3.1 You may need to be able to use your knowledge and understanding to ...

• The thermal conductivity of a material is a measure of how efficient the material is at transferring thermal energy from a higher temperature region to a lower temperature region.

  • Metals are good conductors of thermal energy - useful for cooking pans.

  • Non-metallic materials like stone, glass or plastics are poor conductors of thermal heat energy, these materials tend to be good insulators.

• Appreciate that whatever 'heat system' we are dealing with, thermal (heat) energy is always lost.

  • You can never get from an energy store, a 100% efficient conversion to useful energy.

  • Therefore, it is of great importance to minimise heat losses and save money in the process!

  • A good example is how to save money in the home or any other building where heating systems of some form are used.

• Compare ways in which energy is transferred in and out of objects by heating and ways in which the rates of these transfers can be varied.

• Evaluate the design of everyday appliances that transfer energy by heating, including economic considerations eg reducing unwanted heat energy transfers - heat losses cost money!

  • Examples you should be familiar with include radiators and heat sinks.

• Evaluate the effectiveness of different types of material used for insulation, including thermal conductivity (eg U-values - a measure of the rate of heat transfer) and economic factors including payback time.

  • You should have studied examples like loft insulation and cavity wall insulation.

• Reminder of particle theory: There is always a net transfer of thermal energy from hot materials to colder ones by ...

  • ... conduction of thermal energy through the bulk of a substance, where higher kinetic energy particles either bump into (liquids or gases) or vibrate against, lower kinetic energy particles, so that thermal energy is transferred.

  • ... convection involves the bulk movement of particles, the hotter higher KE particles in gases or liquids space out more lowering the density of them and so will rise with respect to the surrounding cooler fluid. These convection currents are effectively a 'buoyancy' current because the less dense warmer fluid is trying to float on the cooler more dense fluid.

  • ... infrared - thermal radiation - surface particles of a material at a higher temperature will emit more infrared radiation than a colder material surface. All material surfaces are constantly absorbing and emitting infrared, but there will be net transfer of thermal radiation from a hotter thermal energy store to a cooler one.