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  1. J

    An upper bound in temperature?

    Roughly speaking, the temperature depends on the average energy of the molecules, not the average speed, so there would be no upper limit.
  2. J

    Are conduction electrons localized in space?

    I see why you were puzzled. Could you elaborate? Does this assume that each electron exists in a definite energy rather than a superposition of different Bloch waves?
  3. J

    Are conduction electrons localized in space?

    Forget the models. I mentioned the infinite square well model as an example of a model where electrons are delocalized, but that doesn't mean I'm wedded to it. I used it to show people my basis for believing that electrons were spread across the solid. For that purpose, I assumed the ISW model...
  4. J

    Are conduction electrons localized in space?

    Thank you, I think you've provided the best answer so far. I'll look into what you said to Manchot about the relationship between quantum mechanics and the Boltzmann equation. Ditto. Perhaps it would be clearer if I asked a couple of related questions: How can we reconcile the image of...
  5. J

    Are conduction electrons localized in space?

    You're arguing that for bulk solids, it physically doesn't matter whether you choose the infinite square well's sine waves or the free particle models's complex exponentials, but that using the latter is much simpler to deal with. I'll concede that, though only the ISW has a non-zero fermi...
  6. J

    Modeling a plastic cup

    It does make sense that the liquid would cause more stress on the middle of the cup's bottom than at a point near the rim, even if the force is the same in both places. The stresses should be radially symmetric while in either position. I'm guessing, though, that the bottom probably contorts to...
  7. J

    Are conduction electrons localized in space?

    Thank you, everyone. I didn't expect to go to physicsforums and find a month-old thread of mine with 15 replies. I don't doubt that an infinite square well is a bad approximation for a solid, missing behaviors such as band gaps and such, but it seems like a good way to discuss whether...
  8. J

    Rotational degrees of freedom I don't get it.

    I think you have to use quantum mechanics to get a rigorous reason for not getting a contribution for rotations about the axis of a diatomic molecule. Classically, even a diatomic molecule with a very small (non-zero) moment of inertia about its axis would contribute the same amount to the...
  9. J

    Series LC ckt question

    I feel like I'm missing information. What's "2V" referring to? Is there a power source in this circuit? Do we charge the capacitor, then add it to the circuit after it's charged?
  10. J

    Am I right to conclude this about mass?

    You are saying a force on a mass exists if and only if there is an acceleration (in your terms, if "it opposes the motion"). This is Newton's First Law. It doesn't make sense to "apply a force on a force". Instead, we should say "apply a force to an object causing that force". As Pengwuino said...
  11. J

    Electric Circuits

    Let's start with an easier example: one resistor and one battery connected to the circuit by a switch. We turn on the switch. Now, we learn in class that there is a voltage increase across the battery and a voltage decrease across the circuit, and that in general, when you sum the voltage...
  12. J

    Quantum mechanics replacing bohr model

    Here's what happens when you rub two objects together: 1. You add kinetic energy to the object by rubbing the objects together, striking them together, etc. The atoms in the object gain kinetic energy. Soon enough this internal kinetic energy reaches thermal equilibrium. If you dropped the...
  13. J

    Quantum mechanics replacing bohr model

    When you rub two objects together and generate heat from the friction, the energy comes from you, and is transformed into vibrational/heat energy in the object. You can't cool an object by rubbing it. Is this what you meant?
  14. J

    Finding What's Missing In A Circuit

    How much do you know already?
  15. J

    Electric Attractions

    "More energetic particles" means "particles with a higher momentum". It's similar to things orbiting around the sun. If an object has a velocity that's too high, it'll leave the solar system and never return, but if the velocity's not too high, it'll just orbit around the sun in an ellipse, or...
  16. J

    Ukrainian writer in need of your knowledge

    I'll contribute a few comments: 1. What happens if Black controls light and someone else is trying to control it in some other way? 2. Wouldn't denying light make you invisible? 3. I'd think you need to create consistent, detailed rules as to how "denying" things works.
  17. J

    Double Slit Question

    Here's a page with an applet where you see that there's reflection of part of the plane wave approaching the wall with the slits.
  18. J

    Is this property of lines of force true for E.M?

    Thank you, I stand corrected.
  19. J

    If helium is 'superfluid' at low temperatures

    Superfluidity is a quantum effect, where the glass transition is a classical effect, as far as I know.
  20. J

    Are chaotic systems really determinitistic?

    The classical world could in principle be 100% predictable, but the real world involves quantum uncertainty as well, so, no, the underlying reality isn't completely deterministic. If we didn't know about quantum mechanics, I supposed it'd be reasonable to stop short of saying the world is...
  21. J

    Momentum transferred even though no collapse?

    I'm confused. I thought we were talking about momentum applied perpendicular to the walls, not transversely. Jaketodd, could you clarify your example?
  22. J

    Is this property of lines of force true for E.M?

    I don't think it's true. For electric fields, lines of force originate with positive charges and end at negative charges, unless they begin or end at infinity when the net charge of the system is non-zero. You could pick a location near the charges, where a test charge would experience a very...
  23. J

    Are chaotic systems really determinitistic?

    There are three worlds we're talking about: 1. the classical world, 2. the quantum world, and 3. reality. Both #1 and #2 are mathematical models for #3. You seem to be criticizing scientists for saying that #1 is perfectly deterministic, but #1 is a mathematical model, so it should be...
  24. J

    Momentum transferred even though no collapse?

