P2 - Mains Electricity

Question 0

What makes a material an electrical conductor?

Answer 0

Freely moving charged particles

Question 1

Why are electrical insulators needed?

Answer 1

The human body conducts electricity and this needs to be prevented

Question 2

What do we do to prevent electric shocks when a live wire is touching a metal casing?

Answer 2

Attach an earth wire to the metal casing to provide a safe route for the current

Question 3

Why does the earth wire from the casing have a low resistance?

Answer 3

If the live wire touches the casing, there would be an increase in current in the live wire which would cause the fuse in the live wire to melt

Question 4

What are the colours of the live, neutral and earth wires?

Answer 4

Live is brown
Neutral is blue
Earth is green and yellow

Question 5

If facing an electric plug, what is each socket? (Live, neutral and earth)

Answer 5

The bottom-right is the live
The bottom-left is the neutral
The top is the earth

Question 6

What are the live, neutral and earth wires for?

Answer 6

The live carries the electricity to the socket of the plug
The neutral completes the circuit
The earth is connected to the casing of the appliance providing a safe route for the current

Question 7

Why do some plugs have a plastic earth pin?

Answer 7

The casing is not conductive meaning the earth wire is not needed

Question 8

What is difference between AC and DC?

Answer 8

AC can change direction but DC travels in one direction only

Question 9

How often does mains current alternate?

Answer 9

50 Hz

50 times a second

Question 10

What type of electricity do batteries and cells supply?

Answer 10

Direct current

Question 11

What is the voltage of mains electricity?

Answer 11

230 volts

Question 12

Describe how cathode ray oscilloscopes work

Answer 12

If you plug in an AC supply into an oscilloscope, you get a trace on the screen that shows how voltage changes with time

You get a sine wave that goes positive then negative then positive

With a DC supply, you get a straight line

Question 13

How do you calculate the voltage from an oscilloscope?

Answer 13

For an AC supply, the peak of the sine wave is the voltage

For a DC supply, the height of the straight line is the voltage

Question 14

How do you calculate the frequency of an AC supply using an oscilloscope?

Answer 14

Calculate the time period by looking at the time between 2 consecutive peaks

Calculate the frequency by doing 1 / time period

Question 15

On an oscilloscope, what do the gain and timebase dials do?

Answer 15

The gain dial controls how many volts each centimetre division represents on the vertical axis

The timebase dial controls how many milliseconds each division represents on the horizontal axis

Question 16

What are the 9 electrical hazards in the home?

Answer 16

Long cables

Frayed cables

Cables in contact with something hot or wet

Water near sockets

Shoving things into sockets

Damaged plugs

Too many plugs in one socket

Lighting sockets without bulbs in

Appliances without their covers on

Question 17

What is the voltage of the neutral wire?

Answer 17

0 volts even though it carries the electricity back

Question 18

How should fuses be rated?

Answer 18

So that their maximum current is slightly higher than the current of the circuit

Question 19

How are earth wires different to the love and neutral wires?

Answer 19

They are thicker and haveess resistance so that a higher current can flow in order to melt the fuse

Question 20

What is double insulation?

Answer 20

When an appliance has a plastic casing and no metal parts showing, it is double insulated

This means that no earth wire is needed because there is no metal casing

Question 21

What are circuit breakers?

Answer 21

An electrical safety device that detect surges in current and break the circuit by opening a switch

Question 22

What is the most commonly used circuit breaker and how does it work?

Answer 22

Residual current circuit breakers

Normally, the same current flows through the live and neutral wires but if someone touches the live wire, the neutral wire will carry back less current than the live wire

The device detects this difference in current and quickly opens a switch

Question 23

What are the 3 advantages of circuit breakers?

Answer 23

Can be reset by flicking a switch where fuses have to be replaced

Break the circuit much faster than fuses

Work for tiny current changes that might not be large enough the melt a fuse

Question 24

What is the disadvantage of circuit breakers?

Answer 24

Much more expensive than fuses