question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>In the compression stroke of a petrol engine, the mixture is compressed by the upward movement of the piston. From where does the piston get energy to compress the mixture?</p>
<p>Similarly, in the exhaust stroke, the piston again moves upward to expel the gases. Where does the piston get energy from?</p> | 2,628 |
<p>Say you have an electron departing from point A and reaching poing B after a time t.</p>
<p>According to some helping friend, the <em>Partition Function</em> for that electron going from point A to B can be written as</p>
<p>$$Z = \int_{A \to B} [\mathcal{D}x]~ e^{iS[x]}$$</p>
<p>where $\mathcal{D}x$ is the measu... | 2,629 |
<p>I was reading <a href="http://www.weizmann.ac.il/home/kblum/IPC/tutorial%20notes/TutKfirSZ.pdf" rel="nofollow">this</a> writeup on the Kompaneets equation and the Sunyaev-Zel'dovich effect. </p>
<p>On page 3, section 2 the author states </p>
<blockquote>
<p>There is no way to increase the mean energy of a planck... | 2,630 |
<p>We start our question we a definition </p>
<p>A subbundle $P\subset TM^{\mathbf{C}}$ of the complexified tangent bundle is called a complex polarization if
\</p>
<ol>
<li><p>$P$ is Lagrangian</p></li>
<li><p>P involutive</p></li>
<li><p>dim$P\cap\bar P \cap TM$ is constant
Now, Introduce an hermitian form on $P$ ... | 2,631 |
<p>It seems that Planck's constant was made from fitting a curve for blackbody radiation, is it just experimental-further more his assumption that energy comes in quanta seems to have been a guess. Why would energy come in quanta? <a href="http://en.wikipedia.org/wiki/Planck%27s_law">Wikipedia</a> says that he didn't t... | 2,632 |
<p>This question came up when I was talking about the atmosphere. Someone had mistakenly claimed that as temperature increases, the density of the atmosphere should increase as well. I reasoned from the ideal gas law that, as temperature ($T$) increases, then so should $V$, leading to lower density. But I realized that... | 2,633 |
<p>What would the size of the universe be if it were physically possible to remove all of the empty space, leaving only matter?</p> | 2,634 |
<p>The heliopause is now estimated to be something around 100 AU (1 AU = Astronomical unit = about the earth sun distance). See the wikipedia article:<br>
<a href="http://en.wikipedia.org/wiki/Heliosphere">http://en.wikipedia.org/wiki/Heliosphere</a></p>
<p>From reading a book on NASA's Voyager mission, I learned that... | 2,635 |
<p>How little mass can a black hole contain and still be a "stable" black hole? What would the diameter be, in terms of the event horizon?</p> | 624 |
<p>This question arose in a seminar today about the solar wind...</p>
<p>This is my vagueish understanding of the problem - please correct if you see errors!</p>
<p>The 'classical' picture of atmospheric electricity is that the Earth as a whole is neutral, but that thunderstorms maintain a voltage of around +300kV at... | 2,636 |
<p>In classical (Newtonian) mechanics, every observer had the same past and the same future and if you had perfect knowledge about the current state of all particles in the universe, you could (theoretically) compute the future state of all particles in the universe.</p>
<p>With special (and general) relativity, we ha... | 2,637 |
<p>I've watched this video on YouTube by Sixty Symbols entitled "<a href="http://www.youtube.com/watch?v=UV0wtX9AXq4" rel="nofollow">Currents and Magnets</a>". In the video, the professor demonstrates the expansion of a wire due to current heating it up and he also demonstrates how the current interacts with a magnet. ... | 2,638 |
<p>I have come across two similar definitions of primary fields in conformal field theory. Depending on what I am doing each definition has its own usefulness. I expect both definitions to be compatible but I can't seem to be able to show it. By compatible I mean definition 1 $\iff$ definition 2. I will write both defi... | 2,639 |
<p>Could this device theoretically continue in motion forever? If not, why not? (click below for images):</p>
<p><a href="https://docs.google.com/open?id=0B1BhNZS4yw5pVGFqNWFvbk14UG8" rel="nofollow">Device description.</a></p>
<p><a href="https://docs.google.com/open?id=0B1BhNZS4yw5pYlQ1RWxqMjZyRDQ" rel="nofollow">De... | 2,640 |
