question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>I fear my textbook is teaching an incorrect concept of <a href="http://en.wikipedia.org/wiki/Work_%28physics%29" rel="nofollow">Work</a>. I am very frustrated right now since I was struggling to understand the concept in the way that was explained in the textbook and my instincts tell me it is wrong. Also note that ... | 3,180 |
<p>I recently remembered that someone worked out <a href="http://faculty.washington.edu/jcramer/BBSound_2013.html" rel="nofollow">what the big bang sounded like</a> and that got me thinking...</p>
<p>About 377,000 years after the Big Bang, electrons became bound to nuclei to form neutral atoms. Because of (?) this, th... | 3,181 |
<p>How did Nima Arkani-Hamed come up with the following nonzero space-like commutation relation in string theory?</p>
<p>$$\left\langle \left| \left[ \hat{\mathcal{O}}(\vec{x}), \hat{\mathcal{O}}(\vec{y}) \right] \right|^2 \right\rangle \sim \exp \left( -\frac{k\left| \vec{x} - \vec{y} \right|^{D-2}}{8\pi G} \right)$$... | 3,182 |
<p>In free space, the linear momentum density of an EM wave is given by the Poynting vector $\vec S$ over the speed of light squared, $\vec g=\frac{\vec S}{c^2}$.</p>
<p>In a medium, $S$ is generally not directed along the wave vector $\vec k$. If $v_g$ is the group velocity and $W$ is the density of EM wave energy, ... | 3,183 |
<h3>Cost per Power Capacity</h3>
<p>For a number of energy technologies I'd like to know what the minimum costs are to install a given power capacity. Are there any such comparisons available?</p>
<h3>Research Spendings</h3>
<p>Technologies often own much to publicly funded research efforts so I'm also interested i... | 3,184 |
<p>My 8-year-old daughter's school report says that she's good at understanding the basic science she's doing, but she's having trouble seeing how experimental results lead to conclusions. Specifically, it says she struggles to appreciate how changing parameters in an experiment can be used to prove or rule out a hypot... | 3,185 |
<p>Imagine a variation on the double slit experiment. I'll describe it in 2D using the $x-y$ plane. The $x$-axis is impenetrable other than the two slits, which are positioned at $(-1,0)$ and $(+1,0)$. Detectors will be positioned in the upper half plane, say along $y=10$ or something. Here's the twist: the negative $y... | 3,186 |
<p>In the linear sigma model, the Lagrangian is given by </p>
<p>$$ \mathcal{L} = \frac{1}{2}\sum_{i=1}^{N} \left(\partial_\mu\phi^i\right)\left(\partial^\mu\phi^i\right) +\frac{1}{2}\mu^2\sum_{i=1}^{N}\left(\phi^i\right)^2-\frac{\lambda}{4}\left(\sum_{i=1}^{N}\left(\phi^i\right)^2\right)^2 \tag{11.65} $$
(for exampl... | 3,187 |
<p>I know that for parallel RLC circuits, the $Q$ factor is given by: </p>
<p>$$ Q = R \sqrt {\frac{L}{C}} $$</p>
<p>But now suppose it is connected in series to a resistor $R_2$ and capacitor $C_2$. Would the $Q$ factor be changed?</p>
<p><img src="http://i.stack.imgur.com/dpLzs.png" alt="enter image description he... | 3,188 |
<p>The mass of air bubble in any medium is considered as negative. Is the air bubble is massless. I m in confusion. can we not neglect the mass of air bubble in material medium. But i have found in many books the mass of air bubble in material medium as negative. please discuss.....</p> | 3,189 |
<p>I put a nucleus in a magnetic field. It spins. Does the electric charge distribution remain homogeneous or does the charge redistribute? Can this be measured? Will accept reference as answer.</p> | 3,190 |
<p>There are a lot of different models of inflation in cosmology, with plenty of different features and issues.</p>
<p>I read that the problem with some models is that they introduce a never-ending inflation, but I can't find an example of it.</p>
<p>Anyway, I thought that indefinite inflation could only happen in a ... | 3,191 |
<p>Newton's theory of gravity supports "gravity waves" in that moving objects cause changing gravitational fields. For example, two bodies rotating around their center of mass will have a stronger gravitational field when they are longitudinally oriented than when they are transverse oriented. Given two masses of mass ... | 3,192 |
<p>I have trouble imagining how two point-particles can have different properties.</p>
<p>And how can finite mass, and finite information (ie spin, electric charge etc.) be stored in 0 volume?</p>
<p>Not only that, but it can also detect all fields without having any structure.
