All Questions
222,323
questions
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43
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Derive partition sum for ideal gas (sum to integral)
Having the grand canonical partition sum:
$$\mathrm{Z} = \sum_{\{n\}}\exp{\left(-\beta \{\varepsilon_n(N_n) -\mu\,N_n\}\right)}$$
how to arrive at: $$\mathrm{Z} = \sum_{N = 0}^{\infty} \dfrac{V^N}{h^{...
- 428
0
votes
1
answer
51
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Statistical proof of the principle of least action
In statistical thermodynamics we can prove that the evolution of a system minimises some potential with units of energy (e.g. energy). This can be done purely statistically, by using the first two ...
- 77
0
votes
0
answers
22
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Mean first passage time (MFPT)
I want to know the mean first passage time (MFPT) on a unit interval for two boundary conditions (please see attached figures a and b for your reference). This is in the context of the hydrodynamic ...
- 84
-1
votes
0
answers
30
views
Is inside the eye of hurricane safe? [closed]
According to the pressure and velocity diagrams of a hurricane, can we say that inside the eye of the hurricane is somewhere safe?
- 1,946
1
vote
1
answer
65
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Would a black hole instantly form when a neutron star slips below the phantom event horizon?
So lets say we have a neutron star that is just few inches away from the phantom horizon and only needs 500 kg before collapsing. So lets say hypothetically that a ship that is designed to survive the ...
- 851
1
vote
1
answer
32
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An explicit form for the co-BRST operator?
Take a theory with 1st class constraints $M_{\alpha}$. We gave ghosts $c^\alpha$ and their conjugates $b_\alpha$ for every constraint. The BRST operator $\Omega$ has ghost number $+1$ and has an ...
- 459
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votes
1
answer
40
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Why electric potential at each point in a uniform electric field is not same?
why potential at any point in a uniform electric field is not same i.e. why potential difference between any two points in the uniform electric field is not zero?
According to me if i bring a test ...
0
votes
2
answers
45
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Conservation of energy when 2 unequal forces are acting on a body [closed]
How will energy be conserved if two unequal and opposite forces (force1 &force2) are acting on a body, and the net force led the body to be displaced? What is the work done by force1 and force2 (...
1
vote
2
answers
45
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Moment of inertia of hollow body and solid body
I read in my textbook about the various results of moment of inertia for different geometrical shapes like solid and hollow cylinder, sphere, disc and ring etc. Something general I noted is that $M.I$ ...
1
vote
3
answers
70
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ELi5- How do pions hold nuclei together if they are so short-lived?
I need help understanding how particles do what they do and maintain the structures they maintain if so many of them exist for such a short time?
In the case of the nucleus and pions, pions only exist ...
- 173
3
votes
2
answers
63
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Why is the alpha particle beam in the Rutherford experiment not affected by the electrons?
The force exerted by the nucleus on the alpha particles is electrostatic in nature. So, why does mass has an effect? How do we conclude that the electrons don't effect the alpha particles from the ...
0
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0
answers
12
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How is the specific impulse of a gridded ion thruster derived?
How is the specific impulse of a gridded ion thruster derived?
I've searched and got a variety of answers and Bard gives a different answer each time I ask it.
Too the best that I can reckon it's:
1/9....
- 1
0
votes
0
answers
20
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$6\times 6$ linearly independent matrices for Hamiltonian fourier expansion
I want to use the Slater-Koster method to Fourier expand a $6 \times 6$ Hamiltonian and for this, I need 36 linearly independent basis matrices. In two dimensions, we use Pauli matrices, and in 4 ...
- 73
-1
votes
2
answers
93
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Is signal photon independent of idle photon in Delayed Choice Quantum Eraser?
--The experiment of Kim et al. (1999)
I want to know whether anything we do to the idler photon affects the landing position of the signal photon on the screen at $D_0$ in the Delayed Choice Quantum ...
- 159
-1
votes
0
answers
19
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Solving a set of DE in any method, boundary conditions at $\rho\rightarrow -\infty$ [migrated]
I want to solve the following set:
$$
y'(\rho) = y(\rho)*g'(\rho)+a
$$
$$
f'(\rho) = -\frac{1}{a}*y(\rho)*f(\rho)
$$
$$
g'(\rho)=-y(\rho)*\left[\frac{1}{a}f(\rho)^2 +a\right]
$$
with the condition of $...
1
vote
1
answer
30
views
Does off-shell graviton in 3+1D still have two degree of freedom?
In general relativity, on-shell spin 2 graviton has 2 degree of freedom in 3+1D due to gauge symmetry, which is because of einstein equation, however, for off-shell graviton, does it still have 2 ...