<p>When I searched on the Internet for the reason of formation of <a href="http://en.wikipedia.org/wiki/Rainbow">rainbows</a>, I got many explanations like <a href="http://www.wisegeek.com/why-do-rainbows-form.htm">this</a> one & <a href="http://www.scientificamerican.com/article.cfm?id=why-do-rainbows-form-inst">t... | 2,641 |
<p>If the range of energies of cosmic rays is not so far away respect to gamma's, why those are not found commonly in a nuclear reaction?</p> | 2,642 |
<p>It has been argued the exponential size of the wavefunction can be interpreted as many parallel worlds, and this explains how quantum computers can factor large integers and compute discrete logarithms (although on closer examination, Shor's algorithm doesn't work at all by trying out a superposition of all candidat... | 2,643 |
<p>Given a classical field theory can it be always quantized? Put in another way, Does there necessarily need to exist a particle excitation given a generic classical field theory? By generic I mean all the field theory variants, specially Higher Derivative QFT(particularly Lee-Wick field theories).<br>
I ask this que... | 2,644 |
<p>Does quantum fingerprinting really argue for the exponential size of wavefunctions? Quantum fingerprinting is the idea that an exponentially long classical string can be encoded in a linear number of entangled qubits using quantum fingerprints. To an exponential degree of accuracy, but not exactly, the fingerprints ... | 2,645 |
<p>How exactly does gravitational compression, or compression in general, increase temperature? It seems counter-intuitive seen as temperature usually increases from the solid to the gas phase. </p> | 2,646 |
<p>How many photons in one Planck volume would it take to form a tiny black hole?</p>
<p>A photon doesn't have mass but it does have energy, $1.0101 \times 10^{-37}$ Joule for red $650$ nm wavelength light if I'm correct. A photon is a point-like boson so an infinite number of photons can fit into any given area. </p>... | 2,647 |
<p>Given a higher derivative classical/quantum field theory with say one scalar field, particularly the Lee-Wick standard model. It has been shown that such a field theory encompasses two kinds of fields, one normal and other a ghost field, in a particular limit, generally the high mass limit of the ghost field. We sta... | 2,648 |
<p><em>I am aware that there are plenty of questions regarding book recommendations, however, I have not found one that fully matches what I intend to ask. I have provided a list of links to some similar questions under my question.</em></p>
<hr>
<p>I am 2nd year Undergrad doing physics. So far we have completed thre... | 53 |
<p>I am currently a senior in high school. I have spent the past four years participating in physics research at a local lab. Are there any journals in which I as a high schooler can publish my research?</p> | 2,649 |
<p>Assuming water volume ($V$), initial water temperature ($T_0$) and environment temperature ($T_e$) are known, what is the easiest way to calculate temperature of water in given time ($T$)?</p>
<p>For the sake of question, let's assume water container is so thin it doesn't matter. Also, water container is airtight (... | 2,650 |
<p>How can the integral form of <a href="http://en.wikipedia.org/wiki/Gauss%27s_law" rel="nofollow">Gauss's law</a> for magnetism be described as a version of general <a href="http://en.wikipedia.org/wiki/Stokes%27_theorem" rel="nofollow">Stokes' theorem</a>? How does it follow?</p> | 2,651 |
<p>It seems that this question has not really been explored in the literature. Do isolated neutron stars (which do not accrete material) emit stellar wind? If yes, what composition would it have? If yes, what will be the rate of mass loss for the star?</p>
<p>One might also think up that the process of Hawking radiati... | 2,652 |
<p>If Alice wants to send one <strong>bit</strong> of classical information she can use a <strong>qbit</strong>. Then Bob needs to know which axis to measure to get the information. This needs an extra agreement between Alice and Bob before starting communication. I can figure out why up or down agreement has no import... | 2,653 |
<p>According to some theory, currents in <a href="http://en.wikipedia.org/wiki/Quantum_Hall_effect#Integer_quantum_Hall_effect_.E2.80.93_Landau_levels" rel="nofollow">IQHE</a> are due to the edge states. The edge current on the upper edge flows from left to right, that on the lower edge flows from right to left.</p>
