Heat

December 29, 2025

The chapter on “Heat” in establishes the fundamental difference between heat, which is energy in transit driven by temperature disparities, and temperature, a gauge of an object’s warmth related to molecular motion. It elucidates how the addition or removal of heat can induce thermal expansion in substances, with each material exhibiting a unique degree of expansion. Furthermore, it explains the transitions between the states of matter (solid, liquid, and gas) that occur at specific temperatures when heat is absorbed or released, and briefly touches upon the role of heat in initiating chemical transformations. Grasping these concepts provides a framework for understanding numerous everyday occurrences.

A significant portion of the chapter is dedicated to the mechanisms of heat transfer: conduction, where thermal energy propagates through direct molecular collisions, primarily in solids and with varying efficiency depending on the material’s properties; convection, involving the bulk movement of heated fluids (liquids and gases), resulting in the formation of currents; and radiation, the transmission of heat energy via electromagnetic waves, which can occur even through a vacuum. Each of these transfer methods operates under distinct principles and underlies a wide array of thermal phenomena we encounter daily, from the warming of a spoon in hot soup to the Earth receiving energy from the sun.

Concluding the chapter, the curriculum typically emphasizes the practical implications of understanding heat transfer. This includes examining the design and functionality of common items like cooking utensils, crafted from materials that efficiently conduct heat, and thermos flasks, engineered to minimize heat exchange. Additionally, it often explains natural occurrences such as coastal breezes, which are driven by convective air currents. The overarching aim is to bridge the theoretical concepts of heat and its transfer with tangible, real-world examples, thereby fostering a more intuitive and engaging learning experience for young students.

Test Yourself

A. Objective Questions

1. Write true or false for each statement

(a) On touching a lump of ice, we feel cold because some heat passes from our body to the ice.
Ans: True.

(b) Heat flows from a body at a high temperature to a body at a low temperature when they are kept in contact. .
Ans: True.

(d) Telephone wires are kept tight between the two poles in summer.
Ans: False.

(e) Equal volumes of different liquids expand by different amounts when they are heated to the same rise in temperature.
Ans: True.

(f) Solids expand the least and gases expand the most on being heated.
Ans: True.

(g) A mercury thermometer makes use of the property of expansion of liquids on heating.
Ans: True.

(h) Kerosene contracts on heating.
Ans: False.

(i) Water is a bad conductor of heat.
Ans: True.

(j) Medium is necessary for the transfer of heat by radiation.
Ans: False.

(k) Land and sea breezes are convection currents of cold and warm air.
Ans: True.

(l) Liquids are heated by conduction and radiation.
Ans: False.

(m) Black surfaces are the poor absorbers of heat radiations.
Ans: False.

2. Fill in the blanks

(a) Heat is a form of——-.

Ans : energy
(b) ——– determines the degree of hotness or coldness of a body.

Ans : Temperature
(c) On heating a body, its temperature ——.

Ans : rises
(d) We use a ——— for measuring the temperature of a body.

Ans : thermometer
(e) The S.I. unit of temperature is ——–.

Ans : kelvin
(f) In a thermometer, the commonly used liquid is ——-.

Ans : mercury
(g) The temperature of a normal human body is——–.

Ans : 37 °C
(h) A person is said to have fever if his body temperature is more than ———.

Ans : 98.6
(i) A hot metallic piece is placed in tap water contained in a bucket. Heat will flow from ——– to ——-.

Ans : metallic piece , water
(j) The temperature of boiling water is —–.

Ans : 100°C
(k) Liquids expand ——- than the solids.

Ans : more
(l) Gases expand ——- than the liquids.

Ans : more
(m) Heat transfer in solids is by ——-.

Ans : conduction
(n) Heat transfer in liquids and gases is by ——–.

Ans : convection
(o)Metals are ——– of heat.

Ans : conductors
(p) Still air is an ——– of heat.

Ans : insulator
(q) Black and dull surfaces are ——- of heat.

Ans : good absorbers

3. Match the following

4. Select the correct alternative

(a) If we add a lump of ice to a tumbler containing water,

heat flows from water to ice
heat flows from ice to water
heat flows from water to ice if water is more
heat flows from ice to water if ice is more

(b) The temperature of pure melting ice is

0°C
100°C
95°C
98.6°F

(c) A thermometer uses

water
mercury
air
none of the above

(d) Which of the statement is correct

Iron rims are cooled before they are placed on cart wheels
A glass stopper gets tight on warming the neck of the bottle
Telephone wires sag in winter, but become tight in summer
A little space is left between two rails on a railway track

(e) Heat in a liquid is transferred by

conduction
convection
radiation
conduction and radiation

(f) In the process of convection, heat travels

sideways
downwards
upwards
in all directions

(g) The vacuum kept in between the walls of a thermos flask reduces the heat transfer by

conduction and convection
conduction only
convection only
radiation only

B. Short/Long Answer Questions

Question 1.
What is heat ? State its S.I. unit.
Ans:

Heat fundamentally represents energy in transit, specifically moving from regions of higher temperature to those of lower temperature. This transfer occurs due to the inherent thermal imbalance between the systems or objects involved. The standard international unit (S.I. unit) used to quantify this energy transfer, or heat, is the joule, abbreviated as J. Being a form of energy, heat is measured using the same unit that applies to all other forms of energy and work done.

