The motion of the electrons around a circuit determines whether we say the current is direct or alternating. If the electrons start out at the negative side of the circuit and move steadily around the circuit to the positive side without changing direction then we call this 'direct current'. If the electrons continually change direction backwards and forwards because the positive and negative sides of the circuit keep changing for some reason then we call this 'alternating current'. Alternating current has a frequency. This is how many times the current charges direction every second. In the UK this is 50 Hz (50 cycles per second) and in the US, Canada and some other countries it is 60 Hz.
The live wire in the domestic mains supply alternates between being a high positive and negative voltage. This gives rise to mains electricity being alternating current. The neutral wire never changes and is always at zero volts. Electric current normally flows in and out of these two wires.
The earth wire works in conjunction with the fuse and/or a circuit breaker to provide a safety mechanism in the event that there is a fault in the circuit. The end of the earth wire which is not connected to the plug is connected to any exposed metal parts of the appliance. If the appliance has a metal casing this is where the earth wire is attached. If there is no metal casing then there will be no earth wire and the appliance is said to be double insulated.
If for some reason the live wire were to become detached inside the appliance and touch the metal casing of an appliance this could be potentially dangerous if someone was to touch the case. Instead the earth wire carries this electricity down to earth. In the process of doing this a very large current is produced in the live wire. If a fuse is fitted the fuse will get hot because of this high current and will melt. Because the fuse forms part of the circuit from the live wire the electricity will stop flowing when the fuse melts as the circuit will be broken. This then removes the danger of the case being a danger to anyone touching it.
Circuit breakers work in a similar way but they monitor the amount of current flowing in the earth wire. If this reaches above a pre-set amount then this indicates there is a fault. The circuit breaker will switch off the mains supply. A circuit breaker can be part of the domestic electricity installation at the distribution board or can be a separate unit that is plugged into a mains socket between the socket and the appliance being used.
When something like a resistor opposes the flow of electrons in an electric current then the resistor gets warm or even hot.
Some times this is useful to us and is used in electric fires, showers (for heating water), electric cookers, hair dryers, water heaters, kettles etc... This heating effect also produces enough heat in a filament light bulb for it to give out light.
The rate at which energy is transferred in a component or appliance is called the power rating. In your home you will have some sort of electricity meter. This allows the electricity company to measure how much electrical energy you are using in all of your appliances. however, you can work our for yourself how much electrical energy a particular appliance is using if you know its power rating. Electricity companies use a special unit of energy called a kilowatt hour (kWh). This is equivalent to a 1 kW (1000W) appliance being used continuously for one hour. The kWh may also be referred to as a 'unit' of electricity on your electricity bill. The electricity company then decide how much to charge for a kWh of energy and work out your bill by multiplying the number of kWh used by the charge per kWh.
To work out how many kWh an appliance is using you can do the following. For example, a 100W light bulb uses energy at the rate of 100W or 0.1kW. If it is switched on for 20 hours it will use 0.1 x 20 kWh. This is 2 kWh. If you are charged 7 pence per kWh then this would cost you 14p.