- 509
2
votes
0
answers
34
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The difference between $E-k$ diagram and simplified Energy band digram
I know form the the Kroning-Penny Model the $E-k$ diagram solution which something like this
or like this
but While dealing with semiconductors we always draw this energy band diagram
My question ...
- 525
0
votes
0
answers
28
views
Can Newton’s system of particles be used to solve this constrained motion? How? [closed]
Thanks the picture from quora.
No friction on pulley.
I would like to know how to solve detailly with the Cartesian coordinate and vectors clearly.
- 247
0
votes
2
answers
65
views
What happens to those parts of the amplitude that are not inside the black hole? [closed]
Assuming that a sinusoidal wave with an amplitude a million times bigger than the diameter of a black hole enters a black hole, what happens to those parts of the amplitude that are not inside the ...
- 1
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votes
0
answers
40
views
Why does the transverse delta function appear in the definition of the free photon propagator?
I'm working on the coupling of electron and photon kinetics in the non-equilibrium Green's function formalism.
When modeling such a systems, something you encounter are the non-interacting propagators ...
- 1
1
vote
0
answers
34
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Wald: 2-dim Covariant Derivative for Null Hypersurfaces
On pp. 221-222, Wald introduces the 2-dim "hatted" manifold of null geodesics. He moves from 9.2.30 to 9.2.31 and he is allowed to do so because the tensors have the special properties that ...
- 41
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0
answers
30
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How to derive the general Horndeski's action?
I have seen that Horndeski's theory can be written in terms of an action as :
$$
S\left[g_{\mu \nu}, \phi\right]=\int \mathrm{d}^4 x \sqrt{-g}\left[\sum_{i=2}^5 \frac{1}{8 \pi G_{\mathrm{N}}} \mathcal{...
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0
answers
67
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Schrödinger equation is also not exact/breaks down? [closed]
we know for sure that Maxwell's equations are incorrect in detail - the model only works in the classical limit and linearised approximation. Deviations from it are experimentally known.
My question ...
3
votes
1
answer
58
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The relation between $F^2=f_{\mu\nu}f^{\mu\nu}$ and $H^2=H_{\mu\nu\lambda}H^{\mu\nu\lambda}$
there are two papers https://arxiv.org/abs/hep-th/0411105 and https://arxiv.org/abs/hep-th/9406216. What is relation between the $H^2=H_{\mu\nu\lambda}H^{\mu\nu\lambda}$ in the second paper and $F^2=...
- 61
9
votes
2
answers
1k
views
Reverse blood flow in an IV
When the IV drip for a patient is completed, the patient's will flow back.
This is apparently due to the pressure difference.
However, IV needles are inserted into veins in the direction of blood flow....
- 245
2
votes
3
answers
96
views
Shouldn't Electric Potential energy be $U=2kQq/r$?
Let's take 2 charges $Q$ and $q$ a meter apart. Say you want them to collide. You'll have to apply force $F$ on charge $q$ (which I know is changing with distance). I know we integrate this force and ...
- 31
2
votes
0
answers
42
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Deriving the FLRW metric from the cosmological principle?
I was reading some notes (page 10/54 of pdf or page 214 of document) which state:
A more formal approach to the derivation of the spatial FLRW metric
from the cosmological principle starts with the ...
- 1,013
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votes
3
answers
50
views
Is it possible for a single torque to rotate a sphere of uniform density at rest from one arbitrary orientation to another?
If I had an object at rest in some arbitrary rotational position, is it possible to apply a single force to it in order to rotate it to a second rotational position? This would be assuming the object ...
0
votes
1
answer
36
views
Light wave/photon doppler effect
So I understand the explanation/derivation of doppler effect from the perspective of wave crests emission being stretched out as the source moves. But how does this work from the photon point of view?
...
- 141
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votes
0
answers
13
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Sky coverage plot using Python/Astropy
I want to recreate a plot where the area that is not visible from a certain point on earth (latitude and longitude) trough a full year. An example of this plots for example can be:
Where in white is ...
- 479
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votes
2
answers
53
views
Proving that $y$-component of the electric field of a uniformly charged ring is zero without using symmetry
It seems this case is always worked out by means of symmetry in standar Physics textbooks: arguing that for any two opposite segments in the ring, their perpendicular electric field components cancel ...
0
votes
1
answer
31
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Hamilton's Principle extended to deal with non-conservative forces [closed]
I have seen both authors stating that there is an extension of Hamilton's Principle to systems with non-conservative forces, and others stating that there is no variational principle available for ...
- 235
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votes
1
answer
24
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SETI: Are there holes in the EM spectrum that are quiet enough to communicate at decently large distances?
So this is a variant of this other question.