Question 2.

What is meant by the term temperature.

Ans:

Temperature quantifies the degree of hotness or coldness of a substance, directly indicating the average kinetic energy of its constituent particles. Essentially, it tells us how vigorously the atoms or molecules within a material are moving. Crucially, temperature dictates the natural direction of heat transfer between objects in thermal contact; heat invariably flows from areas of higher temperature to those of lower temperature until thermal equilibrium is reached. While the kelvin (K) serves as the fundamental unit of temperature in the International System of Units, the degree Celsius (°C) is also widely employed in everyday contexts and various scientific fields.

Question 3.

State the three units of temperature.

Ans:

The three main temperature units are:

Celsius (°C): Common worldwide, with water freezing at 0°C and boiling at 100°C.
Fahrenheit (°F): Primarily used in the US, with water freezing at 32°F and boiling at 212°F.
Kelvin (K): The scientific standard, starting at absolute zero; water freezes at 273.15 K and boils at 373.15 K.

Question 4.

Name the instrument used to measure the temperature of a body.

Ans:

The device employed to determine the temperature of an object is known as a thermometer.

Question 5.

Name two scales of temperature. How are they inter-related?

Ans:

Two temperature scales are Celsius (°C) and Fahrenheit (°F).

Conversion formulas:

°F = (9/5) × °C + 32
°C = (5/9) × (°F – 32)

Question 6.

How is the size of a degree defined on a Celsius scale ?

Ans:

The unit of temperature on the Celsius scale, the degree Celsius (°C), is defined by dividing the temperature span between the freezing point of water (assigned 0°C) and its boiling point (assigned 100°C) under standard atmospheric conditions into exactly one hundred equal segments. Each of these identical divisions represents the temperature change of one degree Celsius.

Question 7.

How is the size of a degree defined on a Fahrenheit scale?

Ans:

A degree on the Fahrenheit scale is the result of dividing the gap between water’s freezing point (32°F) and boiling point (212°F) into 180 equal parts.

Question 8.

State the temperature of (i) ice point and (ii) steam point, on the Celsius scale.

Ans:

On the Celsius temperature scale:

(i) Ice point: The temperature at which water transitions from a liquid to a solid state, under standard atmospheric conditions, is defined as 0 °C.

(ii) Steam point: The temperature at which water transitions from a liquid to a gaseous state (boils), under standard atmospheric pressure, is defined as 100 °C.

Question 9.

Write down the temperature of (i) lower fixed point, and (ii) upper fixed point, on the Fahrenheit scale.

Ans:

On the Fahrenheit temperature scale:

(i) Lower fixed point (ice point): The temperature at which water begins to solidify into ice, under standard atmospheric conditions, is designated as 32 °F.

(ii) Upper fixed point (steam point): The temperature at which water reaches its boiling point and begins to vaporize into steam, under standard atmospheric pressure, is designated as 212 °F.

Question 10.

What is the Celsius scale of temperature ?

Ans:

The Celsius temperature scale, often referred to as the centigrade scale, establishes its framework based on the behavior of water under standard atmospheric pressure. Specifically:

The point at which water transitions from a liquid to a solid state (freezing) is precisely defined as 0 degrees Celsius (0 °C).
Conversely, the point at which water transitions from a liquid to a gaseous state (boiling) is precisely defined as 100 degrees Celsius (100 °C).

The temperature interval spanning between these two fundamental reference points is uniformly subdivided into one hundred equal segments, with each individual segment representing a temperature difference of one degree Celsius.

It is crucial to understand that the Celsius scale operates as a relative scale. The designation of 0 °C does not signify the complete absence of thermal energy within a system. Rather, it serves as a convenient reference point. The Celsius scale enjoys widespread adoption globally for routine temperature measurements and is a standard within scientific disciplines.

Furthermore, the Celsius scale is an integral component of the metric system and maintains a close mathematical relationship with the Kelvin scale, which serves as the International System of Units (SI) base unit for thermodynamic temperature. The conversion formula between these two scales is expressed as:

Temperature in Celsius (°C) = Temperature in Kelvin (K) – 273.15

Question 11.

What is the Fahrenheit scale of temperature ?

Ans:

The Fahrenheit scale sets water’s freezing point at 32 °F and its boiling point at 212 °F. The range between these is divided into 180 equal degrees. It’s mainly used in the United States.

Question 12.

What is the Kelvin scale of temperature ?

