Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
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... | Look my friend you can consider incident photon as a bigger ball having more energy(in motion) and the electron of the metal surface be a smaller ball kept at rest in a muddy area.
when the both balls collide whole energy from bigger ball is tranferred to the smaller one.
The energy of the bigger ball gets divided into... | {
"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": 2
} |
Why don't the molecules in the air "observe" the electron in the double slit experiment? Question says it all. The air should also interact with the electrons which should result in wave function collapse.
| The double-slit experiment with electrons can only be done in vacuum, unless the dimensions of your double-slit experimental set-up is much smaller than the free mean path of the electrons in air, which is below $0.5\mu m$ in air under normal conditions. Otherwise only very few electrons will reach the screen. Inelasti... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/391564",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"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?
| Yes as Pieter mentions being fermions with Pauli exclusion you can get a spin up and a a spin down in the same ground state for both n and p. Like in atomic physics you can put 2 electrons in the s orbital.
In addition a geometric effect that 4 particles can be arranged in a tetrahedron shape with each ball touching th... | {
"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": 1
} |
Why should the vacuum be unique? Among the Wightman axioms is the requirement that there is a unique Poincare-invariant state called the vacuum.
We know that QFT vacuua are not necessarily unique, for example in situations where spontaneous breaking of symmetry is relevant.
So, why is that? Why do we demand uniqueness ... | Let $G$ be a group, represented on the C*-algebra of quantum observables. By Haag's theorem, if in the C*-algebra of quantum observables there are two $G$-invariant states that are also $G$-abelian (an asymptotic condition that it would be too long to explain here), then they are either equal or disjoint (the latter me... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/392389",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Separation distance of Michelson Interferometer Suppose I would like to use Michelson Interferometer to observe fringes of equal thickness by creating an angle between the mirrors. Why is it vital for the path difference between the mirrors to be small in order to observe the fringes?
| Imagine the laser beam used for the interferometer. Picture it shining onto a distance wall. The beam expands with distance, like a cone. You can then see how the center of the beam has the shortest distance to the target. Now picture them in the interferometer. When the beams are at the exact same length you will get ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/392511",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
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 ... | Lagrangian of a free point particle may depend on $\vec{r}$, as a matter of fact, but this dependence is of no meaning and normally is reduces to a full time derivative of something that contains this dependence.
Lagrangian seves to obtain differential equations supplied with the initial conditions. Through the initial... | {
"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": 1
} |
Why we can assert, in general, that physical processes have the behaviour of low-pass filter? Consequently, why is it not allowed to produce physically some controllers for processes that are described by a transfer function that is an improper function?
A simple example is the driven harmonic oscillator. So the equati... | Exponential fall-off in frequency domain is equivalent to smoothness in time domain. Since physical processes usually happen smoothly, they act as a low-pass filter.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/392711",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
If gravity isn't a force, then how are forces balanced in the real world? Consider a simple situation like this- an object is sitting on a table. In classical mechanics, we say that the net force on the object is zero because gravity (treated as a force) and normal reaction force are equal and opposite to each other, a... | You can describe the physical situation either in a Newtonian way or according to GR (general relativity), but you have to be consistent in each description.
In a curved spacetime a free-falling body follows a geodesic, that is a path with nil acceleration. In the example the object on the table does not follow a geode... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/392863",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "22",
"answer_count": 2,
"answer_id": 1
} |
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... | For an infinitely long wire of infinitesimal thickness carrying a steady current $I$ the magnetic field at a distance $r$ from the wire is $$B =\frac{\mu_0 I}{2\pi r}.$$ This result can be derived from Ampere's Law $$\int {\bf B . d s} = \mu_0 \times \text{current enclosed by path},$$ where the current enclosed by the ... | {
"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": 1
} |
Would a magnet be able to attract an object through a sheet of stainless steel? For example, if there were a neodymium magnet at position x=0, a sheet of stainless steel at position x=1, and a magnetic object at x=5, would the magnet still attract the object?
Is the attraction force less than if the stainless steel she... | Most stainless steel is austenitic alloys of iron, which is not ferromagnetic. In practice, there are always some other phases, and that will results in a relative magnetic permeability that is a bit larger than unity. This will result in a force that is slightly less than if one had wood instead of stainless steel. Bu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394140",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"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... | the condition you describe is analogous to the state in which the generator is spinning but the load resistance to which it is connected is extremely large, so the current being pushed by the generator through the load goes to zero. In this case, the generator still applies its full rated voltage to the load, but this ... | {
"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": 1
} |
Why external chemical potential? My textbook Thermal Physics by Kittel and Kroemer [1, p. 124] says:
When external potential steps are present, we can express the total chemical potential of a system as the sum of two parts:
$$\mu = \mu_\mathrm{tot} = \mu_\mathrm{ext} + \mu_\mathrm{int}\tag{16}$$
Here $\mu_\mathrm{ext... | The energy (per particle) is defined up to a global constant/reference. If we scale the energy per particle (of all particles!) by a constant $\mu_{\text{ext}}$, then if we add another particle to our system while holding temperature and volume constant, the free energy raises by
$$\begin{align}\Delta F&=\mu_{\text{in... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394517",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
The path difference when a block covers one slit in Young's double slit experiment A modification of the simplest case of Young’s double slit experiment is when the path length for one of the slits is changed.
I've been told that if a strip of material of thickness $ t $ and refractive index $ n $ is placed over one s... | Earlier that light had to travel the distance $t$ in vacuum with refractive index 1. Now the refractive index is $n$ so the path will be $n*t$ The additional path is $n*t - t$ which is $(n-1)*t$. So if the path difference between the two rays was some $x$, $(n-1)*t$ would be added to it.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394791",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Velocity of light in Galilean transformation What is the velocity of light in Galilean transformation? Is it infinity?
| Maybe this reading of your question gets to what you are really asking.
It's not really about the "speed of light" [electromagnetic radiation], but about the maximum signal speed.
