General Introduction:

Ohm's law: If physical conditions such as temperature, stress, strain remains unchanged in the materials, then potential difference (V) across two ends of a conductor is proportional to current (I) flowing through a conductor, i.e.,

           V ∝ I (or)

           V = I R

The constant of proportionality, R, is called resistance of the material.

Resistivity:

At a given constant temperature, the resistance R of the conductor is (i) proportional to its length (L) and (ii) inversely proportional to its area of cross-section (A), i.e.,

           R ∝ L / A (or)

           R = ρ L / A

The constant of proportionality, ρ, is called resistivity of the material, which is defined as the resistance offered by a wire of this materials of unit length and unit cross-sectional area.

Units of resistance and resistivity are Ohm (Ω) and Ohm-meter (Ω -m), respectively.