low-grade heat that is difficult to harness for work. It’s the reason why perpetual motion machines are impossible.

In simpler terms, energy loses its “quality” and becomes scattered.

Example 1: Using a Phone

• The Process: Your phone battery stores chemical energy, a highly usable form. When you use it, this energy doesn’t just power the screen and processor; it also generates heat.
• The Degradation: The useful, concentrated chemical energy degrades into a mix of light, sound, and mostly waste heat. This heat dissipates into the air around your phone, becoming so scattered and low-temperature that it’s impossible to gather back to charge your battery. The energy is still in the room, but it’s now useless for its original purpose.

Example 2: Riding a Bicycle

• The Process: Your body converts the chemical energy from food into the kinetic energy of the moving bicycle.
• The Degradation: A significant portion of that energy isn’t used for motion. It is immediately degraded due to friction in the gears and against the road, and air resistance. This friction doesn’t make the energy vanish; it converts it directly into heat that warms the tires, the brakes, and the surrounding air. This heat is dispersed into the environment and is lost for propelling the bicycle forward.

Question 22. 

The conversion of part of energy into a useful form of energy is called ___________

Ans:

The conversion of part of energy into a useful form is called energy utilization.

Exercise 6 (C)

Question 1. 

The ultimate source of energy is : 

1. wood
2. wind
3. water
4. sun

Question 2. 

Renewable source of energy is :

1. coal
2. fossil fuels
3. natural gas
4. sun

Exercise 6 (D)

Question 1. 

What do you mean by greenhouse effect? 

Ans:

Think of our planet wrapped in a cozy blanket high up in the sky. This isn’t a wool blanket, but a natural layer of gases in our atmosphere, like carbon dioxide and water vapor.

Sunlight, which is solar energy, passes straight through this blanket to warm the Earth’s surface. As the Earth heats up, it gives off its own energy, a different kind of heat. But this “Earth-heat” can’t easily escape back through the atmospheric blanket. The gases trap it, much like the glass of a greenhouse keeps warmth inside for plants to grow.

This entire process is the greenhouse effect. It’s entirely natural and absolutely essential—without it, our world would be a frozen, lifeless ball of ice.

The central concern today is that human activities, such as burning fossil fuels, are dramatically thickening this blanket. With a thicker blanket, too much heat is being trapped, causing the planet to warm up at an unnatural rate. This is the enhanced greenhouse effect driving modern climate change.

Question 2. 

Name three greenhouse gases.

Ans:

Three gases that are particularly effective at trapping heat in our atmosphere are:

1. Carbon Dioxide (CO₂): This is the most widely discussed greenhouse gas, primarily released through activities like burning fossil fuels (coal, oil, natural gas) and deforestation.
2. Methane (CH₄): A very potent gas that traps heat much more effectively than carbon dioxide, though it remains in the atmosphere for a shorter time. Significant sources include livestock digestion, landfills, and natural gas leaks.
3. Nitrous Oxide (N₂O): Often called “laughing gas,” it is a powerful, long-lived greenhouse gas. Its release is heavily linked to agricultural practices, especially the use of nitrogen-based fertilizers on farmland.

Question 3. 

Which of the following solar radiations pass through the atmosphere of Earth: X -rays, ultraviolet rays, visible light rays or infrared radiation?

Ans:

How Earth’s Atmosphere Screens the Sun’s Rays

Sunlight arrives at our planet as a broad mixture of energy, but we on the surface experience only a carefully filtered version of it. Our atmosphere functions not as a simple window, but as a sophisticated shield, allowing certain types of radiation to pass while actively blocking others. This selective process is fundamental to life as we know it.

The Unseen Danger: High-Energy Radiation

The most energetic and destructive parts of solar radiation are stopped high above our heads.

• X-Rays: These potent rays are intercepted in the upper layers of the atmosphere. Upon collision with gas molecules, they are completely absorbed, preventing this biologically harmful radiation from ever reaching the ground. This is a primary reason why astronomers must use satellites in space to study cosmic X-ray sources.
• Ultraviolet (UV) Rays: The famous ozone layer, located in the stratosphere, serves as a vital sunscreen for the planet. It efficiently absorbs the majority of the sun’s ultraviolet radiation. The small fraction that does trickle through is the very reason we experience sunburn and must protect our skin, highlighting the critical importance of this atmospheric filter.

