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  1. S

    The famous Institute of Theoretical Physics?

    I can contribute this: (Amsterdam)
  2. S

    Mass dimensions

    If you work in units where c = 1 and h-bar = 1, all physical quantities can be described as having a dimension (unit) which is some power of the mass. Examples: Mass, energy, and momentum have a dimension of mass (i.e. mass dimension = 1). Length and time have a dimension of 1/mass (i.e...
  3. S

    Spinor fields and spinor wave functions

    Hi tomato, Let me try. I have three answers: 1) What makes you think that x and y should anticommute? - There is NO principle that says that for fermions all comutators are blindly replaced by anticommutators. 2) We have the same in Dirac fermions: x and y (which are usually denoted by u...
  4. S

    Quantum computers-the poll!

    So I understand that there is no simple example. One cannot understand how quantum mechanics helps to the computation without considering details of some complicated algorithms...
  5. S

    Dark matter, dark energy

    Could anybody list (and write a few explanatory words about) the various theories which explain the origin of the dark matter and the dark energy? Thanks!
  6. S

    Gravity Lift

    What is "gravity lift"?
  7. S

    Quantum computers-the poll!

    Could you please give me a simple example of a mechanism in which the quantum nature of qubits is used to do any calculation in a non-classical way? (Please write it here.) Thanks!
  8. S

    Quantum computers-the poll!

    I still haven't seen a single (simple) example of how a quantum computer uses quantum mechanics to do anything useful. And it's not that I didn't ask. :cry:
  9. S

    I was just thinking

    Maybe you are referring to the difficulty in explaining superconductivity in high-temperature superconductors by the BCS theory. However, this does not mean that BCS does not explain superconductivity in simple materials.
  10. S

    I found a lepton mass ratio formula:

    Interesting! How did you actually find this? Any comments from the high-energy people?
  11. S

    About quantum mechanics textbook

    Interesting, clear, starts from the basics: The Principles of Quantum Mechanics / Dirac. (Some people consider this book old-fashioned or difficult, but I don't think so.)
  12. S

    What determines the colour? Frequency or wavelenght?

    That's not a complete answer: the wavelength depends also on the medium through which the light passes. The color we see is determined by the frequency.
  13. S

    Thesis help

    A professor who read my recently completed M. Sc. thesis commented that the thesis was good enough so that it was worth eliminating English mistakes from there. However, I can't do it better than I did, since I am not a native English speaker. So I am looking for someone who would be interested...
  14. S

    Why does Isotropy of L imply L(v^2)?

    Don't read that book!
  15. S

    Can M theory/strings explane dark matter/enegy?

    Hi Neotrekkie! Paul Steinhardt from Princeton discusses this possibility in his papers about the cyclic model of the Universe. The papers are easy to understand, and you can find them on his website:
  16. S

    Hund's rule

  17. S

    Basics of the local spin density approximation?

    Does anyone know the basics of the local spin density approximation?
  18. S

    Magnetic susceptibility

    Hi QuantumNet, All materials are like this in some sence: the electrons are mobile, but the protons are not. And in a diode the situation is uni-directional. (Of course this has nothing to do with my original question).
  19. S

    Hund's rule

    Suppose there are n electrons with angular momentum quantum numbers L1, L2, L3, ..., Ln. The total angular momentum L of the atom can be any number between the minimum and the maximum of all non-negative combinations +/- L1 +/- L2 +/- L3 ... +/- Ln (in steps of 1). Same about combinations of the...
  20. S

    Capacitors with dielectrics

    Your multi-plate capacitor is equivalent to n 2-plate capacitors connected in parallel. The capacity of each 2-plate capacitor is given by C = e0 e A / d where e0 is a constant (epsilon-0), e is the dielectric constant of the paper (assuming that it fills all the volume between the...
  21. S

    Solid State Physics

    "Calculate the energy of the electron measured from the conduction band edge." Energy is defined up to an additive constant. The instruction above means that the energy of the conduction band edge should be considered as zero. :smile:
  22. S

    High voltage lines

    What is commonly the voltage in the high voltage lines used to transport electric energy over large distances? What is their resistance and inductance? What is the magnitude of the current flowing there? Yevgeny.
  23. S

