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It seems to me that an electromagnetic field is nothing more than a collection of photons, which as I've heard, extends through space infinitely. Why is it, then, that an inductor such as simple copper wire loop, can "store" energy in it as an electromagnetic field? Wouldn't the photons or waves of EMF just fly away into space and be lost (the energy would be lost, not stored), how is it that this energy is stored as if the photons would fall back down and hit the wire to create current when the field collapses?

user3704920
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Your argument that the energy should radiate away would be true if your inductor were a good antenna, in which case it would be a bad inductor! The problem is an impedance mismatch: The inductor produces a magnetic field (which stores the energy you inquire about), but little electric field. That is the wrong ratio, or impedance, to couple to the vacuum where photons travel at the speed of light.

You obviously are correct in arguing that this is nevertheless electromagnetic energy that must be quantized as photons. But these photons are localized, essentially trapped inside or in the neighborhood of the inductor.

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    great answer I am still puzzled by how that magnetic field from the inductor prevents the photons from radiating? – user3704920 Mar 24 '15 at 15:02
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    It's not so much the magnetic field but the mismatch in the (huge) ratio of magnetic to electric field (which, after all, is mostly just generated as the current changes) to the ratio of magnetic to electric field of photons in free space. That means you cannot translate (much) of the magnetic field of the inductor into a propagating free-space photon (where would the required electric field come from?). –  Mar 24 '15 at 17:53
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Wouldn't the photons or waves of EMF just fly away into space and be lost (the energy would be lost, not stored)

One must distinguish between electromagnetic waves and, e.g., static electric and magnetic fields.

Essentially, ('real') photons are associated with electromagnetic radiation (radio, light, x-rays, gamma rays). Electromagnetic waves transport energy and momentum far away from where they are generated. Quantizing electromagnetism results in quanta, photons, that have both energy and momentum.

But static or (relatively) slowly varying electric and magnetic fields are not electromagnetic radiation. A static electric and / or magnetic field does not transport energy but we can associate an energy due to the configuration of charges and / or currents.

In the case of an inductor, work is done to establish the magnetic field (due to the current through the inductor) and the energy is stored there, not delivered to electromagnetic radiation ('real' photons which would indeed transport the energy and momentum elsewhere).

Now, you may wonder how such static fields are treated quantum mechanically and this necessarily involves the notion of 'virtual' photons which is beyond the scope of this answer.

Alfred Centauri
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  • So the photons stored in an optical resonator are virtual photons in your opinion? –  Mar 23 '15 at 23:06
  • @pyramids, why would you think that? – Alfred Centauri Mar 23 '15 at 23:12
  • Well, I can arrange for them to have zero net momentum, for example by making the resonator so small as to only allow a single mode. You argue momentum would be necessary to have "'real' photons." –  Mar 23 '15 at 23:14
  • @pyramids, what is your intention? – Alfred Centauri Mar 23 '15 at 23:20
  • To understand how the energy in the (mostly) magnetic field of an inductor could be quantized as virtual rather than real photons, as you assert. I view this field as transitory, hence having a (very small) electrical field, and only really fitting the notion of a (very unusual) real photon. But you assert otherwise and avoid giving an explanation by calling the issue "beyond the scope of this answer." Enlighten the reader, please. –  Mar 23 '15 at 23:30
  • @pyramids, as I suspected. No, thank you, I'm not interested in your view or in discussing your view with you. – Alfred Centauri Mar 23 '15 at 23:35
  • -1 not logically consistent, not helpful –  Mar 23 '15 at 23:36
  • @pyramids, your down vote is impotent and irrelevant to me. – Alfred Centauri Mar 23 '15 at 23:37
  • @AlfredCentauri I really appreciate your answer but pyramids is correct in the fact that you mention that the field is not made of "real" photons and leave me hanging as to what those virtual or pseudo photons might be? Which wouldn't be a problem but I am left with more questions than answers. – user3704920 Mar 24 '15 at 15:00
  • @user3704920, I quoted, at the top of my answer, precisely the question that I wished to address in my answer. – Alfred Centauri Mar 24 '15 at 15:10
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A moved charge - in this case an electron - which is accelerated in a circle (the inductive coil) will induce a magnetic field. How does the electron induce the magnetic field? The electron has a magnetic moment and it spins. The movement of the electron in the coil align the magnetic moment and all moved electrons induce the common magnetic field of the coil. By this the electrons get slower, in macroscopic terms the resistance of the coil increases. At the moment when the current will be interrupted the magnetic moments fall back and when starts the self induction, the magnetic field collapses and give back the energy to the electrons.

More about the Lorentz force see How does the Lorentz force work?

HolgerFiedler
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  • really appreciate your answer; though I can't say I understand it fully, nor can I visualize what you said. Still very puzzled. – user3704920 Mar 23 '15 at 21:02