Class 10 Physics — Chapter 11: Transfer of Thermal Energy

Punjab Board — Exercise Solutions

A. Multiple Choice Questions (MCQs)

11.1 Which method of heat transfer requires direct contact between particles?

(a) Convection
(b) Radiation
(c) Conduction
(d) Evaporation

Correct Answer: (c) Conduction

11.2 What causes convection currents in liquids and gases?

(a) The continuous movement of particles in all directions
(b) The rising of warm regions and sinking of cooler regions
(c) The conduction of heat through the medium
(d) The reflection of heat waves

Correct Answer: (b) The rising of warm regions and sinking of cooler regions

11.3 The natural phenomenon caused by convection currents is:

(a) Earthquakes
(b) Land and sea breezes
(c) Magnetic fields
(d) Solar eclipses

Correct Answer: (b) Land and sea breezes

11.4 How does heat transfer occur through radiation?

(a) Through direct contact between objects
(b) Through the movement of heated fluid
(c) Through electromagnetic waves
(d) Through the absorption of water molecules

Correct Answer: (c) Through electromagnetic waves without needing a medium

11.5 The surface which absorbs and emits the most radiation is:

(a) Smooth and shiny surfaces
(b) Dark and rough surfaces
(c) Transparent and thin surfaces
(d) Polished surfaces

Correct Answer: (b) Dark and rough surfaces

11.6 What is the role of the greenhouse effect in Earth’s temperature regulation?

(a) It allows all heat to escape into space
(b) It blocks heat from entering the atmosphere
(c) It traps heat to maintain a stable temperature
(d) It causes immediate cooling of Earth’s surface

Correct Answer: (c) It traps heat to maintain a stable temperature

11.7 The Sun’s heat reaches the Earth:

(a) Through conduction between air particles
(b) Through convection in space
(c) Through radiation via electromagnetic waves
(d) Through reflection by the Moon

Correct Answer: (c) Through radiation via electromagnetic waves

B. Short Answer Questions

11.1 What are the three primary types of heat transfer?

Answer:

The three primary modes of heat transfer are:

1. Conduction: Heat transfer through direct contact between atoms or molecules without bulk movement of the medium. It is dominant in solids.

2. Convection: Heat transfer through the actual bulk movement of fluid (liquid or gas) caused by density differences.

3. Radiation: Heat transfer through electromagnetic waves, such as infrared waves, without requiring any material medium.

11.2 Why does heat travel faster in metals compared to non-metals?

Answer:

Heat travels faster in metals because they contain a large number of free or delocalized electrons. In non-metals, thermal energy is transferred mainly through lattice vibrations from atom to atom.

In metals, free electrons gain kinetic energy at the hot end and move rapidly through the material. They collide with cooler atoms and transfer thermal energy, allowing heat to travel more quickly.

11.3 How does convection transfer heat in liquids and gases?

Answer:

When a liquid or gas is heated, its particles gain thermal energy and move faster. The fluid expands, decreasing its density. The warmer, less dense fluid rises, while the cooler, denser fluid sinks to take its place.

This continuous circulation forms a convection current, which transfers heat throughout the fluid.

11.4 What natural phenomenon causes land and sea breezes?

Answer:

Land and sea breezes are caused by convection currents resulting from the different rates at which land and water heat up and cool down.

• Sea Breeze (Daytime): Land heats up faster than the sea. Warm air above the land rises, and cooler air from the sea moves toward the land.

• Land Breeze (Nighttime): Land cools down faster than water. Warm air above the sea rises, and cooler air from the land moves toward the sea.

11.5 How do birds and gliders take advantage of convection currents?

Answer:

Birds such as eagles and hawks, along with glider pilots, use upward-rising columns of warm air called thermals. These currents form when the ground heats the air above it.

By entering these rising air currents, birds and gliders gain height and remain airborne for long periods without continuous flapping or engine power.

11.6 What is radiation, and how does it transfer heat?

Answer:

Radiation is the transfer of heat energy from a warmer body to a cooler body through electromagnetic waves, particularly infrared radiation.

It does not require a material medium and can travel through a vacuum. For example, solar energy travels through space and warms the Earth.

