Section 21 - Electricity and circuits - a.c. and d.c., electricity in homes, fuses and earthing

Question 1

Names of the two types of electricity supply?

Answer 1

Alternating current (a.c.) and direct current (d.c.)

Question 2

Difference between a.c. and d.c.? What are each produced by?

Answer 2

a. c. - current constantly changing direction and produced by alternating voltages (positive and negative ends of p.d. keep alternating)
d. c. - current always flowing in same direction and produced by direct voltages (a p.d. that’s only positive or negative, not both)

Question 3

What voltage is the UK a.c. mains supply at? What frequency is the a.c. mains supply at?

Answer 3

230V and 50Hz

Question 4

Name of the three wires in electrical appliances? Colours of each one?

Answer 4

Neutral (blue), live (brown) and earth (green and yellow) wires

Question 5

Purpose of neutral wire? What voltage is it at?

Answer 5

Wire completes circuit - electricity normally flows in through live wire and out through neutral wire. Neutral wire always at 0V

Question 6

Purpose of live wire? What voltage is it at?

Answer 6

Wire carries voltage. It alternates between a high positive and negative voltage of about 230V

Question 7

Purpose of earth wire? What voltage is it at?

Answer 7

Wire is for safety and protecting wiring. It carries current away if something goes wrong and stops appliance casing becoming live. It’s at 0V

Question 8

Which two wires have no p.d. at both 0V?

Answer 8

Neutral and earth wires

Question 9

Why do plug sockets have switches which are connected in the live wire of the circuit?

Answer 9

So the circuit can be broken - appliance becomes isolated and risk of electric shock is reduced

Question 10

How can you get an electric shock by touching the live wire?

Answer 10

Body is at 0V so if you touch live wire, a large p.d. produced across body and a current flows through you, which causes a large electric shock

Question 11

How is there still danger of an electric shock even if a plug socket or light switch is turned off?

Answer 11

A current isn’t flowing, but there’s still a pd. in live wire. If you make contact with live wire, body would provide link between supply and earth, so current would flow through you

Question 12

How can a fire start from a live and neutral wire?

Answer 12

Any connection between live and neutral wire creates a low resistance path to earth so a huge current will flow, creating fire

Question 13

2 ways surges in current can occur and what do they lead to?

Answer 13

Can occur because of changes in a circuit or because of a fault in an electrical appliance. Can lead to circuits and wring in your appliance melting or causing a fire, and faulty appliances can cause deadly electric shocks

Question 14

How do earth wires prevent a fault if one develops in which live wire somehow touches metal case?

Answer 14

Because case is earthed, too great a current flows in through live wire, through case and out down earth wire

Question 15

Why does the fuse rating needed for cables increase with cable thickness?

Answer 15

The larger the current, the thicker the cable needed to carry it to stop cable getting too hot and melting

Question 16

3 ways circuit breakers are safer/better than fuses? 1 con?

Answer 16

• Instead of melting a fuse, a large current may instead trip a circuit breaker
• Turn off quicker than time taken for fuse to melt
• Can be reset, which is much easier than having to replace a fuse
• More expensive than fuses

Question 17

What is double insulating?

Answer 17

If an appliance has a plastic coating and no metal parts showing

Question 18

What are two-core cables?

Answer 18

Cables that only carry the live and neutral wires