Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Manometer physics? How to keep nozzles from draining? Picture two liquid spray nozzles spaced vertically by some distance. Both nozzles are connected via flexible tubing to a single valve via a tee. Once the nozzle lines are full of fluid how can i prevent the fluid from draining out the bottom nozzle when the valve ... | Add another valve onto one of the tees? Inject an air-gap into the lines (with an appropriate configuration of lines)?
Once you have a continuous and unbroken line of liquid that is free to move, the entire system simply acts as a container in which the liquid will find the lowest level (and draw air in at the highest)... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Magnetic force direction Good day All!
while trying to solve this question
I used the right hand rule and according to it the Force should be directed outwards (pointing toward me)
but here is the answer that puzzeld me
I really don't get why it is down , and would feel very grateful if someone can explain me the rea... | We note that from the symmetry of the problem, there can be no force directed out of the plane of the page (i.e. it cannot be towards/away from you, as the components from opposite ends of the coil cancel out in this direction). We should also note that to find the direction of the force, we use the Left Hand Rule, not... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/384200",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
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Why is quantum mechanical momentum the derivative of the wave function with respect to the position? In classical mechanics the momentum is defined as mass times the time-derivative of position.
In quantum mechanics, however, the time-derivative of the wave function is the hamiltonian, while the momentum is defined as ... | This interlinkage between classical and quantum mechanical momentum can be thought of as a consequence of the De Broglie's law - that the velocity of the wave depends on it's wavelength. Though the Hamilton operator is obtained from the same law, looking things at this perspective could help in realizing that this rela... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Magical equations in statistical mechanics Studying ensembles in Statistical Mechanics I have found some formulas such as
$$ S = k\frac{\partial}{\partial T} (T\ln Z) $$
or
$$ \langle E \rangle = -\frac{\partial}{\partial\beta} \ln Z $$
or
$$ \langle \Delta E ^2 \rangle = - \frac{\partial \langle E \rangle}{\parti... | The partition function is defined as a sum over microstates at a specific temperature (with $\beta = 1/kT$)
$$Z(\beta) = \sum_s e^{-\beta E_s}$$
where $E_s$ is the energy of the microstate $s$.
Recall that if our system is connected to a large heat bath at temperature $T$, the probability for our system to be in state ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Running or walking up stairs = same work? I have a question belonging to the picture below. It is mentioned that
whether you walk up or run up stairs the same work is done.
When work equals (force in the moving direction) times the way, then I dont understand why it should be correct. When I run up the stairs I def... | The confusion arises from the conflation of acceleration and work, and the intuition that higher acceleration means more energy. It is more clear if you examine the starting and ending states; they are equivalent.
Examining the acceleration component, this is counterintuitive because there is more energy exerted per u... | {
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Reproducing Ramond's sunset diagram calculation for $\phi^4$ theory I am unable to reproduce the calculation of the sunset diagram for $\phi^4$ theory in Pierre Ramond's Fied Theory a Modern Primer. This is the second edition chapter 4.4. He starts with eq. (4.4.19)
\begin{equation}
\Sigma(p) = \frac{\lambda^2 (\mu^2)^... | Silly me, it is the 't Hooft Veltman regularisation scheme explained 5-6 pages earlier. I guess that's what happens when you start reading a section in the middle of the book.
| {
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Electric Motor Project Not Working Long time reader, first time poster :)
I'm a first year physics teacher in a high school, and I'm running a science fair where students research their own topics and create demonstrations. One group has chosen electric motors, and they are making a motor out of a battery, paperclips, ... | You have to check on the supporting ends of the wire how the shaved-off (blank) side of the wire is oriented with respect to the coil plane. If this is not done right, you will not get any sustained rotation.
It might perhaps be better do shave off the insulation completely and put on a new half insulation with a marke... | {
"language": "en",
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Does the potential $V(φ)$ of a scalar field decrease with the expansion of space? If a scalar field (eg. inflaton field) starts with a high potential.
Does the potential $V(φ)$ of the scalar field decrease with the expansion of space?
If it doesn’t decrease, would it mean that extra energy is created to fill in the ad... | You're probably used to a Minkowski-space Lagrangian density such as $\frac{1}{2}\eta^{\mu\nu}\partial_\mu\phi\partial_\nu\phi -V(\phi)$. In curved spacetime, this generalises to $\sqrt{|g|}(\frac{1}{2}g^{\mu\nu}\partial_\mu\phi\partial_\nu\phi -V(\phi))$ with $g:=\det g_{\mu\nu}$.
