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1
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2answers
29 views

A question on heat conduction

I was looking up the definition of heat conduction, but every website has a different answer. In addition to several different answers, some seemed wrong, or at least oversimplified. From ...
1
vote
1answer
29 views

Isometry of Riemann sphere?

The complex metric on the Riemann sphere is given in the Wikipedia article to be $$ds^2=\frac{4}{(1+\zeta\bar \zeta)^2}d\zeta d\bar \zeta$$ while the sphere should be mapped to itself under $SL(2,\...
0
votes
0answers
8 views

I know that if $σ$ is the conductivity of the wire, then, by Ohm’s law in its general form we have: $\vec{J}=\sigma \vec{f}$

I know that if $σ$ is the conductivity of the wire, then, by Ohm’s law in its general form we have: $\vec{J}=\sigma \vec{f}$ that $\vec{f}=\vec{f}_{em}+\vec{f}_{app}$ where where $\vec{f}_{em}$ is the ...
1
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0answers
27 views

Calculating CFT on 2D curved manifolds

In 2D we can always choose coordinates in a coordinate patch so that the metric is conformally flat $$g_{\mu\nu}(x)=\kappa(x)\delta_{\mu\nu}$$ A simple example is the sphere $S^2$ in stereographic ...
0
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0answers
9 views

Electric cryogenic jet propulsion plausible?

Can air quickly be cooled to a liquid form then expelled from an aircraft as a form of propulsion? Could this liquid air also be fed into a turbine specifically made for the superheating of liquid air?...
0
votes
0answers
10 views

Why is sound attenuation greater in dry air than humid air?

I have just learned the surprising fact that the attenuation coefficient of sound in air depends strongly on relative humidity: at any given frequency, it can be several times larger in very dry air (...
0
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1answer
18 views

Ray Tracing in Concentric Spheres

Consider the following model: a) Assume a spherical earth. b) Assume that the ionosphere is a spherical mirror that is concentric with the center of the earth. For the purpose of this question, we ...
0
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1answer
8 views

Critical Current Density vs Critical Current

What is the difference between critical current (Ic) and critical current density (Jc)? How do they relate to one-another?
0
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0answers
13 views

How does a single satellite like the upcoming BIOMASS use interferometry?

The future satellite BIOMASS, from the ESA, will supposedly use p-band frequency interferometry to detect changes in forests. By itself? How?
0
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0answers
17 views

How would gravity affect a flat object with a slightly larger area than Earth's surface area, but lower in mass?

Imagine a flat object slightly wider than the Earth's surface area whose mass is much lower than that of Earth. If it landed on Earth, would the curvature of gravity bend it, or possibly even break it?...
0
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1answer
67 views

2019 Higgs Boson Mass?

Wiki, based on the 2018 Particle Data Group, gives the Higgs boson mass as $125.18 \pm 0.16$ GeV$/c^2$. OK. However, 2017 ATLAS gives a combined value of $124.98 \pm 0.28$ GeV$/c^2$. So what is ...
2
votes
3answers
38 views

Is a force exerted in all reference frames (inertial and noninertial?)

Let's say we got a toy car equipped with an electric engine. The engine exerts a force on the toy car and accelerates it when the car is at rest. Let's say I'm sitting stationary on a bench and I ...
2
votes
2answers
92 views
+100

Shouldn't $CO_2$ forcing depend on the surface temperature?

When I read about radiative forcing due to CO2 it says: "Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-...
2
votes
0answers
45 views

Confusion on Quantum Harmonic Oscillator Eigenvalues

In standard PDE theory, one generates eigenvalues to Sturm-Liouville problems over a finite domain. So, for a wave equation, we have an infinite number of eigenvalues $\lambda_n$ for a Dirichlet ...
1
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3answers
35 views

Change of buoyant force as object moves deeper

When reading about Fluid Dynamics, I saw two questions that presented similar situations but had very different answers, and I'm trying to understand why, since they seem contradictory: A beach ...
-2
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0answers
29 views

Wavefunction description [on hold]

Describe how a wavefunction summarizes the dynamical properties of a system and how those properties may be predicted. Somewhat open ended but I am having trouble contextualizing and summarizing what ...
4
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1answer
66 views

If I surrounded a black hole with gravitometers and observed the gravitational field as mass passed the event horizon, what would I observe?

