-3
votes
0answers
30 views

Please read the problem below,the approach and explain where it's wrong and what's the true concept behind? [on hold]

Where should (A B C or D) charge $Q$ be placed so that the flux of electric field through the hemisphere remain unchanged? there is a charge $q$ at the center of hemisphere's base. Here's the ...
11
votes
1answer
481 views

Calculating $\mathrm{Tr}[\log \Delta_F]$

I am stuck with this problem for quite sometime. I have a propagator in the momentum representation (from this question), which looks like $$ \widetilde\Delta_F(p) = ...
0
votes
1answer
55 views

Bloch sphere representation of $\sigma_x$ operator on $|1\rangle$

I am trying to visualize a Hamiltonian H=$\hat{\sigma_x}$ $$ \hat{\sigma}_{x} = \left( \begin{array}{cc} 0 & 1 \\ 1 & 0 \end{array} \right) $$ acting on the state $| 1 \rangle$. I can write ...
16
votes
4answers
4k views

If water is essentially incompressible, why are there tides?

So recently we were taught in school that tides are formed because the moon 'cancels out' some of the earth's gravity, and so the water rises because of the weaker force. But if water is not ...
3
votes
1answer
195 views

What equation predicts at what point a stretched object comes apart?

I am creating a simulation and am interested in pulling stretchy things and when they break, like taffy. I imagine this is a bit tougher then a simple equation like gravity, but I have no idea. Is ...
1
vote
1answer
60 views

The $T\rightarrow \infty $ limit in quantum field theory

I am new to quantum field theory. Prior to this, I have been using quantum mechanics for a few years. I am reading the book by A. Zee, ''quantum field theory in a nutshell'', 2nd Ed.. On page 18, ...
1
vote
0answers
32 views

Definition for a trajectory in phase-space

When we say "a trajectory in phase space", when the parameter is time, do we mean the set of points in phase-space corresponding to a continuous segment in time? Does it have to be continuous? Does it ...
0
votes
2answers
202 views

Find the force needed to accelerate body to a certain velocity for a certain time with respect to drag force

So, the problem is straightworward when we suggest that air resistance force is constant: $$ \vec F = \frac {\vec V_1 - \vec V_0} {t} m / b $$ $$ \vec V_0, \vec V_1 - \text {initial and final ...
0
votes
2answers
41 views

What does J cm-1 stand?

For an optical component, I got following number as an optical damage threshold from the company, damage threshold = 10 Jcm-1,20ns, 20Hz It looks like 20ns is ...
-4
votes
0answers
38 views

Is the magnetic field caused by the lenght contraction, as seen by us, in the frame of a fast moving charged particle?

If for example an electron moves very fast relative to us, then the length contraction as seen by us, causes the electric field lines to compress themselves in the direction of motion. Is this the ...
-2
votes
0answers
21 views

Why are cirrus cloud those that contribute the most to climate forcing?

I read everywhere that most of the forcing produced by clouds is done by cirrus but I can't figure out why. It's obviously related to their IR absorption - emission...
3
votes
2answers
221 views

Weak interaction violate charge conjugate

How can we show that the weak interaction violates the charge conjugation symmetry?
1
vote
1answer
31 views

What is the gravitational/electric/inverse-square field inside a cylinder?

I've read from the shell theorem that an inverse-square potential has zero field inside a spherical shell. What about the field inside a cylinder? Are objects inside a long cylinder attracted to the ...
0
votes
0answers
15 views

How to calculate light transmission in dBm if it goes through SMF to MMF or MMF to SMF?

The question might be very basic, but I could not conclude it after thinking and searching about a couple of hours. If a light in fiber optics is launched in an SMF, therefore, it is connected to an ...
0
votes
0answers
30 views

Density matrix and basis

Given an hermitian operator $\hat{A}$ I can span the Hilbert space in terms of the eigenvectors $\lvert a_i\rangle$. For example in a spin $1/2$ system I can choose to work in a basis from the ...
4
votes
2answers
114 views

Why do we learn only two computations, cross sections and decay rates, in such a fundamental theory as QFT?

