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Magnetism seems absolute despite being relativistic effect of electrostatics 
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#73
Feb1712, 11:54 AM

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#74
Feb1712, 02:07 PM

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There was a discussion here about almost the same topic a long time ago:
http://www.physicsforums.com/archive.../t327854.html I agree with what I read there on the first page; I haven't read the whole discussion. Note: Also dalespam participated. Dalespam, do you agree with your comments of then? 


#75
Feb1812, 01:57 PM

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Some questions: why aren’t the electrons allowed to bunch together in the bottom left but they are allowed to bunch in the bottom right picture? Referring to the bottom left picture, BiotSavart tells me there’s a magnetic field present. Can you show me how length contraction is responsible for this magnetic field? 


#76
Feb1812, 02:56 PM

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PF Gold
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The bottom right diagram is not symmetrical: in the bottom wire the electrons are at rest and in the upper wire the electrons move faster than the ions. The bottom rest diagram is obtained by considering Lorentz contraction between the two lower diagrams, as indicated by the yellow arrows. If you accept the bottom left diagram is correct, then the bottom right diagram must be correct too. Note that I could have drawn another diagram showing the frame in which the electrons in the upper wire were at rest. This diagram would look like the bottomright diagram drawn upside down, with two static electrons in the upper wire and 14 moving rapidly to the right in the lower wire. 


#77
Feb1812, 03:41 PM

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From post #2 in this thread: 


#78
Feb1812, 04:40 PM

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PF Gold
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In the bottom right diagram, a static (relative to the frame) electron near to but outside the lower wire will be attracted to it due to the net positive charge on the wire. As the electron is static, magnetism is irrelevant to it. Translating that to the bottom left picture, the electron is now moving but the wire is not charged, so there is no electrostatic force. Nevertheless, there is still an attractive force, as we proved using the bottom right picture. The explanation for this force is magnetism. If you already knew about electrostatics and relativity but knew nothing about electromagnetism, this argument would effectively define for you what electromagnetism was. 


#79
Feb1912, 04:08 AM

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#80
Feb1912, 06:31 AM

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#81
Feb1912, 06:33 AM

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#82
Feb1912, 08:23 AM

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#83
Feb1912, 11:18 AM

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@DrGreg. It is perhaps a useful exercise to look at what happens in different frames with test charges and so on but as you stated there’s a magnetic field present in the left bottom picture. Now we have to find out why this magnetic field is there. I do not need any test charges travelling or not. Fact is we have a magnetic field. So we have to find out why the power supply had to inject an extra amount of energy. We have to find out why the energy contribution of the 2 parts of wires is increased as we increase the distance between those parts. 


#84
Feb1912, 12:17 PM

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Furthermore, the magnetic field does not have a rest frame, whereas the magnetic force always acts on a particle which does have a rest frame. So, in general, you can always transform to a frame where the particle is at rest and be guaranteed that the magnetic force is 0, but in that frame the magnetic field may be nonzero. In general, there is not necessarily any frame where the magnetic field is 0. 


#85
Feb1912, 03:09 PM

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It's a bit similar to time dilation and length contraction which according to SR are caused by speed, and while for special cases all relevant speeds can be transformed away, in general this is not possible. This it should perhaps not surprise that the same type of "absolute" vs. "relative" discussions can arise about magnetic fields as with for example the twin paradox. 


#86
Feb1912, 04:36 PM

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#87
Feb1912, 04:46 PM

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#88
Feb2012, 04:25 AM

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To expand on this: What happens when we Lorentz boost an electric field (E0)? Well we get E’=γE0. Same question regarding a magnetic field (B0). Similar, B’=γB0. How then do we get from one to the other? Clearly not by Lorentz boosting! For the purpose of transferring between electric and magnetic fields we have 2 equations which deal with moving fields. The following 2 formulas are copied from “Introduction to electrodynamics 3rd edition D.J.Griffiths” Formula 12.108. Ey’=γ(Ey – vBz) and By’=γ(By + v/C^2 Ez) where v is in the x direction. (Some time ago I lost LaTex for Microsoft Word due to a virus, does anyone know where to buy a copy?) When we say “moving fields” I think the correct expression is “time varying fields”. But I also visualise them as moving. I’m fairly confident that these 2 formulas can also be derived from Dale’s fourvectors equation (# 30) but not sure. 


#89
Feb2012, 03:07 PM

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\tilde{F}^{\tilde{\mu}\tilde{\nu}}=\Lambda^\tilde{\mu}_\alpha F^{\alpha\beta} \Lambda_\beta^\tilde{\nu} [/tex]where [itex]\Lambda^\tilde{\mu}_\alpha[/itex] is the Lorentz boost matrix. 


#90
Feb2212, 03:35 AM

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