All Questions
229,653
questions
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A Hotwheels Problem
I have a Hotwheels track set. It has a looped structure such that at some point in the car's trajectory, it becomes upside down. But in such a situation, it doesn't fall to the ground. I understand ...
- 196k
-1
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0
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33
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Measuring the energy of a photon with E = mc²
Clearly light has energy because the photoelectric effect or photosynthesis would not work.
The formula E = mc² doesn’t work for measuring the lone energy of light as a massless particle, but it works ...
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votes
1
answer
234
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What happens if I send light from two light sources through a single slit?
I have two identical point light sources A & B emitting the same frequency. They are equidistant from the single slit and from the line passing perpendicularly through the middle of the slit ...
CommunityBot
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votes
2
answers
56
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Are black holes 4-dimensional balls of spacetime? If so, will they have 3-sphere surfaces?
If black holes are 4-dimensional balls of spacetime, they will have a 3-sphere surface with a 3-dimensional volume. Would this allow infalling matter to remain within this surface?
- 260
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1
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29
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Determining Whether a Collision is Elastic or Inelastic
I am having a lot of trouble with this problem.
The answer key says that the collision is inelastic, but I am not sure how they came to this conclusion given that there is no indication of whether ...
- 351k
4
votes
1
answer
159
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+50
What is the meaning of $\vec H$ with respect to the total field?
Now before saying anything, I have seen the similar questions in this page regarding $\vec H$ but no one has fully convinced me yet, since I will try to give another perspective to this question.
We ...
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0
answers
5
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How,in induction of electric charges, the field developed due to separation of charges exactly cancels out the external field?
I have been learning electrostatics, and I didn't quite understand why charges reside only on the surface. The charges are moved until they reach the surface after which they cannot move further and ...
0
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0
answers
8
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Predicting atomic dipole transition properties (reference request?)
At the moment, I'm interested in the question of how to predict the strengths of atomic transition dipole elements for atomic transitions. A quantum 101 approach to solving this problem involves ...
- 12.8k
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1
answer
2k
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Sun light collimation
Is it possible to make sun light collimated in such a way, to increase energy density and keep it more less parallel?
like on the Earth we have intensity of sun approximately 1300 watt/m2. If we use ...
CommunityBot
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1
vote
2
answers
16
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Conflicting lensmaker's equation representation
I keep finding these two conflicting forms of the lensmaker's equation. I understand that (n2-n1)/n1 is the same as (n-1) if referring to a lens in air, but I can not make sense of why the reciprical ...
- 36.3k
1
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0
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16
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Transformation rules for components of tensor fields
In general relativity, when asked to prove that an object is (resp. is not) a tensor, we are often expected to show that its components follow (resp. don’t follow) the so-called tensor transformation ...
- 11
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28
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Ejected bodies, dynamical friction and dark energy?
I have a question after reading a couple of papers (https://arxiv.org/abs/1707.06220 and https://www.arxiv-vanity.com/papers/1102.0007/).
Here, the authors seem to indicate that when bodies like stars ...
- 1,980
1
vote
3
answers
338
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Transformation of states and operators under Quantum Lorentz Transformation
For any Homogenous Proper Orthochronous Lorentz Transformation $$x\to\Lambda x$$ , there is a unitary linear operator $$U(\Lambda)$$ [Bogoliubov(1980),Srednicki(2007),Weinberg(1995)] which can be ...
CommunityBot
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0
answers
13
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Buckling Question from Hibbler Textbook
Hello! I have a question from the Hibbler Textbook on Mechanics of Materials! The exact statement of the problem reads:
"A W150 * 24 steel column is 8 m long and is fixed at its ends as shown in ...
- 196k
2
votes
0
answers
22
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When do pairs of quarks form jets vs mesons?
For certain processes with low momentum transfer, such as the Kaon decay shown below, quarks will form bound states of mesons.
Whereas for higher momentum-transfer processes, such as the decay of an ...
- 196k
2
votes
2
answers
40
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How to take derivative of density operator?
I was just trying to confirm to myself that the following density operator
$$\rho(t) = e^{-iHt/\hbar} \rho(0) e^{iHt/\hbar}$$
fulfills the Liouville-von Neumann equation:
$$\frac{d}{dt}\rho(t) = - \...
- 196k
0
votes
0
answers
27
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Question on height of a jump in terms of Power (from WPE chapter, JEE level) [closed]
This is an interesting question I came about and I am not sure how to proceed.
I used the equation P = Work done/time $\rightarrow P = \frac{1}{2t} mv^2$.
I then multiplied by s on both sides and got:
...
