Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Is nuclear power desireable in the long term, given the fact that it's an unnatural heat input to our planet? I've been reflecting on whether we want nuclear at all in the long term (compared to renewables like wind, solar, and hydro). There's a certain amount of heat (energy) entering our planet and leaving it. Greenh... | All known other energy sources including solar and wind energy produce extra waste heat into the planet that would otherwise be lost. All human energy needs are adding extra tiny amounts of energy- nuclear is not special.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/378506",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
"answer_count": 6,
"answer_id": 5
} |
Interval Preserving in Minkowski Space The squared line element in any spacetime is given as $$ds^{2}=g_{ab}dx^{a}dx^{b}.$$ The use of tensors helps us to infer that the line element in some other frame would be $$ds'^{2}=g'_{ab}dx'^{a}dx'^{b}$$ where simply $dx'^{a}=\frac{\partial x'^{a}}{\partial x^{b}} dx'^{b}$.
My... | Here why $\eta_{ab} = \eta_{mn} \Lambda^m_a \Lambda^n_b$ in SR (special relativity).
$ds^2 = \eta_{ab} dx^a dx^b$ (1)
$ds^2 = \eta_{mn} dx^m dx^n$ (2)
but:
$dx^m = \Lambda^m_a dx^a$ Lorentz transformation
so:
$\eta_{mn} dx^m dx^n = \eta_{mn} \Lambda^m_a \Lambda^n_b dx^a dx^b$ (3)
by equating (1) and (2), taking count o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/378824",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Why must the wavelength of light be smaller than uncertainty in position? I have read that to measure the position of a particle to an accuracy $\Delta x$, we need light of wavelength $\lambda < \Delta x$. Is it true? Why is it so?
| This may not be the answer expected, however it is not true if you have a light source that is very narrow bandwidth and stable.
You can interpolate between fringes on an interferometer. For example, a commercial laser interferometer using a 633nm source can achieve a 1nm resolution.
Even if you simply bounce light o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/379301",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 4,
"answer_id": 3
} |
Why does it take time to melt ice when the loss of magnetization of a material at its Curie temperature is immediate? Everything is in the title: Why does it take time to melt ice when the loss of magnetization of a material at its Curie temperature is immediate?
Have you an explanation for this difference?
| In addition to Valerio answer, while liquid water has a higher energy level than ice (which you need to provide for the phase transition. Magnets, on the contrary store some potential energy (You can observe it if you break a magnet parallel with the field lines, parts will turn to make a shorter magnetic circuit, and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/379440",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Is it possible to fall into a black hole, then come back out? Everybody knows that you can only fall into a black hole and you can never get out. But on second thought, this flies in the face of time reversal invariance, which says that every process that goes forward and also go backwards. Moreover, the standard Schwa... | From the point of view of an outside observer, the object never reaches the event horizon, so it can come back. Also, it's possible for the black hole to emit Hawking radiation that coalesces into a spaceship emerging from the black hole. The probability of this happening is astronomically small. Thus, this provides a ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/379549",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 3
} |
Why is static friction providing centripetal force? This could be tagged as a duplicate, but I couldn't understand why static friction causes centripetal acceleration in a car taking a circular turn which is moving. Mustn't it be kinetic /sliding friction as the object is moving?
| Regarding things that are rolling such as wheels of a car, remember one key thing: kinetic friction is not about moving, but about sliding.
Even though a wheel is moving, it isn't sliding over the surface. There is no kinetic friction going on. Only static friction which holds the contact point still while it is in con... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/379678",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What does Mobius group/transformations have to do with special relativity? The group of Mobius transformations, denoted by ${\rm Mob}(2,\mathbb{C})$, is isomorphic to ${\rm SL}(2,\mathbb{C}))/\mathbb{Z}_2$ which in turn is isomorphic to the Lorentz group ${\rm SO}^+(3,1)$.
This connection, to me, seems very intriguing... | *
*A future-directed light-ray $$n^{\mu}~=~(1,{\bf p}/|{\bf p}|), \qquad {\bf p}~\in~\mathbb{R}^3\backslash\{\bf 0\}, \tag{1}$$ can be identified with a non-zero future-directed light-like 4-vector $$p^{\mu}~=~(E,{\bf p})\tag{2}$$ if we mod out with the energy $E\equiv |{\bf p}|>0$.
*Therefore the set of future-direc... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/379945",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 0
} |
Electric potential just outside a spherical shell We know that as we get closer and closer to a point charge, the electric potential approaches infinity.
Since electric potential at the surface of a spherical shell is finite (Gauss law) , so on moving away from the surface it would fall. In other words, it would be fin... | The problem here is symmetry. Use Gauss's law on an infinite sheet of charge. You get a finite (and constant field):
$$ E = \frac{\sigma}{2\epsilon_0} $$.
($\sigma$ is the charge density). It does not matter how close you get, or how far you get: the problem is scale independent, because at any point, you always see th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/380080",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Does in theory spacetime holds all the data of everything? A Question that is bothering me lately.
In my mind spacetime has been in contact with all existing particles and continues to be in contact.
If mass bends spacetime then in quantum scale no matter how small it is there is got to be bend in spacetime and those b... | There is active research on a correspondence between spacetime and information, but it goes the other way round, with the idea that information may give rise to spacetime via entanglement.
The catch words are "It from Qubit"; see an introduction in this Scientific American article. It is yet another venue from the AdS... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/380251",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Heat produced when dielectric inserted in a capacitor When a capacitor is connected to battery, it stores $\frac{C V^2}{2}$, while battery supplied $CV^2$ energy. Therefore, $\frac{C V^2}{2}$ energy gets lost as heat. When a capacitor is already charged and a dielectric is inserted in this charged capacitor (which is s... | Yes, because when you add the dielectric more charge will flow onto the plates of the capacitor.
The energy on the capacitor is initially:
$$E_{c,i} = \frac{1}{2}CV^2 $$
After you add the dielectric:
$$E_{c,f} = \frac{1}{2}C'V^2 $$
So the change in energy is:
$$\Delta E_{c} = \frac{1}{2}V^2 (C'-C)$$
However, for th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/380558",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
How can we explain the linear relation between the temperature scales? In the proof of the relation F=(9/5)C+32 (degrees celsius to fahrenheit) we assume that there is a straight-line (linear) relation between the two temperature scales: F=mC+b. Then we need two "points" to find the equation for that line.
How can we e... | If we demand that our temperature scale respects the zero'th law of thermodynamics and it is an integrating factor to the heat vectorfield, then our temperature scale is unique up to linear rescaling (affine rescaling not even allowed): the elaboration required for such a statement is adequately contained in my answer ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/380892",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 6,
"answer_id": 5
} |
Gravity in vector We know that gravity is a force. But what is it's direction? Can it be expressed by vector and how can we do that? This question can also be asked for Coulomb's Law.
| Imagine you have only two masses in the universe $M$ and $m$, then the gravitational force that $m$ feels due to $M$ is indeed a vector that points towards $M$. This is called a central force, and as you point out Coulomb's force also behaves like that.