Maybe it can check curvature of spaceti... | 3,193 |
<p>I just learnt what absorption and emission spectrum are. And our teacher introduced us to what <a href="https://en.wikipedia.org/wiki/Quantum_dot" rel="nofollow">Quantum Dots</a> are. And showed us their absorption and emission spectra.
And they look something like this
<img src="http://i.stack.... | 3,194 |
<p>Some of the major challenges that heralded the need for quantum mechanics we're explaining the photo-electric effect, the double-slit experiment, and electrons behavior in semi conductors.</p>
<ol>
<li><p>What are some of the predictions we can expect to see from a theory of quantum gravity?</p></li>
<li><p>What ty... | 3,195 |
<p>I am not professional physicist; but I am curious about Stephen Hawking's "imaginary time". It would be better to elaborate exactly what it is. I am not confused because of the word "imaginary" but I find it confusing to imagine a two dimensional "plane time". If we express time in a plane instead of a one dimension... | 77 |
<p>If a plane is flying at a constant speed at some altitude like 5-6 km and it releases a bomb:</p>
<ul>
<li>does the bomb move forward at the same horizontal speed as the airplane?</li>
<li>or does its horizontal speed decrease due to drag and no thrust to push it forward?</li>
<li>or does it outrun the airplane bec... | 3,196 |
<p>In my laboratory, I have already obtained a polarization-entangled photon source. It was created via the Spontaneous Parametric Downconversion process of 2 BBO crystals. The next question is that would I be able to use this source to create qubits? If so, how?</p>
<p>Thank you in advance.</p> | 3,197 |
<p>I'm having some trouble of getting a grasp of this term, so I hoped someone could enlighten me.</p>
<p>As far as I understand, the idea behind it is, that cells have different cycles in their "life", and in each cycle there is different phases where the radiosensitivity is different compared to the others. Then, by... | 3,198 |
<p>I have to give a 10 minute physics talk that have to involve a fair bit of mathematics -- i.e. not just qualitative/handwaving material to some undergrads. I have wasted the last 3 hours looking for appropriate topics and have pretty much lost the will to live. Any suggestions would be greatly appreciated.</p> | 3,199 |
<p>I understand an electric quadrupole moment is forbidden in the standard electron theory. In <a href="http://arxiv.org/abs/gr-qc/0412064v3">this paper</a> considering general relativistic corrections (Kerr-Newman metric around the electron), however, there is a claim that it could be on the order of $Q=-124 \, \math... | 3,200 |
<p>The question inspired by an upcoming <a href="http://physics.berkeley.edu/index.php?option=com_dept_management&act=events&Itemid=444&task=view&id=1235">colloquim at UCB</a>.</p>
<p>A naive interpretation of quark asymptotic freedom seems to imply that at high enough energies they should be weakly i... | 3,201 |
<p>What does it mean to say that mass "approaches infinity"?</p>
<p>I have read that mass of a body increases with the speed and when the body reaches the speed of light, the mass becomes infinity. </p>
<p>What exactly does it mean to say that the mass "approaches infinity" or "becomes infinity"? I am not able to get... | 3,202 |
<p>In the double slit interference pattern for the wave of an electron, what will happen if I make the slits to be smaller than the size of an electron ? Will I still observe an interference pattern on the opposite side of the screen or no electron will be able to cross the slit? </p>
<p>If no, then how can a quantum ... | 3,203 |
<p>Or more specifically, does the CMB radiation have an observer effect on us in our local system due to gravitational lensing? Acknowledging this effect, if any would be nearly negligible I have an also: would this same effect be "felt" in the void spaces between galaxies?</p> | 3,204 |
<p>This paper describes black holes as space flowing inward (the rotating hole also twists in a weird way):</p>
<p><a href="http://arxiv.org/abs/gr-qc/0411060" rel="nofollow">http://arxiv.org/abs/gr-qc/0411060</a></p>
<p>The proper time given by the objects is the same as special relativity except for that fact that ... | 3,205 |
<p>How does the uncertainty principle limit the accuracy of atomic clocks. I know line width and measurement time are important but not exactly why? </p> | 3,206 |
<p>Layman here,</p>
<p>Stumbling through some physics stack posts and started reading the Wikipedia for the <a href="http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang">chronology of the big bang</a>. In it, it states</p>
<blockquote>
<p>The very earliest universe was so hot, or energetic, that initially no
