Energy

This chapter likely introduces energy as the ability to do work. It emphasizes that energy exists in various forms, and these forms can be converted from one to another. The primary forms of energy usually covered are:

• Mechanical Energy: This includes kinetic energy (energy due to motion, like a moving car or flowing water) and potential energy (stored energy due to position or state, like a stretched spring or water stored at a height).
• Heat Energy (Thermal Energy): Energy associated with the temperature of an object, arising from the motion of its particles. Examples include the heat from a fire or the warmth of the sun.
• Light Energy (Radiant Energy): Energy that travels in the form of electromagnetic waves, like sunlight or the light from a bulb.
• Sound Energy: Energy produced by vibrations that travel through a medium (like air or water) and are detected by our ears.
• Electrical Energy: Energy associated with the flow of electric charge, used to power many devices.
• Chemical Energy: Energy stored in the bonds of chemical compounds, released during chemical reactions (like burning wood or the energy in food).
• Nuclear Energy: Energy stored in the nucleus of atoms, released during nuclear reactions (like in nuclear power plants or the sun).

The chapter would then likely discuss the conservation of energy, stating that energy cannot be created or destroyed, but it can be transformed from one form to another. Examples like a falling object converting potential energy to kinetic energy and then to sound and heat upon impact are usually given.

Finally, the chapter might touch upon sources of energy, broadly categorizing them into renewable (like solar, wind, and hydro energy, which can be replenished naturally) and non-renewable (like fossil fuels such as coal, oil, and natural gas, which are finite resources). The importance of energy conservation and using energy wisely might also be mentioned.

Test Yourself
A. Objective Questions 

1. Write true or false for each statement

(a) A man going up has potential energy and kinetic energy both.
Ans: True.

(b) A gum bottle lying on a table has no energy.
Ans: False.
Correct — A gum bottle lying on a table has energy.

(c) In an electric fan, electrical energy changes into the mechanical energy.
Ans: True.

(d) Potential energy changes into kinetic energy when it is put to use.
Ans: True.

(e) One form of energy cannot be converted into another form.
Ans: False.
Correct — One form of energy can be converted into the other form.

(f) There is always some loss of energy in conversion from one form of energy to another form, so the total energy is not conserved.
Ans: False.
Correct — There is always some loss of energy in conversion from one form of energy to the other form, so the total energy is conserved.

(g) The energy of flowing water can be converted into electric energy (electricity).
Ans: True.

2. Fill in the blanks

(a) An electric fan converts electrical energy into——– energy.

Ans : mechanical
(b) Cooking gas converts——– energy into heat energy.

Ans : chemical
(c) Energy possessed by a compressed spring is——– energy.

Ans : potential
(d) The ability to do work is called——-

Ans : energy
(e) The energy possessed by a body due to its position is called ——-energy.

Ans : potential
(f) The energy possessed by a body due to its motion is called energy.

Ans : kinetic

(g) Green plants convert energy into chemical—– energy.

Ans : light
(h) The S.I.unit of energy is—–

Ans : joule
(i) An object falling freely from the roof of a multistory building has —— and—— energy when halfway down the building.

Ans : potential energy & kinetic

3. Match the following columns

4. Select the correct alternatives 

(a) When we rub our hands

1.  kinetic energy changes into potential energy
2.  mechanical energy changes into heat energy
3.  potential energy changes into kinetic energy
4.  heat energy changes into mechanical energy.

(b) A ball rolling on the ground possesses

1.  kinetic energy
2.  potential energy
3.  no energy
4.  heat energy

(c) The energy stored in an electric cell is

1.  chemical energy
2.  electrical energy
3.  heat energy
4.  mechanical energy.

(d) When a bulb lights up on passing current, the change of energy is

1.  from electrical energy to heat energy
2.  from electrical energy to light energy
3.  from electrical energy to heat and light energy
4.  from electrical energy to mechanical energy.

(e) The correct statement is

1.  Both work and energy have the same units
2.  Potential energy of a body is due to its motion
3.  Kinetic energy of a body is due to its position or state
4.  Kinetic energy can change into potential energy, but potential energy cannot change into kinetic energy.

