Mains electricity

Question 1

Direct current (DC)

Answer 1

Electric current in a circuit that is in one direction only

Question 2

Alternating current (AC)

Answer 2

Electric current in a circuit that repeatedly reverses its direction

Question 3

Frequency

Answer 3

The number of wave crests passing a fixed point every second

Question 4

Live wire

Answer 4

The wire of a mains circuit that has a potential that alternates from positive to negative and back each cycle

Question 5

Neutral wire

Answer 5

The wire of a mains circuit that is earthed at the local sub-station so its potential difference is close to zero

Question 6

Oscilloscope

Answer 6

A device used to display the shape of an electrical wave

Question 7

f=1/t

Answer 7

frequency of an arc (Hz) = 1 ÷ time for one cycle (s)

Question 8

Socket

Answer 8

A mains socket is used to connect the mains plug of a mains appliance to a mains circuit

Question 9

Cable

Answer 9

Two or three insulated wires surrounded by an outer layer of rubber or flexible plastic

Question 10

Three-pin plug

Answer 10

A three pin plug has a live pin, a neutral pin and an earth pin - the earth pin is used to earth the metal case of an appliance so the case cannot become live

Question 11

Fuse

Answer 11

A fuse contains a thin wire that melts and cuts off the current if too much current passes through it

Question 12

Circuit breaker

Answer 12

An electromagnetic switch that opens and cuts the current off if too much current passes through it

Question 13

Residual current circuit breaker (RCCB)

Answer 13

An RCCB cuts off the current in a live wire when it is different from the current in a neutral wire

Question 14

P=IV

Answer 14

power (W) = current (A) x potential difference (V)

Question 15

Q=It

Answer 15

charge (C) = current (A) x time (s)

Question 16

E=VQ

Answer 16

energy (J) = potential difference (V) x charge (Q)

Question 17

What is mean by direct current and alternating current?

Answer 17

Direct current is in one direction only, whereas alternating current repeatedly reverses its direction

Question 18

What do we mean by the peak voltage of an alternating potential difference?

Answer 18

The peak voltage of an alternating potential difference is the maximum voltage measured from zero volts

Question 19

What is the frequency of the UK mains system?

Answer 19

50 Hz

Question 20

What is the peak voltage of the UK mains system?

Answer 20

±325 V

Question 21

What is the direct voltage of the UK mains system?

Answer 21

230 V

Question 22

What do we mean by the live wire and the neutral wire of a mains circuit?

Answer 22

A mains circuit has a live wire that is alternately positive and negative every cycle and a neutral wire at zero volts

Question 23

How do we use an oscilloscope to measure the frequency of an alternating current?

Answer 23

Measure the time period taken by the waves then use the formula f=1/t

Question 24

What is the casing of a mains plug or socket made from and why?

Answer 24

Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical conductor

Question 25

What is a mains cable?

Answer 25

Mains cables are three core cables consisting of two or three insulated copper wires surrounded by an outer layer of flexible plastic material

Question 26

What colour is the live wire?

Answer 26

Brown

Question 27

What colour is the neutral wire?

Answer 27

Blue

Question 28

What colour is the earth wire?

Answer 28

Green and yellow striped

Question 29

Why does a 3-pin plug include an earth pin?

Answer 29

The earth wire is connected to the longest pin and is used to earth the metal case of a mains appliance

Question 30

Why is brass better than copper for the parts of three-pin plug?

Answer 30

While copper is more conductive, brass is harder, and so more durable

Question 31

What do we use a fuse for?

Answer 31

A fuse contains a thin wire that heats up, melts and cuts off the current if the current is too large

Question 32

Why is a fuse always on the "live" side of an appliance?

Answer 32

A fuse is always fitted in series with the live wire, thus cutting the appliance off from the live wire if the fuse blows

Question 33

What is a circuit breaker

Answer 33

A circuit breaker is an electromagnetic switch that opens ("trips") and cuts off the current if too much current passes through the circuit breaker

Question 34

Why are appliances with plastic cases not earthed?

Answer 34

A mains appliance with a plastic case does not need to be earthed because plastic is an insulator and cannot become live

Question 35

What is the relationship between power and energy?

Answer 35

The power supplied to a device is the energy transferred to it each second

Question 36

How can we calculate the correct current for a fuse?

Answer 36

The correct rating for a fuse, in amperes, equals the electrical power divided by the potential difference

Question 37

What is an electric current?

Answer 37

An electric current is the rate of flow of charge

Question 38

What energy transfers take place when charge flows through a resistor?

Answer 38

When an electrical charge flows through a resistor, energy transferred to the resistor makes it hot

Question 39

What can we say about the electrical energy supplied by the battery in a circuit and the electrical energy transferred to the components?

Answer 39

When charge flows round a circuit for a certain time, the electrical energy supplied by the battery is equal to the electrical energy transferred to all the components in the circuit

Question 40

Why are electrical faults dangerous?

Answer 40

Electrical faults are dangerous because they can cause electric shocks and fires

Question 41

How can we prevent electrical faults?

Answer 41

Never touch a mains appliance with wet hands, and never touch a bare wire or a terminal at a potential of more than 30 V. Check cables, plugs and sockets for damage regularly, and check smoke alarms and infrared sensors regularly

Question 42

When choosing an electrical appliance, what factors should in addition to cost should we consider?

Answer 42

When choosing an electrical appliance, the power and efficiency rating of the appliance need to be considered

Question 43

How do different forms of lighting compare in terms of cost and energy efficiency?

Answer 43

Filament bulbs and halogen bulbs are much less efficient than low energy bulbs