Question

Changing magnetic fields?!

The induced E is caused by a changing magnetic field according to Faraday's Law right? Then how is a magnetic field changing? I don't understand. Aren't magnetic field's usually constant?

Answer 1

Aha. The magnetic field is not changing at all. The magnetic FLUX is changing.

Answer 2

Do you get the difference?

Answer 3

So the changed magnetic FLUX induces E? Kinda confused could you clarify please.

Answer 4

@Jemurray3 Help me out with this one please.

Answer 5

flux is the amount of change. so [the amount of magnetic change] changing is what induces an electric field.

Answer 6

\[\Phi_B=BS\]\[\left| E \right|=\left| d \Phi_B/dt \right|=\left| d(BS)/dt \right|=\left| SdB/dt +BdS/dt\right|\]
Then, if B=constant, it is dS/dt what counts

Answer 7

And, by the way, where is that idea of magnetic fields usually constant coming from?

Answer 8

Magnetic fields change all the time, I'm not sure what the question is asking.

Answer 9

Sorry for not answering in a long time guys got busy! Well I read this post that said: "The changing magnetic fields will induce the E". So I was a bit confused saying to myself that magnetic fields are constant and I think I'm wrong. I think they're constantly changing like you guys have stated.

Answer 10

@hope99 Still not quite correct. There are magnetic fields that change and magnetic fields that do not. Let us see, the case of a transformer, all parts are fixed, they do not move therefore the induction from primary to secondary and viceversa and self induction and losses come from this changing magnetic field and the variable flux it produces over fixed sections. This is the reason for transformers working with variable voltages and currents.
However, in the case of a dynamo, for instance, the field produced by the magnet (stator) is always the same and what you change is the geometry of the section this field crosses by changing the angle of the mobile part (rotor) in this way you get a variable flux and hence an induced emf. No doubt that that the induced emf if feeding an external circuit will produce a current that when passing through the coil which generated it will also produce a field (in this case variable) and the corresponding self inductance of the rotor coil will have its effect too. Hope it clarifies

Answer 11

@CarlosGP Thanks for that.