most recent 30 from http://physicsoverflow.com 2010-09-05T15:20:51Z http://physicsoverflow.com/feeds http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://physicsoverflow.com/questions/211/how-to-compute-the-path-integral-in-chen-simons-theory Hao Yu 2010-09-04T16:43:17Z 2010-09-04T16:43:17Z

<p>How to compute the path integral in Chen-Simons theory?(the one Witten used to explain Jone polynomial)? Is there any measure in there? Thanks!</p>

http://physicsoverflow.com/questions/112/equilibration-of-bosons-and-fermions Anirbit 2010-04-30T18:44:15Z 2010-09-04T11:23:42Z

<p>Firstly is there an intuitive argument to see why the chemical potential of a system of free bosons should be negative and why for free fermions at high temperatures the chemical potential should go negative?</p> <p>Secondly suppose that a system of free fermions and free bosons are equilibrating then I would like to know how the chemical potential behaves with temperature? </p> <p>As temperature falls closer to zero the bosons would tend to form a BEC (assume that there are enough number of bosons to do that) with 0 chemical potential but the fermion chemical potential would tend to reach the fermi energy as temperature tends to 0. But for equilibration needs the chemical potential for both the species to be the same.</p> <p>How to resolve this apparent conflict? </p>

http://physicsoverflow.com/questions/44/velocity-of-spin-electron-velocity truthinc 2009-11-21T15:12:27Z 2010-09-04T01:23:45Z

<p>Do we know the answer s to some or all of these equations?</p> <p>1.) The radius from the center of a nuclear to the different possible orbit of the electrons 2.) The velocity of a protons spin 3.) The velocity of the electrons in the different electron orbits </p>

http://physicsoverflow.com/questions/209/whats-the-motivation-of-segals-axiomation-of-conformal-field-theory Hao Yu 2010-09-02T14:35:12Z 2010-09-03T13:07:03Z

<p>What's the relation of it with Wightman axioms ans etc.? Thanks! Or which one is the one we prefer?</p>

http://physicsoverflow.com/questions/50/what-is-actually-phase-velocity truthinc 2009-11-21T15:20:03Z 2010-09-03T04:23:48Z

<p>When a body moves aren't we supposed to say that each particles on the body moves with same velocity and so the de-broglie wave corresponding to each particle must have the same velocity. Then why should a wave representation of moving body correspond to a wave packet?? </p>

http://physicsoverflow.com/questions/62/how-to-solve-quantum-particle-in-a-uniform-magnetic-field-with-a-symmetric-gauge Benazhack 2010-01-21T21:52:04Z 2010-09-03T04:23:48Z

<p>Hi, i'm new but i cannot found out how to solve this problem. I can solve it with an asymetric gauge (obtaining Landau levels) but cannot solve the ODE of the Hamiltonian in a symmetric gauge.I read something about Airy functions or Frobenius method.</p>

http://physicsoverflow.com/questions/53/does-size-matter-in-wave-to-particle-momentum-transfer truthinc 2009-11-21T15:24:31Z 2010-09-03T03:23:48Z

<p>Let's say you have an electron wave that hits something and let's assume the wave does not collapse to an electron and collide with that something. Is the amount of momentum transferred to that something dependent on the diameter of that something (in addition of course to the eV of the electron wave)? </p>

http://physicsoverflow.com/questions/43/is-it-possible-to-controll-spin-of-an-entangled-particle-causality truthinc 2009-11-21T15:05:40Z 2010-09-03T02:23:41Z

<p>Everything I've read has said that causality in the universe isn't violated because entangled particles don't transmit information (because measured values are random compliments). All of this spoke about measurement.</p> <p>If entangled atoms are created (such as those 2006 on), can properties such as spin be directly controlled on one of the atoms (and therefore measured on the other)? I thought spin could be influenced by fields.</p> <p>Thanks</p>

http://physicsoverflow.com/questions/42/how-planks-constant-and-avogadro-number-was-determined truthinc 2009-11-21T15:01:56Z 2010-09-03T02:23:41Z

<p>How plank's constant and Avogadro number was determined? </p>

http://physicsoverflow.com/questions/40/how-does-electromagnetic-radiation-propagates-through-space neophilosopher 2009-11-21T14:50:58Z 2010-09-01T04:23:33Z

