A student plugs a laptop charger into the mains supply at home. The cable connecting the charger to the plug contains three wires: a live wire, a neutral wire, and an earth wire. Explain the role of each wire and describe how the earth wire, together with the fuse, protects the user if the live wire accidentally touches the metal casing of the charger.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The live wire carries the alternating current from the mains to the charger.
The neutral wire completes the circuit, returning current to the supply at approximately 0 V.
If the live wire touches the metal casing, the earth wire gives a low‑resistance path to earth, so a large fault current flows through the earth wire.
The large fault current melts or blows the fuse, breaking the circuit and making the casing safe, preventing electrocution.
The neutral wire completes the circuit, returning current to the supply at approximately 0 V.
If the live wire touches the metal casing, the earth wire gives a low‑resistance path to earth, so a large fault current flows through the earth wire.
The large fault current melts or blows the fuse, breaking the circuit and making the casing safe, preventing electrocution.
Examiner tips
- Use the exact terms ‘live’, ‘neutral’, ‘earth’ and ‘fuse’. Show the fault‑current path: live → casing → earth → fuse → mains. Explain that the fuse opens because the current exceeds its rating.
- common_mistakes
- :
- Confusing neutral with earth. Forgetting to mention the fuse’s role in breaking the circuit. Using vague phrases like ‘protects the user’ without describing the fault current and fuse action.
Mark scheme (4 marks)
- The live wire carries the alternating current / high-voltage supply to the appliance
- The neutral wire completes the circuit / returns current back to the supply at approximately 0 V
- If the live wire touches the metal casing, the earth wire provides a low-resistance path to earth, so a large current flows through the earth wire
- The large fault current melts / blows the fuse, breaking the circuit and making the casing safe / preventing electrocution
Key terms in this question
live wire · neutral wire · earth wire · fuse
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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