Mains electricity — AQA GCSE Physics
Test yourself on Mains electricity with AQA GCSE practice questions.
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Mains electricity explained
A three-core cable contains three separately insulated copper wires, live, neutral and earth, inside one flexible outer sheath.
Read the full explanation
The cores are copper because it is a good electrical conductor that bends without breaking; each core is covered in plastic because plastic is an electrical insulator, and the outer sheath adds a second insulating layer and holds the three cores together. Live and neutral carry the working current, and the earth core is a safety connection to the metal case. Not every appliance needs all three: an appliance with a plastic case, such as a hairdryer or a phone charger, is double insulated and uses a two-core cable, because a plastic case cannot become live even if a wire works loose inside.
The insulation covering each wire is colour coded for easy identification: live wire – brown neutral wire – blue earth wire – green and yellow stripes.
The colour is on the plastic insulation, not on the copper, and it is standardised so that anyone wiring a plug can identify each core without testing it. Live is brown, neutral is blue, and earth is green and yellow stripes, the only core with two colours, which is deliberate because it is the safety core and must never be confused. Inside a UK plug the cores always land in the same places: earth goes to the longest top pin, live to the pin on the right as you look into the open plug, and neutral to the pin on the left, with the fuse fitted between the live pin and the live core. Learn the three colours as a set, because questions often give a table of wires to match, one mark per correct row.
The live wire carries the alternating potential difference from the supply.
The live wire is the core connected to the alternating supply, so its electrical potential swings above and below earth potential 50 times a second, and the potential difference between it and earth is about 230 V. That is what makes it the dangerous core. Your body sits at roughly earth potential, so touching the live wire puts about 230 V across you, and charge then flows through you to earth as a shock. The live core stays at that potential all the way to the switch even when the switch is open, because opening a switch only breaks the loop, it does not disconnect the supply. This is why an electrician isolates a circuit at the consumer unit rather than trusting a wall switch before starting work.
The neutral wire completes the circuit.
The appliance is only part of a loop: charge driven out along the live wire has to get back, and the neutral wire is that return path. Without it the circuit is not complete and there is no current, which is why a plug needs at least two cores. Because live and neutral are in series with the appliance, the current in the neutral wire is exactly the same size as the current in the live wire, since the appliance does not use charge up. The neutral wire is held at or very close to earth potential, 0 V, by the supply company, so the potential difference between neutral and earth is nearly zero and touching neutral is far less dangerous than touching live. It is still not safe, because a fault can raise it above 0 V.
The earth wire is a safety wire to stop the appliance becoming live.
The earth wire joins the metal case of an appliance to the ground through a very low resistance path, so the case is held at 0 V. In normal use no current flows in it. Its job shows up in a fault: if the live wire works loose and touches the metal case, the case would otherwise rise to about 230 V and anyone touching it would complete a path to earth. Instead the earth wire gives the charge a much easier route, so a very large current passes through the live wire and the earth wire. That surge melts the fuse in the live line, or trips the circuit breaker, which disconnects the live supply and leaves the case safe to touch. The earth wire and the fuse work as a pair; neither protects the user alone.
The potential difference between the live wire and earth (0 V) is about 230 V. The neutral wire is at, or close to, earth potential (0 V).
Potential is measured from a chosen zero, and for mains wiring that zero is the earth itself. The live wire alternates about that zero, and the potential difference between live and earth is about 230 V. The neutral wire is deliberately connected to earth back at the substation, so it sits at or very near 0 V. Three consequences follow. The potential difference between live and neutral is also about 230 V, and that is what drives the current through the appliance. The potential difference between neutral and earth is close to zero, so no appreciable current passes between them. And a person standing on the ground is at 0 V, so bridging live to earth places the full 230 V across them.
The earth wire is at 0 V, it only carries a current if there is a fault.
In normal operation both ends of the earth wire sit at 0 V, so there is no potential difference along it and no current in it: the working current goes out along live and back along neutral. The earth wire only acts when something goes wrong. If the live core touches the metal case, one end of the earth wire is suddenly at the live potential while the other is still at 0 V, and that potential difference drives a large current through it because the path has very low resistance. The size of that current is the point, because it is far larger than the appliance's normal current and the fuse melts quickly. A residual current device spots the same fault differently, by detecting that the live and neutral currents no longer match.