    That's a good question. Here's my educated guess of the answer: Let me make sure I understand your question. You're talking about a single particle approaching a wall with two slits, where some of its wavefunction passes through both slits and reaches the back wall and produces an...
  25. J

    Are chaotic systems really determinitistic?

    I think chaos is defined by what happens to differences in initial conditions over time. In nonchaotic systems, nearby points tend to reach a common destination and the error decreases with time, while with a chaotic system, nearby points often reach different destinations, and the error...
  26. J

    Is this property of lines of force true for E.M?

    Could you provide us with more detail? What's a "line of force", and what's "reving"?
  27. J

    Modulation ?

    Let's say the signal we want to transmit is s(t). AM modulation is the following: s(t) A \sin(\omega t) Here \sin(\omega t) is the carrier signal, \omega is its angular frequency, A is a constant, and t is time. FM modulation is the following: A \sin(B s(t) t) B is a constant, and the...
  28. J

    Multi-space instantaneous existence.

    You can have a particle have a probability of being in two places at once, but if you measure the particle with 100% accuracy, then you'll find that the particle is either in one location or another. To be more precise, a particle has a wavefunction which gives the probability that it will be in...
  29. J

    Modulation ?

    You need to give us a lot more information before we can tell what you're asking for. Where did you hear about "modulation"? Is it in reference to "amplitude modulation" or "frequency modulation", both of which describe ways information is encoded in radio waves?
  30. J

    About Anharmonic Oscillator

    Where are you stuck?
  31. J

    Conventional Current in a circuit

    I'm confused. You seem to be assuming that the electric field is applied equally to all portions of the wire loop, where I'm saying that it's applied only to a small portion of the wire. I was thinking of a battery, which a potential difference within its region and doesn't directly exert an...
  32. J

    Conventional Current in a circuit

    Maybe I should've used a better word than "nudge". I did not mean to imply that electrons bounce into each other like billiard balls, or line up behind each other one-by-one, but rather that: when you apply an electric field to a small portion of a wire loop, the electrons will move in the...
  33. J

    Electron oscillation frequency, what pushes them away?

    What do you mean by the oscillation of the electron?
  34. J

    Circulating electrons causes a current

    I'm not quite sure what you mean, but I think you're talking about atoms and molecules in a solid, especially a crystal, which is a solid where the atoms or molecules are stacked in a regular order. Now, even if the magnetic moments of each atom are oriented randomly at a certain time, over time...
  35. J

    Conventional Current in a circuit

    What happens is that the current through the resistor would be high enough to heat it up hot enough to cause a fire. The flow of electrons is responsible for the heating up, but not the burning. Now, when electrons flow in a solid, they replace other electrons, so that almost all of the...
  36. J

    Can water be compressed so much that it freezes?

    For a substance to be a liquid, its atoms or molecules have to be able to move around each other with ease. The more you compress atoms, the less room they have to move.
  37. J

    Excited state to ground state

    I suspect that quantum electrodynamics explains it, though I don't know too much about it.
  38. J

    Sound Intensity of a Whisper

    Why would you be asked for a precise answer if they don't give you a value for P? Is this a book problem or something your teacher gave you?
  39. J

    Are conduction electrons localized in space?

    Let's say we use a very simple model consisting of non-interacting electrons in a 3D infinite square well, perhaps a cube of a single metal crystal. If an electron is in a particular energy state, then its wavefunction is spread across the entire crystal. However, electrons are said to...
  40. J

    The Sound Of Music

    Seems right to me.
  41. J

    Electric Field and Magnetic Field

    Where does your second equation come from? If we let the "positive direction" be the x-direction, which direction is the velocity pointing in?
  42. J

    Admissions Graduate admissions

    Overall. My physics GPA was actually 3.23, but not every college asked for it.
  43. J

    Admissions Graduate admissions

    I think many grad programs would be satisfied with less than a 3.5 GPA. I got accepted to four colleges with a 3.45 GPA. I don't know if these colleges are "strong" by your definition (the most reputable was probably SUNY-Stony Brook).
  44. J

    Programs Whats the record in number of PhDs?

    Michael Griffin, who was the administrator of NASA until Obama's inauguration, has 1 B.S., 5 masters degrees, and 1 PhD.
  45. J

    Schools Compare universities

    I visited UCONN recently (I, too, got accepted to UCONN, among other places). It seemed pretty good, though I can't compare it with any of the schools you listed. They seem to have a strong AMO (atomic, molecular, and optical physics) group, and when I toured the AMO labs I saw work done on BEC...
  46. J

    Moon -> Earth (Gravitational Pull help any comment welcome

    The question is at what radius r will the gravitational forces of the Earth and the Moon be equal. Do you know the formula for the gravitational force between two objects? The object will always be affected by both the Earth and the Moon's gravity. The question is which force is larger.
  47. J

    Transfer question

    I'd think playing catch-up with two different majors would be too much work.
  48. J

    Ball falling on to parabolic track

    You weren't too clear on what the question states, so I'll just mention that the potential energy is the same for any x, if we have the same y. Also, when a ball rolls to the top of a hill or something, its kinetic energy must be zero because, for a brief moment, it isn't moving.
  49. J

    Find the boat's speed as a function of time

    This problem seems a little bit ambiguous. Do you need the equation of the boat's motion before it stops, as it's stopping, or for both? Is this a freshman-level physics problem, or intermediate (where you deal with differential equations)?
  50. J

    Is this ok with Polar coordinates?

    I don't think de_r is a unit vector. It is the difference between two unit vectors, but it itself is not a unit vector.