ALTERNATING CURRENT P2 6.1
Cells and batteries supply current that passes round the circuit in one direction. This is called direct current (D.C.)
The current from the mains supply passes in one direction the reverses and travels in the other direction. This is called alternating current (A.C.)
The frequency of the UK main supply is 50 Hertz. I alternates current direction 50 times each second. The voltage of the mains is 230V.
The live wire of the mains supply alternates between positive and negative potential difference. The neutral wire stays at 0V. The live wire alternates between +325 and -325 Volts.
CABLES AND PLUGS P2 6.2
Mains electricity can be dangerous:
- Overlong frayed cables
- Using electricity near sources of heat or water
- Overloading sockets with too many adaptors and plugs
- Cables consist of three insulated copper wires insulated by flexible plastic
- Sockets and plugs are made of stiff plastics which enclose electrical connections
- Live brown wire connected to the live pin connected to the fuse
- Neutral blue wire connected to the neutral pin
- Earth green and yellow wire is connected to the earth pin
- Appliances must be earthed if metal cases
FUSES P2 6.3
Appliances must have a fuse. Otherwise if the earth wire is not in a metal case and a fault happens the live wire touches the metal case and becomes live giving a shock to anyone who touches it.
If a fault develops in an earthes appliance a large current flow to earth and melts the fuse disconnecting the supply.
A fuse must be put in the live wire so is that if it melts it cuts of the supply. The rating of a fuse must be slightly higher than the normal working current of the appliance. If it is much higher it will not melt soon enough is it is not enough the normal current will melt it.
A circuit breaker can be used i 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.
ELECTRICAL POWER AND POTNETIAL DIFFERENCE P2 6.4
Power is when an electrical device transforms electrical energy to other forms and transfers energy from one place to another.
Power (Watts W) = energy transformed (J) / time (s)
Power = current (A) x PD (V)
Electrical appliance have their power shown on them.
ELECTRICAL ENERGY AND CHARGE P2 6.5
When charge flows through an appliance, electrical energy is transformed to other forms e.g. in a resistor to heat.
Energy transformed (J) = potential difference (V) x Charge (Coulombs)
Charge (C) = current (A) x time (S)