# Resistivity dependance on temperature

I'm just starting to study electricity and magnetism. The resistivity $\rho$ of a conductor is defined as

$$\rho = \frac {\mathbf E}{\mathbf j},$$

where $\mathbf E$ is the electric field and $\mathbf j = \sum_in_iq_i\mathbf v_i$ is the current density ($n$= concentration of carriers, $q$ their charges and $\mathbf v$ their velocities). I know that there's a linear approximation of $\rho(T)$, $T$ the temperature of the conductor, that says the resistivity rises when the temperature rises; but how from the definition of $\mathbf j$ can I say that an increase of the temperature means an increase in $\rho$?