教学研究心得选讲

1. 教学研究心得选讲 电磁感应 ―Master’s perplexity 06.10.2014.

2. 考题的争议 问题的澄清 大师的困惑

3. We know of no other place in physics where such a simple and accurate general principle requires for its real understanding an analysis in terms of two different phenomena. Usually such a beautiful generalization is found to stem from a single deep underlying principle. Nevertheless, in this case there does not appear to be any such profound implication. We have to understand the “rule” as the combined effects of two quite separate phenomena. (17-1)

4. 4. A circular wire loop of radius r falls under the influence of gravity and in the presence of a magnetic field given by …… Solution:

5. Controversy rises: The contribution comes from z-component only, and the loop “cuts” the r -component lines. Or the charges in the loop experiences Lorentz force when the loop falls.

6. Same value of B0apr2v makes us doubtif these two terms are same thing indeed.

7. The electromotive force is of nonstatic origin: curl electric field force, Lorentz force and chemical reaction (battery), etc. For a closed filamentary loop, the induction emf is

8. The change rate of flux is unique, the surface it passes through may be different (boundary is same) and it might come from different component of the magnetic field.

9. For example, one of my TAs Mr. Chen Hongyi suggests that when the loop moves, the surface is stretched and top surface is kept still as shown in the figure. In that case the only flux change is

10. In lab system There is only the motional emf.

11. When the loop reaches to terminal speed (t = 0 and somewherel0), we can take it as an inertial system. In our case

12. When the speed is not so large compared to the light speed, g ≈ 1.

13. The induced emf can be calculated according to ( ) According to Faraday’s law, it is equivalent to the contribution from curl electric field:

14. So in lab system we have motional emf and in the loop system we have induced emf. “Two quite separate phenomena” are indeed not so separate. Though we can not prove, even in principle, that two emfs can be expressed in a single form, our example does show the correlation between them and the possibility of transformation between them.

15. 谢 谢 dxlu@nju.edu.cn