Transfer Of Thermal Energy

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Thermal Conduction

Conduction is the transfer of thermal energy through a solid via particle vibrations and the movement of free electrons.
Metals are good conductors because they have many free electrons that transfer energy through collisions.
Non-metals and organic materials are poor conductors (insulators) because they lack free electrons.
In conduction, atoms at the hotter end vibrate more and collide with neighbours, transferring energy until thermal equilibrium is reached.

Conduction: the atoms in a solid vibrate and collide, transferring energy

Convection

Convection is the main method of thermal energy transfer in fluids (liquids and gases).
When a fluid is heated, it expands, becomes less dense, and rises; cooler, denser fluid sinks to replace it, creating a convection current.
Convection cannot occur in solids because particles are fixed in position.
Common examples: radiators heating a room, steam rising from hot coffee.

When a liquid or gas is heated, it becomes less dense and rises

Thermal Radiation

Thermal radiation is the transfer of energy by infrared waves; it does not require a medium and can travel through a vacuum.
All objects emit thermal radiation; hotter objects emit more radiation.
Black, dull surfaces are good absorbers and good emitters of radiation.
White, shiny surfaces are poor absorbers and poor emitters (they reflect radiation).

Effect of surface colour and texture on thermal radiation

Investigating Thermal Radiation

Experiment: fill identical flasks painted black, grey, white, and silver with hot water at the same initial temperature.
Measure temperature at regular intervals (e.g., every 30 s for 10 min) to compare cooling rates.
The black flask cools fastest (best emitter), the silver flask cools slowest (poorest emitter).
Control variables: same amount of water, same starting temperature, same time interval.

Different coloured flasks for investigating infrared radiation

The Greenhouse Effect

The Earth absorbs radiation from the Sun and emits its own thermal radiation.
Greenhouse gases (e.g., CO₂, water vapour) absorb and re-emit some of Earth's radiation, trapping heat and warming the surface.
Without greenhouse gases, Earth's surface would be extremely cold.
Excess greenhouse gases increase the amount of radiation reflected back, leading to global warming.

The greenhouse effect: radiation from the Sun and Earth

Consequences of Thermal Energy Transfer

Conduction applications: metal pans for quick heating; plastic handles and double-glazed windows as insulators.
Convection applications: radiators use convection currents to heat rooms; steam rising cools hot liquids.
Radiation applications: black car bodies absorb sunlight and become hot; fire radiates heat to surroundings.

Double-glazed windows use a layer of air to reduce heat loss

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Practice questions

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1.Which method of thermal energy transfer does not require a medium?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
2.Convection can occur in solids.
Easy
True or false?
3.State one factor that affects the rate of thermal radiation emitted by a surface.
Easy
4.Complete the sentence about conduction in metals.
Easy
In metals, thermal energy is transferred by ____ and by the movement of ____.
5.A black surface and a white surface are placed in sunlight. The black surface absorbs 90% of incident radiation and the white surface absorbs 20%. How many times more radiation does the black surface absorb compared to the white surface?
Medium
- A6.5
- B4.5
- C5.5
- D3.5
6.In a convection current, a fluid expands and becomes less dense when it is heated. What happens next?
Easy
- AIt sinks
- BIt rises
- CIt stays in place
- DIt condenses
7.Arrange the following steps in the correct order to describe how a radiator heats a room by convection.
Easy
- Hot air rises
- Cooler air moves in to replace it
- Air near the radiator is heated
- Hot air cools and sinks
8.Match each surface type to its property regarding thermal radiation.
Easy
- Black, dull surface
- White, shiny surface
- Silver, polished surface
- Good absorber and good emitter
- Poor absorber and poor emitter
- Very poor absorber and very poor emitter

