Mains Electricity Flashcards
All 13 cards in this deck
What is direct current (d.c.)?
Movement of charge in one direction only.
What is alternating current (a.c.)?
Current in which the movement of charge keeps changing direction (it flows both ways).
What is the difference between a direct voltage and an alternating voltage?
A direct voltage keeps the same polarity (positive stays positive); an alternating voltage repeatedly swaps between positive and negative.
True or false? Cells and batteries supply alternating current.
False. Cells and batteries supply direct current (d.c.), in one direction only.
What type of supply, frequency and voltage does UK domestic mains electricity have?
a.c., 50 Hz, about 230 V.
What is the difference in function between the live wire and the neutral wire in a mains cable?
The live wire carries the alternating potential difference from the supply; the neutral wire completes the circuit back to the supply.
What are the potential differences between the live, neutral and earth mains wires?
Live–neutral: about 230 V. Live–earth: about 230 V. Neutral–earth: 0 V.
What is the function of the earth wire in an appliance with a metal case?
It protects a person from electric shock if a fault develops: it carries the current from the case safely to earth.
How does a fuse keep an appliance safe?
If the current is too large the fuse wire melts, which breaks the circuit and stops the current, preventing overheating or fire.
Why must switches and fuses be connected in the live wire of a domestic circuit?
So that when the switch is off or the fuse blows, the appliance is cut off from the 230 V live supply and cannot give a shock.
Why is a connection between the live wire and earth dangerous?
There is about 230 V across it, so a very large current flows, which can cause overheating and fire, or an electric shock to anyone touching it.
True or false? A circuit breaker must be replaced every time it switches off the current.
False. A circuit breaker can be reset and used again; it is a fuse that must be replaced.
How should the rating of a fuse compare with the normal working current of the appliance?
Slightly higher than the normal working current.
e.g. an appliance taking 2 A is given a 3 A fuse.