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KS4 Physics
Mains Electricity
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Contents
Mains Electricity
Alternating current and direct current
Electricity in the home
Fuses and circuit breakers
Buying electricity
Summary activities
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What is direct current (d.c.)?
Direct current (d.c.) is an
electric current that always
flows in one direction.
Direct current is produced by
cells and batteries.
Electronic circuits such as
those in computers and
stereos need direct current
electricity in order to work.
Direct current cannot be
transferred efficiently over
large distances.
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What is alternating current (a.c.)?
Alternating current (a.c.) is
an electric current that is
constantly changing direction.
Alternating current is produced
by most generators and is
used in mains electricity.
Motors often work using
alternating current. The
voltage of alternating
current is easily changed
with a transformer.
Alternating current can be
transferred efficiently over
large distances.
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Comparing a.c. and d.c.
d.c.
time
voltage
voltage
The difference between alternating current (a.c.) and
direct current (d.c.) can be seen using an oscilloscope.
For each current, the oscilloscope trace is a graph showing
how the voltage of an electricity supply varies with time.
peak forward
voltage
a.c.
time
peak reverse
voltage
The voltage of a d.c. supply
is steady and always in the
same direction.
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The voltage of an a.c. supply
follows a repeated pattern: it
rises to a peak, returns to zero
changes direction and so on.
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Frequency of alternating current
voltage
The frequency of a.c. electricity is the number of complete
cycles per second, which is measured in hertz (Hz).
1 complete cycle
peak forward
voltage
a.c.
time
peak reverse
voltage
The a.c. frequency can be determined from an oscilloscope
by counting the number of complete waves per unit time.
If the frequency is increased the number of complete waves
shown on the screen increases. For example, if the
frequency is doubled the number of waves doubles.
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Mains electricity in the UK
In the UK, the frequency of
mains electricity is 50 hertz:
this alternating current flows
backwards and forwards
50 times per second.
This frequency is the same
at any point in the electricity
supply system but the
voltage varies in different
parts of the national grid.
The voltage of mains electricity supplied to UK homes is 230 V.
This is an effective voltage which is equal to the voltage of
a d.c. supply that would produce the same heating effect.
The peak (maximum) voltage is higher than this.
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Comparing a.c and d.c.
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Which type of current – a.c. or d.c.?
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Contents
Mains Electricity
Alternating current and direct current
Electricity in the home
Fuses and circuit breakers
Buying electricity
Summary activities
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Electrical sockets
The mains electricity system
in your house is made up of
three types of wires: live,
neutral and earth.
Electrical sockets are a
convenient and safe way
of making the electricity
available for use.
Remember the electricity in a plug socket is at 230 V and
can easily kill you if you get a shock.
NEVER PUT ANYTHING OTHER THAN A PLUG
IN AN ELECTRICAL SOCKET!
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Plugs and electrical cable
An electrical plug is inserted into
a socket and the cable which is
attached to the plug carries
electricity to the appliance.
The plug has three pins which
fit the three holes on the socket
in one way only, so the plug
cannot be connected wrongly.
The pins are made of brass,
which is a good conductor but
is hard and will not bend.
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Inside an electrical cable
An electrical cable contains
three wires.
copper
These wires are made of copper
because it is a good conductor of
electricity. Each wire is made of
thin strands of copper to keep
the cable flexible.
Each wire is encased in plastic
to stop the wires touching and
causing a short circuit. Plastic is
used as it is a good insulator as
well as being tough and flexible.
The whole cable is encased in
another layer of plastic.
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insulating plastic
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Materials in a plug
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Live, neutral and earth
The three wires in electrical cables have different functions.
The colour of the plastic casing tells you what the wire does.
live
earth
neutral
This colour-coding is a safety feature that makes the
wires easy to identify in poor lighting conditions.
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What does each wire do?
The live wire carries a current
that alternates between a
negative and positive voltage.
The neutral wire completes the
circuit. It is kept at a zero voltage
by the electricity company.
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The earth wire is a
safety wire that is needed
to earth appliances with
a metal case.
For example, if the live
wire becomes loose and
touches the metal case,
a very large current flows
to earth and blows the
fuse. This makes it safe
to touch the appliance.
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Parts of a plug
earth
terminal
live
terminal
fuse
neutral
terminal
cable
grip
cable
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Wiring a plug
When wiring a plug, always check the following:
 Make sure that the wires
are connected to the
correct terminals:
blue to neutral
yellow/green to earth
brown to live
 There are no loose or
bare wires.
 The grip should firmly
hold the cable in place.
 A fuse of the correct
value is fitted.
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Wrongly wired plugs
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Plugs – true or false?
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Contents
Mains Electricity
Alternating current and direct current
Electricity in the home
Fuses and circuit breakers
Buying electricity
Summary activities
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What is a fuse?
A fuse is a safety device
that protects an electric
cable from overheating so
that the insulation does not
catch fire.
A plug contains a fuse to
prevent the case of a metal
appliance from becoming
live if a fault develops.
All plugs contain a fuse like
the one shown and are
simple to change if they
‘blow’ and break the circuit.
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How does a fuse work?
A fuse is a built-in weak point in a circuit. It contains a thin
wire with a higher resistance than normal wire.
terminals
thin wire with
high resistance
case made of
insulating material
When a large current flows the wire becomes hot. If too
much current flows, the wire overheats and melts, which
breaks the circuit.
