P2 Mains Electricity

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  • Created by: Phoebe
  • Created on: 04-04-13 10:02

5.1 Alternating current

Cells and batteries supply current that passes round the circuit in one direction. This is called direct current (d.c).

The current from the mains supply passes in one direction, then reverses and passes in the other direction. This is called alternating current (a.c).

The frequency of the UK mains supply is 50 hertz (Hz), which means it changes direction 50 times each second. (remember: 50th anniversary hertz)

The peak voltage of an alternating potential difference is the maximum voltage measured from zero volts.

A mains circuit has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts.

To measure the frequency of an a.c. supply, we measure the time period of the waves then use the formula: frequency = 1/time taken for 1 cycle

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5.2 Cables and plugs

Plastic is used in sockets and plug cases because it is a good electrical insulator.

Mains cables consist of two or three insulated copper wires surrounded by an outer layer of flexible plastic material.

In a three-pin plug or a three-core cable:

  • the live wire is brown (live = dead = brown, CIRCLE OF LIVE WIRE)
  • the neutral wire is blue (blue swiss cheese = neutral)
  • the earth wire is green and yellow (sick is green and yellow, the earth is sick)

The earth wire is connected to the longest pin and is used to earth the metal case of a mains appliance.

Appliances plugged into the same mains circuit are in series with each other.

The fuse in a plug is parallel to the live wire.

DIAGRAM OF A THREE-PIN PLUG:

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5.3 Fuses

A fuse contains a thin wire that heats up, melts and cuts off the current if it is too large.A fuse should have a slightly higher current than the normal current - too low would blow too soon, too high is too big to protect the appliance

A fuse is always fitted in series with the live wire. This cuts the appliance off from the live wire if the fuse blows.

A circuit breaker is an electromagnetic switch that opens (i.e. trips) and cuts off the current if too much current passes through the circuit breaker.

A mains appliance with a plastic case does not need to be earthed because plastic is an insulator and cannot become live.

The earth wire protects the user and the fuse protects the appliance and the wiring of the circuit.

A residual current circuit breaker (RCCB) cuts off the current in the live wire if it is different to the current in the neutral wire.

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5.4 Electrical power and potential difference

The rate at which an electrical appliance transfers electrical energy into other forms is called POWER.

Power = energy transferred per second 

Electrical power supplied (watts) =                        current (amperes) / potential difference (volts)

Correct rating (amperes) for a fuse =                       electrical power (watts) / p.d. (volts)

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5.5 Electrical energy and charge

An electric current is the rate of flow of charge. 

Charge (coulombs) = current (amperes) x time (seconds)

When an electrical charge flows through a resistor, energy transferred to the resistor makes it hot.

Energy transferred (joules) = potential difference (volts) x charge flow (coulombs)

When charge flows round a circuit for a certain time, the electrical energy supplied by the battery is equal to the electrical energy transferred to all the components in the circuit.

When a charge flows in a circuit the components will heat up - this means that most electrical appliances have vents to keep them cool.

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5.6 Electrical issues

Electrical faults are dangerous because they can cause electric shocks and fires.

Never touch a mains appliance (or plug or socket) with wet hands. 

Never touch a bare wire or a terminal at a potential of more than 30V.

Check cables, plugs and sockets for damage regularly.

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