(f) According to law of conservation of energy, energy changes from one form to another form, but the total energy of that system

1. remains the same
2. increases
3. decreases
4. alternates

B. Short/Long Answer Questions

Question 1.
Define the term energy.
Ans:

Energy is fundamentally understood as the inherent capability within a physical system to execute work or bring about alterations. ¹ This core concept in physics signifies the latent capacity to perform actions or instigate transformations within the physical world.

Question 2.

State the unit of energy and define it.

Ans:

The fundamental unit for measuring energy within the International System of Units (SI) is the joule, represented by the symbol J.

A single joule is precisely defined as the quantity of work accomplished when a force equivalent to one newton (N) acts upon an object, causing it to move a distance of one meter (m) in the exact direction of the applied force.

This relationship can be expressed as: 

1 Joule = 1 Newton × 1 Meter 1

 J = 1 N⋅m

Furthermore, when broken down into the base SI units, one joule is equivalent to:

 1 J = 1 kg⋅m²/s²

The joule serves as a universal unit for quantifying all manifestations of energy, encompassing kinetic, potential, thermal, electrical, and various other forms.

Question 3.

Name five different forms of energy.

Ans:

1. Kinetic Energy: This is the energy inherent in an object by virtue of its movement. Examples include a vehicle in motion, a stream of flowing water, or a spinning gyroscope.
2. Potential Energy: This represents stored energy that an object possesses due to its specific location or physical state. Examples include a spring that has been stretched, water held at an elevated position by a dam, or a book resting on a high ledge (gravitational potential energy).
3. Thermal Energy (Heat Energy): This form of energy is associated with the internal state of an object, arising from the random motion of its constituent particles (atoms and molecules). Examples include the warmth emanating from a fireplace, the heat generated within the Earth (geothermal energy), or the energy radiated by the sun.
4. Light Energy (Radiant Energy): This energy travels in the form of electromagnetic waves and is perceptible to the human visual system. Examples include the illumination from the sun, the light emitted by an incandescent bulb, or the focused beam of a laser.
5. Chemical Energy: This is the energy stored within the bonds that hold atoms and molecules together. This energy can be liberated when these bonds are broken and new ones are formed during chemical reactions. Examples include the energy contained within food, fuels such as wood and gasoline, or the energy stored in electrochemical cells (batteries).

Question 4.

What are the two kinds of mechanical energy.

Ans:

1. Kinetic Energy: This is the energy an object possesses as a direct result of its movement. The magnitude of kinetic energy is determined by both the object’s mass and the speed at which it is traveling; a heavier or faster-moving object will have greater kinetic energy. Examples include a ball in motion, an aircraft in flight, or the flow of a river.
   
2. Potential Energy: This represents the stored energy that an object has due to its specific position or physical state. It signifies the potential to do work. Potential energy can manifest in various ways, including:
   

• Gravitational Potential Energy: Energy stored by an object due to its vertical position relative to a reference point within a gravitational field, such as a book placed on a high shelf.
• Elastic Potential Energy: Energy stored in an elastic material that has been deformed, such as a stretched rubber band or a compressed spring.

Question 5.

What is potential energy ? State its unit.

Ans:

Potential energy is the energy stored in an object due to its position relative to other objects or its internal condition. It represents the capacity to do work that is not currently being used.

The standard unit of potential energy in the International System of Units (SI) is the joule, symbolized by J.

Question 6.

Give one example of a body that has potential energy, in each of the following : (i) due to its position, (ii) due to its state.

Ans:

(i) Potential energy arising from position:

• A water reservoir situated at an elevated height: The water held in the reservoir possesses potential energy due to its vertical position relative to a lower level. As the water flows downwards, this stored energy is transformed into kinetic energy, which can then be used to generate electricity.

(ii) Potential energy arising from state (configuration):

• A compressed spring within a toy: When the spring is squeezed or compressed, mechanical work is done on it, and this energy is stored within the spring’s deformed structure as elastic potential energy. Upon release, this stored energy is converted into kinetic energy, causing parts of the toy to move.

Question 7.

State two factors on which the potential energy of a body at a certain height above the ground depends.

Ans:

The amount of gravitational potential energy an object possesses when it’s lifted to a certain height is determined by two main things: how much stuff it’s made of (its mass) and how high up it is (its height). If you have a heavier object at the same height, it will have more potential energy. Similarly, if you lift the same object to a greater height, it will also have more potential energy stored.

Question 8.

