All Questions

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3 views

How to convert cc to bar?

In astronomy/astrophysics, medium density is often given in cc, particles per cubic centimeter. Also, the temperature of the medium is usually given, in Kelvins. For some materials the melting point ...
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0answers
3 views

Simplify QCD Lagrangian

How we can derive the electric charge of the field theory for each field ? For example lets say that we have the u quarks(3-colors) and electron that we know that has charge -e) free Lagrangian ...
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0answers
4 views

Determine coefficients so that $f(x,y)$ is a conserved quantity

Given the function $f(x,y) = ax^2y + by + ct^2 $ with $x(t)=2t$ and $y(t)=3t^2$ I shall determine the coefficients a,b,c so that $f(x,y)$ is a conserved quantity. My approach is $$\frac{df}{dt} = ...
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0answers
24 views

The integral is zero! $\int \frac{\mathrm{d}^d k}{(2\pi)^d} = 0$

In using dimensional regularization in QFT calculations, one comes across integrals over propagators, they might look like $(d = \text{dimension of spacetime}, n = \text{a number})$ ...
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0answers
3 views

Dependence of capacitance on its build material

We know that the capacitance of a capacitor depends on the dielectric material in between the plates as $C=\dfrac{K\epsilon_0 A}{d}$ But what if the material of the capacitance is ITSELF changed with ...
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0answers
5 views

Coordinate system for crystallographic groups

In the International Tables for Crystallography for each crystallographic group an asymmetric unit is supplied (mathematicians call this a fundamental domain of the group). This region is a bounded ...
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0answers
17 views

Derivation of the Riemann tensor confusion

I'm trying to understand the derivation of the Riemann curvature tensor as given in Foster and Nightingale's A Short Course In General Relativity. They start by giving the covariant derivative of a ...
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1answer
12 views

How much pressure do wheels exert on the ground?

I would like to know how much pressure 4 "rigid" wheels (as opposed to tyres) would exert on the ground; I've been on the case for hours now and the best I've come across to calculate this was the ...
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0answers
6 views

Stop CO2 laser firing through holes in glass

How would you prevent a CO2 laser with interlocks used to mark glass from firing if there were holes in the glass. Usually the interlocks prevent the laser from firing unless it is pressed against ...
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0answers
12 views

Why aren't purely Dirac neutrinos rules out?

Its common knowledge that in neutrinos can be Dirac particles without any Majorana masses as given a mass matrix, \begin{equation} \left( \begin{array}{cc}\nu _L & \nu _R \end{array} \right) ...
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1answer
11 views

Why is the Fermi coupling constant always expressed in units of $(\hbar c)^3$?

Everywhere I've looked so far (such as NIST) the Fermi coupling constant $G_F$ is always expressed as $$\frac{G_F}{(\hbar c)^3} = 1.166 364(5) \times 10^{-5} \textrm{ GeV}^{-2}$$ never as just ...
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0answers
22 views

Computation of spin connection 1-form

If stationary metric is given by $$ds^2 = e^{2U}(dt+\omega_idx^i)^2 - e^{-2U}dx^2$$ and I want to try to solve this. After doing the effort to find the metric and its inverse, I used the tetrad ...
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0answers
11 views

Force and Torque being applied off-center due to magnetic forces

Say that I have two magnetic dipoles, one of which is rigidly attached to a freely movable inflexible body at some point that is not at the body's center of mass, while the other is fixed in space. ...
5
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3answers
190 views

Can all fundamental forces be fictitious forces?

After reading many questions, like this and this, I wonder: is it possible to consider also the other fundamental forces, the electroweak interaction and the strong interaction or ultimately the ...
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0answers
6 views

Determining range of integration for electric filed of a semi-circle contour

Suppose we have a semi-circle contour in $xy$ plane ( it is oriented so that all of it lies at $y<0$) , and an arbitrary point on $z$ axis. It has linear charge density $\alpha$ and radius $a$ The ...
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0answers
22 views

Deduce the 3 dimensional Anti de Sitter space from the 4 dimensional case

From $$AdS^4=\big\{(u,w,x,y,z) \in \mathbb{R} | -u²-w² +x² + y² + z²=-1 \big\}$$ parametrized by $$f(t,\rho, \theta, \phi) = (\sin t \cosh \rho, \cos t \cosh \rho, \sinh \rho \cos \theta, \sinh \rho ...
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1answer
18 views

How does temperature affect flammability of materials?