Ans:

The Kelvin scale serves as the foundational unit for measuring thermodynamic temperature within the International System of Units (SI). Distinguished as an absolute temperature scale, its zero point, designated as 0 Kelvin (0 K), corresponds to absolute zero. Absolute zero represents the theoretical lower limit of temperature, a state where all forms of molecular motion are minimized to their quantum mechanical zero-point energy.

Key attributes of the Kelvin scale include:

Absolute Null Point: The value of 0 K is equivalent to -273.15 degrees Celsius (°C) and -459.67 degrees Fahrenheit (°F).
Equivalent Degree Interval: The magnitude of a single Kelvin unit is identical to that of a single degree on the Celsius scale. Consequently, a temperature variation of 1 °C is precisely equivalent to a temperature variation of 1 K.
Absence of Negative Values (Ideal): Based on classical thermodynamics, temperatures below 0 K are not considered physically attainable, rendering the Kelvin scale devoid of negative values.
Scientific Norm: The Kelvin scale is the standard temperature measurement in scientific and engineering domains due to its absolute nature. This characteristic simplifies thermodynamic calculations and the formulation of gas laws.

The conversion between the Kelvin and Celsius scales is mathematically direct:

Temperature in Kelvin (K) = Temperature in Celsius (°C) + 273.15

and

Temperature in Celsius (°C) = Temperature in Kelvin (K) – 273.15

Fundamentally, the Kelvin scale can be understood as the Celsius scale shifted downwards by a constant value of 273.15 degrees, positioning its zero point at the theoretical absolute zero.

Question 13.
The fig. shows a glass tumbler containing hot milk which is placed in a tub of cold water. State the direction in which heat will flow.

Ans:

Based on the principle of heat transfer, heat will always flow from a region of higher temperature to a region of lower temperature.

In this scenario:

The hot milk in the glass tumbler has a higher temperature.
The cold water in the tub has a lower temperature.

Therefore, heat will flow in the direction indicated by the arrows in the diagram:

From the hot milk in the glass tumbler outwards towards the cold water in the tub.

This process will continue until thermal equilibrium is reached, meaning the hot milk and the cold water will eventually reach the same temperature (assuming the system is insulated from the surroundings).

Question 14.
Draw a neat labelled diagram of a laboratory thermometer.
Ans:

Question 15.
Write down the body temperature of a healthy person.
Ans:

The typical core body temperature of a healthy human adult is generally cited as 37 degrees Celsius (°C) or 98.6 degrees Fahrenheit (°F).

However, it is crucial to recognize that this value represents an average, and the actual body temperature of an individual can exhibit minor fluctuations influenced by several physiological and environmental factors. These influencing factors include:

Circadian Rhythm: Body temperature naturally follows a daily cycle, typically being lower during the early morning hours and peaking slightly in the late afternoon or early evening.
Physical Exertion: Engaging in physical activity can lead to a temporary elevation in body temperature as the body generates heat.
Age-Related Variations: Infants and young children often present with a slightly higher average body temperature compared to adults.
Individual Physiological Baselines: Healthy individuals can possess slightly different inherent baseline body temperatures that fall within a normal range.
Measurement Methodology: The site at which body temperature is measured (e.g., oral, rectal, axillary, tympanic) can yield slightly different readings, with rectal measurements often considered the most accurate reflection of core body temperature.

Therefore, while 37 °C (98.6 °F) serves as a common reference point, a healthy individual’s body temperature typically resides within a range of approximately 36.5 °C to 37.5 °C (97.7 °F to 99.5 °F). Consistent temperature readings outside this range may suggest the presence of a fever (elevated temperature) or hypothermia (abnormally low temperature), potentially indicating an underlying health concern that may necessitate medical evaluation.

Question 16.

What do you understand by thermal expansion of a substance ?

Ans:

Thermal expansion is when a substance gets bigger in volume because it gets hotter. Its particles move more and spread out. Cooling causes it to contract and get smaller. Solids expand least, then liquids, then gases the most. Different materials expand by different amounts for the same temperature change.

Question 17.

Name two substances which expand on heating.

Ans:

Here are two examples of materials that exhibit an increase in volume when subjected to a rise in temperature:

Copper: As the temperature of a copper object increases, its constituent atoms gain kinetic energy, leading to greater vibrational motion. This increased movement causes the average separation between the copper atoms to increase, resulting in an overall expansion of the copper’s dimensions. This property is a fundamental characteristic of metallic solids.
Ethanol: When liquid ethanol is heated, the kinetic energy of its molecules increases. This heightened molecular motion overcomes some of the intermolecular forces holding the ethanol molecules together, causing them to move further apart on average. Consequently, the overall volume of the ethanol sample increases. This expansion is a typical behavior of liquids upon heating.

Question 18.

Why do telephone wires sag in summer ?