The Life-Giving Spectrum: Radiation That Reaches Us

The radiation that successfully navigates the atmospheric gauntlet defines our environment.

• Visible Light Rays: This narrow band of the solar spectrum passes through the atmosphere with remarkable ease. It encounters little obstruction, traveling directly to the surface to illuminate our world and provide the energy that drives photosynthesis, the foundation of nearly all ecosystems on Earth.
• Infrared Radiation: Often experienced as heat, a substantial portion of solar infrared radiation makes the journey to the surface. While certain atmospheric gases like water vapor and carbon dioxide do absorb some of it, a significant amount penetrates through to warm the Earth’s land and oceans.

Question 4. 

What results in the increase of carbon dioxide contents of earth’s atmosphere?

Ans:

The blanket of air surrounding our planet is getting thicker with a specific gas—carbon dioxide. This significant rise is not from a single source, but from a combination of human activities that disrupt the planet’s natural carbon cycle.

The primary driver is our overwhelming reliance on burning ancient fuels. Coal, oil, and natural gas, formed from prehistoric plant and animal matter over millions of years, are being extracted and combusted at a staggering rate. This process, which powers our electricity, industries, and vehicles, rapidly releases vast stores of carbon that had been locked safely underground back into the air as carbon dioxide.

Simultaneously, we are systematically dismantling the Earth’s natural systems for absorbing this gas. Rampant deforestation, especially of the vast tropical rainforests that act as the planet’s lungs, is a major contributor. When forests are cleared by burning, the carbon stored within the trees is immediately emitted. Even when land is just cleared for agriculture, we permanently remove living filters that would have continuously drawn carbon dioxide out of the atmosphere for decades.

Furthermore, modern agricultural practices themselves add to the problem. Certain farming methods disturb the soil, which is a massive reservoir of carbon, releasing it over time. The production of cement for concrete, a cornerstone of global construction, involves a chemical reaction that directly releases a significant amount of carbon dioxide.

Question 5. 

Name the radiations which are absorbed by the greenhouse gases.

Ans:

The Radiations Trapped by Our Atmospheric Blanket

To understand what greenhouse gases absorb, it’s helpful to first think about sunlight. The Sun bombards Earth with a wide spectrum of energy, but for this process, two specific types are crucial: Infrared Radiation and, to a lesser but important extent, Terrestrial Thermal Radiation.

1. The Primary Target: Long-Wave Infrared Radiation

This is the most significant player in the greenhouse effect.

• What it is: Infrared radiation (IR) is a form of invisible energy that we perceive as heat. After the Sun’s energy heats the Earth’s surface—the land and oceans—our planet doesn’t hold onto that heat forever. It re-radiates the energy back towards space. However, because the Earth is much cooler than the Sun, it releases this energy as long-wave infrared radiation.
• The Absorption Process: Greenhouse gases like carbon dioxide (CO₂), water vapor (H₂O), methane (CH₄), and nitrous oxide (N₂O) have a unique molecular structure. Their atoms are arranged in a way that allows them to vibrate at the same frequency as specific wavelengths of this outgoing long-wave IR. When the radiation’s frequency matches the molecule’s natural vibrational frequency, the gas absorbs the energy, much like a tuned radio antenna picks up a specific signal. This captured energy increases the kinetic energy (motion) of the gas molecules, effectively warming the surrounding atmosphere.

2. The Initial Source: Short-Wave Solar Radiation

While the primary “trapping” is of outgoing heat, some greenhouse gases also interact with incoming solar energy.

• What it is: This is the broad spectrum of energy emitted by the Sun, which includes visible light, ultraviolet (UV) light, and a portion of short-wave infrared radiation.
• The Absorption Process: Not all greenhouse gases absorb significant amounts of short-wave radiation. However, some, like ozone (O₃) and water vapor, do absorb specific bands within the solar spectrum. Ozone, for instance, is famous for absorbing the Sun’s harmful ultraviolet radiation in the stratosphere. This initial absorption plays a role in heating different layers of the atmosphere and contributes to the overall energy balance before the Earth even re-radiates it as long-wave heat.

