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1answer
57 views

Is the stress finite at the centre of a spherical continuous charge distribution?

At the centre of a spherical continuous charge distribution with no external electric fields, the electric field is zero from symmetry arguments. But does the stress at the centre remain finite?
2
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0answers
550 views

fun computational physics projects [closed]

I want to learn how to code in Java and am in need of some cool computational physics projects that would provide motivation to do so. I was wondering if you guys could list some medium-level ...
13
votes
2answers
562 views

Can $10^{23}$ stars be treated with methods of statistical mechanics?

Statistical mechanics is used to describe systems with large number of particles ~$10^{23}$. The observable universe contains between $10^{22}$ to $10^{24}$ stars. Can we treat those many stars as a ...
3
votes
2answers
590 views

How can Hilbert spaces be used to study the harmonics of vibrating strings?

The overtones of a vibrating string. These are eigenfunctions of an associated Sturm–Liouville problem. The eigenvalues 1,1/2,1/3,… form the (musical) harmonic series. How can Hilbert spaces be ...
2
votes
1answer
3k views

How to calculate Temperature Humidity Wind Index?

I would like to know how to calculate Temperature Humidity Wind Index (THW Index)? I know how to calculate Heat Index and Wind Chill. I am asking this because my weather station Davis Vantage Pro2 ...
10
votes
3answers
862 views

Is hydrogen the same everywhere?

Silly thought. Feel free to shoot it down Does a hydrogen atom undergo any kind of change subject to it's environment? If one were to study a hydrogen atom on the surface of Mercury, another above ...
1
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3answers
872 views

Should the term Watt's Law be used for $P = IV$?

I'm revising some electrical curriculum for a technical training program. In the curriculum students have to calculate values using Ohm's law and the equation $$\text{Power = Current * Voltage}$$ ...
2
votes
2answers
4k views

Doping of pentavalent atom with hexavalent atom

We know that a tetra valent element can be a semiconductor and doping of pentavalent and trivalent impurities would give a extrinsic semiconductor. I got a question now. Can we use an element of ...
10
votes
2answers
5k views

What happened to Apollo's Saturn-third-stage rockets?

I read recently the original Apollo 11 press release and it mentions that the Saturn V's third stage (used for Trans-Lunar Injection) was deployed into a solar orbit of some kind: (Source: Press Kit -...
0
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4answers
2k views

What isotope has the shortest half life?

Question: What isotope has the shortest half life?
1
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0answers
573 views

How do I calculate the Radiation length of a Molecule

I want to calculate the Radiation length in a Molecule with the Formula given on wikipedia. How do I calculate Z and A for a molecule to put it into the Formula?
13
votes
8answers
14k views

Is it theoretically possible to reach $0$ Kelvin?

I'm having a discussion with someone. I said that it is -even theoretically- impossible to reach $0$ K, because that would imply that all molecules in the substance would stand perfectly still. He ...
3
votes
1answer
461 views

Why do strings split in string theory?

In string theory, we are told strings can split and merge if the string coupling is nonzero, even while the worldsheet action remains Nambu-Goto or Polyakov plus a topological term. However, a ...
2
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2answers
2k views

Why does tension not do work in this pulley system? etc

I have a slight difficulty understanding the solution to the following problem: A light inextensible string with a mass $M$ at one end passes over a pulley at a distance $a$ from a vertically fixed ...
16
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3answers
7k views

PhD Research Areas in String Theory [closed]

I'm thinking of applying to do a PhD in String Theory, starting in September 2013. I'm gradually learning more about the subject through external reading, but still most papers are impenetrable! Could ...
2
votes
1answer
284 views

What role does memory robots play in the many worlds interpretation?

Ron Maimon gave a very interesting answer to a question about the many worlds interpretation here. He mentions the essential role of memory robots in MWI, which is something I have never ever heard ...
2
votes
1answer
2k views

How do I solve these integrals of wave function and operator?

First integral $$\int \Psi^*({\bf r},t)\hat {\bf p} \Psi({\bf r},t)\, d^3r,$$ where the $\Psi({\bf r},t)=e^{i({\bf k}\cdot{\bf r}-\omega t)}\,\,\,$ and $\hat {\bf p}=-i\hbar \nabla$. Second one $$\...
1
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2answers
66 views

Is it feasible for an unmanned vehicle to travel from outside the atmosphere of one planet to another without additional propulsion?

