Mains electricity

Question 1

Cells and batteries supply what type of current?

Answer 1

d.c.

Question 2

What is d.c. current?

Answer 2

Current that always passes in the same direction. Stands for direct current.

Question 3

What is a.c. current?

Answer 3

Current that constantly changes direction. Stands for alternating current.

Question 4

Mains electricity is what type of current?

Answer 4

a.c.

Question 5

What is the frequency of the mains electricity in the UK?

Answer 5

50 cycles per second (50 Hz).

Question 6

What is the potential difference of UK mains electricity?

Answer 6

230V.

Question 7

Most electrical devices in the UK are connected to the mains with what type of plug?

Answer 7

Three-pin plug.

Question 8

What are the three pins for in a three-pin plug?

Answer 8

Live (supply in), neutral (supply out) and earth.

Question 9

What types of electrical cable structure are used in the UK?

Answer 9

Three-core and two-core cable structure.

Question 10

What colour is the live wire in a plug?

Answer 10

Brown.

Question 11

What is the colour of the neutral wire in a plug?

Answer 11

Blue.

Question 12

What colour is the earth wire in a plug?

Answer 12

Yellow/green.

Question 13

What happens in a plug/device if there is an electrical fault that causes too great a current?

Answer 13

The circuit is disconnected by a fuse or circuit breaker in the live wire.

Question 14

What happens in a fuse if the current exceeds the rating of the fuse?

Answer 14

The fuse will melt and break the circuit on the live connection.

Question 15

What is a Residual Current Circuit Breaker (RCCB)?

Answer 15

It is a way of protecting a circuit. It measures the current flow in the live and neutral wires and if there is a difference the circuit is broken. A difference in current would mean there is current outside of the device making it potentially dangerous.

Question 16

Advantage of RCCBs over fuses?

Answer 16

RCCBs operate far faster than fuses.

Question 17

Advantage of fuses over RCCBs?

Answer 17

Fuses are far cheaper than RCCBs.

Question 18

What types of devices are earthed?

Answer 18

Devices with metal cases.

Question 19

Why are devices earthed?

Answer 19

If the live wire touches the casing the device’s case becomes charged. Earthing the case provides a safe route for the current to flow in case this were to happen.

Question 20

What is meant by double insulating?

Answer 20

The device’s casing is made of an insulating material meaning if the live wire were to touch the casing it would not become charged so there is no need to earth it.
The device may also have been designed in a way that means the live wire is never able to touch the casing.

Question 21

What happens to a resistor when electrical current flows through it?

Answer 21

It heats up.

Question 22

What is meant by power?

Answer 22

The rate at which energy is transferred by a device.

Question 23

Equation for power:

Answer 23

P = E/t

Where:
P is power in W.
E is energy in J.
t is time in s.

Question 24

Power, potential difference and current equation:

Answer 24

P = IV

Where:
P is power in W.
I is current in A.
V is potential difference in V.

Question 25

Energy transferred, potential difference and charge equation:

Answer 25

E = VQ

Where:
E is energy transferred in J.
V is potential difference in V.
Q is charge in C.