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Comment from S. Pollock (CU Boulder, visiting OSU and teaching Paradigm “Vector Fields), Nov 2009:

I used this activity on day 1. It was quick, simple, and worked very well. I used a small hula-hoop for the “area”, and passed out rulers to just one small cluster of 4 students. I gave them half a dozen rulers (4 of one size, 2 of another), and said “make a uniform field”. Happily, 2 picked small rulers, 2 picked big ones, and they held them up all parallel to each other. So other students immediately chimed in that this wasn't uniform in magnitude.

Next, I asked them to make a more jumbled field, (but their hands were all on the desk), so I purposefully lowered the hoop just *above* their hands, and asked what flux this represented. (So the rulers poked through, but the “field points”, the hands, were below the plane of the hoop) There was a mix of responses, many “positive”, and a couple of “zero”, and one of the 4 participants spontaneously moved his hand up so that IT was in the plane of the hoop. So I asked the class “why do you suppose T. just moved his hand”? Got a good discussion going about how flux is evaluated IN the plane, the E arrows are represented by rulers but don't have physical length.

Lastly, asked one student “could you adjust your field so that it contributes zero flux”? And she quickly turned it parallel to the loop.

Pretty brief and easy activity, no problems, seemed to get out several of the key ideas in a visual way.

-S