Domestic electricity and safetyIB MYP Physics: Revision notes

A cell or battery supplies direct current (d.c.): the current always flows the same way round the circuit. A cell gives only a small voltage, for example 1.5 V.

The mains supply is alternating current (a.c.): the current repeatedly reverses direction. In the UAE, the UK and many other countries the mains is about 230 V and has a frequency of 50 Hz, which means 50 complete cycles every second.

Mains a.c. is used in homes because it is easy to transmit over long distances by the national grid. d.c. from cells is used in portable devices.

A three-pin plug connects an appliance to the mains using three wires, each with its own colour:

• Live (brown): carries the alternating potential difference from the supply, about 230 V. It is the dangerous wire.
• Neutral (blue): completes the circuit and stays at about 0 V.
• Earth (green and yellow stripes): a safety wire that does not carry current in normal use.

The fuse is fitted in the live wire. The wires are made of copper because it is a good conductor, and the plug casing and cable covering are made of plastic because it is an insulator.

A fuse contains a thin wire. If the current gets too large, the wire heats up and melts, breaking the circuit. The fuse is placed in the live wire so that, when it blows, the appliance is cut off from the 230 V supply.

Fuses come in standard ratings such as 3 A, 5 A and 13 A. Choose a fuse rating just above the normal current of the appliance. A fuse with a rating below the normal current would blow straight away. A fuse with a much higher rating would not protect the cable.

A circuit breaker is an automatic switch. When the current is too large, an electromagnet pulls the switch open. Unlike a fuse, it can be reset and it switches off faster.

Many appliances with a metal casing, such as kettles and toasters, are earthed. The earth wire connects the casing to the ground. If a fault lets the live wire touch the casing, a large current flows through the earth wire (a low-resistance path) to earth. This blows the fuse, so the casing cannot stay live and shock the user.

Appliances with a plastic casing, such as hairdryers and drills, are often double insulated. The live parts are covered by two layers of insulating material, so a fault in one layer cannot make the user touch a live part. These appliances have only two wires and no earth wire. They carry the double-square symbol.

A current through the body can cause a painful or fatal shock, and the 230 V mains is dangerous. Common hazards and how to prevent them:

• Damaged or frayed cable: exposed live wire can be touched. Replace the cable.
• Water near electricity: water conducts and lowers the resistance of your body. Keep appliances and wet hands apart.
• Overloaded sockets: too many appliances draw a large current and overheat the wiring. Do not plug adaptors into adaptors.
• Wrong fuse: too high a rating will not protect the circuit.
• Pushing metal objects into sockets: use socket covers around young children.

Switch off and unplug an appliance before repairing it.

Criterion D questions ask you to weigh up the benefits and risks of electrical technology. Mains electricity is convenient, powerful and cheap per unit, but it can be lethal and is often made by burning fossil fuels. Cells give a low, safe voltage and work anywhere, but supply little power and old batteries cause waste. A good answer gives points on both sides and then reaches a justified conclusion.