In a Nutshell:

Think of it as a two-step process:

1. The Sun delivers energy (including short-wave IR and visible light), which passes through the atmosphere and warms the Earth.
2. The warmed Earth sends heat back towards space as long-wave infrared radiation. It is this outgoing long-wave heat radiation that is predominantly absorbed by greenhouse gases, acting like an insulating blanket that slows the escape of heat and maintains our planet’s habitable temperature.

Question 6. 

What would have been the temperature of earth’s atmosphere in absence of greenhouse gases in it?

Ans:

To understand the Earth’s temperature without its blanket of greenhouse gases, we can look at a simple principle of planetary energy. A planet’s baseline temperature is set by a balance: the solar energy it absorbs from the sun must equal the planetary energy it radiates back into space as heat.

Scientists perform this calculation by treating Earth as a theoretical “black body.” The result of this energy balance gives us an average global temperature. Without the atmospheric layer that traps heat, this calculated temperature would be the actual condition at the surface.

When we run the numbers, accounting for the fraction of sunlight our planet reflects away, the result is strikingly low. The Earth’s surface would settle at an average of approximately -18° Celsius (or about 0° Fahrenheit).

This is a profoundly frigid environment. For context, water freezes at 0°C. An average of -18°C means our world would be locked in a deep, global ice age, with liquid water existing only in rare, transient circumstances under the sun’s direct glare. Most life as we know it could not survive.

This stark reality highlights a crucial fact: the natural greenhouse effect is not an enemy. It is a vital planetary heating system. Greenhouse gases like water vapor and carbon dioxide act as an insulating layer, absorbing some of the heat the Earth radiates and warming the surface to our familiar, life-sustaining average of around 15°C (59°F). This difference of about 33 degrees Celsius is the gift of our natural atmosphere.

Question 7. 

State the effect of greenhouse gases on the temperature of earth’s atmosphere.

Ans:

The Delicate Balance of Our Planet’s Natural Heating System

Our planet’s climate is governed by a delicate and natural heating process, one that relies on a specific group of gases in our air. These gases don’t act like a solid barrier but function more like a thermal manager for the Earth. They are crucial for maintaining the stable temperatures that allow life to flourish. To grasp how this system works, and why its recent changes are so significant, we can follow the journey of energy from the sun to the ground and back again.

Step 1: The Arrival of Solar Power
Our sun constantly sends energy streaming toward Earth. This energy travels mostly as visible light and ultraviolet rays. The majority of our atmosphere, which is composed of nitrogen and oxygen, is largely transparent to these specific wavelengths. Think of it like sunlight passing through a clean window; it meets little resistance and efficiently reaches the planet’s surface, warming the land, oceans, and air.

Step 2: Earth’s Response and Energy Transformation
After being heated by the sun, the Earth doesn’t simply store that energy. It releases it back outward. However, the energy that leaves the Earth’s surface is different from what arrived. The planet re-radiates this absorbed energy as infrared radiation, which we perceive as heat. This is the same type of warmth you feel radiating from asphalt on a summer evening.

Step 3: The Interception by Specialized Gases
This is the critical stage where certain atmospheric gases come into play. While common gases like nitrogen and oxygen allow this outgoing heat to pass them by, trace gases such as carbon dioxide, methane, and water vapor behave differently. Their molecular structure has a unique property: it vibrates when it comes into contact with specific wavelengths of infrared radiation. This means they effectively capture and absorb the heat energy that the Earth is trying to release into the cold vacuum of space.

Step 4: The Creation of a Warmth Layer
Once these greenhouse gas molecules have absorbed the infrared energy, they don’t hold onto it permanently. They become temporarily energized and then re-emit the heat radiation in all directions. A portion of this re-radiated heat travels back toward the Earth’s surface. This process effectively recycling a portion of the planet’s warmth is the heart of the natural greenhouse effect. It’s not that heat cannot escape at all, but that the process is slowed down, creating a stable and livable global temperature range.

This natural mechanism is what prevents Earth from being a frozen, inhospitable rock. The current concern among scientists stems from human activities that are artificially adding more of these heat-absorbing gases into the atmosphere. It’s akin to adding an extra-thick layer of insulation to a house on a mild autumn day; the basic principle of retaining warmth is the same, but the intensified effect disrupts the system’s delicate balance, leading to a problematic and sustained increase in global temperatures.

