● Showing the plot of variation of resistivity ● Expression for resistivity ● Explaining variation of resistivity for conductor and semiconductor ● $ρ=\;\frac{m}{n e^2 \tau}$ ● In case of conductors with increase in temperature, relaxation time decreases, so resistivity increases. ● In case of semiconductors with increase in temperature, number density $(n)$ of free electrons increases, hence resistivity decreases.