New general expressions are given which extend theorems governing the behaviour of the potential vorticity (Ertel) pseudoscalar in continuum mechanics and the potential magnetic field (Hide) in electrodynamics. The laws of thermodynamics (involving entropy and potential temperature) are readily incorporated into the expressions, the application of which should facilitate both prognostic and diagnostic studies of basic magnetohydrodynamic processes, including those underlying the magnetic fields of stars and planets and other phenomena encountered in astrophysics and geophysics.
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R. Hide (1996) studied this question.
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