Text 1. Practical units

The main unitsin electrical engineeringare those relating to current, pressureor voltage, resistance, powerand energy.

Current is that which flows along the conductors forming the electric circuit. It is measured in amperes. Pressure, potential, voltage, or electromotiveforce is that which causes a current to flow between two points when they arejoinedby a conductor. The unit is the volt.

The total pressure generated by a cellor generator is called its electromotive force (e.m.f.). The difference in pressure between any two points in a circuit is simply known as the potential difference, voltage, or pressure of the circuit. The opposition which a substanceoffersto the flow of current through it is called its resistance. Substances having a small resistance, such as metals and most liquids, are called conductors, those offering a high resistance are called insulators. The unit of resistance is the ohm, represented by the Greek letter fi. A megohm equals one million ohms.

When resistances are connected insuccessionto form a circuit, they are said to be connected in series. The total resistance of such a circuit is the sum of all the resistances. Resistances connected to the same terminals are said to be inparallel.

In a circuit in which a steady directcurrentis flowing there is a direct relationbetween the current, voltage, and resistance, temperature remaining constant, and this is expressed by what is known as Ohm's law.

The law is represented by the following equation:

I – current in amperes R – resistance in ohms E – voltage in volts

The power in a d. c. circuit is found from the productof the amperes flowing in it and the pressure at its terminals. The unit of power is the watt. It is the power in a circuit when a current of one ampere flows under a pressure of one volt. The practical unit of electrical energy is the kilowatt-hour (kW/h). It is the energy transformedin a circuit when the power is one kilowatt and the time taken is one hour. In general practice this valueis spoken of as a unit, and is the basis of charges for electrical energy.