Domestic electricity

Revision notes

UK mains electricity is an alternating supply. The live conductor carries the alternating potential difference, neutral completes the circuit and is close to earth potential, and earth is a safety connection that normally carries current only during a fault. A live wire can remain dangerous even when a switch is open. Electrical power can be calculated using P = IV and P = I²R. Energy transferred by an appliance can be calculated using E = Pt or E = QV. The National Grid links power stations to consumers using cables and transformers. High transmission potential differences reduce current for a given power transfer and therefore reduce heating losses in transmission cables.

Mains electricity and conductor roles

Domestic mains electricity is an alternating supply. The live conductor carries the alternating supply potential, the neutral conductor completes the normal circuit and is close to earth potential, and the earth conductor is a protective connection for exposed conductive parts.

The earth conductor is not intended to carry the appliance's normal operating current. Its safety role matters when a fault could make exposed metal live.

Electrical power and energy transfer

Electrical power is the rate of energy transfer. For an appliance, P = IV links power, current and potential difference. A higher-power appliance transfers more energy each second.

Total energy transferred depends on both power and operating time, so two appliances with different powers cannot be compared fairly without also considering how long they operate.

Safe explanations

A strong safety answer describes a complete mechanism: identify the dangerous fault, explain the conducting path that could form and state how the protective arrangement reduces the risk.

Exam technique

Keep power and energy separate. Power is measured in watts and describes a rate; energy is measured in joules when using SI units. In conductor questions, use the names live, neutral and earth accurately rather than referring to all three simply as 'wires'.

Comparing appliance use

Power tells you how quickly an appliance transfers energy, but total energy use also depends on operating time. A lower-power device running for much longer can transfer more energy overall than a higher-power device used briefly.

When a question combines electrical power with energy, identify the time unit before calculating and keep all units consistent. Explanations should distinguish the normal current path from the protective role of the earth connection.