Question 8. 

What do you mean by global warming?

Ans:

Think of our planet as wrapped in a cozy, invisible blanket we call the atmosphere. This blanket is perfect—it traps just enough of the sun’s warmth to keep our world livable. But what happens if that blanket gets too thick?

Global warming is precisely that: a gradual, long-term thickening of Earth’s atmospheric blanket, leading to a rise in the planet’s average surface temperature.

This thickening isn’t a natural change. It’s primarily caused by a sharp increase in certain gases, like carbon dioxide and methane, which we release by burning fuels like coal, oil, and gas for energy, transport, and industry. These gases are experts at trapping heat. Once they are in the atmosphere, they act like extra layers added to the blanket, preventing more heat from escaping back into space.

The “global” part is key. It’s not about one place having a hot day. It’s about the entire Earth’s climate system steadily accumulating more energy, which then disrupts long-standing weather patterns, melts ice caps, and raises sea levels. It’s the overall heating of our shared planetary home due to human activity that has thrown its delicate temperature balance out of sync.

Question 9. 

What causes the rise in atmospheric temperature?

Ans:

The Mechanics of a Warming Atmosphere: It’s All About the Blanket

Think of our atmosphere not as empty space, but as a vast, invisible blanket wrapped around the planet. A rise in its temperature happens because this blanket gets thicker. To understand why, we need to look at the delicate balance of energy coming in and going out.

The primary driver is the Greenhouse Effect, and it’s a natural process that is essential for life. Without it, Earth would be a frozen ball of ice, about -18°C (0°F) on average. The problem we face today is an enhancement of this natural effect, causing the blanket to trap too much heat.

Here’s a breakdown of the process, from the global scale down to the molecular level.

1. The Sun’s Energy Arrives

It all starts with the sun, which bombards Earth with a constant stream of energy. Most of this energy is in the form of shortwave radiation, which includes visible light. This light passes through our atmospheric blanket with ease, as if it were clear glass.

2. The Earth Absorbs and Re-radiates

The Earth’s surface—the land, oceans, and everything on them—absorbs this incoming solar energy. Once absorbed, the planet heats up. But a warm object doesn’t just stay warm; it re-radiates that energy back outward. However, it doesn’t send it back in the same form. The Earth radiates energy as longwave infrared radiation, which we feel as heat.

3. The “Greenhouse Gases” Act as the Blanket

This is the critical step. Our atmosphere is made up of mostly nitrogen and oxygen, which are transparent to this outgoing infrared heat. However, a small but powerful group of gases, known as greenhouse gases, are not.

Key players include:

• Water Vapor (H₂O): The most abundant and natural greenhouse gas.
• Carbon Dioxide (CO₂): Released naturally but massively by human activities like burning fossil fuels (coal, oil, gas) and deforestation.
• Methane (CH₄): A very potent gas from livestock, landfills, and natural gas leaks.
• Nitrous Oxide (N₂O): From agriculture and industrial processes.
• Synthetic Gases: Like chlorofluorocarbons (CFCs).

When the outgoing infrared heat tries to escape back into space, these greenhouse gas molecules absorb it. Think of them like tiny magnets for heat energy.

4. The Blanket Gets Thicker: The Enhanced Greenhouse Effect

After absorbing the energy, these excited gas molecules don’t hold onto it forever. They re-radiate the heat energy, but they do so in all directions—some upwards into space, but a significant amount back downwards towards the Earth’s surface.

This is the core of the issue. By sending a portion of the heat back, these gases warm the planet’s surface for a second time. It’s like the Earth’s own heat is being reflected back at it by the atmosphere.

Human activities, particularly since the Industrial Revolution, have dramatically increased the concentration of these gases, especially CO₂. We are, in effect, weaving more threads into the atmospheric blanket. A thicker blanket traps more heat, leading to a steady rise in global average temperatures—a phenomenon we call global warming.

Question 10. 

State the cause of the increase of the green house effect.

Ans:

The enhanced greenhouse effect, which leads to planetary warming, is primarily caused by a thickening of Earth’s atmospheric blanket. This thickening results from a significant increase in the concentration of certain heat-trapping gases released by human activities.

