BEEZYREVISOR

GCSE Physics Revision

Learn it. Recall it. Revise it.

GCSE Physics revision

Mains electricity

Domestic uses and safety

AQA 4.2.3.2
Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Explain why a live wire remains dangerous with an open switch and why connecting it to earth is hazardous.

MOST 🎯🎯

Apply the scientific explanation of mains electricity to a relevant example.

SOME 🎯🎯🎯

Analyse a new situation involving mains electricity and explain the scientific reasoning.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

Alternating Current (AC) vs Direct Current (DC)

  • Alternating current (AC) is an electric current that repeatedly reverses direction, as supplied by the mains.
  • Direct current (DC) flows in one direction only and is provided by a cell or a battery.
  • A battery is defined as two or more cells connected together.
  • The potential difference of a DC supply varies depending on the cell or battery used, unlike the fixed 230 V of the mains.
  • Some appliances are designed to convert AC from the mains into DC for their internal use.

The Three-Pin Plug & Three-Core Cable

  • Mains electricity is delivered to appliances via a three-pin plug, which contains a three-core cable with three separate wires.
  • The three wires inside a plug are the live wire, the neutral wire, and the earth wire, each connected to a different pin.
  • The live wire is brown and is positioned on the right-hand side of the plug.
  • The neutral wire is blue and is positioned on the left-hand side of the plug.
  • The earth wire has green and yellow stripes and is connected to the top (longest) pin of the plug.

The Role of Each Wire

  • The live wire carries the alternating potential difference of 230 V from the mains supply to the appliance.
  • The neutral wire completes the circuit, allowing current to flow through the appliance and back to the supply.
  • The earth wire is a safety wire that prevents the outer casing of an appliance from becoming live.
  • The live wire is always connected to the fuse inside the plug, which is an important safety feature.

Potential Differences in the Plug

  • The earth wire and the neutral wire are both at a potential of 0 V.
  • The live wire is at approximately 230 V, creating a potential difference of 230 V between the live and neutral wires.
  • This 230 V potential difference between live and neutral is what powers connected appliances.
  • There is also a potential difference of 230 V between the live wire and the earth (the ground).

Insulation & Safety Features of a Plug

  • The outer casing of a plug and the insulation around each wire are made of plastic or rubber, which are good electrical insulators.
  • Each wire is colour-coded with plastic insulation to prevent the wires from touching each other and causing a short circuit.
  • The ends of the wires inside the plug are stripped of their plastic insulation to expose the copper metal, allowing electrical contact with the pins.
  • A cable grip is a plastic component inside the plug that clamps the cable, preventing it from being pulled out and disconnecting the wires.

Electric Shock Hazard

  • A potential difference of 230 V exists between the live wire and the earth (ground), which can drive a dangerous current through a person.
  • If a person bridges the live wire and the earth — for example, by touching a live wire whilst standing on the ground — a complete circuit is formed through their body.
  • This causes an electric current to flow through the person, resulting in a potentially fatal electric shock.
  • It is therefore critical to ensure that live wires are never exposed or accessible, and that appliances are properly earthed.

The Purpose of the Earth Wire as a Safety Device

  • If the live wire inside an appliance becomes loose and touches the metal casing, the casing could become live at 230 V.
  • The earth wire connects the metal casing of an appliance directly to the ground (earth) at 0 V.
  • If the casing becomes live, a large current flows safely through the low-resistance earth wire to the ground, rather than through a person.
  • This large current also blows the fuse, disconnecting the appliance from the live supply and removing the danger.

The Fuse as a Safety Device

  • A fuse is a safety device found inside a three-pin plug, connected in series with the live wire.
  • If the current in the circuit exceeds the fuse's rated value, the fuse wire melts and breaks the circuit.
  • Breaking the circuit disconnects the appliance from the live supply, preventing damage to the appliance and reducing the risk of fire or electric shock.
  • The fuse is always placed on the live side of the circuit so that the appliance is fully isolated from the high-voltage supply when the fuse blows.