Explanation of EM-fields using SR

I don't immediately get why the separation of the negatively charged particles doesn't contract from the man's reference frame, as they are moving relative to him, and therefore there would be a negative overall charge.

 

It seems like an attempt to popularze Purcell.

I have a suspicion that it will confuse it's target's audience , but I'm not sure if that can be helped, considering that it's an attempt to reach as many people as possible (and hence put the "target" as low as possible).

Personally, I'd save this sort of explanation for someone who is sophisticated enough to work out what charge densities are required (as measured in the lab and comoving frames, the comoving frames of the electrons being different from the lab frame) to ensure electrical neutrality in the lab frame. Perhaps I'm being pessemistic, perhaps not.

 

Purcell should be popularized in my opinion. It's the best EM text I know of.

As for Noyhcat, take a look here: http://physics.weber.edu/schroeder/mrr/MRRtalk.html as it explains things in a much more lucid manner than does the video commenter in my opinion.

 

This is related to what I was trying to say earlier.

There are three possibilities:

1) Ignore the issue, which is what the video has done. Then you'll get questions like Noyhcat's.
2) Try to explain this in the video - which will raise the bar on the target audience
3) Raise the bar on the target as far as the "target audience" is concerned.

Overall, I favor 3, because ignoring the issue doesn't really work, and I'm afraid I don't know how to do 2) (explain the issue) without violating 3) (raising the height of the target audience).

 

You're right A.T. in that the link doesn't answer that particular question in a straightforward manner. Do you have access to Griffiths book? Perhaps his explanation would be to your liking. Check out section 12.3.1.

 

That's what Griffiths does in the aforementioned section.

 

Introduction to Electrodynamics by David Griffiths: https://www.amazon.com/Introduction-Electrodynamics-Edition-David-Griffiths/dp/013805326X

 

I like the video and DaleSpam's answer. One has to specify what is happening in one frame of reference, eg. there is a current in the wire, and the wire containing the current is uncharged in the lab frame. Relativity is a relationship between frames of reference, so if you specify what happens in one frame, it tells you what happens in another frame.

 

Maybe something like this:

We know that in the lab frame we can set up a wire with a current in it. We know we can do this in a way such that the wire is electrically neutral, because the wire neither attracts nor repels a charged particle that is stationary in the lab frame. Since we have set up the wire to be electrically neutral in the lab frame, the distance between positive charges is the same as the distance between negative charges in the lab frame. Now, will the wire attract or repel a charged particle that is moving in the lab frame? If the only force that affects charged particles is the electric force, since the wire is electrically neutral in the lab frame, it will neither attract nor repel a moving charged particle.

 

Yeah.

 

And how do you explain, if the current carrying wire is made of highly doped n-type semiconductor, when you don't have any holes to go by.

 

I think role of physics is in finding relations(mostly logical) between observations, for example what should we observe if we stop the current, and we all know it is an observed fact that the wire still remains electrically neutral. The point is, according to the SR length contraction explanation, it should not be neutral when we stop the current if it were to be neutral when there was a current.