Domestic uses and safety (2.3) (R)

Question 1

What type of supply is mains electricity

Answer 1

ac

Question 2

What is the frequency of the domestic electricity supply in the UK?

Answer 2

50Hz

Question 3

What is the voltage of the domestic electricity supply in the UK?

Answer 3

230V

Question 4

What is the difference between direct and alternating potential difference?

Answer 4

Alternating p.d. constantly changes direction. Direct p.d. always goes in same direction.

Question 5

What does an ac supply look like on oscilloscope/voltage-time graph?

Answer 5

Question 6

What does an dc supply look like on oscilloscope/voltage-time graph?

Answer 6

Question 7

What is often used to connect most electrical appliances to the mains?

Answer 7

three-core cable

Question 8

What does a three-core cable look like? Which way does each wire point?

Answer 8

neutral to the left, earth down the middle, live to the right

Question 9

What colour is the live wire?

Answer 9

live wire – brown

Question 10

What colour is the neutral wire?

Answer 10

neutral wire – blue

Question 11

What colour is the earth wire?

Answer 11

earth wire – green and yellow stripes.

Question 12

Why is the insulation covering each wire colour coded?

Answer 12

for easy identification

Question 13

What is the role of the live wire?

Answer 13

The live wire carries the alternating potential difference from the supply

Question 14

What is the role of the neutral wire?

Answer 14

The neutral wire completes the circuit

Question 15

What is the role of the earth wire?

Answer 15

The earth wire is a safety wire to stop the appliance becoming live

Question 16

What is the potential difference between the live wire and earth (0 V)?

Answer 16

230V

Question 17

What is the voltage of the neutral wire?

Answer 17

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

Question 18

What is the potential difference of the earth wire?

Answer 18

0V

Question 19

When will the earth wire only carry a current?

Answer 19

if there is a fault

Question 20

Is the live wire still dangerous when a switch in the mains circuit is open?

Answer 20

yes

still carries alternating potential difference of high voltage (230V), you may complete a circuit between the live wire and the earth (because you’ll be standing on the floor), so you get a shock.

Question 21

What happens if the live wire touches metal casing?

Answer 21

the appliance becomes live

Question 22

What is the danger of providing a connection between the live wire and earth?

Answer 22

This would make a complete circuit of 230V from your mains supply to the ground (earth). A shock or fire would be highly likely.

Question 23

Why is ac sometimes used over dc supplies? (2 ways)

Answer 23

Cheaper to generate ac is more useful for transporting current over a long distance as there are less energy losses

Question 24

Why do plastics not need an earth wire?

Answer 24

They are insulators (so the appliance will not become live)

symbol for double insulation (with insulator/plastic outer layer) shown below

Question 25

How do fuses work once an appliance has become live?

Answer 25

The earth wire is connected to the metal casing

so that the current goes through the earth wire instead of causing an electric shock.

A strong current surges through the earth wire because it has a very low resistance.

This breaks the fuse and disconnects the appliance.

Question 26

How does earthing work once an appliance has become live?

Answer 26

The earth wire is connected to the metal casing

so that the current goes through the earth wire instead of causing an electric shock

Carries current to ground with 0V, thus earthing it

Question 27

How does a fuse work?

Answer 27

The fuse contains a (low-resistance) thin wire that will melt if the current gets too high.

Once the fuse has melted, the circuit is broken and no more current flows through the device