On what factors does the resistivity of a material depend ?

Explanation

The resistivity of conductor is given by
`rho=(m)/(n e^(2)r)`
So (i) `rhoprop(1)/(n)` (n is number of electrons per unit volume)
(ii) `rhoprop(1)/(tau)` (`tau` is average relaxation time).
And n depends upon nature of material and `tau` depends upon the temperature of material.
Since copper is good conductor of electricity and hence its resistance is very low, so copper wires are used as connecting wires so that they do not get heated when currennt is passed through them.