When I learned Newtonian mechanics I found a vast variety of computations that I could do and that was so interesting. And it was so when I learned Maxwell theory. When I started learning QFT I hoped ...
1
vote
1answer
32 views

p-cycles and Fluxes

I would like to ask why the existence of a non-trivial p-cycle leads to a non-trivial flux. I would say that e.g. for a five-form $F_{(5)}$ field strength , the flux is: ...
22
votes
4answers
2k views

Difference between sunrise and sunset?

Other than knowing which direction is east and which direction is west, or observing for a sufficient timespan (to determine the direction of motion), is there any way of telling whether what one is ...
10
votes
2answers
631 views

Non-separable solutions of the Schroedinger equation

I'm studying an undergraduate Quantum Mechanics course and I have some doubts about the solution of the Schroedinger equation by the separation of variables method. If we suppose that the solutions ...
2
votes
1answer
108 views

Dirac delta function definition in scattering theory

I'm studying scattering theory from Sakurai's book. In the first pages he gets to the following expression: $$\langle n|U_I(t, t_0)|i\rangle=\delta_{ni}-\frac{i}{\hbar}\langle ...
7
votes
1answer
129 views

How to properly use Perturbation Theory in classical systems?

Context: If we consider a particle in upwards motion near the Earth's surface and acted by a quadratic drag we get the non-linear eom: $$\frac{dv}{dt}=-g-\frac{b}{m}v^2.$$ We can solve it ...
1
vote
0answers
64 views

World's Largest Radio Telesope : How accurately does it need to be figured?

In Guizhou Province, China, the worlds largest radio telescope is almost complete, measuring 500 metres across. I am aware that todays largest optical telescope require very accurate figuring and ...
0
votes
2answers
126 views

About de Broglie relations, what exactly is $E$? Its energy of what?

Well, you may know de Broglie relations, here they are: $$ E = h\nu, \quad\quad p = \frac{h}{\lambda} $$ My question is simple: What exactly is $E$? Is it the total energy? Maybe only kinetic energy? ...
-2
votes
0answers
52 views

Is it possible for humans to jump from such an height and still be fine? [on hold]

This is the game Assassin's Creed Syndicate and here's a video showing how Evie Frye jumped from the Big Ben into a haystack - https://www.youtube.com/watch?v=JfCQe7Lqe60 Those who do not want to ...
-3
votes
2answers
44 views

Electric field $E$ due to a dipole moment

Why is the formula of $$E = kp/z^3~ ?$$ Why is it $z^3$ ? I want a physics answer not math please.
-2
votes
0answers
21 views

Book on thermodynamics for International Physics Olympiad? [duplicate]

I'm a junior in high school studying for the IPhO and I've started learning thermodynamics as part of my preparation. I have a guide to the exam that says things like this will be tested: Carnot ...
2
votes
1answer
144 views

How do the different types of energy apply to voltage in a circuit

I was just wondering what happens in a circuit in terms of different types of energy transformations. If you apply a voltage to a circuit then electrons start moving (very slowly). Since the electrons ...
2
votes
2answers
67 views

Experimental confirmation of the textbook explanation for local entropy reductions

Clarification: In my original wording of this question, regrettably, I did not make it clear that I am interested in the the Boltzmann/Gibbs/statistical (BGS) interpretation of it, as opposed to the ...
0
votes
0answers
44 views

Sum rule and dense limit

Is the finite size of lattice constant important for derivation of sum rules, for example, the sum rule for diagonal conductivity $\sigma_{xx}$: $$ \int \limits_{-\infty}^{\infty} ...
2
votes
1answer
89 views

Two solutions, A+ and A-, for masses connected via springs

My prof gives an example of trying to solve the equations of motion for a series of gliders each connected by springs, with the same spring constant. From looking at the $j-1$ through $j+1$ glider, he ...
-1
votes
2answers
20 views

Electricity and electric fields equations [on hold]

Whats the difference or relationship of $q$ between the equations $E=F/q$, $v=W/Q$ and $Q =ne$ ?
1
vote
0answers
20 views

What shape does magnetic dust take under the influence of its own magnetic field?