If you add another mass $M'$ into the picture the problem becomes ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/381533",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
L-band for search radars? I'm making my way through the Linesman-related section's of Gough's Watching the Skies. Linesman was developed to counter the carcinotron jammer.
The main solution was the Type 85, which had 12 frequencies switching each pulse at random.
However, they also deployed a second radar, the Type 84.... | If the radar’s mission is to search a given solid angle in a given period of time, then there is no real advantage (other than accuracy) to using a higher frequency, because a narrower beam will not be able to dwell as long on any given position. If the design features a phased array antenna, it is less costly to buil... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/381791",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
soft $Z_2$ symmetry breaking in the context of the Two-Higgs doublets model Imposing additional symmetries to the Higgs doublets in the Two-Higgs doublets model, like the $Z_2$ symmetry, aims to reduce the number of parameters in the potential. But it is said that the mass term $m^2_{12}$ term breaks it "softly" what i... | Actually, imposing $Z_2$ on the two Higgs doublet model (2HDM) is usually a strategy to prevent flavor changing neutral currents, which are very restricted by experiment. Alternatively, it is also a way of impose CP invariance in the model.
With this in mind, $Z_2$ symmetry removes the lagrangian parameters $\lambda_6$... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/382234",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Harnessing permanent magnetism? Putting aside any energy generating schemes that would break the laws of thermodynamics, is it possible or is there a motor which generates power using a permanent magnet? So that the energy wouldn’t be coming from nothing but from the atoms in the magnet being misaligned.
| The idea of a "magnetic motor" pops up on the interwebs almost weekly, accompanied often with youtube clips of the device "actually running", conspiracy theories about the idea being suppressed by government agents, and so forth. Too bad it is impossible.
The best disproof I have read about the fundamental idea of bui... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/382341",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Original 1925 paper by Einstein on Bose-Einstein Condensation? Does anyone know if it is possible to retrieve the original 1925 paper by Einstein on Bose-Einstein Condensation? Possibly a translation into english, but german would be fine if no translation is available. I have managed to find a translation of Quantenth... | I found the following translations :
*
*Quantum Theory of the Ideal Monoatomic Gas (1925).
*The first paper (1924) can be found in translation here Quantum Theory of a Monoatomic Ideal Gas.
As I don't understand German I can't speak to the accuracy of the translations, but they do seem to have been done by someone... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/382713",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
What is the equivalent of piezoelectricity for optics/photonics? Piezoelectricity can be considered as the electromechanical transduction behaviour, where a material can convert mechanical deformation/energy directly to voltage difference / electricity or vice versa. Are there any materials which have the same behaviou... | Photons don't interact the same way with matter as electrons in a crystal lattice. It's the rearrangement of electrons that causes the material to deform. Photon's on the other hand are packets of energy that are said to be mass-less while they travel through space and are converted or release their energy when they co... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/382967",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Milky Way: where does it end? The milky way galaxy, as is typical for spirals (and others?), has a relatively flat rotation curve except very near the center:
Eventually, at great enough distances, almost the entire mass of the galaxy will be enclosed and the rotation curve must become nearly Keplerian and fall off (d... | Galaxies generally have quite flat rotation curves even at very large distances (much farther away from all the stars forming the galaxy).
At such distances most if the ordinary matter of the galaxy is just a heated gas. The rotation velocity of this gas is measured by observing the Doppler shift of some particular spl... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/383161",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Force without magnetic or electric field In a long primary solenoid,if we introduce a secondary one on its outer surface,due to mutual inductance-an emf is generated in the secondary one according to Lenz's law if there is a time varying current in the primary coil.
But,
*
*1.we know that for an infinite solenoid,th... | According to Faraday's Induction Law, the integral form of the Maxwell-Faraday equation $$ \oint_{C} {\bf E} \cdot d{\bf l} = -\frac{d}{dt} \int_S {\bf B} \cdot d {\bf S}$$ (valid for stationary surface/boundary), you don't need a magnetic field at the location of the outer coil for an electric field to be induced ther... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/383269",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
In what sense are quasiholes and quasiparticles "excitations" in Fractional Quantum Hall (FQH) systems? In the theory of Fractional Quantum Hall states, one often sees quasi-holes and quasi-electrons (or quasi-particles) being called "excitations" from the ground state initially given by Laughlin (Jastrow-Laughlin styl... | The full microscopic Hamiltonian of the Hall effect
$$ H = \sum_j \frac{1}{2m} \left [\frac{\hbar}{i} \nabla_j + \frac{e}{c} A(\mathbf{r}_j) \right ]^2 + \sum_{i<j} \frac{e^2}{|\mathbf{r}_i - \mathbf{r}_j|}$$
In the quantum Hall effect, the dynamics is restricted to the lowest Landau level due to the application of ver... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/383450",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Why Hubbard model cannot be solvable exactly? The Hubbard model is a model to describe electrons in a lattice. In general, the Hubbard model Hamiltonian $H$ contains two terms:
*
*The kinetic term:$$T=-t\sum_{\langle ij\rangle\sigma} [c_{i\sigma}^\dagger c_{j\sigma} + h.c.] $$
*The onsite Coulomb interaction term:
... | As for the question of dimensionality: Yes, the two sets of solutions for $T$ and $H$ have the same dimensions (Because they act on the same Hilbert space).
The pedestrian reason why you cannot solve the Hubbard model analytically is that the $U$ term contains quartic interactions ($c^\dagger c^\dagger c c$), and it is... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/383854",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Moment of Inertia of an Equilateral Triangular Plate I was reading about moment of inertia on Wikipedia and thought it was weird that it had common values for shapes like tetrehedron and cuboids but not triangular prisms or triangular plates, so I tried working it out myself. I will post my attempt below, but for some ... | Another solution is to integrate the triangle from an apex to the base using the $\iint{r^2 \text{d}m}$, which becomes $(x^2+y^2)\text{d}x\text{d}y$. Using the limits of $x$ to be $0$ to $h$, and the limits of y to be $-x\tan30°$ and $+x\tan30°$, you get the moment of inertia about an apex to be $0.32075h^4M/AL$, wher... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/384336",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 0
} |
Is there a type of magnet that will only attract one type of metal? Is there a type of magnet that will only attract one type of metal. For example if I were to me in a room full of small metal objects and I turned on a 'special' magnet, would it be possible that only one of those metals in that room react?
| All magnets producing the same magnetic field $B$ have the same effect on other objects. Under normal circumstances you will only be able to sort out (attract) ferromagnetic objects with a magnet. Some of these ferromagnetic objects (materials) might be more strongly magnetized and thus attracted than others. With ea r... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/384707",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Is there evidence that a = dv/dt and a = F/m are always equivalent? If the rate of change in velocity in a particle (of mass m) caused due to a force F is dv/dt, then
F = m dv/dt
It may be argued that this is how we define force. But my question is:
Can there be any kind of force, which is so strange that no matter ho... | If $F = \frac{d(mv)}{dt}$ fails in just one example, then you have disproven Newton's 2nd law by falsification. Then you will get the next Nobel price.
