Electrical Engineering ⇒ Topic : Unit of resistance

Unit of resistance. The practical unit of resistance is ohm and is represented by the symbol Q.

It is defined as under :

A wire is said to have a resistance of 1 ohm if a p.d. of I volt across its ends causes 1 ampere to flow through it See Fig. (a).

                                          figure (a)

There is another way of defining ohm.

A wire is said to have a resistance of 1 ohm if it releases ]joule (or develops 0.24 calorie of heat) when a current of I A flows through it for I second.

A little reflection shows that second definition leads to the first definition. Thus 1 A current flowing for 1 second means that total charge flowing is Q=/x t=1 x 1=1 coulomb. Now the charge flowing between A and B (See Fig. 1.10) is 1 coulomb and energy released is 1 joule (or 0.24 calorie).

Obviously, by definition, p.d. between A and B should be 1 volt