Energy Efficiency

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When we use any device, we have to put energy in than we get out as energy. It is useful to know what of the energy input is useful. We do this by measuring the input energy and the output energy, then working out the .

Energy efficiency = input energy (J) ÷ output energy

This gives us a fraction that is always less than 1. It has no , as it's joules divided by joules.

We often express the efficiency as a by multiplying the fraction by 100.

We can never get efficiency greater than %, because that means that energy is created. That would be against the Rule of of Energy, which states that energy can neither be created nor , but is transformed from one form to another.

No device is 100 % efficient. Energy is lost as heat due to , or sound. The wasted heat is difficult to use.

Car engines have become more efficient than they were, and use less as a result. However they are only about % efficient, meaning that 60 % is . Work has to be done moving a piston up and down. There are rotary internal combustion engines, such as Wankel engines and turbines. The gas is very inefficient at low power, so it is useless in cars. Aeroplane engines in general work at nearly full power during flight; at half power, they hardly produce any (forward force) at all. Therefore the turbine is used a lot in aviation, as it is quite efficient at full power.

Electric motors are much more efficient. Electric cars are not new; they have been around for about 130 years. However the batteries run down quickly. Even with modern batteries, the range is very limited, and charging up takes a long time. Diesel cars can be fuelled up in a few minutes, and have a range of 1000 km