- 196k
5
votes
1
answer
101
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+50
Understanding a supersymmetric quantum mechanical gauge theory model
I'm studying this paper on supersymmetric ground state wavefunctions. In section 5 "quantum mechanical gauge theories", it says:
"We begin with the ${\cal N} = 2$ gauge theory which ...
- 611
0
votes
0
answers
17
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Thermodynamics and entropy
In a thermodynamics process, when heat is exchanged ($\Delta H$), it causes a change of system from state 1 to state 2. As a result, the entropy of a system is changed irreversibly.
Can the change in ...
- 611
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votes
2
answers
223
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Why does work done by light not equal energy absorbed?
Question: Suppose a spherical particle of mass $m$ and radius $R$ in space absorbs light of intensity $I$ for time $t$. (a) How much work does the radiation pressure do to accelerate the particle from ...
CommunityBot
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answers
22
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Justification of the relationship $s/S<\lambda/d$ for interference fringes to be seen
[My book] (NCERT Class 12 Physics text; Chapter no.:10 - Wave Optics)
(https://www.google.co.in/url?sa=t&source=web&rct=j&opi=89978449&url=https://ncert.nic.in/ncerts/l/leph202.pdf&...
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1
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2
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771
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Retarded time and corresponding position
EDIT:
Added Clarification.
By various methods, it's possible to determine that in the formula for Electric potential for the position of a charge in uniform motion,
$V=\frac{q}{4\pi\epsilon_0\kappa \...
CommunityBot
- 1
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votes
1
answer
113
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Ehrenfest paradox and the link to general relativity
I've seen several sources stating that Einstein was partially inspired by the Ehrenfest paradox when thinking about general relativity. However in the Ehrenfest paradox the non-Euclidean nature of the ...
- 111
2
votes
2
answers
42
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How does the Dzhanibekov Effect change if replaced with a propeller?
Every explanation and example uses a t-bar. But what if the "bar" was a propeller? I can understand there being no difference if it's a vacuum, but I would think the aerodynamics would ...
- 37.4k
2
votes
3
answers
318
views
Why was it cold during inflation?
During inflation, spacetime is approximately de Sitter and there is a cosmological horizon; in analogy with Hawking radiation, there should be a de Sitter temperature $T \sim H$. This temperature has ...
- 1,341
2
votes
3
answers
109
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Derivation of Hamiltonian by constraining $L(q, v, t)$ with $v = \dot{q}$
I am trying to reconstruct a derivation that I encountered a while ago somewhere on the internet, in order to build some intuition both for $H$ and $L$ in classical mechanics, and for the operation of ...
- 3,930
4
votes
2
answers
411
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Being Frictionless, surface of contact
I frequently hear references to a smooth surface, or frictionless pulley.
Can being frictionless be obtained if only one of the 2 surfaces has 0 coefficient of friction?
Or is it for the contact of ...
CommunityBot
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-1
votes
3
answers
425
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Does wave function describe particle annihilation?
I don't study physics, this is a layman question. From some online sources, some probability would be given by squaring the parameters of a wave function. Some sources also claim that wave function ...
0
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1
answer
82
views
+100
What is the relevant phenomenon behind Undulatus/Radiatus cloud formations?
I am seeing many people claiming that cumulus clouds that sometimes form periodic wavy patterns (see images for "altocumulus undulatus" or "Radiatus" for instance) have no ...
- 8,280
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0
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15
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Why does a bullet precess in the opposite direction from gyroscope diagrams? [duplicate]
I am reposting this because it was not resolved when I first posted.
Bullet spin causes a bullet to become a gyroscope. Specifically, bullets have their center of pressure in front of their center of ...
- 11
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2
answers
519
views
+500
Can we regard metric as the Higgs field of gravity?
The longer version of the question is: should we regard special relativity just as a spontaneous symmetry breaking phase of general relativity, driven by the non-zero vacuum expectation value (VEV) of ...
- 122k
3
votes
2
answers
872
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Could Earth's Zircon (used in geological dating) have formed in extraterrestrial events before the Earth was formed?
This is a follow up question asked and answered (Radio-dating and the age of the earth)
The answer was given that the mineral Zircon is formed under high pressures and temperatures. How do we know, ...
- 128k
7
votes
1
answer
528
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Is the hierarchy problem definitely a "problem"?
There have been a bunch of questions related to the hierarchy problem, but I still can't help but feel that an assumption is being made that is not backed by any example of correctness and is thus ...
- 3,355
1
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3
answers
2k
views
Influence of an external electric field on an uncharged capacitor
Influence of an external electric field on an uncharged capacitor.
Fig A shows an uncharged capacitor with no external electrical field. There is no net charge on each of the plates. Since the ...
