A2L Item 226
- Description: Identify the objects experiencing a force as a bar magnet moves past a point charge.
- Goal: Reasoning regarding the Lorentz force
- Source: 283-620 Moving bar magnet and charge
- Keywords: Electromagnetism, Forces, Magnetic Field, Newton's Third Law
The question for students:
A bar
magnet moving with speed V passes below a stationary charge q. What can
be said about the magnitude of the magnetic force on the bar magnet and
the charge q.
-
Fbar and Fq are both zero.
-
Fbar is zero and Fq is not zero.
-
Fbar is not zero and Fq is zero.
-
Fbar and Fq are both non-zero.
Commentary for teachers:
Answer
(4) Many students have a lot of difficulty with this one. All of their past experience has been with a moving charge in a magnetic field. They may not think that it is equivalent to view the interaction from the bar’s frame. Of course, they are correct, but the difference is unimportant for purposes of recognizing that the force on the charge is non-zero. They may invoke the third law by rote, without perceiving any mechanism that could provide a force on the magnet. Discussing this in some detail is a good idea.