We model the expansion of 3-dimension... | {
"language": "en",
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Question about calculating |Wavefunction|^2 In one of my homework in Quantum mechanics, I was asked to find $|Ψ(x,t)|^2$, where
\begin{align}
Ψ(x,t)&=1/\sqrt{10}[3ψ_1(x)e^{-iE_1t/ħ}-ψ_3(x)e^{-iE_3t/ħ}]\, ,\\
&=1/\sqrt{10}[3\sqrt{2/a} \sin(\pi x/a)e^{-iE_1t/ħ}- \sqrt{2/a}\sin(3\pi x/a)e^{-iE_3t/ħ}]
\end{align}
So, from... | First note that in general the wavefunction is complex, not real. This answers the first two questions you asked, since for any complex number $z = a + ib$, $|z|^2 = z^*z$. Then,
\begin{equation}
\psi_1^*(x) \psi_1(x) = |\psi_1(x)|^2 \quad \text{and} \quad \psi_3^*(x) \psi_3(x) = |\psi_3(x)|^2.
\end{equation}
You can s... | {
"language": "en",
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Feshbach -Resonance why are hyperfine structures important? The hyperfine structure of energy levels around the ground state seem to enable Feshbach and be intrinsic to it. Why do we need hyperfine levels?
I.e. why is Feshbach specific to ultracold atoms in ground stae, why would a magnetic field in any gas of atoms n... | I think what you are forgetting is that a given Feshbach resonance is between two atoms in a certain scattering channel. This means, in particular, that it only occurs for one partial wave. As far as I know, all observed Feshbach resonances have been either s-wave or p-wave, but presumably they also exist for higher pa... | {
"language": "en",
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"source": "stackexchange",
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How do photons interact with a very fine edge? Suppose you have some material which has a low reflectance and is opaque to an incomming photon when the angle of incidence is small.
Now take that material and make a narrow (20:1 width:length ratio or narrower) wedge with a fine edge - 1/10th the wavelength of said photo... | For the reasons explained by @WetSavannaAnimal aka Rod Vance you're just asking a classical optics question, and I'll supplement that by trying to answer the classical optics question.
A small fraction of light will get scattered or absorbed at the sharp tip, but as the tip gets sharper and sharper, that fraction gets ... | {
"language": "en",
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Why do we use superposition instead of tensor product in interferometer? In the description of a neutron interferometer here, it says:
In an interferometer the incident beam is split into two (or more) separate beams. The beams travel along different paths where they are exposed to different potentials (which results ... | The confusion comes from "and allowed to interfere". Superposition is not interaction, and they are describing a superposition of two beams of neutrons.
It is similar to the superposition of two laser beams split from the same original, which show interference fringes due to the superposition of the two beams, and it ... | {
"language": "en",
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How is particle in a ring is in bound state? Particle in a potential well forms a bound state which gives rise to discrete energy levels. But states for particle in a ring also have discrete energy levels even where there is no potential well. In what aspect hence the system is 'bound'? Or is it that it's not necessary... | Discrete energy levels usually appear due to the presence of boundary conditions in solving Schrödinger's equation. Boundary conditions can come from the shape of the potential, but also from symmetries of the problem for example.
If we immagine a unidimensional infinite potential well, the confined particle do not ha... | {
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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... | I'm not really sure to have understood your question, however you have to consider the sign of the charge inside the formula:
$$\mathbf{F_{Lor}}=q\mathbf{v}\times\mathbf{B}$$
and so if:
$$q=|q|$$
$$\mathbf{F_{Lor+}}=|q|(\mathbf{v}\times\mathbf{B})$$
instead if:
$$q=-|q|$$
$$\mathbf{F_{Lor-}}=-|q|(\mathbf{v}\times\mathb... | {
"language": "en",
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What is "Symmetry of Infinity" in electricity and magnetism? I have this problem from my E&M textbook:
Two infinitely long wires running parallel to the x axis carry uniform charge densities $+\lambda$ and $-\lambda$ (see photo). Find the potential at any point $(x,y,z)$, using the origin as your reference.
The solut... | To my knowledge, this is not a technical term which you don't know, but merely a hand-wavey and brief way of pointing out that the charge distribution is independent of x and so the potential must also be independent of x. "Infinity" is evocative of this fact because if the wires were not infinite in length, then the ... | {
"language": "en",
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Tension in the string of a pulley
In the diagram above why is the tension of the string attached to the pulley at "A"(the string attached to roof) equal to 2T?