<... | 2,654 |
<p>Angular momentum is defined from linear momentum via $\vec L = \vec r\times\vec p$, and is conserved in a closed system. Since energy is the time part of the linear four-momentum, is there a quantity defined from energy that's also conserved?</p> | 2,655 |
<p>The position center of gravity of a bicycle and its rider is known, and the distance from it to the point of contact of the front wheel with the ground, in terms of horizontal and vertical distance (x cm and y cm), are both known, as well as the total weight of the bike and the rider. The brakes are then applied, ma... | 2,656 |
<p>How can you have a negative <a href="http://en.wikipedia.org/wiki/Voltage" rel="nofollow">voltage</a>? I don't really understand the concept of negative voltage, how can it exist? </p> | 2,657 |
<p>When trying to solve the Schrödinger equation for hydrogen, one usually splits up the wave function into two parts:</p>
<p>$\psi(r,\phi,\theta)= R(r)Y_{l,m}(\phi,\theta)$</p>
<p>I understand that the radial part usually has a singularity for the 1s state at $r=0$ and this is why you remove it by writing:</p>
<p>$... | 2,658 |
<ol>
<li><p>What exactly are <a href="http://www.google.com/search?as_epq=crystal+plane" rel="nofollow">crystal planes</a> and how do they reflect x-rays?</p></li>
<li><p>Are crystal planes real physical planes or just an abstract concept? </p></li>
<li><p>What are these planes made of? </p></li>
<li><p>If they are an ... | 2,659 |
<p>Suppose that there are negative charges (e.g. electrons) only. There are more negative charges on left than on right. How would electric field be constructed? (So, What would be the direction?) And how would electrons flow (because of electric field)? The text says that it flows from left to right, but I am not gett... | 2,660 |
<p>If I have a Lagrangian of the form:</p>
<p>$$ \mathcal{L} = k \bar{\psi} \varepsilon^{\mu \nu} \lambda^a \phi G^a_{\mu \nu} + h.c. $$</p>
<p>[where $\phi, \psi$ are fermions, $\lambda^a$ are Gellmann matrices, $\varepsilon^{\mu \nu} $ is some antisymmetric tensor and $G^a_{\mu \nu}$ is the gluon field stength tens... | 2,661 |
<p>I have a doubt about the electric potential of a body. Well, I know that given a continuous distribution of charge we can find the potential at a point $a$ using the following relation:</p>
<p>$$V(x,y,z)=\frac{1}{4\pi\epsilon_0}\iiint_{S} \rho(a') \frac{1}{|a-a'|}dV'$$</p>
<p>However, this gives a function that at... | 2,662 |
<p>In statistical physics, mean-field theory (MFT) is often introduced by working out the Ising model and it's properties. From a spin model point of view, the mean-field approximation is given by requiring that :</p>
<p>Eq.(1)$\hspace{75pt}$$\langle S_i S_j \rangle = \langle S_i \rangle \langle S_j \rangle $ for $i\n... | 2,663 |
<p>In a series or parallel circuit, if two bulbs have the same resistance, do they have the same voltage drops? The problem I am asking about is below. Do A, B, and C have the same voltage drops since they have the same resistance? If so, how do I determine the current?</p>
<p><img src="http://i.stack.imgur.com/2wyv... | 2,664 |
<p>As explained for example in <a href="http://profmattstrassler.com/2012/08/15/from-string-theory-to-the-large-hadron-collider/">this</a> article by Prof. Strassler, modern <a href="http://physics.stackexchange.com/q/32491/2751">twistor methods</a> to calculate scattering amplitudes have already been proven immensely ... | 2,665 |
<p>I was thinking of making a simple 2D model of the solar system, with planets moving along ellipses like</p>
<p>$$x(t) = k_x \sin(t + k_t) (\sin(k_\phi) + \cos(k_\phi))$$</p>
<p>$$y(t) = k_y \cos(t + k_t) (cos(k_\phi) - \sin(k_\phi))$$</p>
<p>and, for earth at least, a angle that some longitude (say the Greenwich ... | 2,666 |
<p>I have hit a bit of a roadblock in my simulation. In the equation I require the aperture size for the laser source.</p>
<p>$$I_a ≈ \frac{\lambda d}{A}$$</p>
<ul>
<li>$I_a$ = aperture atop the atmosphere ($m$)</li>
<li>$\lambda$ = wavelength of laser (in this case 1023nm)</li>
<li>$A$ = aperture of the laser transm... | 2,667 |
<p>Is it a myth that yelling to a coffee mug will heat it? I have been hearing my friend saying that screaming will heat coffee or water.</p> | 2,668 |