ma... | 801 |
<p>During its formation, the Sun contracted under the force of its own gravity, until hydrostatic equilibrium was established. In this contraction, its temperature increased from $3\times10^4\text{ K}$ to $6\times10^6\text{ K}$. How to find the energy radiated during this contraction?</p> | 3,207 |
<p>Is gauge pressure <a href="http://en.wikipedia.org/wiki/Pressure_measurement#Absolute.2C_gauge_and_differential_pressures_-_zero_reference" rel="nofollow">always zero-referenced against ambient air pressure?</a> Or is it referenced against the sum of all pressures acting on a fluid, which usually just happens to be ... | 3,208 |
<p>When a short circuit occurs it's obvious that there is fire. How come electric energy turns out to be heat energy? What causes the conductors to get hot when short circuit is present.</p> | 3,209 |
<p>I'm currently learning the mathematical framework for General Relativity, and I'm trying to prove that the Lie derivative of the Riemann curvature tensor is zero along a killing vector.</p>
<p>With the following notation for covariant differentiation, $A_{a||b} $ (instead of $\nabla_b A_a$ ), I have the following:<... | 3,210 |
<p>Yes, this is a homework question, but I've already failed to solve it enough times that the online system hosting it isn't going to give me any marks, so I figure it's a good time to stop hitting the wall and actually ask. The question is as below:</p>
<blockquote>
<p>A spectroscopist uses a spectrometer that has... | 3,211 |
<p>In this paper about <a href="http://dx.doi.org/10.1109/IROS.2006.282433" rel="nofollow">Backstepping controll of a quadrotor helicopter</a> an algorithm for control is described, but I have hit a dead end.</p>
<p>In equation 15 it is described the part of state space for the angular and translation motion of a rigi... | 3,212 |
<p>I was watching a movie. A spaceship was forced into "warp speed". The co-ordinates could not be set. The spaceships trajectory was that of a nearby sun. Forcing the spaceship to power down was the solution. Now out of "warp speed" and with no computer aid (steering etc) the spaceship was seen to be spinning toward t... | 3,213 |
<p>I'm very new the topic of SPPs and have been trying to understand this particular method of exciting surface plasmons using a 1D periodic grating of grooves, with distance $a$ between each groove. If the light incident on the grating is at an angle $\theta$ from the normal and has wavevector ${\bf k}$, then apparent... | 3,214 |
<p><strong>Intro:</strong> Few hours ago, there was a storm. We heard some constant banging which couldn't be explained by thundering. Then we found out, it was a sewer lid jumping. Maybe it's normal in other parts of the world, but for me it was like the first time in my life.</p>
<p>I've captured <a href="http://you... | 3,215 |
<p>What determines what pitch an object such as a bell or tuning fork produces when struck? I have heard that the box in the "king's chamber" of the great pyramid at Giza is tuned to 438 Hz. I know that in hand-bell choirs, the bigger the bell, the lower the tone, but I have noticed that size does not seem to be the de... | 3,216 |
<p>From what I understand, an object entering the atmosphere will start to burn up from the tremendous resistance of the atmosphere. Presumably, for asteroids under a certain size, they will burn up completely and never impact the surface of the earth. </p>
<p>Do we have a way of determining the minimum size needed ... | 3,217 |
<p>My Question is how to explicitly move into the "Coulomb gauge" in Yang-Mills theory.</p>
<p>Using the answer provided by QMechanic, one can move into the "temporal gauge" for Yang-Mills fields: <a href="http://physics.stackexchange.com/questions/33133/gauge-fixing-choice-for-the-gauge-field-a-0">Gauge fixing choice... | 3,218 |
<p>I have been studying decoherence in quantum mechanics (not in qft, and don't know how it is described there) and renormalization in QFT and statistical field theory, I found at first a similarity between the two procedures: on one side decoherence tells us to trace over the degrees of freedom we don't monitor, in so... | 3,219 |
<p>I don't understand why when first launched Space X's Dragon capsule had to orbit the Earth many times in order to match up with the <a href="http://en.wikipedia.org/wiki/International_Space_Station">ISS</a>? Was this purely to match it's speed, or to get closer (as in altitude) to the ISS?</p>
<p>In the stages when... | 3,220 |
<p>I've seen questions on how small you can make a tokamak. But I haven't yet seen any "physical" upper limit on the tokamak design.</p>
