Cambridge IGCSE0625

Transfer of thermal energy

Physics 0625 Chapter Notes

What this chapter covers

Transfer of thermal energy - ConductionTransfer of thermal energy - ConvectionTransfer of thermal energy - RadiationTransfer of thermal energy - Consequences of thermal energy transfer
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In this chapter

  1. 1Thermal Conduction
  2. 2Thermal Convection
  3. 3Thermal Radiation
  4. 4Applications of Heat Transfer

1. Thermal Conduction

Conduction is the transfer of thermal energy through a substance without any overall movement of the substance itself. It happens primarily in solids. When one part of a solid is heated, its particles (atoms or ions) gain kinetic energy and vibrate more vigorously. These particles then collide with their neighbours, passing on the energy. In metals, which are excellent conductors, this process is helped by 'free' electrons. These electrons are not tied to any single atom and can move throughout the metal, carrying thermal energy quickly from the hot end to the cold end. Materials that are poor conductors, like wood, plastic, and air, are called thermal insulators.

Key term

Thermal Conductor: A material that allows thermal energy to be transferred through it easily.

Examiner insight

Examiners award full marks for explanations of conduction in metals that mention both lattice vibrations and the role of free electrons.

Common pitfall

Stating that particles themselves travel along the material during conduction. It is the energy that is transferred through vibrations and collisions, not the particles themselves (which remain in their fixed positions).

Fun fact

Diamond, a form of carbon, is one of the best thermal conductors at room temperature, even better than metals like copper. This is why a real diamond feels cold to the touch – it rapidly conducts heat away from your skin.

Worked example 14 marks

A metal saucepan has a wooden handle. Explain why the metal pan gets hot quickly when placed on a stove, but the wooden handle remains cool enough to touch.

  1. 1
    1. The metal part of the saucepan is a good thermal conductor.
  2. 2
    1. Heat is transferred efficiently through the metal via vibrations of the fixed ions and, more importantly, by the movement of free electrons.
  3. 3
    1. The handle is made of wood, which is a poor thermal conductor (an insulator). It has no free electrons and transfers energy very slowly through particle vibrations only.
  4. 4
    1. Therefore, heat does not travel quickly to the handle, allowing it to be held safely.

Worked example 23 marks

Woollen jumpers are effective at keeping you warm in winter. Explain how the wool achieves this.

  1. 1
    1. The wool fibres trap many small pockets of air.
  2. 2
    1. Air is a very poor thermal conductor (an excellent insulator).
  3. 3
    1. By trapping the air, the wool prevents it from moving and forming convection currents, further reducing heat transfer.
  4. 4
    1. This significantly reduces the rate of thermal energy loss from the body to the colder surroundings.

Recap

  • Conduction is the transfer of thermal energy through vibrations and collisions of particles.
  • Metals are good conductors because they have free electrons that can transport energy quickly.
  • Non-metals, liquids, and gases are generally poor conductors.
  • Materials that are poor conductors are called insulators.
  • Insulators often work by trapping air, which is a very poor conductor.

Quick check

  1. Name the two ways in which thermal energy is conducted in a metal.2 marks
  2. Why is plastic a suitable material for the casing around electrical wires?1 mark

2. Thermal Convection

Convection is the main way thermal energy is transferred through fluids (liquids and gases). It involves the movement of the fluid itself. When a part of a fluid is heated, its particles gain energy, move faster, and spread further apart. This causes the heated part of the fluid to become less dense. Because it is less dense than the surrounding cooler fluid, it rises. The cooler, denser fluid then sinks to take its place, gets heated, and rises in turn. This process creates a continuous circulation of fluid called a convection current, which distributes thermal energy throughout the fluid.

Key term

Convection Current: A circulating flow of a fluid (a liquid or gas) that is caused by a difference in temperature and density.

Examiner insight

To score well, you must clearly link the change in temperature to a change in density, and then explain how this density difference causes the fluid to move.

