Newest Questions
241,273 questions
4
votes
2
answers
952
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Pound-Drever-Hall frequency stabilisation technique
I am having some difficulty understanding the Pound-Drever-Hall frequency stabilisation technique, when locking a laser to a stable cavity.
As far as I understand:
We emit the laser frequency, f0.
...
- 41
16
votes
1
answer
7k
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What does it mean to say "internal symmetry"?
What does it mean to say "internal symmetry"? Let me try to express the way I see it, so you can have it as a starting point.
There are spacetime symmetries, which are global since any Lorentz ...
- 1,915
8
votes
5
answers
8k
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Minimum frequency of an electromagnetic wave [duplicate]
Is it possible to create an electromagnetic wave of near zero frequency?
An electromagnetic wave carries energy. If we can make the frequency of an EM wave vanishingly small and make it practicality ...
- 2,581
2
votes
2
answers
415
views
Energy of electric field and magnetic field
I am studying about inductors and capacitors. They store their energy in the form of electric field and magnetic field.Energy is required to create fields.But if we take an isolated charge and nothing ...
- 504
3
votes
1
answer
414
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Why doesn't a uniformly moving particle radiate?
When considering a uniformly moving charged particle, we have the following fields:
$$\vec E = \frac{q(1-\beta^2)}{4\pi\epsilon R_a}\vec R$$
$$\vec B = \frac{1}{c^2}\vec u \times \vec E$$
With $\vec ...
- 1,373
4
votes
1
answer
118
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Finding $\pm 2 \pi$ defects in 2-D lattice nematic simulation
I'm working on a Monte Carlo simulation of a two-dimensional nematic system (XY-like model with even-order Legendre polynomial interactions, such that the director angle $\theta$ obeys $\theta \equiv \...
3
votes
3
answers
3k
views
Pressure just before the hole in a draining tank
I get extremely confused in fluid dynamics problem where the pressure in some points is required.
Consider the ideal fluid in the tank in the following picture. I would like to understand what is the ...
- 2,637
0
votes
1
answer
254
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How do I differentiate between units and variables on my notebook? [closed]
On computer systems, variables are written in italics, as $m$ or $s$, and the units as $\mathrm{m}$ or $\mathrm{s}$. It is very incovenient to do italics for variables and upright for units while ...
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1
vote
2
answers
1k
views
How to find the axis of rotation or location given the angular velocity?
Say I have the angular velocity vector of a body as a function of time. How can I determine the axis of rotation/location of the body?
we have the equation:
$\frac{d\vec{r}}{dt}=\vec{\omega}(t)\...
- 1,447
2
votes
0
answers
296
views
OPE coefficents and commutation relations, and OPE with stress tensor
Basic question about conformal field theory: In a conformal field theory in $d\geq 3$ dimensions, what is the relation between commutation relations and OPE coefficients?
In particular, because ...
- 21
1
vote
3
answers
3k
views
Dipole Moment and separation
My understanding of a dipole moment is it defines the strength of various dipole interactions, eg the common example of torque in an electric field. My confusion stems from the fact that the magnitude ...
- 71
3
votes
0
answers
723
views
Non-canonical transformation
I would like to know any method to transform a known non-canonical set of variables to a canonical set for a given system. The Lagrangian and Hamiltonian are known in the non-canonical variables. I ...
- 81
4
votes
2
answers
303
views
Weight Distribution of granular piles
I was recently at a farm watching piles of grain being formed in the shape of enormous cones. Each of these grain stacks weighs in excess of 50 tonnes. I am not a physicist but I am very curious as to ...
- 143
1
vote
0
answers
142
views
Second derivative of Kahler potential [closed]
Does the following second covariant (in terms of Kahler geometry) derivative of Kahler potential vanish?
\begin{equation}
K_{ij}\equiv\nabla_i\nabla_j K=0,
\end{equation}
\begin{equation}
K_{i^*j^*}\...
- 2,736
0
votes
1
answer
154
views
Why isn't mass still being created in the universe? [closed]
I was watching a lecture by Alan Guth and wished I was in the classroom to be able to ask a question. He explained that the total energy in the universe could be very close to zero since mass and ...
- 257
0
votes
2
answers
104
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Could vacuum chambers lighten the load for space bound technology?
Would cavities of near vacuums in the head (or in the 6 x 6 blocks surrounding the rocket [for reentry temperatures]) of a rocket make it lighter so it can be sent out easier?
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5
votes
1
answer
2k
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Electric Potential Field of Parallel Electrodes within Grounded Shell
Any help would be greatly appreciated
I think I need to solve this using boundary potential conditions and Laplace equations.
