Electricity and magnetism, Year 10: mains electricity

An alternating potential difference

提示The mains supply is of this kind; a torch cell is not.

为什么It drives a current that flows first one way and then the other, over and over. Its short form is a.c. The opposite kind, from a cell or battery, always acts in the same direction and is called direct (d.c.).

50

提示It is half of one hundred.

为什么The mains potential difference goes through this many complete cycles every second, reversing direction twice in each cycle. AQA expects the figure to be known, along with the mains potential difference of about 230 V.

230

提示It is a little under a quarter of a thousand.

为什么This is far higher than the few volts from a cell, which is why mains electricity can drive a dangerous current through a person. AQA expects pupils to know this value and that the supply is alternating.

A direct potential difference (d.c.)

提示Its two-letter short form ends in 'c', like the mains kind, but starts with a different letter.

为什么Cells and batteries have a fixed positive and negative terminal, so the potential difference they provide never changes direction and the current is one-way. Mains electricity is the other kind, where the direction keeps reversing.

Its potential difference goes through 50 complete cycles every second

提示Recall what one hertz means for any repeating event.

为什么Frequency is the number of complete cycles each second. In one cycle the potential difference rises one way, falls back, rises the other way and falls back again. So the direction of the mains reverses 100 times a second, far too quickly to see a filament lamp flicker.

brown; blue

提示One is the colour of soil; the other is the colour of a clear sky.

为什么The colours let anyone wiring a plug tell the three wires apart at a glance. The live wire brings the alternating potential difference from the mains, the neutral wire is the return path that completes the circuit, and the earth wire is there for safety.

The live wire

提示Its name is also the word for something that is 'switched on' or 'alive'.

为什么It is at about 230 V compared with earth, so it is the one that can give a shock. Switches and fuses are placed in this wire so that opening them cuts the appliance off from the high potential difference.

It is a safety wire that stops the appliance's casing becoming live

提示It normally carries no current at all; think about when it would.

为什么The earth wire joins the metal casing to the ground at 0 V. If a fault lets the live wire touch the casing, the current flows away through the earth wire instead of through a person who touches the appliance. It carries a current only when there is a fault.

0

提示It is joined to the ground itself.

为什么The earth wire is held at the potential of the ground. The neutral wire is at, or close to, the same value, while the live wire is at about 230 V relative to it. A potential difference of 230 V between live and earth is what drives a current through anything that connects them.

It is still at about 230 V, so touching it lets a current flow through you to earth

提示An open switch breaks the path via the lamp, but ask what it does to the wire on the supply side.

为什么Opening a switch breaks the circuit through the appliance, but the live wire up to the switch stays connected to the supply. A person standing on the ground is at 0 V, so touching that wire puts 230 V across their body and they complete a circuit to earth. This is why appliances are unplugged before any repair.