Transfer Of Thermal Energy

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Notes

Demonstrating Conduction

**Good thermal conductors** (e.g., aluminium, copper) transfer heat easily; **insulators** (e.g., wool, cardboard) transfer heat poorly.
Tiles feel colder than a rug at the same temperature because tiles conduct heat away from the foot faster.
In the wood–metal rod experiment, paper over **metal** stays unburnt because metal conducts heat away; paper over **wood** chars because wood insulates.
Relative conductivity of metals can be compared by timing when wax melts and ball bearings drop from metal strips heated at a common centre.

Thermal Conduction (Extended)

Conduction in solids occurs via **atomic vibrations** and, in metals, via **free electron collisions**.
Metals are the best conductors because they have many free electrons that transfer energy rapidly.
Liquids and gases are **poor conductors** because particles are too far apart or slide past each other, hindering vibration transfer.
Thermal conductivity varies widely among materials; conductors are typically metals with delocalised electrons.

Convection

Convection is the main heat transfer method in **fluids** (liquids and gases); it cannot occur in solids.
Heating a fluid causes it to **expand**, become **less dense**, and **rise**; cooler fluid sinks, creating a **convection current**.
Cooling a fluid makes it contract, become denser, and sink, also driving a convection current.
A convection current can be demonstrated using potassium permanganate crystals in water: the purple dye reveals the rising and falling motion.

Radiation

All objects emit **thermal radiation** (infrared); hotter objects emit more.
Thermal radiation can travel through a **vacuum** (e.g., from the Sun to Earth).
**Black/dull surfaces** are good absorbers and good emitters of radiation; **white/shiny surfaces** are poor absorbers and poor emitters.
At **thermal equilibrium**, an object absorbs and emits radiation at the same rate, so its temperature remains constant.

The Greenhouse Effect

Without an atmosphere, Earth's average surface temperature would be about **−18 °C**.
**Greenhouse gases** (e.g., CO₂, water vapour, methane) absorb and re-emit longer-wavelength infrared radiation from Earth, trapping heat.
This process keeps Earth warm enough for life; increasing greenhouse gases raises the rate of radiation absorbed, leading to **global warming**.

Investigating IR Radiation

Experiment: use identical flasks painted black, grey, white, and silver, filled with hot water at the same initial temperature.
Measure temperature at regular intervals (e.g., every 30 s for 10 min).
The black flask cools fastest (best emitter); the silver flask cools slowest (poorest emitter).
Plot temperature vs. time; the curves show that surface colour affects the rate of infrared emission.

Consequences of Thermal Energy Transfer

**Conduction** applications: metal pans heat food quickly; plastic handles insulate; double-glazed windows use trapped air as an insulator.
**Convection** applications: room radiators heat air which rises and circulates; steam rises from hot drinks.
**Complex transfers**: a hot drink loses heat by conduction through the cup, convection from the surface, and radiation from the cup's sides.
A **car radiator** uses conduction from engine to liquid, then to radiator, and radiation from the dark, large-surface-area radiator to air.

Particle arrangement in solids, liquids, and gases. Solids have closely packed, vibrating particles; liquids have close but sliding particles; gases have widely spaced, fast-moving particles.

Convection current: fluid near heat source expands, becomes less dense, and rises (red arrow); it then cools, becomes denser, and sinks (blue arrow), creating a circular flow.

Four surfaces (black, grey, white, silver) with hot water inside. Black emits the most infrared radiation (largest arrow), silver the least.

