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# Energy loss and efficiency

Energy can be transferred in and out of a closed system. In most real systems, not all of the energy input into the system gets output into useful energy/work as some energy is lost to the surroundings as heat.

Some of the energy obtained by burning fuel in a car engine is lost as heat to the surroundings instead of being converted into kinetic energy to drive the car forward.

This means that most real systems are not $100\%$ efficient. The efficiency of a system is the percentage of input energy/work (or power) that gets transformed into useful output energy/work (or power). This is given by: \begin{align*} \text{Efficiency}&=\frac{E_{\text{out}}}{E_{\text{in}}}\times 100\%\\ &=\frac{W_{\text{out}}}{W_{\text{in}}}\times 100\%\\ &=\frac{P_{\text{out}}}{P_{\text{in}}}\times 100\% \end{align*} $W_{\text{in}}$, $E_{\text{in}}$, $P_{\text{in}}$ and $W_{\text{out}}$, $E_{\text{out}}$, $P_{\text{out}}$ are the input and output work, energy and power respectively.