The eigenvectors of the Lorentz boost transformation are light-like [null] vectors in spacetime. In special relativity, they represent a fin... | {
"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": 3
} |
Does the mass of a bicycle directly affect stopping distance? In this answer on the cycling SE, the claim is made that adding more mass to a bicycle increases the stopping distance. I was under the impression that mass should not affect the stopping distance so long as all the other factors remain the same (balance, co... | when we stop a bike,we convert its kinetic energy(energy due to motion) into heat due to friction of brakes,now kinetic energy is dependant on mass
(1/2mv^2) so increasing the mass increases the kinetic energy and thus the stopping distance should INCREASE,assuming a constant braking force is applied,because the amount... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/394975",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 8,
"answer_id": 6
} |
How strong would a magnetic field need to be to go nuclear A sufficiently strong magnetic field would rip electrons out of their orbits triggering a chain reaction causing a nuclear reaction, with that said, how strong would it need to be before I would have to start worrying about this?
| There has been modelling and simulation work on atoms in magnetic fields up to $10^{13}$ Gauss, the kind of environment you get around neutron star surfaces. As was first noted by Ruderman, superstrong magnetic fields make electron orbitals near-cylindrical and enable superstrong atom chains - quite the opposite of a n... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395204",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
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) ... | Let the atmospheric pressure be $P_0$
Therefore
$$P_1=P_0+\rho gh_1 \quad \text{and}\quad P_2=P_0+\rho gh_2$$
Subtracting we get the required equation
$$P_1-P_2=\rho gh$$
| {
"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": 2
} |
Understanding the statcoulomb conceptually I've just learned about the statcoulomb, which is basically a way to express charge when we didn't have units of charge (I think), with the definition $$1 \mathrm{statC} = 1 \mathrm{dyne}^{1/2} \mathrm{cm} = \mathrm{cm}^{3/2} \mathrm{g}^{1/2} \mathrm{s}^{-1},$$ but I don't und... | As you probably know, it comes from Coulomb's Law written without any constants: $F=q_1q_2/r^2$. Since we've already defined force and distance, the units of charge are fixed. There's nothing more to it.
Well, almost. There are at least four "cgs" systems of units: those based on magnetic force, those based o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395477",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why is it said that density of nucleons in a nucleus is constant? Question:Why is it said that density of nucleons in a nucleus is constant?
I am studying an introductory course in nuclear and subnuclear physics. Based on the context in which it is cited (I cannot cite the notes because they are private notes) I do not... | Robert Hofstadter received a Nobel Prize for his research regarding The electron-scattering method and its application to the structure of nuclei and nucleons
Hofstadter fired high energy electrons at the nuclei of atoms and from the resulting scattering and diffraction effects he was able to map out the charge de... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395646",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Goldstone modes $\hat a^\dagger(k) \left|0\right>$ for small $k$ or $\hat a^\dagger(0) \left|0\right>$ Consider the Hamiltonian:
$$H=\sum_{\vec k} \varepsilon (\vec k)a_{\vec k}^\dagger a_{\vec k}$$
with $\varepsilon(\vec k) \rightarrow 0$ as $|\vec k|\rightarrow 0$. I know that this has gapless excitations and theref... | The massless field is called the Goldstone 'mode'. The term 'mode' does indeed make one think of a particular momentum mode and in that it is a misnomer. For simplicity lets take the $U(1)$ case with $\phi=|\phi| e^{i\theta}$. After symmetry breaking the Goldstone part of the Lagrangian is
$$
\mathcal L = \frac{1}{2} \... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395792",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
For an electron, how does QED maintain a constant rest mass and remove its classical radius from CEM? From classical electromagnetism, the classical radius of the electron is calculated to be
$$r_\text{e} = \frac{1}{4\pi\varepsilon_0}\frac{e^2}{m_{\text{e}} c^2} = 2.817 940 3227(19) \times 10^{-15} \text{ m}$$
Secondly... | The issue is solved, in a way, by separating the "bare mass" that appears in the Lagrangian from the "physical mass" that we observe in experiments. The bare mass diverges under renormalization, irrespective of concerns about the energy content of the electric field around a point-like electron. A standard response to ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/395913",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What does self-closing bra-ket mean in Robetson-Schrodinger Uncertainty Relation? I was reading:
https://en.wikipedia.org/wiki/Heisenberg%27s_uncertainty_principle#Robertson–Schrödinger_uncertainty_relations
Where an inequality is presented:
$$ \sigma_A \sigma_B = | \frac{1}{2} \langle \lbrace \hat{A}, \hat{B} \rbrace ... | So the expression has three different pieces. First, you have the operators which are represented here by hats, e.g. $\hat A$ and $\hat B$. Second, the curly brackets or braces represent the anti-commutator operator
$$\{\hat A, \hat B\}\equiv\hat A\hat B + \hat B\hat A.$$
The final piece is the "enclosed bra-ket" notat... | {
"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": 1
} |
Singlet gluon interaction If a ninth singlet gluon existed (U(3) instead of SU(3)) it would propagate freely interacting strongly. My question is: only with hadrons or, perhaps, with leptons (I.e, electrons) as well?
| If there were a ninth gluon ... The precise wording of your question predetermines the answer. If the supernumerary gluon were the ninth generator of U(3), then it would couple to baryon number alone; but if it were the additional diagonal generator of a grand SU(4) broken to SU(3)xU(1), then it would couple to hyper... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/396429",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
A way to increase photovoltaic cell efficiency with fiber optics? A way to increase photovoltaic efficiency using fiber optics?
One of the claimed inhibitors of efficiency was the band gap of materials which contrasts to that of a photon. My idea is a way to possibly mitigate this loss by increasing the frequency of th... | What you looking for is cooper pairing
https://en.wikipedia.org/wiki/Cooper_pair
Means that a single photon trapped can atract electrons from the walls of the tube and create enegy (better creating heat) ? Is that what you looking for?
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/396733",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Why runners lean forward? Why runners tend to lean forward prior to start running? How does it help run faster? What is the physics behind his leaning?
| Leaning allows for higher acceleration. If a runner at rest tries to run standing perfectly normal to the ground, he will never be able to move without falling over. Leaning is necessary for accelerating, for a person whose centre of mass is higher up from the ground.
Walking requires applying a force parallel to th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/396879",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
How to Change Coordinate Systems in General Relativity Let me preface by stating that I have no experience with General Relativity. I am working on a project for school that requires a little knowledge of it, so I am hoping to find some help. I do have experience with Special Relativity.