I'm making a game that will include temperature, among other features, and I'm wondering how temperature interacts with flammability. I would appreciate both detailed answers, and ones that would just ...
2
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1answer
18 views

1-particle non-interacting Green function

At $T=0$ in the non-interacting case the $1$-particle Green function for an electron in the excited state $\lambda$ (empty band) is of the form \begin{eqnarray} G^{(0)}(\lambda,t-t') = -i \theta(t-t') ...
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0answers
16 views

Larmor Precession of a macroscopic number of electrons

I know that there are some similiar questions out there, but I'm still quite puzzled by the following problem. Say i have a box full of interacting electrons ( I'm not sure if it would change anything ...
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0answers
32 views

Interpretation of all eight solutions of the Dirac equation

There are eight solutions of the Dirac equation. $u_1, u_2, u_3 , u_4$ and $v_1,v_2,v_3,v_4$. Conventionally the four solutions ($u_3 , u_4,v_3,v_4$.) following from $E=- \sqrt{ (\vec p)^2 +m^2}$ are ...
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3answers
51 views

Why Light isn't an Acoustic wave?

I just wanted to know why light isn't an Acoustic wave.Is it because light wave doesn't obey acoustic properties?
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1answer
24 views

When you are standing in a running train if you jump means will you be in same place or will you be in front or back? [duplicate]

When you are standing in a running train if you jump means will you be in same place or will you be in front or back?
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0answers
14 views

Eigenvalue summation in casimir effect

In the following link there is an article about calculation of Casimir force: http://www.phys.utk.edu/courses/fall-2011/physics611/ajpcasimir1990zeta.pdf I'm having trouble in equation (29), ...
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1answer
15 views

Ideas for physics-related simulations to code up as weekend (or monthly) projects [on hold]

What are some good ideas for simulations that I could try to implement/code in my spare time just for educational purposes? I'm thinking that it would be a good way for me to delve into the underlying ...
1
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1answer
6 views

What is difference between anisotropy and inhomogeinity of this type of composite material?

I am studying some types of composite materials having 2 phases - fibers and matrix. I have some questions and confusions. Any help is appreciated. The composite has fiber along length and I am ...
0
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1answer
28 views

Want to know about divergence [duplicate]

Can anyone please explain how to know whether a vector field has divergence or not by seeing its diagram? I have read that a vector field must change for having divergence but why is divergence zero ...
0
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1answer
38 views

Uncertainty Principle and single-slit diffraction [on hold]

A beam of 50eV electrons travel in the x direction towards a slit of width 6 micro metres which is parallel to the y direction. The diffraction pattern is observed on a screen 2 metres away. Use ...
0
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1answer
48 views

Does varying centripetal force produce circular motion?

Centripetal force $\vec{F}$ is responsible for circular motion and is given by $$ \left|\vec{F}\right| = \dfrac{m\left|\vec{v}\right|^2}{r}$$ If the linear velocity $\vec{v}$ varies, the centripetal ...
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1answer
37 views

Passage of time on massive Earth

Say a planet has the same orbital period as Earth (365 days in a year, 24 hours in a day, etc.) but the planet has double the mass. So time must pass more slowly on that planet compared to Earth, ...
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0answers
7 views

Tunable ferroelectric material

Is there any tunable ferroelectric (whose permittivity changes by applying electric field) material for visible or near IR applications?
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0answers
36 views

What is really interacting in weak interactions?

Only particles with chirality $-1$ do interact weakly. The corresponding eigenstate in the Dirac basis is $ \Psi_L = \begin{pmatrix}f \\ -f \end{pmatrix} = \begin{pmatrix}u_r {\mathrm{e}}^{-imt} \\ ...
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0answers
12 views

capacitors - Square circuit of two capacitors charging in a changing magnetic field

I've been studying capacitors and DC circuits, and I received this question as part of my homework: The above circuit is a square circuit with side a. E and F are the midpoints of AB and CD ...
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0answers
19 views

Why are lubicants like grease or oil slippery?

I know that grease or oil are slippery in nature but never thought of why? I want to know what causes this tendency.
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1answer
32 views

Does the $\bf{1+3}$ representation of $SU(2)$ also represent $SU(2)\times SU(2)$?

