1
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1answer
71 views

Why must the final state be stationary?

I faced the following sentences: We consider a gravitational collapse taking place in this spacetime. The singularity theorems assure us that a singularity will form. The assumption that the ...
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3answers
386 views

Reflections in Rearview Mirror

Why is it that in the reflection in the rear view mirror of your vehicle in the car behind you The driver is on the drivers side and the passenger is on the passenger side and when they raise thier ...
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2answers
90 views

Classical Mechanics - Allowed systems

I've edited my question to clear any possible confusing parts: This an exercise from the book "The Theoretical minimum", I'm paraphrasing. In classical mechanics, dynamical laws must be reversible ...
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1answer
684 views

Can a long salt bath increase water retention?

So it's Thanksgiving here in the states and an odd combination of things are on my mind. In the past day, I've Brined a turkey whole, skin on Taken a long epsom salt bath (Same thing, right? What ...
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1answer
96 views

Faradays law for a falling magnet

If experimentally it can be proven that the velocity of a falling magnet through a coil is proportional to the emf induced, has it also been proven that dB/dt is proportional to the emf induced and ...
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1answer
77 views

What is the true electric potential at the terminals of a battery?

I recently studied Laplace's equation and numerical methods for solving it, and so I'm trying to simulate an electron gun. (assuming cylindrical symmetry on the Wehnelt, anode, and beam.) The setup ...
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1answer
42 views

What are the criteria to have rutherford backscattering?

Assuming there is an incident beam(i.e. $p$ or $\alpha$) and a target. How can I be sure if a rutherford backscattering will take place? I know that for high $Z$ it more likely to happen as well as ...
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1answer
71 views

Why does a particle fall in a straight line?

In Lagrangian Mechanics we choose the path of least action. Given a uniform gravitational field, and a particle of finite mass; and fixing two points the start & end-point we consider all paths ...
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1answer
35 views

Molecular formula of a compund [closed]

I have no idea on how to get information out of this one. A compound contains only hydrogen and nitrogen. Nitrogen makes up for 87.4% of the mass. A sample of this have a density of 0.977 g/L at ...
1
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1answer
247 views

Spinors and Probabilities of Electron-Positron Pair

Question: An electron and positron are moving in opposite directions, and are in the spin singlet state. Two Stern-Gerlach machines are orientated in some ...
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2answers
57 views

Temperature-induced wavelength shift of optical coatings?

Optical coatings designed for reflection or anti-reflection are made of many thin layers which will expand when heated. What will the effect be on the wavelengths the coating will reflect when the ...
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2answers
132 views

How to derive inverse Fourier transform for periodic functions (in crystal lattice)?

I would like to derive the following two well-known formulas that work for crystal lattice [1]: $$ F[f(\mathbf{x})] \equiv \tilde f(\mathbf{G}) = {1\over\Omega_\mathrm{cell}} ...
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1answer
4k views

Outdoor Active Noise Canceling Speaker

I would like to understand how to create an active noise canceling system to reduce the noise coming from a street road bump. The advantage is that the area the noise is coming from is very small.
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1answer
86 views

Lorenz gauge in the Gupta-Bleuler Method

Greiner in his book Field Quantization page 180 & 181 wrote: As shown in (7.24) the Lorenz condition cannot be enforced as an operator identity. Instead we will use it as a condition for the ...
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1answer
491 views

Tension on either side of a massive pulley?

When a pulley has mass, why is tension on both the sides different? Why don't we consider rotation of pulley when it is massless?
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1answer
236 views

Is temperature-induced change in the volume of balloons dependent on gas?

Which balloon will have higher relative change in volume, helium balloon immersed in liquid nitrogen or air balloon immersed in liquid nitrogen? Since volume is directly proportional to temperature ...
1
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1answer
202 views

Determining spectra of edge states numerically

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. Also in the ...
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2answers
232 views

Why do orbital speeds decrease further away from the focus?

Why do orbital speeds decrease further away from the focus? A simple question, but I want to make sure I am understanding this correctly: Is it ONLY a function of the gravity well? As in, the ...
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1answer
371 views

how does the mirror equation works and what lead to using of sign convention?

