All Questions
34,116
questions
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answers
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How to model energy loss in a rotating body?
I recently asked a question about modeling instability in a rotating rigid body. I now realize that I was mentally confounding two different effects:
The "Dzhanibekov effect" in which a rigid object ...
4
votes
1
answer
2k
views
Gyroscope's motion explained by internal forces
I'm having a hard time trying to figure out the inner mechanisms behind a gyroscope's precession and nutation. To be very specific, I want to consider gyroscopes like this one:
(source: wordpress.com)...
4
votes
3
answers
636
views
Why general relativity over other similar theories?
Since all theories where gravity is seen as a result of space curvature automatically satisfies the equivalence principle, why should one prefer GR over all other alternatives? Surely you can make the ...
4
votes
1
answer
617
views
Why do we assume simply connected domains and continuously differentiable fields in electromagnetism theory?
In many textbooks, including Griffiths', they erroneously claim that a field is irrotational if and only if it is conservative (there exists a scalar potential).
This is true only if the domain of ...
4
votes
2
answers
729
views
If a photon has no mass, how can it be attracted by the Sun?
I read that the photon doesn't have mass, but my teacher says that the photon has mass because the sun can attract it (like in the experiments to prove the theory of relativity).
I think that there ...
4
votes
4
answers
364
views
Why is the speed of light used to define the fourth axis of spacetime?
The four axes of spacetime are $x, y, z$ and $ct$, where $c$ is the speed of light, and $t$ is time. Why is the speed of light (not any other speed) used to define the fourth axis of spacetime? If ...
4
votes
1
answer
4k
views
Why does Hauksbee's electrostatic machine produce light?
I'm reading on the history of the discovery of electricity and the electron, and I've went from reading about Rutherford's gold leaf experiment all the way back to Francis Hauksbee's spinning glass ...
4
votes
8
answers
44k
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Do particles always flow from high to low pressure?
In a recent question, it was stated that
particles in high pressure air always flow to lower pressure.
In a pipe with a constriction, fluid flows from from low to high pressure after the ...
4
votes
3
answers
588
views
How does energy transfer between B and E in an EM standing wave?
I'm trying to understand how an electric field induces a magnetic field and vice versa, its associated energy, as well as relating it to my understanding of waves on a string.
Using a standing wave ...
3
votes
1
answer
1k
views
How to interpret phase diagrams?
I find quite difficult to interpret phase diagrams in general, for example I see people discuss them along the following lines:
Here we see the coexistence line between liquid-solid phases..
a ...
3
votes
0
answers
795
views
Gauge freedom in Lagrangian corresponds to canonical transformation of Hamiltonian
I want to show that the gauge transformation
$$L(q,\dot{q},t)\mapsto L^\prime(q,\dot{q},t):=L(q,\dot{q},t)+\frac{d}{dt}f(q, t)$$
corresponds to a canonical transformation of the Hamiltonian $H(p, q, ...
3
votes
2
answers
399
views
If electrons are waves, how do they repel each other? [closed]
Louis de Broglie said that electrons are waves. But how they repel each other?
3
votes
0
answers
311
views
Non-local gravitational energy tensor
The well-known derivation of the Landau-Lifshitz gravitational energy pseudotensor, relies on several requirements:
1) that it be constructed entirely from the metric tensor
2) that it be index ...
3
votes
1
answer
2k
views
What is the difference between unit and dimensions?
If I say Height of a block = 2m, then would "Height" be called as a dimension
3
votes
2
answers
2k
views
What all has intrinsic spin?
What does and does not have intrinsic spin?
Wikipedia Spin (Physics)
https://en.wikipedia.org/wiki/Spin_(physics)
says:
“In quantum mechanics and particle physics, spin is an intrinsic form of ...
3
votes
1
answer
907
views
Rayleigh-Jeans Law
My question is simple, why do we believe Rayleigh-Jeans law to be absurd? Is the Ultra-violet catastrophe incorrect or is it only because we can not create or know of a perfect emitter? I am a bit ...
3
votes
1
answer
180
views
Why is the central charge $c=9$ supersymmetry in the internal manifold?
In [2] (abstract [here]) (https://inspirehep.net/record/245643?ln=en) they say that, when compactifying any superstring theory, the six dimensional internal manifold must have $N=2$ supersymmetry with ...
