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P5: Electricity in the home (5.2: Cables and plugs (Most electrical…
P5: Electricity in the home
5.1: Alternating current
Cells and batteries supply current that passes round the circuit in one direction only - this is called direct current (d.c.)
The current from the mains supply repeatedly reverses its direction - this is called alternating current
Frequency is the rate at which and alternating current reverses its direction - the frequency of the UK mains supply is 50 hertz (Hz), which means that mains current reverses its direction of flow 50 times each second
If you measure the time taken for the current to complete one full cycle, you can calculate the frequency using the equation: frequency = 1/time taken for 1 cycle
The live wire alternates between peak p.d.s of +325V and -325V - in terms of electrical power, this is equivalent to a direct current power supply with a p.d. of 230V
The national grid is a system of cables and transformers linking power stations to consumers
Step-up transformers are used to increase the p.d. from the power station to the transmission cables
Step-down transformers are used to decrease the p.d. to a much lower value for domestic use
5.2: Cables and plugs
Most electrical appliances are connected to the sockets of the mains supply using a cable and a three-pin plug
The outer cover of a three-pin plug and the outer casing of sockets are made from a stiff plastic material that is a good insulator
The pins of the plug are made of brass - brass is a good electrical conductor - it is also hard and will not rust or oxidise
The green and yellow striped wire (of a three-core cable) is connected to the earth pin - a two-core cable doesn't have an earth wire
The brown wire is connected to the live pin and the blue wire is connected to the neutral pin
The outer coverings of the wires are made from a flexible plastic material
The plug contains a fuse between the live pin and the live wire - if the current in the fuse becomes too big, it will melt and cut off the current to the live wire
Appliances with a metal case must be earthed - the case is attached to the earth wire in the cable, which stops the metal case becoming live if the live wire breaks and touches the case
Appliances with plastic cases do not need to be earthed - they are said to be double insulated, and are connected to the supply with a two-core cable containing just a live wire and a neutral wire
Cables of different thicknesses are used for different purposes - the larger the current to be carried, the thicker the cable needs to be