I've attached an image to a conceptual problem I'm having a little trouble understanding. It shows part a, b, c, and d (but d is cut off). a and b are one inertial frame while c and d are another. c and d makes sense, but I'm having a little trouble with a and b. Based on the image for c, there is length contraction for the + charges in the wire causing there to be more localized charge and thus an electric field. This makes sense since the + charges are moving in this inertial frame. Now I don't quite see why this doesn't occur in part a/b (i.e. why is there no length contraction of the - charges? If the + charges are at rest while the - charges are moving at speed v in this case, shouldn't there be a localization of - charges also causing for an electric force AND magnetic force in a/b? Any clarification on why the length contraction and thus electric force is not present in a and b would be very helpful!