Common pitfall

A very common mistake is to say that 'heat rises'. This is incorrect and will lose marks. It is the hot, less dense fluid (e.g., hot air) that rises.

Fun fact

The spectacular patterns seen on the surface of the Sun, called granules, are the tops of huge convection cells. Each 'granule' is about the size of France!

Worked example 14 marks

Explain why a room heater is usually placed on the floor, while an air-conditioning unit is placed high up on a wall.

  1. 1
    1. The heater warms the air near the floor. This air expands, becomes less dense, and rises.
  2. 2
    1. Cooler, denser air from the top of the room sinks to take its place, gets heated, and also rises.
  3. 3
    1. This sets up a convection current that circulates warm air throughout the room.
  4. 4
    1. Conversely, an air-conditioning unit cools the air near the ceiling. This air becomes denser and sinks, displacing the warmer, less dense air from the floor upwards. This creates a convection current that cools the entire room.

Recap

  • Convection is heat transfer by the mass movement of a fluid.
  • It occurs in liquids and gases, but not in solids.
  • When a fluid is heated, it expands, becomes less dense, and rises.
  • Cooler, denser fluid sinks to replace the risen fluid, creating a convection current.
  • Convection is responsible for weather patterns, ocean currents, and heating systems.

Quick check

  1. In which states of matter does convection occur?1 mark
  2. What happens to the density of a liquid when it is heated?1 mark

3. Thermal Radiation

Thermal radiation is the transfer of energy by electromagnetic (EM) waves, specifically infrared (IR) radiation. Unlike conduction and convection, it does not require a medium (particles) to travel and can pass through a vacuum. All objects with a temperature above absolute zero (-273 °C) emit thermal radiation. The hotter an object is, the more radiation it emits. The surface of an object is crucial: dull, black surfaces are good absorbers and good emitters of radiation. Shiny, light-coloured surfaces are poor absorbers and poor emitters, but are good reflectors of radiation.

Key term

Infrared Radiation: A type of electromagnetic wave, often associated with heat, that can travel through a vacuum at the speed of light.

Examiner insight

Always specify that thermal radiation is a type of electromagnetic wave, and use the term 'infrared radiation' where possible. When comparing surfaces, state clearly whether they are good/poor absorbers AND emitters.

Common pitfall

Confusing emission and reflection. A shiny surface is a poor emitter (it doesn't give out its own heat well) but a good reflector (it bounces back heat that falls on it).

Worked example 14 marks

Two identical teapots, one with a matt black surface and one with a shiny silver surface, are filled with boiling water and left to cool. Which teapot will cool down faster? Explain your answer.

  1. 1
    1. The teapot with the matt black surface will cool down faster.
  2. 2
    1. This is because matt black surfaces are good emitters of thermal radiation.
  3. 3
    1. The shiny silver surface is a poor emitter of thermal radiation.
  4. 4
    1. Therefore, the black teapot loses heat to the surroundings by radiation at a higher rate than the shiny teapot.

Worked example 22 marks

Why are the pipes at the back of a refrigerator often painted black?

  1. 1
    1. The pipes contain hot fluid and need to transfer heat away to the surroundings efficiently.
  2. 2
    1. Black surfaces are good emitters of thermal radiation.
  3. 3
    1. Painting the pipes black maximises the rate of heat loss by radiation, helping the refrigerator to work effectively.

Recap

  • Thermal radiation is heat transfer via infrared (IR) waves.
  • It is the only method of heat transfer that works in a vacuum.
  • Dull, black surfaces are good absorbers and good emitters of radiation.
  • Shiny, light-coloured surfaces are poor absorbers and poor emitters.
  • Shiny surfaces are good reflectors of thermal radiation.