The thing is that Newton's 2nd law is not provable. But after infinitely many experiments, no one has ever been able to disprove it with normal size scales and speeds. ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/385010",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Friction during pure rotation I have already read
*
*Role of friction during pure rolling
*What is the direction of the friction force on a rolling ball?
*Rolling resistance and static friction
*and many others
But I still have the following problem:
*
*On one hand I've been told that if pure rotation takes ... | *
*Pure wheel rotation in outer space: will rotate forever.
*Pure rotation in outer space plus CM linear velocity: will rotate forever and CM will move forever.
*Same as (1) on a frictionless table: rotate forever.
*Same as (2) on a frictionless table: Rotate forever and move forever.
*Behavior on a table with fr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/385115",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 2
} |
What would happen if the cathode ray experiment by jj Thompson was done in complete vacuum?
Cathode tube has complete vacuum and potential difference of 10,000. Volts
There is a hole in anode for electrons to pass.
| I can't find the details of the equipment J. J. Thomson used, but I believe it did not use a heated electrode i.e. it did not use thermionic emission as the source of the electrons.
In that case the electrons were generated by an avalanche process in the gas in the tube. Any stray electrons, for example produced by cos... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/385280",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Software for simulating gas of hard particles (i.e. polyhedra?) I was wondering if there existed a kind of software I could use to simulate a gas of polyhedra, such as tetrahedra. They would interact through entropic interactions only, i.e. excluded volume.
I'm not an expert in simulation so any kind of suggestion is w... | LAMMPS is a good option, as suggested by lemon.
The region command can be combined with the create_atoms command in order to fill a region of space of defined shape with atoms, with the crystal arrangement that you prefer. You have to
The region command allows you to define the following shapes:
*
*cube
*cone
*c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/385433",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Proof of constructing action-angle coordinates on Hamiltonian system By Liouville-Arnold Theorem, we know we can construct action-angle coordinates such that the Hamiltonian system, when described in these coordinates, will have a form that is integrable by quadratures. I am looking at a proof of the construction of th... | It seems to me that they are making the identification $F_i = I_i = y_i$ where the $y_i$ are the ones discussed in Theorem 5.3 (page 174) and, as they mention, they assume the hypotheses of Theorem 5.3 to hold. As a consequence, they say that Hamilton's equations with respect to all $F_i$ have the form $\dot{\phi}_m=\l... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/386062",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Energy in simple harmonic motion ─ where is the kinetic energy stored, and where is the potential energy? When a mass connected to a spring is in simple harmonic motion and somewhere between the mean and extreme positions the mass is cut from spring. Then instantaneously after cutting the mass will only have its kineti... | You are quite correct. Elastic potential energy is always stored in the spring. Then why does the block move when compressed though it does not have any energy?
Let us consider a situation in which a spring is compressed by a (non attached) block. When compressed the spring will acquire elastic potential energy given ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/386298",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Why do charged particles deflect one way but not the other in a magnetic field? I am well aware that a charged particle moving in a magnetic field will experience a force perpendicular to that magnetic field. But why is it that positive and negative particles experience a force in opposite directions?
What exactly dete... | Electromagnetism is symmetric with respect to parity. That symmetry is broken by the convention we choose to use for defining the magnetic field vector. Aliens on another planet could define magnetic fields to point in the opposite direction compared to our definition. They would then use a left-handed rule $\textbf{F}... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/386391",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 1
} |
Is this a black hole? I came across this metric definition:
$ds^2=-\left(1-\frac{r_s}{r}\right)^2dt^2 + \left(1-\frac{r_s}{r}\right)^{-2}dr^2+r^2d\Omega^2$
I was trying to figure out if it describes a black hole, but I was unable to find the proper definition.
My hints so far:
*
*The time coordinate does not change... | We need to be careful about terminology because the phrase black hole tends to be used to mean a vacuum solution. The only two vacuum solutions are the Schwarzschild and Kerr metrics, so in everyday use black hole means either the Schwarzschild or Kerr metric.
But if we relax the requirement for the geometry to be a va... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/386551",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Thermal energy generated by collision observed from two different frames of reference An isolated system is composed of two bodies $A$ and $B$, with masses $m_A$ and $m_B$, $m_A \ne m_B$, which are in route of collision. The relative velocity between them is $v$. The collision is inelastic and I want to calculate how m... | The usual laws of kinematics are valid in inertial frames of reference only. In particular, inertial frames of reference move with constant velocity. So you can't apply the laws of physics as you've learned them if your reference frame is actually fixed to a body that undergoes acceleration. You can have the referen... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/386652",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Counting massive degrees of freedom after gauge fixing Consider the theory of scalar QED with the Lagrangian
$$\mathcal{L} = - \frac14 F^{\mu\nu} F_{\mu\nu} + (D^\mu \phi)^* (D_\mu \phi) - m^2 \phi^* \phi \tag{1}$$
where $\phi$ is a complex scalar field with mass $m$. Counting the degrees of freedom, we have
*
*two ... | Before we can compare with the second Lagrangian (2),
the first Lagrangian (1) should include a gauge-fixing term ${\cal L}_{\rm gf}$, e.g. ${\cal L}_{\rm gf} = \lambda~ {\rm Im}(\phi),$ where $\lambda$ is a Lagrange multiplier. After integrating out $\lambda$ and ${\rm Im}(\phi)$ the Lagrangian (1) becomes the Lagrang... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/387049",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 2,
"answer_id": 1
} |
Noether charge for Lagrangian with higher-order derivatives I'm trying to find the Noether charge for the symmetry
$x\rightarrow x+f\left(x\right)$
This transformation should leave the action invariant, so
\begin{align*}
dS&=S\left(x+f\left(x\right),\dots\right)-S\left(x\right)=0\\
&=\int dt\ \mathcal{L}\left(x+f\left(... | In this answer, we will just list the result without a proof. For a higher-order action
$$ S[q]~=~\int\! dt~ L(q(t), \dot{q}(t),\ddot{q}(t),\dddot{q}(t),\ldots,t) \tag{1} $$
with a vertical infinitesimal quasi-symmetry
$$ \delta q^i~=~\varepsilon Y^i(q, \dot{q},\ddot{q},\dddot{q},\ldots,t) , \tag{2} $$
the bare Noether... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/387320",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Gravitational potential at the center of a uniform sphere Feynman in second Messenger Lecture said that potential at the center of ball with small radius $a$ is equal to average potential on surface of ball minus $G$ times mass inside the ball divided by $2a$. You can see it on the picture. I don't understand that. I h... | Late answer but I'll bite.
Feynman's talking about a ball, which means that he is talking about a solid sphere, with uniform density, which I shall call $\rho$. You can apply Gauss's law for gravity to then calculate the potential.
Gauss's law states that:
$$ \int FdA = -4\pi GM$$
where F is the g-field, A is a surf... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/387439",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Trouble in understanding AM modulation Amplitude modulation is in fact, superimposing the low frequency transmission signal into a high frequency carrier signal, right?
