Today in physics class we were taught about the equivalence of the electric force and magnetic force with respect to a current carrying wire. If an observer is stationary with respect to the system (a current carrying wire and a moving charge along the direction of the wire) then there will be a magnetic force of attraction between the moving charge and the wire, and we can assume that in this state the positive or negative charge is in the wire are balanced so there is no electric force of attraction.
Now switch the frame of reference so that the moving charge is stationary within the frame. Here there is no “current” In the wire because the relative motion of the electrons is zero with respect to the now stationary charge. From this we can apply a length expansion to the now stationary electrons and a relativistic contraction of the positive charges which are now apparently moving in the opposite direction, and from this difference in charge density the stationary charge now perceives an equivalent electric force.
I have no problem understanding the equations and achieving the correct solution. But my confusion is why relativity is necessary at all. In the first case we have stationary positive charges and a flow of negative charges to the right (relative to the test charge) which produces a magnetic field. In the second case we have stationary negative charges and a flow of positive charges to the left (relative to the test charge) which produces an identical magnetic field. The basis of amperes law is symmetric so that a flow of positive charge carriers in the opposite direction will produce an identical magnetic field with respect to our charged particle. No relativity or equivalence of magnetic and electric fields are required to demonstrate this.
The teacher explains this by saying that only electrons could be current carriers that satisfy amperes law, but this is totally unconvincing to me. Wouldn’t a beam of electrons in space produce a magnetic field? The same would be true of a beam of protons.
I know this has to be correct as formulated, but I hope someone can give me some insight to understand the theory in the terms that I have explained.