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Energy revision notes

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Introduction

Energy revision notes

Sun gives out vast quantities of energy in form of electromagnetic waves, much of it as light energy.

Sound energy also travels as waves, carrying energy away from the vibrations of a noisy object.

Nuclear power stations generate electricity from the nuclear energy possessed by atoms, by splitting large atoms into 2. The sun also releases nuclear energy by joining small atoms together.

A cyclist has energy stored in his muscles. The chemicals in his muscles supply the energy and change when the muscles are used. This form of stored energy is called chemical energy. A motorbike uses energy stored in the petrol. The petrol has chemical energy because its chemicals change when it is burnt.

A skier at the top of the hill has gravitational potential energy. This is energy that is there ready to be used because of the pull of gravity.

Energy is also stored in stretched elastic or springs.

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Middle

Total energy input = useful energy output + wasted energy output

The efficiency of a device is the fraction of the energy supplied to the device, which is usefully transferred.

% Efficiency = useful energy transferred by device divided by total energy supplied to device X 100%

The useful energy got out is usually less than the total energy put in, due to friction in most machines, and so efficiency is less than 100%. It cannot be greater than 100%!

Is someone is doing work there is movement against an opposing force.

Energy is transferred to the object that person is moving.

The amount of energy transferred depends on the size of the force and the distance the object moves in the direction of the force.

If the person pushes against a larger force or moves it a greater distance she will do more work.

Work done (J) = force (N) X distance moved (M)

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Conclusion

Weight (N) = mass (kg) X gravitational field strength (N/kg)

To lift the object a vertical height, the work which must be done on the object, (energy transferred) is weight X vertical height.

Gravitational potential energy gained (J) = weight (N) X vertical height raised (m)

= Mass (kg) X gravitational field strength (N/kg) X height (m)        

When work is done to change the shape of an object, the energy transferred is stored as elastic potential energy.

The greater the force needed to squash a spring or to stretch elastic or a spring, and the greater the movement (compression or extension), the more work is done, and so more energy is transferred.

Kinetic energy is the energy an object has due to its movement. The greater the mass of the object and the greater its speed, the more kinetic energy it has.

Kinetic energy, mass and speed are related by:

Kinetic energy (J) = ½ mass (kg) X (speed)  (m/s)

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