Practical units

The main units in electrical engineering are those relat­ing to current, pressure or voltage, resistance, power and energy.

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

The total pressure generated by a cell or 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 substance offers to 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 high resistances are called insulators. The unit of resistance is the ohm, represented by the Greek letter Ω. A megohm equals one million ohms.

When resistances are connected in succession to 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 in parallel.

In a circuit in which a steady direct current is flowing there is a direct relation between 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. с. circuit is found from the product of 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 (kwh). It is the energy transformed in a circuit when the power is one kilowatt and the time taken is one hour. In general practice this value is spoken of as a unit, and is the basis of charges for electrical energy.