So, if the transmission signal can be represented as $c(t)=A_c \sin (\omega_ct )$ and the carrier wave can be represented as $c(t)=A_m \sin (\omega_mt)... | You are right that by superimposing a constant modulation on the carrier wave you cannot transmit a signal because you get a constant single frequency carrier wave plus two constant sideband frequencies. For the transmission of a signal, you need a modulation changing in time (frequency and/or amplitude) like in speech... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/387564",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
How is semicolon derivative notation defined for multiple derivatives? I have a covector $\eta_\mu$. Then I have some notation which says $$\eta_{\alpha;\beta\gamma}$$ What does this mean? I understand that given a vector $A^\alpha$, that $$A^\alpha_{;\beta}=\nabla_\beta A^\alpha=(\nabla A)^\alpha_\beta$$This makes sen... | Actually, $$\eta_{\alpha;\beta \gamma} = \eta_{\alpha;\beta;\gamma} = \nabla_{\gamma} \nabla_{\beta} \eta_{\alpha}$$ Your result is only the same in the flat spacetime, where the Riemann tensor (which is related to the commutator of covariant derivatives) vanishes.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/387689",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Is the double slit experiment performed measuring single photons? As a hobby physicist I don't understand how the double slit experiment is performed in terms of single photons. Is the experiment really conducted by sending and measuring single photons? Is there not always interference of some kind (except for some dev... | Yes, there are single photon through a double slit experiments , as also single electron ones.
Here is one single photon at a time:
. Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.
You ca... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/388026",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How do convective clouds form? What are necessary conditions to form convective clouds with their flat cloud base, like this one:
I know that convective instalibity at the ground is at start of the process, but I have realized that I don't understand what exactly determines the height of the cloud base. Usually, it is... | There is no bubble and Convective instability isn't used to explain the cloud base.
It starts with a mixture (wet air) of dry air and water vapor. Hot mixture moves up due to buoyancy, is cooled, and moves down, establishing a vertical convective circular flow.
Temperature above the earth surface decreases over heigh... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/388154",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Definition of stress-energy tensor
The image from the wiki article on the stress energy tensor gives $T_{00}$ as $1/c^2$ times the energy density. I believe this is incorrect and that the $1/c^2$ factor should be dropped. Am I missing something?
| The stress-energy tensor can be written with $T_{00}$ as an energy density or a mass density. The latter is of course just $E/c^2$ in accordance with Einstein's famous equation $E=mc^2$. Both forms are used and neither is more correct that the other.
In any case general relativists usually choose units where $c=1$ and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/388442",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Are all waves either transverse or longitudinal? So I recently searched up "em wave transverse proof", and I understood it pretty well enough I think.
After that, I just started to wonder if all waves are either transverse/longitudinal. If there are waves that are neither one of them, how do we put that in mathematica... | Short answer: no.
For example, gravity waves (i.e. ripples) on the surface of a liquid have both transverse and longitudinal motion, so they are not purely either.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/388543",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
Why is the internal energy of a real gas a function of pressure and temperature only? While studying thermodynamics, I read that the internal energy of an ideal gas is a function of temperature only. On searching the internet, i found an article which stated that the internal energy of a real gas is a function of tempe... | First off, let's start with a fundamental assumption: the internal energy of an gas consists of only its kinetic energy. This is the basic assumption of kinetic gas theory, and is a reasonable assumption for real gases. The temperature of any system is simply the average kinetic energy. Therefore, the kinetic energy of... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/389127",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 5,
"answer_id": 2
} |
What is the shortest distance between electron and positron before they are annihilated? I just want to know how does an electron felt the presence of a positron before they are converted into energy? Also how does the electron tell if it is positron or proton if this makes any difference?
| I don't think there is a simple answer to this.
The problem is that in quantum electrodynamics the particles we call electrons and positrons are the states described by the quantum field in the limit of negligible interactions i.e. when the particle is too far from any other particles for any significant interaction to... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/389213",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 3,
"answer_id": 1
} |
$Q$ value of beta plus decay (positron emission) I am unable to understand Q value for positron emission. The general reaction is as follows:
$$p \to n + e^+ + \nu$$
$$ ^A_ZX \to ^A_{Z-1}Y+ e^+ + \nu \tag{1a}$$
This reaction $(1a)$ was giving in my text. First question is that where did one electron go? We began with ... | You should write the reaction in terms of atom and ions as
$$^{\rm A}_{\rm Z }\rm X \Rightarrow ^{\,\,\,\,\,\rm A}_{\rm Z-1 } Y ^-+ e^+$$
noting that the daughter is a negative ion which has a net negative charge because it still has $Z$ elections orbiting the nucleus.
Think of it as a change happening in the nucle... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/389442",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Is potential energy a type of energy at all? Is potential energy, whether it be that of a charge in an electric field or a mass in a gravitational field or anything like that, actually an energy that the particle itself contains, like kinetic energy? Or is it just a measure of its ability to do work?
Is it the case th... | Maybe we can say that a particle "contains" chemical energy, since we have to break its chemical bonds to access to chemical energy, and maybe we can say that a particle "contains" nuclear energy, since we have to break nucleons to access to it.
Kinetic energy, however, depends on movement, which means that it depends ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/389496",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 0
} |
What is the reason for the edge effect in capacitors? The electric field lines bend at the edges of the capacitors like this:
What is the reason for this? Any quick explanation as to why they bend?
| From answer:
This means that the electric field near the edges of the plates is actually larger than the electric field between the plates
Possible correction: the electric field just above the curvature ($E_1$) is the same as the electric field between the parallel plates ($E_2$). $E_1$ from the same plate reduces ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/389766",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 4,
"answer_id": 3
} |
Why is the singlet state for two spin 1/2 particles anti-symmetric? For two spin 1/2 particles I understand that the triplet states ($S = 1$) are:
$\newcommand\ket[1]{\left|{#1}\right>}
\newcommand\up\uparrow
\newcommand\dn\downarrow
\newcommand\lf\leftarrow
\newcommand\rt\rightarrow
$
\begin{align}
\ket{1,1} &= \ket{\... | There are at least 2 approaches. One is just show that it is symmetric by applying the lower operator for total spin to the maximal $S_z$ state which should satisfy ($\hbar=1$):
$$ S_-|1,1\rangle=\sqrt 2 |1, 0\rangle$$
so
$$|1, 0\rangle =\frac{1}{\sqrt 2}S_-|1,1\rangle=\frac{1}{\sqrt 2}(S_{1-}+S_{2-})\uparrow_1\uparrow... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/389946",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 5,
"answer_id": 0
} |
How is thermal energy and dipole related? My textbook states :
The extent of polarization of dielectrics depends on the relative strength of two mutually opposite factors - the dipole potential energy tending to align the dipole with the external field and thermal energy tending to disrupt the alignment.
