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Author Topic: Can we remove the electric wave from light?  (Read 7218 times)

Offline kprimo

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Can we remove the electric wave from light?
« on: 26/04/2005 09:41:27 »
:)As light is an Electromagnetic wave, can we "remove" one of the waves (electric or magnetic) and leave only the other?


 

Offline gsmollin

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Re: Can we remove the electric wave from light?
« Reply #1 on: 26/04/2005 11:54:55 »
We can remove it. We do that all the time. Some real life devices are magnetic and some are electric in nature. It was the genius of 19th century scientists to connect the two forces, and Maxwell to organize one form of the mathematics that combined them. Now we know that permanent magnets have spinning electric charges in the atoms to produce the magnetism.

Ra
 

Offline kprimo

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Re: Can we remove the electric wave from light?
« Reply #2 on: 26/04/2005 12:34:27 »
So, is there a recommanded way/method to take a light, remove the Magnetic/Electric wave and leave the other wave unharm.
 

Offline daveshorts

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Re: Can we remove the electric wave from light?
« Reply #3 on: 26/04/2005 14:29:23 »
The reason why a light wave propagates is that

a changing magnetic field produces an electric field
and
a changing electric field produces a magnetic field

repeat until the end of the universe or you hit something whichever comes sooner.
(because the above is an infinite loop and if you are in a vacuum no energy is lost form the process it will keep propagating for billions of years)

so if you remove one component the propagation stops pretty quickly, and you have an electric field that is changing but no light.


If you think about it all waves need two forms of energy to propagate,
Water wave - Gravitational potential energy and kinetic energy
Sound wave - gas compression potential energy and kinetic energy
etc

So no you can't get light without the magnetic component - or at least not for very far anyway
 

Offline kprimo

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Re: Can we remove the electric wave from light?
« Reply #4 on: 26/04/2005 15:10:38 »
From your answer I aderstand that I can't remove one wave without removing the other one in any electromagnetic wave that's because one create the other.
If I correct I can't take light (or any other electromagnetic wave) and break it to two waves - one electric and one magnetic (knowing that the light properties will lost)?
 

Offline gsmollin

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Re: Can we remove the electric wave from light?
« Reply #5 on: 26/04/2005 15:14:17 »
I got interrupted, and didn't get to post the caveats.

A radiating wave, such as a light beam or a radio transmission is composed of two parts, which are orthogonal to each other, the magnetic vector, H, and the electric field vector, E. We could, in theory, remove one component, but the math may become a problem with singularities. I don't thing you can perform that feat in practice, because the required filtering is not realizable. In certain situations, the E field, for instance, may be neglible, and we simply drop it from the equations for simplicity. This is true in magnetic device design, such as transformers, for instance. We design those with only half of Maxwell's eqiations, and leave out the electric parts. However, there is in truth a very small E component present in the transformer, and in other types of transformers, such as microwave, it can no longer be neglected.

Your question may be more pragmatic, however. You may be looking for a way to filter one of the components. That is not too easy. You can suppress it, by passing the wave through a medium with a high magnetic permeability, or high electric permitivity. That's what happens in the transformer. However, to totally remove one component of a traveling wave is not going to be successful. As the wave travels, it continuously regenerates itself; the electric part collapses and generates a magnetic wave, then the magnetic wave collapses and generates the electric part. The energy carried by the wave is the sum of the E and H energies, or D and B energies in a given medium, and that must be conserved to continue traveling the wave.
 

Offline kprimo

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Re: Can we remove the electric wave from light?
« Reply #6 on: 27/04/2005 07:35:11 »
Q1: If the problem is conservation of energy, can we take an EM wave and seperate it to one E wave and one M wave keeping the total energy of the EM as the sum of the energy of the E wave and the M wave?

Q2: If the answer to Q1 is that we can seperate the waves - are the seperated wave will continue in space as they are or will they regenerate a new M and E waves? Is that means that we can take one EM wave and create two EM waves from it (which mean that we can continue to infinite)? Doesn't it a conflict with conservation of energy?

