Mains Electricity (AQA GCSE Physics): Revision Notes
2.2.5 Mains Electricity
Mains:
- Mains electricity is an AC supply. In the United Kingdom, the domestic electricity supply has a frequency of 50 Hz and is about 230 V.
- AC is alternating current, which comes from the mains o Current continuously varies, from positive to negative (charge changes direction)
- DC, direct current, is the movement of charge in one direction only o Cells and batteries supply direct current
Direct and Alternating Currents recap
There are two different ways that electricity can be supplied. These are direct current (DC) and alternating current (AC).
In DC supplies, the current is always flowing in the same direction. Cells and batteries supply dc current.
In AC supplies, an alternating current is supplied by the application of alternating voltages. This means that the current is constantly cycling between positive and negative directions. UK mains supply is an AC supply.
UK mains electricity is provided at 50 Hz. This means 50 cycles of positive and negative currents per second.
Electrical Plugs
Most electrical appliances are supplied electricity by three core cables. Each cable is made from a copper wire and a coloured plastic coating. The colours are always the same for each type of wire so that they can be easily identified:
Cabling:
-
In a plug, there are 3 wires:
- Live Wire
- Brown, at 230 V
- Carries the alternating potential difference from the supply.
- This wire can be dangerous even if the mains circuit is off, as current may still be flowing through it.
- Neutral Wire
- Blue, at 0 V
- Completes the circuit.
- Earth Wire
- Green and yellow stripes, at 0 V
- It only carries a current if there is a fault.
- Acts as a safety wire to prevent the appliance from becoming live.
- It is connected to the earth and the casing.
- If the live wire touches the metal casing of the appliance, the casing will become live. Touching it could result in a serious electric shock, as the current would flow through you to the ground.
Mains Electricity
Definition
- Mains electricity is an alternating current (AC).
- Flow of Electrons: Moves in the forwards and then reverse directions.
- Frequency and Voltage: 50Hz, 230V
Wiring and Components
Wire Composition
- Materials:
- Copper: Good conductor of electricity.
- Plastic Coating: Does not conduct electricity.
Types of Wires
- Live Wire:
- Function: Carries the alternating P.D. from the supply (230V).
- Safety: Connected to a fuse in the plug.
- Neutral Wire:
- Function: Completes the circuit.
- P.D.: 0V
- Earth Wire:
- Function: Safety wire to stop the appliance from becoming live.
- P.D.: 0V
Safety Mechanism
- Metal Case:
- Attached to the Earth wire.
- Earth Wire Connection:
- Connected to the ground with a metal rod.
- If Case Becomes Live:
- A huge current flows to Earth.
- The fuse melts and shuts off the current.
- Prevents electric shock from touching the case.