Average thermal velocity of free electrons in any direction remains zero .
Drift Velocity of Free Electrons
When a potential difference is applied across the ends of a conductor , the free electrons in it move with an average velocity opposite to direction of electric field . which is called drift velocity of free electrons .
Drift velocity vd = eEτ / m = eVτ / ml where , τ = relaxation time , e = charge on electron , E = electric field intensity , 1 = length of the conductor ,
V = potential difference across the ends of the conductor m = mass of electron . Relation between electric current and drift velocity is given by vd = I / An e
Mobility
The drift velocity of electron per unit electric field applied is mobility of electron . Mobility of electron ( μ ) = vd / E Its SI unit is m 2 s -1 V -1 and its dimensional formula is [ M -1 T 2 A ].
Ohm ’ s Law
If physical conditions of a conductor such as temperature remains unchanged , then the electric current ( I ) flowing through the conductor is directly proportional to the potential difference ( V ) applied across its ends .
I ∝ V or V = IR
where R is the electrical resistance of the conductor and R = Ane 2 τ / ml .
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