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Confused by my modern physics class

  1. Nov 15, 2006 #1
    My lecturer is currently addressing modern physics. "we" (more like "he") solved the schrodinger equation for hydrogen, and then into all the quantized angular momentum, intrinsic momentum.... now we are into quantum statistics...

    Seriously, I have absolutely no idea what my professor is talking about in class, and I doubt that if anyone else is following (at least none of my friends are). It seems like my professor just keeps throwing ideas at us without explaining why or where they come from. Example: electrons have intrinsic angular momentum, when "adding" wavefunction, femions is +, boson is - (i forgot, it may be the other way around...) four quantum numbers of hydrogen atoms n,l,m_l,m_s, j, m_j...

    My professor does explain things (somewhat) but they do not really make sense to me. Am I supposed to be that confused? Is this the way one learns intro quantum/modern physics?????
    Last edited: Nov 15, 2006
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  3. Nov 15, 2006 #2
    You will be a bit lost the first time these concepts are explained to you....you just need to make an effort on your own time to try and comprehend the stuff.

    And typically, a "modern physics" course is a general survey of many different topics and the scope isn't to go into great depth on any one topic.

    Explanation of things like intrinsic spin is not trivial. The spin quantum number comes out of the solution of the relativistic Schrodinger equation, which I assume your professor hasn't touched.
    Last edited: Nov 15, 2006
  4. Nov 15, 2006 #3


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    In my first "modern physics" course as a sophomore, our professor warned us that it was impossible to fully understand any particular part of quantum mechanics unless you first understood some other part of it! :bugeye:

    leright is right, at the level you're at now, lots of times a modern physics course just has to give you results with only a sketchy derivation or justification. The full treatment requires a lot of math and is usually done in a senior-level or graduate-school course. I didn't see the complete solution of Schrödinger's equation for the hydrogen atom (including the radial part with associated Laguerre polynomials) until graduate school.

    It's a lot different from classical mechanics and E&M where you can see most of the basic derivations even in the freshman general physics course.

    You do have a textbook, right? It's often helpful to look in other textbooks for alternate ways of presenting the material, too.
  5. Nov 15, 2006 #4
    Yes, there is a textbook for my class. But the textbook is just as terrible... it doesn't have any "real" justifications for anything...for example, like jj coupling (or whatever, dunno the name), it just says that some angular momentum may interact and put a whole bunch of selection rules and quantized number which I have no idea why they are quantized... I guess I can't blame the text for not going into full depths of complicated math...
    anyway, I've picked up some intro quantum physics books, relativity and such from the library... but its just that the class is covering so much in so little time. When I begin to get a firm understanding of some topic, the professor has already moved two chapters ahead... We are on Chapter 8 and I haven't even fully understood Chapter 6 yet.... : (

    Trying to understanding something from the text is really painful... For instance, the last Saturday I was trying to spend most of my day studying (Yes, I study on Saturday...), but I ended up trying to verify the equation for magnetic moment... which wasted me 3 hours (trying to go into generalization)...Ultimately, I didn't sucessfully verify the equation and my studying goes to waste.
    Last edited: Nov 15, 2006
  6. Nov 15, 2006 #5


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    Here's some advice someone once gave me when I started college:

    Try your best to learn what's going on... but if you get stuck trying to really understand things and you just have to move on [because of time constraints], press your "I believe" button and move on. In my experience, a deeper comprehensive understanding doesn't come during that first exposure. You have to revisit the subject several times, at various levels [e.g. as a tutor, as a more advanced student, as a teaching assistant, as an instructor, etc...].
  7. Nov 16, 2006 #6
    I got through my Modern Physics class by using this motto: "Just shut up and calculate."

    I'm now taking an Intro Quantum Mechanics class, and I'm kind of starting to see the logic behind this stuff, but just barely.
  8. Nov 16, 2006 #7
    Y'all are making me glad Modern was not a requirement for my undergrad, and I didn't take it -- I opted to just go hard core to take more of the upper-level stuff. But -- I am tending to think the quality of Modern depends significantly on the instructor/book/curriculum, since it is inherently of a survey-nature. I hear really good things about Modern from some schools, and really bad things from other schools.

    So my questions: Is Modern a requirement in most programs nowadays? If so, what is the reasoning? For those that HAD modern -- what are their impressions? I feel I did OKAY (even fine) without it... but maybe in retrospect having a survey class would be good for perspective... maybe I'm lacking some perspective on the nature of physics in the modern world.
  9. Nov 16, 2006 #8
    Yep, it's pretty much a requirement everywhere. And yep, you will do a lot of calculating without knowing WHY. Modern physics texts are teasers for the material you'll encounter in later semesters, and most of them let you know beforehand that some of the derivations are beyond the scope of the text. It's a shame, since first impressions are usually lasting ones, and I know my impressions of certain subjects were tainted. If you're a go-getter, find yourself more advanced, undergrad texts that cover the material you're going through in your modern class and appreciate the Why.
  10. Nov 16, 2006 #9
    I took a 2 semester modern course using Kenneth Krane's Modern Physics. As a physics major, I was frustrated by the lack of mathematics in the book and a serious lack of challenging end of chapter exercises. The first semester was also for chemistry, materials science, and EE majors so it was conceptual since everyone's backgrounds were so varied. I think that physics departments see these courses as an extension of basic freshman physics. You learn mechanics and EM at a shallow level as a freshman, and then take the upper division courses. I think the same concept applies to these modern physics courses.
  11. Nov 16, 2006 #10