<p>If you take a wind turbine for example, doubling the linear dimensions will increase the sweep area by a factor 4 but the structural mass with a factor 8, which clearly explains w... | 3,221 |
<p>Can someone write down the <a href="http://en.wikipedia.org/wiki/Boltzmann_equation" rel="nofollow">Boltzmann equation</a>, not neglecting any of the variables of the involved functions and integrals? Specifically, how to concisely capture the "primed" variables in a sensible manner?</p> | 3,222 |
<p>The Pressure of a static spherical object (say star), which has the Schwarzchild metric outside it, satisfies the following differential equation called the <a href="http://en.wikipedia.org/wiki/Tolman%E2%80%93Oppenheimer%E2%80%93Volkoff_equation" rel="nofollow">TOV equation</a>.</p>
<p>$$\frac{\mbox{d}P}{\mbox{d}r... | 3,223 |
<p>I'm trying to understand how Hamiltonian matrices are built for optical applications. In the excerpts below, from the book "Optically polarized atoms: understanding light-atom interaction", what I don't understand is: Why are the $\mu B$ parts not diagonal? If the Hamiltonian is $\vec{\mu} \cdot \vec{B}$, why aren't... | 3,224 |
<p>I know that photons and electrons and such are said to have a wave particle duality, but what does that mean for a photon? When light strikes an object, are many photons emitted, enough to draw infinitely many rays, is only one emitted, or something in between?</p>
<p>In particular, I'm having trouble with thin fil... | 3,225 |
<p>The question is not so much about the theorem, but more about what it means in this context: <a href="http://en.wikipedia.org/wiki/Berry_connection_and_curvature#Applications_in_crystals" rel="nofollow">see this link</a>.</p>
<p>So yes, because of Bloch's theorem the Hamiltonian eigenstates in a crystalline system ... | 3,226 |
<p>Why does it get hotter (feel hotter) in a sauna when one pours water over the hot stones?</p>
<p><a href="http://en.wikipedia.org/wiki/Sauna">Wikipedia says</a> that the water condenses onto the skin, but the actual air humidity is so low that I doubt anything is condensing there. The water (sweat) evaporates from ... | 3,227 |
<p>If I had a flexible tube sealed at both ends and I submerged it in water (held vertical) Would the bottom half of the tube compress and the top half expand? What would the pressure in the tube be? Say its a 2" length of tube with the bottom being in 12" of water</p> | 3,228 |
<p>In the two Phys.SE questions <a href="http://physics.stackexchange.com/q/466">What is the mechanism behind the slowdown of light/photons in a transparent medium?</a> and <a href="http://physics.stackexchange.com/q/7437">Why glass is transparent?</a> transparent media were discussed. But I'd like to clarify one detai... | 3,229 |
<p>Can someone explain in both layman's terms and also technically why when I pulled my glass filled with liquid soda from the freezer, the liquid soda quickly froze?</p>
<p>Doesn't this violate the 2nd law of thermodynamics since heat moved away from the glass with soda and to the ambient?</p>
<p>Thanks</p> | 3,230 |
<p>In CE configuration of NPN transistor, collector emitter junction is reverse biased.
But how is a N-N junction reverse biased?</p> | 3,231 |
<blockquote>
<p>The fact that photons emitted from an electric-dipole active atom
cannot be spatially localized better than to the near-field zone of
the atom is seen as the origin of genuine superluminality.</p>
</blockquote>
<p><a href="http://www.ncbi.nlm.nih.gov/pubmed/11309086" rel="nofollow">http://www.ncb... | 3,232 |
<p>I have a question about the tensor decomposition of $\mathrm{SU(3)}$. According to Georgi (page 142 and 143), a tensor $T^i{}_j$ decomposes as:
\begin{equation}
\mathbf{3} \otimes \mathbf{\bar{3}} = \mathbf{8} \oplus \mathbf{1}
\end{equation}
where the $\mathbf{1}$ represents the trace. However, I do not understand ... | 3,233 |
<p>In a <a href="http://en.wikipedia.org/wiki/Cathode_ray_tube" rel="nofollow">CRT</a>, where do the ejected electrons go after they cause fluorescence on the screen, have they lost most of their energy, or do they actually go through the glass?</p> | 3,234 |
<p>The metric for the BTZ black hole is</p>
<p>$ds^2=-N^2dt^2+N^{-2}dr^2+r^2(N^\phi dt +d\phi)^2$</p>
<p>where $N^2=-M+\frac{r^2}{l^2}+\frac{J^2}{4r^2}$ and $N^\phi=-\frac{J}{2r^2}$.</p>
<p>It is often said that BTZ black hole is asymptotically AdS$_3$, but if I take $r\rightarrow \infty$ limit, then the BTZ metric,... | 3,235 |
<p>I'm trying to simulate the degenerative Anderson model. So depending on an energy difference first orbital and afterwards spin magnetism occurs. First i try to solve an easier ansatz with a limitation of only two orbitals to $l_z = \pm 1$.
