P2.4 Using Mains Electricity Safely And
The Power Of Electrical Appliances
1 P2.4.1 Household Electricity
1.1 Cells and batteries supply current that
always passes in the same direction.
This is called direct current (d.c.). An
alternating current (a.c.) is one that is
constantly changing direction.
1.1.1 Mains electricity is an a.c. supply. In the
UK it has a frequency of 50 cycles per
second (50 hertz) and is about 230 V.
126.96.36.199 Most electrical
the mains using
cable and a
188.8.131.52.1 A mains electricity cable contains two or
three inner wires. Each has a core of copper,
because copper is a good conductor of
electricity. The outer layers are flexible
plastic, because plastic is a good electrical
insulator. The inner wires are colour coded:
blue is neutral, brown is live and green and
yellow stripes is the earth wire.
184.108.40.206.1.1 The case of a plug is made from tough plastic or rubber, because these
materials are good electrical insulators. The three pins are made from
brass, which is a good conductor of electricity. There is a fuse between
the live terminal and the live pin. The fuse breaks the circuit if too much
current flows. The cable is secured in the plug by a cable grip. This
should grip the cable itself, and not the individual wires inside it. The blue
neutral wire goes to the left, the brown live wire to the right and the green
and yellow striped earth wire is on top. The fuse fits next to the live wire.
220.127.116.11.1.1.1 When the current in a fuse wire
exceeds the rating of the fuse it will
melt, breaking the circuit. Some
circuits are protected by Residual
Current Circuit Breakers (RCCBs).
RCCBs operate by detecting a
difference in the current between
the live and neutral wires.
18.104.22.168.22.214.171.124 Appliances with metal cases are usually
earthed. Some appliances are double
insulated, and therefore have no earth
wire connection. The earth wire and fuse
together protect the wiring of the circuit.
2 P2.4.2 Current, Charge And Power
2.1 When an electrical charge flows through
a resistor, the resistor gets hot.
2.1.1 The rate at which energy is transferred by an appliance is called the power. P=Et
where P is power in watts, W, E is energy in joules, J and t is time in seconds, s.
126.96.36.199 P=I×V where P is power in watts, W, I is current
in amperes (amps), A and V is potential
difference in volts, V.
188.8.131.52.1 E=V×Q where E is
energy in joules, J, V is
potential difference in
volts, V and Q is
charge in coulombs, C.