question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>In General Relativity is there a TE symmetry similar to CPT symmetry in the Standard Model ? It's pretty easy to understand that by flipping charge and parity you merely get a time reversed equivalent of your system, so flipping time as well would lead to an equivalent description. Similarly, since metric perturbati... | 6,751 |
<p>Statistical mechanics is restricted to the postulate of the equal a priori probability, but this postulate does not need to be considered for thermodynamics, so the valid ranges of statistical mechanics and the second law of thermodynamics are different.</p>
<p>For example, consider a freely falling body, the poten... | 232 |
<ol>
<li><p>What is the relation between entropy and mass of black hole?</p></li>
<li><p>And what is the relation between symmetry of physics operation and entropy?For instance,measuring or doing measure on state of quantum system especially pure state of a system has to lead to increase in entropy of the system,and t... | 6,752 |
<p>What happens when You make a contact junction between normal metal with some fermi energy $E_{F}$ and superconductor with fermi level $E_{SC}$, but this levels are not necessarily the same ? </p>
<p>There is some analogy between p-n junction or contact potential in metals?</p> | 6,753 |
<p>Simple question: as the title says, what makes a real Feynman diagram real, and what makes a virtual diagram virtual? Or in other words, how do I tell whether any given diagram is real or virtual? I've never gotten a really satisfying explanation of this. I would imagine it has something to do with virtual particles... | 6,754 |
<p>When solving Schrödinger's equation for a 3D quantum well with infinite barriers, my reference states that: $$\psi(x,y,z) = \psi(x)\psi(y)\psi(z) \quad\text{when}\quad V(x,y,z) = V(x) + V(y) + V(z) = V(z).$$ However, I cannot find any rationale for this statement. It may be obvious, but I would appreciate any elucid... | 6,755 |
<p>I saw <a href="https://www.youtube.com/watch?v=1Xp_imnO6WE">this video</a> by <a href="https://www.youtube.com/channel/UCHnyfMqiRRG1u-2MsSQLbXA">Varitasium</a> and I was not 100% sure about the third phenomena, a statically charged object attracting a stream of water, especially because he explicitly mentioned that ... | 6,756 |
<p>Accounts of the "secret" of the hydrogen bomb describe Xrays from a primary fission explosion reflecting off of the bomb case (occasionally passing through polystyrene foam) and compressing and heating the fusion fuel. No mention is made of gamma rays doing the same thing. Is there a physical explanation for why X-... | 6,757 |
<p>I'm solving problem in classical field theory and I have some difficulties. I'm trying to study small oscilations of heavy string with fixed points. </p>
<p>First of all I wrote down this Lagrangian: </p>
<p>$$S=\int dt ds \left[\frac{\rho}{2}(\dot{x}^2+\dot{y}^2)-\rho g y(s,t)+\frac{\lambda(s,t)}{2}\left(\left(\f... | 6,758 |
<p>QCD and confinement specify that hadrons must be color-neutral. My understanding is that this means you can have mesons (quark + antiquark) or baryons with 3 quarks, one of each color: Red+green+blue=neutral.
In a meson, does the color of the quark and antiquark matter? Must a red quark be paired with a red antiqua... | 6,759 |
<p>An explosion is a rapid increase in volume and release of energy in an extreme manner.
A blast wave in fluid dynamics is the pressure and flow resulting from the deposition of a large amount of energy in a small very localised volume.
