Can light cause eddy currents?

  • #1
iVenky
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This may sound dumb..

We know metals reflect microwave. One way to look at this is considering an eddy current formed inside the metal that cause the reflected EM Flux.

When glass reflects light, does something similar happen? Can we have eddy currents (or something analogous) with light since that's also an EM wave?
 

Answers and Replies

  • #2
berkeman
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We know metals reflect microwave. One way to look at this is considering an eddy current formed inside the metal that cause the reflected EM Flux.
Can you give a reference for this? EM reflection at a conducting surface has to do mainly with the E-Field, not the B-Field, AFAIK...

https://www.scientificamerican.com/article/what-is-the-physical-proc/

1600213347985.png
 
  • #3
iVenky
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Can you give a reference for this? EM reflection at a conducting surface has to do mainly with the E-Field, not the B-Field, AFAIK...

https://www.scientificamerican.com/article/what-is-the-physical-proc/

View attachment 269476
I think it's just another way of looking. Even eddy current is an electric current flow that's causing an opposing magnetic flux.
I can take a look, but image something like this. What happens when you place an inductor right above a metal which has 0 resistance (perfect conductor)? eddy currents are formed in the metal that cancels the magnetic flux from the inductor. This is in effect like reflection of flux from inductor at the metal interface.
 
  • #4
berkeman
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I think it's just another way of looking. Even eddy current is an electric current flow that's causing an opposing magnetic flux.
No. Please review what Eddy Currents are and how they are generated. Thank you.

https://en.wikipedia.org/wiki/Eddy_current

Eddy currents (also called Foucault's currents) are loops of electrical current induced within conductors by a changing magnetic field in the conductor according to Faraday's law of induction. Eddy currents flow in closed loops within conductors, in planes perpendicular to the magnetic field. They can be induced within nearby stationary conductors by a time-varying magnetic field created by an AC electromagnet or transformer, for example, or by relative motion between a magnet and a nearby conductor. The magnitude of the current in a given loop is proportional to the strength of the magnetic field, the area of the loop, and the rate of change of flux, and inversely proportional to the resistivity of the material. When graphed, these circular currents within a piece of metal look vaguely like eddies or whirlpools in a liquid.

By Lenz's law, an eddy current creates a magnetic field that opposes the change in the magnetic field that created it, and thus eddy currents react back on the source of the magnetic field. For example, a nearby conductive surface will exert a drag force on a moving magnet that opposes its motion, due to eddy currents induced in the surface by the moving magnetic field. This effect is employed in eddy current brakes which are used to stop rotating power tools quickly when they are turned off. The current flowing through the resistance of the conductor also dissipates energy as heat in the material. Thus eddy currents are a cause of energy loss in alternating current (AC) inductors, transformers, electric motors and generators, and other AC machinery, requiring special construction such as laminated magnetic cores or ferrite cores to minimize them. Eddy currents are also used to heat objects in induction heating furnaces and equipment, and to detect cracks and flaws in metal parts using eddy-current testing instruments.
 
  • #5
iVenky
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No. Please review what Eddy Currents are and how they are generated. Thank you.

https://en.wikipedia.org/wiki/Eddy_current

I think there is a dot connecting these two processes (it's just different way of looking at it). I can try to find a reference and get back.

For example, why is the reflected light (or EM wave) 180 deg out of phase at the mirror interface? That's the lenz's law.
 
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  • #6
berkeman
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I can try to find a reference and get back.
Thank you, that would be great. :smile:
 
  • #7
davenn
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I think there is a dot connecting these two processes (it's just different way of looking at it).

As has been pointed out, eddy currents in an object have nothing to do with reflection
It's not a different way, it's just a plain wrong way
 
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  • #8
DaveE
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I think part of the confusion in comparing these in the vernacular is that they have such different time/wavelength scales. Eddy currents work in the classical EM or maybe statistical mechanics world, large scale compared to the materials; currents flowing through metals. I never hardly ever hear people talking about RF photons. Light photons are usually considered more at the atomic level, eg. individual photons interacting with atoms/particles.

Yes at the large scale you can compare reflection at interfaces due to impedance/index changes in all scales of EM, but this is a classical approximation at large scale of a quantum world. It works great, but it's a little hard to explain if you question it deeply.
 
  • #9
iVenky
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12
I think part of the confusion in comparing these in the vernacular is that they have such different time/wavelength scales. Eddy currents work in the classical EM or maybe statistical mechanics world, large scale compared to the materials; currents flowing through metals. I never hardly ever hear people talking about RF photons. Light photons are usually considered more at the atomic level, eg. individual photons interacting with atoms/particles.

Yes at the large scale you can compare reflection at interfaces due to impedance/index changes in all scales of EM, but this is a classical approximation at large scale of a quantum world. It works great, but it's a little hard to explain if you question it deeply.

Yes, I was thinking along those lines. On a side note, there is a branch of physics called RF photonics :P though it's not exactly the same thing that we are talking about I guess.
 
  • #10
artis
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@iVenky I think you are confusing different modes of how EM waves propagate.

If you are thinking of a solenoid coil wound around an iron core and the coil energized with AC, then if you would put such an electromagnet's iron core end close to a conducting surface there would indeed be strong eddy currents generated within the surface. This is the reason why transformer and AC electromagnet cores are thin laminated metal sheets instead of a single chunk of metal. But this is all the so called "near field" phenomena.
The picture you seem to envision in your question has to do with high frequency EM waves like radio waves. The way they reflect off conducting surfaces is different than a B field from an electromagnet or induction coil would reflect from a surface that is very near the source.
 
  • #11
iVenky
212
12
I think part of the confusion in comparing these in the vernacular is that they have such different time/wavelength scales. Eddy currents work in the classical EM or maybe statistical mechanics world, large scale compared to the materials; currents flowing through metals. I never hardly ever hear people talking about RF photons. Light photons are usually considered more at the atomic level, eg. individual photons interacting with atoms/particles.

Yes at the large scale you can compare reflection at interfaces due to impedance/index changes in all scales of EM, but this is a classical approximation at large scale of a quantum world. It works great, but it's a little hard to explain if you question it deeply.

I see. Is it mainly because the wavelength is shorter compared to RF signals some of the RF concepts don't work here.

Given that for 100% reflection we need good conductive materials, is it related to the fact that perfect conductors have intrinsic impedance ~0 << air impedance that results in 180 deg reflection? (more like image theory in electromagnetics).
 

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