P2- Mains Electricity

Question 1

What is alternating current?

Answer 1

Alternating current means the current repeatedly reverses its direction.

Question 2

What is the mains electricity supply?

Answer 2

Alternating current at about 230V, 50Hz.

Question 3

How can frequency be calculated?

Answer 3

f= 1/T.

f= frequency in hertz (Hz).

T= time for one cycle in seconds (s).

Question 4

What colour and side of a plug is the live wire?

Answer 4

Brown on the right.

Question 5

What colour and side of a plug is the neutral wire?

Answer 5

Blue on the left.

Question 6

What colour and side of the plug is the earth wire?

Answer 6

Green and yellow in the middle(ish).

Question 7

Why are the pins of a plug made of brass?

Answer 7

Brass is a good conductor and does not rust or oxidise.

Question 8

What is direct current?

Answer 8

Direct current means the current only flows in one direction.

Question 9

What is a fuse?

Answer 9

A thin wire which heats up and melts if too much current passes through it. This cuts off the current.

Question 10

Why don’t appliances with plastic cases need to be earthed?

Answer 10

They are said to be double insulated and connected to the supply with two-core cable containing just a live wire and a neutral wire.

Question 11

An appliance operates at a 6A current.

What fuse size should it be and why?

Answer 11

13A because if it was the 5A, it would melt as soon as the appliance is switched on and if it was any higher than 13A, it would not melt soon enough.

Question 12

What’s a circuit breaker?

Answer 12

A circuit breaker is an electromagnetic switch that opens and cuts off supply if the current is bigger than a certain value.

It can be used in place of a fuse.

Question 13

What is a residual current circuit breaker?

Answer 13

An RCCB cuts off current in the live wire if it’s different to the current in the neutral wire.

Question 14

What is the advantage of using an RCCB instead of a fuse?

Answer 14

• RCCB’s can be reset, unlike fuses which have to be replaced.
• RCCB’s protect people from electrocution, whilst fuses only protect the household wiring.

Question 15

Define power.

Answer 15

Amount of energy transferred each second.

Question 16

How can power be calculated?

Answer 16

P= E/t

P= power in watts (W).

E= energy transferred in joules(J).

t= time in seconds (s).

Question 17

How can power be calculated using current and potential difference?

Answer 17

P= I x V

P= power in watts (W).

I= current in amperes (A).

V= potential difference in volts (V).

Question 18

How can the correct rating for a fuse be calculated?

Answer 18

Power/ Potential difference.

Question 19

What is an electric current?

Answer 19

The rate of flow of charge.

Question 20

How can charge be calculated?

Answer 20

Q= I x t

Q= charge in coulombs (C).

I= current in amperes (A).

t= time in seconds (s).

Question 21

When a charge flows through a resistor, what happens?

Answer 21

Electrical energy is transferred to the resistor so the resistor becomes hotter.

Question 22

How can the amount of energy transferred be calculated?

Answer 22

E= V x Q

E= energy in joules (J).

V= potential difference in volts (V).

Q= charge in coulombs (C).

Question 23

Why are electrical faults dangerous?

Answer 23

They can cause electric shocks and fires.

Question 24

Why should you never touch a mains appliance, plug or socket with wet hands?

Answer 24

This is dangerous because it reduces resistance therefore increases current.

Question 25

Explain, in terms of electrons and ions, why both the temperature and resistance increase in the metal filament inside a bulb (2 marks).

Answer 25

Ions vibrate faster (1). Electrons collide more frequently with the ions (1).

Question 26

Going down the slide causes the child’s hair to stand on end.

What conclusion about the electrical charge on the child’s hair can be made from this observation? (2 marks)

Answer 26

The charges are the same (1) because the hairs are repelling (1).

Question 27

Why would the child not become electrically charged if the slide was made from metal? (1 mark)

Answer 27

The charge would pass through the metal to the earth.