Mains Electricity (Edexcel GCSE Physics)

Mains AC

• Mains electricity is the electricity generated by power stations and transported around the country through the National Grid

  • Everyone connects to the mains when plugging in an appliance such as a phone charger or kettle

• Mains electricity is an alternating current (a.c.) supply

• In the UK, the domestic electricity supply has a frequency of 50 Hz and a potential difference of about 230 V

  • A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second

• Mains electricity, being an alternating current, does not have positive and negative sides to the power source

  • The equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit

The live and neutral wires deliver the electricity to the device. The Earth wire is purely for safety

Live & Neutral

• All electrical appliances are connected to the mains using at least a live wire and a neutral wire

• In the UK, most electrical appliances are connected to the mains using a three-core cable consisting of

  • A live wire

  • A neutral wire

  • An Earth wire

• The insulation covering each wire is colour coded for easy identification:

  • Live wire – brown

  • Neutral wire – blue

  • Earth wire – green and yellow stripes

A diagram showing the three wires going to a mains powered appliance: Live, Neutral and Earth

The Live Wire

• The live wire:

  Carries the alternating potential difference from the supply to a circuit

• It is the most dangerous of the three wires

• If it touches the appliance without the Earth wire, it can cause electrocution

The Neutral Wire

• The neutral wire:

  Forms the opposite end of the circuit to the live wire to complete the circuit

• Because of its lower voltage, it is much less dangerous than the live wire

The Earth Wire

• Many electrical appliances have metal cases

• This poses a potential safety hazard:

  • If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted

• The earth wire is an additional safety wire that can reduce this risk

• If this happens:

  • The earth wire provides a low resistance path to the earth

  • It causes a surge of current in the earth wire and hence also in the live wire

  • The high current through the fuse causes it to melt and break

  • This cuts off the supply of electricity to the appliance, making it safe

• Every country has a slightly different configuration, with many using two-pin plugs and plug sockets such as in the USA and mainland Europe

  • The earth wire is still present in the two-pin plugs, just more hidden

• The earth wire:

  Acts as a safety wire to stop the appliance from becoming live

Fuses & Circuit Breakers

• Fuses and circuit breakers are safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

The circuit symbol for a fuse (not to be confused with a resistor)

• Fuses usually consist of a glass cylinder containing a thin metal wire

• If the current in the wire becomes too large:

  • The wire heats up and melts

  • This causes the wire to break, breaking the circuit and stopping the current

• A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value

• This has a major advantage over a fuse because:

  • It doesn't melt and break, hence it can be reset and used again

  • It works much faster

• For these reasons, circuit breakers are used in mains electricity in homes

  • Sometimes they are misleadingly named "Fuse boxes"