Magnetostatibs treats the "source current" (the one that sets up the field) and the "recipient current" (the one that experiences the force) so asymmetrically that it is by no means obvious that the magnetic force between two current loops is consistent with Newton's third law. Show, starting with the Biot-Savart law (5.32) and the Lorentz force law (5.16), that the force on loop 2 due to loop 1 (Pig. 5.62) can be written as In this form it is clear that F2 = ?? F1, since π changes direction when the roles of 1 and 2 are interchanged. (If you seem to be getting an "extra" term, it will help to note that d12 ?? π = dπ. )

F2 = -11 112 - dly-dh.