Mains Electricity Flashcards
All 17 cards in this deck
In direct current (d.c.), how does charge move?
In one direction only.
In alternating current (a.c.), how does charge move?
The movement of charge keeps changing direction (it flows both ways).
What type of current do cells and batteries supply?
Direct current (d.c.).
What is the frequency of the UK domestic mains supply?
50 Hz.
What is the voltage of the UK domestic mains supply, and is it a.c. or d.c.?
About 230 V, and it is a.c.
What is the function of the live wire in a mains circuit?
It carries the alternating potential difference from the supply to the appliance.
What is the function of the neutral wire in a mains circuit?
It completes the circuit, carrying current back to the supply.
What are the potential differences between the live, neutral and earth mains wires?
Live–neutral about 230 V; neutral–earth about 0 V; live–earth about 230 V.
What is the function of the earth wire in an appliance with a metal case?
It connects the metal case to earth, keeping the case at the same potential as earth so a person touching it cannot get a shock.
What are the steps by which an earth wire and fuse make an appliance safe when the live wire touches the metal case?
e.g. a metal kettle with a 13 A fuse
- The metal case is a conductor connected to earth, so the resistance between live and earth is very low.
- A very large current flows to earth through the low-resistance earth wire.
- The large current melts the thin fuse wire, which breaks and disconnects the mains supply to the kettle.
How does a fuse work?
It contains a thin wire whose temperature depends on the current in it; a very large current raises it beyond its melting point so the wire breaks and disconnects the supply.
Why must switches and fuses be connected in the live wire?
So that when they open, the appliance is disconnected from the mains voltage and no part of it can be at mains voltage.
How do you choose a suitable fuse rating for an appliance?
e.g. a radio with a normal current of 2.3 A, choosing from 2 A, 5 A, 10 A, 13 A
Pick the fuse just above the expected current, not below it and not much higher.
e.g. the 5 A fuse.
Why is it dangerous to provide a connection between the live wire and earth?
The live wire is at about 230 V while earth is at 0 V, so a large current flows through the connection — causing electric shock, burns or fire.
True or false? Touching only the neutral wire of an appliance connected to the mains is as dangerous as touching only the live wire.
False. The neutral wire is at about 0 V, the same as earth, so there is almost no potential difference to drive a current through you.
What is the difference between a direct voltage and an alternating voltage?
A direct voltage keeps the same polarity (always positive or always negative); an alternating voltage keeps reversing between positive and negative.
What does a circuit breaker do when the current in a circuit becomes too large?
It trips, switching itself off automatically to break the circuit and cut off the supply.