Transfer of thermal energy

O Level / IGCSE Physics · Thermal physics. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Thermal energy moves from a hotter place to a colder place in three ways.

Conduction happens mainly in solids. Vibrating particles pass energy to their neighbours. Metals are the best conductors because they also have free electrons that carry energy quickly through the metal. Convection happens only in liquids and gases. The heated part expands, becomes less dense and rises, and cooler, denser fluid sinks to take its place. This makes a convection current.

Radiation is energy carried by infrared waves. It needs no medium, so it can cross a vacuum. This is how energy from the Sun reaches the Earth.

Rules to remember

Common mistake

Students write that heat rises. It is the hot liquid or gas that rises, because it is less dense; thermal energy itself can travel in any direction.

Worked example

The cooling unit of a refrigerator is placed at the top of the food compartment. Why does this cool all the food, including the food at the bottom?

  1. Air touching the cooling unit loses thermal energy and becomes colder.
  2. Cold air contracts, so it is denser than the warmer air around it and it sinks.
  3. Warmer, less dense air from the bottom rises to the top, where it is cooled in turn.
  4. This convection current carries cold air to every part of the compartment.

Answer: Cold, dense air sinks from the top and warmer air rises to replace it, so a convection current cools the whole compartment.

Practice questions

Try each one, then open the answer.

1. Which surface is the best absorber of infrared radiation?

  1. A
    dull black
  2. B
    shiny silver
  3. C
    white gloss
  4. D
    polished metal
Show answer

Answer: A. Dull black surfaces are the best absorbers and emitters of infrared; shiny silver surfaces are the best reflectors and poorest absorbers.

2. Thermal energy from the Sun reaches the Earth through the vacuum of space. Which process transfers it?

  1. A
    conduction
  2. B
    convection
  3. C
    infrared radiation
  4. D
    evaporation
Show answer

Answer: C. Conduction and convection need particles. Radiation is electromagnetic waves (mostly infrared) and needs no medium.

3. Two identical cans of hot water are left to cool. Can A is painted dull black, can B is polished silver. Which cools faster, and why?

  1. A
    A, because black surfaces emit radiation better
  2. B
    B, because silver surfaces emit radiation better
  3. C
    A, because black surfaces conduct better
  4. D
    they cool at the same rate
Show answer

Answer: A. A dull black surface is a good emitter of infrared radiation, so can A loses energy faster.

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