# P2.4 Mains electricity

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## Alternating and Direct Current

Cells and batteries supply current in one direction, this is known as direct current (D.C)

A.C ( alternating current) is the opposite of this and constantly changes direction.

Mains electricity in the UK is A.C and has the frequency of 50 Hz (hertz) and a peak voltage of 230-240 Volts

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## Plugs

A plug has three wires, these are the earth, live and neutral wires. These different wires have different colours so the user can tell the difference.

The earth wire is commonly green and yellow, it carries and goes to the top pin.

The neutral wire is blue and carries

The live wire is brown and

A plug also contains a cable grip, outer insulation and a fuse

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## Fuses

A fuse is a thin piece of wire. As the current increases so does the flow of electrons, this causes the fuse to heat up and the metallic bond breaks. This is why the fuse turns a bright orange/red colour.

When an appliance develops a fault, the current becomes very high, this high current melts the fuse. This breaks the circuit therefore no current flows. This prevents the plastic which covers the wires from overheating, thus the risk of fire or further damage to the appliance is prevented.

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## Electrical energy, power, charge and p.d.

The unit of charge is coulomb.

Power = energy/time

voltage = energy/charge

current = charge/time

power = voltage x current

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## RCCB

Residual current circuit breakers (RCCBs) protect circuits. They detect a difference in the current between the live and neutrals wires. These work much faster than fuses as no time is wasted waiting for the fuse to break. A circuit breaker can be reset easier than a fuse by flicking a switch on the device making them more convenient.However they are more expensive to buy.

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## Earthing

Earthing prevents electrical overloads, many electrical items are earthed to reduce the danger of an electric shock. Earthing is when the earth wire is attatched to the case. An earthed conductor can never become live.

Double insulation is when an appliance has only plastic casing and no metal is shown. Anything with double insulation does not need an earth wire . Cables that only carry live and neutral wires are known as two-core cables.

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## Oscilloscope traces

A period of an AC supply is the time taken for one complete oscillation. This can be found by calculating the time between one peak and the next.

The frequency of an AC supply is the number of oscillations per second.

Frequency = 1÷ period

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