Principles of electricity and some definitions

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 and similar devices. Alternating current (ac) 

It is sometimes easier to understand the basic principies of eiectricity by comparing its movement with that of water in a pipe fiowing downhill. The flow rate of water through the pipe (measured in litres/s) is similar to the current flowing through the conductor which is measured in amperes, normally abbreviated to amps. 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. 

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 currertt and the lower the current becomes. Electrical resistance is measured in ohm s. 

These basic formulae enable simple calculations to be made so that, for example, the correct size of fuse may be ascertained for a particular piece of electrical 

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