The equation for a Friedlander waveform describes the pressure of the blast wave ... | 6,760 |
<p>Going to as little details as possible, here is a statement from Wald's text on QFT in curved spacetimes(I am not quoting the book)</p>
<p>He considers two vector spaces ${\cal S}$ and ${\cal H}$.</p>
<blockquote>
<p>Note - For more details about ${\cal S}$ and ${\cal H}$, read this box. I believe for most part ... | 6,761 |
<p>What does it mean by oscillating neutral for <a href="http://en.wikipedia.org/wiki/Three-phase_electric_power" rel="nofollow">3-phase electrical</a> system? Is it any effect of harmonic production due to magnetic core hysteresis nature?</p>
<p>I have been searched a lot about this through internet, but I failed. So... | 6,762 |
<p>In the hydrogen atom, the energy does not depend on l. This degeneracy is sometimes called "accidental" (because it does not correspond to some symmetry?). However, there is l dependence in the energy multi-electron atoms. Since the original degeneracy was "accidental", is it no longer correct to talk about what bre... | 6,763 |
<p>I have a small motor connected to DC source, and use the ampere meter to survey the current through the motor. While the motor is rotating, I hold the motor by hand so that it stop. I see the current through the motor increases. Could you help me explain this problem?</p>
<p><img src="http://i.stack.imgur.com/98Y8Q... | 233 |
<p>What determines the form of the intensity spectra of different particle species in Laser-Induced Fluorescence (LIF) measurements? See e.g. </p>
<p>I figure that bigger particles have more ways to get excited and so the intensities accumulate and make the curve wider? But how exactly do I derive an expected curve fo... | 6,764 |
<p>I was not able to find an answer for this question...</p>
<p>Some radioactive elements have half-life measured in thousands of years and some others even in millions, but over 4.5 billion years all the radioactive material that was part of the initial material that formed the planet earth should have decayed by now... | 6,765 |
<p>(c.f Conformal Field Theory by Di Francesco et al, p39)
From another source, I understand the mathematical derivation that leads to eqn (2.126) in Di Francesco et al, however conceptually I do not understand why the equation is the way it is. The equation is $$\Phi'(x) - \Phi(x) = -i\omega_a G_a \Phi(x)$$ G is defin... | 6,766 |
<p>I'm trying to prove that, under the gauge transformation $$A_{\mu} \rightarrow A_{\mu}^{\prime} = g^{-1} A_{\mu} g + g^{-1} \partial_{\mu} g,$$ the non-abelian Chern-Simons Lagrangian density:</p>
<p>$$\mathcal{L}_{CS} = \kappa \epsilon^{\mu \nu \rho} tr \left( A_{\mu} \partial_{\nu}A_{\rho} + \dfrac{2}{3} A_{\mu}A... | 6,767 |
<p>According to Hubble's law, light and other kinds of electromagnetic radiation emitted from distant objects are redshifted. The more distant the source, the more intense is the redshift. Now, the expansion of the universe is expected to explain the redshift and its nearly linear dependence on distance between source ... | 6,768 |
<p>This question seems to be naive, but I really want some intuitive way of understanding the reason.</p>
<p>Neglecting some trivial explanations, I have only one idea about this: the vacua state.</p>
<p>Consider the harmonic oscillator problem in quantum mechanics, there are the creator (operator) $a^\dagger$ and th... | 6,769 |
<p>I thought for fun I might measure how my ceiling fan slows down when turned off, and maybe find out what forces act on it when it slows down. My problem is that for that I need to measure its rotational speed when it's turned on. When it's in the lowest speeds it's easy because you can see it rotate, but for higher ... | 6,770 |
<p>Background Information:
I'm doing an experiment in which I place a bare tungsten wire in to various liqids, to measure a coefficient $\alpha $ in the equation
$$ Power Dissipated = \alpha * \Delta T $$
I was also given the equation:
$$R=R_0*(1+0.0045 \Delta T)$$
I decided to measure $R_0$ at a low voltage and curre... | 6,771 |
