All Questions

0
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0answers
10 views

Is Smoluchowski equation for aggregation conservative?

The Smoluchowski equation for the aggregation with no breakage can be written as $$\frac{dn_k}{dt}=\frac{1}{2}\Big(\sum_{i+j=k} n_i \beta_{ij}n_j\Big) - n_k \sum_{i=1}^N \beta_{ki} n_i$$ where $n_k$ ...
1
vote
1answer
49 views

How large would the Moon have to be such that I could not see it in its entirety from earth?

The [^circumference of the] moon can be seen in its entirety from Earth [^during a full moon] taking up a very small portion of the visible sky. The Moon is ~1/4 the diameter of the Earth. If for the ...
0
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0answers
25 views

What is a “Fourier transform limited pulse”?

I have some doubts about the definiton of a Fourier transform limited pulse. For example if I consider a generic pulse: $$E(z,t)=\frac{1}{\sqrt{2\pi}}\int_{-\infty}^{+\infty}A(w)e^{i(-\beta(w)z+wt)}dw$...
2
votes
0answers
21 views

Fluid mechanics, symplectic structure, the Hamiltonian, and vorticity

Consider an inviscid irrotational fluid in two dimensions. There are some explicit connections with symplectic geometry that I do not understand. I am not well versed in the later topic, so please ...
-1
votes
0answers
26 views

Are all 4D Ricci flat manifolds locally Euclidean? [on hold]

If a 4D manifold with metric signature (++++) is Ricci flat. Does this mean that locally the space is Euclidean? Does this mean that the only difference between these manifolds is there global ...
-1
votes
1answer
23 views

Potential divider

I am just wondering if a potential divider circuit is used to vary the pd across the other resistor from zero to maximum, why can't the variation be done straight from the source?
1
vote
1answer
42 views

How is atomic position expressed in QM?

I am trying to understand how DFT works. I understand how to express the position of an atom or molecule in terms of the positions of the nuclei and electrons by setting up a Hamiltonian expressed in ...
0
votes
1answer
13 views

Why sample is placed after the interferometer in FTIR?

In all image of schematics of FTIR I found (Wikipedia for example), the sample is placed after the interferometer: Wouldn't this miss some emergent effects, such photo-induced absorption, since the ...
0
votes
1answer
41 views

How do you expand a wavefunction in the basis of eigenfunctions of the free particle?

If we have an initial state given by $ \Psi(x,0) $ and we want to find $ \Psi(x,t) $, we would expand the function in the basis of eigenstates of the Hamiltonian, $\{\psi_n\}$: $ \Psi(x,t)=\sum _nC_n ...
0
votes
1answer
59 views

What physical law(s) are violated by two masses occupying the same space at the same time?

The two masses have equal mass and equal volume. I think the 2nd law of thermo is violated (because there are less ways to arrange a single mass in space than two), but what other laws could be ...
1
vote
0answers
42 views

I have proved that radioactive disintegration is first order using probability. Is my proof correct? [on hold]

I am a high school student and this is just a small attempt to prove why radioactive disintegration is first order. I don't even know if it has already been proved or not. Please let me know. I later ...
0
votes
2answers
31 views

De Broglie Wavelength of a particle at the turning point of a fountain

If you think about an atom which is launched in some kind of straight fountain, upward in the gravitational field. At the turning point the velocity and therefore the momentum of the particle seems to ...
0
votes
3answers
43 views

Non-ohmic conductors

Non ohmic conductors are said to be the conductors that do not obey Ohm's Law. The V-I graph for them is not a straight line unlike ideal ohmic conductors. According to me Ohm's Law states: The ...
-1
votes
0answers
27 views

Solutions of inverted harmonic oscillator [on hold]

I have the following Hamiltonian: $H(x,p)=p^2/2m-\alpha x^2/2$ This leads to: $\dot{x}=p/m$ $\dot{p}=\alpha x$ I'm supposed to solve these but can't find a solution. I tried sin and cos but it ...
0
votes
0answers
9 views

What is the theoretical resolution limit of magnetometers / magnetoencephelography?

I recently read this article about a group of physicists and neurobiologists working on developing advancements to magnetoencephelography, presumably based on new quantum metrology and quantum ...
1
vote
0answers
62 views

What is the Lorentz group composition of two electrons?

We know that the wavefunction of an electron transforms as Dirac spinor $(1/2,0)⊕(0,1/2)$ under the Lorentz group $SO(3,1) \sim SU(2)\times SU(2).$ Which representations can we form with two ...
1
vote
1answer
77 views

Zero-energy universe - What is nothing?

