(a) Define the term 'conductivity' of a metallic wire. Write its SI unit.
(b) Using the concept of free electrons in a conductor, derive the expression for the conductivity of a wire in te'rms of number density and relaxation time. Hence obtain the relation between current density and the applied electric field E.

Explanation

Conductivity of the substance signifies tendency to move the charge (electrons). It is inverse of resistivity. It's SI unit is `Omega^(-1)m^(-1) or Sm^(-1)`.