<p>Reading about nuclear models, nuclear physics and the mythical ``stability island'' I just wondered about the next question: </p>
<p><strong>How can the lifetime of any undiscovered superheavy element be calculated or estimated theoretically?</strong> </p>
<p>Moreover...Does it correspond to a numerical recipe or ... | 2,669 |
<p>I was going through the concept of designing a Nuclear Reactor that uses Spent Nuclear Fuel(SNF) to generate power as proposed by Transatomic Power .</p>
<p><a href="http://transatomicpower.com/white_papers/TAP_White_Paper.pdf" rel="nofollow">http://transatomicpower.com/white_papers/TAP_White_Paper.pdf</a></p>
<p>... | 2,670 |
<p>Imagine that there is a cube box that has mirrors all 6 faces in . If we use a strong laser and enter in the box from a small hole on the box. The laser light travels in the box long time that we can detect the laser via a detector on other hole of the box. </p>
<p>1) Is it possible to simulate the long light trave... | 2,671 |
<blockquote>
<p>Show that in the WKB approximation, the mean kinetic energy $T_{n1}$ in a bound state $\psi_n$ in a potential $V(x)$ is given by</p>
<p>$\langle T_n \rangle = \frac{1}{2}\left(n+\frac{1}{2}\right) \frac{dE_n}{dn}$</p>
</blockquote>
<p>This is a homework problem and I'm having trouble understandi... | 2,672 |
<p>I have a solver for Poisson's equation and it works nicely. It uses finite differences. It works in the presence of multiple dielectrics.</p>
<p>It also solves the <a href="http://en.wikipedia.org/wiki/Poisson%E2%80%93Boltzmann_equation" rel="nofollow">Poisson Boltzmann equation</a>. That is, fixed charges with fre... | 2,673 |
<p>On the plane z=0 there is a superficial charge distribution such that $\sigma$ is constant. </p>
<p>Near to the plane, there is a bar, charged uniform with total charge q. At the extremities the bar has two constraints, so it can't turn.</p>
<p>If I want to find the constraints force and the force momentum needed ... | 2,674 |
<p>In Sun-Earth system, what transformation law allows me to find the Hamiltonian H in an arbitrary inertial system knowing the same in the center of mass?</p> | 2,675 |
<p>I can't seem to think of any way to envision electron spin. Can it be thought of as the uncertainty in angular momentum?</p> | 2,676 |
<p>Consider a single coil that is turning in a constant and uniform magnetic field thanks to a motor. The normal to the coil is given by:</p>
<p>$${\bf u}(t)=\sin (\omega t){\bf u_x}+\cos(\omega t){\bf u_z}$$</p>
<p>How can we obtain the energy that the motor has to spend in a period $T=\frac{2\pi}{\omega}$?</p>
<p>... | 2,677 |
<p>I don't mean <a href="http://en.wikipedia.org/wiki/Line_integral" rel="nofollow">line integrals</a>, I am talking about <a href="http://en.wikipedia.org/wiki/Path_integral_formulation" rel="nofollow">path integrals</a> or <a href="http://en.wikipedia.org/wiki/Functional_integration" rel="nofollow">functional integra... | 2,678 |
<p>When you measure the position of an electron that is in a pure energy state, what happens the energy becomes non-deterministic. That is future measurements of energy can only be predicted with respect to a probability distribution. </p>
<p>This seemingly violates the conservation of energy since future measurements... | 2,679 |
<p>These papers describe a phenomenon referred to as "atomic collapse" and "supercritical charge" in graphene: <a href="http://www.sciencemag.org/content/340/6133/734.abstract" rel="nofollow">Wang et al.</a>, <a href="http://prl.aps.org/abstract/PRL/v99/i16/e166802" rel="nofollow">Pereira et al.</a></p>
<p>"Atomic col... | 2,680 |
<p><strong>Problem:</strong><br />
<em>A block B of 1,5 kg is attached to the right of a spring (not deformed, with its right side attach to a wall) with a constant of $k = 80 N/m$ and, at rest, the block enter in collision with another identical block A. The block A moves to the left with a velocity of 5 m/s before th... | 2,681 |
<p>Photons have no mass. Yet they interact gravitationally, as all energy does, with other energetic and massive particles. This means that if you put multiple photons in a system, you get something that appears to have mass, even though none of the constituent particles do have mass.</p>
<p>That makes me wonder:</p>
... | 2,682 |
<p>Trying to work out some pesky flywheel dynamics for a project I'm working on, would love some for your assistance to better understand the underlying concepts. </p>