I am a layman, so excuse me in advance for the stupidity of my questions, and I hope you can answer them in a way that I can understand. I have read, here and there, that the Universe might have a ...
-1
votes
0answers
15 views

Dynamics general problem question

In problems like the one below, where you have to draw multiple free body diagrams and solve for the forces and angular accelerations etc by solving a system of equations, could you also calculate the ...
-1
votes
1answer
21 views

Rotational kinetic energy of a body

How to find total kinetic energy of a body which is rotating about multiple axes at the same time? For example consider a wheel with a shaft, and that the shaft is hinged. So the wheel is rotating ...
-1
votes
0answers
22 views

Convergent Flow [on hold]

Need some help with this fluid dynamics question please. A high-speed drill required cooling to ensure that it does not overheat while in use. To achieve this, high-pressure air is directed through ...
0
votes
0answers
6 views

How to calculate the corotation radius?

I am studying this paper: P.Ghosh & F.K.Lamb, APJ, 232, pag.259 (1979) I do not understand how the authors calculate the corotation radius, $r_{co}$, where the matter of the disk has the same ...
0
votes
5answers
57 views

Confusion about ket states and bra with position

I am very confused about the bra-ket notation of states and the fact that $$\psi(x) = ⟨x|\psi⟩$$ and $$⟨x|x'⟩ = \delta(x-x')$$ are true. What does this mean? What is the ket $|x⟩$, is it just some ...
1
vote
0answers
6 views

Why are there no excitations by collision for the visible transitions in the Franck-Hertz experiment?

The minima of the I(V) graph for a Neon Franck-Hertz tube are approximately 20V apart from each other, this is usually explained by the fact that excitation by inelastic collision happens at about 20 ...
0
votes
1answer
54 views

Must the total orbital angular momentum quantum number $L$ be less than the principal quantum number $n$? If so, why?

I am studying LS coupling and term symbols. In my textbook, there is an exercise: Why is it impossible for a $2\ ^{2}\text{D}_{3/2}$ state to exist? The answer says, the total orbital angular ...
-1
votes
0answers
26 views

Superconducting Super Collider vs Future Circular Collider [on hold]

I wonder, which one would provide better science and applications such as humanitarian aid science and technology when and if they were completed ?
0
votes
0answers
13 views

Finding the expectation value for the z component of the angular momentum

I'm trying to find the expectation value $<\hat L_z>$ of the angular momentum at an initial state given by the wavefunction: $\psi(\vec r , t_0=0) = sin(r\beta^{1/2})e^{-\beta r^2}(Y^0_0+2Y^1_1-...
1
vote
1answer
27 views

Thermal equilibrium of objects in space

I've read somewhere (not a reliable source though) that an object in space and far enough away from any heat source other than the CMB would eventually cool to 3K, the CMB's temperature, but no colder....
1
vote
0answers
26 views

Does a body shaped like a cone travel faster through water than a double cone?

I read somewhere recently that a cone weightd so that it falls with point forward through water, falls faster than a double cone, of the same mass. Is this true, and if so, why?
0
votes
0answers
22 views

Is it possible to use laser-like technology to create effective force field?

By laser-like lechnology I mean a coherent ray of particals or waves with the same amplitude/energy. This ray would be emited on limited distance (lets say 2meters) where it will be absorbed by ...
1
vote
0answers
24 views

Why hydrogen bond forming has lifetimes on picosecond scale?

According to this Research http://www.kinetics.nsc.ru/mds/downloads/2009_JSC_Naber_Vol_LifeTime.pdf hydrogen bonds are formed and destroyed within a timescale of a few picoseconds. Why hydrogen ...
-1
votes
1answer
24 views

Electric field of two equal charges of the same sign [on hold]

Two equal positive charges are at distance d, -d from the origin on the y axis. What is the distance on the x axis beyond which a small perturbation in y will move a particle away from the x axis? ...
0
votes
2answers
45 views

Is spark from spark plug in SI engine (IC engine) not counted as a hot reservoir in Heat Engine?

Are the Heat Engine (HE) and Internal Combustion Engine (ICE) not the same because ICE only exchanges the heat to the surrounding[only one reservoir]? And I have a question what if it's an SI engine ...
3
votes
0answers
26 views

Gapless modes at the boundary between topological insulator and normal insulator

I am currently learning about topology in condensed matter physics. I think I understand most of how topological indeces come about and differences between Z and Z2 indeces and the symmetries that ...
0
votes
2answers
29 views

Double silt experiment with one circular polarizer in front of only one slit in the which-way perspective?