CommunityBot
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5
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0
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129
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+300
How do I find the kernel of the shift operator in the solution of 2D Ising model?
Okay, this is a second part of my previous question. Again, I'm following Itzykson's book. The fermionic solution for the 2D Ising model is described in terms of a matrix $T = \theta \tilde{\theta}$, ...
- 545
1
vote
1
answer
218
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Ionization vs Bremsstrahlung for Electrons?
According to Kolanoski's & Werme's book Particle Detectors. fundamentals and applications, page 63, for electrons, we can write the energy losses for ionization and Bremsstrahlung as follows:
$$ \...
CommunityBot
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0
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39
views
4
votes
1
answer
193
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Why Curie temperature is bigger for smaller lattice in 2D Ising model
Using Metropolis algorithm, Curie temperature was calculated for square lattices with different sizes 4x4, 8x8, 16x16 and 32x32. Here Curie temperature was estimated as temperature of maximum of heat ...
- 2,434
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votes
0
answers
47
views
Why temporal and spatial term is multiplied to get the wave equation?
When deriving the wave equation we have:
$$y_{spatial} = A\exp(ikx)$$
$$y_{temporal} = A\exp(-i\omega t).$$
Then the overall wave equation is obtained by multiplying both time and spatial component. ...
- 196k
0
votes
1
answer
201
views
What does elasticity with torque and twist depending on normal strain and normal stress look like?
A twisted rope has a relation between normal stress, along the rope, and twist of the rope. If a rope is twisted more it becomes shorter. If it is untwisted it becomes longer. Likewise for coils and ...
- 900
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votes
0
answers
48
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Time evolution operator with chemical potential
In the Bruus and Flensberg textbook, section 1.5, it is mentioned that
Basically, the result obtained from the canonical ensemble is carried over to the grand canonical ensemble by the substitution $...
- 196k
5
votes
1
answer
440
views
Semiflexible discrete polymer chain
Suppose we have a 2D polymer model described by a set of 2D vectors {$\mathbf{t}_i$} ($i=1,2,\dots N$) of length $a$.
The energy of the polymer is given by:
$$
\mathcal{H}~=~-k\sum^N_{i=1}\mathbf{t}...
CommunityBot
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3
votes
0
answers
26
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Theoretical expression for the capacitance of interdigitated electrodes
I am trying to determine the capacitance of interdigitated electrodes, like these:
According to the paper, it can be calculated by:
$$C=\varepsilon \frac{(2l_0-a+2w_0)t_0}{d_0}(2n-1) \tag{1}$$
Where $...
- 196k
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votes
0
answers
25
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Find magnitude of cars acceleration [closed]
A student constructs an accelerometer by attaching cork balls to strings anchored to the bottom of a fish tank. When the student fills the tank with water, the balls float to the surface. When the ...
- 196k
1
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0
answers
55
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Proving that Retarded K-G Propagator is Green function (Peskin & Schroeder 2.56) [closed]
I am trying to derive Peskin & Schroeders expression $2.56$:
$$(\partial^2 +m^2)D_R(x-y)=-i\delta^{(4)}(x-y)\tag{2.56}$$
with $$D_R(x-y)=\theta(x^0-y^0)\langle 0|[\phi(x),\phi(y)]|0\rangle.\tag{2....
- 196k
1
vote
0
answers
16
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"Boltzmann" equation for radiation (Reheating period)
These equations are given in many papers as the "Boltzmann equations" (without derivation) governing the reheating period, where $\rho_\phi$ is the energy density of the decaying inflaton ...
- 355
3
votes
2
answers
226
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Particle density operator in Fetter and Walecka
In Fetter and Walecka page 20 they say in order to find the second-quantized operator $\hat{J}$ of the first-quantized one-body operator $J$ you need to calculate
$$\hat{J} = \int\mathrm{d}^3x\,\hat{\...
- 196k
5
votes
2
answers
2k
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How much more effective is it to stir in both directions?
I have been told that industrial mixing machines (say, for cake batter) switch directions periodically, first stirring in one direction, then the other, because this mixes the material more thoroughly....
- 11
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0
answers
29
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Types of Mechanical Waves
When knocking on a door, does that create a transverse wave or a longitudinal wave? I know it's one of these mechanical waves, but which one?
- 196k
-2
votes
0
answers
15
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Finding the quantum number of highest energy level for which balmer series would be just resolved for a given spectrometer and resolving power [closed]
A spectrometer of power $5\times10^5$ is used to observe the emission spectrum of a sample of atomic hydrogen. The quantum number of highest energy level for which the Balmer series would be just ...
- 351k