The main drivers are:

1. Burning of Ancient Carbon: Our reliance on coal, oil, and natural gas for energy and transport releases vast amounts of carbon dioxide that had been locked underground for millions of years. This single action is the largest contributor, overloading the atmosphere’s natural composition.
2. Widespread Deforestation: Forests act as vital carbon sinks, absorbing carbon dioxide from the air. Large-scale clearing of these forests, both through burning and logging, permanently removes these natural absorbers while simultaneously releasing the stored carbon back into the atmosphere.
3. Agricultural and Industrial Output: Modern farming practices, including livestock digestion and rice cultivation, generate potent gases like methane. Furthermore, industrial processes and the use of synthetic fertilizers release nitrous oxide and other industrial gases, all of which are far more effective at trapping heat than carbon dioxide.

Question 11. 

What will be the effect of global warming at the poles?

Ans:

The polar regions, the Arctic in the north and the Antarctic in the south, are experiencing the consequences of a warming planet more intensely than any other part of the world. They act as the Earth’s natural refrigerators, and as they heat up, the changes are both dramatic and far-reaching.

The most direct and visible impact is the rapid loss of ice. The great ice sheets covering Greenland and Antarctica are melting at an accelerating pace, pouring enormous volumes of meltwater into the ocean. Similarly, the sea ice that floats on the Arctic Ocean is not only shrinking in area but also becoming significantly thinner. This loss of bright, white ice creates a dangerous feedback loop. Since ice reflects sunlight back into space, its disappearance means the darker ocean or land underneath absorbs more solar heat, which in turn leads to even more warming and melting.

This large-scale thaw is fundamentally altering polar ecosystems. For animals like polar bears, seals, and walruses that depend on sea ice as a platform for hunting, resting, and breeding, the loss of their habitat is a direct threat to their survival. Their food becomes harder to find, leading to malnutrition and population decline. Indigenous communities, whose cultures and ways of life have been intertwined with the ice for millennia, are facing immense challenges as the predictable, frozen landscape they rely on becomes unstable and unfamiliar.

The changes at the poles do not stay at the poles. All the meltwater from the ice sheets contributes directly to rising sea levels worldwide, threatening coastal cities and island nations. Furthermore, the intense warming at the poles reduces the temperature difference between the equator and the poles. This difference is a key driver of global wind and ocean current patterns. As it weakens, it can lead to shifts in weather systems, potentially causing more persistent and extreme weather events—such as prolonged heatwaves, droughts, and cold spells—in populated mid-latitude regions.

Question 12. 

State the effect of global warming in coastal regions.

Ans:

Global warming acts like a triple-threat to the world’s coastlines, drastically reshaping them.

First, and most obviously, rising sea levels from melting ice and expanding warm water permanently claim land. This leads to the erosion of beaches, the submergence of low-lying islands and wetlands, and the contamination of freshwater aquifers with saltwater.

Second, the warming atmosphere fuels more intense and frequent storms. Coastal communities now face hurricanes and cyclones with stronger winds and significantly heavier rainfall, causing unprecedented storm surges and catastrophic flooding.

Third, a less visible but critical impact is ocean acidification. As the ocean absorbs excess carbon dioxide, its water becomes more acidic. This harms marine life, particularly creatures with shells or skeletons like corals, oysters, and crabs, disrupting the entire coastal food web and the fisheries that depend on it.

Question 13. 

How will global warming affect the sea level?

Ans:

Global warming acts on our planet’s oceans like heat applied to a glass of water, causing the water level to rise through two primary and interconnected processes.

The first is the straightforward expansion of seawater. As the atmosphere traps more heat, the ocean absorbs the vast majority of this excess warmth. Water, like most substances, expands when it gets hotter. This phenomenon, known as thermal expansion, means the same mass of water takes up more space, pushing sea levels higher across the globe. This is similar to the liquid in a thermometer rising as it warms.

The second major contributor is the massive infusion of meltwater from land-based ice. Global warming accelerates the melting of our planet’s frozen reservoirs. This includes the immense ice sheets covering Greenland and Antarctica, along with thousands of mountain glaciers worldwide. When ice that is supported on land melts, that water eventually runs off into the ocean, adding to its total volume. It’s akin to adding more water to an already full bathtub.

