(i) Why do the 'free electrons', in a metal wire, 'flowing by themselves', not cause any current flow in the wire ? Define 'drift velocity' and obtain an expression for the current flowing in a wire, in terms of the 'drift velocity' of the free electrons. (ii) Use the above expression to show that the 'resistivity', of the material of a wire, is inversely proportional to the 'relaxation time' for the 'free electrons' in the metal.