I know that ... | Thermal energy CAUSES the random motion of particles. The more thermal energy there is, the more the dipoles will jiggle around randomly. This makes their orientation more random, so less line up with the external electric field. This effectively lowers the polarization.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/390018",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Where can I find values for apparent brightness of stars? I'm in high school and doing a project. I want to calculate the distance to stars using their luminosity and apparent brightness, from the equation $b=\frac{L}{4 \pi d^2}$. I have found values for luminosity and apparent magnitude from the Hipparcos dataset. How... | First, you need to use distance modules derive the absolute magnitude M (like in v band) for the star using its apparent magnitude m (in the same band like v):
$$\underbrace{m-M}_\text{Distance Modulus} = 5\log_{10}\left(\frac{r}{10}\right)$$
where r is the distance (d) and then you need to use the driven absolute magn... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/390148",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What is the definition of the charge conjugation? I seem to have troubles finding definitions of the charge conjugation operator that are independant of the theory considered.
Weinberg defined it as the operator mapping particle types to antiparticles :
$$\operatorname C \Psi^{\pm}_{p_1 \sigma_1 n_1;p_2 \sigma_2 n_2;... | There is no natural definition of charge conjugation that works for all QFTs. Rather, you should understand the CPT theorem instead as a combination of reflection-positivity and Wick rotation. See this paper, Appendix A.2.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/390342",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 0
} |
What experimental bounds do we have on big $G$? I know that there has been a large amount of controversy surrounding the exact value of the gravitational constant $G$, but I know that there is not a substantial difference in the measured value. So I was wondering what experimental bounds we have on it so far?
| This page shows the range of some measurements from 1982 to 2014. Basically, it looks like arguments could be made for any value between -460 to +250 parts per million below or above the commonly accepted value of 6.67408 x 10^-11 m^3 kg^-1 s^-2.
http://iupap.org/working-groups/wg13-newtonian-constant-of-gravitation/
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/390541",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
When wood absorbs water, expands, and breaks stones, how is conservation of energy working? There is a traditional stone cutting method consisting of making a series of small holes in the stone, then inserting wood into the holes, and then adding water to the wood. When the wood expands, it will break the stone.
How i... | I think the deleted answer by @naiad (wet wood has higher entropy than [water] and [dry wood] separately) may make sense. It is well-known that wood dimensions change depending on the humidity of the surrounding air. If water is concentrated in some area, its contribution to entropy is lower than if it is dispersed. Th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/391141",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Why does the kinetic energy of a photo-electron vary? Provided it is above the threshold frequency of the metal, when electromagnetic radiation is shone onto a metals surface photo-electrons are emitted. This occurs because 1 photon is absorbed by 1 electron giving it enough energy to be ejected.
We know that the ener... |
"We know that the energy of the incident photons are all equal"
True only if the light shone is monochromatic. Look at the plot used to determine the photoelectric effect:
it uses the maximum electron energy for a given frequency, so as to not depend on the kinematic dispersion discussed in the chosen answer.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/391464",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
} |
What can be the simplest way to find the thickness of a soap bubble? Is it possible to measure the thickness of a soap bubble without using any sophisticated instruments such that anyone can do it?
| One can measure temperature and pressure of the air used to blow a bubble, the diameter of the bubble, and its vertical speed in air (using a camera). Then one can use the Stokes law, calculate the mass of the bubble and the mass of the air in the bubble, their difference is the mass of soap water, then one can calcul... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/391563",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 4,
"answer_id": 3
} |
Melting of ice on applying pressure Can someone explain why ice melts on increasing pressure based on Le Chatelier's principle? By the principle, ice will try to oppose increase in pressure. It can do so by increasing volume right? But on melting volume decreases
| You almost answered your question, except that you deviated a bit. What you got wrong was the condition for equilibrium.
Equilibrium is that state at which any the system will try to counteract a an external change to the system. That is the primary definition of Le Chatelier's Principle also. So when we apply an exter... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/391717",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
How do I calculate Holevo information when information is knowable in principle, but unknown? I am trying to better understand the meaning of Holevo information $\chi$. Suppose Alice starts with data encoded on qubits in the $\{0,1\}$ basis. She takes one of these qubits, originally in pure state $\lvert0\rangle$; rand... | One should consider the states associated with each of the plaintext messages Alice could send, in this case 0 and 1. For both of these cases, the state that reaches Bob is effectively mixed: even though it is actually pure, it is indistinguishable from a mixed state (e.g., one qubit of an entangled pair).
I found Pres... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/391938",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Why is Helium 4 so stable? I've been looking at stuff to do with binding energies and was wondering why Helium 4 is so stable. The fact everything up to carbon is less stable seems a bit odd. Is there a reason for this or another, that's how the universe works?
| If one regards the nucleus as a potential well for nucleons, there is one lowest level. It can contain:
*
*a spin-up proton
*a spin-down proton
*a spin-up neutron
*a spin-down neutron
Then that lowest level is full. Any additional nucleons can only find a place in levels with higher kinetic energy.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/392175",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
What is a "decade" as a unit of measure (ex. a decade of the EM spectrum)? Reading through papers and online sources about radio galaxies, I kept stumbling across a term--a "decade" of the electromagnetic spectrum. Radio galaxy emission encompasses "11 decades of the EM spectrum". Or this quote from NASA:
Astronomers ... | A decade is a factor of $10$, so it's a way of assigning a unit to the common logarithm ($\mathrm{log}_{10}$). It's also frequently assigned the unit symbol $\mathrm{dex}$.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/392508",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "31",
"answer_count": 3,
"answer_id": 2
} |
Why can't the Lagrangian for a free point particle depend on distance? I have looked thorough the derivation of Lagrange equations in Landau and Lifshitz, Vol 1, $\S 3$, p.5. They argues that the lagrangian of a free particle cannot explicitly depend on position vector $\vec{r}$ because of the isotropy and homogeneity ... | Because of the homogeneity of space.
If it depended on $r^2$, then the origin with respect to which you're taking that distance $r$ would have a privileged status in the theory. For a free particle this cannot happen, so you cannot have that dependence.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/392635",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Magnetic field due to a current carrying wire at the wire Why do we take zero magnetic field at a point on the axis of a current carrying wire.
When i use $B =\frac{\mu_0 I}{2\pi r}$ formula for magnetic field calculation at a distance $r$ from the wire then for $r\to 0$, $B$ becomes infinity. What I am missing here?
I... | Complementing jim's answer in order to explicitly address the questions that possibly motivated the original post:
*
*does the field intensity diverge at the wire?
*where does it point to?
The answer is that in practice we have a current density ($I(A)/A$); and, as long as this density is finite, considering a po... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394016",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Torricelli's Law and number of holes Trying to determine if the number of holes at the bottom of a bucket will change time it takes for water to empty the bucket.
Looking at the equation, it would seem that as long as the area of the hole(s) are the same, the time for the water to flow out should be the same?
