Download Safe Electricals – Revision Pack (P4) Resistance: Voltage (potential

Safe Electricals – Revision Pack (P4)
Resistance:
Resistance slows the flow of electrical current or
flow of electrons.
A variable resistor, or rheostat, changes the
resistance. Longer lengths of wire will have more
resistance and shorter lengths will have less
resistance.
Thinner wires have higher resistance and thicker
wire have lower resistance (see left).
Voltage (potential difference) is measured in volts on a voltmeter which must be
connected in parallel to the resistor.
For a fixed resistor, as the voltage increases so does the current.
For a fixed power supply, as the resistance increases, the current decreases.
The formula for resistance is:
RESISTANCE = Voltage / Current
Resistance is measured in Ohms (Ω).
Voltage is measured in Volts (V).
Current is measured in Amps (A).
Live, neutral and earth wires:
The live wire carries a high voltage into and
around the house. This wire is brown.
The neutral wire completes the circuit; it provides
a return path for the current which enters through
the live wire and exits via the neutral wire. This
wire is blue.
The earth wire is there as a safety measure. It is
connected to the case of the appliance to
prevent it from going ‘live’. This is yellow and
green.
A fuse contains a wire which breaks the circuit if
the current gets too high. It is a safety feature.
The fuse is in the live wire. If there was a
fault, like the live wire touches the case,
then the earth wire allows a huge current
to surge through the live wire which melts
the fuse and breaks the circuit.
The earth wire and fuses work together to prevent
people from experiencing an electric shock. If the
live wire was to touch the casing of the
appliance, like an electric cooker, then you
would get a shock when you touched the
appliance. However, the earth wire is connected
to the metal casing so takes the current away
from it, stopping it from becoming ‘live’.
Safe Electricals – Revision Pack (P4)
The image to the left highlights what was discussed
before; the earth wire will take the current away
from the casing if it comes into contact with the live
wire.
NOTE – A re-settable fuse (circuit-breaker) doesn’t
need to be replaced to restore power, it can be reset.
Electrical Power:
The rate at which an appliance transfers energy is its power rating:
(Electrical) Power = Voltage x Current
NOTE – the mains voltage of a home in the UK is always 230V
The formula for electrical power can be used to calculate the correct fuse to use in
an electrical device; for example:
A microwave oven has a power rating of 900W. Which of these fuses is the most
appropriate for this microwave which will be used at home?
7A fuse
2A fuse
4A fuse
13A fuse
ANSWER
Power = voltage x current
Current (in Amps) = Power / Voltage
Voltage at home = 230V
Power = 900W
900 / 230 = 3.913 A -SO, the 4A fuse is the most appropriate fuse to use.
Double insulation:
An appliance with a outer plastic case doesn’t need to be
earthed, this is because their outer case is an electrical
insulator NOT (like metal) a conductor – so it CANNOT
become live.
The symbol for a double insulated appliance is shown the
left.
Safe Electricals – Revision Pack (P4)
Past Papers:
PPQ(1):
PPQ(2):
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PPQ(3):
Safe Electricals – Revision Pack (P4)
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PPQ(1):
PPQ(2):
PPQ(3):