Energy cannot be created or destroyed but it can be transferred into different forms.

Whenever energy is transferred, only part of it is usefully transferred into the form it is needed in.

The rest of the energy is transferred into a non-useful form, so it is wasted.

Both the wasted energy and the useful energy eventually end up being transferred to the surroundings, which become warmer.

Heat is therefore the most common form of wasted energy.

The energy becomes more and more spread out and increasingly more difficult to use for further useful energy transfers.

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The energy transfers of a machine can be shown in a sankey diagram.

This shows the form and amount of energy used by the light bulb and what it is transferred to.

The amount of energy used by the bulb must equal the total amount of energy produced.

The width of the arrows represents the amount of energy.
 

This light bulb uses 500 J of electrical energy, which it transfers into 75 J of useful light energy and 425 J of wasted heat energy.

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Efficiency Calculations

The more efficient a device is, the more of the energy supplied to it is transferred into useful energy.

The efficiency of a device is calculated using:

                           efficiency =  useful energy transferred by the device
                                                 total energy supplied to the device

So, for the lightbulb:

                        efficiency =  75 
                                            500

                                         = 0,15

Sometimes, efficiency is quoted as a percentage.

This is calculated by multiplying the efficiency by 100

                       percentage efficiency = 0,15 x 100
                                                           = 15%

Therefore 85% of the energy transferred by the light bulb is wasted as heat.

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