All Questions
206,017
questions
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How much energy from underwater explosion can become tsunami?
As the title suggests: what is the energy efficiency of generating gravity waves through an underwater explosion? How do the various parameters (depth of detonation, depth of water, gross energy of ...
- 117
2
votes
1
answer
30
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Utility of Burger's equation in the study of shock waves
Consider the viscous Burger's equation,
$$
\frac{\partial u}{\partial t}+u \frac{\partial u}{\partial x}-\nu \frac{\partial^{2} u}{\partial x^{2}}=0, \tag{1}
$$
with $\nu>0$ the kinematic viscosity....
- 888
2
votes
1
answer
141
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Contravariance and covariance of vectors
My main source of confusion is the following.
Suppose I have a scalar potential $V(x,y,z)$. The electrostatic field for this potential is $ -\vec{E} =\vec{\nabla}V = \frac{\partial{V}}{\partial{x}}\...
2
votes
1
answer
86
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Question on computation with the bra-ket-notation
I have a rather silly question I am afraid... I am just getting to know the bra-ket-notation and still think I did not quite get it...
I want to compute a certain term, which contains the braket ...
- 379
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votes
1
answer
37
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Calculation of interaction of gravitational waves with a black hole using AdS/CFT
We have a well understood interacting electromagnetic system, an electromagnetic wave interacting with an atom. We use perturbation theory to calculate what happens in such a system. The result is of ...
- 59
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2
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97
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Is it true that the Poynting theorem $P=E\times H$ is quite valid for DC circuits?
Can we assume that Poynting's vector theorem $P=E\times H$ is one of the universal laws of physics that applies to electromagnetic fields in AC and DC circuits.Is there A rigorous analysis of ...
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votes
2
answers
51
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Book for a complete review of the basics of physics
Having listened to lectures on the fundamentals of physics, I would be interested in a book/book series that presents the fundamentals of physics completely and in depth.
The following books have ...
- 83
1
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1
answer
30
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Can a quantum field theory be completely simulated by a quantum computer? [closed]
I heard a talk on quantum computing and black hole. In this talk Leonard susskind raised a question: can QFT be completely simulated by using a quantum computer? But he said he was not going to answer ...
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1
answer
41
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Electric Power and Energy
I was asked a question three times this week and I have been giving the same explanation over and over again, but I am starting to question myself.
if you have data on a PV panel production in the ...
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votes
2
answers
43
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Would a black teapot midway between the temperatures of the tea and air cool down or heat up? If neither, then how do the 3 reach thermal equilibrium?
Say you had a matt black teapot with hot tea inside it but cool air around it. Originally, I’m guessing the matt black teapot should absorb more joules of IR energy than it emits per second. This is ...
- 105
2
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2
answers
46
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Doubts about direction of normal contact force acting on a tilted rod supported by another body
I'm confused about the direction of the normal contact force acting on a body supported by another body. I have read the following :
If a body is being supported by a surface, the normal force acting ...
- 35
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1
answer
22
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How many more neutrons are flying around in a fast reactor vs thermal reactor?
I'm a bit confused on the number of neutrons in a fast reactor.
So the average neutron in a thermal reactor is 0.05ev and 700kev in a fast reactor. This means fast reactor neutrons are travelling sqrt(...
- 101
4
votes
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answers
27
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Quantum mechanical description of diamagnetism in hydrogen atom
I am trying to understand the source of diamagnetism for atoms. Classically, we can explain diamagnetism using Lenz's law. I wanted to see whether a full quantum mechanical description gives the same.
...
- 51
2
votes
1
answer
33
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Showing that the integration measure is preserved under gauge transformation in the non-Abelian case
I am trying to show that the integration measure we use in the Fadeev-Popov method of quantisation of non-Abelian gauge theory is invariant under a gauge transformation.
I am using Peskin & ...
- 1,709
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25
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Why are the electric and magnetic fields of a charged particle a single entity and should be mutually orthogonal?
I think Maxwell's four well-known equations dictate the correct answer.
I guess the question is a general rule (no specific problem) in electromagnetism as well as Maxwell's 4 equations themselves.
...
-1
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1
answer
30
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Circuit with multiple cells homework question [closed]
I'm having trouble determining the reading on the voltmeter in this problem. I've tried applying Kirchoff's loop law to the large outer loop including both cells. When I did that, It gave me a current ...
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3
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1
answer
50
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Earth is spiraling away from Sun at rate of 1.5cm per year due to mass loss of Sun? How it was calculated? [closed]
My physics teacher asked if we could calculate the rate at which Earth moves away from the Sun due to the mass loss of the Sun. It's very sensible for me to understand that Earth is spiraling away ...