Here is the system I'm attempting to model:
The red rods are metal ...
user121904
0
votes
0
answers
123
views
What is the purpose of condenser lenses in the slide projector shown below? [duplicate]
I searched a lot on internet about this.I could not find the working of these condenser lenses in detail. Some websites say that condenser lenses make the light rays parallel and to pass through ...
- 591
2
votes
2
answers
721
views
Mechanism behind "Hoot Tube" demonstration
For years I've been puzzled by how the "Hoot Tube" demonstration shown at the end of this video (around 2 min 45 sec) actually works.
I've done the demo myself with a double walled stovepipe from the ...
- 524
5
votes
2
answers
4k
views
Double slit experiment; evidence of wavefunction collapse
This video shows the change of a photon's interference pattern in real time of the Young's single and double slit experiment.
In this video it is claimed that by adding a detector to view how the ...
- 1,455
1
vote
1
answer
87
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Given this graph from Wikipedia concerning the cosmogenic origin of each element, how are Lithium, Beryllium, and Boron formed from cosmic rays?
So I found this article from another answer, and it's math is way above me but the gist is that some elements heavier than Iron can form in a star, but this path is not the primary source of these ...
- 1,066
1
vote
3
answers
13k
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what is the purpose of condenser lens in a slide projector?
What if we don't use any condenser lens? can't we use a single convex lens as a condenser? what difference will it make? kindly explain it in detail.
- 591
4
votes
6
answers
6k
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Air flight and Earth's rotation
I read a response asking why flights of equal distance east and west take roughly the same time (disregarding wind actions). I have trouble visualizing part of the answer; "the speed of the rotation ...
- 53
4
votes
2
answers
1k
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Boltzmann factor and ratio of number of states
When using Boltzmann factor for two states, we see that $$N_1/N_2 = \exp\left(\frac{h\omega}{k_BT}\right)\;.$$ Where $N_1$ is the number of atoms in state with lower energy $E_1$ than atoms in state $...
- 41
1
vote
4
answers
168
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Friction-Newtonian Mechanics
Here we are having two blocks of mass $2~\rm kg$ and $4~\rm kg$ on an inclined angle with angle of inclination being $30~\circ.$ The block of mass 2 kg has a coefficient of friction $\mu_1=0.2$ and ...
user118752
0
votes
1
answer
244
views
If we were able to manipulate enough energy, could we use the higgs field for any practical application? [closed]
My question is, the fields in the standard model interact with each other, and when we are able to manipulate some of them through different engineered devices, we can transform the energy field into ...
user83548
1
vote
0
answers
212
views
Deriving the field equations for a dark energy / modified gravity effective field theory
Question
I'm trying to derive the modified gravity EFT field equations and, from their 00 component, this Friedmann equation:
\begin{equation}
H^{2}+H\frac{\dot{\Omega}}{\Omega}=\frac{\kappa \rho_{m}+...
- 11
28
votes
5
answers
4k
views
Can elements heavier than iron be present in a star's core?
My understanding is that elements heavier than iron and nickel are not formed in a star but, can heavy elements such as lead and others be present/found in a star's core ?
I ask because the following ...
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1
vote
2
answers
497
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Special Relativity - oscillator paradox
I am reading about the Special relativity and the original Einstein papers from 1905 and 1920 where he derives the Lorentz transformation and the effects of the time dilation and space contraction ...
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0
votes
1
answer
370
views
Why anti-reflective coating of silicon nitride colour varies with thickness?
Why the silicon nitride colour varies with the thickness?
1
vote
0
answers
69
views
Mixed Gas Absorbtion
Consider a gas misture that contains two type of atom, A and B. The gas is in equilibrium at temperature $T$. If on the surface of the gas container there are M sites that can absorb an atom gas, and ...
- 23
2
votes
1
answer
3k
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Why does laminar flow turn turbulent(*) in external fluid flow and why doesn't a fully developed laminar flow turn turbulent in flow through pipes?
(*:with increase in distance from the leading edge.)
I would appreciate if you give detailed explanation for the first question and then apply the same logic to answer the second.
1
vote
1
answer
365
views
rotation of a state in Bell basis
Suppose I have a state in Bell basis. For example
\begin{equation}
\rho =
\begin{pmatrix}
\rho_{11} &0 & 0 & \rho_{14} \\
0 &\rho_{22} & \rho_{23} & 0 \\
0 &\rho_{32} &...
2
votes
3
answers
596
views
Work done by an ideal gas expressed as change in potential energy of fluid
I get confused in problems where it is necessary to evaluate the work done by a gas that someway moves a mass. I'll make an example.
