Search results

  1. L

    I want to self teach physics

    For first year physics, I would recommend "Physics for Scientists and Engineers" by Serway and Jewett. It is a very readable text with plenty of examples and chapter summaries. As far as math goes, Thomas' Calculus is very good. It covers calculus of a single variable and multivariable...
  2. L

    Angular momentum commutator

    fzero, I didn't even think of that! A free particle would have zero potential energy, so for L to be conserved it would still have to commute with the Hamiltonian, and that means L and P^2 must commute (2m factor is irrelevant). I thought for sure I was making a mistake somewhere since my mind...
  3. L

    Angular momentum commutator

    Oh, I found one mistake but it still leads me to the same conclusion. The second equality should be plus, not minus: [A,BC] = [A,B]C+B[A,C]: \left[ \vec{L} , P^2 \right] = \left[ L^k , P_i P_i \right] = \left[ L^k , P_i \right] P_i + P_i \left[ L^k , P_i \right] = \left( - i \hbar...
  4. L

    Angular momentum commutator

    I have been told that L and P^2 do not commute, but I don't see why. It seems like the commutator should be zero. \left[ \vec{L} , P^2 \right] = \left[ L^k , P_i P_i \right] = \left[ L_k , P_i \right] P_i - P_i \left[ L_k , P_i \right] = \left( - i \hbar \epsilon_{i}^{km} P_m \right)...
  5. L

    Simplifying expression

    Ah! I should have thought of that. It's a problem in Griffith's EM chapter 12. They want the percent error using Galilean vs Einstein velocity addition for two things moving 5mph and 60mph. I guess it's to show this is a non-relativistic speed? Kind of ridiculous if you ask me. Thank you for...
  6. L

    Simplifying expression

    I'm finding the percent error in a S.R. problem and getting a really small number. How can I find the exact percentage? This is the result that needs to be simplified: 6.7x10^(-16) / (1 + 6.7x10^(-16)) If I do an order of magnitude approximation, then the bottom becomes 1, but that will...
  7. L

    Riemann-Christoffel tensor

    Thanks for the reply (and the latex sample!). It is acting on a vector A: \Gamma^{\tau}_{\alpha\nu} \frac{\partial A^{\alpha}}{\partial x^{\sigma}} - \Gamma^{\tau}_{\alpha\sigma} \frac{\partial A^{\alpha}}{\partial x^{\nu}} I've tried rewriting the partials as...
  8. L

    Mass creates Volumetric Space

    I would have to say mass does not create space. The 'curving' of spacetime is a description of the motion of an object near a gravitating body. It would follow a path given by the geodesic equation (which is a geodesic). I have never heard of space being a physical thing that can bend or distort...
  9. L

    Infinite speed of time

    No, time would not be infinite. It would be "normal" time (no time dilation). Just let r go to infinity in the equation and you will see that time is not changed.
  10. L

    Riemann-Christoffel tensor

    Or more simply put: {alpha nu, tau}*d/dx^sigma - {alpha sigma, tau}*d/dx^nu = 0 How can I show this is true? Is there some way of writing this with the nu and sigma switched in one of the terms? Thanks.
  11. L

    Riemann-Christoffel tensor

    I'm trying to work through getting the Riemann-Christoffel tensor using covariant differentiation and I don't see where two terms cancel. I have the correct result, plus these two terms: d/dx^(sigma) *{alpha nu, tau}*A^(alpha) and d/dx^(nu) *{alpha sigma, tau}*A^(alpha) Sorry, I couldn't...
  12. L

    That's a soul?

    I don't think humans have souls. If a soul was real, then you would have all types of questions like what is a soul made of, where does it come from, how long does it exist, what attaches it to a body, etc.. If you accept that a soul is imaginary, then your life is on equal ground to the...
  13. L

    Would you put off graduating?

