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My textbook says that Diamagnetic substances align their longest axis perpendicular to the field in uniform magnetic field while paramagnetic substances align their longest axis parallel to the field.

The paramagnetic case makes sense because the torque on the material would be zero when it is inclined along the magnetic field.

But why is it not in the case of diamagnetic materials?

All I could find on net was this PDF.

But the math in the PDF is too complex for me to understand. Is there any simple intuitive explanation for the fact?

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  • $\begingroup$ I think a good solution would have to account for interactions, any simplified independent magnetic moments approximation will fail $\endgroup$
    – Mauricio
    Commented May 19, 2022 at 17:21
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    $\begingroup$ February 2023: Link does not load: The connection has timed out. The server at www.physics.princeton.edu is taking too long to respond. $\endgroup$
    – Qmechanic ♦
    Commented Feb 10, 2023 at 12:00

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Well, I searched for the answer on some websites and found this -

In case of diamagnetic substances, their atoms have now per moment dipole moment. When kept in external magnetic field, flux through electron loop orbits change and to oppose the change of flux electrons speed up or down inducing a net dispolemet whose direction is dictated by Lenz's law and it must be in the direction to oppose the change of flux and induced dipoles align opposite to applied field. [Not per perpendicular to the applied field].

But if you want a more simpler answer then, my answer may help you ( I hope so ! ).

Diamagnetic Substances are feebly repelled, not attracted, by the external field gradients and therefore they align themselves in such a way that the repulsion offered by the poles on the substance is maximum and thus, they align themselves perpendicular to the field.

Note : While I was searching for the answer, I also found on some websites that this observation is experimental.

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    $\begingroup$ Welcome to Physics SE! Please add a reference to the website you have quoted from. $\endgroup$
    – Dodo
    Commented Apr 17, 2022 at 18:50
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it is because diamagnetic substances are weakly repelled by external magnetic field therefore they align themselves away from the poles where repulsion is maximum and does align themselves perpendicular to the fields between the poles.

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  • $\begingroup$ diamagnetic or paramagnetic objects are repelled or attracted by field gradients, not by fields. $\endgroup$
    – Roger Wood
    Commented Mar 12, 2021 at 6:46

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