PHYS 3220 – Fall 2008
Observations from Homework Help Sessions
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Homework #2 – Due 09/03/2008
No HW Help on 09/01/2008 (Labor Day)
NOTES FROM STEVE P — 09/02/2008
I was there for the 2nd half of the session, it seemed that I got far fewer questions than I
was expecting, mostly I just hung out while they worked.
Question 1, statistics of discrete distribution: I saw almost no questions on this. One
student wanted to know why his probability of finding an object between <L>+sigma and
<L>sigma was NOT closer to the exact result predicted by his statistics book, indeed he
wondered if you should "round" sigma to the nearest integer (since the bins come in
integer chunks), which would also have moved that probability closer to what he
expected.
Question 2, classical probability distrubition: I got almost no questions on this. One
student struggled with the fact that his rho(x) was coming out negative (he had chosen a
coordinate system for which dx/dt was negative). He also was having difficulty getting to
v(x), starting not from energy arguments but the equation of motion (though he did work
it out)
Question 3, the Bohr model: Here I saw the most questions. Many students were reading
their old 2170 book, some could not find the explicit statement of the assumptions of the
Bohr model (Knight, e.g., does not explicitly state quantization of angular momentum!
(It's more a "discussion of" than the real details behind the model)
Part c generated the
most questions, there was a lot of discussion about what was meant by "classical
frequency of motion". One student was bothered by the fact that we USED the Bohr radii
(quantized by "n") to find the "classical frequency", which didn't seem purely classical to
him.
Several people did not understand what we were comparing for this correspondence
principle, or how the result was showing a quantumclassical connection. Several people
had no idea why you might expect the classical radiation to have a frequency at (or
centered on) the classical rotation frequency.
Question 4, on normalization, probability interpretation, and partial differentiation:
Again I saw very few questions. Some students were struggling with the algebra in part d
(just small careless errors, usually), and there was a little distress about not being able to
do part c (the numerical integration) analytically. I saw two sketches of the Gaussian
which looked like a simple exponential on both sides (with a cusp in the middle, basically
like exp(x).
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 Fall '08
 STEVEPOLLOCK
 Work, Steve G, STEVE P

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