Mains electricity

Mains electricity is the supply that comes out of the sockets in your home. In the UK it is alternating current (ac) with a frequency of 50 Hz and a potential difference of about 230 V. Appliances connect to it through a three-core cable: a brown live wire, a blue neutral wire and a green and yellow earth wire.

Direct and alternating current

Direct current (dc) flows in one direction only. Cells and batteries supply dc. In alternating current (ac), the charge keeps changing direction, because the potential difference of the supply keeps reversing. Mains electricity is ac.

A frequency of 50 Hz means 50 complete cycles every second, so one cycle takes 1 ÷ 50 = 0.02 s, or 20 ms. The graph shows the mains pd (up, in volts) against time (across, in milliseconds) for two cycles:

Why 325 V and not 230 V? The 230 V is the root mean square (rms) value, the dc pd that would transfer energy at the same rate. At A-level, AQA's specification uses V_rms = V₀ ÷ √2 for mains, which puts the peak at about 325 V (230 multiplied by √2). At GCSE you just need about 230 V.

Sort some supplies into ac and dc, then press Mark to check them in the course.

Live, neutral and earth wires

Most appliances connect to the mains through three-core cable. AQA's GCSE specification gives the colour of each wire's insulation and what it does:

The colours make the wires easy to tell apart, which matters because connecting them wrongly can make an appliance dangerous.

Why the live wire is dangerous

Your body is at about 0 V, the same as earth. If you touch the live wire, there is a pd of about 230 V between your hand and the ground, so a current can pass through you. That can cause a fatal electric shock.

AQA asks students to explain that a live wire can be dangerous even when a switch is open. Turning off an appliance's switch stops the current, but if the switch were in the neutral wire the live wire up to the appliance would still be at 230 V. Edexcel's specification makes the same point: switches and fuses go in the live wire. Any connection between the live wire and earth is also dangerous: if that connection is a person, the current flows through them.

The earth wire and fuses

Appliances with a metal case have the case connected to the earth wire. If a loose or damaged live wire comes into contact with the case, a large current flows from live, through the case, to earth instead of through a person. That large current melts the thin wire inside the fuse, which sits in the live wire, so the circuit breaks and the supply is cut off. A circuit breaker does the same job with a switch that trips and can be reset.

Power and energy from the mains

AQA expects you to recall and use these equations:

Example 1: current drawn by an oven

A 3.0 kW oven runs on the 230 V mains. Using P = VI, I = P ÷ V = 3000 ÷ 230 = 13 A. Its resistance, from R = V ÷ I, is 230 ÷ 13.0 = 17.6 Ω.

Example 2: energy used

A 1.5 kW iron is used for 20 minutes. E = Pt = 1500 × (20 × 60) = 1 800 000 J = 1.8 MJ. The amount of energy depends on both the power and how long the appliance is on.

Example 3: charge flow

An appliance on the 230 V supply transfers 4600 J. Using E = QV, Q = E ÷ V = 4600 ÷ 230 = 20 C of charge has flowed through it.

The Work, Energy and Power Calculator checks power and energy answers, and what is potential difference covers E = QV in more depth.

The National Grid

Mains electricity reaches homes through the National Grid, which AQA describes as a system of cables and transformers linking power stations to consumers. Step-up transformers raise the pd from the power station for the transmission cables, and step-down transformers lower it again to a much smaller value for homes.

Why bother? For the same power, P = VI means a higher pd needs a smaller current. Since the heating in the cables is I²R, a smaller current wastes far less energy. Because the loss depends on current squared, a current ten times smaller wastes a hundred times less energy in the same cables, which is what makes the grid an efficient way to transfer energy.

Common mistakes

• Mixing up the colours. Live brown, neutral blue, earth green and yellow.
• Saying the neutral wire is at 230 V. The live wire is at about 230 V; neutral is at or near 0 V.
• Saying the earth wire carries current all the time. It only carries current when there is a fault.
• Thinking a switched-off appliance is always safe. The live wire can still be dangerous.
• Leaving kW and minutes in E = Pt. Use watts and seconds for an answer in joules.
• Saying ac is at 50 V. 50 Hz is the frequency; about 230 V is the pd.

Mains electricity is in both GCSE Physics and Combined Science. Practise it on today's GCSE Physics Higher or GCSE Combined Science Foundation short timed paper, and see plugs and fuses in action in the electricity course.

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