If one puts in front of a double slit two quarter-wave circular polarizers (QWP) in front of each slit, say one with 45° and the other with -45° orientation (fast and slow axis - i.e. clock- and anti-...
1
vote
1answer
31 views

Is 1 lux unrealistically low for a cell phone camera flash?

I have a light source which is similar to a typical cell phone camera flash. Based on a measurement and some calculations, I estimated that its illuminance is about 1 lux, or close to twilight values ...
0
votes
1answer
59 views

Question about special relativity in Carroll lecture note

In the lecture of General relativity by Carroll, page 7 is written that: Notice the distinction between this situation and that in the Newtonian world; here, it is impossible to say (in a ...
0
votes
1answer
51 views

How is mass related to the Poincare group?

I have studied about Poincaré group and some QFT, read that the Casimir elements are $p^{u}p_{u} = m^2$ and Pauli-Lubanski vector (or pseudovector). How does the mass come into picture and spin come ...
0
votes
1answer
33 views

Level set of Hamiltonian are the orbits?

Just a small question : If $x(t)=(p(t),q(t))$, then the position $x(t)$ of a particle is given by $$\dot p=-H_q(x(t))\quad \text{and}\quad \dot q=H_p(x(t)).$$ In particular, if $x$ solve the previous ...
2
votes
0answers
19 views

Can an Opposing Current Create another Opposing Current?

In Inductors when current increases it's magnetic field induces a voltage which causes an opposing current that slows down the rise of the current that initially creates it, but can this opposing ...
1
vote
1answer
36 views

How to know what type of diagram contributes to a two-to-two process?

There are 3 types of diagrams that can contribute to a two-to-two process; the $s$-channel, $u$-channel and $t$-channel. How do I know what diagrams can contribute to a process? I know that in QED, ...
2
votes
2answers
59 views

Is there a connection between the energy distribution and time dilation?

Can anyone please help me understand what is descibed bellow? Scenario 1. We have a pair of atomic clocks. Let's call them clock A and clock B. We switch both of them on at the same time. Clock A ...
1
vote
1answer
32 views

Self-induction and conservation of energy [on hold]

back EMF when switch is open is greater than back EMF when it is on; is that opposes the law of conservation of energy?
1
vote
4answers
31 views

Water mixture at different temperatures

Let's say that we mix homegeneously and instantly cold water at $t^\circ $ C and hot water at $T^\circ$ C (like in a water tap) in ratio $p:1$. My question is the following: What is the instant ...
1
vote
0answers
26 views

How to find a CSCO (Complete Set of Commuting Operators/Observables) of an eigenfunction?

So, say you have an eigenfunction defined as: $\psi \left(x_1,x_2\right)=\int _{-\infty }^{+\infty }\:e^{p\left(\frac{2\pi i}{h}\right)\left(x_1-x_2+x_o\right)}dp$ (The spinless 2 particle system ...
-4
votes
0answers
13 views
0
votes
1answer
31 views

When a particle moves perpendicularly to it's projection direction

Could anyone please help me with the following: This is concerning the flight of a projectile. Specifically the point in it's flight at which it's velocity vector becomes perpendicular to its initial ...
1
vote
0answers
23 views

conventional matrix notation for distance interval

Why matrix notation for distance interval is represented by this? $$g_{\mu \nu}\Delta X^{\mu}\Delta X^{\nu}=(\Delta X)^Tg (\Delta X)=\Delta X^{\mu}\Delta X^{\nu}g_{\mu \nu}$$ Could you explain ...
3
votes
1answer
26 views

Cutkosky's rule for vertex correction

I don't understand how to evaluate the diagram for electron-photon vertex with help of Cutosky's rule: According to the rule, one should cut diagram "in the middle". I denote final state as F and ...
1
vote
0answers
20 views

How can I beforehand determine whether a material is a good candidate for a microwave sensor?

Is there a table of some sort or formula where I can determine before I test the material under a microwave sensor whether it will allow the microwaves to pass through and if so at what depth. for ...
0
votes
1answer
8 views

Rolling without slipping around a fixed axis (practice exam) [on hold]

Everything is pretty much given in the image. We had to find the relation between omega-p and omega-s The answer is omega-p * (r+a*cos(alpha)) = omega-s * a It says that the in the inertial frame ...

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