Suppose a virgin unmagnetized ferromagnetic sample is magnetized to complete saturation. Usually, when we reverse the magnetic field, the system does not retrace its path in the M-H plane and there is also a remnant magnetization when the field the made to vanish from the opposite direction. So the process is not reversible. But if the reversed magnetic field is changed very very slowly, as slowly as we can think of, do we expect the process to become reversible and the system to retrace its path? Thanks!
Asked
Active
Viewed 401 times
2 Answers
3
Ferromagnetic hysteresis is the prime example of having a thermodynamic process such that it is dissipative and thus is irreversible at all (macroscopic) time scales showing that despite what you were frequently and erroneously told in freshman physics class a quasi-static process is not necessarily reversible. You may find a beautiful analysis of this problem in Bridgman: "The Thermodynamics of Plastic Deformation and Generalized Entropy",doi:10.1103/RevModPhys.22.56
hyportnex
- 21,193
1
No. If the material shows a remnant magnetization, going slow will not avoid it. To demagnetize a material, gradually remove it from an alternating field. (In a tape recorder, the tape is demagnetized before recording by moving past an AC electromagnet.) This takes the material around a hysteresis loop which shrinks a bit on each loop, ending at zero.
R.W. Bird
- 12,280