I'm a bit confused about this following issue concerning representations of $SU(2)$. Denote by 1 the 1-dimensional representation of the group $SU(2)$ (=the spin 0). Similarly, denote by 2 and 3 the ...
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0answers
16 views

Why are flight feathers asymmetric?

I've always heard that flight feathers were asymmetric because the shape generates lift, like an airplane wing airfoil shape. But, the feathers are not oriented with the shaft perpendicular to the ...
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0answers
37 views

Please help with quantum mechanics [on hold]

Let the Hamiltonian of two nonidentical spin 1/2 particles be where and are constants having the dimensions of energy. Find the energy levels and their degeneracies.
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0answers
22 views

Translational acceleration of cylinders [on hold]

Two cylinders with total mass $M$ and radius $R$ are connected by a massless rod along their axis of rotation and rest on a horizontal surface. A frictionsless ring at the center of the rod is ...
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0answers
29 views

Background field expansion in normal coordinates

Background field expansion following form $Y= X+\pi$ where $X$ is my background field and $\pi$ is the fluctuation. From the Normal coordinates we have the expansion of $\pi^{\mu} = ...
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0answers
14 views

Wigner Formalism: Operator Correspondences

Given the following operator correspondences between density matrix $\rho$ and Wigner function $W$, $$X\rho \leftrightarrow (x+(i\hbar/2) \partial_{p}) W$$ $$\rho X \leftrightarrow (x-(i \hbar/2) ...
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0answers
27 views

White light is line specific

Here I only consider the geometric picture of optics. (not the quantum optics or wave optics) If A and B are two pins placed and a white light is passed in its path AB , after refractions leaves ...
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0answers
7 views

Why molecules in this potential have a common average distance $r_m$ to their neighbors?

A commonly used potential describing intermolecular interactions in liquids and real gases is the so-called Lennard-Jones potential. In this model the interaction of atoms and molecules with each ...
0
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1answer
21 views

Why can the twin plane be uncoincide with the composition plane in reflection twin?

In the book Elements of X-Ray Diffraction, p67, I found the following sentence: In the case of a reflection twin, the composition plane may or may not coincide with the twin plane. But I think ...
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1answer
40 views

Block on an inclined plane, spring at bottom of incline [on hold]

Problem: A block of mass $m = 5.00kg$ is on an inclined plane with an angle $\theta$ = 16.26 degrees with respect to the horizontal. The coefficients of static and kinetic friction are: $\mu_{k}$ = ...
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0answers
22 views

Magnetic flux, and magnetic field shielding?

A magnetic field is shielded by a high permeability ferromagnetic material,assume this:The magnet's field lines are all in the ferromagnetic material(95%+ shielded), an we "slide" off the shield to ...
1
vote
1answer
38 views

Calculate the mass of a Schwarzchild black hole with Komar integral

In Wald's GR, Komar integral is Eq. (11.2.9): $$M=-\frac{1}{8\pi}\int_S\epsilon_{abcd}\nabla^c\xi^d$$ $S$ can be chosen as a 2-sphere, the boundary of a spacelike hypersurface $\Sigma$ such that the ...
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1answer
17 views

Is it more difficult to create higher energy photons, given there is excess available energy?

Im not talking about pair production, I mean a single photon created from an energy transition between two electron shells. I'm studying K(alpha) and K(beta) fluorescence transitions in metals, and ...
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1answer
21 views

Calculating estimated HP from velocity, auto weight, and constant acceleration

I am working on a simulation program that runs theoretical performances of different cars and was wondering if there is a way to estimate the HP at any running RPM? the problem is (and this may be me ...
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0answers
17 views

Deriving an Expression for Helmoltz free energy

Given the equations of states for an isolated system: $$E=\frac32 pV$$ $$p=aVT^4$$ I was asked to find the Helmoltz free energy per particle, $F=E-TS$, as a function of $T$ and $V$. I began with the ...
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0answers
9 views

How $Z_{2}$ topological order is introduced to distinguish states?

In the $Z_{2}$ topological order and the quantum spin hall effect:Here is what the authors said" the states withe zero and on flux quantum threading the cylinder...However, the two states are ...
2
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2answers
62 views

A question over the reality of $\sin x$

Harmonic functions are in widespread use in physical descriptions of natural real phenomena. I am just wondering therefore how we can define $\sin(x)$ to be part of a real physical quantity (with ...

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