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1
answer
31
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Paradox of light passing through prism and glass slab
When white light passes through a glass slab, Newton's assumption is that a slab is made of 2 glass prisms placed in opposite direction, such that dispersion of both is cancelled out and a white light ...
- 73
2
votes
1
answer
74
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Do-It-Yourself physics experiment [closed]
Is there any simple experiment in physics of the first half of the 20th century I could do at home? I tried to make a cloud chamber, but it didn't work at all... A spectrometer from a CD disk is ...
2
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0
answers
60
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Computing Gravitational constant $G$ from Cavendish's data
[I asked this at History of Science and Mathematics but there was no answers. I'm asking here too, after all $G$ is fundamental in physics.]
I'm trying to compute the value of $G$ from Cavendish's ...
- 47
4
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1
answer
54
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Electric potential vs electromagnetic potential questions
We were all taught about the electric potential $V$, which is defined up to a constant, and can be measured with a voltmeter or an oscilloscope.
On the other hand, in electromagnetism are defined the ...
- 266
3
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1
answer
73
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What does $\delta/\delta t$-derivative represent in tensor calculus?
Some texts, such as Pavel Grinfeld's, talk about a $\delta/\delta t$-derivative whose role (in trajectory analysis of particles using tensor calculus) is pretty obscure to me. For example, the ...
- 290
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1
answer
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Peskin & Schroeder QFT，eq. (2.56) derivation
I'm trying to derive the eq.2.56 of P&S's QFT textbook step by step:
$$(\partial^2+m^2)D_R(x-y)=-i\delta^{(4)}(x-y). \tag{2.56}$$
I have no problem with the first step:
$(\partial^2+m^2)D_R(x-y)=(\...
- 11
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1
answer
35
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Energy and pressure relationship for cosmic rays [closed]
How to prove $E = 3P$ for cosmic rays? In other words $\gamma = \tfrac43$ for cosmic rays. Whereas for $\gamma=\tfrac53$ we have $E = \tfrac32 P$.
user292327
-7
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0
answers
51
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Why not study quantum mechanics rather than waste time on relativity? [closed]
Relativity is a dubious tweak to Newtonian Gravity why study it?
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23
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6
answers
3k
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Is it correct to say that it is theoretically impossible for perfect rigid bodies to exist?
If perfect rigid bodies were to exist, then consider a scenario in which two rigid bodies of equal masses moving with velocities of equal magnitude but opposite in direction colliding against one ...
- 398
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10
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Static Equilibrium in fluids and calculating pressure in a moving fluid
Can the below figure support a static state? I presume that is what the question wanted but intuitively the fluid in the diagonal would want to go down and raise the level of the right arm. Am I ...
- 95
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0
answers
14
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How to deduce the Schrödinger equation by assuming a one-dimensional finite potential well? [closed]
I know the deduction of the Schrödinger equation by assuming a one-dimensional infinite potential well but I can't find the one-dimensional finite potential well.
2
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3
answers
925
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Do electrons have inertia?
I don't know quantum mechanics and know a little bit about mechanics and a very little bit about subatomic particles but I am just a curious to know the answer to my question. My question is if '...
- 145
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20
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Berry Phase Computation
Equation $(6.7.16)$ in the book Lectures on Quantum Mechanics (second edition) by Weinberg reads
$$\gamma_{n}[C]=n \iint_{A[C]} d A \frac{\mathbf{B} \cdot \mathbf{e}[\mathbf{B}]}{|\mathbf{B}|^{3}}\tag{...
- 19
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15
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What energy is produced in cable connecting the two terrestrial magnetic poles following this discovery of a variable magnetic field around the earth?
First thank you for trying to understand the content of the message not the form because I do not speak English.
Principle of operation with scientific sources:
Earth's interior is a far from quiet ...
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0
answers
37
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Cosmological redshift
We have three of them (and combinations):
gravitational, when light travels near masive object (red shift)
Doppler's, when light is emited by moving object (red or blue)
cosmological - all far ...
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28
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Commutators between momentum and total angular momentum [closed]
Do $p^2$ and $r^2$ commute with $J^2$? Where $p$ is the momentum, $r$ is position coordinate and $J=L+S$ is the total angular momentum. I think the answer is yes due to the fact that the angular ...
- 35
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0
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17
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Help with the following circuit [duplicate]
Considering a simple bridge circuit as shown in the figure:
Given that $I_i\ |\ i \in \{0,1,2,3,4,5,6\}$ are the different currents present in any branch of the circuit, and that $I_{1,2,3,4,5}$ are ...
