P2.4 Using Mains Electricity Safely And The Power Of Electrical Appliances

P2.4 Using Mains Electricity Safely And The Power Of Electrical Appliances

1. P2.4.1 Household Electricity

   • Cells and batteries supply current that always passes in the same direction. This is called direct current (d.c.). An alternating current (a.c.) is one that is constantly changing direction.
     • Mains electricity is an a.c. supply. In the UK it has a frequency of 50 cycles per second (50 hertz) and is about 230 V.
       • Most electrical appliances are connected to the mains using cable and a three-pin plug.
         • A mains electricity cable contains two or three inner wires. Each has a core of copper, because copper is a good conductor of electricity. The outer layers are flexible plastic, because plastic is a good electrical insulator. The inner wires are colour coded: blue is neutral, brown is live and green and yellow stripes is the earth wire.
           • The case of a plug is made from tough plastic or rubber, because these materials are good electrical insulators. The three pins are made from brass, which is a good conductor of electricity. There is a fuse between the live terminal and the live pin. The fuse breaks the circuit if too much current flows. The cable is secured in the plug by a cable grip. This should grip the cable itself, and not the individual wires inside it. The blue neutral wire goes to the left, the brown live wire to the right and the green and yellow striped earth wire is on top. The fuse fits next to the live wire.
             • When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit. Some circuits are protected by Residual Current Circuit Breakers (RCCBs). RCCBs operate by detecting a difference in the current between the live and neutral wires.
               • Appliances with metal cases are usually earthed. Some appliances are double insulated, and therefore have no earth wire connection. The earth wire and fuse together protect the wiring of the circuit.

2. P2.4.2 Current, Charge And Power

   • When an electrical charge flows through a resistor, the resistor gets hot.
     • The rate at which energy is transferred by an appliance is called the power. P=Et where P is power in watts, W, E is energy in joules, J and t is time in seconds, s.
       • P=I×V where P is power in watts, W, I is current in amperes (amps), A and V is potential difference in volts, V.
         • E=V×Q where E is energy in joules, J, V is potential difference in volts, V and Q is charge in coulombs, C.