Recently, I came across some very powerful neodymium magnets (the flat cylinder kind), and I accidentally broke one by snapping two of them together quickly. What I noticed is that the two halves ...
0
votes
2answers
81 views

Molecular Dynamics simulation question

In molecular dynamics simulations you apply and solve Newton’s equations of motion for a set of atoms to obtain real dynamical information on how individual atomic positions change as a function of ...
5
votes
3answers
3k views

Why can't Iron fusion occur in stars?

It is said that iron fusion is endothermic and star can't sustain this kind of fusion (not until it goes supernova). However star is constantly releasing energy from fusion of elements like Hydrogen ...
6
votes
1answer
403 views

Biot-Savart law from Ampère's with multivariate calculus

Let us assume the validity of Ampère's circuital law $$\oint_{\gamma}\mathbf{B}\cdot d\mathbf{x}=\mu_0 I_{\text{linked}}$$where $\mathbf{B}$ is the magnetic field, $\gamma$ a closed path linking the ...
-1
votes
1answer
27 views

Does closing the shades at night make the room brighter?

Does closing the shades at night make the room brighter? Is the light kept in, or is the dark kept out?
13
votes
5answers
18k views

If I take a bottle of air into space, and open it, where does it go?

It seems to me that space doesn't have any/much air, and if my bottle is full of air, when I open it, where does the air go?
-4
votes
0answers
52 views

What is hyperbolic metamaterial? [on hold]

What is hyperbolic metamaterial? Could you explain in a few words what is hyperbolic metamaterial?
-1
votes
0answers
17 views

How to Determine mass of the ball that is being hit by another ball on the pendulum? [on hold]

I know mass of ball on the pendulum M, initial height of that ball h+H, height of the table on which pendulum is and I know how far away from the table the ball mass m falls. I have the following ...
1
vote
1answer
27 views

Heat loss by radiation reference

In our lab practice, we measured the rate of heat loss of a copper plate in atmospheric pressure and the rate of heat loss of the same plate in high vacuum ($\sim 10^5 torr$). So all I could find for ...
10
votes
5answers
9k views
1
vote
3answers
43 views

Equations of motion derivation doubt

I just started on learning how to derive the equations of motion using calculus. This is my first time and I have a basic doubt. So $v = \large{\frac{dx}{dt}}$ Then, $dx = v dt $ What does this ...
-1
votes
2answers
54 views

Is it possible that there are stars working on fission?

Most/all stars are getting their energy from fusion of small atoms like our sun. But is it possible according to the laws of physics that there are stars getting their energy from fission fe with ...
3
votes
1answer
44 views

$W^{++}$ / $W^{--}$ Bosons in theory and experiment

I wonder whether there is any theoretical interest in and/or experimental search for double charged bosons, probably to be called $W^{++}$ and $W^{--}$. The latter would obviously turn an electron ...
1
vote
1answer
379 views

Magnitude of the average velocity vector (not the average speed)

Thank you ahead of time for taking to look at this. For this following problem we were given an answer however I am almost positive the given answer is wrong. It doesn't even make sense. So here is ...
-1
votes
0answers
15 views

Warm and Hot dark matter density profiles

for Cold dark matter, density profiles are well known and easy to find information about - NFW,Burkert,Einasto, and others. but for some reason i couldn't find explicit expressions for the density ...
2
votes
1answer
72 views

Why is Wick contraction a $c$-number?

It is mentioned in Fetter's Quantum Theory of Many-Particle Systems (in contraction part of section 8 Wick's Theorem), that: contractions are c numbers in the occupation-number Hilbert space, not ...
16
votes
4answers
2k views

Is the statistical interpretation of Quantum Mechanics dead?

I'm sure this question is a bit gauche for this site, but I'm just a mathematician trying to piece together some physical intuition. *Question:*Is the statistical interpretation of Quantum ...
8
votes
3answers
505 views

Polarization of the lowest mode of a Gaussian beam

Most introductory analyses of Gaussian beam optics work within Helmholtz scalar optics, and therefore they ignore the beam's polarisation. Because of that, I'm not clear on what are the possible ...
0
votes
0answers
20 views

How can I calculate the time domain correlation function in the frequency domain?

I have an operator $a(t)$, its correlation function is: $$<a(t)a^{\dagger}(t^{\prime})>=\delta(t-t^{\prime})$$ Now I need to find the correlation function in the frequency domain. i.e. to ...

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