# All Questions

132,850 questions
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### “Regularization” of the stationnary solution for diffusion in spherical coordinates

In spherical coordinates $(r,\theta,\phi)$, the stationnary diffusion equation is a Laplace equation $\Delta f =0$. The solution in a radial symmetry : $f=f_{\infty}-r_0/r$ where $f_{\infty}$ and $r_0$...
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### What is prime focus of a telescope

Telescope here refers to a system of two convex lenses. In one of the problems I wanted to solve, it is given that "a Barlow lens of focal length $f_{b}$ is placed at a distance $D < f_{b}$, from ...
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### Approximage grid-sampled field as graph with nodes and edges

The following quiver plot shows principal direction and magnitude of a vector field in every point of a 2d grid. I'd like to condense the representation to nodes and graph edges -- unoriented edges ...
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### How to calculate air density on Mars based on specicif pressure and Temperature?

I am trying to solve a Physics question which includes air density of Mars and with my calculation, I'm finding value around 0.01045: ρ_(mars,air)= P/RT= 600/((287.05).(200))=0.01045. But my ...
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### Why is it that a stationary bike will fall but a moving one won't? [duplicate]

No matter how much I think about this I can't come up with a reason to me there is no difference In the forces acting against the tilting of the bike in both the cases as mothion is in the ...
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### Relationship between physical and information entropy

This question is inspired by the following example given in Barnett's book on quantum information: (PDF here, starting at the bottom of page 5) I am not a physicist, but I'll accept that the ...
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### Average number of particles in quantum well

An ideal gas of non-relativistic particles is in a box with perodic boundary conditions. I've shown that its chemical potential is given by: \mu=-kT\ln \left( \left( \frac{2\pi m}{h^2} \right)^{3/...
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### Big Bang, Entropy, Electromagnetic Radiation, Energy/Mass Equivalence, Gravity, General Relativity

If the photon in electromagnetic radiation does not have mass then does this mean that electromagnetic radiation is not subject to gravitational forces. Therefore it would continue at the speed of ...
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### Relativity- In a given Scenario, is FTL communication possible?

Take a black hole singularity of sufficient size that a planet could be placed in a stable orbit with a high amount of time dilation observed from the outside. Let’s assume it’s a pretty big black ...
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### Worked examples of Lie derivative [migrated]

I'm trying to find the Lie derivative of a 2 form $\sin(\theta)d\theta d\phi$ with respect to a vector field given in a differential basis and I think the way to go here is to use Cartan's formula but ...
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### Falling electric dipole contradicts equivalence principle?

Consider an electric dipole consisting of charges $q$ and $-q$, both of mass $m$, separated by a distance $d$. If the dipole is given an acceleration $a$ perpendicular to its moment the total ...
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### Is string theory inconsistent with the Standard Model of Elementary Particles?

According to (super)string theory, each fundamental particle represents a different vibrational mode of a relativistic string and each type of vibration pattern represents a different particle. But ...
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### How do all chemical reactions result in a product whose potential is less than the reactants if all reactions are not exothermic?

We are told by our chemistry teacher that all chemical reactions take place as reactants (that have bigger potential) change into products (that have smaller potential) hence instable substances ...
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### Static electric field

What mean: "An electric field (which is static) is not a source of energy (which comes from the electron), it is a 'store' of potential energy."
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### There are a component proportional to speed in gravitional force?

Reading some introductory texts to general theory of relativity (GR), I understand that the gravitational force between two objects has, in addition to the component in the direction of the vector ...
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### Discharging a photoconductor from the substrate side

Is it possible to discharge for example an OPC drum without exposing the surface? Since they are usually build on aluminum tubes (and belt type OPCs on metallized PET or other polymer) could one drop ...
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### Are there supersymmetry algebras with higher spinor representations?

The super-Poincare algebra contains supersymmetry generators $Q^I$ which satisfy fermionic anticommutation relations. By the higher-dimensional analogue of the spin-statistics theorem, they must ...
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### What is the spin-statistics theorem in higher dimensions?

In $d = 3+1$ dimensions, the spin-statistics theorem states that fermionic particles have half-integer spin and bosonic particles have integer spin, in a well-behaved relativistic quantum field theory....
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### Why do composite bosons form a BEC?

I found this question here but it does not fully answer my question. The answer there was that "composite bosons can occupy the same state when the state is spatially delocalized on a scale larger ...
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### Find force between the 2 charges? [on hold]

There are 2 charges q1 and q2. The distance between them is divided into 2 parts: one has a medium with dielectric constant K1 and the other has a medium with dielectric constant K2. The length of the ...
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### Notation and concepts of Yang Mills Theory

I am studying loop quantum gravity using the book by Pullin and Gambini. I am having some trouble understanding and getting past the chapter on Yang Mills theory, mainly because I am confused about ...

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