# All Questions

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### Interface condition for heat exchange

I would like to compute the heat distribution of a piece of metal with some surrounding material. The heat is assumed to propagate by diffusion, so inside the metal piece and also on the outside, the ...
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### Is the diffusion coefficient for a macromolecule sensitive to mass?

Suppose I have two neutrally-buoyant macromolecules diffusing in water. They have the same radius of gyration (i.e. same root-mean-square distance from their center of mass), but one of them is ...
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### Fraunhofer diffraction simulation for a hexagonal aperture, what are the typical units?

Kostya answered a question that was asking what the diffraction pattern looks like for a hexagonal aperture in front of a lens. He lists an equation which was derived using a Heaviside function to ...
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### Is it possible to measure outside magnetic field from inside a car?

I know that I can detect a vehicle by using magnetic field. I can see obviously difference in signal when a vehicle pass. But I don't see that when measuring in a car. Is it possible to measure ...
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### Why is the energy spectrum continuous for a plane wave when it has energy less than the potential barrier?

Please explain it in the context of this task: we have a potential barrier that looks like $\prod$, with $E<U$. There are 3 regions: 1) no field 2) barrier 3) no field Solution could be ...
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### Change of variables, Fermi Integral

This is a really basic question, but I'm kind of confused. I have this integral $$\int_{0}^{\infty}\frac{p^{2}dp}{e^{\alpha+\beta p^{2}/2m}+1}$$ where ...
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### Why does the index of refraction change the direction of light?

