Mains electricity

Question 1

What do cells and batteries supply?

What is this type of current called?

Answer 1

Cells and batteries supply current that passes round the circuit in one direction.
This is called direct current, or dc.

Question 2

What is different about the current supplied by the mains supply and what is this type of current called?

Answer 2

The current from the mains supply passes in one direction and then reverses and passes in the other direction.
This called alternating current, or ac.

Question 3

What is the frequency of the UK mains supply and what does this mean?
What is the ‘voltage’ of the mains?

Answer 3

The frequency of the UK mains supply is 50Hz, which means it changes direction 50 times per second.
The ‘voltage’ of the mains is 230V.

Question 4

What does the live wire of the mains supply do?

What ‘voltage’ does the neutral wire remain at?

Answer 4

The live wire of the mains supply alternates between a positive and negative potential difference with regard to the neutral wire.
The neutral wire remains at zero volts.

Question 5

The live wire alternates between which peak voltages?

What is this equivalent to in terms of electrical power?

Answer 5

The liver wire alternates between peak voltages of +325V and -325V.
In terms of electrical power this is equivalent to a direct potential difference of around 230V.

Question 6

The frequency of an ac supply can be determined by an oscilloscope, using what equation?

Answer 6

f = 1/T
f is the frequency of the ac in hertz, Hz
T is the time for one cycle in seconds, s

Question 7

How are most electrical appliances connected to the mains supply?

Answer 7

Most electrical appliances are connected to the sockets of the mains supply using cable and a three pin plug.

Question 8

What is the outer cover of a three-pin plug made of and why?

What are the pins of the plug made from and why?

Answer 8

The outer cover of a three-pin plug is made of rubber or plastic because both of these materials are good electrical insulators.
The pins of the plug are made from brass, because it is a good electrical conductor, it is also hard and will not rust or oxidise.

Question 9

To which pin is the green and yellow wire of a three core cable connected?
To which pin is the brown wire connected?
To which pin is the blue wire connected?

Answer 9

The earth wire is connected to the longest pin which is the earth pin.
The brown wire is connected to the live pin.
The blue wire is connected to the neutral pin.

Question 10

What three things are important to ensure about cables and plugs?

Answer 10

1) It is important that the cable grip is fastened tightly over the cable.
2) There should be no bare wires showing inside the plug.
3) the correct cable must be connected firmly to the terminal of the correct pin.

Question 11

What must be done to appliances that have a metal case?

Answer 11

Appliances that have a metal case must be earthed, the case is attached to the earth wire in the cable.

Question 12

What is said about appliances that have a plastic case, how are they connected to the mains supply?

Answer 12

Appliances with a plastic case are said to be double insulated and are connected to the mains supply with a two core cable containing a live and neutral wire.

Question 13

Why are cables of different thicknesses used for different purposes?

Answer 13

The more current needed to be carried, the thicker the cable needs to be.

Question 14

Why does a mains appliance with a plastic case not need to be earthed?

Answer 14

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

Question 15

Why do appliances with metal cases need to be earthed?

Answer 15

Appliances with metal cases need to be earthed because if a fault develops and the live wore touches the metal case, the case becomes live and could give an electric shock to anyone who touches it.

Question 16

What is always fitted in series with the live wire and what does this do?

Answer 16

A fuse is always fitted in series with the live wire, this cuts the appliance off from the live wire if the fuse blows.

Question 17

What happens if a fault develops in an earthed appliance?

Answer 17

If a fault develops in an earthed appliance, a large current flows to earth and melts the fuse, disconnecting the supply.

Question 18

What should the rating of the fuse be set to, relative to the normal working current of the appliance and why?

Answer 18

The rating of the fuse should be set slightly higher than the normal working current of the appliance. If it is much higher it will not melt soon enough. if it is not higher than the normal working current, it will melt as soon as the appliance is witched on.

Question 19

What can be used in place of a fuse, what is this?

Answer 19

A circuit breaker can be used in place of a fuse, this is an electromagnetic switch that opens and cuts off the supply if the current is bigger than a certain value.

Question 20

What is the function of a residual current circuit breaker?

What is the benefit of an RCCB?

Answer 20

A residual current circuit breaker (RCCB) cuts off the current in the live wire if it is different to the current in the neutral wire.
It works faster than a fuse or an ordinary circuit breaker.

Question 21

What is the function of an electrical appliance?

What is the power of an electrical appliance?

Answer 21

An electrical appliance transfers energy into other forms of energy, the power of an electrical appliance is the rate at which it transfers energy.

Question 22

How can the power of an electrical appliance be calculated?

Answer 22

P = E/t
P is the power in watts, W
E is the energy transferred in joules, J
t is the time in seconds, s

Question 23

What equation uses current and potential difference to calculate power?

Electrical appliances have their power rating shown on them.

Answer 23

P = I x V
P is the power in watts, W
I is the current in Amps, A
V is the potential difference in volts, V

Question 24

For what is the equation P = I x V used for?

Answer 24

It is used to calculate the normal current through an appliance and so work out the size of fuse to use.

Question 25

What is an electric current?

What equation is used to calculate charge?

Answer 25

An electric current is the rate of flow of charge.
Q = I x t
Q is the charge in coulombs, C
I is the current in amps, A
t is the time in seconds, s

Question 26

What happens when charge flows through an appliance?

What is electrical energy transferred to in a resistor?

Answer 26

When charge flows through an appliance, electrical energy is transferred to other forms.
In a resistor electrical energy is transferred so that the resistor becomes hotter.

Question 27

How can energy transferred to an appliance be calculated?

Answer 27

E = V x Q
E is the energy transferred in joules, J
V is the potential difference in volts, V
Q is the charge in coulombs, C

Question 28

Why may electrical faults occur?

Answer 28

Electrical faults may occur as a result of damage to sockets, plugs, cables or appliances.

Question 29

What should electrical equipment be regularly checked for, what should be done subsequently?

Answer 29

Electrical equipment should be regularly checked for wear, worn or damaged equipment should be replaced or repaired by a qualified electrician.

Question 30

What may overlapping sockets cause?

How should electrical appliances be handled?

Answer 30

Overlapping sockets may cause overheating and a risk of fire.
Electrical appliances should be handled with care and never used in a bathroom or with wet hands.

Question 31

When choosing an electrical appliance, what two things should be considered as well as cost?

Answer 31

1) The power rating

2) The efficiency rating

Question 32

What can be said about filament and halogen bulbs?

Answer 32

Filament and halogen bulbs are much less efficient and do not last as long as low-energy bulbs.