2.2.5Electricity
Mains Electricity — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 16 of 23
Section 1 of 6Three-core cable
| Wire | Colour | Function | Potential |
|---|---|---|---|
| Live | Brown | Carries the alternating p.d. from the supply to the appliance | About 230 V |
| Neutral | Blue | Completes the circuit by returning current to the supply | About 0 V |
| Earth | Green and yellow stripes | A safety wire that does not normally carry current | 0 V |
Section 2 of 6Parts of a three-pin plug
Pins
- Pins Made of brass, a good conductor that resists corrosion.
- The earth pin is longer so that it connects first, and it opens shutters in the socket.
Casing
- Casing Made of tough plastic or rubber, which are good electrical insulators, so a person touching the plug cannot be shocked.
Fuse
- Fuse Fitted in the live side and rated to match the appliance, for example 3 A, 5 A or 13 A.
Cable grip
- Cable grip Holds the outer insulation firmly so that a pull on the cable does not strain the wire connections.
Section 3 of 6How earthing and the fuse work together
1Fault occurs
- Fault occurs The live wire touches the metal casing of an appliance.
2Earth wire gives a path
- Earth wire gives a path The metal case is connected to the earth wire, so a very large current flows through it instead of through the user.
3Fuse heats and melts
- Fuse heats and melts The large current causes the thin fuse wire to overheat and melt.
4Appliance isolated
- Appliance isolated The circuit is broken and the casing no longer remains live.
Section 4 of 6Choosing the right fuse
1
Find the power rating of the appliance, for example 1.5 kW = 1500 W.
2
Calculate the normal current using I = P ÷ V; with 230 V this is about 6.5 A.
3
Choose the fuse whose rating is just above this value, here 13 A rather than 3 A.
4
A fuse that is too low will blow in normal use; one that is too high will not protect the cable.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Live wire | The wire that carries the alternating p.d. of about 230 V to the appliance. |
| Neutral wire | The wire that completes the circuit and stays near 0 V. |
| Earth wire | A safety wire that gives a low-resistance route to the ground if a fault occurs. |
| Fuse | A thin wire in a cartridge that melts when the current exceeds its rating. |
| Insulator | A material that does not allow current to flow through it. |
Section 6 of 6Exam technique
1
Mixing up the coloursbrown is live, blue is neutral, green and yellow is earth.
2
Saying the earth wire is part of the normal circuit; it only carries current during a fault.
3
Putting the fuse in the neutral wire — it must be in the live wire.
4
Choosing a fuse rating below the normal working current, so that it blows each time the appliance is switched on.
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