On to the question. I know that... | You can write $ds^2$ as $a^2$ times a static metric, introducing a new time coordinate viz. $d\eta=dt/a$. We say the full metric is conformal to the simpler one (this adjective refers to angle preservation upon the rescaling), so $\eta$ is called conformal time. The $\eta r\theta\phi$ coordinate system fits your criter... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397086",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Is the superfluid state a coherent state? In the normal to a superfluid phase transition, U(1) symmetry related to particle number conservation is spontaneously broken which seems to imply that the superfluid state is a state in which there is no definite number of particles? This property is shared by that of a cohere... | The ground state of a superfluid can be indeed (very precisely) approximated by a coherent state. More accurately by a squeezed coherent state. Please see Zhang equation (72):
$$|\{z_k \beta_k\}\rangle = \prod_k \exp\{ z_k a^{\dagger}_k - \bar{z}_k a_k\} \exp\{ \beta_k a^{\dagger}_ka^{\dagger}_{-k} - \bar{\beta}_k a_ka... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397260",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"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 $\vec{E}$ also exists outside the wire. There is however no current there as there are no charges. Inside the wire, for simplicity assumed to be an isotropic conductor, $\vec{E}$ is related to the current density $\vec{j}$ by $\vec{j} = \sigma \vec{E}$, where $\sigma$ is the conductivity. It is not z... | {
"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": 1
} |
Quantum teleportation reversible? The following figure displays a circuit implementing quantum teleportation.
It uses Haramard transform (H), controlled not (+), measurements (meas), a Not gate (N) and a phase shift gate (P) to teleport quantum state q from Alice to Bob.
I have learned that all quantum computations s... | Quantum teleportation is not reversible since it exhausts its resource - the entangled Bell state initially shared by Alice and Bob. At the end of the teleportation Alice and Bob don't share anymore an entangled state.
Any unitary operations (here H and CNOT) are reversible. The measurements, hovewer, are not. But the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/397841",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How to calculate thermal equilibrium temperature of three different substances? I am trying to calculate the thermal equilibrium temperature of three different substances (with three difference specific heats). The substances are:
Substance 1 - mass: 0.6 kg, specific heat: 4170 J/kg°C, initial temperature: 90°C
Substan... | Think about it: the specific heat is, in principle, something characteristic of the substance. If you add substances 1 and 2, you will get a resulting mixture, let's call it 1/2. Do you think that it's specific heat will be more like 1's or 2's?
I think it is intuitive to say that it will be closer to the specific hea... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398003",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
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... | As an Engineer I can assure you that many electronic devices work just because there are materials that are good thermal conductors but excellent insulators as well.
Electronic device and equipment, especially those designed for high power, must get rid of the excess heat produced by electrical power dissipation. Most ... | {
"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": 2
} |
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... | I do absolutely agree with @knzhou, but going to provide a couple of points here, more intuitive than scientific.
A satellite can spin for quite a long time since the kinetic energy of it doesn't decrease significantly (it doesn't mean that the energy doesn't decrease at all - it does, and for that reason a satellite w... | {
"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": 1
} |
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?
| Color is not something that happens in the physical world. Color happens in your brain. In the physical world, every light source has a spectrum. The topic of how your brain and eyes reduce the spectrum of some light source to what we call "color" is quite deep.
https://en.wikipedia.org/wiki/Color_vision
The short answ... | {
"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": 2
} |
Symmetry of Spin Function I have a question concerning the symmetry of the spin function in multiple identical particle systems. In the solutions of one of the quizzes, my professor said that the $s=3/2$ spin function is completely symmetric, which is why we need an antisymmetric spatial component (by a Slater matrix) ... | I'll assume your system contains three spin-1/2 particles. With the notation $\vert s,m\rangle$, it is clear that
$$
\vert 3/2,3/2\rangle = \vert \textstyle\frac{1}{2},\frac{1}{2}\rangle_1
\vert \textstyle\frac{1}{2},\frac{1}{2}\rangle_2 \vert \textstyle\frac{1}{2},\frac{1}{2}\rangle_3
$$
is symmetric under permutat... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398678",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Does an event horizon happen abruptly, and if so, what would happen if I was floating in front of it and shined a flashlight in its general direction? Does an event horizon happen abruptly, and if so, what would happen if I was floating in front of it and shined a flashlight in its general direction? Would it appear as... | A black hole's event horizon is a set of events which only has global significance. Locally, within a region of spacetime around some event on an event horizon, as long as the region of spacetime is small enough that tidal forces are negligible within that region, spacetime behaves like ordinary, flat spacetime. An eve... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/398981",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Source of the Chemical Potential I know that the chemical potential is the energy required to add another particle to the system but what is it's origin on the microscopic level? Take for example the case of the ideal Fermi gas, the Hamiltonian:
$$H=\sum_i \frac{p_i^2}{2m}-\mu \hat N$$
clearly the first term is the kin... | There is a misconception in this question about the nature of the chemical potential. Although the chemical potential is often described as 'the energy associated with adding a single particle' this is has to be interpreted with extreme care. In this saying 'energy' refers to the 'mean energy' and 'adding a single par... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/399177",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"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 ... | In general, it's best to give all potentially relevant information. So, in general, that quote provides poor advice.
However, it's important to write your number correctly. For example, a number understood to be approximately represented as "$1.2$" means "$1.2{\pm}0.05$" or "$\left[1.15,\,1.25\right)$".
If you want t... | {
"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": 1
} |
According to Conservation of Momentum, a gun in a sealed box should not have recoil? According to the law of Conservation of Momentum, there is no way to increase the momentum of a system, except by momentum transfer from interactions with the external. If I fire a rifle while sitting on a go kart, the go kart is going... |
haven't we momentarily broken the conservation of momentum?
No. The box/rifle is only moving backward during the period when the bullet is moving forward. If you sum their momenta, you will find it sums to zero.
Just because you cannot see the bullet from the outside does not mean that you can ignore its contributi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/399642",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 1
} |
Why the Fermi temperature isn't zero? The fermi temperature is defined as
$$ k_BT_f = E_f$$
But the fermi energy is the energy at $T=0$, where the energy level is the highest occupied for electrons. So, why is the fermi temperature defined as $\neq 0$?, What temperature $T_f$ is measured? Over who is T measured?
| The temperature is set to zero to define Fermi energy because $T=0$ corresponds to the ground state of this electronic system. The electrons are still moving around with their zero-point motion, and the Fermi temperature corresponds to this motion. Considering that electrons don't contribute much to the heat capacity o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400072",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
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... | Polarizers don’t just filter photons, they also change the polarization of the photons that make it through. If you send 1000 incoherent photons through a polarizer 500 on average will make it through and all of them will become parallel with the Polarizer. Thats why polarization filters do not dim (coherent) light com... | {
"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": 1
} |
How do we know yo-yo is rolling without slipping? How do we know a prior that a typical yo-yo rolls without slipping when released?