Ans:

Telephone wires sag in summer primarily due to thermal expansion. Here’s a breakdown:

Heating Effect of Summer: During the summer months, the ambient temperature rises significantly. The telephone wires, being exposed to direct sunlight and the warmer air, absorb this heat.
Expansion of Materials: Most materials, including the metals (typically copper or aluminum alloys) used to make telephone wires, expand when heated. This is because the increased thermal energy causes the atoms within the wire to vibrate more vigorously, increasing the average distance between them.
Increase in Length: As the wires heat up and their constituent atoms move further apart, the overall length of the telephone wires increases.
Sagging Due to Increased Length: The telephone poles supporting the wires remain at roughly the same distance from each other. When the wires become longer due to thermal expansion, there’s more length of wire between the poles. This excess length causes the wires to sag downwards under their own weight.

Think of it like holding a rope taut between two points. If you suddenly increase the length of the rope without moving the points, the rope will have slack and will hang lower in the middle.

Therefore, the sagging of telephone wires in summer is a direct consequence of the thermal expansion of the wire material due to the increased temperatures. When the temperature cools down in winter, the wires contract, and the sag decreases. Engineers account for this thermal expansion when installing telephone wires, ensuring they are not strung too tightly in cooler temperatures to prevent snapping during contraction.

Question 19.

Iron rims are heated before they are fixed on the wooden wheels. Explain the reason.

Ans:

Iron rims are heated to make them temporarily expand. This allows the slightly smaller rim to easily fit over the wooden wheel. As the rim cools, it contracts, creating a very tight and secure fit.

Question 20.

Why are gaps left between successive rails on a railway track ?

Ans:

Railway tracks have gaps to allow the steel rails to expand when they get hot in summer. Without these gaps, the expanding rails would push against each other and buckle, which could cause train derailments. The gaps provide space for the rails to lengthen safely.

Question 21.

A glass stopper stuck in the neck of a bottle can be removed by pouring hot water on the neck of the bottle. Explain why ?

Ans:

Pouring hot water on the bottle neck heats and expands the glass of the neck more than the stopper inside (initially). This creates a tiny gap, loosening the stopper so it can be removed.

Question 22.

Why is a cement floor laid in small pieces with gaps in between?

Ans:

Cement floors are laid in small pieces with gaps to allow for expansion and contraction due to temperature changes and curing. These gaps, called control joints, prevent the floor from cracking randomly by providing space for movement and directing any cracks along the joints.

Question 23.

One end of a steel girder in a bridge is not fixed, but is kept on roliers. Give the reason.

Ans:

One end of a bridge’s steel girder rests on rollers to allow the girder to expand and contract with temperature changes. This movement prevents the immense forces that would build up if both ends were fixed, which could cause the bridge to buckle or be damaged. The rollers enable the bridge to handle temperature fluctuations safely.

Question 24.

Describe one experiment to show that liquids expand on heating.

Ans:

Fill a flask with a colored liquid and seal it with a stopper through which a narrow glass tube passes. Mark the initial height of the liquid within the glass tube. Gently apply heat to the flask. Observe that the column of colored liquid ascends within the glass tube, indicating an increase in the liquid’s volume as its temperature rises. Upon cessation of heating and subsequent cooling, the liquid column will descend towards its original marked level, further substantiating that liquids undergo expansion when heated. This simple setup visually confirms the principle of thermal expansion in liquids.

Question 25.

State one application of thermal expansion of liquids.

Ans:

A significant real-world application of the principle of thermal expansion in liquids is found in the functionality of liquid-in-glass thermometers. These instruments are designed to measure temperature by exploiting the consistent and predictable change in volume that liquids exhibit in response to temperature variations. Typically employing substances like mercury or colored alcohol, these thermometers consist of a sealed glass tube connected to a reservoir containing the liquid. When the thermometer is exposed to a temperature source, the liquid inside absorbs thermal energy, causing its constituent molecules to move more vigorously and occupy a larger volume. This volumetric increase manifests as a rise in the liquid column within the narrow bore of the glass tube. Conversely, when the temperature decreases, the liquid loses thermal energy, its molecules move less, and its volume contracts, resulting in a fall in the liquid column. A calibrated scale etched onto the glass tube allows for a direct reading of the temperature corresponding to the height of the liquid column. The reliable and quantifiable relationship between a liquid’s temperature and its volume due to thermal expansion forms the basis for accurate temperature measurement in a wide array of applications, from medical diagnostics to environmental monitoring.

Question 26.

Describe an experiment to show that air expands on heating.

Ans:

Put a balloon on a flask. Place the flask in hot water. The balloon will inflate because the air inside the flask expands when heated. When cooled, the balloon deflates. This shows air expands on heating.

Question 27.

An empty glass bottle is fitted with a narrow tube at its mouth. The open end of the tube is kept in a beaker containing water. When the bottle is heated, bubbles of air are seen escaping into the water. Explain the reason.

Ans:

When the glass bottle is heated, the air inside warms up and expands. Because the air needs more space, it moves out through the narrow tube and forms bubbles in the water. This demonstrates that air expands on heating.

Question 28.