3
votes
2
answers
448
views
Is the triple point of water exactly 0.01 $\deg$C?
Is the triple point of water exactly $0.01 \deg$C, or is this an empirical value/is there some uncertainty to it? If so, how do we know it is exact, and why?
3
votes
2
answers
1k
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What is Timelike Quantum Entanglement?
I came across a New Study at http://arxiv.org/abs/1101.2565. Which talks about Timelike quantum entanglement. What does that mean?
Comment added by L.Motl: The same preprint has been discussed in ...
3
votes
2
answers
860
views
Distinction between state space and space of functions
In Quantum Mechanics a particle is described by its wave function $\Psi : \mathbb{R}^3\times \mathbb{R}\to \mathbb{C}$. In that sense, the state of a particle at time $t_0$ is characterized by a ...
3
votes
2
answers
428
views
Determination of the ground state of a field theory
Consider the Spontaneous symmetry breaking in the theory
$$\mathcal{L}=\frac{1}{2}\partial_\mu\phi\partial^\mu\phi-\frac{\mu^2}{2}\phi^2+\frac{\lambda}{4!}\phi^4.$$
By the ground state of a classical ...
3
votes
4
answers
26k
views
Is heat always associated with Light?
I have found that light always produces heat. The only cases I think heat is absent with light is Fluorescence and Phosphorescence (maybe because they emit low energy but maybe the heat is still ...
3
votes
2
answers
2k
views
Factor of 4 discrepancy between integral of Planck's law vs Stefan-Boltzmann law
Consider an absolutely black sphere with unit surface area and temperature $T$. Planck's law states for the power emitted per unit area of the emitter per unit solid angle of emission per unit ...
3
votes
1
answer
2k
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What is the advantage of using exponential function over trigonometric function in analyzing waves?
A.P.French in his book Vibrations and Waves writes:
. . . Why should the exponential function be such an important contribution to the analysis of vibrations? The prime reason is the special ...
3
votes
2
answers
873
views
Charging by induction
When we charge an conductor by induction and grounding, we first bring a negative charge to the conductor. As a result the mobile electrons of the conductor get repelled and stay far from the negative ...
3
votes
2
answers
549
views
The Michelson-Morley experiment
A fundamental assumption in special relativity is that the speed of light is constant in all inertial reference frames, which was first established empirically via the Michelson-Morley experiment. ...
3
votes
3
answers
1k
views
General Relativity 2-Body Closed Form
Is there a closed form solution in general relativity to the 2-body orbit problem?
3
votes
4
answers
4k
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Stress in a rod clamped between two rigid walls when the temperature is increased
The usual approach to calculate stress is to equate thermal expansion in the unclamped condition to the magnitude of contraction caused by strain produced due to the walls. I have some questions about ...
3
votes
3
answers
5k
views
How can we see the moon while it's between the Earth and the Sun? [duplicate]
I know this sounds like (and probably is) a stupid question, but I can't figure it out.
As far as I know, the crescent shape of the moon is when the moon is on the sunny side of the Earth, but that ...
3
votes
4
answers
548
views
Entropy production in isolated system
My question relates to the stereotypical example for understanding the first and second laws: an isolated system filled with a gas of non-interacting molecules with constant ($E$,$V$,$N$) has two ...
3
votes
4
answers
2k
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How to explain what an electron is to someone new to physics? [closed]
I've got asked by someone who just graduated school and is about to start studying physics, what exactly is an electron, if it is not "a small ball rotating around the core of an atom". I couldn't ...
3
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3
answers
1k
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Why are there $1 / 1.602176634 \times 10^{-19}$ electrons in a coulomb?
Why that exact number of electrons in one coulomb? who decided it? there is nothing wrong with the number, it just seems slightly messy. Why didn't the scientific community just settle on an easier ...
3
votes
1
answer
461
views
Angular equation for Schrödinger's equation in 3D
In Griffith's book, section 4.1.2,
the angular equation for Schrödinger equation in 3D is expressed as $$\sin\theta \frac{\partial}{\partial \theta}(\sin \theta \frac{\partial Y}{\partial\theta})+\...
3
votes
2
answers
43k
views
Difference between Speed and Velocity
What is the difference between Speed, Velocity and Acceleration? Could any one describe it pictorially?. I am more over confused even after investigating many times. I am unable to relate myself ...
