Download Alternating current

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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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What is the difference between 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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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 fuse also makes appliances
with a metal case safer.
If the live wire becomes
loose and touches the metal
case, a very large current
flows along the earth wire
and blows the fuse.
This makes it safe to touch the
appliance.
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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 lower resistance than normal wire.
terminals
thin wire with low
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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How do you choose the correct fuse?
The circuit symbol for a fuse is:
A fuse is labelled with the
maximum current that it will
allow to flow through it.
To choose the correct fuse for an
electrical device, always choose
the one with the closest rating
that is greater than 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? You can choose
from fuses of 3 A, 5A and 13A.
5 A fuse
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You choose which fuse to use!
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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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What is a circuit breaker?
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.
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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switch
electromagnet
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What is a residual current device (RCD)?
A residual current device (RCD), also called a residual current
circuit breaker (RCCB), is another type of circuit breaker.
It prevents electric shocks
when using extension cables
to appliances like lawnmowers.
This safety device compares
the current in the live and
neutral wires, which are the
same when the appliance is
working properly.
If the current is leaking, the RCD detects a difference between
the live and neutral wires and instantly shuts down the power.
The advantages of an RCD are that it is very sensitive and
quickly cuts off current before it is too large to cause injury.
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What is a transformer?
The voltage of an alternating current can be changed using
a device called a transformer.
A transformer contains two
coils that are wound around
a soft iron core.
iron core
The alternating current in
the primary (input) coil
produces an alternating
magnetic field.
This alternating magnetic
field induces an alternating
current in the secondary
(output) coil.
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primary
coil
secondary
coil
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How does a transformer change voltage?
The voltage induced in the secondary (output) coil depends
on the number of turns on the primary and secondary coils.
A step-up transformer
has more turns on the
secondary coil and so
increases voltage.
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A step-down transformer
has fewer turns on the
secondary coil and so
decreases voltage.
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Where are transformers used?
A television needs a very high
voltage to operate. It contains
a step-up transformer,
which increases the voltage
of the electricity supplied to
the television.
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This outdoor transformer
decreases the voltage of
the electricity carried by
the national grid. It is an
example of a step-down
transformer.
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How are transformers used to supply electricity?
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How does electricity get to your home?
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Glossary (1/2)
 alternating current – A current that constantly changes
direction. It is produced by most electrical generators.
 circuit breaker – An electromagnetic device that cuts off
the electrical supply, if the current is too large.
 direct current – A current that always flows in the same
direction. It is produced by cells and batteries.
 earth wire – This safety wire connects the metal case of
an appliance to the earth.
 fuse – A length of wire that heats up and melts (‘blows’) if
the current is too large, and so cuts off the electrical supply.
 mains electricity – The domestic electrical power supply
with a voltage of 230 V.
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Glossary (2/2)
 national grid – The network of high-voltage power lines
that carries electricity from power stations across the country.
 step-down transformer – A transformer that decreases
the voltage of alternating current.
 step-up transformer – A transformer that increases the
voltage of alternating current.
 substation – A place where the high voltage electricity
from the national grid is reduced by transformers so that it
can be used in homes.
 transformer – A device that increases or decreases the
voltage of alternating current.
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Anagrams
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Multiple-choice quiz
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