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A.T.
Science Advisor
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What do you think about this explanation of EM-fields using SR?
https://www.youtube.com/watch?v=1TKSfAkWWN0
https://www.youtube.com/watch?v=1TKSfAkWWN0
That tends to be the point that confuses most of the people that actually understand the argument being presented, so kudos on understanding the argument.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.
Thanks for the link, but I don't think it adresses Noyhcat's point, as it also starts out with the current already flowing and the wire being neutral in the lab frame.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.
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.
That tends to be the point that confuses most of the people that actually understand the argument being presented, so kudos on understanding the argument.
The spacing of the electrons in the wire frame is determined by the observed fact that the wire is uncharged in the wire frame. This is a "boundary condition" that can be experimentally controlled.
What do you think about my idea in post #7, with both charge types moving in opposite directions in the wire's frame? Does it work out quantitatively? I don't think it would make the video more difficult to understand.I'm afraid I don't know how to do 2) (explain the issue) without violating 3) (raising the height of the target audience).
What is the title of the book?That's what Griffiths does in the aforementioned section.
Thanks, I will try to check it out. But the idea with both charge types moving is basically correct?Introduction to Electrodynamics by David Griffiths: https://www.amazon.com/dp/013805326X/?tag=pfamazon01-20
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.
Thanks, I will try to check it out. But the idea with both charge types moving is basically correct?
Thanks, I will try to check it out. But the idea with both charge types moving is basically correct?
Yeah.
The spacing of the electrons in the wire frame is determined by the observed fact that the wire is uncharged in the wire frame. This is a "boundary condition" that can be experimentally controlled.
Once the spacing is determined in the wire frame, then it is determined in all frames.
That would only be true if the electrons were rigidly attached to each other, which they are not. Charge carriers in a conductor are, by definition, very mobile and able to change their position and spacing in response to any fields.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.
Yes, it works out quantitatively. Using units where c=1 the current four-vector or four-current is ##J=(\rho,\mathbf{j})## where ρ is the charge density and j is the current density.What do you think about my idea in post #7, with both charge types moving in opposite directions in the wire's frame? Does it work out quantitatively?
That would only be true if the electrons were rigidly attached to each other, which they are not. Charge carriers in a conductor are, by definition, very mobile and able to change their position and spacing in response to any fields.
In the wire frame the wire is electrically neutral. The spacing of the charges must reflect that boundary condition.
If any additional electrons are required then they would come from the battery or other power source. However, usually no additional electrons are required. Usually, charge is just redistributed around the loop.So if it is the spacing of the electrons that keeps changing while switching on and off the current, what do you suggest happens for a closed loop of current wire! ,where does the extra electrons go or come from ?
How does a particular redistribution of charges(electrons), as a result of the presence or absence of net electric field in the wire in a single direction, cancels out the effect of length contraction due to motion of electrons everywhere. A particular redistribution can only cancel out the effects at a particular point and not everywhere.If any additional electrons are required then they would come from the battery or other power source. However, usually no additional electrons are required. Usually, charge is just redistributed around the loop.
Consider an uncharged square loop of unit length (units where c=1) and width carrying a unit current clockwise. In the top wire the four-current is Jt=(0,1,0,0). In the right wire the four-current is Jr=(0,0,-1,0). In the bottom wire the four-current is Jb=(0,-1,0,0). And in the left wire the four current is Jl=(0,0,1,0).How does a particular redistribution of charges(electrons), as a result of the presence or absence of net electric field in the wire in a single direction, cancels out the effect of length contraction due to motion of electrons everywhere. A particular redistribution can only cancel out the effects at a particular point and not everywhere.
Clearly not. I just mentioned it because there are times when there is a net charge and in such cases the charge comes from the power source. I never said that batteries are always needed, and in fact, I specifically said that usually the power source is not needed for the explanation.And you already know we don't always need the batteries to produce the current !!
So if it is the spacing of the electrons that keeps changing while switching on and off the current, what do you suggest happens for a closed loop of current wire! ,where does the extra electrons go or come from ?
Seriously! I thought we were discussing switching the current on and off, what you are describing is Lorentz invariant nature of current in two different frames.Consider an uncharged square loop of unit length (units where c=1) and width carrying a unit current clockwise. In the top wire the four-current is Jt=(0,1,0,0). In the right wire the four-current is Jr=(0,0,-1,0). In the bottom wire the four-current is Jb=(0,-1,0,0). And in the left wire the four current is Jl=(0,0,1,0).
Now, if we go to a frame moving at .6 c wrt the wire then the four-currents become Jt'=(-.75,1.25,0,0), Jr'=(0,0,-1,0), Jb'=(.75,-1.25,0,0), Jl'=(0,0,1,0). So the excess negative charge density on the top is balanced by an excess positive charge density on the bottom, with no net charge for the whole loop.
I think i was rather vague in my last post, what I meant was where do you get the extra electrons when the current is produced by the changing magnetic field, or let's say a current carrying superconducting wire, where current is due to changing magnetic field.Clearly not. I just mentioned it because there are times when there is a net charge and in such cases the charge comes from the power source. I never said that batteries are always needed, and in fact, I specifically said that usually the power source is not needed for the explanation.
Your posts are rather vague. You should carefully specify what you are interested in including the reference frame. I believe that I answered the question you asked.Seriously! I thought we were discussing switching the current on and off, what you are describing is Lorentz invariant nature of current in two different frames.
What extra electrons? The point of the exercise is to show that there aren't any extra electrons.what I meant was where do you get the extra electrons when the current is produced by the changing magnetic field, or let's say a current carrying superconducting wire, where current is due to changing magnetic field.
In the rest frame of the wire, the spacing of electrons does not change, so no extra electrons are needed.
In an inertial frame in which the wire is moving and the electrons along one straight part of the wire are at rest, electrons in a different part of the closed loop (the return wire, if you like) are not at rest. So, in this frame, the electron density isn't constant: it's lower where the electrons are at rest and higher where the electrons are moving. Averaged out over the whole loop, the total number of electrons is unchanged.
I posted the diagram below over 18 months ago in a thread you took part in:
Click here for explanation in old thread