When a potential difference is applied across a conductor. then a electric field is formed across it. There are free electrons in conductor which should accelerate because of there is electric field. Since all we know that Force =charge*Electric_field. But it is not so. Inside a conductor electrons even not able to reach speed of a meter per second. Why it is so?Why not they accelerate? When this thought come in my mind, firstly i think that there is inelastic collision between the electrons that causes heating of conductors and maintains constant speed of electrons. But collisions between the electrons is elastic and elastic collisions doesn't convert mechanical energy to heat. So finally we can say Center of mass of all the electrons should accelerate but they move with constant speed. that's all i want to say. Thanks if you reply.