The reciprocal quantity of resistance is called conductance. Like that, the reciprocal quantity of specific resistance is called conductivity. Conductivity is expressed by the letter σ. The value of σ depends on the type of material of the conductor and its temperature.
Say, the specific resistance of the material of a conductor = ρ
Therefore, the conductivity of its material is, σ = 1/ρ
As unit of ρ is Ω m, Therefore, the unit of σ is (Ω m)-1.
Example: The specific resistance of the nichrome wire used in an electrical heater is 100 x 10-8 Ω m. What will be the resistance of 15 m long wire having cross sectional area 2.0 x 10-7 m2?
Specific resistance, ρ =100 x 10-8 Ωm
Cross sectional area, A = 2.0 x 10-7 m2
Length of the wire, L = 15 m
Resistance, R = ?
R = ρ (L/A)
[(100 x 10-8 Ωm) * (15 m)] / (2.0 x 10-7 m2)
= 75 Ω (Ans)