Unit 2: 5 - Mains Electricity

Question 0

What is alternating current (ac)?

Answer 0

When the mains supply passes the current in one direction, then reverses it in the other direction and repeats.

Question 1

What is direct current (dc)?

Answer 1

When cells/batteries pass the current round the circuit in one direction.

Question 2

What is the frequency of UK mains electricity?

Answer 2

50Hz

Question 3

What does the frequency mean in ac?

Answer 3

How many times the flow changes direction. E.g. 20Hz means it changes direction 20 times a second.

Question 4

What does the live wire in a plug do?

Answer 4

Gives the mains supply alternating between a positive and a negative potential, with respect to the neutral wire.

Question 5

What ‘voltage’ is the mains supply in the UK?

Answer 5

230V

Question 6

What ‘voltage’ is the neutral wire at?

Answer 6

0V

Question 7

What peak voltages does the live wire alternate between?

Answer 7

+325 and -325 volts

Question 8

How can the frequency of ac be determined from an oscilloscope?

Answer 8

f=1/T

Frequency = 1 / Time

Question 9

What colour is the live wire?

Answer 9

Brown

Question 10

What colour is the neutral wire?

Answer 10

Blue

Question 11

What colour is the earth wire?

Answer 11

Green and yellow

Question 12

The more current carried, the _____ the cable needs to be.

Answer 12

Thicker

Question 13

Why are the pins in a plug made of brass?

Answer 13

Brass is a good electrical conductor, yet also hard and it will not rust or oxidise.

Question 14

What is the difference between a two-core and a three-core cable?

Answer 14

A 2-core does not have an earth wire, whereas a 3-core does.

Question 15

What sorts of appliances must be earthed? And why?

Answer 15

Appliances with metal casings. If there is a fault, the live wire could touch the metal case and make it live, causing a shock to anyone who touches it.

Question 16

A fuse is fitted in ____ with the live wire.

Answer 16

Series

Question 17

How does a fuse work?

Answer 17

If a fault develops in an earthed appliance, a large current flows to earth and melts the fuse, cutting off the supply.

Question 18

How must the rating of a fuse compare to the normal working current of the appliance?

Answer 18

It should be slightly higher (else it would straight when the appliance is switched on).

Question 19

What is a circuit breaker?

Answer 19

A device that can be used instead of a fuse. It is an electromagnetic switch that cuts off if the current is bigger than a certain value.

Question 20

What is a residual current circuit breaker (RCCB)?

Answer 20

A device that cuts off the current in the live wire if it is different to the current in the neutral wire.

Question 21

What are the benefits of an RCCB?

Answer 21

It works faster than a fuse or an ordinary circuit breaker.

Question 22

Why don’t appliances with a plastic casing need to be earthed?

Answer 22

Because plastic is an insulator so will not deliver a shock.

Question 23

How do we work out the charge in Coulombs (using current)?

Answer 23

Q=I*t

Charge in coulombs = Current in amperes * time in seconds

Question 24

What is coulombs a unit for?

Answer 24

Charge

Question 25

Give 5 ways in which to avoid fire or shock with electrical appliances.

Answer 25

• Don’t overload sockets
• Check for damaged and worn cables regularly
• Don’t use electrical appliances in a bathroom or with wet hands
• Don’t touch a bare wire or terminal of more than 30V
• Call in an electrician for mains repair