In this case i end up with a set of 4 equations:</p>
<p>$\Delta \tan^{-1} (1... | 3,236 |
<p>I have a hemispherical bowl in which I roll a small particle around the edge, starting from the top at point <em>A</em> with a velocity $v_o$. It travels halfway around the sphere and reaches point <em>B</em>, which is a vertical distance <em>h</em> below <em>A</em>, with a velocity $v_f$. Point <em>A</em> is a radi... | 3,237 |
<p>Why can’t information travel faster than the speed of light, if the two endpoints to and from which the information is being sent are moving relatively to each other, as long as the information travels slower than the inverse of the velocity which the planets are moving apart at.</p>
<p>According to the velocity ad... | 3,238 |
<p>I have read explanations of this but haven't really understood. Given a spacetime $(M,g)$ I have read that if I represent the metric in some coordinates $(x,y,z,t)$ as $g(x,y,z,t)$ and then in another coordinate system as $g'(x',y',z',t'),$ that $g'(x,y,z,t)$ (now using the old coordinates) will also solve the Einst... | 3,239 |
<p>While reading Hobsen et al.'s "General Relativity: An Introduction for Physicists", I came across a bit confusing derivation. Multiplying the 4-force and 4-velocity, the following derivation can be made</p>
<p>$
\boldsymbol{u} \cdot \boldsymbol{f} = \boldsymbol{u} \cdot {d\boldsymbol{p} \over d\tau}
= \boldsymbol... | 3,240 |
<p>For example I have a dielectric solid with a small charged ball in it. And I have external electric field $E$. So what force is acting on this ball? </p>
<p>The field in dielectric is $\frac{E}{\epsilon}$, so the force should be $\frac{Eq}{\epsilon}$.</p>
<p>On the other hand. If I remove a small piece of dielectr... | 3,241 |
<p><em>How does water carve rock?</em></p>
<p><em>And more generally, how does a soft material carve a hard material?</em></p>
<p>Obviously it happens, but is it a continual process (every drop of soft water carries away a minute amount of hard material) or is it a stochastic thing (every once in awhile, the force of... | 3,242 |
<p>The Morning Glory roll cloud (pictured below):</p>
<p><img src="http://i.stack.imgur.com/I5c0J.jpg" alt="enter image description here"></p>
<p>from <a href="http://en.wikipedia.org/wiki/Morning_Glory_cloud">this Wikipedia page</a>, which briefly describes the landform and meteorological causes, but states</p>
<bl... | 3,243 |
<p>I'm not sure whether this question is well defined, but I am interested in the volume of the (obviously not linear) subspace of qubits
$$
\left|\Psi\right\rangle = \alpha \left|\uparrow\right\rangle + \beta\, e^{i\varphi} \left|\downarrow\right\rangle$$
where $\varphi \in [0, 2\pi)$ and $\alpha$ and $\beta$ are fixe... | 3,244 |
<p>Consider the path $x^\mu(u)$ in Minkowski space; such that:</p>
<p>$$t = \frac{a}{c} \sinh(u) , \quad x = a \cosh(u) ,\quad y = 0 ,\quad z = 0 $$</p>
<p>where $a$ is a positive constant and $u$ is a parameter</p>
<p>Use equation:
$$ c \nabla \tau = ds = \sqrt{\eta_{\mu\nu} \dot{x}^\mu \dot{x}^\nu} du $$
to fin... | 3,245 |
<p>If the lattice types are categorized according to the point group symmetries, then what is the difference, for example, between sc and bcc structures?</p> | 3,246 |
<p>A few days ago I went for a walk in the evening. We're having winter with a little snow and freezing temperatures. We're in a quiet, shallow valley with a train station about 1km from us. I heard a train coming so I wanted to wait for it to watch it arriving to the station. To my surprise, although I was hearing the... | 3,247 |
<p>What does the term <a href="http://en.wikipedia.org/wiki/Equations_of_motion" rel="nofollow">equation of motion</a> refer to? If I am asked a question of the form 'What is the equation of motion of this object?', what should I write?</p> | 3,248 |
<p>How does e.g. sodium chloride (aq) conduct electricity? By accepting electrons (unlikely since they already have a full outer shell)? But they can't be hopping around themselves, can they? I mean, if I have two poles made of metals inserted into a beaker with this solution, and I try to let a current go through from... | 3,249 |
