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Two identical charges travelling parallel with same velocity (thus, relatively at rest) will experience Coulomb repulsion. However, if considered from rest frame of reference, they represent two currents travelling in same direction. Thus, they should get attracted.

What will actually happen in this situation?

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  • $\begingroup$ You would need to explain, in some detail, what your own ideas are. That way ,someone might be able to give you a hint as to your assumptions and ideas. This is not a home work solving site, but people may give you a hint if you show what you have done. $\endgroup$
    – user154420
    Commented Jun 28, 2017 at 14:13
  • $\begingroup$ There's no such thing as relativistic electrostatics, when you make it relativistic, you introduce magnetism and it becomes electrodynamics. $\endgroup$
    – user20250
    Commented Jun 28, 2017 at 14:31
  • $\begingroup$ @ counto. I have already jotted my idea and hrnce asked about the descrepancy in solutions.you can call this a homework for all of us to solve $\endgroup$ Commented Jun 29, 2017 at 2:33
  • $\begingroup$ This is explained in depth in chapter 5 of Ed Purcell's Electricity and Magnetism. $\endgroup$ Commented Jun 29, 2017 at 13:24
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    $\begingroup$ Possible duplicate of Relativistic electromagnetism and electromagnetic forces on 2 protons $\endgroup$
    – ProfRob
    Commented Jun 29, 2017 at 19:09

2 Answers 2

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In the frame of reference where the two charges are stationary you will see only the electric force, which is repulsive.

In the frame of reference where the two charges are moving you will see both the electric force, which is repulsive, and an attractive magnetic force (the electric force will dominate, but both forces are there).

The net force will be repulsive and will satisfy the relativistic force equation in both frames of reference.

This is all a consequence of special relativity.

As another answer points out, when you have steady currents in wires you get different answers. The wires in that case have overall neutral charges so the electric force is not a factor. The magnetic force, which results from the moving charges, is important though and so you get the wires moving towards each other.

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The wires that you describe consist of electrons moving through a solid, such that there is no net charge. If a current is running through the wires, there is no reference frame without a current: in the frame that the electrons are still, the protons are moving. The wires will undergo a force that is proportional to the currents and inversely proportional to the separation; attractive if currents have the same sign.

The point charges you describe have a net charge. The point charges will experience a force that is proportional to their charges and inversely proportional to their separation squared; repulsive if charges have the same sign.

There is no inconsistency in the theory.

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  • $\begingroup$ Wires arent used. Just identical charges. Inconsistency remains as same charges repel, same current attract $\endgroup$ Commented Jun 29, 2017 at 2:30
  • $\begingroup$ Same current attracts for WIRES that consist of electrons in a matrix of nuclei, the result of which is zero net charge. Your assumption that the two constant velocity same-sign-charges will attract is wrong. $\endgroup$ Commented Jun 29, 2017 at 14:31

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