<p>Hello everybody.</p> <p>I've been having difficulty in understanding how an electromagnetic wave propagates through space.</p> <p>Firstly, from what I understand, electromagnetic waves are made up of a constantly varying electric field placed perpendicular and in phase with a constantly varying magnetic field. Now lets set up a source to generate an electromagnetic wave. From the above, the source should consist of a varying electric field and varying magnetic field.</p> <p>Now consider a point P a distance d away from the source. The time taken for the energy from the wave to reach the point P from the moment the source is switched on would then be t = d/c.</p> <p>Isn't the effect of a magnetic or electric field assumed to be instant before relativity showed that information cannot be transfered faster than the speed of light?</p> <p>Even so, how does 2 perpendicular fields explain how energy is transfered through space? From what I've read from texts its because the electric field induces a magnetic field (Maxwell's law of induction), and the induced magnetic field induces yet another electric field (Faraday's law of induction) and so and so forth. But I failed to understand that as it still does not seem to explain how the wave propagates through space. The induced B-fields and E-fields are still induced in the space about the source, and I'm unable to see how it travels.</p> <p>Perhaps someone could point me in the right direction? </p>

http://physicsoverflow.com/questions/16/is-it-possible-to-relate-sound-frequency-to-light-frequency aceth 2009-11-21T14:02:57Z 2010-09-01T04:23:33Z

<p>I heard that the color red has a frequency of 43 trillion hz, and if you divide that by 2 over and over until you get a hz range that's audible to the human ear you could find the equivalent Octave of the musical note to hear what that color sounds like. I also heard that light is vibrating photons and sound is vibrating air molecules so you can compare them... If you had a ear that could hear ultra high frequency's could you hear the photons vibrating and would they intern vibrate the air molecules next to them on the same frequency? I see lots of charts on the web comparing color to sound as if they are the same thing, is there a practical way to look at this? I hear lasers can generate underwater sound dose this relate? And is the frequency of a sound underwater the same as it would be if the sound was in air?</p>

http://physicsoverflow.com/questions/30/do-black-holes-violate-the-2nd-law-of-thermodynamics neophilosopher 2009-11-21T14:35:52Z 2010-09-01T01:23:41Z

<p>I ask because i once read that black holes radiate infrared waves. Is this true? If not, then it could absorb some of the infrared waves from a colder object, perhaps one orbiting the black hole, in apparent violation of the 2nd law of thermodynamics.</p>

http://physicsoverflow.com/questions/131/vacuum-amplification Alex 2010-05-25T00:24:31Z 2010-09-01T01:23:41Z

<p>Tube amplifiers use a control grid that sits in between the anode and cathode. This carries an AC signal received from the guitar. I was wondering how the control grid actually amplifies any kind of electron flow, given that without the control grid I assume the flow would be 100% anyway.</p> <p>Cheers.</p>

http://physicsoverflow.com/questions/181/moon-surface-heat-buildup-and-eventual-problems Ramiro 2010-07-27T21:07:39Z 2010-08-31T02:23:33Z

<p>Hi all! A question I've had for a while:</p> <p>On the surface of the Moon, while under direct sun radiation, is there any way of cooling up an object?</p> <p>There appears to be no natural shadows, and of course no atmosphere.</p> <p>I think that neither conductive nor convective cooling really works in that scenario.</p> <p>I guess that the energy radiated from the Sun must be much more than the energy than an object can emit by means of radiation (the only possible cooling method?).</p> <p>If I am not mistaken, any object on the surface on the Moon must keep getting hotter and hotter until the "Moon night" arrives and the Sun's radiation stop.</p> <p>I am from the computer field, and this is obviously not my area of expertise.</p> <p>Can anyone answer this question no too mathematically?</p> <p>Thanks!!!!!! </p>

http://physicsoverflow.com/questions/38/amazing-wireless-energy-transfer-coil-how-does-it-work neophilosopher 2009-11-21T14:48:16Z 2010-08-29T17:23:36Z

