# All Questions

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### Hysteresis and dissipation

Hysteretic phenomena are often linked to dissipation. When there is an hysteresis loop, the dissipated energy can usually be computed as the area of the cycle. For example, in ferromagnetic ...
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149 views

### Relationship between slit size and wavelength in diffraction

Almost in every book on physics we can find a statement like "diffraction gets stronger when the size of the slit is comparable to the wavelength". Let's say we have a wall in a bathtub with a slit in ...
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77 views

### Time-orded operator in Srednicki

On page 51 Srednicki states, "Note that the operators are in time order...we can insert $T$ without changing anything". This I agree with. But then on the next paragraph he states "The time order ...
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76 views

### In a positively biaed PN junction, where do the injection carriers come from?

I am not quite understand i-v character of PN-junction diode. Here is the model in textbook. The PN junction diode can be divided into three regions. They are One depletion region near the PN ...
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142 views

### Interpretation of rank 2 spinors

While inspecting the $(\frac{1}{2},\frac{1}{2})$ representation of the Lorentz group and defining a right-handed spinor with upper dotted index and a left-handed spinor with lower undotted index and ...
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183 views

### Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
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118 views

### Negative sign in the Dirac term from the SUSY Kahler potential

I want to calculate the Dirac term from the canonical Kahler potential, $$K = \Phi ^\ast \Phi \tag{1}$$ but I'm coming across a pesky negative sign in the final result. I ...
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89 views

### Pipe in a flowing river problem

I'm working on a certain problem in fluid mechanics, which isn't really my strongest area. The problem is as follows: Curved pipe is partially submerged in a flowing river so that one end is pointing ...
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106 views

### How strong is electron degeneracy pressure?

I'm trying to get some specific numbers for electron degeneracy that I can understand, using a concrete example. Take for example this portion of carbon crystal: Exactly how much energy would be ...
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69 views

### Vacuum polarization of dark matter candidates

Can a dark matter particle (axions, for instance, but the question applies to any valid candidate for dark matter) induce QED vacuum polarization effects even if being electrically neutral and with no ...
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79 views

### Motion in a Paul trap: $2n$th harmonic with larger amplitude than $n$th harmonic

Using a Paul trap, we captured the motion of a light charged particle (based on a rotating potential applied by AC current). Our rotational frequency was 50 Hz, and so when used FFT on the data, we ...
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262 views

### Atwood machine problem

Sorry for the bad drawing, but I hope that this will help you get a hold of the problem. Consider an Atwood Machine with a total of two blocks, a mass less pulley, ideal string. One block rests on ...
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88 views

### Sliding ruler on table top

I've got a standard foot-long flat plastic ruler (about an inch wide) and a desk with a smooth formica-like surface. When idly passing time reading on the computer, I will pick up one side of the ...
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240 views

### Measuring voltage drop from induced current

I'm having trouble connecting voltage drops and induced current. Imagine you have a triangular loop make up off three resistors. You place this loop in a constantly changing magnetic field. This ...
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170 views

### Can universal continuity be experimentally falsified?

It is an unresolved question whether the universe is discrete or continuous in its intricate quantum level structure. See for example: Is the universe finite and discrete? How could spacetime become ...
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102 views

### What does it mean that Bruno can hit $6\,\rm{kN}$?

Is it the momentum? But momentum is $\rm{Ns}$, not $\rm{N}$. http://www.skepticforum.com/viewtopic.php?f=32&t=14624 I'm not sure that 3,430 N is necessarily beyond human capability. Boxer ...
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125 views

### Does the logistic map have an attractor for a particular value of the parameter?

Background: Currently I am studying a course on non-linear dynamics. We have been studying about attractors only intuitively, so I do not have a definition for an attractor. Let me give you a couple ...
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55 views

### Force field for Calcium ions, potassium ions and water in molecular dynamics

I want to adapt my existing MD simulation so that it can handle these three species. For the interactions of the ions I thought that I would use the coulomb potential, but I don't know what sort of ...
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246 views

### Surface gravity of Kerr black hole

I'm going through Kerr metric, and following the 'Relativist's toolkit' derivation of the surface gravity, I've come to a part that I don't understand. Firstly, the metric is given by ...
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398 views

### Fresnel Diffraction at a circular aperture

In Fresnel Diffraction at a circular aperture the central image according to 'Optics' by brij lal and subramanyam will be bright if odd number of full half-period zones can be ...
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154 views

### Baryon masses in Wetterich's new cosmology

Christoph Wetterich has put out a paper in which the universe isn't always expanding; it can be static or expanding just some of the time or even shrinking. And then there is an interaction which ...
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### The relation between spectral function and band structure

I am confused by the wavevector in spectral function A(k,w). How to understand this k for a periodic structure? And how is it related to the k (in first Brillouin Zone) we use in the band structure? ...
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93 views

### Can convection cells evolve in stably stratified fluid?

Assume stably stratified fluid but not in equilibrium, e.g. with non-constant temperature gradient for example. Can convection cells be present? Typical example of convection cells is Rayleigh–Bénard ...
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108 views

