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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