Two bodies A and B of masses 10 kg and 20 kg respectively are at the same height above the ground. Which of the two has the greater potential energy ?

Ans:

Body B, with a mass of 20 kg, will have greater potential energy than Body A, which has a mass of 10 kg, assuming they are both at the same height above the ground.  

Here’s why: Gravitational potential energy (PE) is directly proportional to the mass of the object, as given by the formula:

PE=mgh  

where:

• m is the mass of the object  
• g is the acceleration due to gravity (which is the same for both bodies since they are at the same location)
• h is the height above the ground (which is also the same for both bodies according to the problem statement)  

Since g and h are constant for both bodies, the potential energy is directly determined by the mass (m). Body B has a larger mass (20 kg) compared to Body A (10 kg), therefore, Body B will possess greater gravitational potential energy.

Question 9.

A bucket full of water .is on the first floor of your house and another identical bucket with same quantity of water is kept on the second floor. Which of the two has greater potential energy ?

Ans:

The bucket on the second floor has greater potential energy.

Here’s why: Potential energy (PE) due to height (gravitational potential energy) is calculated using the formula:

PE=mgh

Where:

• m is the mass of the object (the buckets have the same quantity of water and are identical, so their mass is the same).
• g is the acceleration due to gravity (which is essentially constant for both buckets since they are both on Earth).
• h is the height of the object above a reference point (in this case, likely the ground level).

Since the bucket on the second floor is at a greater height (h) compared to the bucket on the first floor, and the mass (m) and acceleration due to gravity (g) are the same for both, the bucket at the greater height will have a larger value of potential energy.

Question 10.

Define the term kinetic energy. Give one example of a body which possesses kinetic energy.

Ans:

Kinetic energy is essentially the energy that something has simply because it’s moving. Think of it as the “energy of motion.” Any object that is in motion, whether it’s rolling, flying, running, or falling, has this type of energy. The faster it moves and the more “stuff” it’s made of (its mass), the more kinetic energy it will have.

A good example of something with kinetic energy is a bird flying through the air. Because the bird has mass and it’s in motion, it possesses kinetic energy. This energy allows it to do things like collide with a leaf and make it move, or even just push against the air as it flies. The faster the bird flies, the more kinetic energy it has.

Question 11.

State two factors on which the kinetic energy of a moving body depends.

Ans:

How much “stuff” the object is made of (its mass): A heavier object, moving at the same speed as a lighter one, will have more kinetic energy. The relationship is direct – if you double the mass, you double the kinetic energy.

How fast the object is moving (its speed): The speed of an object has a more dramatic effect on its kinetic energy because the energy is proportional to the square of the speed. This means if you make an object move twice as fast, its kinetic energy becomes four times greater. If you triple the speed, the kinetic energy increases ninefold.

Question 12.

Two toy-cars A and B of masses 500 g and 200 g respectively are moving with the same speed. Which of the two has the greater kinetic energy?

Ans:

If two toy cars are moving at the same pace, but one is heavier than the other, the heavier car (toy-car A, weighing 500 g) will have more kinetic energy.  

Think of it this way: kinetic energy is the energy of motion. The formula for it involves both how much the object weighs (its mass) and how fast it’s going (its speed). Since both cars have the same speed in this case, the only difference is their mass. The car with more mass has more “stuff” that’s moving, so it has more energy in its movement. Even though they are traveling at the same rate, the more substantial car has a greater capacity to do work because of its motion.

Question 13.

A cyclist doubles his speed. How will his kinetic energy change: increase, decrease or remain same ?

Ans:

When a cyclist picks up speed and doubles how fast they’re going, the energy they have due to their motion, known as kinetic energy, doesn’t just double – it actually becomes four times as much.

The reason for this is in the physics of motion. Kinetic energy depends on two things: how heavy something is (its mass) and how quickly it’s moving (its speed). Importantly, the speed has a much bigger impact because the energy is related to the speed multiplied by itself (squared). So, if you double the speed, you’re essentially multiplying its effect on the energy by two, and then multiplying that result by two again, leading to a fourfold increase in kinetic energy. A little extra effort in pedaling to double the speed results in a much larger surge in the energy of motion.

Question 14.

Name the form of energy which a wound up watch spring possess.

Ans:

The form of energy stored in a wound-up watch spring is called elastic potential energy.