By surrounding a black hole with gravitometers I would be able to get a 3-D map of the gravitational field. If I observed this field as an object approaches the event horizon, what would I see? Would ...
0
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0answers
32 views

The Gell-Mann and Low theorem and the expansion of the Green function

If we have a system with Hamiltonian $H = H_{0} + V$, with $| \Phi_{0} \rangle$ being the ground state of the system without the interaction, the Gell-Mann and Low theorem say that the quantite $$ |\...
0
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2answers
37 views

Resistance and electricity

strong text In the answer to this question, it stated that the current would remain unchanged and I was wondering why the current would remain the same for the portion of the wire even though the ...
1
vote
1answer
74 views

What does color represent in quarks?

Im sure I'm pretty undereducated to be asking a question like this, but what do the colors represent in quark chromodynamics? A I understand it, each color represents a sort of charge, but what kind ...
1
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0answers
34 views

If the lagrangian density changes by a total derivative of the lagrangian density

When we derive energy momentum tensor current by actively transforming field. We see that lagrangian ( density) changes by a total derivative of the lagrangian. If a total derivative of the function ...
6
votes
3answers
132 views

Inconsistency of PE to KE conversion in moving reference frames

Here's a nice trick question to keep you amused over the weekend. A trolley of unit mass with light frictionless wheels is released to roll down a ramp onto a smooth level surface. The PE lost equals ...
2
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2answers
78 views

Why does a black hole have much entropy?

Is that just an assumption by Bekenstein etc? For example, what is the different states a non-rotating primordial black hole can be in?
0
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1answer
22 views

What force(s) comparable to material and acoustic damping dampens the ringing of merged black holes?

The "ringdown" that was detected after the merger of two black holes by LIGO in 2015 has been compared to the damping of a rung bell. Why doesn't the merged black hole ring perpetually? What force ...
0
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0answers
13 views

Relation among the dead time, recovery time and resolving time of a GM counter

I have been researching a lot for my assignment about the relation among the dead time, recovery time and resolving time of a GM counter. Most of the articles I came across have varying definitions on ...
0
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0answers
17 views

Time-reversal and its effect in the eigenfunctions of the Floquet operator

When time-reversal is present in a kicked Hamiltonian system (e.g. kicked rotator) it can be shown that there exists a basis in which the associated Floquet operator $U$ is symmetric. One can then try ...
0
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0answers
13 views

Deriving transition amplitude with $S$-matrix

Here is the part that is bothering me: Yeah, already here? So, my question: In the first line we have int picture states at time zero and in the second line we have limit of time evolv operator with ...
0
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0answers
20 views

Derivation of the time-derivative in a rotating frame of refrence

I have so trouble following Goldsteins derivation of the time derivative in the rotating refrence frame, and its use to derive the coriolis force (sec. 4.9-10) Given an intertial frame of refrence, $S$...
1
vote
5answers
100 views

How can atmospheric CO2 absorption of infrared be 100% when its atmospheric concentration is 0.04%?

An absorption spectrum from high in the atmosphere of infrared radiation emitted from the earth, shows that for the 15µm wavelength there is almost complete absorption. This is attributed to ...
-6
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0answers
39 views

Who change is constant? [on hold]

How change is constant? As we know when universe created n till today every thing is changing then who would we say change is constant? And also when a baby is born and till his or her death they ...
0
votes
1answer
34 views

What quantity remains conserved in a calorimeter when two bodies at different temperatures are mixed?

This is the question: Two identical bodies at different temperatures are mixed in a calorimeter. Which of the following quantities remains conserved? 1) Sum of the temperatures of the two bodies 2) ...
0
votes
5answers
51 views

Why is the momentum always conserved in elastic collision?

As the title above, why is the momentum always conserved in elastic collision (no loss of kinetic energy)? Does it related to the conservation of kinetic energy? If so, how do we prove that? Further ...
0
votes
1answer
66 views

Why does a falling object gain horisontal speed?

This really bothers me as I think I should know the answer... I pushed a toilet paper roll so it slowly rolled of a five centimeter edge. On the next surface it had gained a lot speed. But why? It ...
0
votes
2answers
20 views

Could a disturbance to a magnetic field be detected near the source?

