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Download The total free energy of a magnetic substance
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The total free energy of a magnetic substance LL8 Section 32 Magnetizable body Magnetic field For given currents j in some conductors, the thermodynamic potential with respect to T and j is For constant T and j, irreversible processes occur until is minimized. In equilibrium is a minimum with respect to changes in state occurring at constant T and j. For given j and constant T, the change in is the work done by the field on the conductors. In the absence of the body, where is the magnetic field produced by the given j without contribution from the magnetized body. Thing to remember: H is not the same as they are often almost the same. , but for non-ferromagnetic bodies, With the body present, includes the energy of the external field that magnetizes the body. Free energy density of body without field. New definition The difference in old and new definitions does not affect the differential thermodynamic relations of the body. , Field part of the energy density, which includes the contribution from M For a reversible change in at constant T HW Magnetization of the body Change in the original external field that created M For spatially uniform Total magnetic moment of the body When temperature is not constant By analogy with dielectrics (section 11) Absolute, not differential Usually the magnetic susceptibility is small for non ferromagnetic bodies, so that their magnetization does not affect the currents that create the field that magnetizes the body. Then the free energy is linearly proportional to the susceptibility. This is found from the original current distribution (e.g. an electromagnet) without any magnetizable material around.