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P2 PHYSICS
BISHOPSTON COMPREHENSIVE
SCHOOL
PHYSICS DEPARTMENT
P2 PHYSICS REVISION
BOOK 4 – MAINS ELECTRICITY
Name: _______________ Form: ___ Teacher: ______________
BOOK 4 – MAINS ELECTRICITY
P2 PHYSICS
TYPES OF ELECTRICITY
Mains electricity is the electricity that we use in the home. It is produced in power stations
across the country and delivered to us via a network of cables known as the NATIONAL
GRID.
Mains electricity is alternating current (a.c.) whereas electricity from cells or batteries is direct
current (d.c.).
Although a.c. and d.c. are both forms of electricity they are very different.
DIRECT CURRENT (d.c.)
Here the current supplied is constant in size and always flows in the same direction. See
below:
Current
Time
ALTERNATING CURRENT (a.c.)
In this case the current supplied is always changing in size and in direction. See below:
Current
Time
Mains electricity is supplied as a.c. This is due to the way that it is manufactured in the
power stations across the country.
BOOK 4 – MAINS ELECTRICITY – PAGE - 1
P2 PHYSICS
MAINS ELECTRICITY
As mentioned mains electricity is a.c. in nature.

Mains electricity is supplied in the UK at a frequency of 50 Hertz. This means that
there are 50 cycles per second in the supply.
Question
If UK mains electricity is supplied at a frequency of 50 hertz, then how many complete cycles
will be supplied in:
2 seconds
…………………………….
4 seconds
…………………………….
10 seconds …………………………….

The period of a supply is the time taken to supply 1 complete cycle.

If there are 50 cycles supplied in 1 second then one little cycle will only take 1/50 th of
a second.

This means that the period of the mains electricity supply in the UK is 1/50th of a
second or alternatively 0.02 seconds.

An oscilloscope is a device used to display voltage waveforms. An oscilloscope
displaying mains electricity would therefore look like this:
0
0.02

0.04
0.06
Mains electricity in the UK is supplied at voltage of 230V
BOOK 4 – MAINS ELECTRICITY – PAGE - 2
0.08
Time (s)
P2 PHYSICS
Questions
1. Look at the trace from the oscilloscope below. It shows an a.c. supply (not mains)
10V
0
0.1
0.2
0.3
0.4
Time (s)
a) What is the peak supply voltage (potential difference)?
…………………………….
b) How many seconds does it take for one complete cycle?
…………………………….
c) What is the period of the a.c. waveform?
…………………………….
d) What is the frequency of the a.c. waveform?
…………………………….
2. Look at the trace from the oscilloscope below. It shows an a.c. supply (not mains)
22V
0
0.5
1
1.5
2
Time (s)
a) What is the peak supply voltage (potential difference)?
…………………………….
b) How many seconds does it take for one complete cycle?
…………………………….
c) What is the period of the a.c. waveform?
…………………………….
d) What is the frequency of the a.c. waveform?
…………………………….
BOOK 4 – MAINS ELECTRICITY – PAGE - 3
P2 PHYSICS
3. Look at the trace from these two oscilloscopes below, one shows an a.c. waveform, the
other a d.c. waveform.
5V
0
0.5
1
1.5
2
0
0.5
1
1.5
2
Time (s)
8V
a) What is the peak voltage of the a.c. supply?
………………………………..
b) What is the d.c. voltage supply?
………………………………..
Time (s)
4. Look at the trace from these two oscilloscopes below, one shows an a.c. waveform, the
other a d.c. waveform.
4V
0
0.5
1
1.5
2
0
0.5
1
1.5
2
Time (s)
3V
a) What is the peak voltage of the a.c. supply?
………………………………..
b) What is the d.c. voltage supply?
………………………………..
BOOK 4 – MAINS ELECTRICITY – PAGE - 4
Time (s)
P2 PHYSICS
THE 3 – PIN PLUG
Our electrical appliances are connected to the mains using a length of cable and a 3 pin
plug.
The names of the 3 pins in the plug are:

Live

Neutral

Earth
Each of these pins have different jobs so it’s absolutely vital to connect them up to the cable
correctly.
The mains cable has three copper wires inside it, these are connected to the pins inside the
plug.
Each copper wire inside the cable has plastic insulation around it.

