(iii)
The magnetic force is zero if  charge is not
moving (as then |v|= 0) Only a moving
charge feels the magnetic force The expression for the magnetic force helps
us to define the unit of the magnetic field, if one
takes q, F and v, all to be unity in the force
equation F = q [ v × B] =q v B sin q ˆn ,  where q is
the angle between v and B [see  Fig 4