Power and Efficiency

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Power and Efficiency

Power tells us how fast work is done.

In other words, it tells us how much energy is transferred per second.

Power can be calculated using:

power = energy transferred / time taken


power = work done / time taken

Power is measured in watts, W or joules per second, J/s. One watt is the same as one joule per second.

Time is measured in seconds, s. (Not hours!!)

So where does energy go? When an object travels along, the friction or air resistance trying to slow it down changes the kinetic energy of the object into heat energy. This becomes wasted energy. It is lost to the surroundings and it is no longer useful to us. In fact most of the energy that is wasted is lost as heat energy to the surroundings.

As energy is often wasted when work is being done we need to know how efficient a process is. The more efficient a process is the less the amount of wasted energy. The efficiency of a machine is often given as a percentage. If a machine is 75 % efficient then 25 % of the energy put into it is wasted and only 75 % is changed into useful energy.

For example, a car that is 70 % efficient will turn 70 % of its energy into movement and 30 % into noise and heat.

There are two similar ways to calculate the efficiency of something:

Copyright S-cool

In the diagram below, click the circle to change the efficiency of the motor: