Conduction of Heat (AQA GCSE Physics)
Revision Note
Author
LeanderExpertise
Physics
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 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
When teaching thermal conduction to my GCSE students, I would get them to act it out. They would form a line, representing the particles in a thermally conductive substance, and they would sway a little on the spot representing the motion of particles in a solid. I would then tell the first person in the line that I was transferring energy to them by heating them up, and so they needed to make their motion bigger and faster. This causes them to bump into the next student in the line. The first student has now transferred some of their energy to the second student, and now that student sways bigger and faster. We watch as this increased energy transfers along the row. This is a model for thermal conduction. We then use this model to explore why conduction is most effective in solids (rather than liquids and gases) because the particles are closer together in a solid and therefore, energy can be transferred through the substance more quickly.
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
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
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