Conduction of Heat (AQA GCSE Physics): Revision Note

Conduction of Heat

• 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 thermal conductivity of a material tells you how well it conducts energy:

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

• The conductivity of materials can be compared by measuring the time taken to transfer energy from one end to another

  • The material that takes the least time to heat up has the highest thermal conductivity

  • The material that takes the most time to heat up has the lowest thermal conductivity

• This is because a shorter time to change temperature by the same amount means a higher rate of energy transfer, so a higher thermal conductivity

Materials with high and low thermal conductivity

• Examples of substances with high thermal conductivity include:

  • Metals such as copper, iron and steel

  • Diamond

  • Aluminium

  • Graphite

• Metals are good thermal conductors, and copper is a better conductor than most other metals

• Examples of substances with low thermal conductivity include:

  • Air

  • Glass

  • Wood

Factors Affecting Conduction

• 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

  • A better insulator makes the water cool more slowly and stay hotter for longer

  • Describe the result for the material you are asked about, not the converse for the other material

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 mat of thin glass fibres (glass wool)

  • 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 (or foam) that traps air in the cavity between the walls

  • This lowers the conduction of heat through the walls from the inside to the outside

  • The rate of cooling of a building is lower if its walls are thicker

  • The rate of cooling of a building is lower if its walls have a lower thermal conductivity