AQA GCSE Physics

Revision Notes

2.2.6 Dangers of Mains Electricity

Potential Difference of Live, Neutral and Earth Wires

  • The live wire carries most of the power to the circuit and, as a result, it is the most dangerous wire
    • It has a voltage of around 230 V
  • The neutral wire is much safer than the live wire, although it can deliver a small shock
    • It has a voltage close to 0 V
  • The earth wire only carries a current to the ground if there is a fault in the appliance
    • It has a voltage of 0 V

Dangers of Mains Electricity

  • Normally, the earth wire does not carry any electricity – it is there for safety only
  • As a result, in most circumstances, its potential difference is 0 volts
  • Many electrical appliances have metal cases that pose a potential safety hazard:
    • If a live wire (inside the appliance) came into contact with the case, the case would become electrified
    • Therefore if anyone touched it, they would risk electrocution
  • The earth wire is an additional safety wire that can reduce this risk
  • If this happens:
    • The earth wire provides a low resistance path to the Earth
    • This causes a surge of current in the earth wire and hence also in the live wire
    • The high current through the fuse causes it to melt and break
    • This cuts off the supply of electricity to the appliance, making it safe to touch

Electrical danger, IGCSE & GCSE Physics revision notes

Signs, like the above, warn of the risk of electrocution

  • Because of the large potential difference between the live (230 V) and the earth (0 V), if the two are connected together, a very large current can be created
  • If a person provides the connection between live and earth then a large current can pass through them, providing a potentially lethal shock
  • Electricians will always switch off the mains electricity supply to the whole house, or section of a house when they are working with electrical appliances
    • This is because they will come into contact with live wires when they are working
    • The potential difference of the live wire is 230 V and the potential of the electrician is 0 V
    • Therefore, there is a large potential difference between the live wire and the electrician, so, a current would pass through the electrician’s body to reach the earth
    • Even if a device is switched off but the mains supply is on, the live wire can still cause an electric shock

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Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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