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Homework Help: A Couple Exam Review Questions

  1. Apr 13, 2009 #1
    Ok, I'm reviewing for my exam and I'm having issues with a few of the answers they have given... Let me know what you think. This practice exam seems a little iffy to me.... There seem to be mistakes in it all over. One question asked the angular momentum of the earth and said the answer was just 2/5MR^2 (no omega), which is just one half of the angular momentum, the rotational inertia.... I'm supposed to be helping a bunch of guys out preparing for this test too, so any help is greatly appreciated. Anyway...

    First problem.
    1. The problem statement, all variables and given/known data
    Initially a hydrogen atom is in its ground state . An electron with kinetic energy
    10.6 eV collides with the atom and excites it. What is the remaining kinetic energy of the
    A. 13.6 eV
    B. 23.9 eV
    C. 3.0 eV
    D. 0.6 eV
    E. 0.4 eV

    2. Relevant equations
    -13.6/n^2 eV
    Ground = -13.6 eV
    n1 = -3.4 eV
    n2 = -1.51 eV

    3. The attempt at a solution
    So the electron would try to raise the energy state from the ground to the first energy state... correct (won't reach farther ones)? So -13.6/4 - (-13.6/1) = 10.2 eV, which leaves 0.4 eV left over for the electron as far as i can see, but the answer is supposed to be D - 0.6 eV

    Never mind the second problem. We've been working with bike tires which are just MR^2, but I guess they wanted us to use a disk, 1/2MR^2, for this one. So I figured it out, there's just the first one now.

    Thanks again for any help.
    Last edited: Apr 13, 2009
  2. jcsd
  3. Apr 13, 2009 #2
    Think about conservation of total energy of the entire system. That's always a winner.
    E(atom initial) + E(electron initial) = E(atom final) + E(electron final)

    You know three of these. You are given the first two and you are correct as to the final state of the atom. Just solve for the electron's final energy. Keep track of negative signs!
  4. Apr 13, 2009 #3
    Also, by "second problem" do you mean the one about the earth? If so the earth is (almost) a solid sphere which has a different moment of inertia than a disc.
  5. Apr 13, 2009 #4
    Hmmm.... that messes with my head a little bit. I must be tired... haha. I thought that's what I did, but not so simple looking. Strange. Ok, that works, thanks :).

    Edit: Actually, now that I do it again..... -13.6 + 10.6 + 3.4 = .4 What's going on?

    No, I actually was talking about a second one i had that i erased, just in case someone saw it and was going to answer it. The Earth one doesn't make sense though either. They ask for angular momentum of it, and the correct answer (according to them) is actually the rotational inertia of it, calculated directly from 2/5MR^2. So unless I'm REALLY dumb tonight they made a mistake.

    Oh well, thanks for the help. I definitely just need to remember to write everything out. Especially when I'm tired... haha
  6. Apr 13, 2009 #5
    well, you're right about the electron problem. 0.4 is the correct answer.

    I'd use the practice exam to help you know WHAT to study but I wouldn't trust it for the answers.
  7. Apr 14, 2009 #6
    Yep, that's what I'm doing. They had two wrong answers on the first practice exam too.... i wonder who solves them... People would be better off with me doing it... haha.

    Thanks for your help.
  8. Apr 14, 2009 #7
    Ha, I told my professor about those two problems, turns out they were both wrong and he went and changed the answers (or in the case of the earth question, the question itself) so that they were right. That's the second time I've done that in this class this year. There are 1400 students in the class. Does no one else care enough to go to the professor and ask about it? Or do they just go with the wrong answer and assume they don't know what's going on? haha
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