Many electrical appliances used in the home transfer electrical energy to other useful forms.
|Appliance||Useful energy||Wasted energy|
|Electric kettle||Energy that heats the water.||Thermal energy store heating the kettle. Infrared radiation transferred to the surroundings.|
|Hairdryer||Thermal energy store heating the air. Kinetic energy of the fan that blows the air.||Sound radiation. Thermal energy store heating the hairdryer. Infrared radiation transferred to the surroundings.|
|Light bulb||Light radiation given out by the hot filament.||Infrared radiation transferred to the surroundings.|
|TV||Light radiation that creates images for the user. Sound radiation that creates audio for the user.||Thermal energy store heating the TV set. Infrared radiation transferred to the surroundings.|
Sankey diagrams show the energy transfers in a system:
Sankey diagrams are most useful when the amount of energy in each of the energy sources is known. The width of the arrow is drawn to scale to show the amount of energy.
The thermal conductivity of a material is a measure of how quickly energy transfers through it by heating.
A building with walls of a high thermal conductivity will cool down faster than one with walls of a low thermal conductivity. The graph shows an example of this difference.
There are two main ways to reduce unwanted energy transfers by heating:
The table shows some ways in which this is achieved in homes and other buildings.
|Cavity wall||A gap between two brick walls contains air, which has a lower thermal conductivity than brick.|
|Cavity wall insulation||The gap between two brick walls is filled with material that has a lower thermal conductivity than air.|
|Loft insulation||A thick layer of material with a low thermal conductivity reduces the rate of heat transfer through the ceiling.|
|Double glazed windows||A gap between two panes of glass contains air or another gas, which has a lower thermal conductivity than glass.|