<p>I've found this video on the web: <a href="http://www.youtube.com/watch?v=TIhPa...eature=related" rel="nofollow">http://www.youtube.com/watch?v=TIhPa...eature=related</a> and id like to be able to build one myself.</p> <p>But i don't really know how to start with this nor how it works. -The 8 shaped coil is really 8 shaped or just 2 coils bound together? -how do you make the second coil (the one with the light) resonate in the same frequency? -what voltage do you put into the primary coil to make it work?</p> <p>I've looked on the net but can't find anything helpful. Please help me or at least push me in the right direction. </p>

http://physicsoverflow.com/questions/18/why-does-a-high-optical-carrier-feq-provide-higher-transmission-rate aceth 2009-11-21T14:05:35Z 2010-08-29T05:23:32Z

<p>For example , optical carrier in the infra-red region can provide higher capacity than microwave. </p>

http://physicsoverflow.com/questions/26/about-e-field-electric-potential-of-a-metal-conductor aceth 2009-11-21T14:22:17Z 2010-08-29T04:23:33Z

<p>A few questions about electric field here:</p> <ol> <li>Why do charges have to be on the outer surface of the conductor?</li> <li>Why no charge on the inner surface of a hollow conductor?</li> <li>If charges cannot be inside the conductor, how come the electric potential is constant throughout the conductor when an electrostatic equilibrium is reached? Isn't it that charges won't be able to redistribute throughout the conductor?</li> </ol> <p>Must be wrong concepts. Please correct. </p>

http://physicsoverflow.com/questions/32/i-need-help-understanding-how-viscosity-affects-motion neophilosopher 2009-11-21T14:40:36Z 2010-08-29T03:23:32Z

<p>How does the viscosity of a fluid in a circular can affect the way the can rolls? For example, 2 cans of similar size, volume, and mass are rolled down a ramp with the same slope. The can with a viscous liquid will take more force to begin rolling than a can with a less viscous liquid, but once it gets moving the thicker liquid will roll a longer distance. As a result, on a shallow ramp, the more viscous can will hardly begin to roll and will go a lot shorter than the other one, but on a high sloped ramp, the more viscous can will begin to roll very fast and go a lot further than the less viscous one. I have tried looking up viscosity but couldn't really find much related.</p> <p>What are some theories or concepts that can help explain why this occurs? </p>

http://physicsoverflow.com/questions/23/how-can-i-calculate-torque-power-of-an-engine-using-accelerometer-data aceth 2009-11-21T14:16:27Z 2010-08-29T03:23:32Z

<p>I'm using an accelerometer to record the rate of acceleration of a vehicle over time (1500kg). With this data I am calculating speed and distance traveled over time. I'm now trying to figure out a way to calculate either the torque or the power produced by the engine over time (I can calculate the other using recorded rpm @ the time). Drive train losses, rolling resistance and aerodynamic drag will all play their part, but I shall ignore these presently till I figure out this first bit.</p> <p>Thanks much for reading. </p>

http://physicsoverflow.com/questions/196/meta-please-put-this-site-in-bootstrap-mode Arlen Beiler 2010-07-30T12:35:42Z 2010-08-29T01:23:35Z

<p>Admin, could you put this site in bootstrap mode? It would be very helpful.</p>

http://physicsoverflow.com/questions/21/i-need-help-with-free-expansion-of-an-ideal-gas aceth 2009-11-21T14:11:01Z 2010-08-27T18:23:40Z

<p>I know that the work done on the system in any free expansion is 0 since the external pressure is 0. However.. is q necessarily 0? Does the temperature necessarily stay constant for an ideal gas?</p> <p>I've been trying to justify the reasons for which q is necessarily 0, but I can't find a reason... Does anybody know why this is so? I know that U varies only with T for an ideal gas, but that would require justifying that the change in temperature is also 0.. does anybody have a mathematical justification for why q is necessarily 0 for a free expansion of an ideal gas?</p> <p>Thanks. </p>

http://physicsoverflow.com/questions/22/homework-question-about-electromagnetism-and-flux aceth 2009-11-21T14:14:41Z 2010-08-26T18:23:44Z