<p>I understood that quantum field theory is essentially based on a problematic mathematical basis. Can someone please explain what is the fundamental problem to formulate QFT as a rigorous mathematical theory? It's clear that it has to do with renormalization but similar techniques are already exist in analytically nu... | 234 |
<p>If I charge a capacitor and connect one lead to ground keeping the other lead floating, will the capacitor discharge ?</p>
<pre><code> G-------||------ open/floating
+q -q
</code></pre>
<p>(G for ground)</p> | 6,772 |
<p><em>(This question is <strong>not</strong> about coding)</em></p>
<p>I have built a little code in Python that allows the user to plot the energy vs the wave number of particle in a 2D box, depending on what values for the integers $p$ and $q$ they desire. </p>
<p>The energy $E_{p q}$ is given by:</p>
<p>$E_{p q}... | 6,773 |
<p>I'm sure I'm not the only person with this problem at the moment. I have been asked to give a public (not quite public, scientists, just not physicists) about 'this <a href="http://en.wikipedia.org/wiki/Higgs_boson" rel="nofollow">Higgs boson</a> thing'. I am trying to think of a good analogy because even my own rat... | 6,774 |
<p>Can fusion and fission happen at the same time, in the same place? I was talking with a friend, and he thinks that fission and fusion happen at the same time at the sun, is that true? I guess this would cause perpetual motion, then I speculated that they could happen, but in different proportions and I believed that... | 6,775 |
<p>If I define the vector as $V_i=V^T_i+V^L_i$ and the transverse part is defined by $$V^T_i=\Big(\delta_{ij}-\frac{\partial_i\partial_j}{\partial^2}\Big)V_j$$ then is is obvious that $\nabla.V^T=0$ as well as $\nabla \times V^L=0$. What happened if I took the the divergence of the cross product of two different vector... | 6,776 |
<p>After listening of some <a href="http://www.youtube.com/watch?v=2DIl3Hfh9tY#t=782s" rel="nofollow">lectures</a> of Leonard Susskind about black holes, he mentioned that conservation of information is one of the foundations of physics. After searching the web I cannot seem to find how we came up with this theory. Cou... | 235 |
<p>We often hear that Higgs boson is a scalar boson, and that Higgs field is a scalar field. I was always thinking that this means "4-scalar". In other words, it is space-time invariant, .i.e. it's properties are not changing if move relative to it. Unlike photon which changes it's frequency, polarization and other pro... | 6,777 |
<p><strong>Setup</strong></p>
<p>Imagine a two-body system of masses under a classical mechanics model. The separation and mass-ratio doesn't matter for this example. Presume they are initially stationary.</p>
<p>Now suppose that we hold one of the masses, m1, permanently stationary with an external force opposing th... | 6,778 |
<p>Does a complete theory of quantum gravity require anthropic post-selection? Certainly the black hole complimentarity and causal patch conjectures highlights the essential role of observers, at least in the asymptotic future of their future timelike trajectories. Does the measure problem in quantum gravity cosmology ... | 6,779 |
<p>Lately, there has been a war between physicists and philosophers. Some of the exchanges can be found at <a href="http://www.nytimes.com/2012/06/10/opinion/sunday/what-physics-learns-from-philosophy.html?bl" rel="nofollow">http://www.nytimes.com/2012/06/10/opinion/sunday/what-physics-learns-from-philosophy.html?bl</a... | 6,780 |
<p>According to Newtons second law: F = ma, if acceleration is zero then the force must be zero, but assuming you have an object moving with a constant velocity of say 2 m/s, and that object strikes you, then obviously some sort of 'force' would be felt by you, so my question is what do you call that 'force' since it a... | 6,781 |
<p>let be the Schroedinguer equation</p>
<p>$$ - \frac{d^{2}}{dx^{2}}y(x)+ae^{cx}y(x)=E_{n} $$ (1)</p>
<p>here a and c are constants.</p>
<p>i know how to solve it from <a href="http://eqworld.ipmnet.ru/en/solutions/ode/ode0232.pdf" rel="nofollow">http://eqworld.ipmnet.ru/en/solutions/ode/ode0232.pdf</a></p>
<p>bu... | 6,782 |