<p>For a given flywheel (thin-walled cylinder, assume a spoked bicycle wheel) rotating in the x-y plane, I'm trying to calculate the force generated in... | 2,683 |
<p>Given particles A, B, C and D, where:</p>
<ul>
<li>A and B have an equivalent mass</li>
<li>C and D have an equivalent mass, both larger than A (or B)</li>
<li>D is the antiparticle of C.</li>
</ul>
<p>A and B start close to C, but with velocity, such that they have moved away from C, so their kinetic energy has b... | 2,684 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/5456/the-speed-of-gravity">The speed of gravity</a><br>
<a href="http://physics.stackexchange.com/questions/26742/does-gravity-travel-at-the-speed-of-light">Does gravity travel at the speed of light?</a> ... | 4 |
<p>Why do I keep getting a slightly different result from the following two ways of determining the center of mass of a rigid, geometrically simple object? </p>
<p>The object is a rectangular 5(x) by 7(y) sheet of uniform rigid material, with a 2(x) by 3(y) rectangle missing from the upper right corner.</p>
<p><stro... | 2,685 |
<p>I'm trying to calculate the amount of fluid that would flow through an area dependant on the amount of pressure that there is. I'd also like to know the rate at which it would flow.</p>
<p>Essentially I have a very basic model of a well drilling system. At the moment the variables / parameters for each of the obj... | 2,686 |
<p>A uniform cylinder was placed on a frictionless bearing and set to rotate about its vertical axis. After a cylinder has reached a specific state of rotation it is heated without any mechanical support from temperature $T$ to $T+\Delta T$.
Angular momentum will be conserved but rotational kinetic energy will decrease... | 2,687 |
<p>I am a total beginner in the field of Quantum Mechanics. So, the question I am asking may be a silly one. So kindly give me possible answers or advice for modifications. </p>
<p>Recently I am learning the concept of qubit. The quantum theory tells that a $n$-qubit system is represented by a unit vector in $(\mathbb... | 2,688 |
<p>Is there even a relativistic mass or just relativistic momentum? How does one reason to prefer one over another? What is the problem with saying a hot gas will have more mass/inertia to it? </p> | 54 |
<p>I am reading a quantum transport book, where they often mention: phase breaking length and Fermi wavelength. I have looked up and found that:</p>
<p><strong>Phase breaking length</strong>= length over which electron remains its phase.</p>
<p><strong>Fermi wavelength</strong>= Wavelength associated with the maximum... | 2,689 |
<p>I live in a very old house - build 1902 - in the 4th floor in the city of Karlsruhe (Germany). I have a shower and the gas-heater for the water is in it:</p>
<p><img src="http://i.stack.imgur.com/tpiBK.jpg" alt="enter image description here"></p>
<p>If I hold the shower head over some height (see image), the water... | 2,690 |
<p>Let's say that there is a parallel circuit with two identical resistors in parallel with each other. If a third resistor, identical to the other two, is added in parallel with the first two, the overall resistance decreases. </p>
<p>Why does this overall resistance decrease?</p> | 2,691 |
<p>In Peskin & Schroeder (and also Cheng which I have skimmed through) they motivate the <a href="http://en.wikipedia.org/wiki/Operator_product_expansion">Operator Product Expansion</a> with a lot of words. </p>
<p>Is there any way to motivate it mathematically, e.g. Taylor expansion or similar?</p> | 2,692 |
<p>I didn't have much luck getting a response to this question before so I have tried to reword and expand it a little:</p>
<p>In early 2010 I attended this inaugural lecture by string theorist- Prof. Mavromatos entitled 'MAGIC strings'. In it he proposes that some string theory models may violate Lorentz symmetry at ... | 114 |
<p>Given a collection of point-particles, interacting through an attractive force $\sim \frac{1}{r^2}$.</p>
<p>Knowing only $m_1a=\sum_i \frac{Gm_1m_i}{r^2}$ and initial conditions we can deduce the motion of the system.</p>
<p>Consequently we can observe that three quantities remains constant
A) center of mass of th... | 2,693 |
