1
vote
2answers
183 views

Does gravity exert more “power” when an object is traveling faster?

So, this question arose when I was pondering the meaning of horsepower and torque in cars. I thought of the following question: There is a 1 kilogram weight on planet M. Planet M has no atmosphere ...
4
votes
1answer
157 views

Entanglement entropy of 1D chiral Fermion

I was told that the entanglement entropy $S_E$ on the ground state of a (1+1)D conformal field theory (CFT) follows the logarithmic behavior $S_E=\frac{c}{12}\ln L$ where $L$ is the length scale ...
3
votes
4answers
446 views

General relativity in terms of differential forms

Is there a formulation of general relativity in terms of differential forms instead of tensors with indexs and subindexs? If yes, where can I find it and what are the advantages of each method? If ...
0
votes
1answer
110 views

is this heat calculation equation correct?

If I have a line of copper wire (lets say $\textrm{1 meter}$ long, $\textrm{1 mm}$ thick) and one end is a flattened disk of copper about the size of a quarter, and I apply a lot of heat to it (I'm ...
-1
votes
1answer
117 views

How is the current for the Dirac equation derived?

Why is it that the derivative of the current $j^\mu$ is the difference between the Dirac equation and its adjoint?
2
votes
1answer
60 views

Why would the environmental lapse rate be slower near the ground?

I have data taken from over 2300 ascents through the atmosphere at various locations around the world, for every 500 meters vertically upwards I have averaged the temperature and plotted it: Its ...
4
votes
1answer
87 views

Vacuum polarization of dark matter candidates

Can a dark matter particle (axions, for instance, but the question applies to any valid candidate for dark matter) induce QED vacuum polarization effects even if being electrically neutral and with no ...
1
vote
0answers
83 views

Time-Independent Gravitational Equation?

Is it possible to calculate gravitational induced position change without requiring the use of time (and therefore, acceleration) anywhere in the equation? If such an equation were to be discovered, ...
3
votes
1answer
78 views

Noether's theorem for more interesting transformations of the time co-ordinate

According to Wikipedia, Noether's theorem (for the mechanics of a point particle) says that if the following transformation is a symmetry of the Lagrangian $$t \to t + \epsilon T$$ $$q \to q + ...
4
votes
0answers
30 views

What groups of symmetry are most suited for filling uniformely a spherical 3D space, whilst possessing the lowest possible surface-to-volume ratio?

I am looking for the closest known approximate solution to Kelvin foams problem that would obey a spherical symmetry. One alternative way of formulating it: I am looking for an equivalent of ...
1
vote
1answer
318 views

How is NASA's mod II Stirling engine so powerful yet so small?

Is it because of the temperature difference? I just don't understand how it can propel a car. Here's the link to the engine: ...
0
votes
1answer
302 views

How effectively does heat flow through copper wire?

If I have a line of copper wire (lets say 1 meter long, 1mm thick) and one end is a flattened disk of copper about the size of a quarter, and I apply a lot of heat to it (I'm talking 800 Celsius) will ...
0
votes
1answer
28 views

Is using water in a charcoal smoker less efficient than not using water?

I have a charcoal smoker that uses water. My understanding is that the water serves as a buffer and as a way to add moisture to the cooking environment. Some say that using water wastes fuel because ...
2
votes
1answer
115 views

Normalization for QFT single particle destruction operator

I don't understand a particular statement in the QFT book by Klauber. The particular page I'm having difficulty on is page 67 of chapter 3 (PDF link). The big picture is that the author wishes to ...
2
votes
0answers
90 views

Regulating an infinite sum in the $bc$ CFT

The EM tensor of the $bc$-CFT is $$ T(z) = \colon \partial b c \colon - \lambda \partial \colon b c \colon $$ After expanding in a mode expansion, we find $$ T(z) = \sum_{m} \frac{1}{z^{m+2}} \sum_n ...
2
votes
1answer
153 views

Large deflection of cantilever beam

How can I find the amount of point force at the end of a cantilever plastic beam that produces e.g. 45deg slop at the end of the beam? Is this the right equation: $$F=\frac{2EI\theta ^2}{L^2 ...
1
vote
1answer
62 views

Question about the refraction of Fresnel lenses

If you line up the suns rays parallel to a Fresnel lens, the light is concentrated, and the focus directly underneath. However, what happens if the sun is off to the side, making the light hit at an ...
0
votes
2answers
567 views

Friction on an object moving with momentum over a surface

I'm familiar with the equations for friction for a static object and an object moving at steady speed over a surface from high school physics. But we never learned how an object moving only due to ...
3
votes
1answer
190 views

Does the Casimir Effect violate the Quantum Inequalities?

