P2.4 Mains electricity

Question 1

What is the voltage of the UK mains supply?

Answer 1

230 volts

Question 2

What is the current of the UK mains supply?

Answer 2

50Hz

Question 3

What type of current is the UK mains supply?

Answer 3

AC

Question 4

What is ‘direct current’?

Answer 4

Current always passing in the same direction

Question 5

What is ‘alternating current’?

Answer 5

Current constantly changing direction

Question 6

Give one example of where direct current is used.

Answer 6

Batteries

Question 7

How are most electrical appliances connected to the mains?

Answer 7

Using a cable and 3 pin plugs

Question 8

What is the structure of a three core cable?

Answer 8

• Three wires inside them, each with a core of copper and covered with a coloured plastic coating:
• The live (brown) wire - transmits an alternating current
• The neutral (blue) wire - always at 0V
• The earth wire (green-yellow) wire - protects wiring and is for safety.
• It is covered with metal casing.

Question 9

How does the earth wire ensure a safer cable?

Answer 9

It is connected to the casting of the appliance and the earth pin - it carries the electricity to the earth if any of the other wires touch the metal case.

Question 10

What is the structure of a two core/double-insulated cable?

Answer 10

• It only has live and neutral wires

- The plug it is attached to is already covered in plastic so it doesn’t need an earth wire

Question 11

Do all cables need an earth wire?

Answer 11

No, only metal cased plugs

Question 12

How are three pin plugs wired?

Answer 12

• Green-yellow wire is connected to the earth pin
• Blue wire is connected to the neutral pin
• Brown wire is connected to the live pin
• No bare wires should be showing in the plug
• The cable grip is then fastened tightly over the cable outer layer

Question 13

How does a fuse work?

Answer 13

It melts if the current gets too high (higher than its rating), cutting off the live supply and breaking the circuit

Question 14

What rating of fuse should be used for an appliance?

Answer 14

One just higher than the normal operating current

Question 15

What is the relationship between the thickness of a cable and fuse rating?

Answer 15

The larger the current, the thicker the cable thickness needed to carry it, so the higher the fuse rating.

Question 16

What are RCCBs and how do they work?

Answer 16

• Residual Current Circuit Breaker
• It detects a difference between the live and neutral wire
• When there is a surge between them, it trips a switch and breaks the current.

Question 17

What are the advantages of an RCCB?

Answer 17

• More sensitive
• Can be reset by flicking the switch on a device
• They don’t have to be replaced like fuses
• Much faster as no time is wasted waiting for the fuse to melt

Question 18

What is the effect of electric charge flowing through a resistor?

Answer 18

• Electrical energy is converted to heat energy
• The more current flowing, the more heat produced
• A bigger voltage means more heating because it pushes more current through

Question 19

Why do filament bulbs waste so much energy?

Answer 19

Filament bulbs work by passing a current through a very thin wire meaning more heat energy wasted

Question 20

Define ‘power’ (of an appliance)

Answer 20

The rate at which it transforms energy/second