# Electromagnetic wave nature

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• rudransh verma
In summary, the author explains that electromagnetism can be explained by wave theory. The fields around an accelerating point charge are described by the Lienard-Wiechert potentials.

#### rudransh verma

Gold Member
I was reading Six easy pieces from Feynman and I got stuck what is electromagnetism 2 years ago. Recently I came across a video and I think I have figured it out.

The paragraph says like this:

“ If we were to charge a body, say a comb, electrically, and then place a charged piece of paper at a distance and move the comb back and forth, the paper will respond by always pointing to the comb. If we shake it faster, it will be discovered that the paper is a little behind, there is a delay in the action. (At the first stage, when we move the comb rather slowly, we find a complication which is magnetism. Magnetic influences have to do with charges in relative motion, so magnetic forces and electric forces can really be attributed to one field, as two
different aspects of exactly the same thing. A changing electric field cannot exist without magnetism.) If we move the charged paper farther out, the delay is greater. Then an interesting thing is ob- served. Although the forces between two charged objects should go inversely as the square of the distance, it is found, when we shake a charge, that the influence extends very much farther out than we would guess at first sight. That is, the effect falls off more slowly than the inverse square.
Here is an analogy: If we are in a pool of water and there is a floating cork very close by, we can move it “directly” by pushing the water with another cork. If you looked only at the two corks, all you would see would be that one moved immediately in re- sponse to the motion of the other—there is some kind of “interac- tion” between them. Of course, what we really do is to disturb the water; the water then disturbs the other cork. We could make up a “law” that if you pushed the water a little bit, an object close by in the water would move. If it were farther away, of course, the second cork would scarcely move, for we move the water locally. On the other hand, if we jiggle the cork a new phenomenon is involved, in which the motion of the water moves the water there, etc., and waves travel away, so that by jiggling, there is an influence very much farther out, an oscillatory influence, that cannot be understood from the direct interaction. Therefore the idea of direct interaction must be replaced with the existence of the water, or in the electrical case, with what we call the electromagnetic field.”

This is the video:
Watch for 1:18 to 2:40 interval.

Is this kink the electromagnetic wave?
Due to changing electrical field force on the paper bits keep changing and we see it move with a delay because the kink is traveling with some speed c.

Pretty much, yes. To get an actual wave you'd have to oscillate the charge, that is, accelerate it in opposite directions periodically. And don't forget that a changing electric field implies a magnetic field that isn't shown in the video.

The fields around an accelerating point charge are described by the Lienard-Wiechert potentials, if you want to look up a more formal treatment.

rudransh verma and vanhees71
It is also worth remembering that the total charge of the electrons in a piece of metal is very great. We do not see their electric field because it is canceled by the superimposed field of the protons, which is opposite polarity but the protons are heavy, so it does not move very much. But as soon as we cause a slight acceleration, say by applying a voltage across the metal, a kink becomes visible.

Ibix
Ibix said:
Pretty much, yes. To get an actual wave you'd have to oscillate the charge, that is, accelerate it in opposite directions periodically. And don't forget that a changing electric field implies a magnetic field that isn't shown in the video.
Cool! So we can actually see the effects of wave traveling through space. Thanks

I like this graphic. I have no idea about the rest of the video.

rudransh verma