# The energy carried from one winding of a transformer to another, in quantum terms

I have read in wikipedia this statement

"The energy carried from one winding of a transformer to another, in quantum terms is carried by virtual photons, not real photons" (wikipedia src: virtual particle)

Of course anything in wikipedia could be solved "changing it" =), but I wonder, someone put it for a reason, then in the case it have sense, I don't know what is the limit for that, what are the frequencies for virtual or real photon interactions. (if is the frequency what makes the difference, or what ?)

Perhaps it's related with the kind of interaction, I mean photoelectric effect and the induction are different mechanism , but I don't see clearly how "real photons" disappear from the picture, if they are the lesser energy that can be transfered anyway.

I have put that into a comment for this answer Low frequency electromagnetic waves but I think is better to open an specific question.

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## 1 Answer

In your case it is a near field that stands for "virtual" photons. The near field does not propagate like EMW in the sense it is "attached to the charges and currents. In fact, it is the Coulomb and quasi-static magnetic interactions of charges and currents. Despite being time-dependent, the near field decays with distance differently (faster) and does not carry away any energy-momentum. Real photons do.

There is no quantum terms for the near field. It is not quantized. It exists, as I said, as a potential interaction $\propto \frac{1}{|\vec{r}_1 - \vec{r}_2|}$.

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If it's not quantized, its fascinating, but could we then exchange energy "in near field" without plank energy limitations? –  HDE Sep 9 '11 at 16:01
Yes, there is radiationless mechanism of energy exchange. If you scatter one cold atom from another, they may push each other away without any photons, so no $\hbar\omega$ is involved. –  Vladimir Kalitvianski Sep 9 '11 at 16:06
This answer is correct about near field, but it is using an electrostatic near-field interaction to illustrate. For the case of transformers, the near field should be magnetic B-flux inducing an E-field at near zero frequency, which is not described by electrostatics or magnetostatics, but requires induction. –  Ron Maimon Sep 9 '11 at 17:33
@Ron Maimon: You are right, as usual. I am just not sure if the magnetic interaction cannot be completely "explained" via the Coulomb one. –  Vladimir Kalitvianski Sep 9 '11 at 19:48