State which expands more, when heated to the same temperature : solid, liquid or gas ?

Ans:

The underlying reasons for this hierarchy of expansion lie in the nature and strength of the intermolecular forces and the mobility of the constituent particles within each state of matter:

Gaseous State: In gases, the intermolecular forces are exceedingly weak, and the constituent molecules possess substantial kinetic energy, resulting in large average separations and relatively unrestricted movement. Upon heating, the kinetic energy of these molecules increases dramatically, leading to even greater separation and a substantial increase in the overall volume of the gas.
Liquid State: Liquids exhibit stronger intermolecular forces compared to gases, and their molecules are more closely packed, although they still possess the ability to move past one another. When heated, the enhanced kinetic energy enables the molecules to partially overcome these attractive forces, resulting in a moderate increase in the average intermolecular distances and a corresponding expansion in volume.
Solid State: Solids are characterized by the strongest intermolecular forces, which maintain their constituent atoms or molecules in a relatively fixed, lattice-like arrangement, allowing only for vibrational motion around their equilibrium positions. When a solid is heated, the increased kinetic energy intensifies these vibrations, leading to a slight increase in the average separation between the particles. However, the strong cohesive forces that define the solid structure limit the extent of this separation, resulting in the smallest degree of thermal expansion among the three states of matter for a given temperature change.

Question 29.

Name the three modes of transfer of heat.

Ans:

The transfer of thermal energy from a region of higher temperature to one of lower temperature occurs through three distinct mechanisms:

Conduction: This mode of heat transfer involves the direct transmission of thermal energy through a material or between materials in physical contact. Energy is conveyed at the microscopic level via the vibration and collision of adjacent atoms or molecules. Importantly, this process does not entail any macroscopic movement of the substance itself. Conduction is the predominant mode of heat transfer in solid materials.
Convection: Convection is characterized by the transfer of heat through the bulk movement of fluids, which encompasses both liquids and gases. When a portion of a fluid is heated, its density decreases, causing it to rise. This movement of the warmer fluid carries thermal energy to other regions, while cooler, denser fluid descends to replace it, establishing a cyclical flow that facilitates heat transfer. Convection necessitates the physical displacement of the heated substance.
Radiation: Unlike conduction and convection, thermal radiation facilitates heat transfer through the emission and absorption of electromagnetic waves. This mode of energy transfer does not require any intervening medium and can therefore occur through a vacuum, as exemplified by the transmission of solar energy to Earth. All matter with a temperature exceeding absolute zero continuously emits thermal radiation, with the intensity and wavelength distribution of the emitted radiation being directly related to the object’s temperature. Hotter objects emit a greater quantity of radiation at shorter wavelengths

Question 30.
Name the mode of transfer of heat in the following :
(a) solid,
(b) liquid,
(c) gas
(d) vacuum
Ans:

Question 31.

What are the good and bad conductors of heat ? Give two examples of each.

Ans:

Good Conductors: Allow heat to flow easily.

Examples: Metals (copper), Stone (marble).

Bad Conductors (Insulators): Resist the flow of heat.

Examples: Wood, Air.

Question 32.

Name a liquid which is a good conductor of heat.

Ans:

Liquid mercury stands out as a notable example of a liquid that exhibits good thermal conductivity. Unlike most liquids, which are poor conductors of heat, mercury is a metallic element that exists in a liquid state at standard room temperature. This unique characteristic allows it to retain the efficient heat transfer properties typically associated with metals. The mobile electrons within its atomic structure facilitate the rapid transmission of thermal energy throughout the liquid. Consequently, mercury finds applications in devices where efficient heat transfer is crucial, such as some specialized thermometers and heat exchange systems.

Question 33.

Name a solid which is a good conductor of heat.

Ans:

Copper is a prominent example of a solid material that exhibits excellent thermal conductivity. As a metallic element, copper possesses a high concentration of freely moving electrons within its atomic lattice. These mobile electrons are highly effective at transporting thermal energy through the material by readily absorbing kinetic energy from hotter regions and efficiently transferring it to cooler regions via collisions and vibrations within the lattice structure. This inherent property makes copper invaluable in numerous applications where efficient heat transfer is paramount, including the construction of cookware for uniform heating, heat sinks in electronic devices to dissipate generated heat, and heat exchangers for effective thermal energy transfer between fluids. Its superior thermal conductivity, coupled with other favorable properties like malleability and electrical conductivity, solidifies copper’s importance in various technological and everyday applications.

Question 34.
Select good and bad conductors of heat from the following :
copper, mercury, wood, iron, air, saw-dust, cardboard, silver, plastic, wool.
Ans:
Good Conductors of Heat: These materials facilitate the efficient transfer of thermal energy.

Copper: A metallic element known for its high thermal conductivity, attributed to the mobility of its electrons.
Mercury: A metallic element that exists in a liquid state at room temperature and exhibits good thermal conductivity.
Iron: Another metallic element that effectively conducts heat due to its electronic structure.
Silver: Recognized as an exceptionally good conductor of heat, surpassing even copper in its thermal conductivity.

