Cambridge IGCSE0972

Transfer of thermal energy

Physics 0972 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. 1Heat Transfer by Conduction
  2. 2Heat Transfer by Convection
  3. 3Heat Transfer by Radiation
  4. 4Surfaces and Radiation
  5. 5Controlling Heat Transfer
  6. 6Specific Heat Capacity

1. Heat Transfer by Conduction

Conduction is the transfer of thermal energy through a substance from a hotter region to a colder region, without the substance itself moving. It happens primarily through direct contact. In solids, particles are held in a fixed lattice. When one part is heated, its particles gain kinetic energy and vibrate more vigorously. They then collide with their neighbours, passing on this extra energy. This chain reaction transfers heat through the solid. Metals are excellent conductors because, in addition to particle vibrations, they have free-moving electrons. These electrons also gain kinetic energy in hot areas and can travel quickly through the metal, colliding with ions and transferring energy much more efficiently than vibrations alone. Materials that are poor conductors, like wood, plastic, and glass, are called insulators. Gases are very poor conductors because their particles are far apart, leading to infrequent collisions.

Key term

Conductor: A material that allows thermal energy to pass through it easily, typically due to vibrating particles and free-moving electrons.

Examiner insight

Examiners look for a clear mention of both lattice vibrations and the role of free electrons when explaining conduction in metals. Omitting the free electrons will lose marks.

Common pitfall

Stating that particles themselves travel along the material during conduction. It is the energy that is transferred through collisions, while the particles vibrate around fixed positions.

Worked example 14 marks

A metal spoon and a wooden spoon are both placed in a cup of hot soup. Explain why the handle of the metal spoon becomes hot quickly, but the handle of the wooden spoon does not. [4 marks]

  1. 1
    1. The metal spoon is a good thermal conductor, while the wooden spoon is a thermal insulator.
  2. 2
    1. In the metal spoon, heat is transferred by conduction. The particles (ions) in the metal vibrate more vigorously and pass this energy to neighbouring particles.
  3. 3
    1. Crucially, metals also have free electrons. These electrons gain kinetic energy from the hot soup, move rapidly through the spoon, and transfer energy to particles in the colder handle, making the process very fast.
  4. 4
    1. In the wooden spoon (an insulator), there are no free electrons. Heat is only transferred by the slow process of vibrations being passed from one particle to the next. Therefore, the handle remains cool for much longer.

Recap

  • Conduction is the transfer of thermal energy through vibrations of particles.
  • It does not involve the movement of the material as a whole.
  • Metals are good conductors because they have free electrons that speed up energy transfer.
  • Non-metals, liquids, and gases are generally poor conductors (insulators).
  • Heat always flows from a region of higher temperature to a region of lower temperature.

Quick check

  1. Name one material that is a good thermal conductor and one that is a good thermal insulator.2 marks

2. Heat Transfer by Convection

Convection is the main way thermal energy is transferred through fluids (liquids and gases). It happens when the fluid itself moves and carries heat with it. When a part of a fluid is heated, its particles gain kinetic energy, move faster, and spread further apart. This makes the heated part of the fluid less dense than the surrounding cooler fluid. Due to its lower density, the warmer fluid rises. Cooler, denser fluid then moves in to take its place, gets heated, and also rises. This continuous circulation of fluid, driven by differences in temperature and density, is called a convection current. This process cannot happen in solids because the particles are not free to move from place to place.

Key term

Convection Current: The movement of a fluid caused by differences in density that result from temperature variations.

Common pitfall

Applying the concept of convection to solids. Remember, convection requires particles to be free to move, which is only true for liquids and gases.

Fun fact

Large-scale convection currents in the Earth's atmosphere are what drive global wind patterns and weather systems.

Worked example 13 marks

A domestic hot water tank is heated by an electric immersion heater. Explain why the heater is placed at the bottom of the tank. [3 marks]

  1. 1
    1. The heater is placed at the bottom to heat the water at the base of the tank by conduction.
  2. 2
    1. This heated water becomes less dense and rises. Cooler, denser water from the top of the tank sinks to take its place at the bottom, where it is then heated.
  3. 3
    1. This sets up a convection current that circulates the water, ensuring that all the water in the tank is heated efficiently.