Students should be able to explain: that a live wire may be dangerous even when a switch in the mains circuit is open the dangers of providing any connection between the live wire and earth.
Opening a switch only breaks the loop; it does not disconnect the live core from the supply. The length of live wire between the consumer unit and the switch is still at about 230 V relative to earth, so touching it is just as dangerous with the appliance off as with it on. The second danger is any accidental connection between live and earth. You are at earth potential whenever you stand on the ground, touch a metal pipe or lean on a damp wall, so completing that path puts the full 230 V across your body and charge passes through you. Water makes it worse, because damp skin has a much lower resistance and the same 230 V then drives a larger current. Isolate at the consumer unit before touching any mains wiring.
Your focus
- Name the three cores of a mains cable and state the job of each one.
- Explain the choice of copper for the cores and plastic for the covering using the words conductor and insulator.
- Justify why a plastic-cased appliance can be supplied safely by a two-core cable.
Show all 24 objectives
- Name the insulation colour of the live, neutral and earth wires without prompting.
- Label a plug diagram with the correct wire at each pin, including the fuse on the live side.
- Explain why the earth wire is given two colours rather than one.
- Explain why touching the live wire gives a shock, using the potentials of the wire and of the person.
- State the potential difference between the live wire and earth and the type of supply producing it.
- Justify isolating a circuit at the consumer unit rather than at a wall switch before repair work.
- State the function of the neutral wire and the potential at which it sits.
- Explain why the current in the neutral wire equals the current in the live wire of the same appliance.
- Distinguish the neutral wire from the earth wire by what each carries during normal operation.
- Describe the sequence of events after a live wire touches an earthed metal case, ending with the fuse melting.
- Explain why an earthed case cannot give a shock even though a fault has occurred.
- State why the earth wire carries no current while the appliance is working normally.
- Build the four-step shock explanation the mark scheme credits for someone touching the live wire: the potential of the live wire, the potential of the person standing on the ground, the large potential difference between them, and the charge that then passes through the body.
- Calculate the potential difference between any two of the three mains wires from their potentials.
- Explain why a connection from live to earth is more dangerous than one from neutral to earth.
- Predict what an ammeter in the earth wire would read for a working appliance and for one whose live core has touched the metal case, and justify each reading.
- Explain what changes during a fault so that a current appears in the earth wire.
- Use I = V ÷ R to argue why a fault current in a low resistance earth path is large enough to melt a fuse.
- Explain why the live wire on the supply side of an open switch is still at 230 V.
- Describe three everyday situations that connect a person to earth and so make live contact dangerous.
- Explain why damp skin increases the current through a person for the same 230 V.
Mains electricity exam tips
Marking Points
- one mark for naming the three cores as live, neutral and earth
- one mark for copper as the conductor because it is a good electrical conductor
- one mark for the plastic covering as an insulator that prevents contact with the conducting core
- one mark for a double insulated appliance needing only two cores because its case is not metal
- one mark for the earth wire as green and yellow
- one mark for the live wire as brown
- one mark for the neutral wire as blue
- one mark for the fuse being fitted in the live side of the circuit
- one mark for the live wire being at about 230 V because it is connected to the alternating supply
- one mark for the person being at earth potential, 0 V
- one mark for the large potential difference between the live wire and the person
- one mark for charge or current passing through the body to earth
- one mark for the neutral wire providing the return path that completes the circuit
- one mark for the current in the neutral wire equalling the current in the live wire
- one mark for the neutral wire sitting at or close to 0 V, earth potential
- one mark for no current passing when the loop is incomplete
- one mark for the earth wire connecting the metal case to earth so that the case stays at 0 V
- one mark for a fault in which the live wire touches the metal case
- one mark for a large current in the earth wire because it is a low resistance path
- one mark for the fuse melting or the circuit breaker tripping, disconnecting the live supply
- one mark for taking earth as the zero of potential, 0 V
- one mark for a potential difference of about 230 V between the live wire and earth
- one mark for the neutral wire sitting at or close to 0 V
- one mark for the potential difference between live and neutral also being about 230 V
- one mark for no current in the earth wire under normal conditions
- one mark for there being no potential difference along the earth wire when there is no fault
- one mark for a fault putting the case, and so the earth wire, at the live potential
- one mark for the resulting current being large because the earth path has very low resistance
- one mark for the live wire remaining connected to the supply when a switch is open
- one mark for the live wire still being at about 230 V relative to earth
- one mark for a person at 0 V completing a path from live to earth
- one mark for the resulting current through the body, increased by the lower resistance of damp skin
Examiner Tips
- 💡Use the exact names live, neutral and earth; 'positive' and 'negative' score nothing for an ac supply.
