# I Electromagnetic field disturbance if radiation disappears

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1. Jul 3, 2017

### jorgeha

Hi everyone!

I've been reading about these topics (Feynman lectures and more on the internet and some books) but I still have a doubt, maybe because I haven't understood the whole of it.

This is my doubt: Think of an imaginary situation in which we have an accelerating charge. The electromagnetic field will change and the charge will emit radiation. As far as I know, a point which is 300,000,000 meters away form the charge won't feel the change until one second in the future, because of c.
Now, what if, as it is seen in the picture, radiation suddenly "disappears" in an angle? Would the point A feel the change, if it is in that area of space without radiation?
(It is possible that electromagnetic waves could reach the point A by diffraction, so I extended the barrier with the purple lines which go to infinity) I also wanted to ask: if electromagnetic waves don't reach that point directly (because of the blue barrier) but they do by diffraction, would the electromagnetic field in that point of space be the same as if they did directly?

I'd be grateful if you could answer me in both situations: with just the blue barrier, and with both the blue and the purple.

Picture: https://ibb.co/foZ1Hv
White lines -> electromagnetic radiation.
Yellow point -> charge.
Blue line -> hypothetical barrier which absorbs radiation 100%
Purple line -> extended hypothetical barrier.

Thank you in advance.

2. Jul 3, 2017

### tech99

I cannot see how it is a problem for radiation to be absorbed in certain directions. It happens all the time. The source does not care, because once the energy has left, it never comes back. Am I missing something - apologies?

3. Jul 5, 2017

### jorgeha

My doubt is, if light "carries" the information about the change of the electromagnetic field, if we absorb every radiation from that source (the yellow charge) in a region of space, will charges in that region feel the change in the electromagnetic field? If yes, then I don't understand the concept of electromagnetic waves carrying information about the change in the field. ((If you know about some insighting books, please let me know). Thanks!

4. Jul 5, 2017

### tech99

As far as I can see, if a charge is shielded from the radiation, then it will not feel a change in the source.

5. Jul 5, 2017

### Staff: Mentor

I don't think the initial setup is correct, as I don't think there is a barrier that can block an electro-static field. Accelerating the charge should cause a change in the barrier and these changes then propagate towards charge A or point A or whatever that is in your drawing. That's my thoughts at least.

6. Jul 6, 2017

### jorgeha

So we should now think of the electromagnetic field at a point of space like a superposition of fields: the one generated by the moving charge and the one generated by the "barrier" in reponse to the radiation? If the barrier was something like 100% reflexive (theoretically), would the superposition of both electromagnetic fields, generated by charge and barrier, be 0 in a point inside the barrier?

It is not very intuitive, but it makes sense. Thanks.

7. Jul 6, 2017

### NFuller

If the barrier is a perfect conductor and is grounded or a closed surface, then it will block all electric fields. If the material has an infinite magnetic permeability, then it will block all magnetic fields. Lets assume the barrier has these properties for your argument.

For the finite sized shield, electromagnetic waves can diffract around the shield and interact with the second particle. For the infinite shield, the second particle will not feel any fields from the first particle and will not respond to any changes in its motion.

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