Now suppose we get a typical yoyo which is attached to the string (otherwise it would just fall and hit the ground if you released it!), then I cut the string so its not attached the to ... | Rolling without slipping occurs if the rotational velocity of the yo-yo matches that of its angular velocity i.e. v=wr.
This; however, is complicated by the fact that for a yoyo the radius will change as the spooled string unwinds.
In this case you want to determine a relationship between r and the length wound.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400718",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Air driven piston creating vacuum I am looking at generating a vacuum from two interconnected pistons, a driver piston from high pressure and the drivee creating a negative pressure in a container.
I am looking at improving the amount of vacuum I can get with the use of Pascal's principle but am unsure if I am trying t... | Consider you've achieved some pressure in the left cylinder, the system has moved and reached a state of rest. Lets find the pressure in the right cylinder. I'm not sure what exactly you mean by
I have two pistons, with D2 being double D1
I assume you mean the areas of the pistons, so lets say that the area of the le... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/400812",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Can there be general relativity without special relativity? Can General Relativity be correct if Special Relativity is incorrect?
| I wouldn't have thought so.
One solution to the vacuum equations of GR without cosmological constant is Minkowski space, this is the space of Special Relativity. Thus we see that General Relativity generalises Special Relativity
And we also see why Einstein opted for the word 'General' when he named this theory - actu... | {
"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": 5
} |
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... | *
*You have successfully described a downwards force on the cube, being the pressure at its top due to the weight of water above.
*Another downwards force is the cube's own weight.
*An upwards force comes from the water below the cube. Here the pressure is a bit larger than that at the top of the cube, since we ar... | {
"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": 1
} |
What is the relation between a measurement and an observable? Observables are represented by Hermitian operators. First of all, it's a little strange (to me) that some measurable physical quantity is represented by a transformation (or linear map), given that I think of a linear map as a function and I don't think of p... | I give here only the simplest explanation. In quantum mechanics (Schrödinger representation), the state of a physical system is completely determined by its, in general, complex (normalized) wave function $\psi$. An observable $A$ is a physical quantity that can be measured. It is represented by a corresponding Hermit... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401392",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why do objects float in liquids denser than themselves? Why do objects float in liquids denser than themselves? I know that a balloon floats on water because it has air in it, but why?
| Objects sitting in a fluid (liquid or gas) experience a pressure on every surface, equal to the pressure of the fluid.
Horizontally these pressures cancel out - the pressure pushing on the left hand side of an object will be the same as on the right, and the object has no net sideways force.
Vertically however is a dif... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401502",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Red-shifting due to emitting gravitational waves Light waves exert their own gravitational pull and must be emitting gravitational waves, losing energy in the process. Does this mean that light becomes red-shifted as it travels even without the effects of universal expansion?
| First we must clarify:
*
*gravitational waves can be created by constantly changing gravitational fields, like two oscillating neutron stars.
*photons do have gravitational effects, because they have energy
*gravitational effects are because of stress-energy
*photons energy comes from their frequency, E=h*f
*a s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401638",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
How can a body have two axis of rotation at the same time? I m not concerned with rotation of a body with two simultaneous axis but concerned with how we choose the axis,while going through pure rolling I have observed that there are two axis of rotation one is passing through the center of mass and the other is throug... | A body will only have one instantaneous axis of rotation (Chasle's Theorem).
In your example, the center of mass translates, and thus it is not a center of rotation. The only center of rotation is the point that does not move (the contact point).
To find the location of the center of rotation relative to some point A w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/401773",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Doubt on an answer to a mechanics problem Question: A block of Mass $m$ is connected to another block of mass $M$ by a massless spring of spring constant $k$ . The blocks are kept on a smooth horizontal plane and are at rest. The spring is unstretched when a constant force $F$ starts acting on the block of mass $M$ (ho... | Method 1 is correct. The error in Method 2 enters with your expression for "work done by external force". This should be a sum of the the work done on $m,$ which is $x_1mF/(m+M)$ and that done on $M,$ which is $x_2(F-MF/(m+M))$. To solve using this method, you will also need an additional relation: the ratio of $x_1... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/402007",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Does a charged particle travelling with uniform velocity induce a magnetic field? Does a charged particle, an electron say, travelling with uniform velocity induce a magnetic field? I believe it doesn't. In primary school, we all learned how to induce a magnetic field into an iron nail by wrapping coils of wire aroun... | Of course there is a magnetic field, otherwise the ampere would not exist. The ampere is defined by the magnetic attraction of two parallel straight wires carrying DC current.
(This answer is backwards in that it explains the existence of the field in terms of the non-silliness of the SI.)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/402118",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 4,
"answer_id": 3
} |
EM Plane wave, the changing electric field is in all directions right? I just want to confirm this, because this type of diagram seems pretty popular.
The electric field and magnetic field actually surround in all directions orthogonal to x axis, right? It is not just 2d pointing only in the y direction and z direction... | The EM wave in this diagram is just a ray, a geometrical visualization of the EM wave.
The EM wave is in reality a 4D wave, or at least you have to visualize it in 3D, where the wave rotates, that is, it goes from E wave to a M wave, because in the Maxwell equations, a moving E wave creates a moving M wave and this is ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/402251",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 3
} |
Difference between pure quantum states and coherent quantum states In the post What is coherence in quantum mechanics? and the answer by udrv in this post it seems to imply that a pure quantum state and coherent quantum state are the same thing since any pure state can be written as a projector onto the pure state when... | Coherent quantum states are special types of pure quantum states. The term coherent is only meaningful when there is an Fock algebra of ladder operators. Coherent states will be eigenvector of the (non-Hermitian) annihilation operator
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/402510",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 2
} |
AP physics 1 rotation problem could someone help me with this problem?
the correct answers are a and d. one issue i have with it is that i just don't understand what the problem is asking. like what spool? what table? i tried making some sense of the question and the answers, but i can only see d moving the wheel clo... | Not a very clearly worded question! The spool consists of the two wheels and the axle, as shown, and these are rigidly connected. Whoever wrote this will have assumed their readers were familiar with the spool that a camera film came on - not so common today! It is assumed to be resting on a table, which isn't shown. ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/402751",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 2
} |
A gravitational field is path independent. Why does a rocket not fly in serpentine lines? in theory a gravitational field is path independent, a gravitational field is a gradient field and so conservative. why doesn't a rocket fly in serpentine lines to exit the gravitational field of the moon, as said the gravitationa... |
In theory a gravitational field is path independent, a gravitational field is a gradient field and so conservative.
