Mains Electricity, Power and Energy
UK mains electricity is an alternating current supply at about 230 V with a frequency of 50 Hz, produced by generators in which the current constantly changes direction, unlike the direct current supplied by a cell. A three-core cable carries this supply through a live wire, a neutral wire and an earth wire, the last providing a low-resistance safety path to earth if the live wire touches a metal case.
Before you start
Make sure you're comfortable with these topics first:
Method
- State the mains values precisely when asked: alternating current, about 230 V, 50 Hz, and contrast it with direct current from a cell or battery, where the current flows in one direction only.
- Learn the three wires as colour, potential and job: live (brown, alternates about 230 V, dangerous even when a device is switched off because it is still at a high potential relative to earth), neutral (blue, near 0 V, completes the circuit), earth (green and yellow, 0 V, carries current only when there is a fault).
- Explain the earth wire and the fuse together, because the mark scheme wants the sequence: if the live wire touches the metal case, a large current flows through the earth wire to earth, which melts the fuse or trips the circuit breaker, disconnecting the live supply and making the case safe to touch.
- Select the right power equation: use power = potential difference x current when both are given, and power = current squared x resistance when the resistance and current are known, which is the one that explains transmission losses.
- For energy in a circuit, use energy transferred = power x time with time in seconds, or energy transferred = charge flow x potential difference, with charge flow = current x time.
- Explain the national grid in terms of the loss equation: for a given power to be transmitted, a higher potential difference means a smaller current, and because the power wasted in the cables is proportional to the current squared, halving the current quarters the energy wasted by heating.
Worked example
An electric shower is rated at 8.5 kW and runs from the 230 V mains. Calculate the current it draws, and state whether a 30 A or a 45 A circuit is needed.
- Convert the power to watts: 8.5 kW = 8500 W.
- Write the equation: power = potential difference x current.
- Rearrange for current: current = power / potential difference.
- Substitute: 8500 / 230 = 36.96 A.
- Round sensibly: about 37 A.
- Conclude: 37 A exceeds 30 A, so the shower must be on a 45 A circuit; using a 30 A circuit would overload the cable and repeatedly trip the breaker or overheat the wiring.
Practice questions
Try each question, then tap to reveal the answer.
Q1State the potential difference and frequency of the UK mains supply.Show answer
Answer: About 230 V at 50 Hz.
Q2What is the difference between alternating and direct current?Show answer
Answer: In alternating current the direction of the current constantly changes; in direct current the charge flows in one direction only.
Q3Give the colour and function of the earth wire.Show answer
Answer: Green and yellow stripes. It carries no current in normal use, and provides a low-resistance path to earth if a fault makes the metal case live.
Q4Calculate the current drawn by a 1150 W kettle on a 230 V supply.Show answer
Answer: 1150 / 230 = 5 A.
Q5Give the equation linking energy transferred, charge flow and potential difference.Show answer
Answer: Energy transferred = charge flow x potential difference.
Q6Why is the live wire dangerous even when a device is switched off?Show answer
Answer: It remains at a high potential relative to earth, so touching it provides a path to earth and a large current can flow through the body.
Q7Why does the national grid transmit electricity at a very high potential difference?Show answer
Answer: For a given power, a higher potential difference means a lower current, and since the power wasted in heating the cables depends on the current squared, a lower current wastes far less energy.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain how the earth wire and the fuse together protect a user from an electric shock if the live wire comes loose and touches the metal case of an appliance.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 4 available
A power station transmits 100 MW. Explain, using the appropriate equation, why transmitting at 400 000 V wastes far less energy than transmitting at 25 000 V.
Show mark scheme
Tick each line you got. Your score builds from the marks on the scheme.
Nothing ticked yet - 4 available
See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the mains electricity, power and energy worksheet pack - 15 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 12 of GCSE Physics Workbook, the whole course as one free printable PDF.
Next topics
Not quite what you needed?
Tell us what is missing on mains electricity, power and energy, or which topic to write up next. Every request is read, and we reply to every one.
Build a full practice pack.
This topic is one of hundreds in the library - pick the ones a student needs and generate a printable PDF in minutes.