I've been studying in optics the macroscopic maxwell's equations, and how electromagnetic fields propagate through different mediums. Over there, the index of refraction appears, as a complex function ...
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Say I have a multicomponent field $X_a(x,t)$ such that I know it Fourier modes satisfy the following equation of motion, $(\delta_{ab} \partial_t + \Omega_{ab}(t))X_b(k,t) = e^t \int \frac{d^3p ... 0answers 83 views ### CP-symmetry and Ward identities and finite temperature I have a few questions about Ward-identities which I summarize here. For each I am very greateful for answers and references to literature. Wikipedia states about Ward-identities: The ... 1answer 317 views ### Contracting the Riemann tensor issues, p540 hobson I am stuck trying to work through something on p540 in Hobson (General Relativity: An Introduction for Physicists), one is supposed to use the variation of the full Riemann tensor and then contract it ... 1answer 812 views ### What is the difference between the properties of Electron spin and Photon polarization/helicity? What is the difference between a photon's polarization/helicity and an electrons spin half? I know that the photon is spin 1 but isn't its polarization analogous to spin half? This question stems ... 3answers 501 views ### Special Relativity - speed of light question Just a basic question: I know that if you are traveling at$x$speed the time will pass for you slower than to an observer that is relatively stopped. That's all just because a photon released at the ... 0answers 49 views ### Do all the 4 forces of nature act at the same speed? [duplicate] It is believed that gravity, the weakest of the four forces propagates at the speed of light, cf. e.g. this Phys.SE post. One would expect (perhaps erroneously) that the other, stronger, forces acted ... 2answers 366 views ### What is wrong with these ways of determining the mean occupation number? Could anyone point out what went wrong in this argument? Setup: We have a system with 2 energy levels say with energies$0,e$respectively. We consider the grand canonical ensemble for the system ... 3answers 1k views ### Pascal's law: pressure of fluid at different locations I know that's stupid question, but I'm really confused what my teachers says, so I need to check that theory. Here are just two ordinary connected containers, which are full of water. On grounds ... 1answer 232 views ### How do you actually define an orbit? How do you actually define an orbit? I believe, Newtonian Mechanics describes an orbit as one object in free fall around another where projectile paths become elliptical. I think, Einstein describes ... 3answers 15k views ### Is there a compound denser than the densest element? I'm musing about how to give students an intuitive feeling about density by letting them lift a same sized volume of different materials, e.g. 1 liter of water, a 10x10x10 cm cube of iron, lead etc. ... 6answers 763 views ### Why is momentum conserved (or rather what makes an object carry on moving infinitely)? I know this is an incredibly simple question, but I am trying to find a very simple explanation to this other than the simple logic that energy is conserved when two items impact and bounce off each ... 5answers 6k views ### Does everything with mass or energy have a gravitational pull? As small as it may be, does every 'thing' have a gravitational pull? That is, something with mass or energy. No matter how obsolete or negligible it may be, is it there? If so, how is it calculated? ... 1answer 129 views ### How to determine predicted CP violation for a given SUSY point? I'm currently studying at the spectra of some supersymmetric models, and would like to know whether the parameter points I'm looking at are ruled out due to excessive CP violation. I am using SPheno, ... 0answers 119 views ### exercise books for Feynman diagrams [duplicate] I know QFT at graduate level but I'll like to master the skill of working with Feynman diagrams. I'm looking for a book of solved exercises on this topic. Specifically, I'm looking for the kind of ... 1answer 301 views ### Pauli matrices and the Levi-Civita symbol This is just a quick question. I would figure this out myself if I wouldn't have an exam about this tomorrow. I am working on the non-relativistic approximation of the Dirac equation for an electron ... 1answer 648 views ### Shear Flow corresponding to Eccentric Shear Force of a Closed-section Beam (Structural Analysis - Mechanics) Been stumped with this question for way too long... its a beam with a thin-walled rectangular cross-section, and a shear force is acting at a distance from the shear center. I know my decomposition of ... 2answers 743 views ### Independence of frequency in sound waves? Why does the frequency of sound wave depend only on the source? Why is the frequency and not any other "quality" independent of everything but the source? And that said, why is velocity and ... 0answers 280 views ### linear response for a simple harmonic oscillator Really sorry for this simple question, but I think it will be useful/interesting in general. Consider a quantum simple harmonic oscillator. Add a perturbation$H_I = -\lambda \hat{x}$Calculate ... 0answers 125 views ### Feynman Diagram for Interaction of Deuteron with Oxygen-16 I have to draw a Feynman diagram for the interaction of a deuteron with Oxygen-16. The interaction is as follows:$ d + {}^{16}O \rightarrow p + {}^{17}O . $I am generally ok with Feynman ... 1answer 277 views ### Work done by gravity on Water Now according to me we would see change in potential energy of system and equate it to the work done by gravity. But when we see this the first column lowers by$H/2$and right one rises by$H/2$... 2answers 814 views ### How to visualize a 3D scalar function? [closed] In physics, we regularly deal with observables, which are functions$f(x,y,z)$of the position vector$\vec r = (x,y,z)$. How, do you plot such a function? For functions$y=f(x)$of 1 parameter, we ... 1answer 96 views ### Can someone explain this intuitively please? In a static situation we defined voltage as energy/unit charge, or j/c. As the distance between the charged particles increased, the voltage decreased. Now why do we not apply this in a simple DC ... 1answer 91 views ### Is there a material that changes local conductivity I hope this is the right forum to ask this question. Is there a material (preferably thin, like a membrane) that changes its local conductivity (by that I mean the permeability for an electric field; ... 1answer 231 views ### Exploiting the Heisenberg Uncertainty Principle as a means to communicate It seems as though I've come across a rather unusual conclusion that could either simply be a misinterpretation or a contradictory discovery. I seem to have found a way to utilize the Heisenberg ... 2answers 942 views ### A question about Haldane's conjecture Haldane's conjecture states that the integer spin antiferromagnetic Heisenberg chains have a gap in the excitation spectrum. However, the dispersion relation of the antiferromagnetic spin wave is ... 2answers 425 views ### Relativity of Simultaneity Relativity of Simultaneity seems to be about OBSERVING two events simultaneously (please correct me if I am wrong). However, as long as the two events are separated by a distance (any distance) then ... 1answer 329 views ### Phase transition water The water-gas phase transition is said to be similar to the ferromagnetic-paramagnetic phase transition (same set of critical exponents = same universality class). In the former case the order ... 8answers 4k views ### Is there a symbol for “unitless”? I'm making a table where columns are labelled with the property and the units it's measured in: Length (m) |||| Force (N) |||| Safety Factor (unitless) ||| etc... I'd like not to write "unitless" ... 0answers 25 views ### how to explain the upright force for the plane? [duplicate] I remember in the high school physics, my teacher told us that the design of the plane wing is because we want the air above the wing flowing faster than the air flowing below so the pressure above ... 4answers 2k views ### Does the observer or the camera collapse the wave function in the double slit experiment? Ok so if we setup a camera before the slit we will find a single photon and will follow through accordingly, likewise by having a camera setup after the slit, we can retroactivly collapse the wave ... 1answer 583 views ### Simulations of Planetary Motions I wrote a spreadsheet that simulates the trajectory of 3 planets in 2D space. The method is simple: for each moment in space, calculate the force felt and velocity of each planet, then for the next ... 3answers 11k views ### how dense fluid affect the buoyancy force? I read a story regarding the Archimedes' principle in a magazine of popular science and I am thinking of the following question: how does the density of the fluid change the buoyancy force for the ... 2answers 198 views ### About Heisenberg uncertainty principle [duplicate] What would happen if someone invented a way to measure both position and momentum precisely? If it is impossible why? 2answers 1k views ### If the universe is 3D, how is space-time like a “fabric”? [duplicate] I have been taught that space-time should be viewed as a fabric and that objects with a large gravitational influence indent that fabric. My question is, if the singularity of a black-hole punctures ... 1answer 133 views ### Notational techniques for dealing with creation operators on Fock space This question is trying to see if anyone has some simple notation (or tricks) for dealing with operators acting on coherent states in a Fock space. I use bosons for concreteness; what I'm interested ... 2answers 2k views ### Elastic collision and spring Bodies$A$and$B$are moving in the same direction in a straight line with a constant velocities on a frictionless surface. The mass and the velocity of$A$are$2 \text{kg}$and$10 \text{m/s}$. ... 1answer 451 views ### Electric field at a point$P$Problem Charge is distributed uniformly over a large square of side$l$, as shown in the figure. The charge per unit area ($C/m^2$) is$\sigma$. Determine the electric field at a point$P$a ... 2answers 422 views ### Are there “gaps” in light, or will it hit everywhere? Not sure how to word my question. Picture a light source in vacuum, so nothing disturbs the light (or similar conditions), 2d. If I move very, very far away, will it happen that some of the light ... 1answer 696 views ### A water balloon in vacuum: does it boil? If I put water in a vacuum it will boil. But what if I put this water inside a balloon ? I searched for answers and fount this video: http://www.youtube.com/watch?v=9q8F3ClUuV0 It seams that the ... 2answers 413 views ### primary constraints for constrained Hamiltonian systems I would be most thankful if you could help me clarify the setting of primary constraints for constrained Hamiltonian systems. I am reading "Classical and quantum dynamics of constrained Hamiltonian ... 1answer 295 views ### Divergent sum in lightcone quantization of bosonic string theory I had the following question regarding lightcone quantization of bosonic strings - The normal ordering requirement of quantization gives us this infinite sum$\sum_{n=1}^\infty n\$. This is regularized ...
I would want to understand why the density operator in second quantization takes the form: $$\rho_\sigma(\mathbf{r})=\Psi_\sigma^\dagger(\mathbf{r})\Psi_\sigma(\mathbf{r})?$$ Is this a definition or ...