Practice questions

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1.Which of the following is the main method of thermal energy transfer in solids?
Easy
- AConduction
- BConvection
- CRadiation
- DEvaporation
2.Thermal radiation is part of which electromagnetic spectrum?
Easy
- AVisible light
- BInfrared
- CUltraviolet
- DMicrowave
3.In the experiment comparing conduction in wood and metal, a rod made of half wood and half metal is wrapped in paper and heated at the join. What is observed?
Medium
- AThe paper chars only where it touches the metal
- BThe paper chars only where it touches the wood
- CThe paper chars evenly across both materials
- DThe paper does not char at all
4.Which of the following is a greenhouse gas that contributes to the greenhouse effect?
Medium
- AOxygen
- BNitrogen
- CCarbon dioxide
- DHydrogen
5.In the investigation of infrared radiation using flasks painted different colours, which flask is expected to cool the fastest?
Hard
- ABlack flask
- BWhite flask
- CSilver flask
- DDull grey flask
6.Convection currents in fluids are caused by changes in:
Easy
- ATemperature and density
- BPressure and volume
- CColour and texture
- DMass and weight
7.Which of the following is a good thermal insulator?
Medium
- ACopper
- BAluminium
- CWool
- DIron
8.A student places one foot on a tile floor and the other on a rug in the same room. The tile feels colder because:
Hard
- AThe tile is at a lower temperature than the rug
- BThe tile conducts heat away from the foot faster than the rug
- CThe rug radiates heat towards the foot
- DThe tile has a higher specific heat capacity