Why is it not Mg+(M+m)g?(considering that the pulley is mass less)
I have trouble understanding
| It would if the weights weren't accelerating but they are accelerating because they are not of equal mass. So to get the overall force on the pulley you have to take acceleration into account; subtract whatever force the acceleration creates from the force created by gravity
Tension is not going to equal to what the ma... | {
"language": "en",
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If Bohr model is outdated and we know that there is no such thing as an "electron orbital circumference" then how is $2\pi r=n\lambda$ still valid? We know that Bohr model is outdated and we know that there is no such thing as an "electron orbital circumference" then how is $2\pi r=n\lambda$ still valid?
Edit :
If th... | The Bohr model is a semi classical model, treating the electrons like satellites of the proton, in the successful hydrogen atom solution. The success relied that the Bohr assumptions reproduced the series that fitted the hydrogen emission spectra.
The solution of the Schrodinger equation for the hydrogen atom reproduce... | {
"language": "en",
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Energy needed to overcome the coulomb barrier I am in my first year of college and doing an EPQ type project on nuclear fusion and am wondering about Coulomb's Law.
I am looking into the force experienced by a particle due to the Coulomb force and would like to find an equation that describes how much energy is needed ... | Your equation for $E$, the work done by an external force to bring the two charges from a separation of $r_2$ to a separation of $r_1$ should be
$$E = \int_{r_2} ^{r_1} -\dfrac {q_1q_2 e^2}{4 \pi \epsilon_0 r^2 } dr = \dfrac {q_1q_2 e^2}{4 \pi \epsilon_0 } \left( \dfrac {1}{r_1}-\dfrac {1}{r_2}\right)$$
If you take ... | {
"language": "en",
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Is there a SI unit for space-time? Space and time are routinely combined into space-time nowadays, which implies that the SI meter and second should be combined into a single SI unit such as [meter-second]. So far, I haven't come across such a SI unit.
| The units keep being 'meter'. Since the speed of light $c$ is a constant for all inertial observers, there's no problem in multiplying time by this number to get meters, that is
$$
ds^2 = c^2 dt^2 - dr^2
$$
$ds$ has thus units of length
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Electric potential energy and equipotential lines
Consider an electric dipole as in the figure. There is a vertical equipotential line/surface. If I understand correctly, electric potential is the amount of work done per unit charge in bringing a charge from infinity to a distance r from a charge.
But consider moving ... | Equipotential lines are always at right angles with the electric field (most clearly shown in the centermost equipotential line). This implies that if a charge were to move along an equipotential line then throughout the entire journey $F_{electric} \perp dr $ and hence, $F_{electric} \cdot dr = 0$.
To move a charge a... | {
"language": "en",
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"source": "stackexchange",
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Phase of a Wave and Phase Space What relation does the phase of a wave have with the phase space? Namely, how are they related historically and/or physically?
P.S. if it helps, I came across this question while thinking about the phase-space formulation of QM and the pilot-wave theory.
| There is no relation between the phase of a wave and the so-called phase space of a mechanical system which consists of the space of all possible generalized coordinate and conjugate generalized momentum variables.
| {
"language": "en",
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Standing waves on string with different densities I am rather confused with the interference of waves that must occur in a string with different densities.
Say for example we have a string of length 2L. And the first L part has mass per unit length u, while the second part has mass per unit length 9u.
A wave is continu... | Thinking about standing waves in terms of reflections from the discontinuity in the middle is a recipe for confusion. Here’s an easier solution.
Since the string tension will be uniform, but the mass/length ratio changes by a factor of 9 at the midpoint, you will have to match displacement and slope at the discontin... | {
"language": "en",
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Why not quarter-life? The number of nuclei left after time $t$ in radioactive decay is given by:
$$N(t) = N_0 e^{-t/ \tau}$$
Now if we put $N(t)$ as $\dfrac{N_0}2$, we get half-life. But, if we had put $\dfrac{N_0}4$, we would have quarter-life, which is also independent of $N_0$.
Is there anything special about half-... | The decay time $t_{1/2}$ of half the given number $N_0$ of atoms atoms is just convenient and visually appealing. Of the unit fractions it is also nearest to the decay time constant (mean lifetime) $\tau$ $t_{1/2}=0.6931 \tau$. The decay time to a unit fraction $1/n$ given by the positive integer $n$ is $$t_{1/n}=\tau... | {
"language": "en",
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Jeans equation for a spherical equilibrium I am currently studying the Jeans equation for a system in spherical equilibrium. Since the distribution function $f(x,v)$ can be written in depency of energy and angular momentum $f(E, L)$ it seems to follow that most of the velocity moments disappear:
$$
\tag{1}
\left<\upsil... | Jeans equation is just an analog to the Euler equations.