<p>I've got a magnet of a determinate radius and a coil.</p> <p>Which radius should i give to the coil (in function of magnet radius) in order to obtain the maximum intensity?</p> <p>I think that when the coil is closer to the magnet, the flux density is bigger, but i think that the surface has something to do in this problem.</p> <p>Thank you.</p>

http://physicsoverflow.com/questions/149/will-you-join-a-se-2-0-physics-site-replacing-or-merging-with-this-site Tobias 2010-06-02T08:32:08Z 2010-08-26T18:23:44Z

<p>Unfortunately physicsoverflow is quite unused. The framework it is based on, <a href="http://www.stackexchange.com" rel="nofollow">StackExchange</a>, is curently updated to version 2.0 and all rather inactive site, possibly including this one, will be shut down. However, a new site is already under preparation <a href="http://area51.stackexchange.com/proposals/1908/physics?referrer=Ho2q28uHijgN5QfMOt5KEg2" rel="nofollow">here</a>, see <a href="http://blog.stackoverflow.com/2010/06/area-51-we-come-in-peace/" rel="nofollow">here</a> for an explanation of the process.</p> <p>We hope that SE-2.0 will also include LaTeX support, as <a href="http://meta.stackoverflow.com/questions/53436/implement-google-api-call-to-display-latex-as-inline-image">requested</a> more than <a href="http://meta.stackoverflow.com/questions/tagged/latex+feature-request">once</a>. Please feel free to join and support those requests and the new <a href="http://area51.stackexchange.com/proposals/1908/physics?referrer=Ho2q28uHijgN5QfMOt5KEg2" rel="nofollow">physics proposal</a>.</p>

http://physicsoverflow.com/questions/36/is-the-net-charge-density-on-each-platesigmainitial-sigmainduced-the-same-in neophilosopher 2009-11-21T14:45:29Z 2010-08-25T06:23:41Z

<p>Say i have a capacitor and i supply it with a voltage V and after that it has charge density sigmainitial on each plate.</p> <p>Now i remove the voltage source and then introduce a dielectric with area A that it equal to the area of one of the plates in the capacitor to the capacitor.</p> <p>But this dielectric is only touching one plate.</p> <p>Can someone please explain?</p>

http://physicsoverflow.com/questions/125/diffraction-distance-between-slits Aoe 2010-05-16T16:11:29Z 2010-08-25T01:23:32Z

<p>My <a href="http://physicsoverflow.com/questions/121/diffraction-width-and-distance-btw-slits-from-the-wave-pattern" rel="nofollow">last question</a> became bloated so new question with more narrow topic about diffraction. If this question is solved, it solves <a href="http://physicsoverflow.com/questions/126/diffraction-how-to-approximate-the-ratio-of-distance-btw-slits-and-slit-width/127#127" rel="nofollow">another question</a> about approximating the distance btw slits.</p> <p>The terms <code>R, a, C, q, I and N</code> in the <a href="http://scienceworld.wolfram.com/physics/DiffractionGrating.html" rel="nofollow">formula</a>, at the very bottom, do not depend on the distance between slits. Instructions given are:</p> <blockquote> <p>Calculate the widths of slits in slit-system and their distances from one another.</p> </blockquote> <p>I find the last part very confusing.</p> <p><strong>Intensity Formula</strong></p> <p>I am not sure but the intensity formula may quickly kill the question: <a href="http://mathbin.net/47322" rel="nofollow">MathBin</a> or</p> <pre><code>I = I_{0} \left[ \frac{sin \left( \pi b sin \left( \frac{\theta}{\lambda} \right) \right) }{\pi b sin \left( \frac{\theta}{\lambda}} \right) \right]^{2} \left[\frac{sin\left(N \pi a sin\left(\frac{\theta}{\lambda}\right)\right)}{sin \left( \pi a sin\left( \frac{\theta}{\lambda}}\right) \right) \right]^{2} </code></pre> <p><strong>Data about slit-system</strong></p> <p>2 slits</p> <p>width = 0,04 mm</p> <p>right distance btw slits = 0,250 m</p> <p>light wavelength = 632.8 nm</p> <p><strong>Questions</strong></p> <ol> <li>How can you get distance between slits in wave pattern?</li> </ol>

http://physicsoverflow.com/questions/126/diffraction-how-to-approximate-the-ratio-of-distance-btw-slits-and-slit-width Aoe 2010-05-16T23:27:16Z 2010-08-24T11:23:41Z

