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# How does Electromagnetic Wave propagate in vacuum?

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#### magnetra

##### Full Member level 5
How does Electromagnetic Wave propagate in vacuum of outer space?

M

Re: EM wave propagation

There is no good explanation for this. EM-waves consists of electric and magnetic fields and it would then be equivalent to try to understand how the electric field of a point charge expands through vacuum. EM-waves don't need any medium just as the gravitational force acts without any medium. EM-waves carry energy but no mass. In Maxwell's equations it is described as the displacement current. In classical physics waves can be treated as photons. None of the above will give you a satisfactory answer, EM-waves in free space is something you have to accept.

Re: EM wave propagation

None of the above will give you a satisfactory answer, EM-waves in free space is something you have to accept.

Well, just having to accept a phenomena without proper explanation is difficult to perceive. There should be some mathematical description or any explanation to this!
Pls anyone would like to contribute!!

Regards
M

EM wave propagation

The most important result of the maxwell equations is the wave equation. It predicts that time varying electromagnetic fields propagate in space. the medium may change the propagation behaviuor but the propagation phenomena is not dependent on the medium to be filled with a material.

Re: EM wave propagation

For example do you understand how a Coulomb field expands through space? How come that an emf is generated in a nearby conductor when a changing B-field is applied? If you do then you understand waves too since they are just a disturbances in the fields. The Maxwell's equations predict propagation in free space with the speed of light but it is not understood from an intuitive point of view. If your into antennas then you probably have read about how field lines detach from a dipole antenna. This gives some explanation but the fundamental question is still not answered.

EM wave propagation

/// ELECTRIC field and magnetic field are mutually perpendicular to each other while propagating..//

Re: EM wave propagation

Dear Magnetra,

This is quite simple, but it needs lots of intuitive thinking if you want a physical explanation without going through Maxwell's equations.

As you know, any moving current will cause a circular magnetic field around it. This comes due to the motion of the charges. Similarly any magnetic flux would be linked to charges in motion.
What is really interesting that charges also cause the presence of electric field, originating or terminating around them.

So it comes as : Charges already cause electric field, when moving they also cause magnetic field.

Now lets take a look on how a charge would move. Charges would move only in the presence of an external force to move them, which would be mainly due to the presence of the electric field.

So when you have a current carrying DC current, it causes a circular magnetic field around the conductor ONLY due to the presence of current motion which occurs on the application of electric potential difference on the conductor, which in essence creates an Electric field that will move the charge.

This can be modeled as:
Rotating magnetic field comes due to an electric field.

Similarly, by analogy or symmetry, we can say that if we were able to generate a magnetic field, this'll cause an electric field that circulates it, for example like in normal coils, or Helmholtz's coil if you want a clearer image, where the electric field is in the coil's conductor vicinity.

So the summary is:
Electric field causes rotating magnetic flux and vice versa.

What happens in electromagnetic waves propagation in space, or any other medium considering non-guided propagation, is that the wave source creates a circulating magnetic(electric) field, which is bounded in space. It couldn't generate an infinite field, so it generates its complement electric(magnetic) field, which terminates on itself, i.e. the regenerated complement field is circular itself. This causes the regeneration of the source's magnetic(electric) field again and we go into a loop of alternating magnetic and electric field rings generating the propagating wave.

I hope everything is clear now . Tell me if there is anything ambiguous.

### magnetra

Points: 2
Re: EM wave propagation

Hi Everyone -- At this moment I am sitting about 10 minutes walk from Maxwell's birthplace, 14 India Street, Edinburgh, Scotland. That will not help me answer this question, but I think it is kind of neat, so I mention it.

Another way to look at an EM wave is to look at the differential time-harmonic form. In words, a changing E-field makes an H-field. A changing H-field makes an E-field. Thus, each field changing creates the other field, and this can continue forever in free space. No medium is needed. All they need is each other. This is an EM wave.

However this is only an approximation. Maxwell's equations fail for low power. But the power has to be so so low, you can count photons. Now Maxwell's equations fail dramatically.

In Quantum Electrodynamics (QED), what we know as EM wave propagation is treated as particles of energy called photons, that interact with electrons. That is all. By modeling this photon-electron interaction, all effects of Maxwell's equations are predicted (as an average of many many photons). In addition, all low power (i.e., where you can count the photons) is also perfectly predicted, all be it on a statistical basis. (It is impossible to predict exactly what any given photon will do.)

If you want to learn more about QED with no equations, then read Richard Feynman's book, "QED, The Strange Theory of Matter and Light". I consider this required reading for anyone working professionally with Maxwell's equations.

One important point Feynman makes about QED is that we do not know why QED works (and it is a really strange theory!), all we know is that it works amazingly well. In fact, in spite of strong efforts to find a case where QED fails, it, at least up to now, has always been found to work.

The same is true of the, now known to be approximate, Maxwell's equations. We do not know why using a model with the abstract mathematical concepts of E and H (and B and D, and A and F, etc.) works so well (as long as we look only at averages of many photons), it just does.

So, the answer to your question, how does an EM wave propagate in empty space, no one knows that answer. All we can offer are abstract mathematical models that happen to give the right answer, at least approximately so anyway. The answer to why EM waves happen is not yet answered, and it might never be answered.

EM wave propagation

elmolla, that was a very apt explanation. Thanks. Ok I understand how the electric field induces magnetic field and vice versa as they radiate out into space. However I still have some questions. Existance of electric field necessitates the existance of a charge particle (Correct me if im' wrong with this!). So in space what is the source of such a charge? Are there charged particles in space?