11.7 Why does a dull black surface emit and absorb more radiation than a shiny silver surface?

Answer:

• Dull Black Surfaces: These surfaces are non-reflective and absorb most of the thermal radiation falling on them. They are also good emitters of radiation.

• Shiny Silver Surfaces: These surfaces reflect most incoming thermal radiation. They have low absorptivity and emissivity, making them poor absorbers and emitters of heat radiation.

According to Kirchhoff’s law of thermal radiation, a good absorber is also a good emitter under the same conditions.

11.8 How does the greenhouse effect help regulate Earth’s temperature?

Answer:

Short-wavelength solar radiation passes through the Earth’s atmosphere and warms its surface. The Earth then emits energy as long-wavelength infrared radiation.

Greenhouse gases, such as carbon dioxide (\(\text{CO}_2\)), water vapour, and methane, absorb and re-emit some of this outgoing infrared radiation. This helps retain thermal energy in the atmosphere and keeps the Earth warm enough to support life.

C. Constructed Response Questions

11.1 During the Leslie’s Cube experiment, why does the dull black surface radiate more heat than the shiny silver one, even though they are at the same temperature?

Answer:

In Leslie’s Cube, all surfaces are maintained at approximately the same temperature by the hot water inside the cube. However, their rates of thermal radiation differ because of their surface properties.

• Dull Black Surface: It has high emissivity (\(\varepsilon\)), so it emits thermal radiation efficiently.

• Shiny Silver Surface: It has low emissivity and reflects much of the incident thermal radiation, so it emits less radiation than the dull black surface.

Therefore, a thermal radiation detector placed opposite the dull black surface records a stronger reading than one placed opposite the shiny silver surface.

11.2 How does the size (surface area) of a radiator affect the amount of heat it emits through radiation? Explain using a real-world example.

Answer:

The rate of heat radiation is directly proportional to the exposed surface area when temperature and surface properties are the same.

Where:

• \(P\) = Rate of heat radiation

• \(A\) = Exposed surface area

A larger surface area allows more thermal energy to be emitted at the same time.

Example: Household heating radiators and car cooling radiators contain thin metal fins that increase the exposed surface area. This improves heat transfer between the radiator and its surroundings.

11.3 Why is convection not possible in solids, and how do convection currents in liquids and gases compensate for their poor thermal conductivity?

Answer:

Why Convection Is Not Possible in Solids

Convection requires the actual movement of fluid particles from one region to another. In solids, particles are held in fixed positions within a rigid structure and mainly vibrate around their mean positions. Therefore, bulk movement does not occur, and convection is not possible.

How Convection Transfers Heat in Fluids

Liquids and gases are generally poorer conductors of heat than metals. However, when heated, a fluid expands and becomes less dense. The warmer fluid rises while cooler, denser fluid sinks.

This continuous circulation transports thermal energy through the fluid and helps distribute heat efficiently.

11.4 How do birds like eagles and gliders manage to stay aloft without flapping their wings for hours? Explain the scientific principle involved.

Answer:

The scientific principle involved is thermal convection and atmospheric updrafts.

Solar radiation heats different parts of the Earth’s surface unevenly. The air above warmer ground gains heat, expands, and becomes less dense. It rises in upward currents called thermals.

Eagles and gliders use these rising currents to gain height and remain airborne for long periods without continuously flapping their wings or using engine power.

11.5 Why does a hot metal object placed in a cooler room eventually cool down? Explain using the concepts of radiation and thermal equilibrium.

Answer:

All objects continuously emit and absorb thermal radiation.

1. A hot metal object initially emits thermal radiation at a greater rate than it absorbs radiation from the cooler surroundings.

2. As a result, it loses net thermal energy, and its temperature decreases.

3. Cooling continues until the object’s temperature approaches that of the room.

At thermal equilibrium, the rate of energy emitted as radiation equals the rate absorbed under the same surroundings, so there is no net radiative heat transfer.

D. Comprehensive Questions

11.1 Describe how convection occurs in fluids. How do convection currents contribute to natural phenomena such as sea breezes and wind patterns?

Answer:

Mechanism of Convection in Fluids

When a region of fluid is heated, its temperature rises and it expands. This usually decreases its density. The warmer, less dense fluid rises due to buoyancy, while cooler, denser fluid moves in to replace it.