Does the Casimir Effect violate the Quantum Inequalities? From what I understand, the Casimir Effect is able to produce negative energy densities for an indefinite amount of time, or is that wrong?
2
votes
0answers
69 views

A question about polarization in quantum mechanics

We start our question we a definition A subbundle $P\subset TM^{\mathbf{C}}$ of the complexified tangent bundle is called a complex polarization if \ $P$ is Lagrangian P involutive dim$P\cap\bar ...
1
vote
1answer
60 views

Photon number conservation during scattering

I was reading this writeup on the Kompaneets equation and the Sunyaev-Zel'dovich effect. On page 3, section 2 the author states There is no way to increase the mean energy of a planckian ...
1
vote
1answer
160 views

Does the temperature at which water evaporates influence the amount of chalk left behind?

I believe the title is straightforward. Evaporating water leaves behind chalk. Assuming you evaporate the same amount of (the same type of) water, does the temperature at which this happens (and so ...
1
vote
3answers
203 views

pressure in fluid mechanics of incompressible liquid

Most liquids can be approximated to be incompressible, since the Mach-number is much smaller than 1. That means that the density variations are negligible and from the relation between pressure p and ...
1
vote
1answer
151 views

Loop corrections to propagator (QFT of Srednicki)

Perhaps this is a very basic question. In chapter 14 of Srednicki's QFT textbook (2007), $O(g^2)$ loop corrections to the propagator of $\phi^3$ theory is discussed. However, I don't know how to ...
0
votes
1answer
62 views

Second law of thermodynamics and motion

The second law of thermodynamics states that the entropy of an isolated system never decreases, because isolated systems spontaneously evolve toward thermodynamic equilibrium—the state of ...
0
votes
2answers
83 views

How do I know if an object comes from the Solar System or not?

I'm told that if the total energy of an object near the Sun is negative, then it comes from the Solar System; if positive, it is extrasolar. I don't understand why. Can someone please explain this?
3
votes
1answer
235 views

Is the spin-rotation symmetry of Kitaev model $D_2$ or $Q_8$?

It is known that the Kitaev Hamiltonian and its spin-liquid ground state both break the $SU(2)$ spin-rotation symmetry. So what's the spin-rotation-symmetry group for the Kitaev model? It's obvious ...
3
votes
1answer
2k views

How do I predict volume loss due to evaporation when boiling water?

Suppose I have a pot with diameter $D$ containing a volume of water $V$, being heated by a flame under it. If the ambient air temperature is $T$ and relative humidity is $R$, how can I calculate the ...
2
votes
2answers
270 views

Noether's theorem and time-dependent Lagrangians

Noether's theorem says that if the following transformation is a symmetry of the Lagrangian $t \to t + \epsilon T$ $q \to q + \epsilon Q$ Then the following quantity is conserved $\left( ...
0
votes
1answer
124 views

Why will an accelerated electron emit a photon? [duplicate]

Why will an accelerated electron (or a charge) emit a photon? For example normal matter even if it accelerate or not will emit a photon.
2
votes
0answers
37 views

Is the real scalar remained real after analytical continuation in imaginary time formulation?

I am studying thermal field theory. I am being confused while studying analytical continuation of time coordinate made in the real scalar field theory at finite temperature. Apparently it occurs to me ...
0
votes
2answers
118 views

Debye temperature for the steel?