Bad Conductors of Heat (Thermal Insulators): These materials impede the flow of thermal energy.

Wood: Its cellular structure and the presence of trapped air pockets make it a poor conductor of heat.
Air: A gas with widely spaced molecules, resulting in inefficient heat transfer through molecular collisions.
Saw-dust: The fine particles trap air, significantly reducing heat transfer and making it a good insulator.
Cardboard: A porous material that contains trapped air, which acts as an insulator against heat flow.
Plastic: Generally possesses low thermal conductivity due to the nature of its chemical bonds and molecular structure.
Wool: Its fibrous structure traps a substantial amount of air, making it an effective thermal insulator.

Question 35.
Why is an oven made of double walls with the space in between filled with cork ?
Ans:

Ovens are constructed with double walls and an intermediate layer of cork primarily to establish a highly effective thermal barrier, thereby minimizing the transfer of heat between the oven’s interior and the external environment. This design serves crucial functions in enhancing energy efficiency and ensuring user safety.

Cork, the material filling the space between the walls, is an exceptional thermal insulator. Its unique cellular structure is characterized by numerous tiny, air-filled cells. Air itself is a poor conductor of heat, and the confinement of air within these discrete cells significantly impedes the transfer of thermal energy through the process of conduction. Moreover, the double-walled construction inherently creates an air gap, which further contributes to the insulation. This layer of trapped air also acts as a poor conductor, hindering the flow of heat from the hot inner wall to the cooler outer wall and vice versa.

The primary benefit of this insulation system is the substantial reduction of heat loss from the oven’s interior. By effectively trapping the heat generated within, the oven can reach and maintain the desired cooking temperature more rapidly and with less energy expenditure. This enhanced thermal retention translates directly into improved energy efficiency and lower operational costs. Furthermore, the insulation provided by the cork and the air gap works bidirectionally, preventing excessive heat from being transferred to the oven’s exterior surfaces. This ensures that the outer walls remain at a safe touch temperature, mitigating the risk of accidental burns and enhancing the overall safety of using the appliance.

Question 36.

Why do we use cooking utensils made up of copper.

Ans:

Copper is a favored material for crafting cooking utensils primarily due to its exceptional ability to conduct heat efficiently. This high thermal conductivity translates to several key advantages in the culinary process. Firstly, copper cookware heats rapidly and distributes thermal energy uniformly across its entire surface, eliminating localized hot spots that can lead to food burning while other areas remain insufficiently cooked. This even heat distribution is crucial for achieving consistent and optimal cooking results.

Secondly, copper’s responsiveness to temperature adjustments is remarkable. Due to its inherent thermal properties, copper cookware quickly reflects changes in the heat source. When the burner’s intensity is altered, the temperature of the copper pan adjusts almost instantaneously, affording the cook a high degree of precision in controlling the cooking process. This responsiveness is particularly beneficial for delicate cooking techniques that require fine-tuned temperature management.

Furthermore, the efficient heat distribution inherent in copper cookware can contribute to energy savings. The uniform heating allows for effective cooking at lower heat settings, reducing the overall energy input required. While pure copper can exhibit reactivity with certain acidic foods and necessitates regular maintenance to prevent tarnishing, many high-quality copper utensils feature a lining of non-reactive materials like stainless steel or tin, which mitigates these concerns while retaining the superior heat conductivity of the copper exterior. Despite a potentially higher initial cost, the enhanced cooking performance and longevity of well-maintained copper cookware make it a preferred choice for many discerning cooks.

Question 37.

Why is a tea kettle provided with an ebonite handle ?

Ans:

Here’s why this is important:

Safety: When you heat a tea kettle on a stove, the body of the kettle gets very hot due to the transfer of heat from the flame or heating element. If the handle were made of a good conductor of heat (like metal), it would also become dangerously hot, making it impossible to lift or pour the hot water without burning your hand.
Preventing Burns: Ebonite, being a poor conductor, significantly reduces the transfer of heat from the hot kettle body to your hand. This allows you to hold the handle comfortably and safely, even when the kettle is boiling hot.
Practicality: A handle made of a material that doesn’t get hot ensures the practicality and ease of use of the tea kettle. You need to be able to handle it without requiring additional protection like oven mitts every time.

Question 38.

In summer, ice is kept wrapped in a gunny bag. Explain the reason.

Ans:

Encasing ice within a gunny bag during the summer months serves to impede the rate at which the ice transitions from its solid to liquid state. This insulating effect is primarily attributed to the inherent properties of the gunny bag material, typically a loosely woven fabric such as jute or burlap. The coarse weave of the bag creates numerous interstitial spaces that effectively trap a substantial volume of air.