    I have two options right now and I would like to hear what you would do in this situation. I don't have any research experience to put on a grad school application. If I take the GRE this fall, I will just be starting QM, so I probably won't be able to solve many of the QM questions on the...
  14. L

    Vector decomposition (Helmholtz)

    I actually just found an easy way of showing it using projection operators. Thanks for the reply. Consider this question solved.
  15. L

    Vector decomposition (Helmholtz)

    Helmholtz' Theorem starts with the two components in my original post and defines the divergence and curl as: div[V] = s(r) and curl[V] = c(r), where div[c(r)] = 0 But I can't find anything about how we can define a generic vector as two components: V = -grad[phi] + curl[A], where "phi" is...
  16. L

    Vector decomposition (Helmholtz)

    I have to show that a generic vector can be decomposed into an irrotational and solenoidal component: V(r) = -Grad[phi(r)] + Curl[A(r)] I'm not sure how to start. Do I need to take the curl or div of V and use a vector identity? Any help would be greatly appreciated!
  17. L

    Causality and Psychics

    I don't know if this belongs in Philosophy or Skepticism/Debunking... I seen the section of the paper with the daily horoscopes and got to wondering: How would physics change if one of these Miss Cleo or Edgar Cayce type people were real (capable of consistently and accurately stating what...
  18. L

    Confused, stuck, and need opinions.

    I would look into physics more if I were you. Then, if you still feel like it is something you want to do with your life, make it happen. You can do what I did, work two jobs over the summer, get rid of what can't fit into a dorm room, and go to school (out of state if necessary). I moved from...
  19. L

    Physicist title

    "Physicist" title I'm not sure what section to post this in, so I guess this one will have to do. I was recently told by an engineer that they have the title "Engineer" with a bachelor degree. I have always assumed the title "Physicist" was reserved for someone with a phd. But that got me to...
  20. L

    Maxwell's electromagnetic wave equation confusion

    I just realized what my problem was. The RHS is JUST with respect to time, not x or y or anything. That's not the laplacian... Lets just imagine this thread never happened, haha! Thanks for your help.
  21. L

    Maxwell's electromagnetic wave equation confusion

    Since the laplacian is the second partial derivative, you can write the expression as: Laplacian of E = (1/c^2)*Laplacian of E I don't see how you can multiply by a scalar and still get the Laplacian of E back or why it is written as a second derivative instead of the Laplacian. That is...
  22. L

    Maxwell's electromagnetic wave equation confusion

    time Laplacian of E = (1/c^2)* second partial of E with respect to t.
  23. L

    Heat, Work & Internal Energy

    Have you tried calculating the kinetic energy the woman would have, converting it to calories, and finding the number of sprints? I'm not sure if that will get you the right answer, but I come up with about 3.5 sprints. You may want to check the back of the book to see if that is close.
  24. L

    Maxwell's electromagnetic wave equation confusion

    I'm not understanding something here. Maxwell's wave equation is: Laplacian of E = (1/c^2) * second partial of E (sorry, I don't know how to write symbols) But the second partial derivative is the Laplacian. So how can you scale the laplacian of E by a number and get the laplacian of E as...
  25. L

    News Fox News

    Before Fox News Channel came around, the conservative voice was largely muffled. Fox is obviously biased, but that makes up for the liberal 24 hour stations and keeps the news "fair and balanced" if you watch all stations. The closest I have seen to a network that just reports the news without...
  26. L

    College is incredibly lonely

    You could check out to find people to run with. They have groups for everything from athletic activities to dinner and movies. That would be a start. You should also find a hobby that you enjoy. Its kind of hard to spend time with people if you don't have something to do. I would...
  27. L

    Death is

    You have already been dead for an infinite amount of time... before you were born. After you are alive, you die and it is identical to what you experienced before you were born. Absolutely nothing!
  28. L

    Need help with basic specific heat capacity problem

    Check your algebra again when you solve for c. It looks like you may have made a mistake. I'm getting about 0.45.
  29. L

    Schools Joining the military to pay for college

    I can think of a couple other options. You could go part time instead of full time. Get a job and just take one or two classes a semester. If you need to go full time, I had a friend that was a male dancer in Texas. He would make around 300 dollars per night in Dallas or Shreveport LA. I'm not...