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19 February 2010

Writing out the Lenz's Law problem solving process

I think I've done a reasonable job over the years teaching Lenz's Law for the direction of an induced current -- my approach is detailed in this post.  The next obstacle to my students' understanding is to remember and internalize the problem solving process.  This time of year, we've just finished electricity AND magnetism -- it's tough enough to get the class to remember the difference between electric and magnetic fields, let alone the difference between the THREE right hand rules for magnetism and how each one works.

Because the use of Lenz's Law seems so easy on the surface, and because you usually have a 50-50 shot of getting the right direction for an induced current, students tend to give short shrift to using the law correctly.  They don't write out their reasoning, and so they lose opportunities to cement confusing ideas like the difference between magnetic flux and magnetic field. 

I'm going to try something a bit different this year.  On Monday, when I introduce Lenz's Law, I'm going to write out each step in the process on the board, in longhand.  When I assign Lenz's Law problems, then, I am going to require everyone to write out the steps as their solution.  In the past, I've begged, pleaded, and cajoled the class to explain their reasoning.  Well, I'm gonna try modeling the reasoning I want to see, hoping that I get better retention.

Below is an example problem from Serwaybased on the diagram at the top of the page.  I've typed in the template that I want students to use:  they write out the phrases, and fill in the blanks.  This template works with virtually any Lenz's Law problem.

A rectangular loop is placed near a long wire carrying a current I¸ as shown above. The current I is decreasing. What is the direction of the current in the resistor?  Write out the Lenz's law problem solving process:

(1) The direction of the magnetic field is _____________. I know this because___.

(2) The magnetic flux is (increasing / decreasing). I know this because ____.

(3) So I point my right thumb ____________ toward (increasing / decreasing) flux, which means the current flows _______________.

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