Domestic Uses of Electricity & Power

Energy Transferred by Electrical Devices

Every electrical device transfers energy from one store to another. A kettle transfers energy from the electrical store to the thermal store of water. A motor transfers energy from the electrical store to the kinetic store. Understanding how much energy a device transfers — and how quickly — is essential for calculating running costs and choosing appropriate safety features.

The energy transferred by a device depends on three things: the current flowing through it, the potential difference across it, and how long it runs for. These are linked by the equation:

$$E = I \times V \times t$$

where \(E\) is energy transferred in joules (J), \(I\) is current in amperes (A), \(V\) is potential difference in volts (V), and \(t\) is time in seconds (s). For example, a heater drawing 10 A at 230 V for 60 seconds transfers \(10 \times 230 \times 60 = 138{,}000\) J of energy.

Electrical Power

Power tells you how fast a device transfers energy. It is measured in watts (W), where one watt equals one joule of energy transferred per second. A 2000 W kettle transfers 2000 J every second — much faster than a 60 W lamp, which transfers only 60 J per second.

Power is calculated using:

$$P = \frac{E}{t}$$

where \(P\) is power in watts (W), \(E\) is energy transferred in joules (J), and \(t\) is time in seconds (s).

Two more equations link power to current, potential difference, and resistance:

$$P = I \times V$$

$$P = I^2 \times R$$

The second equation shows that doubling the current quadruples the power dissipated, because current is squared. This is why high-current devices need thicker cables — thin wires carrying large currents would overheat dangerously.

Direct Current and Alternating Current

There are two types of electric current. Direct current (d.c.) is the movement of charge in one direction only. Cells and batteries supply d.c. — the current always flows from the positive terminal, through the circuit, and back to the negative terminal.

Alternating current (a.c.) is different: the direction of charge flow keeps reversing. In the UK, the mains electricity supply is a.c. at a frequency of 50 Hz. That means 50 complete cycles each second, so the current changes direction 100 times a second. The supply voltage is about 230 V.

The Three-Pin Plug and Mains Wiring

Domestic appliances connect to the mains through a three-pin plug containing three colour-coded wires:

The live wire (brown) carries the alternating current to the appliance. Its potential alternates between about +230 V and −230 V relative to the neutral wire. This makes it dangerous even when a device is switched off — the live wire is still connected to the supply.

The neutral wire (blue) completes the circuit and carries current away from the appliance. It is at, or very close to, 0 V (earth potential).

The earth wire (green and yellow stripes) is a safety wire connected to the metal casing of an appliance. Under normal conditions, no current flows through it. If a fault occurs and the live wire touches the metal casing, the earth wire provides a low-resistance path for current to flow safely to earth, preventing electrocution.

Fuses and Circuit Breakers

A fuse is a thin wire inside a ceramic or glass case, placed in series with the live wire. If the current exceeds the fuse's rated value, the wire melts and breaks the circuit, cutting off the current before it can cause a fire or damage the appliance. Common fuse ratings are 3 A (for appliances up to about 700 W) and 13 A (for higher-power devices).

Fuses and switches must always be connected in the live wire, not the neutral wire. This ensures that when the fuse blows or the switch is off, no high voltage reaches the appliance — making it safe to touch or repair.

A circuit breaker does the same job but works faster and can be reset by flipping a switch, rather than needing to be replaced like a fuse.

Power Ratings and Energy Use

Every domestic appliance has a power rating printed on it. A 2000 W kettle uses energy much faster than a 10 W LED bulb. Higher power means greater energy transfer per second, which usually means higher running costs.

Different appliances transfer energy in different ways. A toaster transfers electrical energy to thermal energy to heat bread. A washing machine transfers electrical energy to kinetic energy (the drum spinning) and thermal energy (heating the water). A television transfers electrical energy to light and sound energy.

To calculate the current drawn by an appliance, rearrange \(P = I \times V\) to give \(I = \frac{P}{V}\). For a 2000 W kettle on the 230 V mains: \(I = \frac{2000}{230} \approx 8.7\) A. This tells you a 13 A fuse is needed (the next standard rating above 8.7 A).

Dangers of the Mains Supply

The mains supply at 230 V is potentially lethal. Any connection between the live wire and earth creates a direct path for current to flow through whatever — or whoever — completes that circuit. Touching the live wire while standing on the ground can cause a large current to pass through the body, which can cause burns, cardiac arrest, or death.

Even when a switch is off, the live wire inside the appliance may still be at 230 V. This is why you should always switch off and unplug an appliance before attempting any repairs, and why damaged cables or cracked plugs must be replaced immediately.