EVERYTHING HERE IS FOR CONCAVE MIRROR Everywhere I see the derivation of the mirror equation is given by placing an object before the focus and then proving similarity of the triangles to get to the ...
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1answer
176 views

Difference between primitive unit cell and the associated basis?

As I understand it, the basis is the group of atoms whilst the primitive unit cell is the unit space that fits the total space without any gaps, and only containing one lattice point? How do the two ...
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2answers
172 views

Bloch wave function orthonormality?

there is this text book that is giving me a hard time for a while now: It shows that Bloch wave functions can be written as $$\Psi_{n\vec{k}}\left(\vec{r}\right) = \frac{1}{\sqrt{V}}e^{i\vec k \vec ...
1
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1answer
103 views

Mass is rigidity?

In General Relativity, a totally rigid body cannot be accelerated. It will behave like something of infinite mass. Similarly a body of two separated particles which connected to each other with a ...
1
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1answer
150 views

Combining relativistic velocities in the same direction using Lorentz transformation matrices

It is known that when combining the Lorentz Transforms of two frames with velocities $v_{1}$ and $v_{2}$ in the same direction it is equivalent of computing a Lorentz Transform of a single frame with ...
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1answer
60 views

Allowed interactions for the neutral weak vector boson

Just wanted to double check a couple of things for my own sanity! I am looking at scattering amplitudes for 2 partons going to two partons with the emission of a Z boson (eventually decaying to e+ ...
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2answers
105 views

Absorption of radiation due to temperature

I was wondering if the temperature of an object affects the amount of radiation it absorbs. For example, if I have a box that is hotter, will it absorb more energy as compared to the same cooler box? ...
1
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1answer
119 views

Changing effective mass to lower frequency

I'm trying to scale a physical object up in size, either double or quadruple. It's a circular acrylic plate, with multiple weights attached around it's edge, to lower the resonant frequency to a ...
1
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1answer
213 views

Wien's Displacement Law for real bodies

It is known that for perfect blackbodies, $$\lambda T= c$$ where $\lambda= \text{peak wavelength}$ $T= \text{Absolute temperature}$ $c= \text{Wien's constant}$ ...
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1answer
131 views

Experiment regarding myopic correction by a manipulation of fingers?

Here is a small experiment my tutor once told us for just amusement. It works for myopic people at least, and can be a good check to see if you have myopia. With your naked eye, ("remove the ...
1
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1answer
76 views

Period of Interference Pattern on a Substrate

Can anybody explain to me where this equation came from? It's for two point sources at the two listed points, and it's calculating the period of the wave on the substrate. It seems to be ...
1
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1answer
1k views

Ground State Wavefunction of Two Particles in a Harmonic Oscillator Potential

Question: Two identical, non-interacting spin-$1/2$ particles are in a 1D Harmonic Oscillator Potential. Their Hamiltonian is given by ...
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1answer
107 views

Equation for relativistic electron and two-component spinor

Recently I heard that there is some "alternate" equation for the Dirac one. It can be introduced if we refuse some properties of the theory describes the electron, which Dirac used in his original ...
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2answers
86 views

What happens to the physical properties of electrons after diffraction?

Particle Wave duality shows us that waves and particles are the same thing. Therefore electrons can be viewed as both particles and waves. The wave properties of electrons can be seen in the double ...
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2answers
80 views

What is the effect of phase transition on the thermodynamic state variables of a material?

When a material undergoes a phase transition, it releases an amount of heat (under a specific temperature). So the effect of the phase transition on entropy would be equal to: \begin{align} ...
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1answer
203 views

A few simple questions about Grassmann numbers: commutation relations and derivatives

I'm trying to learn about Grassmann numbers from the book "Condensed Matter Field Theory" by Altland and Simons, but I am currently encountering some difficulties. I have several smaller questions ...
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1answer
841 views

Is thermal/heat energy the same thing as internal energy?

