Conduction of Heat (AQA GCSE Physics: Combined Science)

• Thermal conduction is the process where energy is transferred by vibrating particles in a substance
• The vibrating particles transfer energy from their kinetic store to the kinetic store of neighbouring particles

• The direction of energy transfer is always from hot to cold

The higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across the material

• Materials with high thermal conductivity heat up faster than materials with low thermal conductivity

Materials with high and low thermal conductivity

• Examples of substances with high thermal conductivity include:
  • Diamond
  • Aluminium
  • Graphite

• Examples of substances with low thermal conductivity include:
  • Air
  • Steel
  • Bronze

• An insulator is a substance that is a poor thermal conductor
  • Examples include wool, plastic, wood

• Insulators are used to reduce energy transfers, for example, to keep a house warm or build a soundproof room
  • This is why in cold weather, a woollen jumper is worn to retain body heat and keep warm

• The energy transfer through a layer of insulating material depends on:
  • The temperature difference across the material - the greater the temperature difference, the more conduction
  • The thickness of the material - the thicker the material, the less energy will be transferred by conduction
  • The thermal conductivity of the material - the higher the thermal conductivity, the more energy will be transferred by conduction

• Therefore, good insulators which keep the energy transfer through them as low as possible have:
  • A low thermal conductivity
  • Layers that are as thick as possible

Insulation in the Home

• Insulating the loft of a house lowers its rate of cooling, meaning less energy is lost to the outside
• The insulation is often made from fibreglass (or glass fibre)
  • This is a reinforced plastic material composed of woven material with glass fibres laid across and held together
  • The air trapped between the fibres makes it a good insulator

• It has a much lower thermal conductivity than the roof material
• Several layers of insulation make it very thick and therefore decrease the rate of cooling

Less heat is lost from a building with the help of insulation (filled cavity in walls) 

• Another aspect that affects the cooling of buildings is the walls
• Houses in cold countries are fitted with cavity wall insulation which is made from blown mineral fibre filled with gas
  • This lowers the conduction of heat through the walls from the inside to the outside