<p>The idea of the material Maxwell equation is almost clear. But I'm curious about the idea that except for material equation the pure Maxwell equation should work, but in harder sense: more currents and charges.</p>
<p>There are plenty of vacuum in solid body, so, I think, that there should be some precursors of ele... | 6,783 |
<p>all Lorentz observers watching a particle move will compute the same value for the quantity</p>
<p>$$ds^2 = -(c \, dt)^2 + dx^2 + dy^2 + dz^2,$$
$$ds^2 = g_{\mu\nu}dx^{\mu}dx^{\nu},$$
and ''ds/c'' is then an infinitesimal proper time. For a point particle not subject to external forces (''i.e.,'' one undergoing in... | 6,784 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/9415/why-do-electrons-occupy-the-space-around-nuclei-and-not-collide-with-them">Why do electrons occupy the space around nuclei, and not collide with them?</a><br>
<a href="http://physics.stackexchange.com... | 132 |
<p>Everybody knows you can produce electrostatic charge rubbing two different materials together. But have you ever smelt e.g. at the plastic after charging it? There actually is a distinct electrostatic charge smell :-) While normally smelling involves the transport of molecules, what you are smelling here, is the ... | 6,785 |
<blockquote>
<p><strong>Possible Duplicate:</strong><br>
<a href="http://physics.stackexchange.com/questions/13170/particle-physics-plots">Particle physics plots</a> </p>
</blockquote>
<p>I have seen graphs like this in lots of talks by phenomenologists talking about the Higgs search (see for example <a href="h... | 236 |
<p>I have learned (the basics) of how one can incorporate the principles of special relativity to quantum mechanics to obtain quantum field theory. Can the same be done with GR and QM to obtain a new theory? If not, what happends if one tries, what exactly goes wrong? Is there a mathematical inconsistency in any theory... | 6,786 |
<p>I need to draw a graph of measurements of time for a ball to drop on an inclined plane, with error bars, along with lines of best and worst fit. The problem is that I need to do this on the computer and I am unsure about what graphing package I can use to do this and how to go about doing it. I have access to Wolfra... | 6,787 |
<p>I've consulted several books for the explanation of why </p>
<p>$$\nabla _{\mu}g_{\alpha \beta} = 0,$$</p>
<p>and hence derive the relation between metric tensor and affine connection $\Gamma ^{\sigma}_{\mu \beta} $</p>
<p>$$\Gamma ^{\lambda } _{\beta \mu} = \frac{1}{2} g^{\alpha \gamma}(\partial _{\mu}g_{\alpha ... | 6,788 |
<p>Basically I wonder what happens to emitted radiation by douubling a light e.g. twice the surface area of the sun will emit how much more radiation? 4 times more? Is there a formula?</p> | 6,789 |
<p>Particularly in the case of Majorana neutrinos, it seems a little odd that the particle and antiparticle would have differing cross sections.</p>
<p>Perhaps the answer is in here, but I've missed it:
<a href="http://pdg.lbl.gov/2013/reviews/rpp2013-rev-nu-cross-sections.pdf" rel="nofollow">http://pdg.lbl.gov/2013/r... | 6,790 |
<p>I have learned that a torque on an object due to the gravity can be calculated as if the gravity acts on the center of mass of the object when the object is near the surface of the earth.</p>
<p>Now I want to prove it...</p>
<p>So, I have $$\tau=\int d\tau=\int r(dmg)sin\theta=g\int rsin\theta dm.$$</p>
<p>Then, ... | 6,791 |
<p>To quote from Wikipedia on elastic scattering, "In this scattering process, the energy (and therefore the wavelength) of the incident photon is conserved and only its direction is changed."</p>
<p>How does this work? The impacted particle will also start moving to conserve momentum. So it'll also carry some energy.... | 6,792 |
<p>Can someone explain to me how to solve $6.6738410\times 10^{-11}\ \text{m}^3\ \text{kg}^{-1}\ \text{s}^{-2}$, the formula to find the <a href="http://en.wikipedia.org/wiki/Gravitational_constant" rel="nofollow">gravitational constant</a>?</p>
<p>I'm writing a bunch of reports about the scientific perspective about ... | 6,793 |
<p>I am working on some radiative transfer equations, and struggling as I'm fairly new to this field. I have read about the Rayleigh Phase Function which is:</p>
