A magnetic needle is placed in a uniform magnetic field. It experienece

Explanation

a. In either case, one gets , each with a north and south pole.
b. No force if the field is uniform . The iron nail experiences a nonuniform field to the bar magnet. There is induced magnetic moment in the nail, therefore , it experiences both force and torque. The net force is attractive because the induced south pole (say) in the nail is closer to the north pole of magnet than induced north pole.
c. Not necessarily True only if the source of the field has a net nonzero magnetic moment . This is not so for a toroid or even for a straight infinite conductor.
d. Try bring different ends of the closer . A repulsive force in some situation establishes that both magnetised . If the is always attraction , then one of them is not magnestied . In a bar magnet the intensity of the magnetic field is the strongest at the two ends (poles) and weakest at the central region. This fact may be used to determine whether A or B is the magnet . In this case, to see which one of the two bars is a magnet , pick up one, (say,A) and lower one if its ends , first on one of the ends of the other (say, B) and then on the middle of B. If you notice that in the middle of B, A experiences no force , then B is magnetised If you do not notice any change from the end to the middle of B , then A is magnetised .