[QUE/TH-13002] TH-PROBLEM

a) An ion of mass m and electric charge e is moving in a dilute gas of molecules with which it collides. The mean time between collisions is $\tau$. Let there be a uniform electric field $E$ along the x-axis. Show that the mean distance travelled by the ion is
$$ \frac{Ee}{m}\tau^2$$
assuming the velocity of the ion is zero immediately after collision.