Quick check

  1. Which surface is the better reflector of thermal radiation: white paint or polished aluminium?1 mark
  2. How does most of the thermal energy from the Sun reach Earth?1 mark

4. Applications of Heat Transfer

Understanding conduction, convection, and radiation is essential for designing objects that either keep things hot or keep things cold. This involves minimising unwanted heat transfer. A classic example is the vacuum flask (or thermos), which is designed to reduce all three types of heat transfer. Similarly, insulating a house involves using materials and designs that reduce heat loss in winter and heat gain in summer. Key principles include using insulators (like fibreglass), trapping air, creating vacuums, and using reflective surfaces.

Key term

Vacuum Flask: A container designed to minimise heat transfer by using a vacuum, silvered surfaces, and an insulated stopper to reduce conduction, convection, and radiation.

Examiner insight

For questions on applications, you must identify each relevant heat transfer mechanism (conduction, convection, radiation) and explicitly link it to a specific design feature, explaining how that feature reduces heat transfer.

Common pitfall

Simply stating a feature (e.g., 'it has a vacuum') without explaining *how* it prevents heat transfer (e.g., 'the vacuum prevents heat transfer by conduction and convection because there are almost no particles to carry the energy').

Worked example 16 marks

The diagram shows a vacuum flask used to keep drinks hot. Explain how the labelled parts help to reduce heat transfer. (A) Stopper (B) Silvered surfaces (C) Vacuum

  1. 1
    1. (A) The stopper is made of plastic or cork, which are poor conductors (insulators). It is also hollow to trap air. This reduces heat loss by conduction and convection through the opening.
  2. 2
    1. (B) The inner and outer surfaces are silvered and shiny. Shiny surfaces are poor emitters and good reflectors of thermal (infrared) radiation. This reduces heat loss by radiation from the hot liquid.
  3. 3
    1. (C) The vacuum is the gap between the two walls from which most of the air has been removed. Since there are very few particles in a vacuum, it prevents heat transfer by both conduction and convection between the inner and outer walls.

Recap

  • A vacuum is used to stop heat transfer by conduction and convection.
  • Shiny surfaces are used to reduce heat transfer by radiation.
  • Trapped air is an effective insulator as air is a poor conductor and trapping it prevents convection currents.
  • These principles are applied in everyday items like vacuum flasks, home insulation (loft, cavity walls), and survival blankets.

Quick check

  1. What is the main purpose of the gap between the two glass panes in a double-glazed window?2 marks
  2. Why does a foil 'space blanket' help a marathon runner stay warm after a race?2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define thermal conduction and name a material that is a good thermal conductor.2 marks
  2. A bird fluffs up its feathers to stay warm in cold weather. Explain how this helps to reduce heat loss.3 marks
  3. A black car and a silver car are parked in direct sunlight on a hot day. Explain why the interior of the black car becomes significantly hotter than the interior of the silver car.3 marks
  4. Explain, in terms of the movement and energy of particles, how thermal energy is transferred through a copper rod when one end is placed in a flame.4 marks
  5. The diagram shows a solar thermal panel used for heating water. Explain the purpose of: (a) the blackened copper pipes, (b) the glass sheet on top, and (c) the insulating material at the back.6 marks
  6. Explain fully how a convection current heats the air in a room with a radiator placed under the window.4 marks
  7. A student has four identical metal cubes, with surfaces that are matt black, gloss white, polished silver, and dull grey. Describe an experiment to determine which surface is the best absorber of thermal radiation. State the key variables you would control to make it a fair test.5 marks
  8. A person stands near a large bonfire. (a) Name the main process by which they feel the heat. (b) Explain why conduction and convection are not the main ways heat reaches the person.3 marks
  9. The cooling fins at the back of a refrigerator are designed to transfer thermal energy to the kitchen. Explain why they are (a) painted black and (b) have a large total surface area.4 marks
  10. A modern vacuum flask is designed to keep a cold drink cold for many hours. Explain how the following features reduce the rate at which the drink warms up: (a) the vacuum between the double walls, (b) the silvered surfaces of the walls.4 marks

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