<p>$P(\theta) = \frac{3}{4}(1 + cos^2 \theta)$</p>
<p>I can plot this function, and generate values from this function in my computer program (which is what... | 6,794 |
<p>This remark from Lubos puzzles me</p>
<blockquote>
<p>Ramond's string - and Neveu-Schwarz string - wasn't really an "origin of string theory". String theory had "origin" as bosonic string theory which has no fermions. All SUSY/fermioncs strings are "new".</p>
</blockquote>
<p>How to fit this view, which seems to... | 6,795 |
<p>So as part of a video I'm working on, I'm trying to launch a 10kg table about 8m into the air. A machinist friend of mine has provided a solution that involves pneumatic cylinders and a pump to generate the force needed to propel the table. </p>
<p>What we're trying to calculate is the amount of PSI needed to lif... | 6,796 |
<p>Below is a picture of a simple pendulum clock. Suppose that the bob (a rigid disk) on the end of the pendulum can spin without friction about its geometrical axis and is spinning at an angular velocity $\omega=const$. Will the clock gain or lose time compared to the ordinary pendulum clock. Or it does not have any e... | 240 |
<p>What I mean is could it turn out that the world is not described by string theory / M-Theory, but that nevertheless some version of one of these extra-dimensional theories is true?</p>
<p>I have no real background in this area. I just read Randall and Sundrum's 1999 paper "A Large Mass Hierarchy from a Small Extra... | 6,797 |
<p>When a lambda particle decays into proton and a pion, I am told it does not conserve parity. Why?</p> | 6,798 |
<p>Show that: $$\int _v \vec{J} dV = \frac{d \vec{p}}{dt}$$</p>
<p>Here's my attempt: $$\vec{p} = \int _v \rho r dV \rightarrow \frac{d}{dt} \int _v \rho r dV = \int _v \vec{J} dV = \int _v \frac{\partial p}{\partial t}r = \int _V \vec{J} dV$$ </p>
<p>Because of the fact that $\nabla \cdot \vec{J} = - \frac{\partial ... | 6,799 |
<p>I've been reading an introduction to quantum mechanics online, and while constructing the Schrodinger equation for a free particle, the equation $i\hbar \frac{d \Psi}{dt}=\hbar\omega\Psi$ is obtained. </p>
<p>It is then explained that if $i\hbar \frac{d}{dt}$ is considered as an operator, then the equation means "t... | 6,800 |
<p>Consider a real scalar field operator $\varphi$. It can be written in terms of creation and anihilation operators as
$$\varphi(\textbf{x})=\int \tilde{dk}[ a(k)e^{i\textbf{kx}}+a(k)^{\dagger}e^{-i\textbf{kx}}]$$
where $\tilde{dk}$ is a Lorenz-invariant measure.
If $\varphi$ is interpreted as creating a particle at ... | 6,801 |
<p>I am taking an introductory course on quantum mechanics and would like to understand this.
Do forces act in quantum mechanics?
In orbitals, is the charge of an electron dispersed, with a higher charge at places with a higher probability of finding an electron?
Is there any centripetal force/acceleration at the qu... | 6,802 |
<p>I read that the reason solids emit continuous spectra is that they don't have time to let their electrons decay-they are too close together. Given that electrons decay on the order of 100 nanoseconds I find this difficult to believe. Also, do electromagnetic waves move the electrons, or the atom, or both? If it is s... | 6,803 |
<p>For some reason I am suddenly confused over something which should be quit elementary.</p>
<p>In two-dimensional CFT's the two-point functions of quasi-primary fields are fixed by global $SL(2,\mathbb C)/\mathbb Z_2$ invariance to have the form</p>
<p>$$\langle \phi_i(z)\phi_j(w)\rangle = \frac{d_{ij}}{(z-w)^{2h_i... | 6,804 |
<p>I know (from Kinematics) that for an object moving linearly with an acceleration and without air resistance the following equations can be used to determine v(velocity) or x(position of the object) at any time:</p>
<p>$v=v_0+at$</p>
<p>$x=x_0+v_0t+\frac{1}{2}at^2$</p>
<p>Where $x_0$ is the position of the object ... | 6,805 |
<p>I tried to solve the following problem:
There are 2 objects . The object m1 with charge q and the object m2 with charge q.(same charge).The object m2 is connected with a rope to the ceiling. </p>
<p>at the initial state , m1 and m2 were on the same height.
