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Re: [Phys-L] Inference Lab Design

On Aug 13, 2012, at 15:20 PM, Turner, Jacob wrote:

I've been banging my head on how to design a lab which will impart a
sense of using inference. I intend this for our lower level
(non-calculus) freshman labs, so should be fitting for AP High School
physics range as well I imagine.

One that I've used quite successfully is the Automatic Flushing Toilet exercise, which was published in Physics Teacher March 2011. Essentially I tell the students I want to know how an automatic flushing toilet works (i.e. how it knows when to flush), but I won't believe any of the "propaganda" they might find online - I only care about their own direct observations. We spend some time discussing how they *might* work (e.g. sound, light, motion, heat, weight, etc...). Then I tell them that they must design tests which distinguish the possibilities, and they have to actually perform the experiments! Of course this assumes there is ready access to these toilets, which we have at our University.

A few interesting things popped up along the way. For example, students will often switch the lights in the room on and off, notice no flush, and then claim that was a test for light - that light level can't be what determines the flush. I have to point out that they can't assume the engineers were stupid, and that the engineers certainly wouldn't have made the light detector flush when the lights went *out*, that they'd ignore huge swings in light intensity. This leads some students then get the idea of using the toilet in the dark, and when it flushes, ah ha! Can't be light! Yay! Another common one is the "I don't see a microphone, so it can't be sound" argument, or the "it would be too expensive to use x-rays for this device, so it can't do that". I don't accept answers like that, essentially arguments from incredulity or ignorance.

Students also tend to test to their prior conception, which usually is motion (which is not the correct answer).

Clearly this activity explores the notion of falsification, but also the limits of measurement, prior conceptions, and it can be done in the first week, and requires very little equipment. It's also a great ice-breaker, and when I've used it as a homework assignment, I've heard many interesting stories of the students' adventures. :)

hope this helps,

Brian Blais

Brian Blais