Re: EM wave propagation

hi!

i'm new to the forum, so this is my first post, hope it's useful.

magnetra said:
However I still have some questions. Existance of electric field necessitates the existance of a charge particle (Correct me if im' wrong with this!). So in space what is the source of such a charge? Are there charged particles in space?

i think you are getting messed up by the generation and the propagation of an EM field... you need a charged particle to GENERATE an electric field, but the propagation of a wave is something else.

Imagine the electric field as the geometric place where a charged particle could do some electric force on another body. If i put a body in that field, it will notice a force acting upon it (electric force), but as long as there are no bodies in the field, it just exists there, with no interaction (there ain't anybody to interact with).

This means that some particle (or bunch of particles) in the Earth could generate an electric field, and its range could go even to outer space, but there is no need to have charged particles in the space to get that field working, its source is on Earth...

According to EM wave propagation is better explained in previous posts.

Hope this helps

halls said:
This means that some particle (or bunch of particles) in the Earth could generate an electric field, and its range could go even to outer space, but there is no need to have charged particles in the space to get that field working, its source is on Earth...

i forgot to say, that this doesn't mean there aren't charged particles in space, of course there are (space dust, rock fragments, etc), but propagating EM waves do not need of these to exist

Re: EM wave propagation

To confuse things even more you can through special relativity into the mix. On a microscopic level, charges moving down a conducting wire have an electric field attracting them along. However at a macroscopic level we observe the field to be zero since all the charges and their holes cancel out. These moving charges generate a magnetic field, however if you run beside the wire at the same velocity of the charges then you see no magnetic field. So, a magnetic field is really just a phenomenon you observe from being in a different reference frame from the electric field.

This is interesting:
https://www.physics.ucla.edu/demowe...cs/special_relativity/special_relativity.html

We see Lucy and Ringo both moving, approaching each other.

Lucy says:

The loop is stationary and the magnet is moving toward it. There is a magnetic field, but it can't produce any force on my electrons since they are stationary within the loop. Instead, the magnetic field is changing, growing stronger as the magnet gets closer, and this changing magnetic field produces an electric field which causes forces on the electrons, and drives them around the loop and produces the current in the galvanometer.

Ringo says:

The magnet is stationary and the loop is moving toward it. The electrons in the loop, since they are moving with the loop, feel a magnetic force, F = - e/c v X B, which drives them around the loop and produces the current in the galvanometer. There is no electric field.

The Conclusion:

Electric and magnetic fields are not invariant entities themselves, but are aspects of a single entity, the electromagnetic field, which manifests itself differently to different moving observers.

Also, as Rautio brought up QED, I suppose you can think of an antenna pattern as the probability density function of a photon heading off in a certain direction.

Interesting stuff.

I have never seen an explanation of inductance. Sure there is a mathematical definition, but not an intuitive one. I think it relates to the momentum, or kenetic energy, of a charge. The energy it takes to get a charge up to velocity is stored in the magnetic field. When the circuit is broken, that charge wants to keep on moving, the same way the water in a pipe wants to keep on moving. Water hammer (pressure) is generated at the end of a pipe the same way electrical pressure is generated at the end of the cut wire. Changes is direction of mass flow are analogous to changes in direction of current flow (hence the inductance of microstrip bends and discontinuities). It also explains why an infinite ground plane has no inductance. It's like dripping a drop of water on one side of the ocean and watching a single drop wash up on the beach 1000 miles away due to displacement. An infinite amount of charges moved, but the total distance moved was zero, so there was no momentum in the transfer. There is one web site where momentum is mathematically related to inductance, but it's one of those "fringe" science sites; I can't find it now.

### magnetra

Points: 2
Re: EM wave propagation

As halls said, the generation of the electric field needs a charge, but its existence away from the charge is what counts. i.e. The electromagnetic field generation requires a charge as you said, but its propagation is a self-sustained property, independent on the charge but dependent on the material properties of the medium that could attenuate, or amplify, these waves.

The self-sustaining behavior comes from the analogy that if we have a field then the complementary field will be generated if the original field has a non-zero rotation (curl), so it flows as : The antenna charges cause the electric field that causes magnetic field away from the antenna that in turn causes again electric field away from the antenna, due to the magnetic field not because of the charge present on the antenna and so on. So the propagation doesn't need a charge, but its initiation only needs a charge. i.e.

Charge --> E-Field-->H-Field-->E-Field ad so on

Hope its better now . Feel free to answer any more questions.

EM wave propagation

I THINK THAT ALL THE POST HERE ARE INCREDIBLE

HIHI I JUST SAY ABOUT THE TIME YOU EXTRACT FROM YOUR HOUSE JUST PRESENTING THE GREET THINGS ELECTROMAGNETICS HERE

"How does Electromagnetic Wave propagate in vacuum of outer space? "

EM wave propagation

i dont know much about the propagation of em waves in outer space but i think this book will be helpful for those who need to know...
"space and time" by "stephen hawking"... and other books from stephen hawking are gud to read...

EM wave propagation

hi itsfanidot,
I think that you have just got hold of the wrong end of the stick !
"Space and time" that talks abt the sky and universal world..

EM wave propagation

ya when u talk about universe it also includes the outer space... u can understand what real ting is it.
well may be its helpful... and if not then... its a worth reading book...

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