ABSTRACT
In simple terms, electricity is the flow or movement of electrons through a substance which allows the transfer of electrical energy from one position to another. The substance through which the electricity flows is called a conductor. This flow or movement of electrons is known as the electric current. There are two forms of electric current – direct and alternating. Direct current (dc)
involves the flow of electrons along a conductor from one end to the other. This type of current is mainly restricted to batteries, dynamos and similar devices. Alternating current (ac) is produced by a rotating alternator and causes an oscillation of the electrons rather than a flow of electrons so that energy is passed from one electron to the adjacent one and so on through the length of the conductor. It is sometimes easier to understand the basic princi-
ples of electricity by comparing its movement with that of water in a pipe flowing downhill. The flow of water through the pipe (measured in litres per second) is similar to the current flowing through the conductor which is measured in amperes, normally abbreviated to amps (A). Sometimes very small currents are used and these are measured in milliamps (mA). The higher the pressure drop is along the pipeline, the
greater will be the flow rate of water and, in a similar way, the higher the electrical ‘pressure difference’ along the conductor, the higher the current will be. This electrical ‘pressure difference’ or potential difference is measured in volts (V). The flow rate through the pipe will also vary for a fixed
pressure drop as the roughness on the inside surface of the pipe varies – the rougher the surface, the slower the flow and the higher the resistance to flow becomes. Similarly, for electricity, the poorer the conductor, the higher the resistance is to electrical current and the lower the current becomes. Electrical resistance is measured in ohms. The voltage (V), the current (I) and the resistance (R) are
related by the following formula, known as Ohm’s law:
V = I × R (volts)
and, electrical power (P) is given by:
P = V × I (watts)
These basic formulae enable simple calculations to be made so that, for example, the correct size of fuse for a particular piece of electrical equipment can be ascertained.
