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Suppose we have a magnetic material that is well insulated from its environment, and we then ramp up the current in a helmholtz coil that encloses the magnet. We then observe an increase in the magnet's temperature.

Interpretation A: The original disorder in the magnet's constituent spins is eliminated as they align to the applied field. Since the process is adiabatic, this loss of magnetic entropy is compensated by an increase in the lattice entropy, causing a temperature increase.

My question is:

Why do we say the process described in the first paragraph is adiabatic? Why do we exclude the possibility of heat being transferred via the magnetic field?

Of course there are many unavoidable imperfections of experimental arrangement which would prevent any process from being perfectly adiabatic. Rather suppose I were to dispute Interpretation A, claiming instead that the observed temperature increase was due to a transfer of heat to the system (Interpretation B). How would adherents of interpretation A prove to me that the process is indeed adiabatic?

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By definition, heat transfer is the energy transfer due to a temperature difference. Energy transfer by other means, such as piston work or magnetic induction, are not considered as heat transfer processes. For this reason the process in your question is adiabatic.

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  • $\begingroup$ Suppose the Helmholtz coil was hotter than the magnet $\endgroup$
    – creillyucla
    Commented May 26, 2018 at 17:05
  • $\begingroup$ @creillyucla You said "...we have a magnetic material that is well insulated from its environment..." Everything other than the magnetic material, in particular the coil, belongs to the environment. $\endgroup$
    – Deep
    Commented May 26, 2018 at 17:58
  • $\begingroup$ By insulated I mean that we can rule out conduction of heat via the air surrounding the magnet. But we know that there are other mechanisms of heat transfer (radiation for example). How do we know that the temperature rise observed in the magnet is not due to some unaccounted form of heat transfer? What additional tests would you perform on the system to demonstrate that no heat was transferred to the magnet? $\endgroup$
    – creillyucla
    Commented May 26, 2018 at 18:44
  • $\begingroup$ @creillyucla If you are not sure that all forms heat transfer have been prevented, then you shouldn't be making the statement that it is well insulated. Whether all modes of heat transfer have been prevented in an actual experiment is for you to verify, not a question that I can answer. If you want to know how heat transfer occurs and how to prevent it, then read a good engineering book on heat transfer (e.g. Cengel and Ghajar). $\endgroup$
    – Deep
    Commented May 27, 2018 at 3:54
  • $\begingroup$ Thus my question all along was "Is there an experimental method to verify the adiabaticity of some process besides simply preventing all known forms of heat transfer?" What prevents someone from associating any process which results in a temperature increase in a subsystem with some unknown form of heat transfer from the environment to the subsystem? $\endgroup$
    – creillyucla
    Commented May 27, 2018 at 19:02

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