Worked example 23 marks

Explain how a radiator placed on one side of a room heats the entire room. [3 marks]

  1. 1
    1. The radiator heats the air next to it by conduction and radiation.
  2. 2
    1. This heated air expands, becomes less dense, and rises towards the ceiling.
  3. 3
    1. Cooler, denser air from the other side of the room sinks and moves across the floor towards the radiator to replace the rising hot air. This creates a large convection current that circulates air and distributes thermal energy throughout the room.

Recap

  • Convection is the transfer of heat through the bulk movement of fluids (liquids or gases).
  • It occurs because heating a fluid makes it less dense, causing it to rise.
  • Cooler, denser fluid sinks to replace the rising warm fluid, creating a convection current.
  • Convection cannot occur in solids or in a vacuum.
  • Examples include boiling water, room heaters, and sea breezes.

Quick check

  1. Why are freezers placed at the top of a fridge-freezer unit?2 marks

3. Heat Transfer by Radiation

Thermal radiation is the transfer of heat in the form of electromagnetic waves, specifically infrared (IR) waves. Unlike conduction and convection, radiation does not require a medium (particles) to travel through. This is why we can feel the heat from the Sun, even though it has to travel through the vacuum of space. All objects with a temperature above absolute zero (-273 °C) emit thermal radiation. The hotter an object is, the more infrared radiation it emits per second. The rate of heat transfer by radiation also depends on the object's surface area and surface properties (colour and texture).

Key term

Infrared Radiation: Electromagnetic waves emitted by all objects above absolute zero, which transfer thermal energy and can travel through a vacuum.

Examiner insight

Students gain marks for explicitly stating that radiation does not require a medium, especially when comparing it to conduction and convection.

Fun fact

Thermal imaging cameras work by detecting the infrared radiation emitted by objects. This allows firefighters to find people in smoke-filled rooms or police to track suspects at night.

Worked example 13 marks

Explain why you can feel the heat from a bonfire even when you are standing several metres away and the air is still. [3 marks]

  1. 1
    1. The bonfire emits a large amount of thermal energy as infrared radiation.
  2. 2
    1. This radiation travels outwards in all directions as electromagnetic waves.
  3. 3
    1. Unlike conduction or convection, radiation does not need air particles to transfer the heat. It travels through the space between you and the fire and is absorbed by your skin, making you feel warm.

Recap

  • Radiation is heat transfer via infrared electromagnetic waves.
  • It is the only method of heat transfer that can occur in a vacuum.
  • All objects above absolute zero emit thermal radiation.
  • The hotter an object, the more radiation it emits.
  • The Sun's energy reaches Earth through radiation.

Quick check

  1. What is the name of the electromagnetic waves responsible for heat transfer?1 mark

4. Surfaces and Radiation

The surface of an object plays a crucial role in how it interacts with thermal radiation. Different surfaces are better or worse at emitting (giving out) and absorbing (taking in) radiation. The key rules are:

  • Matt black surfaces are the best absorbers and the best emitters of thermal radiation.
  • Shiny, light-coloured or silvery surfaces are the worst absorbers and the worst emitters. They are good reflectors of thermal radiation.

This means a matt black object will heat up quickly when placed in the sun (good absorber) and cool down quickly in a cold room (good emitter). A shiny silver object will stay cooler for longer in the sun (poor absorber/good reflector) and keep its heat for longer when hot (poor emitter). This principle is used in many applications, from solar panels (black surfaces) to emergency space blankets (shiny surfaces).

Key term

Emitter: An object or surface that gives out thermal energy in the form of infrared radiation.

Common pitfall

Confusing the properties of emitters and absorbers. A simple rule to remember is: 'Good at taking it in, good at giving it out'. Black surfaces are good at both.