- 💡If the appliance in the question has a plastic case, expect a two-core answer and say why.
- 💡Link each material to a property: copper conducts, plastic insulates.
- 💡Match-the-wire tables carry one mark per row, so fill in every row even where you are unsure.
- 💡Remember earth is the two-colour wire; that alone rules out wrong options in a multiple-choice list.
- 💡In a plug diagram find the fuse first, because the core it connects to must be the brown live wire.
- 💡Shock explanations need three steps: live at 230 V, body at 0 V, large potential difference driving current through the body.
- 💡Use 'potential' for a single point and 'potential difference' for the gap between two points; examiners mark that distinction.
- 💡Quote the 230 V figure rather than writing 'a high voltage'.
- 💡Separate the two near-zero wires: neutral carries the working current, earth normally carries none.
- 💡If asked why the neutral current equals the live current, say the two are in series with the appliance.
- 💡Use the phrase 'completes the circuit', because a vague 'takes electricity back' may not be credited.
- 💡Earth-wire answers are a chain of four steps, so write them as a numbered sequence and miss no link.
- 💡Always finish with the fuse or breaker disconnecting the supply, because that is where the final mark sits.
- 💡Say 'the case becomes live' rather than 'the appliance breaks' when describing the fault.
- 💡Name both points whenever you quote a potential difference: 'between live and earth', not just '230 V'.
- 💡Sketch the three wires with their potentials written beside them before answering a safety question.
- 💡Remember 0 V is a choice of reference, not an absence of danger; a 0 V wire can still be part of a fault path.
- 💡Answer 'when does the earth wire carry a current?' with 'only during a fault', then name the fault.
- 💡Link low resistance to a large current using I = V ÷ R whenever the command word is explain.
- 💡Do not describe a residual current device unless the question mentions one; the fuse answer is what is expected.
- 💡Explain the open switch case by saying which side of the switch is still connected to the supply.
- 💡For any shock question, state both potentials before you mention current.
- 💡Mention resistance explicitly when water appears in the question, because that is where the extra mark usually sits.
Common Mistakes
- naming the third core as the 'ground wire' or the 'positive wire' rather than the earth wire
- saying the cable has three cores because the appliance needs three separate currents
- claiming every appliance must be earthed, ignoring double insulated plastic-cased devices
- describing the plastic sheath as a conductor that carries stray charge away
- swapping brown and blue, so the live wire is given as blue
- writing 'green' alone for the earth wire instead of green and yellow stripes
- quoting the obsolete red, black and green colours from old wiring
- colouring the copper core rather than the insulation on a labelled diagram
- describing the live wire as positive, which ignores that an ac supply reverses
- saying the shock happens because the live wire has a lot of current in it, rather than because of the potential difference across the body
- assuming a live wire is safe whenever the appliance is switched off
- claiming the person is positively charged rather than at 0 V
- saying the neutral wire carries no current because it is at 0 V
- claiming charge is used up in the appliance so less returns along the neutral wire
- confusing the neutral wire with the earth wire because both sit near 0 V
- describing the neutral core as the 'negative' wire of an ac supply
- saying the earth wire carries a current all the time
- claiming the earth wire absorbs or stores the extra charge rather than conducting it away
- stopping at 'a large current flows' without saying the fuse then breaks the live circuit
- connecting the earth wire to the neutral pin on a labelled plug diagram
- giving the potential difference between neutral and earth as 230 V
- treating 230 V as a property of the live wire alone rather than a difference between two points
- saying the earth is at 230 V because the live wire is connected to the same supply
- quoting 230 V without a unit, or writing it as 230 A
- saying the earth wire carries a small current continuously to keep the case safe
- explaining the fault current from the low resistance alone, never naming the potential difference that drives it
- confusing the earth wire with the neutral wire, which does carry current in normal use
- claiming a fuse melts because of a high potential difference rather than a high current
- saying an open switch makes the whole circuit safe because there is no current anywhere
- arguing that the danger comes from current already in the wire rather than from the potential difference created
- treating the neutral wire as equally dangerous to touch
- saying water conducts so the electricity goes into the water, without mentioning lower resistance and larger current