I'm looking for data on Debye temperature of steel (ideally with a known carbon concentration, structure, and set of phases), but find only data on elements. Do You happen to meet the data in papers, ...
0
votes
2answers
486 views

Why would an electron in an orbit be accelerating continuously and would thus radiate away its energy and fall into the nucleus in a classical model? [duplicate]

I was reading this answer by madame anna v: You are right, the planetary model of the atom does not make sense when one considers the electromagnetic forces involved. The electron in an orbit is ...
1
vote
2answers
2k views

Realistic calculation of heat loss for pipe

Good day everyone, I am new on this site and I hope to find here help, since I am not going anywhere with the literature I have found. I try to calculate realistically the heat loss of a hot, ...
0
votes
1answer
104 views

How to calculate cost of AA accumulator charge? [closed]

I would like to calculate how much it will cost to change AA accumulator. Is it really cheaper than usage of batteries (don't consider ecology question as of now)? Let's say I have the following ...
2
votes
2answers
334 views

Rocket/Thrust/Gas/Free Expansion of Gas

We know, the rockets in space use Newton's 3rd law to increase their velocity and hence move. What I don't understand is how it is possible in space aka vacuum-state without air? From what I know, ...
6
votes
1answer
313 views

What is the percentage of stars with planetary systems

We have discovered quite a number of exoplanets to date. The Kepler spacecraft has examined 150,000 stars and found 1,059 exoplanets. We know that Kepler, as well as all other exoplanet searches to ...
-4
votes
1answer
109 views

From where we can view earth moving?

Earth moves with a speed around $100000 \,\,km/h$, which will be amazing, but from where we can see that moving speed? If we far away from earth (outside of earth gravity) then can view its moving? is ...
2
votes
1answer
77 views

Will a space traveller slow down due to space expansion?

Photons of relic radiation loose their energy as they propagate through space. Will a space traveler loose their peculiar velocity as he travels through vast distances? Will he stop somewhere or still ...
2
votes
1answer
97 views

Physical meaning of the Rindler hyperbola vertex and the Rindler lines

Two questions regarding the Rindler diagram: 1) Does the vertex of a given hyperbola in the diagram have physical meaning? I know it is the inverse of the constant proper acceleration ($\alpha$) ...
2
votes
3answers
223 views

Natural units of information

In physics entropy is usually measured in nats. I wonder is there a possible model of a physical system which has entropy of discrete number of nats? How particles and degrees of freedom should be ...
5
votes
3answers
3k views

Real and imaginary parts of dielectric constant vs refractive index?

So for a complex dielectric constant $\epsilon = \epsilon_a + i\epsilon_b$, the wave vector and index of refraction are related to it through $k = \frac{\omega}{c}n$ and $n = \sqrt{\frac{\mu ...
0
votes
1answer
110 views

How do Stirling engines work?

How do Stirling engines work? I understand the heating and cooling of air, but how much faster (or more force, I'm not sure which to use) does the piston move per degree Celsius that the temperatures ...
-1
votes
1answer
79 views

What is opposite of electric field?

A negatively charged molecule will generate a field, an electric field, and it will repel all negative ions. So what does positively charged molecule generates that causes other positive molecules ...
3
votes
2answers
978 views

Why is there no current flow when i connect two opposite terminals of two separate batteries

I don't understand this..maybe there is something wrong with my understanding of battery or electricity. I am learning about electricity more deeply this time. So please spare with me... So lets ...
5
votes
1answer
97 views

Fundamental group of Calabi-Yau 3-fold in string theory

In string theory, we compactify a 10-dimensional space by a Calabi-Yau 3-fold to reduce the dimension to 4. To get a reasonable theory, a Calabi-Yau 3-fold should satisfy some properties. One is the ...
2
votes
2answers
143 views

Lenses and wave optics?

We all have studied lenses in the framework of geometrical optics, but how do they work within wave optics? I figure that the topic is quite broad, but I would appreciate any hints, like which ...
2
votes
1answer
51 views

The resolution limit for lithography in practice?

For a given wavelength of light $\lambda$, and a given numerical aperture (NA), I always see the statement in papers/etc. that we can perform lithographic patterning at the Rayleigh limit given by ...
2
votes
2answers
204 views

Is speed of sound really constant?

Does not speed of sound actually depend on the frequency and/or amplitude of the waves? If so, why it is constant?
3
votes
2answers
53 views

Temperature of relic radiation and thermal equilibrium

The temperature or relic radiation is 2.7 K. Does it mean that any flying body sufficiently far from any stars will reach this temperature? As I understand it, you can warm up water with sufficient ...

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