Worked example 14 marks

Two identical metal teapots are filled with the same volume of boiling water. One teapot has a matt black surface, and the other has a shiny silver surface. Which teapot will keep the tea hot for longer? Explain your answer. [4 marks]

  1. 1
    1. The shiny silver teapot will keep the tea hot for longer.
  2. 2
    1. The main way a hot teapot loses heat to the surroundings is through thermal radiation.
  3. 3
    1. The shiny silver surface is a poor emitter of infrared radiation.
  4. 4
    1. The matt black surface is a good emitter of infrared radiation, so it will lose heat more quickly, and the tea will cool down faster.

Recap

  • Matt black surfaces are good emitters and good absorbers of thermal radiation.
  • Shiny, light-coloured surfaces are poor emitters and poor absorbers.
  • Poor absorbers are good reflectors of thermal radiation.
  • A good absorber is also a good emitter.
  • The choice of surface depends on whether you want to maximise or minimise heat transfer.

Quick check

  1. What colour and texture should the surface of a solar water heater panel be? Explain why.2 marks

5. Controlling Heat Transfer

In many everyday situations, we want to control the transfer of heat, either to keep things hot or to keep them cold. This is achieved by reducing one or more of the three heat transfer mechanisms: conduction, convection, and radiation. A vacuum flask is a perfect example. It has a double-walled glass container with a vacuum between the walls. The vacuum prevents heat transfer by conduction and convection because there are almost no particles to carry the energy. The inner surfaces are silvered and shiny to reduce heat transfer by radiation. The stopper is made of an insulator like cork or plastic to reduce heat loss by conduction and convection through the opening.

Key term

Insulation: The use of materials (insulators) to reduce the rate of heat transfer between an object and its surroundings.

Examiner insight

When explaining insulation, always state which method of heat transfer is being reduced by which feature. For example, 'The fibreglass traps air, which reduces heat loss by convection'.

Worked example 16 marks

Explain how the main features of a vacuum flask help to keep a hot drink hot. [6 marks]

  1. 1
    1. Stopper: The stopper is made of an insulating material like plastic or cork. This reduces heat loss by conduction and also stops air from entering or leaving, which prevents heat transfer by convection.
  2. 2
    1. Double Walls: The flask has an inner and an outer wall. This design allows for a space in between them.
  3. 3
    1. Vacuum: The air is removed from the gap between the two walls to create a vacuum. Since conduction and convection require particles, the vacuum prevents heat transfer by these two methods across the gap.
  4. 4
    1. Silvered Surfaces: The surfaces of the glass walls facing the vacuum are coated with a shiny, silver layer. Shiny surfaces are poor emitters and poor absorbers of thermal radiation. The inner silvered surface reduces heat loss by radiation from the hot liquid, and the outer one reduces heat gain by radiation from the surroundings.

Recap

  • To reduce conduction, use insulating materials and minimise contact.
  • To reduce convection, stop the fluid from moving, for example by trapping air in small pockets (like in wool or fibreglass).
  • To reduce radiation, use shiny, light-coloured surfaces.
  • A vacuum is the best insulator as it stops both conduction and convection.
  • Home insulation uses these principles, e.g., loft insulation (trapped air) and double glazing (trapped gas/vacuum).

Quick check

  1. Why does wearing multiple thin layers of clothing keep you warmer than one thick layer?2 marks

6. Specific Heat Capacity

Different materials require different amounts of energy to heat up. Specific heat capacity (symbol 'c') is a measure of this property. It is defined as the amount of thermal energy needed to raise the temperature of 1 kilogram (kg) of a substance by 1 degree Celsius (°C). The unit for specific heat capacity is joules per kilogram per degree Celsius (J/kg°C). Water has a very high specific heat capacity (4200 J/kg°C), meaning it takes a lot of energy to heat it up, but it also stores a lot of energy and cools down slowly. Metals have low specific heat capacities, so they heat up and cool down quickly. The energy transferred can be calculated using the equation: Energy transferred = mass × specific heat capacity × temperature change.

