A rise in temperature is caused by the transfer of wasteful energy in mechanical processes. The energy is dissipated into the system.
In a mechanical system, energy is dissipated when two surfaces rub together. Work is done against friction which causes heating of the two surfaces – so the internal (thermal) energy store of the surfaces increases and this is then transferred to the internal energy store of the surroundings.
There are many electrical appliances that are used in the home to transfer electrical energy to other useful forms. Every system will waste some energy, and so the useful and wasted energy can always be identified.
|Appliance||Useful energy||Wasted energy|
|Electric kettle||Energy that heats the water.||Internal (thermal) energy heating the kettle. Infrared radiation transferred to the surroundings.|
|Hairdryer||Internal (thermal) energy heating the air. Kinetic energy of the fan that blows the air.||Sound radiation. Internal (thermal) energy heating the hairdryer. Infrared radiation transferred to the surroundings.|
|Lightbulb||Light radiation given out by the hot filament.||Infrared radiation transferred to the surroundings.|
|TV||Light radiation that allows the image to be seen. Sound radiation that allows the audio to be heard.||Internal (thermal) energy heating the TV set. Infrared radiation transferred to the surroundings.|
Devices can be made to reduce the energy that they waste or 'dissipate' to the surroundings. One example is lubrication being used to reduce the friction between moving parts of a machine. This reduces the thermal energy transferred.
For systems that are designed to transfer thermal energy, the wasteful dissipation of thermal energy to the surroundings can also be reduced. This is often done by using thermal insulation, for example, making a kettle from plastic, which is a thermal insulator.