Q3: If we decide to suppress one wave in oppose to seperate them (for example we suppress the E wave) - for how long the M wave will continue traveling before it will regenerate the E wave?
 

Offline gsmollin

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Re: Can we remove the electric wave from light?
« Reply #7 on: 27/04/2005 16:40:27 »

Q1: If the problem is conservation of energy, can we take an EM wave and seperate it to one E wave and one M wave keeping the total energy of the EM as the sum of the energy of the E wave and the M wave?

Not to my knowledge. The energy in the wave, which we call the Poynting vector incidentally, would not be conserved if we split the E and H parts. As far as I know, the wave would then not exist anymore, and we would have made its energy into something else. I'm sorry I can't satisfy you on this question. As far as I know, whenever there is an oscillating electric field, such as around an antenna, the displacement currents it generates in the surrounding medium give rise to magnetic fields. The is almost the definition of a magnetic field, and I can't see any way to avoid it.

Q2: If the answer to Q1 is that we can seperate the waves - are the seperated wave will continue in space as they are or will they regenerate a new M and E waves? Is that means that we can take one EM wave and create two EM waves from it (which mean that we can continue to infinite)? Doesn't it a conflict with conservation of energy?

We can certainly split an EM traveling wave into two EM traveling waves. You can buy such a device, and it is called a splitter. You may have one on your TV cable. The enrgy of each split wave is 1/2 the energy of the original wave.

Q3: If we decide to suppress one wave in oppose to seperate them (for example we suppress the E wave) - for how long the M wave will continue traveling before it will regenerate the E wave?

The two components of the traveling wave are continuously regnerating each other. This is how they propagate through space and time. If we suppress one component, by making the magnetic permeability of the medium large, for instance, the other component is not removed, just suppressed. It is still present in diminished amplitude. If the wave leaves the medium and returns to free space, the suppressed component is regenerated to its original amplitude, minus any losses in the medium.

We frequently ignore one or the other component in mathematics or design, but really they are both always present in any traveling wave.

As far as I know, what you are trying to do is impossible. Since I am not the last word on the subject of electromagnetics, you should continue to research your question until you satisfy yourself about this issue, one way or the other.

[/quote]
 

Offline kprimo

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Re: Can we remove the electric wave from light?
« Reply #8 on: 28/04/2005 11:36:39 »
Thanks a lot!
Your answers were very helpfull.
 

Offline moth

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Re: Can we remove the electric wave from light?
« Reply #9 on: 29/04/2005 19:16:25 »
I don't think gsmollin's answers are right.

A1&A2:
If you take away one energy component of a traveling wave it will become a standing wave with half the energy of the traveling wave. (In a traveling wave, the peaks and troughs move along, while in a standing wave the the peaks and troughs are stationary, but the peaks flatten out then turn into troughs, while the troughs flatten out then turn into peaks, and they continue interchanging in this way.)
The pattern of electric field that you would see if you could take a 'photo' of a traveling electromagnetic wave can exist without an associated magnetic field, but only for an instant. Like a piece of taut elastic bent into an odd shape, it will try to even itself out, to smooth the pattern away to nothing. So the electric field changes, but a changing electric field generates a magnetic field, so as the electric field wave pattern disappears it creates a magnetic field wave pattern, and there is an instant when there is only a magnetic wave, but magnetic waves aren't stable either, so it tries to smooth out, but a changing magnetic field generates an electric field (one that is the negative of the one we started with), and so on.

(The Poynting Vector describes the flow of energy in a wave, and is zero in a stationary wave because the energy isn't traveling along any more, but there is still energy there.)

A3:
Mathematically, because of the way waves add up, a stationary wave is the same thing as two equal waves (each with half the energy) traveling in opposite directions through the same space. The stationary wave will convert its energy from all electric to all magnetic in the same time it would take the traveling waves to move from having their electric peaks line up, to having the peaks of one line up with the troughs of the other, that is to travel 1/4 of their wavelength.

The difference between traveling waves and standing waves is how the patterns of their energy components line up in space and time.

