The north pole of a permanent magnet is held 8 mm from one end of a soft-iron rod, and the rod is attracted. (a) State the induced pole at the near end and explain the attraction. (b) Explain why attraction alone does not prove the rod was already a permanent magnet. (c) Describe a stronger test for permanent magnetism and a suitable material for a permanent magnet.

Question

Accepted Answer

Near end becomes south; induced unlike poles attract. Repulsion by a known pole identifies a magnetic pole; steel is suitable for retaining magnetism. The external magnetic field magnetises the soft iron by induction. Its end nearest the magnet becomes a south pole, opposite to the approaching north pole, so the two nearby ends attract. Attraction is not sufficient evidence that the rod originally had permanent poles. An initially unmagnetised magnetic material is also attracted because the applied field induces opposite polarity at the near end. Test each end of the object against a known permanent-magnet pole. Definite repulsion is stronger evidence of an existing like pole, because an unmagnetised soft-iron object would acquire a nearby unlike pole and be attracted instead. Steel is suitable when magnetism needs to be retained; soft iron is useful for temporary cores because it is readily magnetised and loses much of its magnetism when the external field is removed. Keep the test magnet away from electronic devices and magnetic storage.

Testing an unlabelled magnetic object 7

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