I think the link given by Gary provides with the most straightforward=
and convincing demonstration for the classroom (at least for estimat=
ion the length of a chain molecule.) Can the same method be used for =
estimation of the atomic size or at least that of a diatomic molecule=
? I think yes, even though the corresponding experiment would become =
more sophisticated.
There are more possibilities, probably also known before Einstein's=
work on Brownian motion and fluctuations.
For instance, in the Van-der-Vaals equation of state
=20
(P + a/V^2)(V-b) =3D RT
the constant a describes the Van der Vaals long-range attraction forc=
e, and b is just the combined volume of all the atoms in a given mole=
of gas. Is it possible that for some gases a few isotherms P(V) can =
be obtained experimentally for a few different respective T not very =
far above the critical point by the students in a school Lab? If so, =
then it would be very instructive for the students to find a and b fi=
tting the experimental curves the best, and then from b and Avogadro'=
s number estimate the size of the gas molecule. Also, this size can b=
e estimated from=20
(sigma) x n x l =3D 1
where sigma is cross-sectional area of a molecule or atom, n is conce=
ntration, and l is the average free path. The latter can be found, e.=
g. from the gas viscosity. But all this requires that the students mu=
st already have sufficient preliminary knowledge of gas theory, and o=
f course, is much less straightforward.
Moses Fayngold
NJIT=20
=20
=20
-----Original Message-----
=46rom:=09Forum for Physics Educators on behalf of G.D. Williams
Sent:=09Sat 7/9/2005 4:42 AM
To:=09PHYS-L@LISTS.NAU.EDU
Cc:=09
Subject:=09Re: Sizes of atoms (was evidence for non-classical behavio=
r)
This used to be a fairly standard experiment in the UK for 11-14 year=
olds,
doesn't seem that popular these days. There are detailed instructions=
at:
I found that the trays used need to be as clean as possible and I pre=
fer to
use chalk dust or talc as I have a suspicion that it was inhaling lyc=
opodium
spores that triggered my asthma.
Even if you don't try and convince pupils about the lining up of the
molecules and the monolayer business it still gives a nice upper limi=
t. I
don't know the origin of the experiment but I note that in "Physics f=
or the
Enquiring Mind" by Eric Rogers it says late 19th century and I also r=
ecall
reading something about Benjamin Franklin realising that oil on water=
would
make a very thin layer but not realising the next step, not surprisin=
g
really when the nature of the atom was still unknown.
Cheers
Gary
G. D. Williams,
Editor, Physics Education
National Coordinator, IoP Teacher Network
gary.williams@iop.org
gdwilliams100@hotmail.com
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