    Suppose you are navigating in Egypt deserts by trusting the compass

    Is there a physical explanation for the daily changes there in Arizona? Has anyone seen UFOs there?
  24. S

    Voltage amplifier

    This is not homework. This is a question from the sample Physics GRE exam (question 72), and the correct answer is (A). When I saw this question I understood that it would be better if I knew something about voltage amplifiers before taking the exam. This is why I asked these questions...
  25. S

    Inifinite potential barrier problem

    After you measure the energy of the system once, it's state becomes the eigenstate of energy corresponding to the value you obtained. If you measure it again and again you will always get the same value.
  26. S

    Suppose you are navigating in Egypt deserts by trusting the compass

    Thank you very much! I just wanted the order of magnitude such declination can be. If you say 5 degrees in UK, it is of the same order of magnitude as I got. I was surprised that it was so large. Before, I thought that I can really trust the compass. I knew that the magnetic and the real...
  27. S

    Wave in a two-part string

    I am interested in the limit where the ratio of the densities of the heavy part and the light part is very large (infinite). Probably the answer to this limiting case can be found without solving differential equations...
  28. S

    Heat conduction

    1. Should it be (dT)^4 or d(T^4)? 2. What would be the answer if the gap were filled with wood?
  29. S

    Voltage amplifier

    Great answer, Warren! Thank you! Now I know much more about the voltage amplifier. But somehow it still isn't enough. I need to find an answer to the following question: In a voltage amplifier, which of the following is NOT usually a result of introducing negative feedback? (A) Increased...
  30. S

    Heat conduction

    Two parallel thin metallic plates with a gap between them are used as a heat insulator. How does the heat flow rate through this device depend on the temperatures on its two sides in the following two cases? Case 1: The gap is filled with air. Case 2: There is a vacuum in the gap.
  31. S

    Wave in a two-part string

    A string has two parts: one with a very high mass density (per unit length), and the other with a very low mass density. A wave with amplitude A moves from the dense part toward the light part. What will be the amplitude of the wave which is transmitted to the light part?
  32. S

    Can beats be created by two waves with different amplitudes?

    Can beats be created by two waves with different amplitudes? How will they look like?
  33. S

    Lasers - pumping

    What is the meaning of the term "pumping a transition" in relation to lasers?
  34. S

    Voltage amplifier

    Could you please tell me what the inputs and the outputs of a VOLTAGE AMPLIFIER are? Also what do these mean (in a voltage amplifier): - voltage gain - bandwidth - distortion - positive/negative feedback ?
  35. S

    Suppose you are navigating in Egypt deserts by trusting the compass

    Suppose you are navigating in Egypt deserts by trusting the compass to determine the South direction. If your destination is 15 km to the North of you, approximately by how much will you miss it?
  36. S

    Magnetic susceptibility

    Suppose there is a line of atoms X and Y. For atom Y, S = L = 0, but not for the atom X. The arrangement is like this: --X-Y---X-Y---X-Y---X-Y-- (periodical) In the following 2 cases, will the magnetic moment depend on whether the magnetic field is applied to the right or to the left? (The...
  37. S

    Hund's rule

    Does Hund's rule also apply when combining the angular momenta of electrons from shells with DIFFERENT quantum number n?
  38. S

    Spin projection

    Sounds great! Is this something you thought about now, or you read/learned this trick somewhere in the past?
  39. S

    Inifinite potential barrier problem

    Hi mmwave! Here are the answers: 1. The new state is not stationary. The book is right. Actually, any time-dependent state can be written as a sum of stationary states. 2. Your result confirms my answer to question 1. 3. <H> is the average energy. Average is (E1+E2)/2, not E1 + E2...
  40. S

    Spin projection

    For a particle with S = 1 in a state of Sz = 1, what are the probabilities to obtain the various values of the spin component if measured along a direction which makes an angle Q relative to the z-axis? Is there a general method for an arbitrary value of S?
  41. S

    Poll: How many elementary fermions?

    Why are all the suggested answers powers of 2?