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Prerequisites for E&M

  1. Oct 8, 2012 #1
    Ok, this class is killing me. The only college physics classes I've taken are physics 1 and 2. Then I walked into this class and it seems like I'm missing some physics and math classes. I meet the prerequisites, which are only Calc 3 and Physics 2. But this book we're using (from 30 years ago, since this is the only subject in the world that doesn't get updated text books) has differential equations in it. That wasn't a prereq for this course.

    And aside from that, I just don't like this course. It's formula after formula and confusing notation after confusing notation. If the rest of my physics courses are going to be like this, I'm switching majors. Physics 2 was interesting and I actually understood what was going on. In this class, I have no idea what the hell is going on.
  2. jcsd
  3. Oct 8, 2012 #2
    Is your question what background are you missing or..?
  4. Oct 8, 2012 #3


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    Yeah, that's E&M :frown:.

    I'm really surprised they don't require differential equations! But beyond that, E&M is div, grad, curl. Div, grad, curl. Say it with me: Div, grad, curl.

    I struggled with it too!
  5. Oct 9, 2012 #4
    I would think the answer to your question depends entirely on the specific E&M course you're taking and how it's being taught.
  6. Oct 9, 2012 #5
    Ideally, physics 2, calc 3, and differential equations (ode and pde) should be the prereqs for E&M.
  7. Oct 9, 2012 #6


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    What textbook is being used? Maybe you should supplement with a better textbook. Whatever knowledge of ODEs you need for intermediate EM you can easily pick up on your own with some self studying and the PDE technique(s) are usually introduced and explained in the text (for griffiths at least). I do agree with you in that I find intermediate mechanics much more interesting.
  8. Oct 9, 2012 #7
    I don't know, I'm kinda just venting. But I really am wondering if there was a mistake. But the teacher has asked a few times who has taken ODE or PDE, so apparently it's not required. I think it should be if there's going to be differential equations in the homework.
    Good to hear I'm not the only one. But a lot of people in my class struggle with it apparently. I don't really associate with anyone in the class, so to me it looks like everyone but me understands this course perfectly. But when the test scores were given back, the teacher announced how he curved the grades, and if you got a 38 on the test, that was a C. So apparently everyone did bad. I still got a D, but I ran out of time and studied the wrong chapter.
    Introduction to electrodynamics third edition by David J Griffiths. It's from 1981 I think.
    To me, it seems like the older the text book is, the less user friendly it is.
  9. Oct 9, 2012 #8


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    What new things have been discovered in classical E&M in the past 30 years? :confused:

    Unless you got your copy of Griffiths from a used-book store, you've probably got the third edition which dates from 1999. The updates between editions have been for pedagogy, based on feedback from users. Different people may have different opinions, but I don't think you'll find another book at this level of E&M that's significantly more "user-friendly."

    As for the differential equations, whether they'll be a real problem for you depends on how the course is actually taught. For example, Griffiths's book introduces Laplace's equation (a partial differential equation!) in chapter 3 and solves it for some simple examples by the method of separation of variables, It's a slow-paced treatment that's obviously aimed at someone who's never seen this before, and has only basic knowledge of partial derivatives. A "lower-intermediate" E&M course might even skip over this stuff completely.

    I suggest you talk to your professor about it and point out the sections in the book that you're worried about. Hopefully he'll say something like "nah, we're not going to do that stuff."

    It's common to learn new math in physics courses, anyway. A lot of students see Schrödinger's equation and learn how to solve it for simple things like the "particle in a box" using separation of variables, before they've had a course in PDEs or even ODEs, just basic calculus.
    Last edited: Oct 9, 2012
  10. Oct 9, 2012 #9
    Get Griffiths' latest edition, but all three are very user friendly.

    You shouldn't be learning new math as you're applying it IMO

    Get a differential equations book or a math methods book and review, but if this is only the first test and you're using griffiths I can't think you've gone over anything past gauss' law, so I don't know what diff eqtns you've been doing.
  11. Oct 9, 2012 #10
    Keep in mind, a prerequisite does not mean you have such a solid background you'll breeze through a course. It's a minimum. It's the least amount of prep you need to do a course (usually).

    Also, Griffiths E&M book is amazing! It's one of the most user friendly books there is imo.
  12. Oct 9, 2012 #11
    Do you have the book? Could you tell me how he goes from equation 3.26 to 3.27?
    It's not explained at all.
  13. Oct 9, 2012 #12


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    I assume you're talking about the second order ODEs that he then gives the solutions to? I guess he is assuming you know how such an equation is solved (roots of the characteristic equation and such).
  14. Oct 9, 2012 #13
    I guess you just have to be gifted to do well in physics, because if this book passes for informative, then I'll just accept that I'm below average and will never succeed.
  15. Oct 9, 2012 #14
    the average GRE score of physics majors FAR outshines that of other science and engineering majors... actually, other majors in general. Trust me, it is very hard to be exceptional in physics. The only fields harder to be exceptional in, measured by average GRE scores, is math and CS.
  16. Oct 9, 2012 #15
    Those are the general solutions of differential equations with real roots and complex roots respectively. Look up 2nd order linear differential equations solutions, these are the some of the easiest ones to solve:

  17. Oct 10, 2012 #16


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    It turns out that (for the purposes of this book, at least) you don't actually need to know how to derive the solutions for X(x) and Y(y) from scratch. Problems 3.12 through 3.15 at the end of the section are all based on that example and on the following two examples which are similar and have the same ODEs after separation of variables. The main thing you have to worry about is applying the boundary conditions for the specific situations.

    I saw the differential equations for X(x) and Y(y) for the first time when learning about the Schrödinger equation in a second-year "intro modern physics" course. We didn't solve them rigorously, but instead made "educated guesses" to arrive at the general solution.

    For example, with the X equation, we want a solution whose second derivative is proportional to the function itself, with a + sign. What function do we know that fits the bill? The exponential, of course! And we can have either a + or - sign in the exponent and it still satisfies the DE, so that gives us two solutions. And we can add them together in a linear combination and it's still a solution. Similarly for the Y equation, except that the - sign leads us to sines and cosines instead of exponentials.

    It's a hand-wavy type of derivation, and it leaves some questions open, but it was good enough for our purposes in that course. We were told we would see the more rigorous method in our later differential equations course which none of us had taken yet.
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