More questions

9.Explain why the Earth's surface temperature would be much lower if there were no greenhouse gases in the atmosphere.
Medium
10.In an experiment to investigate thermal radiation, four identical flasks are painted black, grey, white, and silver, filled with hot water at the same initial temperature, and their temperatures are recorded over time. Which flask is expected to cool the fastest?
Medium
- ABlack
- BGrey
- CWhite
- DSilver
11.Complete the sentence about the greenhouse effect.
Easy
Greenhouse gases such as ____ and ____ absorb and re-emit radiation emitted by the Earth.
12.Name one everyday example of a good thermal conductor.
Easy
13.Which method of thermal energy transfer involves the movement of particles in a fluid?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
14.Thermal radiation requires a medium to travel through.
Easy
True or false?
15.In a convection current, what happens to a fluid when it is heated?
Easy
- AIt becomes more dense and sinks.
- BIt becomes less dense and rises.
- CIt becomes less dense and sinks.
- DIt becomes more dense and rises.
16.Which of the following are greenhouse gases? (Select all that apply)
Easy
- ACarbon dioxide
- BOxygen
- CWater vapour
- DNitrogen
- EMethane
17.Which of the following is the best thermal conductor?
Easy
- AWood
- BPlastic
- CCopper
- DAir
18.Match each method of thermal energy transfer to the correct description.
Easy
- Conduction
- Convection
- Radiation
- Transfer through particle vibrations and free electrons in solids
- Transfer by movement of heated fluid
- Transfer by electromagnetic waves without a medium
19.Conduction is the main method of thermal energy transfer in liquids.
Easy
True or false?
20.In an experiment to investigate thermal radiation, four identical flasks are painted black, grey, white, and silver, filled with hot water at the same initial temperature. After 10 minutes, which flask is likely to have the lowest temperature?
Medium
- ABlack
- BGrey
- CWhite
- DSilver
21.Arrange the following steps in the correct order to explain how a convection current is formed when a liquid is heated from below.
Easy
- Liquid expands and becomes less dense
- Cooler liquid sinks to replace it
- Liquid is heated
- Hot liquid rises
22.Which of the following are good thermal insulators? (Select all that apply)
Easy
- AAir
- BCopper
- CPlastic
- DWood
- EAluminium
23.In an experiment to investigate thermal radiation, four identical flasks painted black, grey, white, and silver are filled with hot water at 80°C. The temperature is recorded every 30 seconds for 10 minutes. Which flask shows the smallest temperature decrease?
Medium
- ABlack
- BGrey
- CWhite
- DSilver
24.Which of the following is a control variable in the experiment to investigate how the colour of a surface affects the amount of thermal radiation emitted?
Easy
- AColour of the flask
- BStarting temperature of the water
- CFinal temperature of the water
- DType of paint used
25.In the thermal radiation experiment, the amount of energy lost by conduction and convection is different for each coloured flask.
Medium
True or false?
26.Which of the following are safety precautions when carrying out the thermal radiation experiment? (Select all that apply)
Easy
- AWear safety goggles
- BKeep water away from electrical equipment
- CUse a heatproof mat
- DCarry out the experiment while sitting down
- ERun burns under cold water for at least 5 minutes
27.Which of the following is the main method of thermal energy transfer in solids?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
28.Which method of thermal energy transfer is the main way heat travels through liquids and gases?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
29.In the experiment to investigate thermal radiation using flasks painted different colours, which of the following are control variables? (Select all that apply)
Medium
- AColour of the flask
- BInitial temperature of the water
- CVolume of water in each flask
- DType of thermometer used
- ERoom temperature
30.In the experiment to investigate thermal radiation, a student uses a data logger with digital thermometers instead of glass thermometers. What type of error is reduced?
Medium
- ASystematic error
- BRandom error
- CParallax error
- DZero error
31.Arrange the following steps in the correct order to describe how a convection current forms when a liquid is heated from below.
Medium
- Cooler liquid sinks and replaces the rising liquid