As this process continues, a circulating flow called a convection current is established.

Contribution to Natural Phenomena

1. Sea Breezes: During the day, land heats up faster than seawater. The air above the land warms and rises, drawing cooler air from the sea toward the shore.

2. Global Wind Patterns: The Earth’s surface receives unequal amounts of solar energy. This creates temperature and pressure differences that contribute to atmospheric circulation and global wind patterns.

Warm air tends to rise in warmer regions, while cooler air tends to sink in cooler regions. The resulting circulation, combined with the Earth’s rotation and other atmospheric effects, helps shape global and regional winds.

11.2 How does radiation differ from conduction and convection in terms of heat transfer? Give examples of radiation in everyday life and explain how different surfaces affect radiation absorption and emission.

Answer:

Differences Between Conduction, Convection, and Radiation

Everyday Examples of Radiation

1. Feeling warmth when standing near a fire.

2. Sunlight warming the Earth through space.

Effect of Surface Properties

• Dark, Dull or Rough Surfaces: These are generally good absorbers and good emitters of thermal radiation.

• Light-Coloured, Shiny or Polished Surfaces: These generally reflect more radiation and are poor absorbers and emitters.

11.3 Discuss the role of the greenhouse effect in regulating Earth’s temperature. What are the potential consequences of an enhanced greenhouse effect due to human activities?

Answer:

Role in Temperature Regulation

The natural greenhouse effect helps keep the Earth warm. Greenhouse gases, such as carbon dioxide (\(\text{CO}_2\)), methane (\(\text{CH}_4\)), and water vapour, allow much of the incoming solar radiation to reach the Earth’s surface but absorb some of the outgoing infrared radiation.

This reduces the rate at which energy escapes into space and maintains a global average surface temperature of approximately \(15^\circ\text{C}\). Without the natural greenhouse effect, the average temperature would be around \(-18^\circ\text{C}\).

Consequences of an Enhanced Greenhouse Effect

Human activities, including burning fossil fuels and deforestation, increase the concentration of greenhouse gases in the atmosphere. This strengthens the greenhouse effect and contributes to global warming.

Major consequences include:

1. Global Warming: An increase in average global temperatures.

2. Melting Ice and Rising Sea Levels: Warming contributes to the loss of land ice and rising sea levels, increasing the risk of coastal flooding.

3. Extreme Weather Events: Increased risks of severe heatwaves, droughts, and certain types of storms and heavy rainfall.

11.4 Compare the absorption and emission of heat by different surfaces. How does the colour and texture of a surface affect its ability to absorb and emit heat?

Answer:

The absorption and emission of thermal radiation depend on the colour and texture of a surface.

1. Effect of Colour

• Black or Dark Surfaces: Generally absorb more incoming radiation and emit thermal radiation more effectively.

• White or Light-Coloured Surfaces: Generally reflect more incoming radiation and absorb less of it.

2. Effect of Texture

• Rough or Matte Surfaces: Generally absorb and emit thermal radiation more effectively than polished surfaces.

• Smooth or Polished Surfaces: Reflect more incoming radiation and generally have lower absorptivity and emissivity.

Therefore, dull black surfaces are generally good absorbers and emitters, while shiny, polished surfaces are generally poor absorbers and emitters.

11.5 Analyze how uneven heating of Earth’s surface leads to the formation of convection currents and influences wind patterns.

Answer:

Uneven heating of the Earth’s surface causes temperature and pressure differences, which contribute to convection currents and wind formation.

1. Differential Solar Absorption: Different regions receive and absorb solar energy at different rates because of variations in surface type, such as land and ocean, and the angle at which sunlight reaches the Earth.

2. Density Decrease and Rising Air: The ground heats the air in contact with it. The warmer air expands, becomes less dense, and rises.

3. Lateral Air Replacement: Cooler, denser air from surrounding regions moves horizontally to replace the rising warm air.

4. Circulation and Wind Formation: Rising air cools as it moves upward and may spread horizontally. Air then sinks in cooler regions, contributing to large-scale circulation patterns.

These processes, together with the Earth’s rotation and differences in atmospheric pressure, help determine regional and global wind patterns.

Notes by Sir Dilawar

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