The voltage across an inductor isn't -Ldi/dt

Mar 04, 2007 27 Replies

Let's assume the secondary is open-circuit so that it has no effect on the the primary. When you apply 450V across the primary, that doesn't mean the primary winding has a voltage of magnitude 450V through it because total EMF = 450V - primary = rate of change of magnetic flux = Lp*d(Ip)/dt. With static I, total EMF = 450 - primary = 0, and so primary = 450V which, as you pointed out, would give an infinite current for a 0R primary.

Where there is a changing magnetic field, the electric field isn't conservative and so you can't assume that components in parallel will have the same volage through them. Suppose you have a ring of 1R in a uniform magnetic field normal to the plane of the ring. Suppose it changes at a uniform rate so that it induces an electric field in the ring with the total EMF around the the ring being 1V. The voltage across a 0.1R segment = 0.1V is in parallel with the rest of the 0.9R = 0.9V segment, yet the voltage drops are not the same. In a conservative field where total EMF around ring = 0V, you can say V_0.1R = V_0.9R.

Some people recommend this paper. If you want I'll email it to you.

R.H. Romer: "What do 'voltmeters' measure? Faraday's law in a multiply connected region", Am. J. Phys., 50, 12 (1982), pp. 1089-93

What are you, a TROLL? You wrote "I always thought the voltage across an inductor was -Ldi/dt. Yet it cannot be ..." Silly boy.

I usually enjoy Am. J. Phys. papers, especially when they cover an interesting topic and demonstrate a clear pedagogic approach. I subscribed for many years, starting as a physics undergraduate but stopped before 1982. Certainly send me a copy. Thanks.

integral_loop_E.dl = integral_area_B.dA. Why doesn't it work to include the inductor?

In a non-conservative field, the voltages across components in parallel doesn't equal one another in general. Have a look at my reply to Win.

Are you that important that I should want to?

With hindsight, I think I have to agree with you. I should have said "I always thought that an EMF = Ldi/dt was generated INSIDE the conductor of an inductor. In fact the EMF is infinitesimally small i.e. 0 if it has a resistance of 0R and carries a finite current."

text -

I sent it and I hope you enjoy reading it, if you have the time.

the problem with that line of reasoning is that the voltmeter is outside the conductor.

Bye. Jasen

How does that create a problem in my reasoning? I'm just using Faraday's law for the loop consisting of the inductor, leads and voltmeter. E.dl through inductance and leads is zero , with E.dl through the voltmeter making up the rest = Ldi/dt.

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