Model

Three years after the discovery of the electron by J. J. Thomson, Drude applied the kinetic theory of the gases to a metal, considered as a gas of electrons. The basic assumptions are:

• When metallic elements are brought together to form a solid, then the valence electrons become freely moving through the metal (conduction electrons), whereas the ions remain intact as nearly immobile positive particles.
• Between collisions the electrons move freely, i. e. there is neither electron-electron interaction (independent electron approximation) nor electron-ion interaction (free electron approximation).
• There is a scattering mechanism leading to instantaneous collisions of the electrons with the scatterers (ions, electrons).
• The collisions are characterized by a relaxation time ; the collision probability per unit time is (relaxation time approximation).
• The electrons achieve thermal equilibrium only through collisions; the velocity after a collision is randomly directed, the speed is as appropriate to the temperature.