# All Questions

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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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### equation related to room temperature and air conditioner temperature and flow rate? [on hold]

i want to find and equation that related the air room temperature and air conditioner temperature and flow rate. i want to model a cooling system. the input to the room are air conditioner supply and ...
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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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### Confirming transformation to polar coordinates is canonical [on hold]

This is exercise 2.7.5 in Shankar's Principles of Quantum Mechanics which I am working through myself. I understand that for the transformation to polar coordinates we have ...
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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

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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### Conflicts between Bernoulli's Equation and Momentum Conservation?

The well known Bernoulli's equation states that $P+\frac{\rho V^2}{2}=c$ However, a simple momentum conservation considering $P_1$ and $P_2$ acting on two sides, and velocity changes from $V_1$ to ...
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### Curl of the gradient [on hold]

The work is defined by the equation: W = 2x ^ 2 y + sin4x. Determine the curl of the gradient of the work and comment on the result.

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