This is my first posting to the list so please be tolerant.
I'm currently reviewing some chapters for a "well-known" cal-based intro physics book. It uses a popular explanation (I've seen it elsewhere) for deriving the emf developed by a conductor moving with uniform velocity through a constant magnetic field. Jsut to be thorough, here's the setup:
Two conducting rods are placed parallel to each other. Let's call them rails and say the left end of both rails are connected by a resistance, R. Another conducting rod of length L is placed across (perpendicular) to the rods and can slide freely along the rails. An external agent acts on the rod to give it a uniform velocity, v, parallel to the rails (and away from the resistance). A uniform magnetic field is applied perpendicular to the plane of the problem (let's say into the page). Assume a current flows through the circuit (through R, along one rail, up the rod and returns along the other rail) such that the charge carries have a drift velocity v_d.
The text says that each charge carrier, q, in the rod has the velocity v and since q moves in a magnetic field it experiences a lorentz force F_M= qv cross B. The text then states that the work done by this force pushing the charges along the rod is F_M * L= qvBL. Since emf is energy per charge, the motional emf between the ends of the rod is E= vBL.
Now this bugs the heck out me because magnetic forces are not supposed to do work. Using this explanation just sets the students up for confusion and puts me in a pickle to try to justify it.
I prefer the explanation of calculating the changing flux, Phi_M= BLx, through the circuit where x is the position of the rod measured along the rails from the resistance. This gives the emf E= -dPhi_M/dt= BL(dx/dt)= BLv.
After all this yacking, my reason for posting is to get a range of opinions of this text's derivation.
Thanks for the time.
Kenny Stephens
---------------------------------
Do you Yahoo!?
Read only the mail you want - Yahoo! Mail SpamGuard.