### Trying to understand mixed states

I took a basic quantum chemistry course (McQuarrie's "Quantum Chemistry"), but it never dealt with mixed states -- only pure states (or if it did, we never got to it in class). So I'm trying to ...
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136 views

### First order coherence through double slit

The state $$|\Psi \rangle = |0\rangle + \sum_j \int d\omega f_j(\omega)\hat{a}^\dagger_j (\omega) |0\rangle$$ is coming from a far field and incident on a double slit setup. Here j is the index of ...
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349 views

### Magnetostatic energy density — derivation without introducing induchance?

I was looking for a derivation of the expression for the energy density at any point in a static magnetic field. I do know that it is $\dfrac {1}{2 \mu_0}\left|\vec{B}\right|^2$ -- I was just ...
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450 views

### Two photons transition

if an atom in its ground state is coupled to an electromagnetic field it can absorb a photon if the EM field contains one with the right frequency. These transitions depends on $⟨f|H_i|i⟩$ (from ...
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212 views

### Tangent bundles and $\mathbb{C}P^n$ and $\mathbb{C}^n$

As discussed here the complex projective space $\mathbb{C}P^n$ is the set of all lines on $\mathbb{C}^n$ passing through the origin. It would seem natural to assume that any $\mathbb{C}P^n$ can be ...
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105 views

### Amount of energy to separate Gases - relationship to concentration

I want to understand the efficiencies of separating mixed gases, and for that I want to understand the thermodynamic limit case. Looking at the wikipedia page for entropy of mixing, I find the ...
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228 views

### Semiconductor problem: how much Boron is required to establish a certain amount of charge carrier density

The problem description is as follows: Boron is used to dope 1 kg of germanium (Ge). How much boron (B) is required to establish a charge carrier density of 3.091 x 10^17 / cm^3. One mole of ...
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129 views

### Secondary constraints leads to the value of lagrange multiplier

From Lagrangian I got two primary constraint $\phi_i$ and $\phi$. And my Hamiltonian in presence of the constraints becomes- $$H_p=p\dot q-L+\lambda_i\phi_i+\lambda\phi$$ here the $\lambda_i$ and ...
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229 views

### What is crystal field anisotropy or effect ? It forces the magnetic moment to point in particular local direction..

Can you give a basic explanation of what is crystal field anisotropy ? What is the reason to arise ? In spin ice it forces the dipoles to point in the local 111 direction. For partially filled rare ...
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468 views

### warp drive with gravitational waves in the nonlinear regime

gravitational waves are strictly transversal (in the linear regime at least), also their amplitudes are tiny even for cosmic scale events like supernovas or binary black holes (at least far away, ...
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99 views

### Are there microscopic theories, which work, but which wrongly predict macroscopic behaviour?

Motivated by this question (and the P. W. Anderson article linked in that question, which I came across here somewhere today and just read) I wonder about something, which is somewhat bordering an ...
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504 views

### Gravitational attraction of triangles

Suppose I have two triangles relatively close together (so they probably shouldn't really be treated as point masses). I want to calculate the gravitational force (and potentially torque?) generated ...
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284 views

### Does a negative line tension in a 3-fluid immiscible interface make sense?

This question is inspired by this question/answer pair: Is this formula for the energy of a configuration of 3 fluids physically reasonable? Consider three immiscible fluids forming contact surfaces, ...
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177 views

### Feynman Diagram for Bragg Equation

Did Feynman ever derive the Bragg equation as a particle representation using Feynman diagrams? And where is it available? I spoke to Freeman Dyson and he couldn't recall.
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99 views

### What ring weave disposition should be the most resistant against stabbing and/or how to determine it

In a website that I am studying so I can build a Chain Mail, I have found a page featuring a lot of different Ring Weaves to build them. I want to determine which one is the best to provide ...
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256 views

### Can we have non continuous models of reality? Why don't we have them?

This question is about Godel's theorem, continuity of reality and the Luvenheim-Skolem theorem. I know that all leading physical theories assume reality is continuous. These are my questions: 1) Is ...
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270 views

### Are black hole states completely mixed?

A completely mixed state is a statistical mixture with no interference terms, and (QMD, McMahon, pg 229): $$\rho = \dfrac{1}{n}I$$ $$Tr(\rho^2) = \dfrac{1}{n}$$ Are black hole quantum states ...
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43 views

### Very short decay: A bare quark?

I keep reading the same phrase about the very short life time of the top quark: Because the t-quark decays on a shorter than the characteristic QCD interaction-time it cannot hadronize. ...
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43 views

### Incompressiblity and speed of sound

I have read that usually if the speed of a fluid is much lower than the speed of sound (in that medium) then it can be treated as an incompressible fluid. Where does this condition come from? Is it ...
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23 views

### Does ordinary refraction exert a couple on the refracting medium?

Refraction gives rise to a momentum change orthogonal to the propagation direction. This must result in an equal and opposite change to the medium at the boundary. Entry and exit cancel, and the ...
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71 views

### Thermodynamics, chaperones : How to model polymer fragmentation

Living polymers are well described by equilibrium statistical physics. Now I would like to consider a case were living polymers undergo fragmentation due to chaperones. I can think of a kinetic ...

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