Q = m × c × ΔT

Key term

Specific Heat Capacity (c): The amount of energy required to raise the temperature of one kilogram of a substance by one degree Celsius.

Common pitfall

Using the total mass in grams instead of kilograms, or using the final temperature instead of the temperature change (ΔT) in the equation.

Fun fact

The high specific heat capacity of water is why coastal areas have milder climates than inland regions. The sea absorbs huge amounts of heat in summer and releases it slowly in winter, moderating the temperature.

Worked example 13 marks

A 0.5 kg block of aluminium is heated, causing its temperature to rise from 20°C to 100°C. Calculate the amount of thermal energy supplied. The specific heat capacity of aluminium is 900 J/kg°C. [3 marks]

  1. 1
    1. Identify the formula: Q = m × c × ΔT
  2. 2
    1. Identify the values: m = 0.5 kg, c = 900 J/kg°C.
  3. 3
    1. Calculate the temperature change (ΔT): ΔT = Final Temperature - Initial Temperature = 100°C - 20°C = 80°C.
  4. 4
    1. Substitute the values into the formula: Q = 0.5 kg × 900 J/kg°C × 80°C
  5. 5
    1. Calculate the result: Q = 36,000 J (or 36 kJ).

Worked example 23 marks

An electric heater supplies 63,000 J of energy to 2 kg of a liquid, raising its temperature by 15°C. Calculate the specific heat capacity of the liquid. [3 marks]

  1. 1
    1. Start with the formula: Q = m × c × ΔT
  2. 2
    1. Rearrange the formula to make 'c' the subject: c = Q / (m × ΔT)
  3. 3
    1. Substitute the known values: Q = 63,000 J, m = 2 kg, ΔT = 15°C.
  4. 4
    1. Calculate the result: c = 63,000 / (2 × 15) = 63,000 / 30
  5. 5
    1. Final answer with units: c = 2100 J/kg°C.

Recap

  • Specific heat capacity (c) is the energy needed to raise 1 kg of a substance by 1°C.
  • The formula for energy transfer is Q = mcΔT.
  • Q is energy in Joules (J), m is mass in kilograms (kg), and ΔT is temperature change in °C.
  • Water has a high specific heat capacity, making it a good coolant and energy store.
  • Metals have low specific heat capacities, so they heat up and cool down quickly.

Quick check

  1. How much energy is needed to raise the temperature of 2 kg of water by 1°C? (c for water = 4200 J/kg°C)1 mark

End-of-chapter exercise

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

  1. Explain in terms of particles how thermal energy is transferred along a metal bar when one end is heated.3 marks
  2. A 2.0 kW electric kettle contains 1.2 kg of water at 20 °C. The specific heat capacity of water is 4200 J/kg°C. Calculate the minimum time it takes for the kettle to heat the water to 100 °C. Assume no energy is lost to the surroundings.5 marks
  3. Explain why birds often fluff up their feathers to keep warm in cold weather. Your answer should refer to the methods of heat transfer.3 marks
  4. A student has two identical cans, one painted matt black and the other shiny silver. She fills both with hot water and records their temperatures over time. Sketch a graph to show the expected results and explain the difference between the two cooling curves.4 marks
  5. Describe an experiment you could perform to investigate which type of surface (matt black or shiny silver) is the better absorber of thermal radiation.5 marks
  6. Explain why placing a lid on a saucepan of boiling water helps it to boil more quickly and stay hot.3 marks
  7. A solar panel used to heat water has a blackened surface and a glass cover. Explain the purpose of both of these features.4 marks
  8. A 400 g block of iron at 90 °C is dropped into a beaker containing a large volume of water at 10 °C. Describe the net flow of energy and what happens to the internal energy of the iron and the water.3 marks
  9. Explain the formation of a sea breeze during a hot day using the principles of heat transfer.4 marks
  10. A house has double-glazed windows. The gap between the two panes of glass is 10 mm wide. Explain how this design reduces heat loss compared to a single pane of glass. Why is the gap not made much wider, for example 100 mm?4 marks

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