Correct. But what this means is that the total work done by gravity is constant over the path.
why doesn't a rocket fly in serpentine lines to exit the gravitational field of the moon, as said the gra... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/403218",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 3
} |
Does increasing tension on a string reduce or increase the harmonic wavelength for a standing wave? I had thought that increasing tension on a string increases the frequency and thus decreases the wavelength. My book says otherwise. Which is correct?
| This question is about standing waves on a string of length $L$ with fixed ends. A string like this will behave differently when it is excited with external vibrations depending on the frequency. Because frequency and wavelength are connected via $c=\lambda f$, there is a corresponding wavelength for each excitation fr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/403336",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Will the weight of a man standing on a scale changes if he throws something straight up? If a man enclosed inside a chamber and the chamber is on a weighing scale and if he throws a ball straight up in air will the weight of the whole system changes or not? If yes, then what is the weight change at
1) just moments aft... | The force on the entire system (measured by the scale), must equal the weight of the entire system, plus any required to accelerate the center of mass upward.
If the center of mass is not accelerating, the scale will measure the weight of all the contents. If the center of mass is accelerating upward (such as when the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/403471",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Confused with heat as a form of energy I have quite a simple question. Energy can be defined as capacity to do work.
But I have read
When energy is exchanged between thermodynamic systems by thermal interaction, the transfer of energy is called heat.
I can't understand what is the work done. For example, what is the... | Heat and work can both be referred to in this context as energy in transit. They are not forms of energy themselves but rather a means to transfer energy. As you have described, heat represents the flow of thermal energy by conduction, convection and radiation. Work describes the change of the energy if the system when... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/403616",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
What additional things should I do to understand language of science I am about to complete my High school. Typically, I study text books of science but it takes a lot of time to understand the concept resulting less time to do questions and due to which I get low marks(not having English as my native language is also ... | Well, one of the best ways is to enroll in a full time University course but of course that is a lot of commitment. If you don't want to do that then I would suggest looking for University lectures on YouTube. Here is a full quantum mechanics module by MIT, it doesn't get much better than that! Don't start with that mo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/403737",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What would qualify as a deceleration rather than an acceleration if speed is unchanged? The instantaneous acceleration $\textbf{a}(t)$ of a particle is defined as the rate of change of its instantaneous velocity $\textbf{v}(t)$: $$\textbf{a}(t)=\frac{\mathrm{d}}{\mathrm{d}t}\textbf{v}(t).\tag{1}$$ If the speed is const... | Acceleration is the correct technical term for the physical quantity you mentioned in the equations you posted (i.e. a).
The term deceleration doesn't describe a rigorously-defined standard physical quantity, it's just a term used differently in different situations that means "handwavily" that the velocity or speed is... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/403864",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 4,
"answer_id": 0
} |
Rayleigh-Taylor instability with negative Atwood number? I was reading a paper entitled "The Rayleigh—Taylor instability in astrophysical fluids" by Allen & Hughes (1984) that indicates the instability can occur for $ \rho_{01} < \rho_{02} $ which would indicate a negative Atwood number. But how is this possible? Does ... | Your intuition is correct; there's no such thing as a Rayleigh-Taylor instability with a negative Atwood number. That would imply that the density of the upper fluid, $\rho_{01}$, is less than the density of the lower fluid, $\rho_{02}$, which is clearly a stable situation with respect to the R-T instability.
So how d... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/404329",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Law of vector addition In one (Why is heat a scalar quantity?) of the questions I answered, I came across this:-
https://physics.stackexchange.com/a/404287/181020
or
A vector quantity should obey the law of vector addition.
But I don't think it is true. Consider a y junction circuit containing 3 identical wires. If i ... | For a wire, regardless of how you orient it, there's forward and backwards only. So, for any multi wire junction, you ARE adding vectorally (just in one dimension.)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/404453",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
What is the difference between electrons and holes in silicon? Electrons and holes behave differently in a silicon semiconductor (e.g. mobility of holes is one order of magnitude smaller than that of electrons, the collection time of holes at the same electric field is larger than for electrons... ). I was wondering, i... | Check that electrons move in the conduction band, whereas holes "move" in the valence band. They have different energies, but they are also at a different border.
So the difference arises that the mass of electrons depends on the second derivative of the energy with respect to $k$. This derivative isn't equal in both ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/404636",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
Hawking radiation calculation breaking apart with mass density $M_{planck}$? Hawking radiations are predicted from semiclassical quantum field theory, and it is sometimes said that calculations break apart at mass density $M_{planck}$.
The question is, is this true only for some Hawking radiation calculations and can b... | Sadly it cannot be overcome in general. The Planck mass marks the scale at which our current theories make sense on their own, that meaning that the techniques used in QFT in curved space-times don't hold anymore. Specially when it has to do with gravity. Non-linear effects (self-interactions) and back-reaction become ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/404721",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Entanglement of formation of the mixture of maximally entangled states Suppose we have two spin-$S$ systems. Let $\left| \psi_{a,b} \right\rangle = \frac{1}{\sqrt{2}}(\left| a,b \right\rangle+\left| b,a \right\rangle)$ be the maximally entangled state. ($a\neq b$ and $-S\leq a,b \leq S$.)
What is the Entanglement of f... | Since you say that any other entanglement measure is fine, let's compute the negativity. Let me denote by $\rho_{ab}:=|\psi_{a,b}\rangle\langle\psi_{a,b}|$.
