Question

If a magnetic needle is fastened to a cork and then lowered into water, then, under the action of the magnetic field of the earth, the needle will turn and point along the magnetic meridian, but it will not move to the north or south. If a bar magnet is placed not far from the needle, the needle will not only turn in the direction of the lines of force, but it will begin moving towards the magnet. What is the reason for the different behaviour of the needle in the magnetic fields of the earth and the magnet? 

Answer 1

The terrestrial magnetic field is practicality constant over a distance equal to the length of the magnetic needle, i.e., its field intensity remains constant in magnitude and direction. Therefore the terrestrial magnetic field acting on the needle can produce only a rotational moment, but it cannot give a resultant force different from zero.

The field of a permanent magnet varies greatly over a distance equal to the length of the needle. The field intensity at one end of the needle is larger than at the other. Therefore the field of the magnet, when acting on the needle, gives a resultant force different from zero and causes not only rotation, but also translational motion of the needle.