Let the number
density of these mobile charge carriers in it be n Then the total number
of mobile charge carriers in it is nlA For a steady current I in this
conducting rod, we may assume that each mobile carrier has an average
drift velocity vd (see Chapter 3) In the presence of an external magnetic
field B, the force on these carriers is:
F = (nlA)q vd ´´´´´ B
where q is the value of the charge on  a carrier