I know stars are big, they radiate a lotta energy, they have spectral lines. But how dense are these spectral lines and is there a noise floor at ...
- 1,600
2
votes
1
answer
106
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Derive 2d wave equation on a membrane: why my result is wrong?
I am trying to derive the 2d wave equation on a membrane. Instead of considering a small $\Delta x\times \Delta y$ rectangle around $x_0$, I consider a small circle with radius $r$ around $x_0$. But I ...
- 123
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0
answers
31
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How does strong nuclear force hold hydrogen vs xenon? does the strong nuclear force have variable distance? [closed]
My understanding is that the strong nuclear force holds the protons and neutrons together as well as the surrounding electrons. give too many electrons and it becomes an unstable isotope due to ...
0
votes
1
answer
50
views
Equating 2 sides of EFE
Can we say if the covariant derivative is zero, the partial derivative is also zero because locally covariant derivative reduces to partial derivatives (since locally spacetime is flat)? Because, in ...
- 715
1
vote
1
answer
38
views
Does the formula $ V =\omega r$ holds in angular frequency
$ V =\omega r$ can I use that formula if $\omega$ is angular frequency instead of angular velocity?
V is velocity
2
votes
0
answers
37
views
QED theta term as pullback of a volume form
The well-known QED theta term is
$S_\theta = \frac{\theta}{4\pi} \int d^4 x F \wedge F $
where $F=dA$ is the field strength 2-form. Altland and Simons (p.547) introduce a more general category of $\...
- 2,397
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votes
0
answers
25
views
Are excited states broader?
Consider the 1D Schroedinger equation with some potential $V(x)$. Let the ground state $\psi_0$ be centered around $x = 0$. If $\psi_1$ is the first excited state, is it true that for all potentials
$$...
- 232
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0
answers
43
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Could Dark Matter have negative pressure?
Is it possible for dark matter to have negative (but negligible) pressure? How small should it be to fit with observations? Dark matter pressure is actually known and measured?
- 77
3
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0
answers
43
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Renormalizability of massless Gross-Neveu theory
We have the following Lagrangian density:
$$ \mathcal{L} = \bar{\psi}_i i \gamma^\mu \partial_\mu \psi_i
+ \frac{g^2}{2} \left( \bar{\psi}_i \psi_i \right)^2 $$
which corresponds to the two-...
- 1,052
3
votes
4
answers
196
views
On the non-quasistatic transfer of heat
Suppose we have two bodies at different temperatures, and we let them interact thermally in such a way that the process is not quasistatic (e.g. two different metal spheres touching). Do we arrive at ...
- 942
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0
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15
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In non-metals, does low specific heat capacity correlates with good thermal conductivity and vice versa?
As i understand, when heat transfers by conduction, there are two main mechanisms that can take place: either by the vibrations of the lattice (mostly non-metals) or with the movement of free ...
0
votes
1
answer
38
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Dependence of Klein-Gordon solution only on spatial coordinates
I am studying QFT with the Peskin and Schroeder textbook, and I am new to this area of physics. I'm struggling with the solution of the Klein-Gordon equation using Fourier integral as a continuum set ...
- 11
1
vote
1
answer
24
views
Which dimensions do self-dual even Lorentzian lattice exists?
I am reading page 499 of Peter West's Introduction to Strings and Branes where he stated 'Self-dual even Lorentzian lattices only exist in dimensions $8n+2$, $n=1,2,...$, the simplest such lattice is ...
- 639
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1
answer
36
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Determining the location of light receivers using signals propagating in anisotropic media
Problem.
I have a set of $16$ light receivers with entirely unknown locations, and a pair of light transmitters with exactly known locations. One light transmitter is stationary, and located near to ...
0
votes
1
answer
58
views
Why should this thermodynamics problem be indeterminate?
Callen asks us to consider the following
A cylinder of length $L$ and cross-sectional area $A$ is divided into two equal-volume chambers by a piston, held at the midpoint of the cylinder by a ...
- 942
2
votes
1
answer
52
views
Why is sequential Stern-Gerlach experiment a complete surprise?
In page 5 of his Modern QM , Sakurai block $S_z-$ from first S-G experiment and sends $S_z+$ atoms to next S-G experimen in $x$-direction. Then sends the result of second experiment to third S-G ...
- 1,169
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votes
0
answers
8
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Viability of generators and electromagnetics based solely on ionic liquids
The background to this question lies in the science fiction novel "The Day the Machines Stopped" by Christopher Anvil (1964), where somehow an experiment gone wrong causes the laws of ...
- 314
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votes
0
answers
40
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Why Tree branches only grow "upwards"? [closed]
Here is a Tree
This is a genuine questions. Sunlight may not be the reason.
Pika-chu!!!
- 7