Mains Electricity
Just think that around 150 years ago the light bulb didn’t exist and now we’re living in towns and cities coursing with electricity, with places like Times Square and Las Vegas glistening with neon signs and billboards 24/7. Electricity can be dangerous so safety features are included on all household appliances to ensure that they are safe to use.
Safety features for household appliances
The following features are present in most domestic appliances to reduce the chance of somebody experiencing an electric shock:
Double insulation: all wires are insulated with a plastic coating but some appliances are also made of an insulating material, such as kettles and electric power drills. Even if the wire becomes loose and touches the outer casing, there is no chance of an electric shock.
Earthing: Anything that has a metal case, such as cookers, washing machines and refigerators, must be earthed otherwise you would receive an electric shock if the live wire becomes loose and touches the metal casing. An earth wire is connected to the metal case and provides a path of very low resistance for the electricity to pass through. The surge of electricity through the earth wire causes the fuse to break.
Fuses: If there is a fault in the circuit, too much current may flow through. Fuses consist of a wire which melts if the current passing through it exceeds a threshold level, preventing the device from overheating and causing a fire. Fuses are made in standard ratings (commonly 3 A, 5 A and 13 A) and it is important that you use a fuse only slightly higher than the device requires. Fuses often have to be replaced after they have blown.
Circuit breakers: these are similar to fuses but can be easily reset so do not need to be replaced.
Conversion of electrical energy to heat
When a current travels through a wire some of the electrical energy is converted into heat energy. This happens because as the electrons pass through the wire, they collide with ions in the wire and cause them to vibrate more. The increased vibration causes the temperature of the wire to increase. This can be harnessed in appliances such as kettles, toasters and hairdryers which contain wires with a high resistance, so that more electrical energy is converted into heat energy.
Power, current and voltage
Current is the rate of flow of electric charge (i.e. how many electrons are flowing through the wire in a certain amount of time). It is measured in Amps using an ammeter.
Voltage (also referred to as potential difference) is a measure of how much energy is transferred between two points in a circuit. It is measured in Volts using a voltmeter.
Power is the rate at which electrical energy is converted into other forms. For example, a washing machine with high power will convert electrical energy into kinetic energy much faster than one with low power. Power is measured in Watts.
Current, power and voltage are linked in the following equation:
Worked example: calculating power from current and voltage
A toaster uses 15 A of current when connected to a 230 V mains supply. Calculate its power and remember to include units.
Power = current x voltage
Power = 15 x 230 = 3,450 W
If we know the power of an appliance and the time taken to transfer electrical energy into another form, then we can calculate the amount of energy transferred in a given time. To do so we use the following equation:
Since we know that power is equal to current multiplied by voltage, this is the same as saying:
Worked example
A radio uses a current of 50 A when connected to a 230 V mains supply. Calculate the energy transferred, in joules, if the radio has been switched on for one hour.
Energy transferred = current (A) x voltage (V) x time (s)
Convert time into seconds by multiplying 60 mins by 60 seconds = 3600 s
Energy transferred = 50 x 230 x 3600 = 41,400,000 J
Direct current (DC) vs alternating current (AC)
Direct current means that the electrons are flowing through a wire in one direction only whereas in alternating current, the electrons are constantly changing direction. AC can be more efficiently transmitted over long distances, so AC current is used in the mains electricity supply whereas simple batteries will use a direct current. If you plotted a voltage-time graph of DC and AC current, the DC current would look like a straight line whereas AC would alternate between positive and negative values.