Think of it like this: when you wind the watch, you’re doing work to twist the spring into a new shape. This work isn’t lost; it’s stored within the spring itself, kind of like how energy is stored in a stretched rubber band. Because the spring is now in a strained or “wound-up” state, it has the potential to do work – in this case, to unwind and make the watch mechanisms move. This stored energy, due to the change in the spring’s shape, is what we call elastic potential energy.

Question 15.

Can a body possess energy even when it is not in motion ? Explain your answer with an example.

Ans:

Imagine a book resting on a high shelf. The book is stationary; it is not moving, so it has zero kinetic energy (the energy of motion). However, due to its position high above the ground, it possesses gravitational potential energy.  

This potential energy is stored because the Earth’s gravitational force is acting on the book. If the shelf were removed, the book would fall, and this stored potential energy would be converted into kinetic energy as it gains speed. The higher the shelf, the greater the potential energy the book possesses.  

Therefore, the book at rest on the shelf has energy (potential energy) due to its position, even though it is not currently in motion. This demonstrates that a body does not need to be moving to have energy; stored energy due to position or condition is a valid form of energy.

Question 16.

Name the type of energy (kinetic or potential) possessed by the following :

(i) A moving cricket ball.

(ii) A stone at rest on the top of a building.

(iii) A compressed spring.

(iv) A moving bus.

(v) A bullet fired from a gun.

(vi) Water flowing in a river.

(vii) A stretched rubber band.

Ans:

Here’s the type of energy possessed by each:

(i) A moving cricket ball: Kinetic energy (due to its motion)

 (ii) A stone at rest on the top of a building: Potential energy (specifically, gravitational potential energy due to its position above the ground)

 (iii) A compressed spring: Potential energy (specifically, elastic potential energy due to its deformed state)

 (iv) A moving bus: Kinetic energy (due to its motion) 

(v) A bullet fired from a gun: Kinetic energy (due to its high speed motion) 

(vi) Water flowing in a river: Kinetic energy (due to its motion) 

(vii) A stretched rubber band: Potential energy (specifically, elastic potential energy due to its deformed state)

Question 17.

Give one example to show the conversion of potential energy to kinetic energy when put in use.

Ans:

A book held stationary above the ground possesses potential energy due to its height in the Earth’s gravitational field.  

When you release the book and it falls, this potential energy is converted into kinetic energy, the energy of motion. As the book falls, its height decreases (decreasing potential energy), and its speed increases (increasing kinetic energy). Just before it hits the ground, most of its initial potential energy has been transformed into kinetic energy.

Question 18.

State the energy changes that occur in the following :

(i) The unwinding of a watch spring.

(ii) Burning coal while operating a steam engine.

(iii) Lighting of a torch bulb.

(iv) An electric generator (or dynamo).

Ans:

(i) Unwinding a Watch Spring: The potential energy meticulously stored within the tightly wound spring of a watch acts as the initial energy source. As the spring gradually unwinds, this stored potential energy doesn’t simply vanish; instead, it undergoes a transformation into kinetic energy. This kinetic energy becomes the driving force behind the intricate mechanisms within the watch, setting its gears in motion and ultimately causing the hands to sweep across the dial, marking the passage of time.

(ii) Burning Coal in a Steam Engine: The operation of a steam engine powered by burning coal involves a sequence of energy conversions. The process begins with the chemical energy locked within the bonds of the coal molecules. Combustion ignites this chemical energy, releasing it in the form of intense heat energy. This heat energy then serves to boil water, transforming it into high-pressure thermal energy in the form of steam. Finally, the expanding steam exerts force on the engine’s pistons or turbines, converting the thermal energy into the mechanical energy that propels the engine and its attached machinery.

(iii) Lighting a Torch Bulb: The illumination of a torch bulb is a result of a straightforward energy conversion process. The chemical energy contained within the torch’s batteries serves as the primary energy reservoir. Through chemical reactions within the battery cells, this chemical energy is transformed into electrical energy, creating a flow of electric current through the circuit. When this electrical energy reaches the bulb’s filament (in older incandescent bulbs) or semiconductor junction (in LED bulbs), it encounters resistance, causing the filament to heat up intensely and emit light energy, along with some accompanying heat energy.