3
votes
2
answers
332
views
Are gravitational waves effected by the curvature of space time (gravitational lensing)?
I have a basic question I can't seem to find anything on (I keep hearing about how gravitational waves and gravitational lensing were both predicted by Einstein).
We all know about the gravitational ...
3
votes
1
answer
4k
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Why isn't a metal pot a faraday cage?
Someone left their cell phone here, it was ringing like crazy. I stuck it in a metal pot with a metal lid to shut it up, it still rang. I later put it in a safe, it still rang, but so muffled as to ...
3
votes
2
answers
1k
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What defines a cold plasma?
Been looking for this very simple answer for a while now, and google returns a face cream with the words cold plasma in it. Very frustrated.
Just wondering, what constitutes a cold plasma in the ...
3
votes
1
answer
2k
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Why is the potential energy of a particle in a travelling wave maximum at the mean position?
I mean, we calculate the speed of a wave (in a string) by considering the tensional force as a centripetal one (that's obviously an approximation), so shouldn't potential energy decrease in the ...
3
votes
3
answers
589
views
Showing that light bends twice as much as newtonian gravity predicts with special relativity
Light bends twice as much due to gravity as Newtonian theory predicts as discussed in this related question. The argument couched in general relativity, quote from here.
Since most objects move
...
3
votes
1
answer
1k
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Physical understanding of Anderson (disorder) localisation
My current understanding is that waves in disordered potentials experience localisation due to interference effects. (eg an electron in a disordered medium tries to take different paths of effectively ...
3
votes
2
answers
310
views
Violation of Stefan's law when shining a light on a black body
Suppose there is a black body in a dark room and the room temperature is constant. Now a ray light is shone upon it. Since a black body absorbs all radiation that falls upon it, it must absorb more ...
3
votes
3
answers
2k
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What is the distance between two objects in space as a function of time, considering only the force of gravity? [duplicate]
What is the distance between two objects in space as a function of time, considering only the force of gravity? To be specific, there are no other objects to be considered and the objects in question ...
3
votes
4
answers
8k
views
Why is the net work done in a pulley-string system zero?
In any pulley system, where the pulleys and strings are massless and frictionless, why is the net work done by Tension zero?
3
votes
3
answers
6k
views
Is the yellow we perceive when our eyes are hit by red and green light at the same time the same yellow that is at the yellow frequency/wavelength?
I am trying to understand how color works, and I am curious whether the yellow we perceive when our eyes are hit by red and green light at the same time is the same yellow that is at the exact yellow ...
3
votes
1
answer
597
views
A basic question about how to apply the gauge covariant derivative in Yang-Mills theory
I am sorry if this question is too stupid...
We know that Yang-Mills equation (without source) can be written as
$$D^\mu F_{\mu\nu}=0,\tag{1}$$
where $$D^{\mu}=\partial^\mu-ig A^{\mu}$$
and $$A^\mu=A^...
3
votes
4
answers
817
views
Why is a temperature gradient set up in a heated rod? [duplicate]
Suppose a cylindrical rod is maintained at 100 degree Celsius and the other at 0 degree Celsius. My book says that after reaching "Steady State" the rod will have developed a constant temperature ...
3
votes
3
answers
16k
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Can Quantum Entanglement and Quantum Superposition be considered the same phenomenon?
Quantum entanglement is known to be the exchange of quantum information between two particles at a distance, while quantum superposition is known to be the uncertainty of a particle (or particles) ...
3
votes
1
answer
322
views
If Fine Structure Constant in not uniform in space what would that imply for cosmic background?
Recently J.Webb submitted paper which has extraordinary claim - that Fine Structure Constant is different in different directions in space! He (with others) measured $\alpha$ using quasar spectrum ...
3
votes
1
answer
2k
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Clarification in deriving the radial momentum operator $p_r$
In deriving an expression for $p_r$, a particle's radial momentum, I am unsure what is happening at a certain step. The derivation given in The Physics of Quantum Mechanics by Binney and Skinner is as ...
3
votes
2
answers
3k
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Electron Double Slit Experiment-de broglie wavelength relation to distance btw slits
In the 2 slit experiment with electrons, is the distance between the slits related to the individual electron's de broglie wavelength?
In other words, if the slits are too far apart which would ...