When the object m2 is placed in distance d from the object... | 6,806 |
<p>Just curious ... </p>
<p>Known facts:</p>
<ul>
<li>When outside the atmosphere levels of radiation are higher</li>
<li>Typically such radiation is CBR, Solar Radiation</li>
</ul>
<p>Question:</p>
<p>If the matter of the atmosphere and magnetosphere as shields is kept aside for a moment, does gravity affect radia... | 6,807 |
<p>I have a differential equation of the form </p>
<p>$ \frac{d^2 y}{dt^2} + f(t) \frac{dy}{dt} + g(t) y = 0 $</p>
<p>where $f$ and $g$ are known functions of time. </p>
<p>Is there a systematic (or otherwise) way of finding the conserved quantities, if there are any?</p>
<p>I've been trying to google this topic, b... | 6,808 |
<ol>
<li>Is there a simple intuitive way to understand the difference between left-handed and right-handed fermions (electrons say)? </li>
<li>How to experimentally distinguish between them?</li>
</ol> | 6,809 |
<p>[sorry, this way below the level of this forum -- flames are most welcome]</p>
<p>When a photon is absorbed by a piece of matter that does not reflect it -- where does the photon "go"? Eg, one shines light at a black object -- clearly the photon arrives at the object and then, well, vanishes. Or does it?</p>
<p>T... | 6,810 |
<p>According to the equation $V = E-Ir$, the gradient of a graph of $V$ against $I$ should be $-r$ (internal resistance) and the Y intercept should be the EMF. Am I right?</p>
<p>In an experiment I used a constant wind speed to drive a small motor, keeping everything constant and changing the resistance in the circuit... | 6,811 |
<p>The world <em>A Song of Fire and Ice</em> series is set on is characterized by seasons that last several years (how years are measured on this planet is not specified to my knowledge). However, they are of unpredictable, variable lengths, though seemingly in balance (long summers are followed by long winters). Rat... | 6,812 |
<p>Consider the Newtonian gravitational potential at a distance of Sun:</p>
<p>$$\varphi \left ( r \right )~=~-\frac{GM}{r}.$$</p>
<p>I write the classical Lagrangian in spherical coordinates for a planet with mass $m$:</p>
<p>$$L ~=~ \frac{1}{2}m (\dot{r}^{2} + r^{2}\dot{\theta ^{2}} + r^{2}\dot{\phi ^{2}}\sin^{2}\... | 6,813 |
<p>Today I had a discussion with a colleague who teaches electricity and magnetism to 2nd year undergraduate physics students. He is seeking the best way to explain how is the <a href="http://en.wikipedia.org/wiki/Electromotive_force#Chemical_sources" rel="nofollow">emf generated inside a battery</a> with a minimal app... | 6,814 |
<p>The conclusion of <a href="http://lenr-canr.org/acrobat/KarabutABanalysisof.pdf" rel="nofollow">this paper</a> (p. 6) discusses some hypothetical Pd(d,f) yields as a possible explanation for anomalous results that the author observed. Suspending disbelief in the data reported in the paper for a moment, what are som... | 6,815 |
<p>Say Silicon and boron doped silicon. Would the doping affect the binding energy? Could I see this in an XPS spectra?</p> | 6,816 |
<p>Is there any facet of the energy emitted by a human body that is consistent and unique - like a fingerprint, but a signal that could be detected by a remote device?</p> | 6,817 |
<p>Adiabatic approximation or the Born-Oppenheimer approximation is used whenever the electronic motion is too fast that the electrons effectively see static nuclei and the nuclei, in turn, see an averaged electronic cloud. My question is: </p>
<ol>
<li><p>As the word 'adiabatic' suggests no heat/energy transfer, does... | 6,818 |
<p>I am facing some problems in understanding what is the importance of a <a href="http://en.wikipedia.org/wiki/Killing_vector_field" rel="nofollow">Killing vector field</a>? I will be grateful if anybody provides an answer, or, refer me to some review or books. </p> | 6,819 |