(iv) An Electric Generator (Dynamo): An electric generator, or dynamo, functions by converting mechanical energy into electrical energy. This mechanical energy, often supplied by an external source such as a rotating turbine driven by water, steam, or wind, or an engine, is used to rotate a coil of wire within a magnetic field, or to rotate a magnetic field around a stationary coil. According to the principles of electromagnetic induction, this relative motion between the magnetic field and the conductor induces the flow of electric charges within the wire, thus generating electrical energy.

Question 19.

Energy can exist in several forms and may change from one form to another. Give two examples to show the conversion of energy from one form to another.

Ans:

Energy, the capacity to do work, manifests in a multitude of forms, each capable of transforming into another under the right circumstances. This fundamental principle of energy conversion underpins countless natural phenomena and technological applications that shape our world. Understanding these transformations is crucial for comprehending the intricate workings of the universe and for developing innovative solutions to meet our energy needs.

One compelling example of energy conversion in action is the solar panel. The journey begins with the radiant light energy emanating from the sun, a virtually inexhaustible source. When these photons of sunlight strike the photovoltaic cells within a solar panel, a remarkable transformation occurs at the atomic level. The energy carried by these light particles dislodges electrons within the semiconductor material of the solar cells, setting them in motion and generating a flow of electric charge. This directed movement of electrons constitutes electrical energy, a versatile form that can directly power homes and industries or be efficiently stored in batteries for later utilization, demonstrating a clean and sustainable pathway for harnessing solar power.

Another familiar illustration of energy transformation can be observed in the simple act of burning wood in a fireplace. Here, the initial form of energy resides as chemical energy meticulously stored within the complex molecular structures of the wood. These organic compounds, primarily cellulose and lignin, hold potential energy within their chemical bonds. The process of combustion, initiated by heat, triggers a chemical reaction where the wood rapidly combines with oxygen from the surrounding air. This energetic reaction breaks the existing chemical bonds and releases the stored energy in the form of both tangible heat energy, which radiates outwards to warm the environment, and visible light energy, which manifests as the flickering flames that characterize a fire. This ancient practice highlights the conversion of stored chemical potential into thermal and radiant forms of energy.

Question 20.

Give one relevant example for each of the following transformation of energy :

(i) Electrical energy to heat energy.

(ii) Electrical energy to mechanical energy.

(iii) Electrical energy to light energy.

(iv) Chemical energy to heat energy.

(v) Chemical energy to light energy.

Ans:

Here are relevant examples for each of the energy transformations you listed:

(i) Electrical energy to heat energy:

• Electric Kettle: When you switch on an electric kettle, electrical energy flows through a heating element (usually a coil of wire with high resistance). This resistance causes the electrical energy to be converted into heat energy, which then heats the water.  

(ii) Electrical energy to mechanical energy:

• Electric Fan: An electric fan uses an electric motor. When electrical energy is supplied to the motor, it creates a magnetic field that interacts with another magnetic field (either from permanent magnets or another set of coils). This interaction produces a rotational force, converting electrical energy into the mechanical energy of the rotating blades, which move the air.  

(iii) Electrical energy to light energy:

• Incandescent Light Bulb: In an incandescent light bulb, electrical energy flows through a thin filament made of tungsten. The resistance of the filament to the flow of electricity causes it to heat up to a very high temperature, emitting light as a result of incandescence. While some heat is also produced, the primary intended conversion is to light.  

(iv) Chemical energy to heat energy:

• Burning Wood: When wood burns, a chemical reaction (combustion) occurs between the wood (containing chemical energy in its bonds) and oxygen in the air. This reaction releases the stored chemical energy as heat energy, which is why a fire feels warm.  

(v) Chemical energy to light energy:

• Glow Stick: A glow stick contains two chemicals that are kept separate. When the stick is bent, the barrier between the chemicals breaks, allowing them to mix. This mixing initiates a chemical reaction that releases energy in the form of light (chemiluminescence) without producing significant heat.

Question 21.

What do you mean by conservation of mechanical energy? State the condition when does it hold.

Ans:

1. Conservation of Mechanical Energy: The total mechanical energy (potential + kinetic) of a system remains constant.
2. Condition: Only conservative forces (like gravity, spring force) are doing work within the system.
3. Meaning: In the absence of non-conservative forces (like friction), energy can transform between potential and kinetic forms without loss or gain to the total mechanical energy of the system.
4. Non-Conservative Forces: If present, they cause mechanical energy to be converted into other forms (e.g., heat), thus violating its conservation.