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    Bingo! That's exactly it... it's a survey course. Most students can't handle a full-blown upper-level QM or nuclear physics or atomic physics course without having seen some of the basic stuff beforehand, just like most freshmen can't jump right into Symon or Marion for mechanics. Also, most upper-level courses don't do much with the history or the experimental side of the subject, and most "intro modern physics" textbooks provide some of that background.

    You have to keep in mind that just by being here, you're not like "most students." How many other students do you know at your school who hang out on Physicsforums or other similar sites? :smile:
  12. Nov 16, 2006 #11


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    That's something I noticed that blows me away. Most of my school bench comarades (and I'm talking final undergrand year) don't like physics! It's probably the scholarly discipline that they dislike the less but they are not passionate about it and they won't talk about it for fun. It's just a chore to them..

    I guess I should consider myself lucky. I get to have fun even when I'm doing my "chores". :smile:
    Last edited: Nov 16, 2006
  13. Nov 16, 2006 #12
    It is funny because it is true... I often check this forum when I go on the internet...

    I bet most of my friends don't even know that this website exists.
  14. Nov 16, 2006 #13


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    Tell them about it and see if they'll visit it. :rolleyes:
  15. Nov 17, 2006 #14
    While it's surely been around for a while NOW, I don't think Physicsforums was around when I was a student... (yep -- :surprised SHOCKING! :surprised)

    I found PF the week I defended my PhD (hence the name... which I'll change to something more modest soon). Since I had lots of free time after defending, I inquiried the undergrads in the research lab -- and a surprising number of them knew and used the forum... but they probably aren't "normal" undergrads, since they are in the lab doing research.

    I guess I'm just surprised Modern is a REQUIREMENT, not an ELECTIVE. I never took a "math methods" for that matter... I just took a lot of MATH.
  16. Nov 17, 2006 #15


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    Did your freshman general physics course include "modern physics" topics? Some schools have a three-semester general physics sequence in which the third semester is mostly modern physics. Or else they have a two-semester sequence and do some modern physics during the last two or three weeks. But I think in those cases they also have a separate modern physics course.

    Next year our separate intro modern course will become part of a three-semester calculus based general physics sequence.
  17. Nov 17, 2006 #16
    I remember during our first Modern Physics Lecture our professor busted out the gradient and derived the EM wave equation from Maxwell's equations. I went home that morning and told my girlfriend that I hit the limit of my academic abilities and could no longer be a physics major.

    As the lectures went on, things got a bit easier to follow.
  18. Nov 17, 2006 #17
    That is exactly what I am talking about! My professor went straight to Fourier series, delta function, and Fourier inverse when we were doing Schrödinger's equation...couple lecturers later, we went over harmonic oscillator and hydrogen atom... I mean they were fascinating stuffs but everyone was totally lost when my professor was deriving stuffs in such a short amount of time.

    Anyway, as time went on, I eventually understood the harmonic oscillator and some of Fourier series...
  19. Nov 18, 2006 #18
    There is a story of a baseball player who struck out the first time at bat in the Majors. The catcher was supposed to have said, "Welcome to the Majors". The player eventually went on to have a fairly good career in baseball inspite of the slow start. Well, when it comes to modern physics, I say welcome to the majors. Keep swinging. Everyone finds it hard.
  20. Nov 18, 2006 #19
    Sometimes I get a little discouraged because I don't know Fourier Transforms, Guassian Integrals and Differential Equations, but I guess we really don't need to know how to solve those things at this time.

    Overall the professor is not that tough on us. We don't have to remember derivations for this class.

    What is annoying is that people in my class like to get the professor off track. We were supposed to have learned the Hydrogen atom already and are supposed to be on spin and atomic physics. There are 3 other chapters where we are supposed to at least touch on having to do with bonding and particles, but I doubt it's happening. :( I will read it myself.
  21. Nov 18, 2006 #20
    I've had quite an interesting experience with my modern physics class this semester. We started out the first couple weeks with special relativity, and a small intro to general. That was all fine. I understood it well, I aced the first test, and we continued.

    Then, all of a sudden, the professor hit the gas. Class started to get hard. Really hard. And the professor has just been breezing through all of the topics. Now, as the semester draws to a close, I only have 4 classes left before the final!

    Yeah, I'd say it's been a pretty tough class.
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