<p>When doing perturbation theory in quantum mechanics, if the perturbation hamiltonian is made of three terms :
$$W = W_1 +W_2 + W_3,$$
can I treat each term separately and performing perturbation theory for each one of them, then adding the perturbed energies together to get the total energy ?</p>
<p>$$ E \stackrel{... | 6,820 |
<p>When one drops a pebble in a body of water, one can observe multiple waves emanating from the point the pebble came in contact with water. Be it because the water "jumps" up and comes back down, creating more waves or because the water does "up and down" motion, there are definitely many waves.</p>
<p>But when one ... | 6,821 |
<p>I am working a problem concerning a 3 axis gryoscope, the spinning mass is a magnet (dipole). This is part of a optical sensing device. The inner gimbal is for pitch rotation, and the outer gimbal is for yaw precession. The outer gimbal is fixed in the sensor casing. The whole assembly is well balanced and not aff... | 6,822 |
<p>We can derive Lagrange equations supposing that the virtual work of a system is zero.</p>
<p>$$\delta W=\sum_i (\mathbf{F}_i-\dot {\mathbf{p}_i})\delta \mathbf{r}_i=\sum_i (\mathbf{F}^{(a)}_i+\mathbf{f}_i-\dot {\mathbf{p}_i})\delta \mathbf{r}_i=0$$</p>
<p>Where $\mathbf{f}_i$ are the constrainded forces and are su... | 6,823 |
<p>It is written in Jackson (page 3) :</p>
<blockquote>
<p>In fact, though there are <strong>recurring attempts</strong> to eliminate explicit reference to the fields in favor of action-at-a-distance descriptions of the interaction of charged particles, the concept of the electromagnetic field is one of the most fru... | 6,824 |
<p>I am confused as to how a particular equation in Thornton & Marion's 'Classical Dynamics of Particles and Systems' was derived. It is equation 9.88, on page 354 of the fifth edition.
<br/>
An incoming particle, $m_1$, is fired with kinetic energy $T_0$ at a stationary particle, $m_2$. After the collision, the ... | 6,825 |
<p>I think about a rotating torus (simplified tire) filled with ideal gas. Mass of gas is $m$ and molar mass is $M$. Pressure in non rotating torus is $p_0$. Temperature is constant $T$. Inner radius of torus is $r$ and outer radius is $R$.</p>
<p>Then the cylinder begins to rotate with angular velocity $\omega$</p>
... | 6,826 |
<p>When a domain is aligned to an exterior uniform magnetic field.
There are no forces acting on it?
Magnetization force = 0, and all other force?</p> | 6,827 |
<p>I am a student who is interested in scientific facts. I want a simple definition for <a href="http://en.wikipedia.org/wiki/Radiation" rel="nofollow">radiation</a>. Neither too scientific, nor too simple.</p> | 6,828 |
<p>Whilst studying for my physics courses, I like to drink tea. Today, I noticed that if you pull a bag of tea out of a hot cup of water, it gradually starts to rotate, picking up speed as time progresses and reaching a asymptotic speed after a while. My first question is: why does it start rotating? Of course it has s... | 6,829 |
<p>How does one go about accurately modeling sound propagation in a room (with reflections, absorption, and diffusion characteristics) from the motion of a loud speaker? More specifically what are the governing equations that are needed? Obviously Naiver-Stokes but this is too general? Is there an easier way?</p> | 6,830 |
<p>Lets say we have a complex vector $\vec{z} \!=\!(1\!+\!2i~~2\!+\!3i~~3\!+\!4i)^T$. Its scalar product $\vec{z}^T\!\! \cdot \vec{z}$ with itself will be a complex number, but if we conjugate the transposed vector we get $\overline{\vec{z}^T}\!\! \cdot \vec{z}$ <em>(this is a inner product right?)</em> and a positive ... | 6,831 |
<p>I am studying quantum physics and I would like to know a bit more in detail about <a href="http://en.wikipedia.org/wiki/Group_velocity" rel="nofollow">group velocity</a>, particle velocity and <a href="http://en.wikipedia.org/wiki/Phase_velocity" rel="nofollow">phase velocity</a>.</p>