More questions

9.Thermal radiation can travel through:
Easy
- ASolids only
- BLiquids only
- CGases only
- DA vacuum
10.Which surface is the best absorber of thermal radiation?
Medium
- AShiny white
- BDull black
- CShiny silver
- DDull grey
11.In a car radiator, thermal energy is transferred from the engine to the surrounding air mainly by:
Hard
- AConduction through the liquid, then radiation from the radiator
- BConvection only
- CRadiation only
- DConduction through the metal directly to the air
12.Which of the following is not a method of thermal energy transfer?
Medium
- AConduction
- BConvection
- CRadiation
- DEvaporation
13.In an experiment to compare cooling rates, a student records the temperature of hot water in a beaker every 30 seconds. The results are: t=0 s, θ=94°C; t=30 s, θ=93°C; t=60 s, θ=92°C; t=90 s, θ=91°C; t=120 s, θ=90°C; t=150 s, θ=89°C. What is the average cooling rate over the first 90 seconds?
Medium
- A0.033 °C/s
- B0.033 °C/min
- C0.1 °C/s
- D1 °C/s
14.A student investigates cooling of hot water in a beaker. The table shows temperatures every 30 s. t=0: 95.0°C, t=90: 79.0°C, t=180: 68.5°C, t=270: 62.5°C. Which statement about the cooling rate is correct?
Hard
- AThe cooling rate decreases as the water cools.
- BThe cooling rate increases as the water cools.
- CThe cooling rate is constant throughout.
- DThe cooling rate is fastest in the last 90 s.
15.In an experiment to compare cooling of water in a beaker vs a metal can painted matt black, the following temperatures are recorded every 30 s. Beaker: 0s=94°C, 30s=93°C, 60s=92°C, 90s=91°C, 120s=90°C, 150s=89°C. Can: 0s=93°C, 30s=91°C, 60s=90°C, 90s=89°C, 120s=88°C, 150s=87°C. What conclusion can be drawn?
Medium
- AThe water in the can cools faster than in the beaker.
- BThe water in the beaker cools faster than in the can.
- CThere is no significant difference in cooling rates.
- DThe can cools faster because it is metal.
16.A student plans to investigate whether the difference in cooling rate between a beaker and a metal can is due to surface colour or material. Which pair of changes to the apparatus would help distinguish these factors?
Medium
- AUse a metal can painted shiny and a glass beaker painted matt black.
- BUse a metal can painted matt black and a metal can painted shiny.
- CUse a glass beaker and a metal can both painted matt black.
- DUse a glass beaker and a metal can both left unpainted.
17.In a cooling experiment, a student uses a beaker with insulation and another with a lid. The temperature drop over 180 s is 14.5°C for the insulated beaker and 7.5°C for the beaker with lid. Which is more effective at reducing cooling rate?
Hard
- AThe lid is more effective.
- BThe insulation is more effective.
- CBoth are equally effective.
- DNeither reduces cooling.
18.A student calculates average cooling rate for beaker A over 180 s as x = (θ0 - θ180)/T. If θ0 = 83.0°C and θ180 = 68.5°C, what is x?
Hard
- A0.0806 °C/s
- B0.0806 °C/min
- C0.0833 °C/s
- D0.0833 °C/min
19.In the investigation of infrared radiation using flasks of different colours, which flask is expected to have the highest temperature after 10 minutes?
Medium
- ABlack flask
- BWhite flask
- CSilver flask
- DGrey flask
20.Which of the following is the best description of a convection current?
Easy
- AHot fluid rises because it is less dense, then cools and sinks.
- BHot fluid sinks because it is more dense, then warms and rises.
- CCold fluid rises because it is less dense, then warms and sinks.
- DHot fluid rises because it is more dense, then cools and sinks.
21.A car radiator is painted black and has a large surface area. Which combination of reasons explains why this helps cool the engine?
Medium
- ABlack surfaces are good emitters of infrared radiation; large area increases radiation rate.
- BBlack surfaces are good absorbers of infrared radiation; large area increases conduction.
- CBlack surfaces are poor emitters of infrared radiation; large area reduces convection.
- DBlack surfaces are poor absorbers of infrared radiation; large area reduces radiation.
22.In the wood and metal rod demonstration, paper wrapped around the rod is heated at the join. Why does the paper not burn where it touches the metal?
Medium
- AMetal conducts heat away from the paper, keeping it cool.
- BMetal reflects heat away from the paper.
- CMetal is a poor conductor, so heat stays in the paper.
- DThe paper is a better conductor than metal.
23.A student places one foot on a tile floor and the other on a rug in the same room. Both surfaces have been at room temperature for hours. Which statement explains why the tile feels colder?
Hard
- ATile conducts heat away from the foot faster than the rug.
- BTile is at a lower temperature than the rug.
- CRug conducts heat away from the foot faster than the tile.
- DRug is at a higher temperature than the tile.
24.Which of the following is a greenhouse gas that contributes to the greenhouse effect by absorbing infrared radiation emitted by the Earth?
Medium
- ACarbon dioxide
- BOxygen
- CNitrogen
- DArgon
25.In the experiment to compare conduction in wood and metal, a rod made of half wood and half metal is wrapped in paper and heated at the join. After unwrapping, the paper is charred where it touched the wood but undamaged where it touched the metal. Which statement explains this observation?
Medium
- AMetal conducts heat away from the paper, preventing it from reaching its ignition temperature.
- BWood conducts heat away from the paper, preventing it from reaching its ignition temperature.
- CThe metal reflects heat away from the paper.
- DThe wood absorbs heat, keeping the paper cool.
26.A student investigates the rate of cooling of hot water in a beaker. The temperature at t=0 is 95.0°C and at t=90s is 79.0°C. Using the equation x = (θ0 - θ90)/T where T=90s, what is the average cooling rate?
Hard
- A0.178 °C/s
- B0.178 °C/min
