When connected to a resistor the coil appears in the magnetic circuit as a "magnetic inductance Lm" obeying mmf=-LmdΦ/dt, that mmf of course coming from the coil current flowing into the load resistor. The value of Lm is N2/R where N is the number of turns and R is the load resistor.
Let's see how that stacks up against reality when R is set to zero as in a superconducting coil. If L m = N 2/R then with a superconducting coil L m = ∞ To me, infinite "magnetic inductance" means that the magnetic flux is frozen through the coil (a fragment of the magnetic circuit in series) and does not vary in time. So far so good... However, the expression mmf=-L m(dΦ/dt) is confusing or wrong because when L m = ∞ then it becomes mmf=-∞(dΦ/dt), which suggests that the mmf reaches infinite levels in response to the slightest change of flux, while in reality the mmf reaches only a level necessary to prevent any change in flux that penetrates that coil, as is shown in this video. In AC waveforms that dΦ/dt is a 90 degree phase shift, the reaction current is shifted 90 degrees from the applied flux. This is perhaps more readily understood when the coil is part of a magnetic circuit having sinusoidal flux Φ driven through it.
That other situation, when R>0, is shown in this video, unfortunately I don't see a lag or 90º phase shift between flux penetrating the loop's area and current induced in this loop. That EM simulator and its author (prof. Belcher) earned a lot of credibility with me and other scientists, by their sheer accuracy.
« Last Edit: 2015-08-19, 22:53:31 by verpies »
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