Question 22.

Give one example to show that the sum of potential energy and kinetic energy remains constant if friction is ignored.

Ans:

Imagine a perfectly frictionless swing set. When you pull the swing back and hold it high, it possesses maximum potential energy due to its elevated position in Earth’s gravitational field. At this instant, it’s stationary, so its kinetic energy, the energy of motion, is zero. The total mechanical energy of the swing is solely its potential energy.

As you release the swing, gravity acts upon it, causing it to move downwards. The swing’s height decreases, and consequently, its potential energy diminishes. Simultaneously, the swing gains speed, and its kinetic energy increases. This illustrates the continuous conversion of potential energy into kinetic energy as the swing descends. At the very bottom of its arc, the swing reaches its lowest point, where its potential energy is at a minimum (ideally zero), and its speed, and therefore its kinetic energy, is at its maximum.

As the swing continues its motion upwards on the other side, the process reverses. The swing’s kinetic energy now does work against gravity, causing it to slow down and gain height. Its kinetic energy decreases, while its potential energy increases. This conversion continues until the swing reaches its highest point on the opposite side, ideally the same height from which it was initially released (in the absence of friction). At this peak, the kinetic energy is again zero, and the potential energy is back to its maximum value.

Throughout this entire idealized motion of the frictionless swing, the total mechanical energy, which is the sum of the potential and kinetic energies, remains constant. Energy is continuously being exchanged between these two forms – potential becoming kinetic on the way down, and kinetic becoming potential on the way up – but the overall amount of mechanical energy within the system is conserved. This exemplifies the principle that when only conservative forces like gravity are at play and non-conservative forces like friction are negligible, the total mechanical energy of a system remains a constant quantity.

Question 23.

A ball is made to fall freely from a height. State the kind/ kinds of energy possessed by the ball when it is

(a) at the highest point

(b) just in the middle

(c) at the ground.

Ans:

(a) At the Highest Point: When the ball is held at its maximum height before release, it is stationary. Consequently, its energy of motion, known as kinetic energy, is zero. However, due to its elevated position within Earth’s gravitational field, the ball possesses its maximum gravitational potential energy relative to the ground. This potential energy represents the stored energy due to its position, ready to be converted into motion. Thus, at the apex of its fall, the ball’s energy is predominantly in the form of potential energy.

(b) Just in the Middle: As the ball commences its descent, the force of gravity initiates its acceleration. As its height above the ground diminishes, so too does its gravitational potential energy. Simultaneously, the increasing speed of the falling ball translates into a growing amount of kinetic energy. At the midpoint of its fall, the ball possesses a blend of both energy forms. The initial potential energy has been partially transformed into the energy of its downward motion, resulting in a significant amount of kinetic energy coexisting with the remaining potential energy.

(c) At the Ground: Immediately before the ball makes contact with the ground, it has reached its lowest possible height, which we consider our reference point for zero gravitational potential energy. At this juncture, the potential energy due to its position has been minimized. Conversely, due to the continuous acceleration under gravity throughout its fall, the ball attains its maximum velocity just prior to impact. This peak velocity corresponds to the ball possessing its maximum kinetic energy. Effectively, the initial potential energy stored at the highest point has been almost entirely converted into the energy of its motion as it approaches the ground.

Question 24.

State the changes in form of energy while producing hydro electricity.

Ans:

The process of generating hydroelectricity relies on a series of energy transformations, beginning with the potential energy inherent in water stored at an elevated position, typically behind a dam. This potential energy is a direct consequence of the water’s height within the Earth’s gravitational field.

As this stored water is released, it flows downwards, and its potential energy is converted into kinetic energy, the energy of motion. The speed and force of the flowing water are directly related to the amount of kinetic energy it possesses.

This high-speed water then strikes the blades of a turbine, transferring its kinetic energy and causing the turbine to rotate. This rotation represents mechanical energy.

Finally, the rotating turbine drives a generator, which utilizes the principle of electromagnetic induction to convert the mechanical energy into electrical energy, the usable form of power. Thus, hydroelectricity production is a sequential transformation of energy from potential to kinetic, then to mechanical, and finally to electrical.