Efficiency

• Most energy transfers involve some waste energy
• Useful devices are only useful because they can transfer energy from one store to another
• Some of the input energy is usually wasted by being transferred to another energy store - usually a thermal store
• The less energy that is 'wasted' in this energy store, the more efficient the device is said to be
• You can improve the efficiency of energy transfers by insulating objects, lubricating them or making them more streamlined
• You can give efficiency as a decimal or you can multiply your answer by 100 to get a percentage
• Efficiency = useful output energy transfer / total input energy transfer
• Efficiency = useful power output / total power input
• A blender is 70% efficient. It has a total input power of 600W. What is its useful output power?


  efficiency = 70% = 0.7 useful power output = efficiency x total power input = 0.7 x 600 = 420W
• Useful energy output isn't usually equal to total energy input
• For any given example you can talk about the types of energy being input and output, but remember: NO device is 100% efficient and the wasted energy is usually transferred to useless thermal energy stores
• Electric heaters are an exception to usual efficiency. They're usually 100% efficient because all the energy put in the electrostatic energy store is transferred to "useful" thermal energy stores
• Ultimately all energy ends up transferred to thermal energy stores. For example, if you use an electric drill, its energy transfers to lots of different energy stores, but quickly ends up all in the thermal energy store