# Using Energy

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## Forms of Energy

Energy exists in different forms such as: light, sound, kinetic (movement), nuclear, electrical, gravitational potential, elastic potential and chemical.

The last three are forms of stored energy

Energy can be transferred from one form to another

Any object abover the ground has gravitational potential energy

A falling object transfers gravitational potential energy to kinetic energy. When it falls and gains speed, its gravitational potential decreases and its kinetic energy increases.

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## Conservation of Energy

It is not possible to create or destroy energy. It is only possible to transfer it from one form to another, or from one place to another.

This means that the total amount of energy is always the same. This is call the conservation of energy and it applies to all energy transfers.

For example, when an object falls, gravitational potential energy is transferred to kinetic energy. Similiarly, stretching an elastic band transfers chemical energy to elastic potential energy. In a solar cell, light energy is transferred to electrical energy.

A swinging pendulum transfers energy from gravitational potential energy to kinetic energy and back again as it swings.

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## Useful Energy

A machine is something that transfers energy from one place to another or from one form to another.

The energy we get out of a machine consists of:

• useful energy, which is tranferred to the place we want and in the form we want it
• wasted energy, which is not usually transferred

Both the useful energy and the wasted energy will eventually be transferred to the surroundings, and make them warm up. As the energy spreads out, it becomes more difficult to use further energy transfers.

Energy is often wasted because of friction between the moving parts of a machine. This nergy warms the machine and the surroundings.

Sometimes friction may be useful, for example in the brakes of a bicycle or a car. Some of the kinetic energy of the vehicle is transferred to energy heating the brakes

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## Energy and Efficiency

Energy is measured in joules (J). This unit is used for all forms of energy.

The energy supplied to a machine is often called the input energy. From the conservation of energy we know that:

input energy (energy supplied) = useful energy transferred + energy wasted

The less energy that is wasted by a machine, the more efficient the machine

We can calculate the efficiency of any appliance that transfers energy, using the equation:

Efficiency = useful energy transferred by the appliance                   (x 100%)
total energy supplied to the appliance

No appliance can be 100% efficient, except an electric heater which usefully transfers all of the electrical energy supplied to it by heating the surroundings.

The energy transfer through an appliance can be represented with a Sankey Diagram

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