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On 07/09/05 16:07, Bernard Cleyet wrote:_______________________________________________
>>As another way of skinning this cat, note that if you know
>>the charge on the electron (‡ la Millikan, just barely doable
>>in a HS class) then you can get kT (as in eV/kT) from the
>>I/V characteristic of a diode (measure it -- easy!) and then
>>get the mass of nitrogen from the speed of sound (dimensional
>>analysis -- easy!).
>
> Less than a class period?
1) I didn't realize that was the criterion.
2) In any case, tying up the loose ends in this version
has got to be much easier & quicker than tying up all the
loose ends in the oleic acid version ... see below.
3) Actually yes, the diode I/V characteristic can be
measured in considerably less than a class period, given
the right equipment and modest preparation. It just ain't
that tricky. (OTOH of course you can't have 30 students
all do it independently in an hour if you only have one
voltmeter.....) Hint: If you've got a nice four-and-a-half
digit voltmeter, just measure V directly. If all you've
got is some Rat Shack meter, use *two* diodes and make
a differential measurement. Measuring at a single
temperature suffices for present purposes; if you want
to gild the lily you can measure at 0C and 100C and compare
results.
I'm assuming they already know the speed of sound, but if
you need to measure it there's lots of ways of doing that
in much less than a class period. For example, video (with
audio) of fireworks one km from the camera ... plus a
stop watch (or video editing-studio software if you want
to get fancy).
And figuring out the scaling law for the velocity of air
molecules takes less time than it takes to tell about it.
Of course they can't do it in September in an intro-level
course, but after a few months of emphasizing scaling laws
at every opportunity they catch on. There's only four
energies even remotely relevant to the problem, and two
of them can be ruled out pretty easily. Set .5 kT equal
to .5 m v^2 ... there's nothing tricky about it.
p.s many of the JD assumptions were discussed when I was presented w/
that demo.
Really? I'm impressed. They sure weren't discussed when
the demo was foisted on me.
1) What is lycopodium powder? Why are we using such an
unfamiliar substance? Why not some more-familiar powder?
use dictionary
Hmmm. We started out with a powder I've never heard
of. Now I know it comes from a plant I've never heard
of. This doesn't answer the question of what properties
it has that are relevant to the present situation.
2) For that matter, most high-schoolers aren't familiar
with oleic acid, either. Given that we are dealing
with *two* unfamiliar substances, how do we know the
behavior we observe tells us the size of oleic acid
molecules ... as opposed to telling us something about
lycopodium molecules? We're taking a lot on faith, here.
If obscure, peculiar substances are required, it makes
me suspect that the result is not robust, not reliable.
do a number of expts. w/, inter alia, other dusts (cork dust used by bc
also) and oils for comparisons and upper limit(s).
In less than a class period?
3) You say the oleic acid molecules are long and skinny,
and stand upright on the surface. How do you know? If
they were to lay down on the surface, how big would the
resulting systematic error be? A factor of ten? More?
4) You say the oleic acid forms a monolayer. How do you
know? There are molecules with rather similar structure
that form thicker layers. Salad oil, for instance.
5) You say the oleic acid molecules congregate densely,
side by side. How do you know? How do you know they
don't spread out and form a 2D gas, with space between
the molecules? The lycopodium powder spreads out.
6) You say the oleic acid doesn't dissolve in water. How
do you know? There are molecules with verrry similar
structure that do dissolve. Soaps and detergents, for
example.
shake mixture in sep. funnel. Sep. and find upper limit on miscibility.
Did you really do that when you were in high school?
I am truly impressed. Didn't the formation of micelles
make it kinda hard for the typical high-schooler to
interpret the observed result?
7) What role does surface tension play in all this?
try other liquids including Hg.
In less than a class period?
Did you really do this in HS? What happened when you
put oleic acid on the Hg?
8) Does the water have to be pure? How pure? How do
you know?
here's a chance to use a conductivity cell and various solutes, etc. etc.
In less than a class period?
The cited reference
http://www.stkate.edu/physics/phys100/MoleculeSize.html
states that oil in general will spread out until it is one
monolayer thick ... which is almost *never* true.
(Even when I was in HS I knew this wasn't true; you
can tell by looking at the coloful Newton rings that oil
thickness is on the order of micrometers, not nanometers.)
I don't entirely subscribe to the rule "falsus in uno,
falsus in omnibus" ... but garbage like this certainly
makes students wonder whether the whole oleic acid demo
is a swindle.
be patient.
Really? How patient? How long does one have to wait
before Exxon-Valdez oil becomes a single monolayer on
the surface?
Try it sometime. Let us know what you observe.