The earth wire has green / yellow plastic insulation around it.

The live wire has brown plastic insulation around it.

The neutral wire has blue plastic insulation around it.
The correct wiring of the 3 pin plug is shown below:
Earth
( green / yellow )
FUSE
Neutral
( blue )
Live ( brown )
The live and neutral supply the electricity to the appliance and the earth is for safety:
BOOK 4 – MAINS ELECTRICITY – PAGE - 5
P2 PHYSICS
Questions:
1. Why do you think the wires inside the cable all have their own colours?
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
2. Why do you think the wires are made of copper?
……………………………………………………………………………………………………………
3. Why do you think there is plastic coating the wires?
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
4. What is the difference between a.c. and d.c.?
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
5. The following diagram shows a three pin mains plug.
A
X
B
C
(a). Complete the following table naming the wires and their colours.
WIRE
NAME
A
B
C
(b). What is X? _________________________________
BOOK 4 – MAINS ELECTRICITY – PAGE - 6
COLOUR
P2 PHYSICS
6. When wiring up a three pin plug, it is vital that you wire it up properly, otherwise it may be
extremely dangerous. Can you think of 5 ways that a three pin plug may not be wired up
properly.
1. ………………………………………………………………………………………………………...
……………………………………………………………………………………………………………
2. ………………………………………………………………………………………………………...
……………………………………………………………………………………………………………
3. ………………………………………………………………………………………………………...
……………………………………………………………………………………………………………
4. ………………………………………………………………………………………………………...
……………………………………………………………………………………………………………
5. ………………………………………………………………………………………………………...
……………………………………………………………………………………………………………
BOOK 4 – MAINS ELECTRICITY – PAGE - 7
P2 PHYSICS
THE FUSE
The fuse is designed to protect the APPLIANCE should there be an electrical fault in the
appliance itself or in any electrical wiring.
The fuse is simply a small hollow cylinder with a metal cap at either end. A thin piece of wire
is stretched from one cap to the other through the middle. The thin piece of wire has a low
melting point.
Metal end caps
Thin fuse wire
When a fault develops and too much current flows into the appliance (through the plug) then
the fuse wire will melt and disconnect the appliance from the supply saving the appliance.
The fuse is always connected in the live part of the circuit. This is because it is only then can
we be sure there is no chance of electrocution. If the fuse were connected in the neutral part
of the circuit then the live wire would still be active all the way up to the appliance (risky!)
Fuses come with different current ratings. These are normally 1A, 2A, 3A, 5A and 13A. It
means that if you were to use a 13A fuse then a current of 13A would melt the fuse and turn
off the appliance.
Question
Say you bought a TV and there was a 5A fuse in the plug. Well you borrow the %A fuse and
lose it. The next time you want to use the TV you can only find a 13A fuse in the drawer.
Should you use it?
……………………………………………………………………………………………………………
Explain your answer
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
For the same reason as the fuse, if you have an on off switch for mains (say a lighting circuit)
you should always connect the switch in the live part of the circuit.
BOOK 4 – MAINS ELECTRICITY – PAGE - 8
P2 PHYSICS
Questions
1. An electric fire consists of an element with a shiny metal reflector behind it.
It is connected to a mains plug by means of a 3 - core lead.
(a). Complete the table below to show the colour of each wire and the terminal A, B or C of
the fire to which it should be connected for maximum safety.
WIRE
COLOUR
TERMINAL
Live
Neutral
Earth
2. The diagram below shows a kettle, containing water, being boiled on an electric hotplate.
The hotplate is connected to a mains plug by a length of electrical cable. The cable
contains three wires that are covered with coloured P.V.C. plastic.
(a). (i). Name the colour on the plastic on the wire that should be connected to the neutral pin
of the plug. _______________________
(ii). State two properties that this plastic has that makes it suitable for covering electric wires.
a)._____________________________________________________________________
b). ___________________________________________________________________
BOOK 4 – MAINS ELECTRICITY – PAGE - 9
P2 PHYSICS
3.
The diagram represents a metal hotplate which is heated with an insulated heating element.
A, B and C are the three leads of the cable which connects the hotplate to a mains plug.
Complete the table below to show