I learned that internal energy of an object is the sum of the kinetic energy and potential energy of the molecules of the object. Is this energy the same thing as the heat energy the object contains?
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1answer
234 views

Partition function for classical particle and quantum particle are the same?

Permutation for classical particle $$\Omega=\frac{N!}{\Pi n_i!}$$ By using Lagrange method of undetermined multiplier, we get $$n_i=Ae^{\frac{-E}{kT}}$$ Probability, $$p=\frac{n_i} {Z}$$ where we ...
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1answer
134 views

What happens to chiral Majorana edge fermions near quantum phase transition in p+ip superconductors?

In the weakly-coupled BCS regime two-dimensional chiral (p+ip) spinless superconductors and superfluids posses a chiral gapless fermionic Majorana state localized near the boundary. This gapless edge ...
1
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1answer
122 views

Amplitudes in renormalized perturbation theory

This question arose while reading Peskin and Schroeder, specifically, it arose in regards to the sum of diagrams above their Eq. (10.20) on pg. 326. The context is $\phi ^4$ theory and they are using ...
1
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1answer
42 views

Pre-gauge-fixed superspace action of the RNS superstring

When writing down the the action of the RNS superstring in superspace, all of the sources I have checked (BBS, GSW, Polchinski) seem to just write down the action in conformal gauge, that is $$ ...
1
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1answer
90 views

Wave vector $\vec{k}$ vs position vector $\vec{x}$

My question is about the $k$-vectors in first Brillouin zone. If I am not misunderstood, the relation k = 2π/(Na) tells that when k goes to zero, we are very very far away from the reference atom and ...
1
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1answer
81 views

Fluid Virtual Mass

In this paper of Lighthill, the author studies the motion of a fish in a constant flow field $U$, modelling the movement of the fish as a deforming, rotationally symmetric, rigid surface in an ...
1
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2answers
403 views

Neither Biot-savart nor Ampere Law can solve this problem?

I'm confused about the use of the Ampere's Law and the Biot-Savart Law due the inconvenience of each law. I want to calculate the magnetic field due to current carrying a circular loop over itself, ...
1
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1answer
73 views

Initialize a traveling wave in a 1D gas?

I am trying to initialize a traveling wave for a 1d simulation as one can see from the attached figure. Such that it will be traveling to the right. However, I cannot initialize the right velocity ...
1
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1answer
68 views

What are the fastest electron orbitals

I read that the mercury has a low melting point because its outer shell electrons are pulled in close by its nucleus (large nucleus, sparse outer shell) and because its outer shell electrons have ...
1
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3answers
229 views

Electric flux for a rectangular surface? [closed]

I have the following homework problem: A line of charge $\lambda$ is located on the z-axis. Determine the electric flux for a rectangular surface with corners at coordinates: $(0, R, 0)$, ...
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2answers
109 views

What is the Definition of Static Electricity?

What is the Definition of Static Electricity? My grade nine textbook states: Static electricity is the imbalance of positive and negative charges. What exactly does this mean? I understand ...
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2answers
410 views

Bound state in a potential well?

Reading from http://quantummechanics.ucsd.edu/ph130a/130_notes/node151.html It says: This means that the solutions separate into even parity and odd parity states. We could have guessed this from ...
1
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1answer
131 views

Is water a gas at critical density, room temperature?

I am quoting Chaikin, Lubensky, Principles of Condensed Matter Physics, p. 4. Now suppose we have a closed container of water vapor at a density of 0.322 g/cc at room temperature. As the ...
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1answer
72 views

Why is th $\hat{r}$ component zero in this integral?

I'm trying to evaluate the magnetic field by calculating the Coloumb integral $\overrightarrow{A}$, and later I will take: $$\overrightarrow{B}=\nabla \times \overrightarrow{A}$$ However, in the ...
1
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1answer
37 views

Double variation of Schwinger action principle

The Schwinger action principle is given by $$\delta_{1}\big\langle b\big|a\big\rangle= i\int_{t_{a}}^{t_{b}}\text{d}t\,\sum_{c,d}\big\langle b\big|c\big\rangle\big\langle ...

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