<p>If in a uniform magnetic field a conducting rod moves on a set of joined friction less rails which are perpendicular to the direction of magnetic field, a potential is developed and current flown this current generates heat, the mechanical energy required to move the rod is converted to heat energy in wires.</p>
<p... | 3,250 |
<p>In the following circuit (that has come up during a BJT DC analysis for what is worth), I'd like to calculate the potential at point B and point E.</p>
<p>My notes say that $V_B = -10 + 0.7 = -9.3V$. Trivial as it may seem, I can't understand it. The only way that I can think of is that the negative pole of the sou... | 3,251 |
<p>What is the maximum ratio in the rate of change in time in reference to object $A$ which is standing still and object $B$ which is moving at the speed of light?</p> | 3,252 |
<p>Did big bang create gravity?
What role gravity is assumed to have played in the formation (starting from the big bang) of large structures of our universe and what other important physical mechanisms and processes probably led to the structure we observe today?</p> | 3,253 |
<p>I'm reading a book (Sun of Suns by Karl Schroeder) that the main location is a planet called Virga, which contains air, water, and floating chunks of rock, and has no or a very small amount of gravity. There is a main 'sun' at the center of the planet, which provides the heat for weather.</p>
<p>Could a 'planet' of... | 3,254 |
<p>I have difficulty understanding exercise 24 in <a href="http://www.physics.leidenuniv.nl/dbpages/em1_test/em1_test.asp" rel="nofollow">this document</a>:</p>
<blockquote>
<p>Two parallel wires I and II that are near each other carry currents i and 3i both in the same direction. Compare the forces that the two wir... | 3,255 |
<p>Why is the sign convention used in the derivation of the lens formula and yet used <em>again</em> when it is applied in numerical problems? Won't the whole idea of sign convention be eliminated if it is used twice?</p> | 3,256 |
<p>I am trying of resolve this exercise: Show that if $|\psi \rangle$ is an entangled state of two Qbits, then the application of a unitary operator of the form $U_1 \otimes U_2$ necessarily generates an entangled state.</p>
<p>Suppose that $(U_1 \otimes U_2)|\psi\rangle$ is not entangled then it must have the form </... | 3,257 |
<p>Would it violate any known laws of physics to construct a universe containing no mass, only energy?</p> | 3,258 |
<p>We've been discussing radioactive decay at school, and I grasped everything except for $\beta +$ decay. When I googled radioactive decay, I immediately found out they dumbed down radioactive decay for us, which is probably why they didn't care to explain what they did, they just showed some calculations. We have nev... | 3,259 |
<p>How quarks electric charge directly have been measured when quarks never directly observed in isolation? (Due to a phenomenon known as <a href="http://en.wikipedia.com/wiki/color_confinement" rel="nofollow">color confinement</a>.)</p> | 3,260 |
<p>I'm studying an interacting bose-einstein condensate using the energy functional proposed in this paper</p>
<p>K. Huang, C.N. Yang, Phys. Rev. <strong>105</strong></p>
<p>$$
E\left[\phi\right] = \int d^3\vec{r} \left\{-\frac{\hbar^2}{2m}|\nabla\phi|^2 + \phi^*\left[U(x,y,z)+\frac{g}{4}\left| \phi \right|^2 + \frac... | 3,261 |
<p>We can get monopole $1/r$, dipole $1/r^2$, quadrupole $1/r^3$ and octupole $1/r^4$ potential falloff by placing opposite point charges at the corners of a point, line, square and cube, respectively. My book cryptically says "and so on", how does one get a $1/r^5$ and higher potential falloff with a finite number of ... | 3,262 |
<p><img src="http://i.imgur.com/VG7izgO.jpg" alt=""></p>
<p>The problem is to find the current on the capacitor. $I''$ should be correct, but I don't know how to construct the formula for $I'$.