(a). the colour of each lead
(b). the terminal A, b or C of the hotplate to which the live, neutral and earth wires should be
connected for maximum safety.
WIRE
COLOUR TERMINAL
LIVE
NEUTRAL
EARTH
4. The following diagram shows an iron connected to a socket.
fuse
WIRE 1
WIRE 2
WIRE 3
Name the three wires:
WIRE 1 = ______________ WIRE 2 = ______________ WIRE 3 = ______________
BOOK 4 – MAINS ELECTRICITY – PAGE - 10
P2 PHYSICS
5. The diagram below shows part of a lighting circuit where A, B, C, D and E are identical
lamps and S1, S2, S3 and S4 are identical switches.
(a). What is the advantage of wiring the lamps in the way shown?
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
(b). State which lamp is wired incorrectly and explain why it is wrongly wired.
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
(c). State which lamps would light when the switches were in the following positions:
S1 open; S2 open; S3 closed; S4 closed
……………………………………………………………………………………………………………
6. If the fuses available to a householder are 2A, 5A, 10A and 13A, calculate the appropriate
fuse to be placed in an appliance rated at 240V, 6A.
……………………………………………………………………………………………………………
7. Use an arrow to match the following items with their correct descriptions of operation.
ITEM
DESCRIPTION OF OPERATION
Fuse
Protects electrical appliance from potential
damage should too much electrical current pass
through it.
Live wire
Protects user from possible electrocution
Earth wire
Used to supply electrical current to electrical
appliance from mains socket
BOOK 4 – MAINS ELECTRICITY – PAGE - 11
P2 PHYSICS
THE CIRCUIT BREAKER
The circuit breaker does the same job as the fuse. That is it protects appliances from
damage by switching them off the current flowing is too high.
Circuit breakers are normally found in a distribution box (old fashioned name – fuse box)
normally just inside you front door mounted on the wall up high.
Circuit breakers are switches which are current controlled. They normally are used to turn off
a whole load of appliances (like downstairs sockets, or maybe upstairs lights). They have
advantages over fuses. These are:
1. They are faster acting (it takes up to a second for a fuse wire to heat up and melt)
2. They are re-usable, just flick them back on (you have to buy a new fuse every time it’s
used)
3. They always work at the perfect current (fuses are more approximate)
THE EARTH WIRE
As explained previously, the earth wire is not used to supply electricity. It is a safety measure
used to protect the PERSON USING THE APPLIANCE.
But how does it work?
Well, it’s quite simple really. Say a fault developed in an appliance with an outer metal
casing. Well it’s quite possible that the outer metal casing may become live (230V). This
means that if you were to touch this live piece of metal outer casing then you would have an
electric shock and would probably die !!
But not if you had an earth wire properly connected. If you had, then the earth wire would
provide an easy path to ground for the electricity to flow down (and not through the user)
WAHAY !! Safe again.
It is for this reason one end of the earth wire is always connected to 0V and the other to the
metal outer casing of the appliance.
Therefore you can say that:
The earth wire and fuse together protect the appliance and the user.
BOOK 4 – MAINS ELECTRICITY – PAGE - 12
P2 PHYSICS
THE VOLTAGES OF LIVE, NEUTRAL AND EARTH
Electrical current can only flow from a more positive voltage point to a less positive voltage
point.
That’s the reason that little birds can quite safely stand on electrical overhead cables without
being electrocuted. As long as both of their little feet are on an identical voltage (the same
cable) then no current can flow, and they will be alright.
Now what voltages are live, neutral and earth held at ?

Earth is always fixed on zero volts.

Neutral is always fixed on zero volts (in fact it’s connected to earth in the sub-station)

Live is the funny one. It is constantly changing. It alternates between a positive
voltage and a negative voltage all the time. It’s voltage has the same shape as the
a.c. waveform on page 1 of these notes.
Now then, when live is positive, current flows into the appliance from live and out of
the appliance from neutral.
But as soon as live become negative, then the zero volts of neutral is more positive
than the negative live. This means that current flows into the appliance from neutral
and out of the appliance from live.
This causes the current direction to change every half cycle as stated was the case
with a.c. supplies.
BOOK 4 – MAINS ELECTRICITY – PAGE - 13