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A bar magnet of length l, pole strength p and magnetic moment m is cut into two equal pieces, each of length l/2. The magnetic moment and pole strength of each piece is respectively ______ and _____.

Asked in GSEB Board March 2018 · Magnetic moment of magnets and loops

• (1) m and p/2
• (2) m/2 and p
• (3) m/2 and p/2
• (4) m and p

Step-by-step solution

Why the other options are wrong

• (1) The two are the wrong way round: a cut across the magnet leaves the pole strength untouched and halves the moment, because it is the length that halves.
• (3) Halves the pole strength too. A transverse cut does not touch the pole faces - each piece keeps the full cross-section and the same magnetisation - so p is unchanged.
• (4) If nothing changed, one magnet would yield two pieces each as strong as the original and the total moment would double. With m=p l and each piece of length l/2, each moment must be m/2.

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