In a traveling wave both wave patterns stay the same except for moving along at the speed of the wave. But the patterns don't line up - one is offset from the other. The peaks and troughs of each component line up with the places where the other component is zero. The direction of the offset decides which way the wave travels.

In a standing wave the wave patterns are constantly changing from being a pattern, to nothing, to the opposite pattern, to nothing, and back to the pattern, and so on. The peaks and troughs of the wave patterns of the two components do line up in space, but not in time. When one component is at its strongest, the other is zero, and the other way round.
 

Offline tweener

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Re: Can we remove the electric wave from light?
« Reply #10 on: 01/05/2005 03:46:56 »
gsmollin is right. 100%



----
John - The Eternal Pessimist.
 

Offline gsmollin

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Re: Can we remove the electric wave from light?
« Reply #11 on: 02/05/2005 13:23:36 »
I was careful to add the adjective "traveling" to the the noun "wave", and did not discuss standing waves. However, the same constraints apply. A standing wave is formed when two waves travel in opposite directions through the same time and space. This is most common at a change in medium. Part of a traveling wave can be reflected back through the path of the incident wave. Their sum is a standing wave. They show up frequently on transmission lines where there is an impedance discontinuity. Another famous place is a cavity resonator, such as in a maser or laser. The E and H field components of the standing wave are still fully functional, and standing waves provide no loopholes for the unification of electric and magnetic fields.
 

Offline moth

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Re: Can we remove the electric wave from light?
« Reply #12 on: 02/05/2005 19:46:16 »
But as i see it, standing waves are the answer. Yes, they have both E and H fields, but not both all the time. There are instants when there is only one, which correspond to having magically vanished one component of a traveling wave.

I can't see any obvious way to do this magic instantaneous vanishing (as opposed to gradually building up a standing wave by exciting a cavity mode).

OK, wrong was too strong a word. I think we are just reading the emphasis of the question slightly differently. I agree with most of the physics in your explanations, except for the bit about the Poynting vector where you say that there is no energy associated with an electric only wave. I say it is just the energy flux that is zero (energy is not a vector, unless you are going to get all space-time on me).

Or maybe i am wrong?
 

Offline chimera

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Re: Can we remove the electric wave from light?
« Reply #13 on: 02/05/2005 20:08:22 »
You do not seem to be the only person with some doubts about the standard answer:

http://www.ee.surrey.ac.uk/Personal/D.Jefferies/poynting.html
 

Offline moth

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Re: Can we remove the electric wave from light?
« Reply #14 on: 02/05/2005 22:57:30 »
Thanks for the interesting link, but you're missing my point. I wasn't challenging the standard explanation, only a detail of gsmollin's exposition of it.
 

Offline chimera

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Re: Can we remove the electric wave from light?
« Reply #15 on: 03/05/2005 10:40:31 »
quote:
Originally posted by moth

 I agree with most of the physics in your explanations, except for the bit about the Poynting vector where you say that there is no energy associated with an electric only wave. I say it is just the energy flux that is zero (energy is not a vector, unless you are going to get all space-time on me).



From the link:

"The integral of the Poynting Flux over a closed surface gives the total flow of energy into or out of a surface, but it has not been proved, and we are not entitled to assume, that there is an actual flow of energy at every point equal to the Poynting Flux.

I see. I'm a total moron. Sorry. I'll now go and commit seppuku. Thank you for your kind words nonetheless.

Or do moths attract flames? Mmm. Maybe something for a nice new nit-wit topic...
 

Offline moth

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Re: Can we remove the electric wave from light?
« Reply #16 on: 03/05/2005 12:27:46 »
I'm sorry if my posts seem agressive, i don't intend them to be. Your link implies that my understanding of the Poynting vector is flawed, so i'm definitely glad you posted it. Maybe i should stop picking at gsmollin's post until i have mended my own house. Sorry gsmollin.
 

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Re: Can we remove the electric wave from light?
« Reply #16 on: 03/05/2005 12:27:46 »

 

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