Between collisions electrons move in straight line (in the absence of a any electromagnetic field) - effect of electron-electron interaction is ignored (independent electron approximation – reasonably valid) – effect of electron-ion in ignored (independent-electron approximation; completely invalid). ;(probability of collision per unit time is 1/
2. Mean free time between collisions is is probability of having a collision in infinitesimal time interval dt is dt/ independent of electron’s position or velocity (a good assumption).
3. Electrons achieve thermal equilibrium by collisions with lattice – they emerge after collision at a random direction with speed appropriate to the temperature of the region where collision happened – the hotter the region; the higher the speed of the emerging electrons