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Magnetism seems absolute despite being relativistic effect of electrostatics 
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#1
Feb1412, 02:48 AM

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I know that magnetic force due to a current carrying wire on a test charge moving w.r.t the wire(along the wire), can be interpreted as the electrostatic force if we use the first order relativistic corrections for Time Dilation or Length contraction of the charges of the wire, in the frame of the the test charge.
But what I don't seem to understand is rather very simple situation. Let's consider a simple model of a conducting wire, + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +                                     Now, let's suppose there is some current in the wire and the electrons are moving at speed 'v' w.r.t the the wire, secondly, a stationary test charge w.r.t the wire lying around. Naming the above scenario as (1) Now, the test charge starts moving in the direction of electrons with the same speed 'v'. This time in the reference frame of the test charge, electrons are stationary and nucleus(positive charge) is moving at speed 'v'. Naming this scenario as (2) And so the question arise, the two scenario are identical w.r.t principle of relativity. That is, in the first case only negative charges are moving, but there is no force on the charge. But in the second case when positive charges are moving there is a force on the test charge(magnetic force towards wire). Whereas, the two cases are essentially identical w.r.t principle of relativity. 


#2
Feb1412, 06:49 AM

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Here is probably the best resource for this question:
http://physics.weber.edu/schroeder/mrr/MRRtalk.html Your scenarios are explicitly covered in the section "Magnetism as a Consequence of Length Contraction". 


#3
Feb1412, 08:06 AM

P: 303

Thanks for the reply and the good link.
The explanation is quite good, but then we have another problem if we assume this explanation to be correct. This is, why then we don't see any force when the situation changes from NO current to some current. Since, due to the motion of charges, which are responsible for current should also go through the length contraction when compared to the stationary state of these charges, when there is NO current. Therefore, I think, according to the above explanation, there should be a magnetic force even if we switch ON or OFF the current. 


#4
Feb1412, 08:36 AM

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Magnetism seems absolute despite being relativistic effect of electrostatics
If there is no current then the electrons are not moving therefore there is only one single reference frame and the test charge is stationary as are all of the charges in the wire. There is no length contraction, no net charge, and no force on the test charge with the current off.



#5
Feb1412, 08:46 AM

P: 303

Yes, that is correct.
But there should be all these effects when the current is ON, that is, when electrons are moving and therefore there should be length contraction and thus Force. 


#6
Feb1412, 09:14 AM

P: 1,011

It is not clear to me what as to which plane would the charges actually length contract toward. Say I have a sequence of charges like this at 0 current condition:
or
Would we have a situation where the field along the wire appears to lack uniformity because of such contraction, with, say, the leading and trailing end of the wire being more positive? This doesn't seem very intuitive or logical if you ask me. 


#7
Feb1412, 09:59 AM

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In the lab frame the force is attributed to the magnetic force on the moving charge due to the current in the neutral wire. In the electron/testcharge frame the force is attributed to the electrostatic force on the stationary charge due to the net charge in the wire. 


#8
Feb1412, 10:54 AM

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P: 17,344

Suppose that you suddenly turn the current on at time t=0 in the lab frame. So at t=0 electrons begin leaving one end of the wire at a certain rate and entering the other end of the wire at the same rate, so there is no net charge. In the moving frame the beginning of the electrons leaving one end is not the same time as the beginning of the electrons entering the other end, so there is a net charge. 


#9
Feb1412, 11:03 AM

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#10
Feb1412, 12:31 PM

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The same conclusion is true in the steady state situation, it is just easier to describe in the charging situation.



#11
Feb1412, 12:57 PM

P: 1,011

Let's be a little more direct here: To what plane does the electron bulk flow actually contract towards as a result of the bulk's relative velocity with respect to the rest frame of the positive charges? 


#12
Feb1412, 01:09 PM

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If that is not good enough for you then you are welcome to pursue the math on your own, but do not (as you did in your ASCII drawings) forget the relativity of simultaneity, and do not forget the charges that are entering and leaving the ends of the wires. 


#13
Feb1412, 01:11 PM

P: 1,011

What does your comment about the relativity of simultaneity have to do with the situation with steady state current? Finally, to what plane do the  charges actually contract in the rest frame of the + charges? 


#14
Feb1412, 01:13 PM

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Uhh, uniform.



#15
Feb1412, 01:28 PM

P: 1,011

Let's keep this REALLY simple. Assuming that the wire is neutral (no net charge) and that the wire is 1 meter long and that I have a length contraction of electrons, why should I get from that a uniform charge distribution when the electrons are drifting through wire (current)? I would TOTALLY expect an ununiform distribution, assuming length contraction applies to the bulk flow of electrons. I STILL don't have an answer to my question as to what do the electrons actually length contract towards. 


#16
Feb1412, 03:24 PM

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#17
Feb1412, 04:50 PM

P: 1,011

SR says that objects (read: multiple particles) will length contract. So, logically speaking, you can treat the + charges and  charges as two separate "objects" at different speeds. I assume this to mean not only the particles by themselves, but the entire bulks of the particles as a whole. For an object to contract, the distance inbetween also has to contract. You don't have just the fundamental particles contracting. In the extreme case, going from 0 current to a very high current would cause the following to occur: This



#18
Feb1412, 06:04 PM

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