Within the Solar System (or any other system for that matter) - Is it feasible for an unmanned vehicle to travel from outside the atmosphere of one planet to another without additional propulsion? ...
3
votes
1answer
4k views

The trajectory of a projectile launched from a hilltop

Here is the problem: A boy stands at the peak of a hill which slopes downward uniformly at angle $\phi$. At what angle $\theta$ from the horizontal should he throw a rock so that it has the greatest ...
16
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4answers
841 views

Can physics get rid of the continuum?

Almost every physical equation I can think of (even though I don't actually feel comfortable beyond the scope of classical mechanics and macroscopic thermodynamics, as that's enough for dealing with ...
4
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2answers
1k views

Need help understanding dynamic and static contact angles

I've run into a conceptual road block. I'm coming to you guys because I think my adviser is getting annoyed with me. The concept involves a meniscus being pulled up a cylinder. I understand that ...
6
votes
2answers
332 views

Partition Function for the Jaynes-Cummings Hamiltonian

I was wondering if anyone here has calculated before the partition function for the Jaynes-Cummings Hamiltonian: $H_{JC}=\omega_0 (a^\dagger a + \sigma^+ \sigma^-)+g (a \sigma^+ +a^\dagger \sigma^-)$ (...
2
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2answers
2k views

Calculating the time of dawn

Knowing that astronomical twilight (i.e. astronomical dawn) is when the sun is 18 degrees below the horizon, I am calculating the astronomical twilight time this way: ...
2
votes
1answer
337 views

Is Connes model a composite Higgs in disguise?

Most of the 5-dimensional Higgs models can be seen, if I understand correctly, as models where the Higgs is a composite. Now, is this true for Connes models? It is a model of extra dimensions too, ...
8
votes
3answers
4k views

Why does thermal resistance go down as temperature goes up?

Here is the thermal resistance data for three speaker coils disengaged from the speaker cone. Any ideas? I would think it would be a horizontal line. http://en.wikipedia.org/wiki/Thermal_resistance ...
3
votes
1answer
1k views

Wilson loops and gauge invariant operators (Part 1)

I guess the Hilbert space of the theory is precisely the space of all gauge invariant operators (mod equations of motion..as pointed out in the answers) Is it possible that in a gauge theory the ...
4
votes
3answers
158 views

uncertainty of fields with many harmonic modes

In most basic level introduction to the quantum harmonic oscillator formulation of fields, it is assumed that the commuting variables for the fields $p_m$, $q_m$ are $$ \lbrack p_m , q_n \rbrack = \...
2
votes
1answer
108 views

In electro-optic material, what is happening to the structure of the material for the index of refraction to change?

I apologize if electro-optic material is not the correct word. As I understand it, when an electric field is applied to an electro-optic material, the index of refraction changes in proportion to the ...
3
votes
5answers
3k views

motion in the body-fixed frame?

This is really basic, I'm sure: For rigid body motion, Euler's equations refer to $L_i$ and $\omega_i$ as measured in the fixed-body frame. But that frame is just that: fixed in the body. So how ...
6
votes
1answer
461 views

Taking the continuum limit of $U(N)$ gauge theories

I would like to draw your attention to appendix $C$ on page 38 of this paper. The equation $C.2$ there seems to be evaluating the sum $\sum_R \chi _R (U^m)$ in equation 3.16 of this paper. I ...
2
votes
1answer
171 views

Where is the critical moment where the microcanonical ensemble enters the justification for the equilibium state?

As explained in many books, for the microscopic justification of the second law of thermodynamics (lets formulate it as the total entropy takes maximum among all possible exchanges of two systems), ...
2
votes
2answers
4k views

Why is Gravitational force proportional to the masses?.

We know that two mass particles attract each other with a force $$F~=~\frac{G M_1 M_2}{r^2}.$$ But what is the reason behind that? Why does this happen?
7
votes
3answers
506 views

Poisson structure comes from hamiltonian?

