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P4C
Electrical circuits
 electrical current is the rate of flow of
electrical charge
 A resistor is added to a circuit to limit
the amount of current in it. Adding a
larger resistor reduces the flow of
current, so the current is smaller.
 A variable resistor/rheostat is made
of a length wire. The length wire of a
circuit can be changed.
 Longer wires allow less current to
pass (Imagine going through a crowd
to reach downstairs)
 thinner wires also allow less current
to pass (imagine going through a thin
corridor with loads of people, will you
get through)
Electric current

In an electric circuit, charge is carried
by negatively charged electrons the
electrons are free to move.They flow
in the opposite direction.
 If a circuit has a large resistance it is
hard for the charge to move.In this
case the rate of flow of charge (off
current) is small.
 A variable resistor (or rheostat) can
be used to change the resistance and
current in a circuit. The slider alters
the length of the wire in the circuit.
The longer the length of the wire:
 the bigger it's resistance
 the smaller the current
 the dimmer the lamp
Resistance = Like friction, Resistance is a
force that is opposing another force

The current in the circuit is measured
in amps (A) using an ammeter, it is
connected in a series circuit (one
circuit)
Voltage
 The force motivating electrons to
"flow" in a circuit is called voltage.
Voltage is a specific measure of
potential energy that is always
relative between two points
 Voltage is measured in volts, using a
voltmeter.A voltmeter is always
connected in a parallel circuit (more
than one circuit)
 For a fixed resistor , as the voltage
across it increases the current (flow
of electrons) increases.
 For a fixed voltage, as the resistance
increases the current decreases.
Live, earth and neutral wire
 The live wire carries the voltage into
and around the house
 The neutral completes the circuit,
because electricity normally flows in
through the live wire and leaves from
the neutral wire
 The earth wire and fuse are for safety
and work together. The earth wire
creates a safe route for the current to
flow through if the live wire touches
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the casing.
You would get an electric shock if the
live wire inside an appliance, such as
a cooker, came loose and touched the
metal casing.
However, the earth terminal is
connected to the metal casing so that
the current goes through the earth
wire instead of causing an electric
shock.
if the application (oven ect)develops
a fault e,g, the live wire touches the
case, there is suddenly as large
current in the live wire and earth wire,
if the current becomes too large the
fuse melts, stopping the flow of
charge
A the fuse is in the live wire , the
appliance is now disconnected from
the mains supply, it is safe to touch.
However the fuse must be replaced
otherwise it would not work.
The flex (cable) cannot overheat and
cause a fire.
Further damage of appliance is
prevented.
Mains electricity ( the thing you put the
plug in)
Double insulation:
a double insulated appliance does not
need an earth connection. It has a plastic
case with no electrical connection (so if
the live wire touched the case nothing
would happen as it would not take the
shock) or non conductor, it cannot
become live.
Circuit breakers
The Re-settable fuses (circuit breaker)
does the same job as the fuse, but it
works in a different way. Whenever
electrical wiring in a building has too
much current flowing through it, these
simple machines cut the power until
somebody can fix the problem.