In the limit you describe, the results of your Equation 1 imply you are working in the center of momentum frame, i.e., the bulk flow rest frame. The results of equation 2 state the pressure tensor can be diagonalized, i.e., there is no viscosity. Viscosity aris... | {
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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... | If we describe in short then you'll find that kinetics theory of gases is stated that
Total energy of gas = kinetic energy
Because there is no attraction between the gas molecules i.e. Potential energy will be zero
And I think you know very well that KTG is applicable only for ideal gases while real gas molecules have... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
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How does a receiving antenna work given that the electric field is 0 in conductors? The question of how a receiving antenna works has been asked on this site before, such as here How does a receiving antenna get an induced electric current? and here How does a receiving antenna work?. I understand the basic principle t... | When a metal antenna wire is put into the field of a propagating electromagnetic wave with time-varying fields, there will be an electric and magnetic field inside the wire and thus also a current but the penetration is exponentially damped. The penetration depth $\delta$ is called the skin depth. In treating boundary ... | {
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"source": "stackexchange",
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Metric tensor in spherical coordinates using basis vector? I'm using these spherical basis vectors but it's not agreeing with other literature when I use the definition of the metric tensor to derive the metric tensor in spherical coordinates.
\begin{align}
{\mathbf e}_r
&=\sin \theta \cos \phi \,\hat{\mathbf x} +
\sin... | Remember that a basis of a vector space only needs to (1) span the vector space, and (2) be linearly independent. In particular, a basis does not have to be orthogonal, and it certainly doesn't have to be normalized. And one of the most common types of basis (a coordinate basis) is usually not normalized.
You're conf... | {
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Distribution of zero-mean, independent, complex-valued, white noise terms In this paper (open access here), in equations (13) and (14) they state that $W(\mathbf{x})$ is a zero-mean, independent, complex-valued, white noise term such that
$$\overline{W(\mathbf{x})W(\mathbf{x'})} = 0$$
$$\overline{W(\mathbf{x})W^*(\mat... | $W(x)$ is a complex-valued Wiener process:
$$
W(x)=X(x)+iY(x)
$$
where $X,\,Y$ are real-valued, independent Wiener processes.
The authors are asserting two conditions:
*
*The product of two Wiener processes is zero
*The product of a Wiener process and its complex conjugate is of order ${\rm d}t$
It seems to me t... | {
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Is potential energy a type of energy at all? Is potential energy, whether it be that of a charge in an electric field or a mass in a gravitational field or anything like that, actually an energy that the particle itself contains, like kinetic energy? Or is it just a measure of its ability to do work?
Is it the case th... | A body's kinetic energy is the work it can do because of its motion (as it comes to rest). Calculate this amount of work and, in Newtonian physics, you find it to be equal to $\frac{1}{2}mv^2$.
A body's potential energy at point P is the work it can do by changing its position (in a conservative field) from P to anothe... | {
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Why is the singlet state for two spin 1/2 particles anti-symmetric? For two spin 1/2 particles I understand that the triplet states ($S = 1$) are:
$\newcommand\ket[1]{\left|{#1}\right>}
\newcommand\up\uparrow
\newcommand\dn\downarrow
\newcommand\lf\leftarrow
\newcommand\rt\rightarrow
$
\begin{align}
\ket{1,1} &= \ket{\... | If $\:\mathsf{H}_{\boldsymbol{\alpha}},\mathsf{H}_{\boldsymbol{\beta}}\:
$ are the 2-dimensional Hilbert spaces of two particles $\:\boldsymbol{\alpha},\boldsymbol{\beta}\:$ with spins $\:1/2\:$ then the composite system lives in the product 4-dimensional Hilbert space
\begin{equation}
\mathsf{H}_{\boldsymbol{f}}\equi... | {
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Generalised representation of a 2x2 Positive Operator-Valued Measure Let {$E_{i}$} be a set of 2x2 POVM operators, satisfying $\sum_{i}E_{i}=\mathbb{I_{2x2}}$.
