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Re: [Phys-l] unbiased experiments +- index of refraction



Hi Phillip-
So the student is justitified in believing that you have set up
a lab in which some mysterious thing called "momentum" is mysteriously "conserved". Where is the insight in that? Insight can come when your setup leads the student to make a prediction that is dramatically different from the outcome of the experiment.
I think that elementary labs should teach the student how difficult it is to make a relieable, repeatable measurement.
Regards,
Jack


On Tue, 12 May 2009, Philip Keller wrote:

Are we sure that there is NO educational value in "verifying" labs? That's a strong claim, and if it is true, I am troubled to hear it because I sure do a lot of them in my first-year high school physics classes.

For example:

We use motion sensors to get velocity data to see if momentum is conserved in a series of collisions. We also check if energy is conserved, and if not, we calculate the fraction that is "lost" in the collision. Is this a pointless exercise for a first year class?

Or here's another: we just used resonance with tuning forks to find speed of sound. Oversimplified? Yes. Handed correction factors without explanations...guilty. But this is a first-year, non-honors class. I think they are more likely to believe and remember what resonance is after raising the glass and hearing the tuning fork magically amplified by the resonating column of air.

As I have argued on other occasions, teaching involves far too many pedagogical judgments to investigate each one with a FCI-linked research project. I'm not looking for religion, but I think one-true-physics pedagogy may have room for verifying labs AND discovery labs, with more of the latter as students advance.

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of Edmiston, Mike
Sent: Tuesday, May 12, 2009 12:26 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] unbiased experiments +- index of refraction

The only problem I have with totally eschewing "accepted-value" labs is
that students should learn to calibrate their instruments. Thus, the
accepted-value lab is the starting place to make sure the equipment is
working properly. The accepted-value lab should not end once the
student has determined how well the accepted value was measured.

One of the things I like about the "refractive index lab/demonstration"
I described is that when measuring sugar content in drinks we don't
actually measure a numerical refractive index and compare it to any
published refractive index values. Rather, students calibrate the
homemade refractometer with sugar solutions they create, and then use
the calibrated refractometer to measure some solutions for which they
don't know the sugar content.

If you have time, you can make the lab even more productive by adding
several components. One component is to have the students calibrate
the
refractometer with solutions they make, then have them measure some
"unknowns" you made. This simulates a common practice performed in
many
commercial labs that periodically have to be recertified. The
commercial lab calibrates their instruments using their usual methods,
then they have to determine several "unknowns" provided by the state or
federal EPA or other certifying agency.

Or, an in-house lab in an industry such as a refinery might test their
waste-water effluent for various things, and they periodically have to
split the samples and analyze them both in-house and by hiring an
outside EPA-certified laboratory, and the numbers need to agree within
some tolerance.

Another component that can be added to the refraction-measured
sugar-water is to find out how much the results differ if the
calibration and unknowns are not measured at the same temperature. Can
you calibrate at room temperature and then measure a can of Coke
straight from the fridge? (No.) Can you calibrate with
refrigerator-temperature standards and then measure a can of Coke
straight from the fridge? (Yes.)

For more advanced classes, students can use a hollow prism on a
spectrometer and find out what things they need to control in order to
get a refractive index measurement that agrees with a published value
within three decimal places. It's not difficult to get this level of
accuracy, but it's not a piece of cake either. For example, if the
students are not told that published values are measured with 589-nm
light, then using a He-Ne laser they might get good agreement with a
low-dispersion liquid such as water, but poor agreement with a
high-dispersion liquid such as toluene. Can they figure this out on
their own?


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu
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Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
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