Mains electricityRevise the following

• Direct and alternating current• 3-pin plugs and electrical cable• Safety – fuses and earth wire• Period and frequency (Higher tier)• Calculating Power

Direct current

Alternating current

neutral terminal

earth terminal

cable grip

fuse

cable

What are the parts of a plug?

live termin

al

Label the plug parts

The names and jobs of each plug part

What’s inside an electrical cable?

• An electrical cable contains three wires.• 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

copperinsulatin

g plastic

What does each wire do?

The neutral wire (blue) completes the circuit. It is kept

at a zero voltage by the electricity company.

The earth wire (green and yellow) is a

safety wire that is needed to earth appliances with a metal case.

This makes it safer to touch the appliance if it

develops a fault.

The live wire (brown)carries a current that alternates

between a negative and positive voltage.

Safety using Fuses and EarthingFUSES

• A fuse contains a thin wire, which melts if the current is too high.

• This breaks the circuit and so electricity is unable to flow through the appliance.

• The appliance stops working and any danger has been averted.

• Fuses act as an early warning system, preventing appliances from being damaged by surges in electricity and warning owners of faults.

EARTHING

• Cables contains an earth wire to provide an alternative path for current if appliances develop faults. • An earth wire is essential for an appliance with a metal case. • If the live wire becomes loose and touches the metal case, a very large current flows to earth and blows the fuse, breaking the circuit• When an appliance is working correctly, no electricity flows through the earth wire.

Frequency and period (Higher tier)

• We can use an oscilloscope to work out the frequency of the ac supply by measuring the period (time taken for one complete cycle)

• Frequency and period are connected by the equation:

Frequency (Hertz) = 1 . period (seconds)

Using oscilloscope traces to work out period and frequency

(0V)Zero voltage

Each division will have a potentialdifference (voltage)

valueThe 0 voltagerepresents the

Neutral terminal(stays at a

potential close to zero –

compared tothe Earth)

Time

Work out the frequency of the supply shown below?

Each time division =1 milliseconds

Period = 4 divisions x1 = 4ms

4 ms = 0.004 seconds

Frequency = 1/Period (seconds)

Frequency = 1/0.004

= 250Hz

• Electric current is the rate of flow of charge.• When an electrical charge flows through a resistor,

electrical energy is transformed into heat energy.• The rate at which energy is transformed in a

device is called the power.

Calculating Power

Power = current x potential difference(Watts/W)) (Amps) (Voltage/V))

energy = potential difference x charge transformed (Joules/J) (Voltage/V) (Coulombs/C)

charge = current x time(Coulomb/C) (Amps) (secs)