We know that a general 2x2 Hermitian matrix (say, $H$) can be represented by
$$
H =
\left[{\begin{array}{cc}
a_{0}+a_{3} & a_{1}-ia_{2} \\
a_{1}+ia_{2} & a_{0}... | By applying extra constraints on the above generalized 2x2 Hermitian matrix for satisfying positive semi-definiteness criteria, we can arrive at a generalized representation for 2x2 POVM operator, say $E_{i}$.
The constraint is that the Hermitian matrix should have only non-negative
eigenvalues [1].
Let $E_{i}$ be
... | {
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Potential Difference due to a infinite line of charge When a line of charge has a charge density $\lambda$, we know that the electric field points perpendicular to the vector pointing along the line of charge.
When calculating the difference in electric potential due with the following equations.
$$\nabla V=-\vec{E}$$
... | No, it's okay. The pontential difference increases as you go farther. The less you move away, the more similar potential you have (little difference).
By the way
*
*You can't integrate in three dimensions that way. You're using cylindrical coordinates (because of the symmetry of the problem), and you integrate alon... | {
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Why does oil float on water? This might be a silly question but I want to know why oil actually floats on water. I tried to explain it to myself using Archimedes' principle but that didn't help.
Archimedes’ principle, physical law of buoyancy, states that any
body completely or partially submerged in a fluid (gas o... | "...because oil and water don't even mix so there's no displacement of water hence no buoyant force is exerted."
This is where you are misunderstanding. There is a displacement. Wood doesn't mix with water either, yet it displaces water and it floats. With oil, there is a slight depression of the lower surface, betwee... | {
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Mass Dropped on Scale When a mass is dropped onto something like a bathroom scale, the reading on the scale temporarily exceeds the actual weight of the mass. How do I explain this using forces and a force body diagram?
Also, let's say instead of a mass and a scale, its just a person, a ball, and a scale. The person is... | Forces must always balance. A force is required to support a stationary mass on a bathroom scale. An additional force is required to effect the deceleration of a mass if it has vertical downward motion as it makes contact with the bathroom scale surface.
Dropping the mass onto the bathroom scale:
$$F = mg + ma \tag1$$
... | {
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Why change in resistivity is proportional to the original resistivity? When there is a temperature change $\Delta T$, the change of resistivity is
(1) proportional to $\Delta T$
(2) proportional to the original resistivity $\rho_0$
Hence we can define the temperature coefficient of resistivity $\alpha$ so that
$$\Delta... | Okay, so if I understand your question properly you're asking for the physical causes behind the change in resistivity being affected by the temperature and why such changes are linearly proportional to the original resistivity.
Well, being just a physics student, I went and, uh, looked at the Wikipedia page on conduct... | {
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Why does the electric field strength for a dipole go as $1/r^3$? I've been given the following graphic to help wrap my head around this.
If the potential can be shown to represent a $1/r^2$ relation, then I'm more than happy to accept that the electric field is hence a $1/r^3$ relation, but I need to accept the first p... | The key point is that $r$ (the distance to the center of the dipole) is not the same thing as the distances $b$ and $c$ from your test charge to the positive and negative charges of the dipole.
If the dipole separation isn't very large, then that isn't a big deal, as $1/r$ and $1/b$ will generally be quite similar, bu... | {
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Are there any algorithms which conserves the energy for quantum harmonic oscillators simulation? well I am trying to simulate the quantum harmonic oscillator. I have tried a few algorithms like using the Rk_4, etc. the energy is not conserved in that. are there any algorithms which will keep the energy constant for the... | You want an algorithm in which every time step is unitary. One sketch of such an algorithm for the generic Schrödinger equation is given by approximating the time evolution operator for a time step as
$$\mathrm{e}^{-\mathrm{i}H\Delta t} \approx \frac{1 - \frac{1}{2}\mathrm{i}\Delta t H}{1 + \frac{1}{2}\mathrm{i}\Delt... | {
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Causal ordering on spacetimes On any spacetime $(M,g)$ we can form the causal ordering $\leq$, where for any two points $a,b \in M$ we have that $a \leq b$ iff there exists some future directed, non-spacelike curve from $a$ to $b$. This causal ordering is transitive, irreflexive, antisymmetry and dense in general. Howe... | While the example of de Sitter space is a classic one, there is an even simpler example that doesn't require computing geodesics.
