# All Questions

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### Photon entering medium

The speed of light in any medium besides vacuum is smaller than $c$. In a classical way, I just look at that as a wave that propagates less fast, the change in EM-field is passed on slower. But how ...
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### Is the Legendre transformation a unique choice in analytical mechanics?

Consider a Lagrangian $L(q_i, \dot{q_i}, t) = T - V$, for kinetic energy $T$ and generalized potential $V$, on a set of $n$ independent generalized coordinates $\{q_i\}$. Assuming the system is ...
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### How do the renormalization enter the actual amplitude calculation in QFT?

I have studied QFT from Peskin and Schroeder and from a few other books and lectures and I think I understand the procedure of renormalizing various parameters in the Lagrangian like mass, coupling ...
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### Mass and wave function renormalization In chiral perturbation theory

Before I put forward my actual question, I think it will be useful to set the context in a clear way and that involves my understanding of a few very basic things of Chiral Perturbation Theory. ...
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### Refrigeration and thermal science

What happens to the weight of the vegetable after keeping it in fridge for few hours? Does it gain the weight or lose the weight?
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### Fillings of dispersion bands (E-K diagram)

I struggle in understanding why in some references the bands filling by electrons in the E-k diagram is shown as an area delimited below by the dispersion curve and above by the Fermi energy (if in ...
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### Assuming the possibility that a ship could reach $c$, how long would it take? [on hold]

A friend wants to know how long at 5gs it would take to reach the speed of light.
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### Wave equation on Schwarzschild background

I am trying to follow the solution of the wave equation for a scalar field on Schwarzschild background from http://batteringram.org/science/gr/scalar_wave.pdf. I have a problem on page 2 where they ...
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### Do quasars exist today?

Are there likely to be any quasars right now, or are all the ones we're seeing old galaxies today, like the Milky Way?
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### Kinematic of linear motion [on hold]

Water flows out horizontalmy at the end of a pipe at a height of 52 cm from the floor.If the horizontal distance before it hits the floor is 100 cm,calculate the velocity of the water at the instant ...
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### Sounds of supersonic objects

Would you, as a stationary observer, hear a sound in reverse when the source of the sound travels with twice the speed of sound? Of course, he wouldn't hear anything at all before the airplane passed ...
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### How can a singularity have infinite density? [duplicate]

I've heard many times that black holes have infinite density at their center, but I've also heard astronomers quote mass estimates for black holes. How could the singularity have infinite density if ...
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### Drag in low earth orbit

So I'm looking at the following problem; Arrow type configuration in orbit. Each end of the arrow has a mass and a cross sectional area (we can assume the head and tail are connected by a ...
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### The fate of Poincaré recurrence with the Big Rip

Recently, there has been a lot of talk in the media about the "Big Rip". It most certainly resulted from the paper by Marcelo M. Disconzi and Thomas W. Kephart where they have figured out a ...
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By considering the superposition of two waves propagating through a string, one representing the original or incident wave and the other representing the wave reflected at the fixed end, if both ends ...
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### Projectile - air drag problem! [on hold]

I know it's not a homework help site, but still, I apace proceeded in this problem, but cannot go further. A body of mass m us thrown at an angle $\alpha$ to the horizontal plane with an ...
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### dependence of dipole moment on charge of dipole

What is the change in the dipole momemt of dipole on reducing its charge by half.Is there any change or not because from expression it becomes half but from understanding on reducing the charges the ...
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### Scalar field in Reissner-Nordstorm metric

I am trying to solve this kind of problem: For a scalar field $\psi$ in Reissner-Nordstorm metric \begin{align} g_{ab} dx^{a} dx^{b} = - f dt^{2} + f^{-1} dr^{2} + r^{2} d \Omega_{S^{2}}^{2} ...
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### Why do night sky appear red when it is cloudy?

Since childhood, I have seen that whenever at night, the sky is cloudy, the sky becomes red. But, clouds are grayish, so it is expected that the sky becomes more darker. But no, it is not! Why does ...
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### How fast can a fluorescent lamp flicker

Using electronic ballasts, the current frequency is boosted up to 60 kHz in some models. Does the fluorescent lamp continue to flicker at that frequency or does it produce continuous light? In this ...
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### Is the Energy of an absorbed photon exactly the energy of the band gap?

