Some energy transfers are more effective than others. During most energy transfers there will be a certain amount of loss, or wastage of energy to useless energy stores. One of the main culprits is the thermal energy store where energy can be wasted due to the effects of friction. We try to combat this loss with effective lubrication, but even then we cannot have 100% efficiency.


Using the term "efficiency" we can define the ratio of energy input into a system, compared to the useful output energy or work. If we take a look at a simple example this will become clearer.


Example:


Q. An electric kettle is supplied with 5000 kJ of energy, due to poor insulation 1400 kJ is lost to the environment. What is the efficiency of the kettle as a percentage?


A. 


You may sometimes be asked to state the efficiency as a decimal number, unlikely but if this is the case just don't multiply your answer by a hundred so in the previous case the decimal answer would be 0.72.


Q. A motor with an efficiency of 0.83 is supplied with 974 J of energy. How much energy is usefully transferred by the motor?


A. This is quite a straightforward question, we simply multiply the efficiency (as a decimal number) by the amount of energy supplied, this gives us an answer of:


The most important thing to take from this is the fact that no device is 100% efficient. There will be wasted energy which is usually transferred to useless thermal energy stores. Devices can be made more efficient by design, for example attempts to reduce friction by whatever means between moving parts, and streamlining to reduce the effects of air resistance.



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