Ans. As resistance is directly proportional to the length of the conductor and inversely proportional to the area of cross-section of the conductor, which can be
written as
i Roc L ...... 1
and I
By combining these two equations, we get
L or R = pT- ...... (iii)
where p is the constant of proportionality, known as specific resistance. Its value depends upon the nature of the conductor. For example, copper, iron, tin, and silver would each have a different values of p.
If we put L = 1m, and A = 1m2 in Eq. (iii), then R = p, i.e. specific resistance or resistivity in defined as:
The resistance of one metre cube of a substance is equal to its specific resistance or resistivity.
Unit: The SI unit of specific resistance is ohm-meter (Qm)