I am interested in studying quantization, but it seems I am lacking the basics of classical mechanics. Any help would be appreciated. I would first like to ask what is necessary to have a sympletic/...
2
votes
2answers
397 views

Average Neighbouring Impurity Separation in a Random 1D chain [closed]

I have a finite and discrete 1D chain (edit: linear chain, i.e. a straight line) of atoms, with unit separation, with a set number of impurities randomly distributed in the place of these atoms in the ...
2
votes
1answer
1k views

Gas Cylinder Life

Say I have a gas cylinder filled with nitrogen to some volume at some pressure. On that cylinder I have a regulator so that I get a steady pressure out. I wish to discharge the cylinder. How many ...
4
votes
1answer
565 views

Conjugate Transpose of Hamiltonian Matrix

I read some notes saying, $$i\hbar \frac{dC_{i}(t)}{dt} = \sum_{j}^{} H_{ij}(t)C_{j}(t)\tag{1}$$ where $C_{i}(t) = \langle i|\psi(t)\rangle$ and $H_{ij}$ is hamiltonian matrix. However, what is ...
1
vote
2answers
909 views

Inner product of four-vectors in special relativity

Reference) "Feynman lectures on Physics Vol.3 , p.7-4 ." With four vectors $x_{\mu} = (t,x,y,z)\ , \ p_{\mu} = (E,p_{x},p_{y},p_{z})$ the inner product of these two four vectors is scalar invariant ...
4
votes
3answers
1k views

Probability vs. degree of belief in facts of nature (“Plausibility”)

I just came across a line in a paper: "Assume the probability that a Lagrangian parameter lies between $a$ and $a + da$ is $dP(a) = $ [...]." This reminded me again of my single biggest qualm I have ...
7
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2answers
265 views

Under what assumptions can we split a Hilbert space into subspaces?

I was thinking about an apparently simple question about quantum mechanics, if I am looking at a quantum system described by a Hilbert space $\cal{H}$ under what hypothesis can I define A and B as ...
3
votes
0answers
86 views

A problem concerning the change of temperature and spectrum of a filament

The spectrum of a filament has been given before, the left one having the lowest temperature, the middle with a medium temperature and the right one with the highest. My question is this: Why does the ...
2
votes
2answers
215 views

Charges of quarks and leptons

Are there any theoretical restrictions within the framework of QFT that fix the relative sign between charged leptons and up-type quarks? We know that in our universe, they have opposite signs -- ...
1
vote
1answer
513 views

Why does the water-pressure of my shower fall if I hold the shower head high?

I live in a very old house - build 1902 - in the 4th floor in the city of Karlsruhe (Germany). I have a shower and the gas-heater for the water is in it: If I hold the shower head over some height (...
10
votes
5answers
3k views

What does symplecticity imply?

Symplectic systems are a common object of studies in classical physics and nonlinearity sciences. At first I assumed it was just another way of saying Hamiltonian, but I also heard it in the context ...
0
votes
1answer
1k views

How do Anamorphic images work? [closed]

I found the following anamorphic image on this site Can anyone explain the physics behind the working of such images? Can we create our own anamorphic images? If so how? Thanks in advance.
4
votes
1answer
3k views

Finding the tension in rope tied to ladder using the principle of virtual work

A ladder $AB$ of mass $m$ has its ends on a smooth wall and floor (see figure). The foot of the ladder is tied by an inextensible rope of negligible mass to the base $C$ of the wall so the ladder ...
2
votes
1answer
907 views

Why is the crust of my meat pie only slightly warm?

I just cooked a meat pie in the oven. Almost immediately after pulling it out of the oven, I felt that the crust was only slightly warm, but when I cut it open the filling felt very warm. I can't ...
3
votes
2answers
403 views

Can a free hydrogen atom constitute ionizing radiation?

Radiation is basically just particles flying around, right? Are free hydrogen atoms just typically not moving fast enough to be considered "radiation"?
4
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1answer
2k views

Combining Proportions to get Newton's Law of Universal Gravitation

I've read a little on the history of Newton's Law of Gravitation and noticed that the formula can be separated into 3 distinct parts that lead to the end result of $F_g = G \frac{m_1 m_2}{r^2}$; the ...
4
votes
1answer
155 views

What Cat States of light have been experimentally produced?

I'm specifically looking for Schrödinger's Cat states involving superpositions of two, or if it's been done more, coherent states, i.e. monomodal states of the form $$|\psi\rangle=a|\alpha\rangle+b|\...
7
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3answers
861 views

Can one make a ball rotate around a vertical axis using only a combination of horizontal axis rotations?

This is a nice problem that I would like to share. Problem: In a public garden, there a statue consisting of a spherical stone and a stone cup. The ball is 1 meter in diameter and weighs at least a ...

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