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Asked in GUJCET 2011 · Dia, para and ferromagnetism
Idea: a diamagnetic sample magnetises against the applied field -- the induced moment ⃗m points opposite to ⃗B, because χ is small and negative.
The energy of a dipole in the field is U=-⃗m·⃗B. With ⃗m antiparallel to ⃗B this becomes U=+mB, which grows wherever the field is stronger.
A body is always pushed towards lower energy, so the sample is driven to where B is smaller.
Hence the resultant force on a diamagnetic specimen points from the stronger part of the field towards the weaker part; this is the feeble repulsion seen when a diamagnetic rod is hung between the poles of a magnet.
So the force is directed from the stronger to the weaker part of the field.
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