1
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0
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46
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Why is the Hubble law so accurate at scales smaller than galactic voids?
It's possible to derive the Hubble law:
$$v = H_0 d$$
from the FRW metric by differentiation.
Experimentally the Hubble law appears to hold for relatively small distances, say 20 Mpc and smaller. ...
- 591
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1
answer
26
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Separability of an Hamiltonian with spin
I'd like to know if this Hamiltonian $\hat{H}=\frac{p^2}{2m}+\frac{1}{2}m\omega^2r^2+\frac{A}{\hbar^2}(J^2-L^2-S^2)$ is separable into two parts $H_1=\frac{p^2}{2m}+\frac{1}{2}m\omega^2r^2$ and $H_2=\...
- 601
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50
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Book which deals with constraint motion using differential forms
I'm interested in books going more over the approach described in this post. Here is a quote of the total answer in case anything happens to it:
Calculating the wedge product $df_1\wedge df_2$ gives $...
1
vote
2
answers
64
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Criterion for stationary density matrix
A density matrix $\rho$ is time independent iff it commutes with the Hamiltonian $H$. I am wondering if there is a criterion to test whether $[\rho, H] =0$ using some trace condition. Specifically, I ...
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1
answer
51
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Why do two magnets faceing eachother with opposite poles but offset horizontally repel?
When you place two magnets with opposite poles facing eachother vertically they attract. However if you move one magnet horizontally by a distance greater than the magnets diameter they will begin to ...
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0
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16
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How (if) can we connect a 2D "throat" piece of a wormhole to two hyperbolic 2D manifolds?
This question wad closed on the mathematics site, as it lacked clarity. So I try my luck here. My question is cosmology-inspired.
Imagine two 2D hyperbolic manifolds. I connect them by a manifold like ...
- 1,653
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Contradiction from Poisson-Boltzmann equations -- Gouy-Chapman model
Recently I have been investigating the problem of a neutral solutions (with equal number of anions and cations) interacting with an infinite surface $z=0$ with electrostatic potential $\phi(z=0)=\...
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1
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Is the identity $\rho=\sum_m M_m\rho M^t_m$ possible for measurement operators with $\sum_m M_m^t M_m=I$?
Nielsen and Chuang quantum information book has the following identity
$$\rho=\sum_m M_m\rho M^t_m$$
Where $M_m$ are measurement operators and $\sum M^t_m M_m=I$.
I suspect it must be a typo. I think ...
- 154
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votes
2
answers
48
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Angular Momentum about a Point
I am confused by two different definitions of the angular momentum of a particle P about a moving point Q, with point O as the origin of the inertial frame. I learned the first definition from my ...
- 31
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0
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10
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Transformation of density with finite deformations
Initially, there is a infinite continuous medium with a given mass density $\rho(\vec r)$, which is a smooth function that vanishes outside of a sphere of radius R. Then, a deformation of medium is ...
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0
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15
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How to predict the time domain of pulse from an amplitude mask in frequency domain?
I am studying ultrafast spectroscopy (pump-probe) and I know that ultrashort laser pulses are used for the pump and probe. These pulses often contain multiple frequencies (i.e. polychromatic pulse) ...
- 97
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0
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27
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Most efficient method to evaporate a metal in a vacuum?
If one had a metal block (lets say tantalum) in a vacuum chamber, what would be the most efficient way of evaporating it in regards to the proportion of the input energy directly going into removing ...
- 83
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2
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34
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What is the current density of a circular current?
Lets consider a wire loop in the $xy$-plane of radius $R$ which carries a current $I$. I want to find the current density $\vec{j}(x)$ of that configuration expressed with delta functions.
Intuitively,...
- 13
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0
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10
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The cavity method of Ising model in infinity dimension and dynamical mean field
In the article "Dynamical mean-field theory of strongly correlated fermion systems and the limit of infinite dimensions" chapter III.A, when discussing the cavity method of Ising model, the ...
- 396
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2
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85
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What would be the value of $c$ where $c$ is the speed of light?
From the equation of $E=mc^2$, where $E$ is the energy, $m$ is the mass and $c$ is the speed of light, so from that we can get $c=\sqrt{Em}$, again from the equation of $\lambda=v/f$, where $\lambda$ ...
0
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1
answer
32
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How would the vacuum look like if no mass/energy is present in it?
The observable universe is thought to inflate faster and faster towards infinity. The energy will be diluted to zero in the far future.
The influence of attractive gravity is getting smaller and ...
- 1,653