4.2.3.2 Mains Electricity

Question 1

How are electrical appliances connected to the mains?

Answer 1

using 3 core cables

Question 2

What is mains electricity in the UK?

Answer 2

alternating current (AC) - flow of electrons moves in the forward and then reverse directions

Question 3

What frequency and potential difference does alternating current have in the UK?

Answer 3

• frequency of 50Hz
• potential difference of 230V

Question 4

How are electrical appliances connected to the main supply?

Answer 4

• using 3 core cable
• each wire is made of copper which is a good conductor of electricity
• the coatings are made of plastic which don’t conduct electricity

Question 5

What colour is the live wire?

Answer 5

brown

Question 6

What is the function of the live wire?

Answer 6

• the live wire carries the alternating potential difference from the supply (230V)
• potential alternates between -230V and 230V
• the live wire is connected to a fuse in the plug

Question 7

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

Answer 7

about 230 V (0V)

Question 8

When may the live wire be dangerous?

Answer 8

• the live wire may be dangerous even when a switch in the mains current is open
• that’s because the first part of the live wire running to the switch still ha a potential difference of 230V

Question 9

Dangers of the live wire:

Answer 9

• if someone touched the live wire a current would flow through the person (230V potential difference from live wire and earth) and into the earth and the person would be electrocuted
• appliances with metal cases can be dangerous - if the live wire comes loose and touches the metal case, the case can become live
  
  • potential of live wire is 230V
  • potential of person is 0V
  • so large p.d. (230V) between live wire and the man
  • large current passes through person’s body giving theme a fatal electric shock if they touched the live metal case

Question 10

What colour is the neutral wire?

Answer 10

blue

Question 11

What is the function of the neutral wire?

Answer 11

the neutral wire completes the circuit with the live wire

Question 12

What is the potential of the neutral wire?

Answer 12

the neutral wire is at, or close to, earth potential (0V)

Question 13

What colour is the Earth wire?

Answer 13

green and yellow stripes

Question 14

What is the function of the earth wire?

Answer 14

• the earth wire is a safety wire to stop the appliance becoming live
• if live wire touches case, current will go through low resistance Earth wire and cause fuse to get blown out breaking the ciruit
• only carries current if there is a fault in the appliance

Question 15

How is the Earth wire used?

Answer 15

• the earth wire is connected into the ground with a metal rod
• if a metal case does become live a huge current flows to the earth
• the fuse melts and shuts off the current
• this prevents anyone from getting an electric shock from touching a live metal case

Question 16

What is the potential of the earth wire?

Answer 16

the earth wire is at 0V - only carries current if there is a fault

Question 17

How could the live wire touching the casing of an appliance give someone an electric shock when they used the appliance?

Answer 17

• live wire touching case causes case to become live
• potential of live wire is 230 V
• potential of man is 0 V
• so large p.d. between live wire and the man
• large current passes through his body giving him an electric shock