Every mains circuit has a live wire and a neutral wire. The current through a mains appliance alternates because the mains supply provides an alternating potential difference between the two wires. An alternating current repeatedly reverses its direction. It flows one way then the opposite way in successive cycles. Its frequency is the number of cycles it passes through each second.
The neutral wire is earthed, the live wire is dangerous as it alternates between +325 volts and -325 volts. Each cycle takes 0.02 second. The UK mains supply is about 230V with a frequency of 50Hz (50 cycles per second).
Alternating Current (cont.)
The potential difference of the live terminal varies between a large positive value and a large negative value. However, the neutral terminal is at a potential difference close to earth, which is zero. In the diagram, the red trace is the live terminal and the blue trace the neutral terminal. Although the mean voltage of the mains supply is about 230V, the peak voltage is higher.
Direct current is the current that flows in only one direction. Batteries and cells supply DC electricity with a typical battery supplying around 1.5V.
Cables and Plugs
A mains electricity cable usually contains three inner wires. Each has a core of copper because copper is a good conductor of electricity.
- Neutral wire – Blue
- Live wire – Brown
- Earth wire – Green and yellow stripes
Features of a plug:
- Case is made from tough plastic or rubber – good insulators
- Three pins are made from brass – good conductor
- Fuse between live terminal and live pin – breaks circuit if too much current flows
- Cable secured in the plug by cable grip
Cables and Plugs (cont.)
Fuses and Circuit Breakers
Fuses protect electrical circuits and appliances. They break the circuit if a fault in an appliance causes too much current flow. A fuse contains a piece of wire which melts easily. Fuses in plugs are made in standard ratings, most common are: 3A, 5A, and 13A. The fuse should be rated at a slightly higher current than the device needs.
A circuit breaker is an electromagnetic switch that detects a difference in the current between the live and neutral wires, it opens and cuts the current off if the current is greater than a certain value. It can then be reset once the fault that made it trip has been put right.
Electrical Power and Potential Difference
When you use an electrical appliance, it transforms electrical energy into other forms of energy. The power of the appliance, in watts, is the energy it transforms, in joules, per second.
Power (watts, W) = Energy Transformed (joules, J) ÷ Time (seconds, s)
In an electric circuit it is more usual to measure the current through a device and the potential difference across it.
Power (watts, W) = Current (amperes, A) x Potential Difference (volts, V)
Electrical Energy and Charge
When charge flows through an appliance, electrical energy is transformed to other forms. In a resistor electrical energy is transformed into heat.
Energy transformed (joules, J) = Potential Difference (volts, V) x Charge (coulombs, C)
When there is a current of one amp for one second the charge flowing is one coulomb.
Charge (coulombs, C) = Current (ampere, A) x Time (Seconds, s)