Domestic Electricity, Power and the National Grid
Domestic Electricity
The UK mains supply is alternating current (a.c.) at about 230 V and a frequency of 50 Hz. This means the supply goes through 50 complete cycles every second. Batteries supply direct current (d.c.), which always flows in the same direction.
UK mains supply: alternating current (a.c.), approximately 230 V, frequency 50 Hz. Batteries supply direct current (d.c.).
The Three-Pin Plug
Most appliances connect to the mains via a three-pin plug. The three wires are:
Live wire (brown) — carries the alternating potential difference from the supply. It alternates between about +325 V and −325 V.
Neutral wire (blue) — completes the circuit. It is at, or close to, 0 V (earth potential).
Earth wire (green and yellow) — a safety wire connected to the metal casing of an appliance. It carries current to earth if a fault occurs, preventing electric shock.
The live wire is dangerous even when the switch is off, because it is still at 230 V relative to earth. If you touch it, a large potential difference exists between you and the ground, causing a current to flow through your body. This can cause serious injury or death. A switch is always connected in the live wire so that when it is off, no current can reach the appliance.
The fuse is connected in the live wire. If too much current flows (due to a fault), the fuse wire melts, breaking the circuit and preventing overheating or fire. A circuit breaker does the same job but can be reset instead of replaced.
Electrical Power and Energy Transfer
The power of an appliance tells you how much energy it transfers per second. Two equations link power to current and voltage:
$$P = IV$$
$$P = I^2 R$$
Where \(P\) is power in watts, \(I\) is current in amps, \(V\) is potential difference in volts, and \(R\) is resistance in ohms.
The energy transferred by an appliance depends on its power and how long it is used:
$$E = Pt$$
Where \(E\) is energy in joules, \(P\) is power in watts, and \(t\) is time in seconds. You can also use \(E = QV\), where \(Q\) is charge in coulombs.
A 2 kW kettle is used for 3 minutes. Calculate the energy transferred in joules and in kilowatt-hours.
In joules: \(E = Pt = 2000 \times 180 = 360{,}000\) J
In kWh: \(E = 2 \times \frac{3}{60} = 0.1\) kWh. If electricity costs 30p per kWh, the cost is \(0.1 \times 30 = 3\)p.
The National Grid
The National Grid is a network of cables and transformers that distributes electricity from power stations to homes and businesses across the UK.
Electricity is transmitted at very high voltages (up to 400,000 V) and low currents. Since power lost as heat in the cables equals \(P = I^2 R\), reducing the current dramatically reduces the energy wasted.
Transformers change the voltage of an a.c. supply. A step-up transformer increases the voltage at the power station before transmission. A step-down transformer decreases the voltage near homes to a safe 230 V.
When voltage is increased, current decreases (assuming power stays the same: \(P = IV\)). This is why high-voltage transmission wastes less energy — the lower current means less heating in the cables.