# All Questions

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### How do electrons get a charge?

Electrons belong to a group of elementary particles called leptons. There are charged and neutral leptons. And electron is the charged one. But how come it got charged? The negative or positive ...
17 views

### Magnetic field direction, Lorentz force direction?

I know that the direction of current flow can change the direction of Lorentz force, what about the direction of the magnetic field? Also, for induced current due to electromagnetic induction is the ...
40 views

### How can electric field representation be obtained from Enge representation using Maxwell's equations?

Suppose we have a long electric capacitor. Let $L$ be its length ($z$ coordinate), $W$ its width ($y$ coordinate), and $D$ its full height (full aperture; $x$ coordinate). Let $L\gg W\gg D$. The ...
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### Running Euler's disk in a superfluid

I was considering the toy Euler's Disk, a video can be found here: http://www.youtube.com/watch?v=mVl2CBG_h2s I was interested in understanding the behavior of the disk particularly in vacuum and in ...
588 views

### What was the entropy of the universe at the time of the Big Bang?

(I asked this question in Philosophy.SE; but I was advised to direct it here, despite it is, in my opinion, somewhat too speculative for physics.SE). High entropy generally means high disorder; and ...
456 views

### Why does weak equivalence principle say gravity is equivalent to acceleration?

I am told that the weak equivalent principle, that $m_i=m_g$ (inertial and gravitational masses are equivalent) is equivalent to the statement that in a small system you can't tell whether you are in ...
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### Can a spacetime solution in GR have no Killing vector fields?

Sometimes Killing vector fields in a given spacetime are described as giving information about a symmetry of that particular spacetime solution. If I look at the requirement of a Killing vector field ...
266 views

### Geodesic equation from Euler - Lagrange

There are several ways to derive the geodesic equation. One of which is the variational method which I seemed to understand it because it was written in great details. Then it was mentioned that the ...
25 views

### Is there a material that could “convert” near infrared frequency to infrared?

I have a laser with a wavelength of 650 nm (visible red light) and was wondering if there is some sort of material that could be used to absorb and disperse a different frequency of light around (900 ...
117 views

### Is WKB really applicable for the ground state?

It is a long time question for me. For me, it seems that WKB is applicable for a given $E$ if and only if $\hbar$ is sufficiently small. Or in other words, WKB is applicable if and only if the ...
25 views

### Making water spin a wheel [on hold]

Let's say that I have a wheel with 8 symmetric arranged rectangular plates of area $A$ and the water, with density $\rho$, in the river it moves with $v\: \mathrm{m/s}$. How do I calculate the ...
12 views

### Do two engines with the same specific impulse always perform the same amount of work?

As the title say, do engines with different thrusts with the same $I_{sp}$ and the same amount of fuel perform the same amount of work?
14 views

### Filtering UV Light to Visible Light [on hold]

I'm wondering if it is physically possible to filter UV light from 400nm to 100nm so that UV light is in the visible range. If we had a UV screen over a visible range screen, could there be two ...
18 views

### Pion-Pion One-Loop Scattering Amplitude? [on hold]

What is the one-loop pion-pion scattering amplitude (with finite pion mass)?
20 views

### What is a good (e-)book on flight dynamics, guidance, and navigation? [on hold]

I want to buy a detailed theoretical foundation book on flight dynamics, particularly geared towards navigation. Is there any solid comprehensive material?
26 views

### Non-Darcy Flow : Forchheimer plotting method

Recalling Darcy's equation for gas flow, assuming ($z\mu_g$) = Constant $$\tag{1} q_{g,sc}=\frac{kA}{L}\left(\frac{T_{sc}}{Tp_{sc}}\right)\left(\frac{1}{2z\mu_g}\right)(p_1^2-p_2^2)$$ For ...
175 views

### Fluctuations of an interface with hammock potential

This question is related to that one. I ask it here since comments are too short for the extended discussion that was going on there. I am interested in a very simple interface model. To each ...
46 views

### Does inducing charge on a grounded conductor do work?

Imagine a grounded conductor as an plane of infinite area. If I bring a charge to a close proximity to the plane, it induces some charge distribution on the conductor. The question is: is any work ...
71 views

### Can you die from a $250 \, \mathrm V$ broken cable? [on hold]

I have a laptop power cable that is broken, my dad taped it so it is safe now. I am just wondering: would it have killed me had I touched it? If you could state your qualification/job to show you ...
257 views

### Scaling with the Ising Model

I am stuck with one formula in the CFT book by Di Francesco and al. Chapter 3. Equation 3.46 third step, for those who don't have the book, he integrates out degrees of freedom from the Ising Model by ...
204 views

### Energy difference between symmetric and antisymmetric wavefunctions [on hold]

Is there any energy difference between a particle in a symmetric wavefunction and an identical particle in an identical potential but in a state with an anti-symmetric wavefunction? Or is it ...
38 views

### Does polarization remove any of the fields completely?

A light is made up of electric and magnetic fields oriented perpendicular to each other. Can polarization lead to the removal of any of the fields completely?
2k views

### Why don't we have a theory of everything?

What is currently stopping us from having a theory of everything? i.e. what mathematical barriers, or others, are stopping us from unifying GR and QM? I have read that string theory is a means to ...
1k views

### Can internal forces do work?

My Mechanics textbook claims that the sum of the work by internal forces is not generally zero. translated to English the paragraph reads: Notice about the work by internal forces: the work by ...
20 views

### What type of glass is used in oven glass doors? [on hold]

I understand that the glass used in convection oven door must be able to withstand high heat. But what should be the exact specification if I am thinking of replacing my broken oven door and ...
75 views

### What's the difference between frequency domain and time domain spectra?

If I have a mechanical oscillator and want to observe the dynamical behavior of the oscillator, is there any additional information to observe it in time domain and frequency domain? Normally, we ...
330 views

### Plants and Quantum Mechanics!

So, I have been working on quantum biology and found something interesting that I would like to write an equation for: Scientists have wondered how plants have such a high efficiency in ...
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### Electron/positron annihilation lines in astrophysics

I have a reasonable understanding of electron/positron annihilation, in that it is a collision between a pair of particles, one matter and one antimatter, that generally produces gamma radiation. ...
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### Interference of sound: how to calculate the sound intensity at each point?

I need help with the following question: Two speakers are sending out sound in phase with the frequency 2,13 kHz. The sound from the speakers is registered in three points A, B and C. See figure. ...