Air, by its nature, exhibits very low thermal conductivity, classifying it as an excellent thermal insulator. The layer of trapped air surrounding the ice within the gunny bag acts as a significant barrier against the direct transfer of heat from the warmer ambient air to the colder ice through the process of conduction. The thermal energy from the environment must navigate through the stagnant layers of air held within the bag’s fibers and weave, a process that occurs with considerable inefficiency due to air’s poor conductive properties.

Furthermore, the gunny bag also plays a role in mitigating heat transfer via convection. By partially enclosing the ice, it restricts the free movement of warm air currents directly across the ice’s surface. While complete prevention of air circulation is not achieved, the bag acts as a physical impediment, hindering the rapid displacement of cooler air in contact with the ice by warmer air from the surroundings. This reduction in convective heat transfer further contributes to slowing down the melting process, thereby preserving the ice for a longer duration compared to its exposure to unhindered airflow.

Question 39.

Explain why

(a) we wear woolen clothes in winter.

(b) the water pipes are covered with cotton during very cold weather.

Ans:

(a) Wool keeps you warm: Its fibers trap air, which acts as insulation, preventing body heat from escaping.

(b) Cotton protects pipes: It insulates the pipes by trapping air, slowing down water’s heat loss and preventing freezing/bursting.

Question 40.

Why are quilts filled with fluffy cotton ?

Ans:

Quilts utilize fluffy cotton as their filling due to the exceptional insulating capabilities inherent in the cotton fibers’ structure. The loose, airy arrangement of the cotton creates a multitude of minuscule air spaces. Air, being a poor thermal conductor, significantly hinders the transmission of heat.

When enveloped by a quilt filled with this airy cotton, the heat generated by your body becomes entrapped within these numerous air pockets formed between the cotton strands. The cotton fibers themselves also contribute by acting as a barrier, impeding the flow of heat away from your body. This captured layer of warm air functions as an effective insulator, preventing the colder ambient air from reaching your skin, thereby maintaining warmth and comfort.

Essentially, the fluffy cotton within quilts operates by immobilizing air, and both the cotton material and the trapped air exhibit poor thermal conductivity. This dual effect minimizes the loss of heat from the body, effectively providing warmth in cooler environments.

Question 41.

State the direction of heat transfer by way of convection.

Ans:

In natural convection, the direction of heat transfer is predominantly upwards. The cooler, denser fluid then descends to replace it, creating a continuous upward flow of warmer fluid carrying thermal energy. While convection involves a circular motion, the net movement of heat is generally from a lower, warmer region to an upper, cooler region.

Question 42.

Why is a ventilator provided in a room ?

Ans:

A ventilator, in the context of room design (not a medical breathing device), is typically a small opening or window positioned high up on a wall, near the ceiling. Its primary purpose is to facilitate ventilation, which is the process of replacing stale, impure air inside a room with fresh air from the outside.

Here’s a breakdown of why ventilators are provided in rooms:

Removal of Warm, Stale Air: Due to natural convection, warm air, which often contains higher levels of carbon dioxide, moisture, and pollutants (like odors), rises and accumulates near the ceiling. A ventilator provides an outlet for this warmer, less dense air to escape the room.
Facilitating Air Circulation: As the warm, stale air exits through the ventilator, it creates a slight decrease in air pressure within the room. This pressure difference allows cooler, fresh air from outside (often entering through windows or other openings at a lower level) to be drawn into the room. This establishes a natural flow or circulation of air.
Maintaining Fresh Air Quality: By continuously replacing the indoor air with fresh outdoor air, ventilators help to maintain a healthy and comfortable environment within the room. This reduces the concentration of pollutants, odors, and excess moisture.
Utilizing Natural Convection: The placement of ventilators high up on the wall is strategic. It takes advantage of the natural tendency of warm air to rise. This passive system of ventilation doesn’t require mechanical assistance (like fans) in many cases, making it energy-efficient.

Question 43.

Why are chimneys provided over furnace in factories ?

Ans:

A chimney is provided over a furnace in a factory for several crucial reasons, primarily related to the principles of heat transfer by convection:

According to the principles of convection, these hot gases are less dense than the surrounding cooler air. This density difference causes the hot, polluted air to naturally rise. The chimney provides a vertical pathway for this upward movement.

Secondly, the height of the chimney enhances this natural convection process, often referred to as the “stack effect” or “chimney effect.” As the hot gases rise within the chimney, they create a lower pressure at the base, which helps to draw in more air for combustion in the furnace and efficiently expel the combustion products out of the factory and into the atmosphere at a higher altitude.

Finally, by releasing pollutants at a greater height, chimneys aid in the dispersion of these substances over a wider area. This dilution in the atmosphere helps to reduce the concentration of harmful pollutants at ground level in the vicinity of the factory, thus contributing to better air quality for the surrounding environment and the people working within and around the industrial area.

Question 44.

What are the land and sea breezes ? Explain their formation.