<p>There are some things I encountered, studying the Bernouilly equation, that I don't understand. I was studying in the following book: <a href="http://www.unimasr.net/ums/upload/files/2012/Sep/UniMasr.com_919e27ecea47b46d74dd7e268097b653.pdf" rel="nofollow">http://www.unimasr.net/ums/upload/files/2012/Sep/UniMasr.co... | 2,694 |
<p>When analysing powder diffraction patterns, the broadening of peaks can be used to estimate crystal sizes. Smaller crystal size gives larger broadening according to the <a href="http://en.wikipedia.org/wiki/Scherrer_equation" rel="nofollow">Scherrer equation</a>:</p>
<p>$$ \beta = {{K\cdot\lambda}\over{D\cdot\cos \... | 2,695 |
<p>I'm not sure if this is the appropriate forum for my question as I actually am studying this as part of electrical engineering and I don't actually study physics. Nonetheless, I shall ask and if need be, move my question to another venue.</p>
<p>My question is with regard to how complex permittivity is defined. A... | 2,696 |
<p>I am looking the derivation of the speed of sound in Goldstein's Classical Mechanics (sec. 11-3, pp. 356-358, 1st ed). In order to write down the Lagrangian, he needs the kinetic and potential energies.</p>
<p>He gets the kinetic energy very easily as the sum of the kinetic energies of the individual particles (the... | 2,697 |
<p>The following passage has been extracted from the book "Modern's abc of Chemistry": </p>
<blockquote>
<p>..Heisenberg in 1927, put forward a principle known as Heisenberg's uncertainty principle. It states that, it is not possible to measure simultaneously both the position and momentum (or velocity) of a micros... | 2,698 |
<p>I am looking for patterns to efficiently disperse reflected ultrasound in the range of wavelengths 1mm to 4mm within the interior of a narrow tube (I do not want wall reflections). For various reasons absorption does not appear to be a solution. Thickness is also critical in that the surface must not be much more th... | 2,699 |
<p>In quantum mechanics, what is the difference between <a href="http://en.wikipedia.org/wiki/Group_velocity#Matter-wave_group_velocity" rel="nofollow">group velocity</a> and <a href="http://en.wikipedia.org/wiki/Phase_velocity#Matter_wave_phase" rel="nofollow">phase velocity</a> of matter wave? How can it also be that... | 2 |
<p>I have read the following: <a href="http://www.feynmanlectures.caltech.edu/I_20.html#Ch20-S1" rel="nofollow">http://www.feynmanlectures.caltech.edu/I_20.html#Ch20-S1</a> </p>
<p>The formula for $\tau_{xy}$ is derived in this chapter: <a href="http://www.feynmanlectures.caltech.edu/I_18.html#Ch18-S2" rel="nofollow">... | 2,700 |
<p>How can we define U(1) local gauge transformation for Dirac spinor field?, like scalar fields? </p> | 2,701 |
<p>I want to be able to modify the height of a quantum locked superconductor. My original plan was to levitate the superconductor over an electromagnet and modify the current to said magnet (increase the current for higher levitation height, decrease for lower). Further research has made it apparent to me that this won... | 2,702 |
<p>In this problem set I have a passage that describes an experiment that looks at the changing temperature as an air filled balloon rises to the surface from the bottom of a water filled tank. The graph they provide shows that as the balloon rises to the surface the temperature of the air in the balloon decreases. (I'... | 2,703 |
<p>This <a href="http://travel.stackexchange.com/a/21886/4868">travel stackexchange answer</a> has kinda got me wondering... how long do experiments involving the large hadron collider usually take? I'd expect you run it for a few seconds and bam - higgs boson detected or whatever. Maybe it'd take a few months to set ... | 2,704 |
<p>$$
\hat{\Omega}_j{(\tilde{q}_j)}=\Omega_j(\tilde{q}_j-\hat{q}_j)
$$</p>
<p>$$
[\hat{q}_j,\hat{q}_l]=ik_{jl}
$$</p>
<p>Implies</p>
<p>$$
[\hat{q}_j,\hat{\Omega}_l]= \frac{\partial\Omega_l(\tilde{q}_l-\hat{q}_l)}{\partial\hat{q}_l}.ik_{jl}
$$</p> | 2,705 |
<p>Consider some 1D Lattice of atoms with nth neighbor coupling of strength k_{n}. I'm looking for the dispersion relation for acoustical phonons under these conditions. </p>
<p>I start with the Lagrangian,
$$L = K- V$$
$$L = \sum^{\infty}_{n} \frac{1}{2}m \dot{x}_{n}^{2} - \sum^{\infty}_{p=1} \frac{1}{2}k_{p} \{(x_n... | 2,706 |
<p>This website <a href="http://www7b.biglobe.ne.jp/~kcy05t/" rel="nofollow">http://www7b.biglobe.ne.jp/~kcy05t/</a> appears to refute Quantum mechanics using some proof.</p>