I managed to get the value for $I_c$ using Thevenin and Norton equivalents, and they're the same, so that should be correct.<... | 3,263 |
<p><img src="http://i.stack.imgur.com/Oo2mp.png" alt="enter image description here"></p>
<p>Can anyone point me to a derivation of this expression? $n_s$ is the number of bosons in a state.</p> | 3,264 |
<p>Normally we write a Bloch Hamiltonian $H(\mathbf{k})$ for the bulk and determine the spectrum which gives us various bands i.e we basically obtain $E=E(\mathbf{k})$ for the bulk only.</p>
<p>Also in the real space if we solve a tight-binding model, we get the energy eigenvalues which have both edge modes and the bu... | 3,265 |
<p>I wish to pursue a career that somehow involves programming, electronics, and physics. What are such careers? </p>
<p>Also, I have heard of some 1 year post-graduate diplomas/courses for specialization in Physics. Which universities/ colleges offer them?</p> | 3,266 |
<p>A Lagrangian is given by,
$$L= \left(\frac{\pi}{2}\right)^2 R^d \left[\frac{1}{2}\dot A^2 - V(A_{max})\right]$$
$$E=\left(\frac{\pi}{2}\right)^2R^d V(A_{max}) $$
where V (A) now includes nonlinear terms and E is the energy which is found by taking the appropriate Legendre transform of the Lagrangian and evaluating i... | 3,267 |
<p>How can one show that the action of dilaton in the String Background Fields must be of the form: $ S_\Phi = \frac1{4\pi} \int d^2 \sigma \sqrt{h} R(h) \Phi(X) $?</p>
<p>Thank you.</p> | 3,268 |
<p>I'm a physics student and I'm attending an introductory course of particle physics. My professor stated that, in center of mass frame, the $\nu_\mu e^- \to \nu_\mu e^-$ elastic scattering has an isotropic angular distribution, while the $\bar{\nu}_\mu e^- \to \bar{\nu}_\mu e^-$ scattering has not.</p>
<p>I can't fi... | 3,269 |
<p>I have a Hamiltonian:</p>
<p>$$H=\dot qp - L = \frac 1 2 m\dot q^2+kq^2\frac 1 2 - aq$$</p>
<p>In a system with one coordinate $q$ (where $L$ is the Lagrangian). One of the Hamilton equations is:</p>
<p>$$\dot q =-\frac {\partial H} {\partial p}$$</p>
<p>But when I try to derive $H$ with respect to $p$, I get ve... | 3,270 |
<p>I have a doubt over the Kelvin and Planck's statement of thermodynamics' second law, in particular applied to a cycle. Let's take a Carnot cycle as an example, and let's call the first two transformations (the isotherm and the adiabatic) done. Now, isn't it obvious that the machine has to give up heat to go back to ... | 3,271 |
<p>This question is actually related to my earlier question ("<a href="http://astronomy.stackexchange.com/questions/948/what-exactly-is-the-definition-of-motion-and-its-relation-to-machs-conjecture">what is motion</a>").
The fact that objects move a lot in the universe and that the universe is expanding, can imply tha... | 3,272 |
<p>If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a <em>spherically symmetric</em> gravitational field, or a <em>dipole</em> field? </p> | 765 |
<p>I'm studding special relativity and there is a famous example where there is a moving train witch has a light source on its floor and a mirror attached to its roof.</p>
<p>an observer in the train sees a pulse of light leaves the source vertically and goes to the mirror and comes back to the source. </p>
<p>the ex... | 3,273 |
<blockquote>
<p>A car is driving down a mountain ($v=90 km/h=25 m/s$, when the driver realizes that brakes aren't working. He try to lose velocity going up an inclined ($20°$) plane, with a friction coefficient of $k=0.60$. How many meters will it take to halt?</p>
</blockquote>
<p>I've tried as following ($s$ is th... | 3,274 |
<p>I'm trying to understand why when you have a muon decay event, the energy of the electron peaks near the maximum kinematically allowed value. Is there an intuitive explanation for why this is the case or is it one of those things where the explanation is that it's just how the math works out?</p> | 3,275 |
<p>This has been a really great confusion for me now ....</p>
<p>Many places i have read in books that when a potential difference is applied across the ends of a wire a constant electric field is generated inside it which drives the current through it...</p>
<p>My question is how is this electric field generated ?? ... | 78 |
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