Electrical Engineering ⇒ Topic : Resistance

Resistance

The unit of electrical resistance is ohm. Its symbols is Ω only. Ohm is defined as the resistance between two points of a conductor when a potential difference of 1 volt, applied between these points, produces in this conductor a current of 1 ampere, the conductor not being a source of any electromotive force.

Alternatively, ohm can be defined as "that resistance in which a current of 1 ampere, flowing for 1 second, generates 1 joule of thermal energy". Hence, if a current of 1 ampere flows through a circuit having a resistance of R ohms for t seconds,

                                   Heat energy generated = I²Rt joules                ............ (1)

Resistance

    The absolute value of ohm can also be determined by means of a copper disc rotated in a uniform magnetic field. Both the above methods require very elaborateand expensive equipment and therefore, are carried out very infrequently.Normally, use is made of the standard resistance in the form of manganin wire resistors and a voltage standard in the form of Weston cells.

         Manganin wire resistors are made of an alloy of copper, nickel and manganese,with sometimes, a trace of iron. The temperature coefficient of resistance of manganin is very low (0.000003 per degree) and hence its resistance value remains largely unchanged

Resistance

The opposition offered by a substance to the flow of electric current is called its resistance.Since current is the flow of free electrons, resistance is the opposition offered by the substance to the flow of free electrons. This opposition occurs because atoms and molecules of the substance obstruct the flow of these electrons. Certain substances (e.g. metals such as silver, copper, aluminium etc.) offer very little opposition to the flow of electric current and are called conductors. On the other hand, those substances which offer high opposition to the flow of electric current (i.e. flow of free electrons) are called insulators e.g. glass, rubber, mica, dry wood etc.

It may be noted here that resistance is the electric friction offered by the substance and causes production of heat with the flow of electric current. The moving electrons collide with atoms or molecules of the substance ; each collision resulting in the liberation of minute quantity of heat