Question

A potential difference of V volts is applied across the ends of a copper wire of diameter D and length l. What will be the effect on drift velocity of free electrons when (i)D is doubled ?

Answer 1

Drift velocity = v_drift

= I / (neA)

= J/(ne)  

=  V/(neA R)

= V/(neρl)

     I = current    

n = num of electrons per unit volume (constant for copper)

     

e = electron charge      

A = area of cross section = π D²/4

      R = resistance of wire      

ρ = resistivity      

  l = length of wire

Thus, 

i)  When D is doubled, drift velocity will be half.

hope it helps