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Choosing the right fuse
The circuit symbol for a fuse is:
Fuses are labelled with the maximum
current that they are designed to allow.
To choose the correct fuse for a
device, always choose the nearest
one rated above the operating
current of the device.
Example:
If a kettle operates with an electrical current of 4.3 A,
what fuse should it be fitted with? 5 A fuse
You can choose from fuses of 3 A, 5A and 13A.
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Choose which fuse to use
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Calculating which fuse to use
A lamp has a potential difference of 230 V
across it and operates at a power of 60 W.
a) What current is running through the lamp?
b) What fuse should the lamp’s plug be fitted
with?
a)
P = IV
I = P/V
= 60 W / 230 V
= 0.26 A
b) The plug should be fitted with a 3 A fuse.
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Calculating which fuse to use
A cooker has a potential difference of 230 V
across it and operates at a power of 9.2 kW.
a) What current is running through the
cooker?
b) Why should the cooker’s plug not be fitted
with a 13 A fuse?
a)
P = IV
I = P/V
= 9 200 W / 230 V
= 40 A
b) A 13 A fuse would blow when the cooker was turned on.
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Steve the student buys a DVD player
Steve bought a second-hand DVD player that did not work.
He changed the fuse for one of the same size but the DVD
player still did not work. What might be the problem?
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Steve the student buys a DVD player
When Steve changed the fuse on his second-hand DVD
player it still did not work. Steve and his flatmates suggested
different solutions to the problem:
Steve: “I think I should throw
it away and buy one from a
shop where I know it will work.”
Fatema: “I would put a bigger
fuse in it because the fuse in
it can’t be large enough if it
keeps on blowing.”
Kieran: “It sounds as if
something is wrong, I would
get an electrician to have a
look at it.”
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Circuit breakers
The electrical wiring in a building must be protected from
being overloaded so that it does not overheat.
This is the job of a ‘fuse box’, which used to contain fuse
wire but now contains circuit breakers to protect the wiring.
Circuit breakers do the same job as fuses but they are
electromagnetic switches which are easy to reset.
A house has several
circuits and each one is
protected by a separate
circuit breaker.
Circuit breakers have
different ratings as shown
by the coloured dots.
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How does a circuit breaker work?
The circuit breakers in a ‘fuse
box’ are some of the most
important safety mechanisms in
your home.
Each circuit breaker is an
electromagnetic switch which is
designed to break the circuit
when the current gets too high.
switch
Too much current makes the
magnetic field produced by the
electromagnet strong enough to
open the switch.
The circuit breaker is said to ‘trip’
and switches off the current.
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electromagnet
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Domestic wiring system
lights
5A
30A
ring main
power points
kWh
meter
live neutral
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earth
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Contents
Mains Electricity
Alternating current and direct current
Electricity in the home
Fuses and circuit breakers
Buying electricity
Summary activities
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Buying electricity
Electricity costs money,
which is why every home
has an electricity meter.
The meter records how much
electricity is used in a house
in units of electrical energy.
The units of electrical
energy are called kilowatt
hours (kWh).
The cost of an electricity bill
is calculated from the
number of units used.
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Cost of electricity
The cost of electricity is the number of units of electrical
energy multiplied by the cost per unit:
cost = number of units x cost per unit
Example:
How much would 10 units of electricity cost at a price of
9p per unit?
cost = 10 units x 9 p/unit
= 90 p
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Calculating the units of electricity
The amount of electrical energy (i.e. the amount of electricity)
used by an appliance depends on its power and how long
the electricity is used for:
electrical energy = power x time
Power is measured in kilowatts (kW) and the time is
measured in hours (h), so what are the units of electricity
measured in?
1 unit of electricity = 1 unit of electrical energy
= 1 kilowatt hour (kWh)
Example:
How many units of electricity is 17.6 kWh? 17.6 units
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Buying electricity problem
A kettle uses 45.2 kWh of energy.
If electricity costs 10 p per unit, how
much does it cost to use the kettle?
Number of units:
number of units of electricity = number of kilowatt hours
= 45.2 units
Cost of electricity:
cost = number of units x cost per unit
= 45.2 units x 10 p / unit
= 452 p or £4.52
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Buying electricity problem
An iron that operates at a power of 3 kW
for 4 hours uses electricity that costs 8p
per unit.
How much does it cost for the electricity
used by the iron in that time?
Number of units:
number of units of electricity = number of kilowatt hours
= 3 kW x 4 h
= 12 kWh
= 12 units
Cost of electricity:
cost = number of units x cost per unit
= 12 units x 8 p / unit
= 96p
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Buying electricity calculations
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Contents
Mains Electricity
Alternating current and direct current
Electricity in the home
Fuses and circuit breakers
Buying electricity
Summary activities
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Glossary
 alternating current – A current that is constantly
changing direction.
 circuit breaker – An electromagnetic device that cuts off
the electrical supply if the current is too large.
 direct current – A current that is always flowing in one
direction.
 earth wire – This safety wire connects the metal case of
an appliance to earth.
 fuse – A length of wire that heats up and ‘blows’ (melts)
if the current is too large and so cuts off the electrical supply.
 live wire – This wire carries a current that alternates
between negative and positive values.
 neutral wire – This wire is kept at zero voltage by the
electricity company.
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Anagrams
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Multiple-choice quiz
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