<p>Can you guys suggest some b... | 98 |
<p>WMAP/Planck results confirm than we live in a de Sitter-like phase, i.e., a Universe with positive acceleration or positive cosmological constant! Therefore, I believe that a way to solve the cosmological constant problem could be to extend QFT from Minkowski to de Sitter space. I would not be surprised if our "mist... | 6,832 |
<p>What is the fundamental basis of resolution of vector. Suppose we have a vector $\vec{mg}$, now we resolve it into two components, horizontal and vertical. My question is what is the basis for telling that this is the horizontal and this is the vertical component since the Cartesian plane can be drawn on any directi... | 6,833 |
<p>Lets say we have a potential step as in the picture: </p>
<p><img src="http://i.stack.imgur.com/HB3XI.png" alt="enter image description here"></p>
<p>In the region <strong>I</strong> there is a free particle with a wavefunction $\psi_I$ while in the region <strong>II</strong> the wave function will be $\psi_{II}$.... | 6,834 |
<p>Today, at the Brazilian news, I heard that a men fell from a cruise into the sea, from a height of 50m, and decided to see if he could have survived. I took his weight for being 80kg. </p>
<p>I did $V^2=V0^2 + 2 G H$, this resulted $V = 31.3 m/s$ or $112.69 km/h$.
Also $F = m a=784 N$. </p>
<p>My question is:</p>
... | 6,835 |
<p>In pictures taken from the summit of Mount Everest (<a href="http://www.peakfreaks.com/images/Everest_summit_view_north.JPG">such as this one</a>), the colour of the sky is a very dark blue or even black in some pictures. I remember from my own experiences of hiking in the Himalayas that the sky appeared to be dark... | 6,836 |
<p>I have some difficulties designing my experimental apparatus:</p>
<p>The goal is creating a Solenoid in air (or iron powdered core) able to produce at least 50G of peak B field on his axis at a distance of 1".</p>
<p>I would like to drive this solenoid in such a way that the B field varies sinusoidally from 0G to ... | 6,837 |
<p>I was recently reading about NASA Faster than Light proposed spaceship, it supposedly will bend spacetime behind the spacecraft, so technically instead of bringing the ship to the destination, "it will bring the destination to you." now, I have an understanding of calculus-based physics, and a basic understanding of... | 6,838 |
<p><strong>Question</strong> Measuring the number of decays per minute $N(t)$ of a radioactive source every four days we have that: $N(t=0):=N_0=200$, $N(t=4)=141$, $N(t=8)=100$, $N(t=12)=71$, where $t$ is measured in days ($d$). ¿Which radioactive isotope is it?</p>
<p><strong>Answer</strong> From the decay law we ha... | 6,839 |
<p>I am designing a <a href="http://externalcombustion.org/thermionic-converters-problems-and-solutions/" rel="nofollow">thermionic converter</a> and one of the biggest inefficiencies is the thermal conductivity and limited electrical conductivity of the leads. I'm trying to mitigate the loss of heat besides finding ot... | 6,840 |
<p>I'm trying to do my prelab for an E&M course, and am asked if, for plotting $V$ vs $I$ with a varying temperature, I should expect a linear slope. I know that both $V$ and $I$ depend on $R$, and since $R$ is proportional to $T$ that I shouldn't.</p>
<p>Trying to show this, I'm having some problems -- probably b... | 6,841 |
<p>The solution to the Dirac equation (in the Dirac basis) are 4 coupled fields. The first 2 of them represent a particle (spin up/down), the other 2 fields are the antiparticle (spin up/down). When the particle is observed from its rest reference frame, the antiparticle solutions are zero. However once the particle is... | 6,842 |
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