Derive Expression for Drift Velocity | Define Relaxation time or mean free time.
In a metal Conductor, there are extremely large number of free electrons. These free electrons move randomly with a thermal speed of the order of $10^{5}$ to $10^{6}$ m/s at room temperature.Im any portion of the conductor, the flow of electrons is so oriented that the average thermal velocity of total number of free electrons in a conductor is zero. That is $U_{av}= \frac{u_1+u_2+u_3+u_4+u_5....u_6}{n}$ $U_{av} = 0$ Where $u_1 , u_2 , u_3 , u_4 ...... u_n$ are thermal velocities of free electrons and n is the total number of free electrons. When an electric field is applied across the conductor , the free electrons accelerate in a direction opposite to the direction of the applied field. Due to this acceleration, the electrons gain extra velocity but for a short time because the accelerated electrons collide with the ions in the conductor and during this collision , the extra velocity gained is destroyed . Again , the electron is Accelerated and comes to rest after collisio...