Hi, I recently came across the idea that relativity can play a role in the repulsive force of a charge outside of a current-carrying wire. The situation described was when a positive charge (q) moves at the same speed and direction as the drift velocity of the electrons in the wire (assume non-conventional current flow). From our frame of reference the magnetic force causes repulsion. In the charge q's frame it is not moving with respect to the electrons but instead the positive charges appear to be moving in the opposite direction. It was argued that this cannot be explained classically as the charge is stationary in its frame so cannot interact with the magnetic field created by the moving positive charges. It was then argued that length contraction of the positive charges caused a higher positive charge density which causes the charge q to be repelled by an electric field from its reference frame. The bit I really don't understand (because it wasn't even mentioned) was if the charge q is at rest but there is still a current flowing in the wire. From the charges frame it is not moving and neither are the positive charges but the electrons ARE moving as there is a current in the wire. Therefore surely you can equally argue that the electrons will length contract from the charge q's reference frame? But this means the wire would be negatively charged from the charge q's reference frame and be attracted to it but this doesn't happen when you put a stationary positive charge outside a current carrying wire! What am I not understanding? Thank you to anyone who can help me!