Bucket 1 ... | In a world without friction, where water is a perfect fluid, as long as the total area of the holes in the bucket is the same, the rate of water flowing out would be the same. However, since friction exists, and water is far from being a perfect liquid, the bucket with the smaller holes would have a lower rate of water... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394134",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Electricity generator We know in an electricity generator , electrons move from negative voltage to positive voltage of the stator winding and we can use that electric power on its way , so is it possible for the stator to loss all of its electrons((because we consumed it )) so it can not produce more voltage and ele... | An electricity generator does not create electrons nor do appliances consume them. The electricity generator creates an electromotive force (emf) by moving the generator rotor's conductors in a magnetic field provided by the stator. This emf drives electrons. The resulting electron motion is called current. Without suf... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394359",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
What motivates the term "State function"? What motivates the term "State function"? Or,in other terms, what is the relation between, potential theory, gradient fields and exact and inexact differentials?
| A rigorous answer would require a good chunk of a calculus course. Assuming that you are quite familiar with exact differentials, the idea behind a State function is that, given certain conditions (coordinates over a manifold), the scalar value of the State function is always the same. That is, if I have a quantity $S$... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394783",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Velocity of light in Galilean transformation What is the velocity of light in Galilean transformation? Is it infinity?
| Let's say a light beam travels at a speed of $c$ in one reference frame $R_0$. Now, consider another reference frame $R_1$ where it moves in parallel to the light beam with velocity $v$, then the speed of the light beam will be $c-v$ in the frame $R_1$.
So under Galilean transformation, we see that the speed of light i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394890",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 2
} |
Can you vibrate an object and split its molecules? Let’s say I have paper (or, I don't know, any other material). Can I with some sort of device vibrate the molecules of those atoms so fast that they separate, and as a result I don’t have paper anymore but just air?
If so what is this called?
Please try to simplify you... | What you're describing is the end result of increasing thermalization, or the addition of energy to the mechanical degrees of freedom of a molecule. These degrees of freedom include rotational or translational ones as well. Energy stored in these degrees of freedom is called heat.
So you're not describing a process tha... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395088",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Concept regarding Venturi Tube-Bernoulli application I was recently studying applications of Bernoulli Equation and came across the Venturi tube. This is diagram I have used to analyse the venturimeter. I understand how we obtain the first equation using bernoulli theorem which is
$$P_1 - P_2 =(1/2)ρ(v_2^2 - v_1^2) ... | You cannot apply Bernoulli's theorem through the two lateral pressure taps (where the vertical tubes are linked to the main tube). In general, it is assumed that the pressure is continuous and the speed is clearly discontinuous at this point. (To justify the continuity of the pressure, one would have to look in detail ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395372",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
} |
Paradox in special relativity involving capacitor In the laboratory reference frame (LRF), a horizontally moving (with constant speed) flat capacitor would have a different size of plates therefore resulting in different capacity $C'$, namely
$$
C' = \frac{1}{\gamma}C,
$$
where C is a capacity in its own reference fram... | If we somehow contract the plates of a charged capacitor, the charges get closer to each other and the repulsive Coulomb forces between charges become stronger. The voltage of the capacitor increases. The capacitance of the capacitor decreases.
If we somehow contract the plates of a charged capacitor, and also the fiel... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395464",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 3
} |
How to show that $\int\nabla^2\psi_n (x)\overline{\psi_m (x)}dx=0$ Let us consider the three-dimensional time-dependent Schrödinger equation that has the general solution $\psi(x,t)=\sum_n c_n\psi_n(x)e^{-iE_nt/\hbar},$ where the functions $\psi_n$ are orthogonal.
How to show that
$$\int[\nabla^2\psi_n (x)]\overline{\p... | I don’t think the result is true in general. It is certainly not true for the harmonic oscillator ground state where $\psi_0^{\prime\prime}(x)$ is proportional to $\psi_0(x)$ and $\psi_2(x)$.
More generally, recall that the kinetic energy $T=-\frac{1}{2m}\nabla^2$
and that, by assumption of your problem,
$$
H\psi_n(... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395644",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Griffith's vector potential identity I am working through some problems in Griffith's text "Introduction to Electrodynamics", and am having some trouble with an identity he uses. In Example 2.7 (and many times throughout the book) he says something along the lines of (with a sphere of radius $R$ constant and $z$ varyin... | There is no mysterious "vector potential identity" involved. As the square root is supposed to be a positive distance, you have to take the appropriate sign of the square root when R-z or z-R is positive.
PS: In example 2.7, Griffiths points this out explicitely himself!
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395893",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Simple explanation for what a torsor is I am studying Chris Elliott's notes on Line and Surface Operators in Gauge Theories (available here).
In the notes, there's a mention of the fact that (for $G = U(1)$),
$$W_{\gamma, n}(A) = e^{in\oint_{\gamma}A}.$$
The gauge field $A$ is not actually a 1-form, but upon choosin... | Oversimplified & in a nutshell:
*
*Recall the slogan
A $G$-torsor is like the group $G$ that has forgotten its neutral element.
*Example: An affine space $A$ is a torsor for a vector space $V$.
*The space of $U(1)$ gauge fields is an affine space, while $\Omega^1(X)$ is a vector space.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/396082",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
What does self-closing bra-ket mean in Robetson-Schrodinger Uncertainty Relation? I was reading:
https://en.wikipedia.org/wiki/Heisenberg%27s_uncertainty_principle#Robertson–Schrödinger_uncertainty_relations
Where an inequality is presented:
$$ \sigma_A \sigma_B = | \frac{1}{2} \langle \lbrace \hat{A}, \hat{B} \rbrace ... | The angled brackets here are not bra-kets. For any operator $\hat{O}$, the expression $\langle \hat{O}\rangle$ denotes the expectation value of the operator with respect to a state - here implicitly assumed to be arbitrary, but the same state for all such expressions occurring.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/396286",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Can all waves interfere with each other? What conditions must two waves have such that they interfere? Do they need to have the same frequency or amplitude? Should they pass through a given space at the same time? Should they have the same sources?
| The conditions are as follows:
1) The first condition is obvious, but should be stated for completeness. The waves must overlap in space and time- a wave crossing Pond A will not interfere with a wave crossing Pond B, nor will a wave on Pond A today interfere with a wave crossing Pond A tomorrow.
2) The waves must not... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/396418",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
How does holding a glass prevent it from falling? When I hold a glass of water,
$\hspace{1.5cm}$,
I am applying a force horizontally, but its weight acts downwards. Should it not fall?
How do you describe the equilibrium?
|
"I am applying a force horizontally ..."
Absolutely, totally wrong.
In the photos, you are applying an upwards force to the mass. Just as you would expect.
The only purpose of the finger pinch force is to increase static friction. Note that if you simply dry the glass and glue the glass to your fingers (or use, say... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/396588",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 3
} |
Tangent vector of photon If I had some line element such as minkowski line element:
$ ds^2 = -dt^2 + dx^2 + dy^2 + dz^2 $
And assuming this photon or beam of photons travels in the x direction, how would one find the components of the tangent vector?
Actual answer should be $k^\mu = B(1,1,0,0)$, but how is this obtain... | For an inertial observer with 4-velocity $t^a$ such that $g_{ab}t^a t^b=t_bt^b=-1$,
let $x^a$ be the unit spacelike-vector orthogonal to $t^a$. So, $g_{ab}x^a t^b=x_bt^b=0$ and $g_{ab}x^a x^b=x_b x^b=1$.
