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Dangers of Mains Electricity (AQA GCSE Physics)
Revision Note
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
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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