Question

Why electrical conductivity decreases when the temperature increases?

Answer 1

The thermal conductivity of any material is dependent on two things:
i. The motion of free electrons
ii. Molecular vibrations
For metals, the thermal conductivity is mainly a function of the motion of free electrons. As the temperature increases, the molecular vibrations increase (in turn increasing the mean free path of molecules). So, they obstruct the flow of free electrons, thus reducing the conductivity.
In case of non-metals, there are no free electrons. So, only the molecular vibrations are responsible for conduction of heat and hence for non-metals, the conductivity increases with increase in temperature.