A critical point is that sea ice, such as the floating ice cap in the Arctic Ocean, does not contribute to sea-level rise when it melts, much like an ice cube melting in a drink doesn’t overflow the glass. The danger lies in the land ice sliding into the sea.

The consequence of these processes is a steady and accelerating creep of the ocean onto land. This leads to the erosion of coastlines, the contamination of freshwater aquifers with saltwater, and the permanent submergence of low-lying islands and coastal communities. The rising sea also allows storm surges to travel farther inland, making coastal flooding from hurricanes and cyclones more destructive and frequent.

Question 14. 

How will global warming affect agriculture?

Ans:

Global warming is set to reshape farming in profound ways, creating a story of winners, losers, and immense challenge. The effects aren’t uniform, but they are widespread.

First, the most direct hit comes from heat stress and water scarcity. As temperatures rise, key crops like wheat, corn, and rice suffer during critical growth stages, such as pollination. This leads to significantly lower yields. Coupled with more frequent and intense droughts, the struggle for water will become a central crisis for farmers, making irrigation more expensive and often impossible.

Second, the chaos of extreme weather becomes a new normal. Agriculture depends on predictability, but climate change delivers the opposite. Unseasonal floods can wash away topsoil and seeds, while powerful hurricanes can flatten entire harvests. A late spring frost after a warm period can kill budding fruits, turning an early season advantage into a total loss.

Finally, we face a silent invasion of pests and diseases. Warmer winters allow destructive insects and fungi to survive and thrive in regions that were once too cold for them. This forces farmers to use more pesticides, increasing costs and environmental harm, while crops face new threats they have no natural resistance to.

Question 15. 

State two ways to minimize the impact of global warming.

Ans:

1. Shift to Plant-Rich Diets: Livestock farming, especially for cattle, is a massive source of methane—a potent greenhouse gas. By consciously reducing our consumption of meat and dairy and filling our plates with more plant-based foods, we can significantly lower the demand for these emissions-intensive products.
2. Practice “Smart Cooling”: Instead of only relying on energy-intensive air conditioning, we can revive traditional methods like strategic tree planting for shade, using light-colored or reflective materials on roofs and walls to deflect sunlight, and promoting natural cross-ventilation in building designs. This directly cuts fossil fuel electricity use while keeping us cool.

Question 16. 

What is carbon tax? Who will pay for it?

Ans:

Imagine a fee charged for releasing carbon dioxide into our shared atmosphere. This is the essence of a carbon tax. It is a financial tool used by governments that puts a direct price on pollution, specifically on the carbon content of fossil fuels like coal, oil, and natural gas.

The core idea is to make activities that generate high greenhouse gas emissions more expensive. By increasing the cost of pollution, the tax creates a powerful financial incentive for businesses, industries, and even individuals to seek out cleaner alternatives. The goal is to nudge the entire economy toward a less polluting future by making green energy, electric vehicles, and energy-efficient technologies more cost-competitive.

Now, who ultimately bears the cost of this tax? The initial payment is typically made by the companies that extract or import the fossil fuels—the first point in the energy supply chain. However, these companies rarely absorb the cost themselves. Instead, they pass it down the line.

Think of it like a chain reaction:

1. The fuel supplier pays the tax to the government.
2. They then add this cost to the price they charge to an electricity utility or a gasoline distributor.
3. The utility, facing higher costs for coal or gas, then raises its electricity prices for a factory or a household.
4. The factory, now paying more for power, might increase the price of the goods it manufactures.

In this way, the cost of the tax ripples through the economy. The end consumers—that is, all of us—ultimately pay for it in various ways. We pay it through higher prices for gasoline, heating, electricity, and for goods and services that are carbon-intensive to produce.

Exercise 6 (D)

Question 1. 

The green house gas is : 

1. Oxygen  
2. Nitrogen
3. Chlorine  
4. Carbon dioxide

Question 2. 

The increase of carbon dioxide gas in atmosphere will cause 

1. Decrease in temperature
2. Increase in temperature 
3. No change in temperature 
4. increase in humidity.

Question 3. 

Without green house effect, the average temperature of Earth’s surface would have been : 

1. -18°C
2. 33°C
3. 0°C
4. 15°C

Question 4. 

The global warming has resulted in : 

1. the increase in yield of crops
2. the decrease in sea levels
3. the decrease in human deaths
4. the increase in sea levels