I was wondering, if the Energy of a Photon which is absorbed by an Electron, hast to be exactly the Energy of the bound gap. So if i have two energy levels in an atom $E_2$ and $E_1$, does my ...
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### Determining path of light through a nanolens

In the case of light passing through stacked liquids, we can use a ray approximation of the path of light to determine the path taken through each layer of liquid using Snell's law. From all the ...
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### Simple Harmonic Motion in Special Relativity

I was trying to see what results I would get if I were to incorporate relativistic corrections into the case of a harmonic oscillator in one dimension. I thought that if the maximum velocity of the ...
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### How do we deal with air resistance?

Suppose a body of mass $m$ is projected and given that the air resistance is $kv$, where $v$ is the instantaneous velocity. I need to find the maximum height. I could do it given that the air ...
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### Interpolation formula for BCS superconducting gap

In BCS theory, the superconducting gap is given by solving at different temperatures the integral ...
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### Material research for fuel cells - what are the fundamental concepts/ topics one has to learn?

As I am looking forward to start a research career in the material research for fuel cells, such as carbon (Graphene) & non-precious metals based catalysts for fuel cells etc.. But I am a ...
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### Parity of $n$-photon system

The $C$-parity (charge conjugation) of an $n$-photon system is given by $(-1)^n$. If I'm not totally wrong, the intrinsic parity of a photon is $(-1)$. What is the parity $P$ of a system of $n$ ...
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### Optimal wavelength for low IR attenuation under normal atmospheric conditions?

I can't find any good data about the transmit attenuation of infrared light in the atmosphere transmitted horizontally (there is a bunch of stuff detailing penetration of radiation from space, but ...
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### Is this a case of the “venturi effect”? What's behind the “Venturi effect” actually?

Some dinghy sailboats have a hole in the bottom of their deck used to automatically bail out water after a capsize. bailer of the laser sailoat The mechanism only works if the boat is moving. The ...
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### Mathematician learning theoretical physics [duplicate]

EDIT: I was aware of the supposed duplicate. But I'm interested in a clear and focused path through the basics to advanced theoretical physics such as string theory - a path that avoids studying ...
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### Should the weaker of two air conditions be switched off?

We have two airconditioning systems in the office. For the sake of this question, let us assume the following current conditions: AC1 (old) outputs air of 26°C AC2 (new) outputs air of 18°C Office ...
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### Is the Klein-Gordon Hamiltonian unbounded below?

This question is about the Klein-Gordon Hamiltonian for simplicity, but the problem seems to remain when dealing with other fields (e.g. Dirac, photon...). One usually writes the Hamiltonian ...
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### Is internal resistance relevant in motional EMF?

When a conductor passes a magnetic field and connected to a circuit, the induced voltage is calculated via the motional EMF($\epsilon$): $$\epsilon=-vBL$$ Is the conductor's resistance (or internal ...
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### Why is universe expanding?

Okay, this question may sound silly: base on the observation Besides an expanding universe, would there be other possibilities? Would it be possible, say, there exists a fundamental repelling ...
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### How are charges formed in clouds during lightning?

How are charges formed in clouds that are responsible for lightning?
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### What's the difference between wave equation in PDE form and wave equation in normal form?

What's the difference between "wave equation in partial derivative form" and "wave equation in y(x,t) form" ? Are they both same? And why "wave equation in in y(x,t) form" is the solution of "wave ...
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### Matter originated out of space? [on hold]

Is it possible that matter has originated through space out of imbalance of space? If yes then is this event same as big bang? Also is it possible that there are multiple non-interfering big bangs? ...
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### Dispersion in crystall for a random direction

Lets say one get a dispersion of electrons or phonons from this website: http://www.matprop.ru Usually dispersion in particular directions is drawn: as for wurtzite crystall it is A to L, L to U and ...
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### Can the math for physics be expressed without any uncountable sets at all?

I am wondering about this and have wondered about it for a short while. Usually physics is modeled using things based off of the Real Number Line $\mathbb{R}$, which is uncountable. (E.g. we may use ...
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### Difference between Enzo & Gadget astronomy simulation codes

Enzo and Gadget are simulation codes used in astronomy. What are the largest differences between them both in terms of physics they simulate and in their implementations?
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### Buying books that are out of print [migrated]

I've just returned the book Principles of Modern Thermodynamics, by Bimalendu Narayan Roy* to the library. It is a terrific book which gave me a much greater understanding of thermodynamics than I ...

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