Ans. Usually a metallic conductor has a crystalline structure in which its various atoms in the form of positive ions are fixed at their respective places. Free electrons move in the spaces between these atoms. When the current follows through the conductor, the opposition to the flow of current is caused by the collision of free electrons w ;h the atoms of the conductor. More frequently electrons collide with the atoms; the larger is the opposition or resistance in the flow of current.
When the temperature of the conductor rises, average speed of the random motion of the free electrons increases, which enhance the rate of collisions of electrons and the atoms. This causes an increase in the resistance of the conductor.