Cells and batteries supply current that passes round the circuit in one direction. This is called direct current or dc. The current from the mains supply passes in one direction then reverses and goes in another. This is called alternating current or ac. The frequency of the UK mains supply is 50 hertz. So it changes direction 50 times each second. The voltage is 230 V.
The live wire alternates between a positive and negative potential with respect to the neutral wire. The neutral wire stays at 0 V. The live wire alternates between peak voltage of 325 V and 325 V.
The frequency of an ac supply can be determined by an oscilloscope by:
F = frequency in hertz. F = 1
T = time for one cycle in seconds. T
Cables and plugs
Most electrical appliances are connected to the sockets of the mains supply using cable and three-pin plug. The outer cover of three-pin plug is made out of plastic or rubber, these are both good electrical insulators. The pins of the plug are made of brass. This is a good electrical conductor. It is also hard and will not rust or oxidise.
There are three wires; the earth, neutral and live wire. The earth wire is connected to the longest pin. The earth wire is green and yellow, the live wire is brown and the neutral is blue.
Appliances with a metal case must be earthed, so that it does not become live and electrocute you. Cables can be different sizes, the thicker the cable needs to be, the more the current needed to be carried. It is important that the cable grip is fastened tightly over the cable. There should be no bare wires inside the plug and the correct pin must be connected to the terminal of the correct pin.
A fuse is always fitted in series with the live wire. This cuts the appliance off from the live wire if the fuse blows. If a fault develops in an earthed appliance, a large current flows to earth and melts the fuse disconnecting the supply.
The rating of the fuse should be higher than the normal working current of the appliance. If it is much higher, it will not melt soon enough. If it is not higher than normal, it will melt as soon as the appliance is turned on.
A circuit breaker can be used in place of a fuse. This is an electromagnetic switch that opens and cuts off the supply if the current is bigger than a certain value.
A residual current breaker (RCCB) cuts off the current in the love wire if it is different to the current in the neutral wire. It works faster than a fuse or an ordinary circuit breaker.
Electrical power and potential difference
An electrical appliance transfers electrical energy into other forms of energy, the rate at which it does it is called power. Power can be calculated by:
P = E P = power watts T = time seconds
T E = energy transferred joules
In an electric circuit it is more usual to measure the current through an appliance and the potential difference across it. We can work it out by:
P = I x V I = current amps V = potential difference volts
Electrical energy and charge
An electric current is the rate of flow of charge. You find charge by:
Q = I x t Q = charge C T = time seconds
I = current A
When charge flows through an appliance electrical energy is transferred to other forms. In a resistor electrical energy is transferred to the resistor so it becomes hotter.
The amount of energy transferred, can be calculated by:
E = V x Q
V = potential difference volts E = energy transferred joules
When a charge flows in a circuit the components will heat up. This means that most electrical appliances have vents to keep them cool.
Electrical faults may occur due to damage of sockets, plugs, cables or appliances.worn or damaged equipment should be replaced or repaired by a qualified electrician. Overloading sockets should be avoided as it can cause overheating and a risk of a fire. Electrical appliances should be handled safely and never used in a bathroom or with wet hands. The cable should be appropriate for the intended use. A two core cable can be used for a hair dryer as the case is plastic so it doesn't need to be earthed.
When choosing an electrical appliance the power and efficiency rating need to be considered as well as the cost. Filament bulbs and halogen bulbs are much less efficient than low energy bulbs. Filament bulbs are very inefficient because they transfer a lot of energy as heat rather than light.