EM question

When you connect the load and current flows in the winding, that current will create a magnetic field that opposses the original magnetic field.

The amount of energy you can draw is a function of how well you can keep the magnetic field. And also the resistance of the winding.

Mark

The peak current available would be the same current that would generate the applied flux, by symmetry, no? You should be able to calculate that.

Ok, of course. So then I guess both the terminal voltage and "ESR" vary with load - the more current you attempt to draw, the greater the opposing magnetic field and the induced voltage gets smaller, along with a smaller energy density?

I don't know what you mean by "how well you can keep the magnetic field." In this situation the magnitude of the original magnetic flux density would remain constant, I.e. not increase with increasing load.

That's what I thought initially, but I forgot that drawing current from the circuit is creating a magnetic field in opposition to the applied flux. So maybe it is not so simple...

The opposing flux reaches a limit when it exactly cancels the applied flux. It doesn't "go round twice", so your concern is unfounded.

If the opposing flux cancels the applied flux, then how is a voltage being induced?

Ok, nevermind, I understand. The situation is like how in an ideal transformer there is no net flux in the core. In this situation when the maximum current is being drawn ideally there will be no net flux in the inductor, as all the energy is being transferred from the field to the secondary and not "stored" in the inductor. Of course in the non ideal case there will be some net flux remaining to create the EMF.

In a nonideal transformer, however, the flux will remain approximately constant from no load to full load because the primary can draw more current as the secondary is loaded down. But in this situation with an applied magnetic field of fixed magnitude, it seems that as the "secondary" is loaded down the total flux must decrease due to the induced flux. So it seems there must be a maximum power point between no load, full voltage, and full load, minimum voltage?

The above is NOT a question. We are waiting with baited breath...

consider the axample where the alternating magnetic field is created by a spinning magnet.

As you take more electrical power out of the coil, more mechanical power will be needed to keep the magnet spinning and the magnetic field moving.

Mark

The induced voltage is 90 out of phase with the magnetic field. The current that flows into your load [if you've made your load correctly] is IN PHASE with that voltage. Take advanatge of that!

Sure. Just like a generator slows down as you load it. I guess as it stands my thought experiment question is too divorced from reality. In reality some kind of generator must exist to provide the energy extracted from the alternating magnetic field. In my thought experiment the alternating magnetic field just exists without any thought as to how it is produced.

Exactly by the word of your setup, the current is unlimited.

Whether you could ever set that up, I don't know.

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

I don't see how the current could be unlimited. Consider a loop antenna that responds only to the alternating B field component of an EM wave. Even if the antenna were lossless, how could you pull unlimited current? There's only a finite energy density in the core.

I guess it depends on whether I'm in the near or far field region of whatever is generating the magnetic field. If I'm in the near field region, I can always load down the generator more to pull more current, if the generator can supply infinite energy and my coil has no losses. In the fsr field region, I will be limited by the energy density of the B field component of the EM field.

Doesn't matter, regardless: if the field inside the loop is absolutely, positively, some known, fixed, absolute B(t), then the voltage must be nothing other than -dPhi/dt.

Your intuition is perhaps starting to tell you something: how nonphysical, or at least unrealistic, such a simplistic textbook example is!

Also, the only "loop" antenna (the shape doesn't matter, of course) which does not respond to E, necessarily does not respond to B either: it is a loop with an effective area of zero. The electric field exists *in space*, regardless of whether there's metal there or not to "experience" it -- the only condition under which E or B can be treated completely separate is at DC (magneto/electrostatics), where antennas don't work.

Tim