Electrical appliances

Many electrical appliances used in the home transfer electrical energy to other useful forms.

ApplianceUseful energyWasted energy
Electric kettleEnergy that heats the water.Thermal energy store heating the kettle. Infrared radiation transferred to the surroundings.
HairdryerThermal 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 bulbLight radiation given out by the hot filament.Infrared radiation transferred to the surroundings.
TVLight 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.

The greater the amount of the supplied electrical energy transferred to useful energy stores, the more efficient the device will be. This can be shown in an energy transfer diagram or Sankey diagram.

Sankey diagrams

Sankey diagram of a child on a slide

Sankey diagrams show the energy transfers in a system:

  • the energy transferred into the system is shown as an arrow
  • this arrow splits into narrower arrows showing other transfers

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.

Thermal conductivity

The thermal conductivity of a material is a measure of how quickly energy transfers through it by heating.

  • a material with a high thermal conductivity transfers energy quickly
  • a material with a low thermal conductivity transfers energy slowly, and may be used as a thermal insulator

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.

A graph shows the temperature inside a building against time. The graph shows that lower thermal conductivity retains heat for the longest time.Buildings with higher thermal conductivity walls will take longer to cool

Reducing unwanted energy transfer

There are two main ways to reduce unwanted energy transfers by heating:

  • use insulator - materials with low thermal conductivity
  • use thicker materials

The table shows some ways in which this is achieved in homes and other buildings.

MethodDetails
Cavity wallA gap between two brick walls contains air, which has a lower thermal conductivity than brick.
Cavity wall insulationThe gap between two brick walls is filled with material that has a lower thermal conductivity than air.
Loft insulationA thick layer of material with a low thermal conductivity reduces the rate of heat transfer through the ceiling.
Double glazed windowsA gap between two panes of glass contains air or another gas, which has a lower thermal conductivity than glass.
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