<p>The question builds on <a href="http://physicsoverflow.com/questions/125/diffraction-distance-between-slits" rel="nofollow">my last question</a> in a way that you need the distance between slits. So how to approximate:</p> <pre><code>\frac{a}{b} = \frac{distance between slits}{slit width} </code></pre> <p>Approximation means without calculations or to use ready data from the 1-slit-system-experiment. The envelopes of 1-slit-system and many-slits-system do fit together so the only difference is the term <code>sin(Nqa)</code>, causing the oscillating below envelope in many-slit-experiment. 1-slit-system does not naturally have a term distance btw slits so I I need to somehow focus on the change in the pattern.</p> <pre><code>System Number of Middle maximums 0th order length Width Space Ratio 1 13 4.0cm 40\mu m 0.25mm 6.3 2 25 4.0cm 40\mu m 0.50mm 12.5 3 7 2.0cm 80\mu m 0.25mm 3.1 4 13 2.0cm 80\mu m 0.50mm 6.3 </code></pre> <p>The space is <code>the distance btw slits</code>. The length must be converted to detector value multiplying by 0.015.</p>

http://physicsoverflow.com/questions/46/does-a-position-operator-exist-for-eigenfunction truthinc 2009-11-21T15:15:52Z 2010-08-24T04:28:22Z

<p>Does a position operator acting on a wavefunction give a position eignenfunction? </p>

http://physicsoverflow.com/questions/163/maxwells-equations-using-4-potential JoeG 2010-06-18T05:29:37Z 2010-08-23T13:23:37Z

<p>Sometimes I see Maxwell's equations in the vacuum written as BOX A_u = 0 where BOX is the d'Alembertian operator.<br> <a href="http://en.wikipedia.org/wiki/Maxwell%27s_equations#Four-potential" rel="nofollow">http://en.wikipedia.org/wiki/Maxwell%27s_equations#Four-potential</a></p> <p>But, I also often see that this is only true when the Lorenz Gauge condition (d_uA^u=0) is asssumed. <a href="http://en.wikipedia.org/wiki/Lorenz_gauge_condition#Description" rel="nofollow">http://en.wikipedia.org/wiki/Lorenz_gauge_condition#Description</a></p> <p>What do solutions BOX A_u=0 represent if the Lorenz gauge condition is not satisfied? Are they also solutions to Maxwell's equations?</p>

http://physicsoverflow.com/questions/206/how-does-youngs-double-slit-expirement-imply-something-passes-through-both-slits Daniel 2010-08-17T14:31:23Z 2010-08-17T16:51:45Z

<p>I'm reading an 'armchair physics' book (A User's Guide to the Universe), and find it fascinating, but am having trouble understanding something that seems pretty fundamental. Concerning Young's double-slit experiment, Wikipedia (and the book I'm reading), say similar:</p> <blockquote> <p>The most baffling part of this experiment comes when only one photon at a time is fired at the barrier with both slits open. The pattern of interference remains the same, as can be seen if many photons are emitted one at a time and recorded on the same sheet of photographic film. The clear implication is that something with a wavelike nature passes simultaneously through both slits and interferes with itself — even though there is only one photon present.</p> </blockquote> <p>To me, it seems like a simpler explanation is that a single particle may just have that pattern due to the way it travels, without traveling through both at the same time. I'm sure I'm not understanding something, but just don't quite grasp it. In layman's terms, why is travel through both slits "the clear implication"?</p> <p>For bonus points, how is a single photon generated in the lab? How does one know that they really are just emitting one photon?</p>

http://physicsoverflow.com/questions/187/meta-who-wants-some-upvotes Arlen Beiler 2010-07-29T14:20:24Z 2010-08-13T09:19:52Z

<p>Since this site is not in bootstrap mode, if anyone doesn't have 15 rep, just leave an answer here, and we will try to vote you up. Even better, could the Admin put this in bootstrap mode?</p> <p><s>Another thing you can do is go around and vote everyone up who has less than 15 rep.</s></p>