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  1. J

    Electric field of a charged disc

    ... with constant charge density σ = Q/((pi)a^2) The Electric field is, after some calculation, is given by E_p below: z is the z-axis, and a is the radius of the disc. Now for the questions at the bottom of the page, here are my thoughts: σ is independent of a because as a->2a, Q->4Q, and...
  2. J

    Evidence that electron is a point particle?

    Most theories (/ when doing experiments we) assume that the electron, quark, etc are really point particles. What is the evidence for this? I don't think, for example, that experiments at cern convey that they are. They could have an internal structure with a stronger force than the strong...
  3. J

    Propagator operator

    So in the attachment, in fact (6), the formula for the propagator rectangled in red... is the Hamiltonian ACTING on (t-t')? is the Hamiltonian a function of (t-t')? or should it be (this is what I think), to be more clear U(t,t') = exp((-i/h)(t-t')H), so that when acting on a state...
  4. J

    Trivia: Potential of the hydrogen atom

    When (what year and by whom?) was it discovered that the hydrogen potential is V=-1/r ? I imagine this was deduced from experimental data... amirite? Quite urgent... From wiki it looks like it was found in teh 1920's but I'm not certain so I just want to check. Thanks in advance
  5. J

    New Video on One trillion frames per second camera

    New Video on "One trillion frames per second camera" Here is the video: Mods, please delete if there is already a thread for this. I think this is a great achievement by the researchers. It's an interesting topic, so maybe people can use it to ask questions on how/why it works, and...
  6. J

    Wavefunction Collapse

    Simple question. So the energy of a particle is observed to be E_1 (for example) at time t=0. At time t=0 the wavefunction psi(x) collapses to phi(x)exp(-i(E_1)t/h). At time t>0 the wavefunction is also in this state (right?). Is it in this state until it interacts with another particle or...
  7. J

    Measurement disturbs the system.

    "Measurement disturbs the system." Okay so I explained the uncertainty principle to my non-scientist friend, and he came up with a basic thought experiment. I know there is something wrong with it, so I read paragraph 8 from here:
  8. J

    Photons with electric charge

    What would happen if all photons had (slight) electric charge? Would we just not be able to see? Or would we not exist at all? Why? If they were negatively charged, would they repel electrons and attracts to the nucleus of an atom? What consequence would this have?
  9. J

    River flow on a planet or large object.

    So let's say we have a planet not too dissimilar in size to those in our Solar System. It doesn't have to be spherical. But let's say we want a river to flow round the object. We want the flow to last as long as possible. We can consider two cases seperately: in our Solar system...
  10. J

    I throw a metal rod (not light) into the air

    It is cylindrical in shape and is solid (not hollow) and has uniform density. Igonre air resistance. It spins with some angular velocity omega and spins in the other possible ways. Apart from omega, how many more degrees of freedom is possible? I can find the MOI of a rod spinning with...