The WebAssign homework problems for SV8 Chap. 28 are now posted and available. The lectures on Chap. 28 in SV8 will be on Tues., 28 Nov. and Thurs., 2 Dec. The Chap. 28 problems will be available on WebAssign until 5 pm, Mon., 6 December. Good luck! Have a nice Thanksgiving break.
Prof. Koch
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26 comments:
Professor(s),
Do any of the homeworks or labs get dropped this semester?
Course policy is in the syllabus, which all students have known about since the first day of class. The only scores dropped are the lowest five clicker scores.
There were many problems with the first homework assignment that we had caused many people problems, and therefore many of us had low grades for it... can anything be done about that?
Example 28.1 page 897 in the text, says to find the longest wavelength photon emitted in the balmer series and determine its freq and energy.
The "worked out" example shows this:
1/lambda=Rh((1/nf^2)-1/ni^2)) the text book somehow comes up with 5Rh/36. Where did these numbers come from? .25-.11111 does not get 5, or 36, or anything close to that.
-Vito
Just wondering,
when will we see the retro-active regrade of our first web assign assignment?
HOMEWORK 27 IS POSTED DUE MONDAY AT 5PM...
I HAD THEM WORKED OUT AND JUST TRIED TO PLUG THEM IN REALLY QUICKLY TO FIND THAT IT WAS SHUT DOWN AT 3:30 PM.
BY THE TIME THIS IS CORRECTED, IT WILL BE PAST 5PM AND I AM NOT ABLE TO ENTER MY ANSWERS.
PLEASE ADVISE
Vito, (1/4 - 1/9) = 9/36 - 4/36 = 5/36. I don't understand your problem.
"These problems are due Monday, November 27, at 5:00 PM. The first six pertain to material that will be covered in lecture on November 18; the rest, November 23."
Is there some reason that my post remained unanswered?
-CA
Sorry about the due date mistake - I'll re-open that assignment for everybody, due 3:00 today.
The reason your post went unanswered is that you didn't post it in the stream for Chapter 27. I was monitoring that, on and off, all day yesterday. There were no posts, so I concluded that nobody was asking for help.
The first assignment was set to reduce credit by a small percentage for each incorrect answer. It was set that way for eveerybody in the class, and that system was announced. It was not a mistake. Subsequently, we decided to change the grade algorithm to allow unlimited (almost) incorrect answers without penalty. We looked into the possibility of retroactively regrading the first assignment that way, but it turns out not to be possible. The grades of the first assignment will stay as they are.
That's wonderful...thanks for letting me know when i spoke to you in class. I checked for a response REPEATEDLY last night and this morning. I was in class from 10:30am until I went to your class at 3:50 - so again, I was left out of those who were allowed to hand in the assignment. Had you mentioned it to me during class, when I asked why there had been no response - this would have been pertinent information.
-sincerely, a disgruntled Chriss a.
If you don't email me and let me know who you are, I cannot answer you personally. I made every effort to publicize the information: posting the change as an announcement in blackboard, and answering in the same blog stream where you had brought the problem to my attention.
I am having problem with question 5, part c.
The question asks, "By what fraction would the Earth-Moon radius have to be increased to increase the quantum number by 1?"
In part b, I calculated a very large quantum number n, to the power of 68.
In part c, if such a large quantum number is increased by 1, a very small number, shouldn't the radius be virtually unchanged and the fraction be close to 1 because the increase of one is negligible?
any input is greatly appreciated. thanks.
I am also having trouble with question 5, part c. As the person above mentioned, increasing n by merely 1 should mean the change in r is pretty much negligible.
Thank you for any help.
Dear Anonymi at 12/3/10 2:49 am and 12/4/10 at 1:16 am:
Your comments mean you understand the gist of the problem. Great! You've found that the moon's "(principal) quantum number" is enormous. Therefore, as I covered in lecture for the Bohr atom, the spacing between energy levels is tiny. To make a change of 1 in the (principal) quantum number requires a verrrrry small (fractional) increase in the moon's orbital radius. The whole idea of problem 5(c) is to calculate how tiny. Hint: You know what n is and you know from the problem statement what Delta n is supposed to be. Hence you know what (Delta n)/n needs to be. From the Bohr-atom relationship between r-sub-n and n , you need to calculate what (Delta r)/r needs to be. It's just like the problems we've been doing all semester in the lab pretest questions! Good luck.
