# All Questions

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### entropy and dispersion

If I know the positions and the speeds of each particle in a box over the time, how can I compute the entropy ? (I`m making a simulation where I want to show that the disperion of the particles over ...
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### Is there any aspect of an explosion resulting from a nuclear weapon test that cannot be simulated using super computers?

This Washington Post news article states that with the advent of computer simulation of nuclear tests, live tests are no longer needed. Generally speaking there are 3 aspects of an explosion ...
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### Is Coulomb's law valid if the electric charge is not constant in time?

If the electric charge is constant the law of Coulomb says that: $E(r) = kQ/r^2$. The question is, if $Q = Q(t)$, can I consider that: $E(r,t) = kQ(t)/r^2$? Update: It appears that the first ...
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### How to determine rotational energy of gas molecules

For an arbitrary polyatomic molecular gas, what is the expected behavior of $U_{rot}(T)$, the rotational kinetic energy of the atoms, in the two limits T high and T low? So far, I have written an ...
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### Is the CMB Cold spot growing or shrinking?

What do we know about the nature of this region of the sky? is that region entirely devoid of galaxies? Could it be a bubble void, and we are simply unable to see the galaxies that are on the ...
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### How can we see the moon while it's between the Earth and the Sun?

I know this sounds like (and probably is) a stupid question, but I can't figure it out. As far as I know, the crescent shape of the moon is when the moon is on the sunny side of the Earth, but that ...
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### Exact (quantitative) definition of “knee” and/or “cut-in” voltage?

Sort of a 2-part question: 1) I know they are used interchangeably but are the "knee" and "cut-in" voltages of a diode actually the exact same thing? I had thought that the "cut-in" voltage was ...
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### Energy release on impact

I was watching this video from a video game showing a space ship travelling towards a planet at 22% the speed of light. The ship system pulls the ship out of this "quantum travel" just in time to ...
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### Which is more mechanically efficient, a bicycle or a unicycle?

Human physiology aside, I wonder which device is more mechanically efficient, a bicycle or a unicycle?
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### Expectation value of two spin 1/2 particles where particle 1 along z axis and particle 2 along another axis?

We are supposed to show that for a two spin 1/2 particles, the expectation value of $\langle S_{z1} S_{n2} \rangle$ is $-\frac{\hbar^2}{4}\cos \theta$ when the system is prepared to be in the singlet ...
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### How to escape freeze-out effects of semiconductors in very low temperature

In very low temperature, semiconductors suffer from freeze-out effects. I did see that degenerately doping eliminates freeze-out, but degenerate semiconductors behave like metals. Is/can there ...
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### Momentum in quantum harmonic oscillator with step up and step down operators [closed]

I'm hitting a wall in my understanding of the momentum operator in a quantum harmonic oscillator. I've showed that $p = (a^\dagger - a)\sqrt{\frac{m w \hbar}{2}}i$ where $a^\dagger$ and $a$ are the ...
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### Temperature of ideal gas in a cylinder

Why is option A and C wrong or correct? My attempted answer : A :A is plausible as if the piston is suddenly moved inwards, the gas molecules bounce off the piston at higher speeds Since Temp ...
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### Direction of Resultant velocity

So we know that an object undergoing horizontal circular motion, has a angular velocity with direction perpendicular to the plane of motion, and also a linear velocity tangential to the circular path. ...
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### Why did the big bang happen when it did?

Was there time before the big bang? I assume there wasn't, so is it even logical to ask if there was anything before the big bang? If there was time before the beginning of the universe, what happened ...
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### Total reflection in a solar collector?

So, I was wondering how radiation is kept inside a solar thermal collector? The sun light (or its photons) penetrates through the glass and is then absorbed by the absorber layer. In turn, the ...
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### Photon behaviour [duplicate]

What happens when a photon hits a mirror? Does the photon decelerate instantaneously to zero and then accelerates instantaneously to the velocity of light when it is reflected from the mirror? How do ...
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### Bending moment in composite beam with internal “active” stress

I am looking at a problem of a bending composite beam with a symmetric cross -section, $A(s)$. Above the neutral axis $s$ is material $U$ and below the neutral axis is material $L$. $U$ has an ...
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### What happens to the wave function of a particle immediately after measuring its energy?

For this question, I will be adhering to the Copenhagen interpretation (since that's what I've learned in university so far). For the sake of brevity/clarity, also, assume the Hamiltonian here has ...
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