Define drift velocity, time of relaxation and current density. Current density is a vector quantity but current has both magnitude and direction still it is considered as scalar. Why?

Explain resistivity, show its variation with temperature for 

(i) conductors (ii) semi-conductors (iii) alloys.

A current of $1 \mathrm{A}$ is flowing through a conductor. How many electrons pass through any cross-section of the conductor per second ?