Mains Electricity

Question 1

Explain direct and alternating potential difference?

Answer 1

1. Cells and batteries have direct current (dc)
2. Alternating current is an ac supply, frequency of 50 hertz (50 cycles per second) and about 230V
3. Most electrical appliances connected to mains using 3 core cables
4. Insulation covering each wire colour coded for identification
5. Live wire = brown, neutral wire = blue, earth wire = yellow and green stripes

Question 2

What does the live wire carry?

Answer 2

The alternating potential difference from the supply

Question 3

What is the potential difference between the live wire and the earth wire?

Answer 3

+/- 230 volts

Question 4

What materials would you use for a cable casing of a plug and why?

Answer 4

Flexible plastic because it is an good insulator

Question 5

What material would you use to make pins or terminals in a plug and why?

Answer 5

Brass because it’s a good conductor and doesn’t rust (doesn’t react with oxygen)

Question 6

What material do you use for a plug casing and why?

Answer 6

Stiff plastic because it is an good insulator

Question 7

The earth wire is connected to the casing of the appliance at one end. Where is it connected at the other end?

Answer 7

The other end of the earth wire is connected to the ground with a piece of metal

Question 8

Which appliances need earth wires?

Answer 8

Appliances with metal casings eg heaters

Question 9

When will the current flow in the earth wire?

Answer 9

When there is a fault on the appliance

Question 10

What happens when a current flows in the earth wire?

Answer 10

The fuse blows

Question 11

Why is it important that the live wire and not the neutral wire is connected to the fuse?

Answer 11

The neutral wire isolates the appliance

The live wire supplies the current to the appliance

Question 12

Why is it important that the cable grip holds the outer casing of the cable?

Answer 12

So that the wires aren’t exposed

To prevent loose wires being pulled from the plug

Question 13

What is the national grid?

Answer 13

A system of cables and transformers
It’s an efficient way to transfer energy because transmitting power at high voltage means transmitting at lower current
At the power station the electricity is high current and low voltage

Question 14

What are the two types of transformers in the national grid?

Answer 14

Step up transformers - used to increase PD from power station to transmission cables from 25000 to 132000V
Step down transformers - used to decrease PD from transmission cables to the consumer-
Factories use 100kv or 33kv
Big potential means low current required and resistance heating cables is reduced

Question 15

Live wire - What colour - where is it connected in a plug?

Answer 15

Brown

Middle right

Question 16

Neutral wire - colour - where in the plug?

Answer 16

Blue

Bottom left

Question 17

Earth wire - colour - where in the plug?

Answer 17

Green and yellow

Top middle

Question 18

What are the parts of the plug?

Answer 18

Earth wire connected to earth pin
Live wire connected to live pin
Neutral wire connected to neutral pin
Cable grip
Plug casing 
Cable 
Fuse

Question 19

How do you measure the frequency of an a.c. Supply

Answer 19

Frequency =1/time taken for one cycle