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18 February 2009

Lenz's Law quiz

I was on duty last night. For those of you out of the boarding school loop, this simply meant that I had the privilege of hanging out on dorm for a few hours after dinner, making sure the guys were studying during the study periods, and supervising check-in and dorm cleanup.

Dorm duty is a useful physics teaching time. I advertise to my students when and where I'll be on duty, so occasionally a group of students will show up to ask questions and to talk physics. If nothing else, I get caught up on my stack of papers to grade.

Problem was, the dorm I was on last night doesn't have internet access for the duty master. Thus, you're stuck with a very short post about Lenz's law.

The multiple choice question above is based on the diagram from Serway and Faughn, 8th ed. It was one of eight similar Lenz's law questions I asked on today's beginning-of-class quiz.

Textbooks will define Lenz's law with complicated verbiage... consider S&F's definition. "The current caused by the induced emf travels in the direction that creates a magnetic field with flux opposing the change in the original flux through the circuit." AARRGH! What first-year high school student can deconstruct that sentence?

I suggest teaching Lenz's law from the practical standpoint of finding the direction of an induced current, as would be useful for today's quiz. The process is straightforward. Here are the steps:

1. Point the right thumb in the direction of the magnetic field.

2. Ask, "Is the flux decreasing?"

2a. If the answer is "yes," then you're done -- curl your fingers, and they point in the direction of the induced current

2b. If the answer is "no," then flip the direction of your thumb. Curl your fingers, and they point in the direction of the induced current.

That's it. Don't you like that better than what the textbook said?

To answer the quiz question above, point the right thumb into the page, because the current I creates a magnetic field into the page by the second right hand rule. Is the flux decreasing? NO. Flux is increasing, because the current producing the field is increasing. So flip the thumb, curl the fingers, and the current runs counterclockwise, right-to-left through the resistor.]

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