More to Anonymi at 12/3/10 2:49 am and 12/4/10 at 1:16 am:
When you calculate how small the Delta r is, it may amuse you to realize that it is a large number of orders of magnitude smaller than the diameter of a proton, in fact smaller than the diameter of any known (elementary) particle that is not "thought to be point-like" (no dimension), like an electron. We cannot say an electron is actually a "point" because we cannot measure a point. We can only set a limit, viz., that if it has a diameter, that diameter is less than such and such in meters. I don't recall what the present limit is on the electron, but somehow something on the order of 10^(-20) meters comes to mind. That induced me to look it up, where I see at http://hussle.harvard.edu/~gabrielse/gabrielse/overviews/ElectronSubstructure/ElectronSubstructure.html that the limit is 10^(-18) meters. My memory wasn't too bad!
As I wrote in the Uncertainty, Error and Graphs manual, "You cannot measure zero." Physics is an experimental science, so the ultimate arbiter of "the truth" is what we measure.
Professor Koch,
I think I now understand how to approach question 5, part c, but I am still having some trouble figuring it out. In solving for (Delta r)/r, there no error in a0, and Z does not apply for this question, correct? So when solving for (Delta r)/r, you only have sqrt(((Delta n)/n)^2)) (which equals just (Delta n)/n ).
Obviously I am missing something, but cannot figure out what it is.
I appreciate your help.
I am also working on problem 5 part c and this is what I have so far:
(deltar/r)=(1/2)(deltan/n)=(1/2)(1/answer from b)
I have calculated it several times but webassign says my answer is incorrect. What am I doing wrong?
Anonymi at 10/5 4:29 am and 10:47 am,
Look at (E.8) in Uncertainty, Error and Graphs. What is S? It's r-sub n. What is A? It's n (the quantum number, not the exponent). What is the power n (the exponent, not the quantum number)? Answer: it's 2. The constant a-sub0 is just that, a constant. Proceed from there. Prof. Koch
Can someone help me with number 9? I don't know how to figure out what Zeff or E2 are for the equation EL=-(Zeff^2)*E2. I saw a worked example on cramster for the version in the book, but they don't explain how they find those numbers. The book doesn't really do a great job of explaining it either.
I am having trouble with problem #9, what should we use for Zeff? I have tried (Z-1), (Z-4) and (Z-9), but nothing has worked. I am using E2 to equal 13.6/n^2 which is 13.6/4. For my particular problem, I have Z=25. I have calculated Ek to be -7833.6eV. I understand how to find the wavelength once I have DeltaE, so I think my problem is in calculating EL. The only example for Zeff in the book describes EM and they use (Z-9) for that transition. Please let me know what we should be using for Zeff in this problem and/or if I am making a mistake elsewhere.
Dear Those Having Problems with Number 9,
I looked at Example 28.4 to use as an example for this problem.
As for Zeff, it is the effective nuclear charge. So, Zeff is Z-#of electrons between the nucleus and the electron that is moving.
Hope this helps. If it is not clear, I can try to give a little more detail.
I got the answer right for number 9 by just plugging in every number I could (ridiculous waste of time). If you could please further explain how you determine the number of electrons I think that would help me more than anything. Thank you for replying!!
For 5A, im calculating the angular momentum using kg(m^2/s), but i keep getting a response that says im off by more than 10%...
Anonymi asking about problem 9:
As 12/5/10 at 11:10 pm said, looking at Example 28.4 is helpful for this problem. You need to use different values of Z_eff for the K shell (as shown in Eq. [28.20]) and the L shell (as mentioned in the right-hand column at the top of Example 28.4.
Prof. Koch
Anonymous at December 6, 2010 12:54 PM:
Yes, the units of angular momentum are what you write, so you must have calculated the wrong number. You need to know the mass, the orbital velocity, and the orbital radius to calculate it.
Prof. Koch
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