<p>An important paper involved is this 'Calculation of Helium Ground State Energy by Bohr's Theory-Based Methods' <a href="http://arxiv.org/abs/... | 2,707 |
<p>For simplicity let us consider one-dimensional quantum-mechanical systems only. Given any state $\rho\in\mathcal{B}(\mathcal{H})$ and its Wigner function $W_\rho(x,p)$, there are several properties it always satisfies,</p>
<ul>
<li>it is real-valued and bounded in absolute value by $2/\hbar$ from above,</li>
<li>it... | 2,708 |
<p>Consider a Pure state of a two dimensional system $|\psi\rangle={1\over\sqrt{2}}(|e_1\rangle|e_1\rangle+|e_2\rangle|e_2\rangle)$ where $\{|e_i\rangle\}$ is an orthonormal basis.</p>
<p>Could any one just confirm me whether the corresponding density matrix $\rho$ is defined as $|\psi\rangle\otimes |\psi\rangle$? If ... | 2,709 |
<p>What is the absorption cross section, how is it measured?</p>
<p>How to convert it to the absorption coefficient (measured in cm$^{-1}$)?</p> | 2,710 |
<p>What is the relationship between earthquakes power and nuclear explosions?</p>
<p>How to compared power of earthquake with nuclear explosion?</p> | 2,711 |
<p>I was referring Electron microscopes and read that the electrons have wavelength way less than that of visible light. But, the question I can't find an answer was that, If gamma radiation has the smallest of wavelengths <em>of all</em>, why can't it be used to reach to even finer details in microscopy?</p> | 2,712 |
<p>Consider that space is uniformly charged everywhere, i.e., filled with a uniform charge distribution, $\rho$, everywhere.</p>
<p>By symmetry, the electric field is zero everywhere. (If I take any point in space and try to find the electric field at this point, there will always be equal contributions from volume ch... | 427 |
<p>"The flag of a ship that is moving northward with 10 km/h, points exactly southwestward. The windsock at the lighthouse points under 30 relative to the western direction southward. Calculate the velocity of the wind at land and on board of the ship".</p>
<p>I have done my homework and solved this problem, so this i... | 2,713 |
<p>I have the following situation in mind: </p>
<p>A big airtight bag of arbitrary shape with a person standing on it. The bag gets inflated with air to lift the person.
Assuming that the bag is much larger than the persons footprint, how do I find the minimal overpressure in the bag that I need to lift the person of... | 2,714 |
<p>My friend said this to me and just want to make sure this is right " when we connect the a battery to a LED and the 2 poles are connected, electrons flow from the (-) to the (+) but with very low velocity, but when you do the same with a capacitor , once the 2 plates are connected electrons flow incredibly fast that... | 2,715 |
<p>I have encountered a hard exercise which i cannot quite solve. Could anyone help me with it? This is the exercise:</p>
<blockquote>
<p>Lets say we have a photon whose energy $W_f$ is equal to the
relativistic energy of an electron $W_{e0}=m_e c^2$. What is the energy of
a scaterred photon $W_f'$ if after the ... | 2,716 |
<p>There is a popular belief that wet skin burns or tans faster. However, I've never heard a believable explanation of why this happens.</p>
<p>The best explanation I've heard is that the water droplets on the skin act as a lens, focusing the sunlight onto your skin. I don't see how this would affect an overall burn, ... | 2,717 |
<p>So i have a force field $F(x,y)$ and i have to find out wether it is a potential or not.</p>
<p>My first idea was to calculate : $dU=Fdr$ (where $r$ is the radius vector) , to integrate on both sides and hence to see if $U$ is path dependent or not. That turned out to be right.</p>
<p>I was thinking about another ... | 2,718 |
<p>For a two-dimensional collision experiment, I must explain in one of the questions the problems in analyzing a non-ideal two-dimensional collision. As the question states:</p>
<blockquote>
<p>Suppose that the projectile marble and target marble do not collide with their centers of mass equidistant from the floor,... | 2,719 |
<p>When a magnet passes through a copper coil and electricity is induced into the coil, is there a magnetic resistance on the magnet as it passes through the coil?</p> | 2,720 |
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