(Forgive my crude MS paint drawing) Imagine we have a large magnetic field coming from a powerful source. At the edge of the field where it is weak (A), we introduce a disturbance using another ...
1
vote
4answers
48 views

The potential at a point

According to my book, 'The potential at a point is said to be 1 volt when 1 joule of work is done in bringing 1 coulomb charge from infinity to that point.' But I wonder how it is possible. As the ...
0
votes
2answers
50 views

How can the annihilation of an electron and a positron create a quark-antiquark pair or a muon-anti muon pair?

The overview of this interaction is that a positron and an electron annihilate with each other and create two gamma photons which contains too much energy and soon materialise into a quark-anti quark ...
0
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0answers
50 views

Trajectories in space

I want to say that a set $T$ of vectors in $R^{\,4}$ is a "trajectory" if there is an interval $I$, and continuous functions $x,y,z$ on $I$, such that $T$ is the set of $[t,x(t),y(t),z(t)]$ for $t$ in ...
0
votes
1answer
8 views

What will be the effects of weight of a fluid containing contain after an external body is submerged in it

Initially taking a beaker containing water of having total mass of water and beaker as W. Now when an external object of weight w' is submerged inside the beaker containing water having density d, ...
0
votes
0answers
16 views

Principal axes of cube at one of it's corner [on hold]

Find the equation of momental ellipsoid at the corner of a cube of side $2a$ referred to its principal axes. I have specific doubts in this question. At any point, angle which the principal axes ...
0
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0answers
18 views

Relation between AC Current and electromagnetic wave in transmission lines?

I m so confused we started by learning about electromagnetic waves in the vacuum , then we went to transmission lines and there the professor mentioned the em waves are traveling through them. I ...
0
votes
3answers
51 views

Can 2 atoms of oxygen be transformed into $H_2O$ using electicity? [on hold]

I am writing a novel in which a scientist is required to design a self sufficient moon base and was hoping to use this idea and whilst I have created a model where this is possible I would like to ...
1
vote
0answers
36 views

Proving that $$ [\phi(\vec{x}, 0), \phi(\vec{x}, t)] \sim e^{-i m t}-e^{+i m t} $$ in QFT

So far, I get the following (for the left term in the integral, $d$=3): \begin{equation} \begin{aligned} \Delta_{+}(x) &= \int \frac{\mathrm{d} \vec{p}^{d}}{(2 \pi)^{d} 2 e(\vec{p})} \exp (-i t e(...
0
votes
0answers
21 views

Surface States and Fermi Level

In work Gelmont, B. L., Shur, M., & Stroscio, M. (1995). Polar optical-phonon scattering in three- and two-dimensional electron gases. Journal of Applied Physics, 77(2), 657–660 when author moving ...
2
votes
2answers
48 views

If a ball is kept on the table, then what forces are being exerted on it?

If a ball is kept on a table then there is gravity acting on it as well as a normal reaction force by the table on the ball. But as both the forces are being exerted on the same object i.e. the ball, ...
0
votes
1answer
16 views

What distance do I use when examining the effect of an electric field on a point outside of a nonconducting sphere with an arbitrary volume charge?

It's easy enough for me to do this for a ring (as I interpret the field lines as "exiting from the centre of the ring") or a point (simply the distance from the point to the other point), but I just ...
0
votes
1answer
36 views

What is the Komar mass of the de-Sitter spacetime?

The Komar mass of some spacetime is defined as an integral (volume or surface, depending on its formulation): https://en.wikipedia.org/wiki/Komar_mass The de-Sitter metric in static coordinates is ($\...
0
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1answer
39 views

Nature of induced magnetic field lines in cylindrical capacitor

What is nature of the induced magnetic field lines inside a cylindrical capacitor which has a time variant potential gradient across it? For a parallel plate capacitor, its straightforward: the ...
0
votes
0answers
17 views

Constructing many-body densities from Kohn-Sham orbitals

In many electronic structure methods, one solves "effective" one-particle Schrödinger equations for one-particle wavefunctions. In my case, I have Kohn-Sham orbitals from a DFT calculation. The ...
-2
votes
0answers
37 views

The use of Ramanujan Summation [duplicate]

What is the real life significance of the famous Ramanujan summation $1+2+3+\cdots = -\frac{1}{12}$ in string theory? How this is coming into play?
0
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0answers
17 views

Is undamped reflection in a pipeline possible, under described circumstances?

Take $\,H_2O\,$ as the compressible medium (water hammer), and if we were to decide to compress it to the limit of breaking down to sub-components, holding in view the non-locallity of protons and ...

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