Conduction

Conduction is how energy passes through a solid. Particles at the hot end vibrate more and pass energy to their neighbours, and in metals free electrons carry it quickly too, so metals are good conductors. Materials that trap air, such as wool, foam and double glazing, are good insulators and slow the energy down.

Key facts

  • Conduction: vibrating particles pass energy to their neighbours; metals conduct best (free electrons).
  • Insulators (trapped air, wool, foam) slow conduction and convection.
  • Liquids and gases are poor conductors: their particles are further apart.
  • Rate of heat loss through a wall or window = U × area × temperature difference.

Worked examples

Example 1

A metal spoon and a wooden spoon stand in a bowl of hot soup. Why does the metal handle get hot much more quickly?

  1. Heat travels along a solid by conduction: hot, fast-vibrating particles pass energy to their neighbours.
  2. Metals are very good conductors (they also have free electrons that carry energy quickly).
  3. Wood is a poor conductor, an insulator, so its handle stays cool.

Answer: metal is a good thermal conductor; wood is an insulator, so heat travels along it very slowly

Example 2

Is steel a good thermal conductor or a good thermal insulator?

  1. Metals conduct heat well; materials that trap air insulate.
  2. Steel is a metal.
  3. So it is a good conductor.

Answer: good conductor

Example 3

How is energy mainly transferred by heating in your hand warming up a cold metal railing?

  1. The energy passes through a solid, particle to particle.
  2. So it is conduction.

Answer: conduction

Example 4

Why does a woolly hat keep your head warm?

  1. Wool traps lots of air between its fibres.
  2. Trapped air is a poor conductor and cannot circulate.
  3. So less heat is lost by conduction and convection.

Answer: it traps air, reducing heat loss by conduction and convection

Example 5

Why do birds fluff up their feathers on a cold day?

  1. Air is a very poor conductor of heat: it is a good insulator.
  2. Fluffing up the feathers traps a thicker layer of still air next to the body.
  3. This slows down heat loss, so the bird stays warm. Duvets and woolly jumpers work the same way.

Answer: fluffed-up feathers trap more air, and air is a good insulator, so the bird loses heat more slowly

Example 6

How does double glazing reduce heat loss through a window?

  1. Double glazing has a gap of gas between two panes.
  2. Trapped gas is a very poor conductor.
  3. So less heat escapes by conduction.

Answer: the trapped gas between the panes is a poor conductor

Example 7

Why does a thick woollen jumper keep you warm on a cold day?

  1. Air is a very poor conductor of heat.
  2. The wool fibres trap the air in tiny pockets, so it cannot move and carry energy away by convection.
  3. So energy leaves your warm body much more slowly.

Answer: wool traps lots of air, which is a poor conductor; trapped air cannot move, so it stops convection too, and less energy is transferred from your body

Example 8

A wall has area 20 m² and U-value 0.30 W/m²°C. It is 21°C inside and 1°C outside. How much energy passes through the wall each second?

  1. Temperature difference: 21 − 1 = 20°C
  2. P = 0.30 × 20 × 20 = 120 W

Answer: 120 W (120 J each second)

Example 9

A double-glazed window has area 2.5 m² and U-value 2.8 W/(m² °C). Inside is 21°C, outside 3°C. Find the rate of heat loss.

  1. Temperature difference across the glass: ΔT = 21 − 3 = 18°C
  2. Heat flows from hot to cold through the window. P = U × A × ΔT = 2.8 × 2.5 × 18 = 126 W

Answer: 126 W

Example 10

A single-glazed window of area 2.0 m² has a U-value of 5.0 W/m²°C. It is replaced by double glazing with a U-value of 2.0 W/m²°C. It is 20°C inside and 5°C outside. How much less power is lost through the window?

  1. Single glazing. 5.0 × 2.0 × 15 = 150 W
  2. Double glazing. 2.0 × 2.0 × 15 = 60 W
  3. Find the difference. 150 − 60 = 90 W

Answer: 90 W