Why resistance of a metallic conductor increases with increase of temperature?

Explanation

If the temperature of metal conductor is raised , the ions/atoms of the metal vibrate with greater amplitudes and greater frequenceis about their mean position .
Due to increase in thermal energy , the frequence of collisiong of free electrons with atoms/ions while drifiting towards positive end of the c onductor will also increase which reduces the average relaxation time `tau` . But the number density (n) of electrons in metal conductor does not change appreciably with temperature . So , the value of electrical resistance R increase with the rise of temperature .