With $^{T_A}$ the partial transpose, we have
$$
\rho_{ab}^{T_A} = \frac12 \Big[
|a,a\rangle\langle b,b|+|b,b\rangle\langle a,a|+
|a,b\rangle\langle a,b|+|b,a\rang... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/404851",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
What is the velocity of a photon through space-time? What is the 4-velocity of a photon? What is the velocity of a photon through space-time? What is the 4-velocity of a photon?
| The problem with the four velocity for anything travelling at the speed of light (i.e. any massless particle) is that we define the four velocity using the proper time $\tau$. If we choose some coordinates $(t,x,y,z)$ then the four velocity in our coordinates is:
$$ \mathbf U = \left(\frac{dt}{d\tau}, \frac{dx}{d\tau},... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/405131",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Are graphene and coal are the same? I recently read about the atomic structure of graphene, which is carbon arrange in hexagon shape but only 1 atomic thick(2d). And then I remember that coal is also made of carbon arrange in hexagon shape but it has more than 1 atom thick(3d). So is coal is made from multiple stacks o... |
As this decomposed peat is buried deeper and deeper, the pressure and temperature to which it is exposed increases causing chemical reactions to occur which reduce the lignin (remove oxygens via expulsion of carbon dioxide and water). In this process, carbon-carbon bonds form between the aromatic rings producing the h... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/405551",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Kirchoff's radiation law So, I have some problems understanding Kirchoff's Radiation law.
My textbook, Transport Processes and Separation Process Principles, by Geankoplis, states that at the same temperature T1 the emissivity and absorptivity of a surface is equal, which holds for any black or non black solid surface... | To add some detail to Pieter's answer:
The absorptivity quoted is an average over all the wavelengths of light incident on the body from the Sun. This light may have an average wavelength of $\lambda \approx 0.5 \mu \text{m}$ or so which presumably has an absorbtivity of 0.9 in your case.
The emissivity is also average... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/405702",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Change of wavefunction due to relativistic speed Imagine a spacecraft which is moving at a speed comparable to the speed of light relative to a reference frame with a hydrogen atom at it's origin. How would the probability distribution function of an electron in 1s orbit look relative to an observer inside the spacecra... | Copy/pasted from https://physics.stackexchange.com/questions/591368/galilean-invariance-of-schrödinger-equation as a reaction on @MikeStone's answer. It is too long for a comment.
The Schrödinger equation is not Galilei covariant, even in the absence of electromagnetic fields. There is a symmetry group, the Schrödinger... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/405778",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 1
} |
Why is $ \frac{\vec{r}}{r^3} = \frac{1}{r^2} $? I know it's surely a beginner's question but I don't see why you can write
\begin{align}
\frac{\vec{r}}{r^3} = \frac{1}{r^2}\cdot \frac{|\vec{r}|}{r}
\end{align}
Could someone explain it please? It would help understand quite a few things ...
| Answer: Equality: $\frac{\vec r}{r^3}= \frac{1}{r^2}$ is not true
Explanation: A vector quantity (having both magnitude & direction) & a scalar quantity (having only magnitude) can never be equated.
Now, $\frac{\vec r}{r^3}$ is a vector quantity while $\frac{1}{r^2}$ is a scalar quantity hence
$$\therefore \frac{\v... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/405861",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 3
} |
Analysing decoupling channels in Hubbard-Stratonovich transformation I have an action defined in terms of fermionic fields $c$ and $d$ that looks like
$$S = - \bar{d}(t)\bar{c}(t') V(t,t') d(t')c(t)$$
where $V$ is an interaction matrix.
Then performing Hubbard-Stratonovich transformation via the exchange channel, say, ... | It seems that there is no reason to discreminate the $c$ and $d$ fields when just the interaction term is given.
I think a more good notation would be helpful to you.
Instead of treating $c$ and $d$ fields separately, let us introduce a two-component spinor $\psi=(c,d)^T$. Then, the interaction you give as an example i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/406009",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Method of Images and Green functions in Quantum Field Theory Traditional Laplace equation solutions techniques for a system of conductors rely on the fact that the Green function for solving Laplace's equation represents the potential due to an image charge distribution ([1] Sections 1.10 and 2.1-2.6, for example). I w... | Method of images is based on the uniqueness theorem: the solution to a differential equation with specified boundary conditions is unique (up to the integration constants, which for PDEs may be functions). That is, if you guess a solution that satisfies both the equation and the boundary conditions, then this is the co... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/406137",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Inertia on a rotating disc? If I toss a ball upwards in a train moving with uniform velocity, the ball will land right back in my hand. This is because the ball has inertia and it continues to move forward at the speed of train even after leaving my hand.
Now consider I'm standing on the outer edge of a rotating disc (... | The first thing to note is that on the train you and the ball are moving with constant velocity whereas on the disc you and the ball are undergoing a (centripetal) acceleration.
The vertical motion of the ball is the same in both cases as the only vertical force acting is the same in both cases - the gravitational at... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/406384",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 5,
"answer_id": 0
} |
Why can a wave be expressed with a sine function? I see many expressions which express waves with the sine function like $y=\sin(kx-\omega t)$.
Waves really look similar to the shapes of a sine or cosine function, but does this guarantee that expressions that show wave-like movement are sine or cosine functions or is t... | This is a slightly different interpretation of the question...
Waves really look similar to the shapes of a sine or cosine function, but does this guarantee that expressions that show wave-like movement are sine or cosine functions or is this just an approximation?
These aren't always approximations. A lot of waves a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/406469",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "23",
"answer_count": 8,
"answer_id": 7
} |
Can air make shadows? I have read about schlieren photography, which uses the ability of non-uniform air to create shadows. Is it really possible that air makes shadows?
| In addition to V.Joe's excellent answer about refraction, I want to add that air does directly block (absorb, not refract) some infrared and ultra-violet light.
If your eyes could see light in the infrared and UV spectrums, air would appear partially opaque in those wavelengths. The surface of the earth is in the infra... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/406604",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "25",
"answer_count": 3,
"answer_id": 0
} |
Why is Ampère's law violated if there are no fringe fields? What's wrong with the following diagram?
Image source: Page 183, NCERT Physics Textbook for Class XII Part I
The reason stated in my textbook is as follows:
Magnetic field lines between two pole pieces cannot be precisely straight at the ends. Some fringing ... | Consider the two paths $ABCDA$ and $EFGHE$.
Path $AB$ contributes a positive value to the $\vec B\cdot d\vec l$ integral but the other parts of the loop contribute nothing, so overall there is a finite value for the $\vec B\cdot d\vec l$ integral but no enclosed current which violates Ampere's law.
Again path $EF... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/406837",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "30",
"answer_count": 1,
"answer_id": 0
} |
Do quantum fluctuations in the inflaton field lead to fluctuations in the potential energy density? During inflation there are quantum fluctuations in the inflaton field. Do quantum fluctuations in the inflaton field lead to fluctuations in the energy density?
|
Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the Universe (see galaxy formation and evolution and structure formation).