- Heated liquid expands, becomes less dense, and rises
- Liquid near the heat source is heated first
- The rising liquid cools, contracts, becomes denser, and sinks
32.Which of the following surfaces is the best emitter of thermal radiation?
Easy
- AShiny silver
- BWhite
- CDull grey
- DBlack
33.In the experiment to investigate thermal radiation using flasks painted different colours, which of the following is a control variable?
Easy
- AColour of the flask
- BInitial temperature of the water
- CTemperature after 10 minutes
- DAmount of thermal radiation emitted
34.A student investigates how the colour of a surface affects the rate of cooling. Four identical flasks painted black, grey, white, and silver are filled with hot water at 80°C. The temperature of each flask is measured every 30 seconds for 10 minutes. Which of the following is a control variable in this experiment?
Easy
- AThe colour of the flask
- BThe initial temperature of the water
- CThe final temperature of the water
- DThe rate of cooling
35.Complete the sentence: The hotter an object is, the ___ thermal radiation it emits.
Easy
The hotter an object is, the ____ thermal radiation it emits.
36.In an experiment to investigate thermal radiation, four identical flasks painted black, grey, white, and silver are filled with hot water at 80°C. After 10 minutes, which flask is expected to have the lowest temperature? (Enter 1 for black, 2 for grey, 3 for white, 4 for silver)
Easy
37.Complete the sentence.
Easy
Thermal radiation does not need a ____ to travel through.
38.A black surface and a white surface are placed in sunlight. The black surface absorbs 90% of incident radiation and the white surface absorbs 20%. If the incident power on each surface is 500 W, how much more power (in watts) does the black surface absorb compared to the white surface?
Medium
39.A black surface absorbs 90% of incident radiation and a white surface absorbs 20%. If the incident power on each surface is 500 W, calculate the power absorbed by the black surface.
Medium
40.The power radiated by a black surface is 200 W. If the surface is replaced by a white surface at the same temperature, and the white surface emits only 20% of the radiation emitted by the black surface, calculate the power radiated by the white surface.
Medium
41.A black surface absorbs 90% of incident radiation and a white surface absorbs 20%. If the incident power on each surface is 500 W, calculate the power absorbed by the white surface.
Medium
42.Explain why adding excess carbon dioxide to the Earth's atmosphere leads to an increase in surface temperature.
Medium
43.Explain why adding excess carbon dioxide to the Earth's atmosphere leads to an increase in surface temperature of Earth.
Medium
44.Explain how a convection current is formed when a liquid is heated from below.
Medium
45.Describe an experiment to investigate how the colour of a surface affects the amount of thermal radiation it emits.
Medium
46.Explain why adding excess carbon dioxide to the Earth's atmosphere leads to an increase in the surface temperature of Earth.
Medium
47.In an experiment to investigate thermal radiation, four identical flasks painted black, grey, white, and silver are filled with hot water at 80°C. After 10 minutes, which flask is expected to have the lowest temperature? (Enter 1 for black, 2 for grey, 3 for white, 4 for silver)
Easy
- A0
- B1
- C2
- D3
48.A black surface and a white surface are placed in sunlight. The black surface absorbs 90% of incident radiation and the white surface absorbs 20%. If the incident power on each surface is 500 W, how much more power (in watts) does the black surface absorb compared to the white surface?
Medium
- A385 W
- B315 W
- C350 W
- D420 W
49.A black surface absorbs 90% of incident radiation and a white surface absorbs 20%. If the incident power on each surface is 500 W, calculate the power absorbed by the black surface.
Medium
- A540 W
- B405 W
- C495 W
- D450 W
50.The power radiated by a black surface is 200 W. If the surface is replaced by a white surface at the same temperature, and the white surface emits only 20% of the radiation emitted by the black surface, calculate the power radiated by the white surface.
Medium
- A48 W
- B36 W
- C44 W
- D40 W
51.A black surface absorbs 90% of incident radiation and a white surface absorbs 20%. If the incident power on each surface is 500 W, calculate the power absorbed by the white surface.
Easy
- A120 W
- B90 W
- C100 W
- D110 W

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