Ans:

Sea Breeze (Day): During daylight hours, the land surface heats up more quickly than the adjacent sea due to differences in their heat capacities. This differential heating causes the air directly above the land to become warmer, less dense, and consequently, to rise. The rising warm air creates a region of lower atmospheric pressure over the land. In response, the cooler, denser air situated over the relatively cooler sea moves horizontally towards the land to replace the rising air, resulting in a wind blowing from the sea towards the land, which is termed a sea breeze or onshore wind.

Land Breeze (Night): As night falls, the land surface cools down more rapidly than the sea because it loses heat more efficiently. This cooling causes the air directly above the land to become cooler, denser, and consequently, to sink, leading to the development of a region of higher atmospheric pressure over the land. Therefore, the cooler, denser air from the land surface flows horizontally towards the relatively warmer, lower-pressure area over the sea, resulting in a wind blowing from the land towards the sea, which is termed a land breeze or offshore wind.

Question 45.

Why is the freezing chest in a refrigerator fitted near its top?

Ans:

Density and Temperature: The freezing unit cools the adjacent air, causing its density to increase. Denser fluids (in this case, air) tend to sink due to gravity.
Downward Flow of Cold Air: The cold, denser air produced by the freezer compartment naturally descends, effectively displacing the warmer, less dense air that resides in the lower regions of the refrigerator.
Upward Movement of Warmer Air: Conversely, the warmer air within the refrigerator, being less dense, rises towards the top, where it comes into thermal contact with the cold surfaces of the freezing chest. This allows the warmer air to be cooled.
Establishment of Convection Currents: This continuous process of cold air sinking and warm air rising establishes convection currents throughout the refrigerator’s interior. These circulating airflows are crucial for distributing the cooling effect uniformly across all shelves and compartments, ensuring that food items stored at different levels are maintained at the desired temperatures.

Question 46.

Explain briefly the process of heat transfer by radiation.

Ans:

Radiation is heat transfer through electromagnetic waves, needing no medium. All objects above absolute zero emit it, with hotter and darker objects emitting more. When absorbed by another object, it increases its temperature. Examples include sunlight and heat from a fire.

Question 47.

Give one example of heat transfer by radiation.

Ans:

A common instance of heat transfer via radiation is the warmth you experience when standing near a campfire. The fire emits electromagnetic radiation, including infrared waves, which propagate through the air. When these waves encounter your body, they are absorbed by your skin and clothing, increasing the kinetic energy of the molecules and resulting in the sensation of heat. This transfer of thermal energy occurs without any direct physical contact with the fire or the involvement of air as a primary heat carrier; it is solely mediated by the emitted electromagnetic radiation.

Question 48.

Why do we prefer to wear white or light coloured clothes in summer and black or dark coloured clothes in winter ?

Ans:

Summer (Light): Reflect sunlight, absorb less heat, keep us cool.

Winter (Dark): Absorb sunlight, retain more heat, keep us warm.

It’s all about how the colors affect the absorption and reflection of radiant heat from the sun and the surroundings.

Question 49.

The bottom of a cooking utensil is painted black. Give the reason.

Ans:

Then is why this is salutary for cooking

Effective Heat immersion When the cuisine instrument is placed on a heat source( like a gas honey or an electric coil), the black bottom efficiently absorbs the heat radiated by the source. This means further of the energy from the heat source is transferred to the instrument rather than being reflected down.
Faster cuisine Due to the increased immersion of heat, the instrument heats up more snappily. This leads to faster cuisine times, saving energy and time.

Livery Heat Distribution While the color primarily affects the immersion of radiant heat from the source, the material of the instrument( generally essence) also conducts this absorbed heat across the base, contributing to further even cuisine.

Question 50.

Draw a labelled diagram of a thermo flask. Explain how the transfer of heat by conduction, convection and radiation is reduced to a minimum in it.

Ans:

A thermos flask minimizes heat transfer through three key features:

Vacuum: The space between the double walls is evacuated, drastically reducing heat transfer by conduction (no medium for heat to travel through) and convection (no air movement to carry heat).
Reflective Surfaces: The inner walls are coated with a shiny material (like silver) to reflect radiation, preventing heat from escaping or entering.
Insulating Stopper: A tight-fitting stopper made of insulating material further reduces heat loss or gain by conduction and convection at the opening.

Essentially, the vacuum stops heat movement by contact and fluid motion, while the shiny surfaces block heat transfer by electromagnetic waves, and the stopper seals the flask with a poor heat conductor.

C. Numericals

Question 1.
The temperature of a body rises by 1°C. What is the corresponding rise on the (a) Fahrenheit scale (b) Kelvin scale?
Ans:

(a) Fahrenheit: A 1°C rise equals a 1.8°F rise.

(b) Kelvin: A 1°C rise equals a 1 K rise.

Question 2.
The temperature rises by 18°F. What is the rise on the Celsius scale ?
Ans:
Since 100 divisions on the Celsius scale =180 divisions on the Fahrenheit scale
∴ 18 divisions on Fahrenheit scale.