Consider the tube containing mercury and an ideal gas in A (...
- 2,637
1
vote
1
answer
33
views
When de-evacuation occurs, what provides energy?
When a syringe piston is pulled from its initial position to spray position by blocking the nozzle with a finger, creating a vacuum inside the syringe, the piston will probably get pulled back to ...
0
votes
3
answers
73
views
Work done on gas problem with movable set
I found two different version of an exercise where the work is considered in a different way. Consider a gas in a tank divided in two parts by a movable set. The set in $B$ is also movable. If a ...
- 2,637
0
votes
1
answer
54
views
When happens to the current during reversed biased?
The main working function of a half wave rectifier is to convert AC current to DC. My problem is to understand during the negative cycle.
The above picture is time versus current. We can see easily ...
user58143
0
votes
2
answers
352
views
Why monochromatic waves exist even theoretically
I was reading this question. Where the author made a statement that
If I take a wave with period T, it is also true that it has a period 2T, 3T and so on. That is, it has frequencies of 2π/T, but ...
- 2,581
0
votes
0
answers
135
views
Time independent Yang-Mills field coupled to scalar field
Let $A$ be a Yang-Mills field with $A_0 = 0$ and we also have time independent scalar field $\phi$ in the adjoint representation of our gauge group with zero potential (no mass too). I have to show ...
- 526
3
votes
1
answer
5k
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How can I derive the Phonon Density of Normal Modes in two and three dimensions?
The density of normal modes in 1D has already been discussed in another post (LINK)
In essence, the formula is:
$$D(\omega) = \frac{1}{2\pi}\int\mathrm{d}q\,\delta(\omega-\omega(q))$$
resulting in a ...
- 859
2
votes
1
answer
6k
views
Energy-band diagram of forward-biased pn junction
When a p-n junction is forward biased then its energy-band diagram looks like this:
What would happen if $V_a>V_{bi}$? ($V_{bi}$ is the built-in potential and $V_a$ is the externally applied ...
- 1,213
1
vote
0
answers
154
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T-duality between $E_8 \times E_8$ and $\text{Spin(}32)/\mathbb{Z}_2$ heterotic strings at the $\sigma$-model level
I would like to understand how T-duality between the heterotic $E_8 \times E_8$ (HE) and heterotic $\textrm{Spin}(32)/\mathbb{Z}_2$ (HO) theories works, at the level of the worldsheet $\sigma$-model.
...
- 2,105
8
votes
2
answers
3k
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Preparatory knowledge to study Quantum Gravity
I am an old (41-Years-old) physics graduate. I pretty much quit to "frequent physics" 10 years ago and my ability to manage physics is a bit rusty.
I was fascinated with Quantum Gravity (QG) theory ...
0
votes
4
answers
3k
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Water evaporation rate: temperature vs humidity
I have a hot tub which I keep at 100 degrees F, and the water has "a lot" of dissolved salts in it. If I leave it open, will it evaporate faster when it is hot and humid outside, or when it is cold ...
- 1,484
0
votes
2
answers
188
views
The counter-intuitive time scales in atomic physics and nuclear physics
Compare atomic physics and nuclear physics. The interaction in the latter is much stronger than that in the former.
However, the typical spontaneous emission time scale in atomic physics is on the ...
- 2,165
0
votes
1
answer
596
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Reduced Density operator in matrix form
I already read book of Quantum Computation and Quantum Information by Nielsen and Chuang according to reduced density operator and I already understand how to do the reduced density using Dirac ...
- 49
0
votes
2
answers
511
views
Canonical partition function and counting
That's a silly silly question, so my apologies, but in this moment I could not reach out!
Let's have a system made of a particle reservoir $R$, and a subsystem $S$. The total particle number is $N$. ...
- 647
3
votes
1
answer
331
views
How to apply conservation of energy to active noise-cancelling headphones?
Active noise-cancelling headphones add the opposite waveforms of the ambient sound in order to cancel out this ambient sound by destructive interference. I do not understand why adding more power in ...
- 199
1
vote
0
answers
57
views
Modelling a flow due to an oscillating hemisphere?
Say I have a long cylinder filled with air, closed at one end and open at the other. Now say I place a compressible hemisphere at the closed end and make it oscillate (compression and expansion of the ...
- 289
-1
votes
2
answers
235
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General Approach and Intuition to Calculus based problems in Physics [closed]
I've done a calculus-based course in Mechanics, Electromagnetism and at the moment I'm studying Strengths of Materials. Time and again, one is faced with problems where Integration is used to sum up ...
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