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 P: 303 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.