# All Questions

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### If an atom emits a visible photon doesn't that mean it's visible?

A visible photon can be emitted and absorbed by an individual atom. Is this process sufficient for saying that we can see an individual atom with visible light? I realize the wavelength of visible ...
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### How does entropy ensure that isolated thermal processes are 100% irreversible?

How can anyone prove that molecules in figure C will never return to their original positions in figure A? It will be more believable to say that the chance is very slim.
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### time distortion

Observer A is standing on a planet, and observer B is on a ship. At t=0 B is 1 light year away from A. Also at t=0 B will accelerate its ship to a 0,999999999...$c$ instantaneously (yes, its a very ...
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### Holographic dual of pure-classical systems

There are classical systems (eg. see Sections VII and VIII of Kogut's review) that shares many of the properties of a pure-gauge SU(N) quantum theory including factorization and mass-gap, but with ...
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### How to calculate the resistance (conductance) between two contacts embedded into a 3D object?

Pouillet's law, $R = \rho \frac{\ell}{A}$, is useful when dealing with a "column" (cylinder) of a conducting material, with cross section $A$ and height $\ell$. Using this equation has an underlying ...
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### Is it true that all processes where entropy increases are permitted by the second law, if the system is isolated?

It is possible to deduce that in a thermodynamic process for an isolated system $\mathrm{d}S$ has to be greater than zero, from this it follows trivially that $\Delta S > 0$. It is usually said ...
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### How to measure resistance of gel electrolyte using multimeter

How can I measure the resistance of a gel electrolyte using a multimeter? I know the formula to measure the conductivity of a gel electrolyte is k = (1/R) * (d/a) where k is the conductivity (S/cm), ...
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### What is the meaning of the point of inflection in this?

Consider the following flow velocity profiles near a boundary, and the boundary profile point of inflection,PI Regarding the effect of pressure gradient on boundary-layer profiles does the point of ...
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### Is it possible that one of the quasars we see is an ancient Milky Way?

As I understand it, even though our current measurements of the Riemann curvature of the universe overlaps with flat spacetime, this doesn't restrict the shape of the universe to being infinite. It's ...
147 views

### Solenoid cannon (coil gun) possibility and recoil?

Similar to my other question If you are to pass a magnet through a solenoid a current is produced in the solenoid due to the magnetic flux. However, if you were to have a magnet sit inside a ...
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### estimate air temperature changes from changes in sea surface temperature

Consider a body of water, where the change in temperature for a given time period can be estimated by: $$\frac{dTw}{dt} = \frac{Q_{net} \times A}{\rho \times C_{pw} \times V}$$ where $dt$ is the ...
398 views

### Calculate pressure from partition function with separated volume geometric parameters?

How does one calculate the pressure from the partition function if it is specified in terms of three parameters defining the space of which the gas occupies, but all three parameters are not always ...
201 views

### How can flux be the number of electric field lines normal to a surface? [duplicate]

Shouldn't the number of field lines through any surface be infinite if there is a charge present. Or is there a finite number of field lines from a unit charge?
886 views

### Rate of flow of electrons in a simple circuit, given the current? [closed]

I have this question: The correct answer is C. I understand that the flow of electrons should be from Y to X because electrons flow opposite to "conventional current", which is from positive to ...
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### Do I have some freedom when I define the quantum SHO ladder operators? [closed]

I tried to solve the quantum harmonic oscillator via the operator method. After doing it and looking up the solution I noticed that for some reason the ladder operators got an additional factor of (i) ...
895 views

### What is the relation between entropy and pressure in strict thermodynamic terms?

I read on the net that it is a general consensus that entropy in a system decreases as pressure increases and vice versa. How can one reach such conclusion using the characteristic equation in terms ...
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### Calculate Potential [closed]

$\vec{w} = \begin{pmatrix}w_r \\ w_{phi} \end{pmatrix} = \begin{pmatrix}\frac{Q_0}{2 \pi r} \\ 0 \end{pmatrix}$ 1) Show that the flow satisfys the continuity equation 2) Show that $\vec{w}$ ...
28 views

### Evaluation of a probability from Fermi Golden rule

In Marc Bee's book, he has described the principle of spectroscopy with reservoir (the material) and the probe as interacting systems with their own hamiltonians $H_R$ and $H_p$ respectively and $H_c$ ...
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### Model of spherical wave fronts in relativity

In relativity, the 3-surface formed by light rays emanating from an event as they evolve in time has the shape of a hypercone wich is flat. I have difficulties seeing how can a spherical wave front, ...
232 views

### Direct vs. Compound reactions

I know there are different types of direct reactions like inelastic scattering and such. But is the main difference between direct and compound reactions that one excites (Or at least can) the whole ...
179 views

### Gravitational lensing on galaxy cluster with given potential

I am having a problem with gravitational lensing question where we are interested in deflection angle of light traveling in potential of galactic cluster, described with tensor ...
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### How does a body accelerate that is put into a flow field with constant speed?

