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 Now nq vd is the current
density j and |(nq vd)|A is the current I (see Chapter 3 for the discussion
of current and current density)