Mains electricity (AQA GCSE Physics): Revision Notes
Mains electricity
What is mains electricity?
Mains electricity is the electrical energy that comes into our homes in the UK. It arrives at 230 volts AC (alternating current). This is much higher than the voltage in small batteries, which is why mains electricity can be very dangerous.
Mains electricity is approximately 40 times more powerful than a typical car battery (12V). This massive voltage difference is what makes household electricity potentially lethal and requires extreme caution.
Most electrical devices in your home connect to mains electricity using a three-core cable. This cable has three different wires inside it, each with an important job. Understanding these three wires is essential for electrical safety and explains how modern electrical systems protect us from harm.
The three wires in mains electricity
Each wire in a mains cable has a specific colour, voltage, and function. Learning to identify these wires and understand their roles is crucial for electrical safety.
Live wire (brown)
- Color: Brown
- Voltage: 230V
- Job: Carries electrical energy to the appliance
- This is the dangerous wire because it has high voltage
- It has a 230V potential difference compared to neutral and earth wires
The live wire remains at 230V even when an appliance is switched off. This is why electrical repairs should always be done with the power turned off at the mains, not just at the appliance switch.
Neutral wire (blue)
- Color: Blue
- Voltage: 0V
- Job: Completes the electrical circuit
- Allows current to flow back from the appliance
- Much safer than the live wire because it's at 0V
Earth wire (yellow and green stripes)
- Color: Yellow and green stripes
- Voltage: 0V
- Job: Safety protection only
- Does not carry current during normal use
- Works with the fuse to keep you safe
The earth wire is like a safety net - it's there to catch dangerous electricity if something goes wrong. During normal operation, no current flows through it, but it's ready to spring into action if needed.
How earthing keeps you safe
Earthing is a vital safety feature that protects you from electric shocks. The earth wire creates a low-resistance pathway that electricity will prefer to take instead of going through your body.
Worked Example: How Earth Protection Works
Step 1: Normal operation
- Current flows: Live wire → Appliance → Neutral wire
- Earth wire carries no current
Step 2: Fault occurs
- Live wire touches metal case of appliance
- Metal case becomes "live" at 230V
Step 3: Earth protection activates
- Large current flows: Live wire → Earth wire → Ground
- This current is much larger than normal operating current
Step 4: Fuse responds
- Fuse detects excessive current and melts
- Circuit breaks, stopping all current flow
- Appliance becomes safe within milliseconds
Key point: Even when a switch is off, live wires can still be dangerous. The earth wire protects you in these situations by providing an alternative path for dangerous currents.
Potential differences in mains electricity
The potential difference (voltage) between different wires varies depending on which wires you measure between:
- Live and earth: 230V
- Live and neutral: 230V
- Earth and neutral: 0V
This is why the live wire is dangerous - it has a big voltage difference compared to both earth and neutral. The earth and neutral wires are both at the same potential (0V), which is why they're much safer to be near.
Safety devices working together
Modern electrical systems use multiple safety devices that work together to protect you. No single device provides complete protection - they're designed to back each other up.
Two main safety devices protect you:
Fuses:
- Melt when current gets too high
- Break the circuit to stop current flow
- Prevent appliances from overheating
Earth wires:
- Provide a safe path for current if something goes wrong
- Work with fuses to cut off electricity quickly
- Stop metal cases becoming "live" and dangerous
Think of fuses and earth wires as a team - the earth wire detects the problem by providing a path for fault current, and the fuse responds by cutting off the power supply. Neither device alone would provide complete protection.
Key Points to Remember:
- Mains electricity in UK homes is 230V AC - much more dangerous than batteries
- Three wires: Brown (live) carries 230V, Blue (neutral) at 0V, Yellow/green (earth) for safety
- Live wire is always dangerous even when switches are off
- Earthing protects you by giving dangerous currents a safe path to ground
- Fuses and earth wires work together to keep you safe from electric shocks
- Never touch exposed wires - mains electricity can kill you