- C1.78 °C/s
- D0.178 K/s
27.In an experiment to compare cooling rates, a beaker of hot water and a metal can (painted matt black) of hot water are used. The temperature drop over 150s is 5°C for the beaker and 6°C for the can. Which conclusion is justified?
Medium
- AThe water in the can has a greater rate of cooling than the water in the beaker.
- BThe water in the beaker has a greater rate of cooling than the water in the can.
- CThere is no significant difference in cooling rates.
- DThe can is a better insulator than the beaker.
28.In the investigation of infrared radiation using flasks of different colours, four identical flasks painted black, grey, white, and silver are filled with hot water at the same initial temperature. After 10 minutes, which flask is expected to have the lowest temperature?
Hard
- ABlack flask
- BGrey flask
- CWhite flask
- DSilver flask
29.A student wants to investigate whether the difference in cooling rate between a beaker and a metal can is due to surface colour or material. Which pair of changes to the apparatus would best isolate the effect of surface colour?
Medium
- AUse a metal can painted shiny silver and a metal can painted matt black.
- BUse a metal can and a glass beaker, both painted matt black.
- CUse a metal can painted black and a glass beaker painted white.
- DUse a metal can and a glass beaker, both unpainted.
30.In a cooling experiment, a student uses a beaker with insulation and another with a lid. The temperature drop over 180s is 14.5°C for the insulated beaker and 7.5°C for the beaker with a lid. Both start at the same temperature. Which conclusion is correct?
Hard
- AThe lid is more effective at reducing cooling than the insulation.
- BThe insulation is more effective at reducing cooling than the lid.
- CBoth are equally effective.
- DNeither reduces cooling significantly.
31.A student calculates average cooling rate for beaker A over 180s as x = (θ0 - θ180)/T. If θ0 = 83.0°C and θ180 = 68.5°C, what is x?
Hard
- A0.0806 °C/s
- B0.0806 °C/min
- C0.806 °C/s
- D8.06 °C/s
32.A student plans an experiment to test how the thickness of a lid affects the cooling rate of hot water. Which of the following should be controlled variables?
Hard
- AVolume of water, initial temperature, room temperature, and material of the lid.
- BThickness of the lid, volume of water, and initial temperature.
- CRoom temperature, material of the lid, and thickness of the lid.
- DVolume of water, initial temperature, and thickness of the lid.
33.In the experiment to compare conduction in wood and metal, a rod made of half wood and half metal is wrapped in paper and heated at the join. After unwrapping, what is observed?
Hard
- AThe paper is charred on the wood side and undamaged on the metal side.
- BThe paper is charred on the metal side and undamaged on the wood side.
- CBoth sides are charred equally.
- DNeither side is charred.
34.A student investigates the rate of cooling of hot water in a beaker. The temperature at t=0 is 95.0°C and at t=90s is 79.0°C. Using the equation x = (θ0 - θ90)/T with T=90s, what is the average cooling rate?
Hard
- A0.178 °C/s
- B0.178 °C/min
- C1.78 °C/s
- D17.8 °C/s
35.In an experiment to compare cooling rates, a beaker of hot water and a metal can (painted matt black) of hot water are used. The temperature drop over 150s is 5°C for the beaker and 6°C for the can. Which conclusion is correct?
Hard
- AThe water in the can has a greater rate of cooling than the water in the beaker.
- BThe water in the beaker has a greater rate of cooling than the water in the can.
- CThere is no significant difference between the rates of cooling.
- DThe can cools slower because it is metal.
36.In the investigation of infrared radiation using flasks of different colours, four identical flasks painted black, grey, white, and silver are filled with hot water. After 10 minutes, which flask has the highest temperature?
Medium
- ASilver
- BBlack
- CGrey
- DWhite
37.A student wants to investigate whether the difference in cooling rate between a beaker and a metal can is due to surface colour or material. Which pair of changes should be made?
Hard
- AUse a metal can painted shiny and a glass beaker painted matt black.
- BUse a metal can painted matt black and a glass beaker painted shiny.
- CUse two metal cans, one painted matt black and one shiny.
- DUse two glass beakers, one painted matt black and one shiny.
38.In a cooling experiment, a student uses a beaker with insulation and another with a lid. The temperature drop over 180s is 14.5°C for the insulated beaker and 7.5°C for the beaker with a lid. Which is more effective at reducing cooling?
Hard
- AThe lid is more effective because it results in a smaller temperature drop.
- BThe insulation is more effective because it results in a smaller temperature drop.
- CBoth are equally effective.
- DNeither is effective.
39.A student investigates cooling of hot water in a beaker. The table shows temperatures every 30 s. t=0: 95.0°C, t=90: 79.0°C, t=180: 68.5°C, t=270: 62.5°C. Which statement correctly describes the pattern of cooling?
Hard
- AThe rate of cooling decreases as the temperature falls.
- BThe rate of cooling increases as the temperature falls.
- CThe rate of cooling is constant throughout.
- DThe rate of cooling first increases then decreases.
40.A student plans to investigate whether the difference in cooling rate between a beaker and a metal can is due to surface colour or material. Which pair of changes to the apparatus would best distinguish between these two factors?
Hard
- AUse a beaker painted matt black and a metal can painted shiny silver.
- BUse a beaker painted shiny silver and a metal can painted matt black.
- CUse a beaker and a metal can both painted matt black, and also a beaker and a metal can both painted shiny silver.
- DUse a beaker and a metal can both made of the same material and painted the same colour.

