Efficiency

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Efficiency Revision

Efficiency

Efficiency tells us how much energy is transferred to useful energy and how much energy is wasted. By reducing wasted energy, the efficiency of an energy transfer can be increased. This will save energy and money.

Efficiency Calculations

If you know the amount of input energy and useful energy from an energy transfer, you can calculate the efficiency using the following equation:

$\color{aa57ff}{\text{efficiency}}$ $\color{aa57ff}{=}$ $\color{aa57ff}{\dfrac{\text{useful output energy}}{\text{total input energy}}}$

Remember that the total energy is equal to the sum of the useful energy and the wasted energy.

$\color{aa57ff}{\text{total energy}}$ $\color{aa57ff}{=}$ $\color{aa57ff}{\text{useful energy} + }$ $\color{aa57ff}{\text{wasted energy}}$

You can also calculate efficiency using the total power input and the useful power input using the following equation:

$\color{aa57ff}{\text{efficiency}}$ $\color{aa57ff}{=}$ $\color{aa57ff}{\dfrac{\text{useful power output}}{\text{total power input}}}$

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Increasing Efficiency

The efficiency of an energy transfer can be increased by reducing the amount of wasted energy. How we do this depends on the system and the types of energy store involved.

For example, by using oil as a lubricant in a car engine, less heat is produced by friction. Therefore, less energy is wasted as heat and so more energy is converted to useful energy and the engine is more efficient.

We always want to make a process more efficient because this will save energy and money.

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Example: Calculating Efficiency

A lightbulb is switched on for a period of time. During this time, the lightbulb uses $100\text{ J}$ of energy and $20\text{ J}$ of heat energy is produced by the lightbulb.

Calculate the efficiency of the lightbulb.

[3 marks]

\text{useful energy output} = \text{total input energy} - \text{wasted energy output} \\= \textcolor{f21cc2}{100}-\textcolor{00d865}{20} = \bold{80\text{ J}}\\ \text{efficiency}= \dfrac{\text{useful energy output}}{\text{total energy input}}=\dfrac{80\text{ J}}{\textcolor{f21cc2}{100\text{ J}}}= \bold{80\%}\,(\text{or }0.8)

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Efficiency Example Questions

Question 1: An electric motor has a power rating of $6\text{ W}$ but it only produces kinetic energy at a rate of $4\text{ W}$ . What is the efficiency of the motor?

[2 marks]

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