I read this example in a book and I'm thinking about what is the "determining factor" of the directions of the magnetic field vector B and the magnetic force vectors F. If we start by looking at wire 1 in picture (a), if we only know the direction of the current (we call this "v"), can we really say anything about the direction of F and B? I would say no. We need to know the directions at least two of the vectors v, B and F in order to find the direction of the third vector. Here we know v only. So which of the other, B or F, do we also know from the start? Is it in this case, determined simply Newtons third law, so that the two wires, will always direct their B and F so that the forces oppose each other? Is that the only thing which is "constant", so to speak? If we compare (a) and (b) we can see that the magnetic field vector created in wire 1 must change direction, since the force vector changes direction but the current direction remains the same. So the B-vector in wire 1 in (b) must be directed downwards. But why does changing direction of the current in wire 2 change the direction of the magnetic field in wire 1? The only explanation I can find is that the directions of B and F in two wires are always directed in a way that Newtons third law holds. Am I correct in concluding that Newtons third law is the "determining factor" of the directions of B and F?