Efficiency

During energy transformations, energy can neither be created nor destroyed. However some energy can be transformed into unwanted forms of energy, meaning that there is less useful energy after the change than there was before. In most situations, some energy is changed into heat or sound and lost to the surroundings.

Energy loss

Electrical energy can be lost during the generation process, during distribution and also as we use it in the home.

Generators have moving parts which, even when lubricated, rub together to produce friction and heat. Also generators are large and noisy so they produce sound energy as well.

As electricity as distributed through the National Grid, energy is lost from the transmission lines and also when it is transferred from the power stations to the lines and the lines to our homes.

Finally we lose energy in our homes. The appliances we use are not perfect converters of electricity and therefore we lose some of the electrical energy. For example, the next time you use your laptop or computer, touch the underneath or side and you'll find it's very warm. This is because as it processes data, the computer chip gives out heat energy. Therefore some of the electrical power going into your computer is lost as heat and noise (from the fan) to the surroundings.

Efficiency

The more energy that is lost as heat or sound, the less useful energy is transformed and the less efficient the appliance is at the conversion process. So, when we boil a kettle, we're losing energy in the form of sound.

We can calculate the efficiency of an appliance in terms of either the energy conversion or in terms of the useful power input and output.

If we know the energy going into the appliance, we can use the following equation for efficiency:

Efficiency\%  = \frac{{{E_{out}}}}{{{E_{in}}}} \times 100

This equation describes the percentage of total energy output compared to the total input energy.

Of course, since we know that;

power = \frac{{energy}}{{time}}

P = \frac{E}{t}

we can also calculate efficiency using power:

Efficiency\%  = \frac{{{P_{out}}}}{{{P_{in}}}} \times 100

Example

Question

If an electric drill uses 300 J of electrical energy each second but only does 100 J of useful work, calculate the percentage efficiency of the drill.

To calculate the percentage efficiency of the drill you need to take the following steps:

\% Efficiency = (useful\,energy\,out \div energy\,in) \times 100

= (100 \div 300) \times 100

= 33\%

\% Efficiency = 33\%

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