Slides

Transfer of Thermal Energy

Thermal energy can be transferred by three processes: conduction, convection, and radiation.
Conduction mainly occurs in solids, convection in fluids (liquids and gases), and radiation can travel through a vacuum.
This lesson covers each process, experiments to demonstrate them, and real-world consequences.

Conduction

Conduction is the transfer of thermal energy through particle vibrations and the movement of free electrons.
It is the main method of heat transfer in solids.
Metals are good conductors because they have many free electrons.
Non-metals are poor conductors (insulators).

Which of the following is the best thermal conductor?

Wood
Plastic
Copper
Air

Demonstrating Conduction

A rod half wood and half metal wrapped in paper is heated at the join.
The paper on the metal side does not burn because metal conducts heat away.
The paper on the wood side burns because wood is an insulator and does not conduct heat away.

In the rod experiment, the paper on the ____ side does not burn because heat is conducted away.

Convection

Convection is the transfer of thermal energy by the movement of fluids (liquids and gases).
It occurs due to changes in density: when a fluid is heated, it expands, becomes less dense, and rises.
Cooler, denser fluid sinks, creating a convection current.

Which of these are true about convection?

Convection can occur in solids.
Convection currents are caused by density differences.
Hot fluid rises because it becomes less dense.
Convection is the main heat transfer method in gases.

Demonstrating Convection

A beaker of water with potassium permanganate crystals is heated gently.
The purple colour shows the path of the convection current as the water rises when heated and sinks when cooled.
This demonstrates how heat is transferred through the liquid.

What drives a convection current?

Differences in temperature causing density changes
Movement of free electrons
Infrared radiation
Particle vibrations

Radiation

Thermal radiation is infrared radiation, part of the electromagnetic spectrum.
All objects emit thermal radiation; hotter objects emit more.
Radiation does not require a medium and can travel through a vacuum.
The amount emitted depends on surface colour and texture: black/dull surfaces are good emitters and absorbers; shiny/white surfaces are poor emitters and absorbers.

Which surface is the best emitter of thermal radiation?

Shiny silver
White
Dull black
Transparent glass

Investigating Infrared Radiation

An experiment uses four flasks painted black, grey, white, and silver, filled with hot water at the same initial temperature.
Temperature is recorded at regular intervals.
The black flask cools fastest (best emitter), the silver flask cools slowest (poorest emitter).
Differences in cooling rate are due to radiation, as conduction and convection are the same for all flasks.

In the radiation experiment, the ____ flask cools fastest because it is the best emitter of infrared radiation.

The Greenhouse Effect

The Earth receives thermal radiation from the Sun and emits its own infrared radiation.
Greenhouse gases (e.g., CO₂, water vapour, methane) absorb some of this emitted radiation and re-radiate it back to Earth.
This keeps the Earth warmer than it would be without an atmosphere.
Increased greenhouse gases lead to more absorption and re-radiation, causing global warming.

Which of these are greenhouse gases?

Carbon dioxide
Oxygen
Methane
Water vapour

Consequences of Thermal Energy Transfer

Conduction: metal pans heat food quickly; plastic handles are insulators.
Convection: radiators heat rooms via convection currents.
Radiation: car radiators are black to emit heat efficiently.
In real situations, multiple processes occur together (e.g., a hot drink loses heat by conduction through the cup, convection from the surface, and radiation from the sides).

A car radiator is painted black to:

Absorb more heat from the engine
Emit more thermal radiation
Reflect sunlight
Reduce convection

Summary

Conduction: heat transfer through solids via vibrations and free electrons.
Convection: heat transfer in fluids via density-driven currents.
Radiation: heat transfer via infrared waves, no medium needed.
Surface properties affect radiation: black/dull = good emitter/absorber; shiny/white = poor emitter/absorber.
The greenhouse effect is caused by gases trapping infrared radiation from Earth.

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