As the inflaton fluctuations are modeled to be the seeds of the cosmic microwave background radiation and th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/406961",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Help with D. Tong example on Noether in QFT In this lectures, example 1.3.2 on page 14 concludes that the Noether current is
But how can the current be a two index object when it is defined in eq. (1.38), which is
as a one index object? If I apply the formula I obtain something of the form $j^\mu$. Can someone mak... | $T^{\mu \nu}$ is conserved because the Lagrangian is invariant under space-time translations, meaning that it is invariant under space translations and time translations. Naturally, all the quantities conserved should be inside $T^{\mu \nu}$.
For example, $Q^0 = \int d^3x T^{00}$ is the first one, and $Q^0$ is the Ener... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/407101",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 4
} |
Does density of light power (w/m²) increase after passing a convex lens ? Does density of light power (w/m²) increase after passing a convex lens ? and if so i need the equation that represent that increase
| In general, power density of a light beam will be increasing or decreasing depending on whether the beam converges or diverges.
A convex lens is referred to as a converging lens because it converges parallel rays. But, if the rays entering a convex lens are not parallel, the light exiting the lens could either converge... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/407221",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Aerodynamics equation I still need to find an equation for the following restated problem: Suppose a full-size glider passes 10 feet over my head at high speed. No doubt I will feel the downward air pressure caused by the angle of attack of the undersides of the wings. But if the same glider flies over me at a distance... | I know of no explicit equation of the sort you seek, but here is a useful estimate which may be of help.
A pilot who is flying her airplane in for a landing on a runway will notice that the proximity of the ground beneath the plane begins to affect the flow of air over the wings on her plane when the plane is within o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/407331",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Connecting a charged capacitor to an uncharged capacitor
I was attending a lecture about capacitors and something confused me.
If I charge a capacitor using a DC supply, the capacitor will gain charge $Q_0$.
Now, if I discharged it along an uncharged capacitor in this arrangement, according to the lecture notes, the c... | When we charge a capacitor, it gains charge q on one of the plates and loses charge q from the other plate, i.e., its total charge remains zero.
Capacitors differ, in that sense, from other objects, like our bodies or spheres and rods used in various electrostatic devices and experiments, which actually gain a net char... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/407459",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Why are there only four fundamental interactions of nature? Is there an answer to the question why there are only four fundamental interactions of nature?
| The answer "because we do not need more" by @rubenvb is fine.
Studying physics, you must realize that physics is not answering fundamental "why" questions. Physics uses mathematical tools to model measurements and these models have to fit new data, i.e. be predictive. As long as the models are not falsified, they are c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/407688",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "47",
"answer_count": 10,
"answer_id": 0
} |
How to think of the components of 4-momentum? Suppose I have some photon with a 4-momentum $p_\mu$, with $\mu = t,r,\theta, \phi$.
There is also a point with coordinates $(x,y,z)$. The 3-vector $x^i$ describes the vector between the origin of the coordinate system and the point.
The photon is at the origin of this coor... | The angle between photon momentum and a vector is only defined in three dimensions. It is given by $\theta = \arccos (\vec p \cdot \vec x)/ (|\vec p||\vec x|))$. It has different values when observed in different reference frames. In other words, it is not a relativistic invariant.
The inproduct of four momentum with s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/407799",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Free surface in fluid dynamics In Computational Fluid Dynamics, if the fluid does not cross the free surface, the relation between fluid velocity at free surface and free surface velocity is given by$$
{\vec{V}}_{\text{surface}} {\cdot} \vec{n} = \vec{v}_{\text{fluid}} {\cdot} \vec{n}
\,,$$where:
*
*$\vec{V}_{\text{... | I think it should be $$\mathbf{V}_{surface}=(\mathbf{v}_{fluid}\cdot\mathbf{n})\mathbf{n}$$ Above equation says that the shape of the free surface of a fluid can change only due to fluid motion normal to the free surface. Fluid motion "in the surface", i.e. tangential to the surface, does not cause a change in the shap... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/407884",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How to visually understand that dislocation moves on a slip plane? When we look at images of edge or screw dislocation, it seems as if the direction in which that dislocation will move is already fixed by the 'way the dislocation is present'. For example, take any image of edge dislocation, and say the top half is movi... | There are cases, when two mobile dislocations (dislocations with Burgers vector pointing into a slip plane, therefore, can move with gliding) contact-interact and form a third dislocation. If this two dislocations split up into two-two Shockley partial dislocations, and one Shockley partial dislocation contact-interact... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/408234",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
What happens while breaking a Bar magnet? If we have a bar magnet and we break it as shown below
Then as per the image we get two bar magnets But does this happen instantaneously or there is some time lag between this process.
Or simply, Can magnetic monopoles exist even for a short time period?
| As long as the two parts are together, and the boundary is not a physical separation, you may consider them as one magnet or as two. There is no difference as the two poles you inserted cancel out and nothing is created. Of course when you pull the two parts things happen but the poles already existed in this sense.
Ma... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/408331",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Which program to analyse and graph laboratory measurements? I'm interested in which program would you recommend for drawing graphs with x and y errors. Also to be able to analyse data and then graph it. My ideal graph would look like this:
Until now I was drawing my graph reports in Excel, but I feel like Excel is ma... | Python is certainly a major language for scientific data analysis today. The three key words are: Numpy, Scipy and Matplotlib.
*
*NumPy: it's the basic numerical analysis package in Python. It allows for array manipulation, matrix calculus, Fourier transforms, statistics, ...
*SciPy: it's the extended version of nu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/408452",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Effect of greater coils on induced current in a solenoid Simple case:-
A bar magnet is inserted into a solenoid with 'n' turns at velocity 'v'. The current is measured using an ammeter that is part of the complete circuit of the solenoid.
My question is that if I increase the number of turns 'n' then will this lead to ... | According to Faraday's law of electromagnetism, emf induced is given by
$\epsilon=\frac{d\phi}{dt}$
But $\phi=B.A$
Here B is the electric field and A is the effective area though which the electric flux changes.
$\therefore$effective area =$n.a$
Here a is the area of each turn and n is the number of turns of the coil.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/408751",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why do punctured balloons fly around chaotically? If an inflated balloon is punctured, it can fly around wildly like in this cartoon @18:07.
Why is this motion so chaotic as opposed to being like a straight line or parabola as with rockets? Is there a mathematical framework for understanding why the balloon performs s... | The balloon moves because of conservation of momentum.