(In this frame, $t^a=(1,0,0,0)$ and $x^a=(0,1,0,0)$.)
So, $k^a=A t^a+B x^a$.
Since the photon tangent vector must sa... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397079",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Problem understanding Fermi - Dirac distribution function I am studying Statistical Mechanics by R K Pathria. There the author tries to calculate the partition function of in canonical ensemble as:
$$Q_N(V,T) = \sum_{[n_\epsilon]} (e^{-\beta\sum_{\epsilon}n_\epsilon \epsilon})$$
(In above the expression the summation r... | You are confusing the two restrictions. The restriction the author is referring to which makes the derivation difficult using the canonical ensemble is the restriction on total particle number $N_0$ in the canonical ensemble, not the restriction on the individual occupancies $n_{\epsilon}$ to be $0$ or $1$.
$$
\sum_{\e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397254",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Strong force gets weaker at small distances yet approximated by -1/r potential My particle physics textbook (by Martin and Shaw) has confused me, it states in ch.7 that the strong force gets weaker at small distances, and that it can be approximated by $V(r) = -\frac{4 \alpha_s}{3 r}$ for $r<0.1fm$, so doesn't this pot... | I understand the strong force to be the result of a competition between the Coulomb electrostatic forces between charged particles, and the magnetic forces resulting from the high speed motion of these charges. Two charges moving parallel to each other develop a magnetic attraction force and the ratio between the two i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397471",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How does a wire bend the electric field? The electrons in the current in a wire are constantly colliding with the metal cations which means there must be a constant cause of acceleration: an electric field. How, why is it that the electric field is identical in shape to that of the wire?
| The electric field has to be provided by a battery for example. There is no electric field within a conductor by itself. If a voltage is applied to a bent conductor, surface charges accumulate in curved parts which leads to a net electric field in axial direction.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397606",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
If light could bend around a person ,would that make the person invisible? If we place an object such as a coin in front of the light source ,e.g a candle we cant see the candle through the coin but if the size of the coin would be comparable to the wavelength of visible light the we can see the candle through the coin... | Well, light can only travel in straight linear paths, so that machine would be implausible and also it can't be plausible because light would have to travel around the person (it wouldn't work as your drawing suggested because the light would have to change its frequency to become a higher wave) and because it can only... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397693",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Pauli's first paper about the spin Wikipedia states, that the spin degree of freedom was first formulated by Pauli in 1924:
In 1924 Wolfgang Pauli introduced what he called a "two-valued quantum degree of freedom" associated with the electron in the outermost shell.
However, there is no reference to this quote. Does ... | Pauli himself talked about this in his Nobel Lecture: https://www.nobelprize.org/uploads/2018/06/pauli-lecture.pdf:
In the autumn of 1924 I published some arguments against this point of view, which I definitely rejected as incorrect and proposed instead of it the assumption of a new quantum theoretic property of the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397824",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Is it possible to make a laser by sending sunlight through an optical apparatus? 1) Is it possible to create a laser from focused sun light by separating and using only one wavelength of light as a laser and using the proper mechanism to polarize it and make it coherent? ... | Even with perfect filtering (answer of S. McGrew), one can still distinguish filtered sunlight from true laser light by using the 2nd-order autocorrelation function $g^2(τ)$.
For a laser: $g^2(0) = 1$.
For thermal light (e.g. sunlight): $g^2(0) = 2$
In words: While the temporal arrival time of laser photons is randomly... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398002",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Are materials which are bad at conducting heat always bad at conducting electricity also? When defining a material's conductivity, we usually consider its conductivity of heat and conductivity of electricity separately. However, I realize that materials like metal conduct both heat and electricity well. In contrast, ma... | The ratio of thermal conductivity to electric conductivity is a constant at a given temperature. This relation is mathematically defined by the Wiedmann-Franz law.
There are exceptions to this. The human body is a bad conductor of heat but is an excellent conductor of electrical.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398266",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 1
} |
Is it possible to harvest the energy from the movements of a satellite in orbit? I was thinking about how energy is harvested on Earth from movements of certain forces like wind and ocean currents. Could similar principles be applied in space?
Satellites are virtually in perpetual motion when orbiting the Earth. Is the... | Energy is 'harvested' from satellites all the time. Air drag converts the satellite's energy into heat in the atmosphere. This is, of course, not a perpetual motion machine. The orbit of the satellite will get closer to Earth over time, until the satellite crashes back down, as the Tiangong-1 did last week.
It should a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398397",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 0
} |
Is Yellow a monochromatic light? I have got a serious doubt.
I have read,
"yellow light from a low pressure sodium vapour is monochromatic"
How can it be monochromatic when yellow light is a combination of red and green primary colours?
| You are right that yellow is a secondary colour in light - where RGB is primary. But you are slightly confusing the difference between how RGB works to create colours in a screen compared to sodium vapour.
You can tell whether an apparently yellow source is made up of a combination of red and green, or is just yellow b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398501",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 1
} |
Where does rest mass come from? Firstly I'll say that I know the current standard terminology is to just refer to "mass", but I wanted to be clear about what I was talking about.
I've heard that in standard model of physics (which is compatible with special relativity), it is necessary to treat mass as an emergent prop... | In field theory, each elementary particle is associated with a field of its own. One writes down a Lagrangian (density) $\mathscr{L}$ for the field. When you quantize the theory you obtain particles. The positive coefficient ($\mu^2>0$) of the term in $\mathscr{L}$ which is quadratic in the field becomes proportional t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398665",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Sending a Signal inside the Photon sphere You and a friend are inside the Photon sphere of black holes(Non rotating and uncharged).
At what angles and speed(Momentum) you need to throw a stone(or anything else )so that it reach him According to the General relativity(schwarzschild metric).
(if he is on the other side w... | The problem with your question is that a physical observer can only be on time-like orbits, whereas the photon sphere is defined by null-like orbits. So you couldn't really exist on the photon sphere.
However, if you (and your friend) somehow accelerate to nearly the speed of light, it might be good enough to stay very... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398972",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Where does the energy of the expansion of frozen water come from? As you cool a system, you are removing energy, yet as water transitions to a solid, it expands, exerting sufficient force to rip through metal, for example in residential copper water pipes that freeze. Where does that energy come from?
| The change from liquid to solid releases some energy as (stronger) bonds are made between the water molecules - latent heat of fusion.
The expansion pushes back the surroundings and work is done at the expense of some of the released energy when the bonds are being made.
The internal energy (potential energy) of the w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/399166",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
A doubt related to Significant Digits Could someone please explain this statement to me
"Reporting the result of measurement that includes more digits than significant digits is superfluous and also misleading since it would give a wrong idea about the precision of measurement."
Also, shouldn't the word accuracy be ... | Suppose I have a ruler marked in millimeters, and I can estimate lengths measured with my ruler to about half a millimetre by estimating by eye how far between marks my length is. So when I measure a length my precision is $\pm 0.5$ mm.
