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As another way of skinning this cat, note that if you know
the charge on the electron (â¡ 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?
p.s many of the JD assumptions were discussed when I was presented w/
that demo.
1) What is lycopodium powder? Why are we using such an
unfamiliar substance? Why not some more-familiar powder?
use dictionary
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).
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.
7) What role does surface tension play in all this?
try other liquids including 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.
The cited reference
http://www.stkate.edu/physics/phys100/MoleculeSize.htmlstates 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.