When the gas molecules diffuse out to conserve momentum the balloon moves. But inside the balloon the gas molecules possess random motion called Brownian Motion . Suppose at time t those molecules escape in some direction. Thus conserving momentum in that direction... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/408897",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What is the cause of wave impedance? As in electrical impedance,
Causes:
Resistance - collision of electrons with atoms and other electrons,
Reactance - Capacitive and inductive effects.
Likewise, what offers opposition to a wave traveling in a medium?
| The "opposition" to the wave or wave impedance or impedance of a medium to a wave is caused by characteristics of the medium analogous to the resistance, capacitance and inductance. While resistance is a pretty generic term, applicable to different types of waves, the energy storing characteristics, capacitance and ind... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/409074",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 0
} |
Non-zero electric field inside a conductor, when applying an large external field I'm probably missing something, or does not understand conductors well enough. But I have a question related to the title of this message.
In many places you read that there can be no electric field inside a conductor. The arguments typic... | If the field gets stronger electrons near the surface of the conductor will feel a stronger residual field, until some fly off in a spark that will hit you voltage source and reduce the voltage it produces.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/409393",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Is it possible for a lightning strike to hit the ground if there are high rise buildings nearby? Say we have pointed conductors connected to the top of the high rise buildings. Will the strikes hit the nearby ground in such a case?
| Yes.
A lightning conductor on top of a high-rise structure is designed to minimize the possibility of lightning hitting the structure by the virtue of its pointed tip giving rise to very high electric fields (the reason). However this doesn't always ensure that lightning will definitely strike the conductor. This link... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/409513",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 3,
"answer_id": 0
} |
Does photon absorption annihilate the associated EM wave instantly? My Understanding
A single photon has an associated electromagnetic wave. The wave is spread out in space, but the photon is considered a point particle. If the photon is absorbed, the entire wave disappears. Photon absorption is instantaneous, so the w... | Light, a classical concept, emerges from zillions of photons in a continuous mathematically demonstrable way, but photons, a quantum mechanical entity, are not light.
At the quantum level photons are mathematically connected to the frequency of light with a complex wavefunction, a solution of a quantized form of Maxwe... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/409617",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Density function in phase space What does density function in phase space physically mean? How does it indicate, the more familiar density that we are accustomed to ( an analogy may be), in phase space?
| It is a probability density, not a density of something like matter or energy.
The probability density $f$ answers the question "how likely is it (what is the probability that) the microstate $\omega$ in one of the points in some set $A$":
$$P(\omega\in A) = \int_A f(p,q)dpdq$$
The most other famous example of probabil... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/409776",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Quantum tunneling versus over-the-barrier ionization I was doing a quantum physics past paper and there was this question about ionizing atoms with laser. My answers made reference to the photons within the laser knocking off or transferring their energy to the electrons and allowing them to escape if this energy excee... | The laser doesn’t provide the barrier, on the contrary, the laser helps to overcome the barrier. That means that when you give visible light, you achieve just an excited state of the atoms, and it’s impossible to get them ionized. But these excited electrons can easily escape through quantum tunneling, something that i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/409915",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Why does plasmon have higher erngy than phonon? In my mind plasmon is movement of electrons and phonon is movement of atoms in an lattice. movement of atoms should have a large energy because atom is larger.
| Like it's said Plasmons describe collective oscillations of charge carriers. One can derive that the energy is given by (with $n$ the charge density, $\varepsilon$ vacuum dielectric constant):
\begin{equation}
E_{p}=\hbar \cdot \omega_{p}=\hbar \sqrt{\frac{ne^{2}}{m\varepsilon} }
\end{equation}
Now you can compare i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/410066",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Which pole will electrons flow towards in a changing magnetic field (generator) In a generator a magnet spins in the middle of a coil of wire and the changing magnetic field causes current to flow, but the current changes direction whenever fields from a new pole of the magnet cross a point on the wire, so will the cur... | The image you presented looks like a DC motor. If thats the case, there would be a DC source connected at the terminals. When current flows through the wire, a temporary magnetic field is formed due to the current flow through the wire. This temporary field repels the magnetic field felt from the magnets and causes the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/410290",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Energy conservation on expanding universe Due to the expansion of the universe, the photons emitted by the stars suffer redshift, Its mean that the energy is lowered a little bit. Does this mean that the energy is lost? Does the expansion of the universe violate some conservation principles according to Noether's theor... | Let us take the simple case, we see the iron spectrum of a star shifted, and we can use energy conservation to assign a velocity to the star that would assure the Doppler shift of the spectrum.
The expansion of space was deduced because a redshift was measured that can only be interpreted as "every cluster of galaxies... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/410392",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 1
} |
Why does a mirror reflect visible light but not gamma rays? Visible light (~500 THz) as well as gamma rays (~100 EHz) are electromagnetic radiation but we can reflect visible light using a glass mirror but not gamma rays. Why is that?
| Look at the electromagnetic spectrum:
Visible frequencies have wavelengths of microns, $10^{-6}$ meters.
Gamma rays have a wavelength of $10^{-12}$ meters, picometers.
In physics, there are two mainframes, the classical frame, which includes Maxwell's electrodynamics, Newton's mechanics, and derivative theories, and th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/410585",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "42",
"answer_count": 3,
"answer_id": 0
} |
What would be the charge distribution of a conducting sphere in front of a positive point charge? What would be the charge distribution of a conducting sphere in front of a positive point charge? I mean if it's a positive charge then it should induce negative charge in the near side and positive on the other side. But ... | This electrostatic problem of a point charge $q$ in vacuum at a distance $L$ from the center of an isolated conducting sphere with radius $R$ can be easily solved by the method of images, which introduces a virtual image charge $$q'= -q\frac {R}{L}$$ at a distance $$l_1=\frac {R^2}{L}$$ from the center in the sphere's ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/410906",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 0
} |
Why acoustic glitches in stars translate into extra oscillatory components in the normal frequencies? Acoustic glitches are locations inside the star where the sound speed changes abruptly compared to the wavelength of the acoustic waves that propagate through. Examples are the ionization zones and also transition betw... | A structural glitch in the star (i.e., an abrupt or sharp variation in the stellar structure) will affect the coupling of the wave with the star at the location of the structural glitch. That impact in the coupling will translate into a change in the eigenfrequencies which are predicted in the absence of structural gli... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/410964",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.