Derive an expression for current in terms of drift velocity. (Derive I = nAev)

(This is being posted as per request from students)

Consider a conductor of length l and area of cross section A. Let the number of free electrons per unit volume be n. When a pd V is applied, let the drift velocity of electrons be v.

The total no. of free electrons in the piece of conductor = nAl (no. of free electrons per unit volume x volume)

Total charge due to free electrons; q  = nAle

The time taken by this charge to completely traverse the conductor, t =l/v

Therefore, the current in the conductor, I=q/t = nAle/(l/v) = nAev

i.e; I=nAev



  1. Paper is very lengthy. Hence no time for revision.
    some of the questions are unexpexted.
    Left dissatisfaction to the brilliant and hard working students since they lost centum in physics.