If I measure a length then report it as $3.14159$ mm this would be be misleading be... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/399313",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Why is it easier to apply torque via short bursts There are two popular tools I use to apply torque to a fastener (bolt, screw, etc.): an impact driver and a drill.
The drill is a motor hooked up to some gears and eventually a bit that fits over the fastener. If I want to apply 40 lb/ft of torque, I feel as though I ha... | the reason you can sustain torque bursts easier with the rotary impact tool than steady torque loading from the nonimpact tool with the same average power rating is the rotary inertia of the impact tool reflects most of the impact shock into the tool bit before it can get transmitted out of the tool and into your hands... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/399457",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 0
} |
How is the centripetal force of a car when turning distributed over the wheels?
The centripetal force can easily be calculated as: $F = (M*v^2)/R = (M*v^2)*sin(\delta)/L$. But how is this force distributed over the (front and rear) wheels? My initial thought was to just divide it by 4 for each wheel, but when you turn... | The two-wheel model shown in the question is good start for determining the loading on the tires. But this would only result with the total load on the front tires, and the total load on the back tires, without any further details on how these loads are distributed left-to-right. To get there, you also need to consider... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/399761",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 0
} |
Question regarding vector direction and Unit Circle
Find the electric field at the origin for a line of charge density λ on the $y > 0$ portion of the unit circle if λ is constant.
The solution provided was:
$$\int_{0}^{\pi}\frac{\lambda d \phi}{4\pi e_0}(-\hat\rho).$$
If it is $y>0$, why is the direction ($-\hat\r... | Note that the electric field points from positive to negative, so if $\lambda > 0$, the electric field points away from the line, towards the origin (negative radial direction). Then the integral is just summing up these contributions. Also, the value of $\hat{\rho}$ depends implicitly on $\phi$, as the radial directio... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/399895",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Arranging solenoids into a sphere to create a magnetic monopole? [I believe that this question is very different than the question about arranging magnets into a sphere. Solenoids are ruled by Ampere's Law and magnets are not. Ampere's Law is what could make it work.]
I am wondering if Ampere's Law can be a loophole ar... | If your solenoids were thin enough to behave approximately as Dirac strings, with an effective monopole at each end, your sphere would have an equal number of negative and positive magnetic charges. A Gaussian surface that enclosed the entire sphere would still have zero magnetic flux. Symmetry considerations might s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400061",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What exactly is deterministic in Schrödinger's equation? I have read the following on Wikipedia but I can't understand it:
In quantum mechanics, the Schrödinger equation, which describes the continuous time evolution of a system's wave function, is deterministic. However, the relationship between a system's wave funct... | TL;DR Schrödinger's equation determines the wave function. The wave function determines probabilities. But a particle's position, for example, is not determined by a probability.
If you know the wave function $\Psi(x,t)$ at some time $t_0$, Schrödinger's equation will give you the wave function for all $t>t_0$. This m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400162",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "28",
"answer_count": 9,
"answer_id": 7
} |
Why polarization filter do not dim the light completely? In a circle there's infinite amount of degrees (eg. 0 deg, 0.00000000000...1 deg etc.) In a ground school we are thought that there's 360 degrees in a circle.
A landscape behind my window is incoherent light source, so it randomly emits photons with all polarizat... | A polarizer does not work by transmitting one very specific polarization angle and blocking all other - even only very slightly different - polarizations completely.
Instead a polarizer obeys Malus' law: If the angle between the axis of polarization of the incident light and the polarizer's polarization angle is $\thet... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400387",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
Why do two-photon interactions only occur at extremely high energies? I've been reading for pleasure about two-photon interaction experiments, and one thing that confuses me is why, for example, two photons in the visible spectrum cannot interact. Is this indeed the case, and if so, why?
| Photons are electrically neutral, so their interaction is by vacuum polarisation. The involvement of virtual charged particles makes the interaction difficult at energies $\ll m_e c^2$, $m_e$ the electron mass. (Difficult, but not impossible; pair production, of course, would be.) Eq. (1) here, first derived in 1935, s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400526",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Does the drift velocity of an electron in a wire with constant current depend on the area of its cross section? Suppose current, $I=nevA$ with the electron density $n$, the cross section of the conductor $A$ and the drift velocity $v = \frac{V}{RneA}$ where $R$ is the resistance of the wire. This equation becomes $v/dl... | $V$ in your equation is the voltage drop over the restive element. It's not the voltage in the circuit.
It depends on the resistance of the element and cannot be presumed to be constant.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400697",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why two opposite circular polarization filters let light pass through? I've been playing with 3D glasses. I punched out one of the filters, stacked them with some space in between and looked through both the filters without flipping anything over. I expected the interaction to be like between linear polarization filter... | These glasses consist of two layers: an ordinary linear polaroid-type polarizer and a quarter-wave retarder plate. The $\lambda/4$ retarder is the front surface. It converts circularly polarized light to linear.
Whay happens then if you put a second analyzer for circular polarization behind the first one? The linearly ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400769",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Can there be general relativity without special relativity? Can General Relativity be correct if Special Relativity is incorrect?
| As far as I know, and I'm not an expert or a physicist, Special Relativity developed the principle of Relativity which stipulates mainly two rules:
*
*All events follow the same laws of physics in every frame of reference.
*The independence of the speed of light, which means that light is always moving at a constan... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400910",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 8,
"answer_id": 4
} |
Comparing work in thermodynamics with work done in mechanics Let us the consider a gas as our system enclosed in a cylinder with piston.
1st case(Expansion of gas):
Here force on the piston is exerted by the gas in upward direction and during expansion piston moves up. So, the work done here is positive(force and displ... | In mechanics as well as in thermodynamics, the work done by the system on the surroundings is equal to the force vector the system exerts on the surroundings dotted with the displacement vector. If the force vector exerted by the system on its surroundings is in the same direction as the displacement (as, for example,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401134",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Where does the buoyant force come from? If I place a cube in water, the force at the top of the cube, $F_1$ will be $Ah\rho_wg$.
Where,
$A =$ cross-sectional area
$h =$ height at the top
$\rho_w =$ density of water
$g$ = acceleration due to gravity
All this made sense, because this downward Force ($F_1$) is actually th... | Lets assume you position a 10 cm cube one meter under the water.
The water pressure on the four sides cancel each other. So the cube is experiencing the difference between opposing pressures on the top and bottom surfaces.
The top pressure is $h g\times\rho_{water}.\times 100cm $
The bottom pressure is $(h+10cm)g\time... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401229",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
are redshift and Doppler effect identity with speed of light? we know that redshift and Doppler effect say wavelength and energy are relative to the speed of cause body, so how can I understand the special relativity which said that the speed of light is same regardless of the speed of body emitting light?
| *
*redshift happens to a photon when it is coming towards us from a far away galaxy that is receding from us, in expanding space
*as the photon travels in expanding space, its wavelength gets bigger, so it gets redshifted
*the speed of the photons is c when measured locally, and it does not depend on the emitting bo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401381",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.