Mains electricity (AQA GCSE Physics Combined Science): Revision Notes
Mains electricity
What is mains electricity?
Electrical energy comes into UK homes as mains electricity. This electricity has a voltage of 230V AC (alternating current). Most electrical devices in your home connect to the mains using a three-core cable. This means the cable has three separate wires inside it.
AC (alternating current) means the direction of the current changes many times per second, unlike DC (direct current) which flows in only one direction. This is different from the electricity you get from batteries, which is DC.
The three wires in mains cables
Every three-core cable contains three important wires. Each wire has a different colour and job:
Live wire (brown)
- Carries the electrical supply to your appliance
- Has a potential difference of 230V compared to the neutral and earth wires
- This wire can be dangerous because it carries the high voltage
Neutral wire (blue)
- Completes the electrical circuit with the appliance
- Has a potential difference of 0V compared to the earth wire
- The current flows back through this wire to complete the circuit
Earth wire (yellow and green)
- Does not carry current normally - it's not part of the main circuit
- Acts as a safety feature to protect you from electrocution
- Connected to the metal casing of appliances
All three wires are made from copper (which conducts electricity well) but are covered in coloured plastic insulation. This prevents you from getting a dangerous electric shock when handling the cable.
The live wire is always dangerous and carries 230V even when an appliance is switched off. Never touch exposed wires, and always treat electrical cables with caution.
How earthing keeps you safe
The earth wire is crucial for your safety. Here's what happens if something goes wrong:
The problem: Sometimes the live wire inside an appliance can come loose and touch the metal casing. This makes the whole appliance dangerous - if you touched it, you could get electrocuted.
The solution: The earth wire prevents this danger by providing a safe path for electricity:
Safety Sequence: How Earthing Protects You
- If the live wire touches the metal casing, a large current flows through the earth wire instead of through you
- This large current heats up and melts the fuse in the circuit
- When the fuse melts, it breaks the circuit completely
- With no complete circuit, there's no risk of electric shock or fire
- You stay safe because the dangerous current goes safely to earth
Potential differences in mains electricity
Different pairs of wires have different voltages between them:
| Wire pair | Potential difference |
|---|---|
| Live and earth | 230V |
| Live and neutral | 230V |
| Earth and neutral | 0V |
The earth and neutral wires are both at 0V, which is why there's no voltage difference between them.
Understanding these potential differences helps explain why the live wire is dangerous (230V difference) while touching the neutral or earth wires is much safer (0V with respect to earth).
Safety devices working together
Your safety depends on two devices working together:
The fuse:
- Melts when current gets too high
- Breaks the circuit to prevent overheating and fires
- Isolates the appliance so it can't cause harm
The earth wire:
- Provides a safe path for dangerous currents
- Ensures the fuse will melt quickly if the live wire comes loose
- Prevents you from getting electrocuted
Both devices must work properly to keep you safe. Without the earth wire, a loose live wire could make an appliance dangerous even when switched off.
This is why it's crucial that both the fuse and earth wire are functioning correctly. They work as a team - the earth wire ensures a large current flows when there's a fault, which makes the fuse melt quickly to disconnect the dangerous appliance.
Key Points to Remember:
- Mains electricity in the UK is 230V AC supplied through three-core cables
- Brown live wire carries the supply, blue neutral wire completes the circuit, yellow/green earth wire provides safety
- Earthing protects you by providing a safe path for dangerous currents to flow to earth
- Fuse and earth wire work together - earth wire ensures large current flows when live wire comes loose, making the fuse melt quickly
- The live wire is always dangerous (230V) even when appliances are switched off