Imagine an infinitely long flow channel filled with water flowing at a constant speed everywhere parallel to the infinitely long dimension of this channel. Furthermore, imagine we place a body (say a ...
435 views

### What is the freezing/melting point tap water?

Yes, I know purity is not a constant and depends on area etc. Does anyone have any experimental results on freezing/melting point of any tap water, as opposed to distilled, de-ionized, saline etc? I ...
163 views

### Why can't liquid nitrogen be sealed in gas cylinders?

By observing the phase diagram of nitrogen, hydrogen and carbon dioxide, I discovered the supercritical region. I can easily buy a gas tank of carbon dioxide at 250 Bars. However I never saw any ...
60 views

### Ground state of 113Sn from shell model

The shell model predicts that the ground state of 113Sn with 63 neutrons would have J=5/2+. But the ground state is taken to be 1/2+. What is the reason behind that? Has it got something to do with ...
155 views

### Why is the Reynolds number “the way it is?” Why is its order the way it is?

Why is the Reynolds number “the way it is?” Why is its order the way it is? I'm not sure if this is an appropriate question for this context, but I would like more intuition on this matter and so ...
32 views

### Help reading power spectrum graphs?

How would I go about figuring out how long a source was measured when given a graph of the power spectrum? From notes I'm following, they just state the total observation time as being roughly ...
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### Why does Landau theory not fail when dealing with a first order phase transition?

Here is a problem where I can do the calculation, but I am not understanding the philosophy behind it. It is about Landau theory: The Landau theory of phase transitions is based on the idea that the ...
316 views

### How do I know what variable to use for the chain rule?

In my textbook the tangential acceleration is given like this: $$a_t=\frac{dv}{dt}=r\frac{dw}{dt}$$ $$a_t=rα$$ I understand that the chain rule is applied here like this: ...
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### Does an iron rich substance stay magnetised?

I am doing research (i.e. playing around) with a dry magnet and a material that has about 10% contained iron. The balance of the material is non-magnetic (i.e. silica and non-magnetic metals). It is ...
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### How is electric dipole moment analogous to mechanical moment?

Diplole moment is q*2l. I don't understand the similarity between dipole moment and ordinary mecanical moment.
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### Difference in induced current, when magnetic field “span” is reduced?

A conductor of known volume $(V)$ passes a uniform magnetic field$(B)$with a constant velocity $(v)$ the conductor is a source of induced EMF, a power source to a circuit. The induced EMF can be ...
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### particles from waves?

If you slide a styrofoam cup containing milky coffee accross a formica tabletop, the friction of the styrofoam on the formica, will produce a wave interference pattern on the surface of the coffee. ...
174 views

### How did people discover the conservation of mechanical energy?

I want to write about the history of energy, and am focusing on mechanical energy at the moment. My question is: Can I attribute the conservation of mechanical energy to the power of maths, ...
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### Why can only asymptotically flat and AdS black hole have the thermodynamics? What's about asymptoticaly dS black hole?

Almost all advanced GR textbooks will have the content of black hole thermodynamics for asymptotically flat black hole. And this paper solve the asymptotically AdS (Anti-de Sitter) black hole ...
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### How to determine the collapse operator for a Lindblad equation

Given a Hamiltonian $H$, how can I relate the collapse operator for the Lindblad equation to a given environmental effect? Also, how can I relate the constant $\gamma$ in front of the sum of the ...
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### Will objects heat up and become hidden at relativistic speed?

Let us assume there is a mirror travelling at extremely relativistic speeds, perhaps few picometers away from speed of light, and let us assume we have a very thin detector which is very small perhaps ...
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### Has there been any research done in fluid dynamics within a mobius strip?

I was wondering if there has been any research done with the flow of fluid within a mobius strip? The reason I am interested is, assuming the fluid would flow around a center axis point, at certain ...
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### Is it degeneracy pressure rather than electrostatic repulsion that stops us falling through the floor? [duplicate]

From Wikipedia: Degeneracy Pressure Freeman Dyson showed that the imperviousness of solid matter is due to quantum degeneracy pressure rather than electrostatic repulsion as had been previously ...