Take two dimensional Minkowski space, and remove the line $\{ (x,t) | x=0, t \geq 0 \}$. Any event $t \geq 0$ will have its future lightcone restricted to either positive or negative $x$. Th... | {
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Basic cut-off regularization I've been reading these notes on regularization by Hitoshi Murayama
here, and on page 3 there's a few lines of calculations on a quick method of regularizing an integral. But I can't follow the steps - where does the $z$ come from? Why is a second integration variable suddenly introduced, ... | I believe you should read up on Feynman's parametrization.
$$ \frac1{AB} = \int_o^1dz\ \frac1{(Az+(1-z)B)^2} $$
Substitute $A= (p^2 + \Lambda^2)$ and $B= p^2+m^2$ and see what follows.
| {
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Notation of Maxwell relations The Maxwell relations are often given as for example
$$\left(\frac{\partial T}{\partial V}\right)_S = -\left(\frac{\partial P}{\partial S}\right)_V.$$
What does the $S$ and the $V$ in the index of the parantheses mean? I guess that $S$ and $V$ should stay constant for the derivation, but i... | Basically in Thermodynamics different functions get the same name if they refer to the same quantity. So, for example, the inner energy $U(p,V,N)$ and $U(T,V,N)$ both are called $U$ although they are not the same function.
To my understanding this is why you write the constant variables next to the brackets. It's used ... | {
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Difference, in relation to minus sign, between the processes: $gg \to q\bar q$ and $qg\to qg$ I have been computing some QCD cross-sections lately and I have followed the instructions given by Peskin and Schroeder. However I stumble upon this situation: Where the minus sign in $qg \to qg$ comes from? Because the crossi... | Gluons are vector bosons, but quarks are fermion. By crossing symmetry, you flip an anti-quark in s-channel $gg\to q\bar{q}$ to quark t-channel $qg\to qg$. By fermion statistics, you must change the sign. So there is a "-" sign in the result.
| {
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Is this interpretation of quantum fluctuation in eternal inflation in Wikipedia correct? Wikipedia's article on inflation says
Although new inflation is classically rolling down the potential, quantum fluctuations can sometimes lift it to previous levels. These regions in which the inflaton fluctuates upwards expand m... | The classical inflaton potential does receive quantum corrections from the other fields in the theory, but quantum fluctuations of the inflaton field itself have much greater significance during inflation. As a quantum field, the inflaton exhibits fluctuations about its classical trajectory, $\phi(x,t) = \phi_0(t) + \... | {
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Using a previously determined detective quantum efficiency for a detector I am doing a radiation damage survey on a few different materials and will need the Detective Quantum Efficiency (DQE) when calculating the dose. I will be using the same detector for each sample and was wondering if it would be sufficient to acq... | Short version: It's better to measure it in situ and shortly before (or better before and after) taking data.
Explanation
Quantum efficiency can be a function of various operating parameters, so you would need to know that the previous measurement was done using the same parameter you plan to use.
Various classes of d... | {
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Electron repulsion force vs gravitational pull I understand the electron repulsion force is 20 orders of magnitude stronger than the force of gravity pull. But in Proton Earth, Electron Moon, where it hypothesized to replace the Moon with 10^52 electrons, it states that
The energy from all those electrons pushing on ... | First of all, the statement in question is NOT a consequence of string theory. It could be, but we don't know the physics of such high energy systems. Accordion to the article "something stringy might happen, we simply don't know".So we will ignore string theory.
The next thing that you have to consider is that Newtons... | {
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2D Ising Landau theory and the term $A_i( m, T)\nabla_i m$? In (Sethan, 2007; pg$\sim$206) it is said that in the 2D Ising terms of the form e.g.
$$A_i( m, T)\nabla_i m$$
are not allowed in the free energy since there is no vector $A_i(m,T)$ that is invariant under $\pi/2$ rotations. I am confused why we need this sin... | In the 2d Ising there are two symmetries we need to concern ourselves with:
*
*Symmetry of the Lattice: The lattice is symmetric under $\pi/2$ rotations as stated in the question.
*Symmetry of the Order Parameter: The order parameter has summetry under $m\rightarrow -m$
It is the symmetry of the lattice that concer... | {
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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?
| "Fringes of equal thickness" probably means equal spacing. You only get fringes of equal spacing in a Michaelson interferometer when the beams in the interferometer are collimated.
The only reason for it might be necessary for the paths in the two arms of the interferometer to be nearly equal is if the light is not te... | {
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Why doesn't a new ball-point pen write as smoothly as one being written for a little? Why doesn't a new ball-point pen write as smoothly as one being written for a little? You will say that the friction is more first up.Then why is that so?
| Friction is higher at the beginning. Usually, a ballpoint pen comes with a tip to protect the ink inside. This causes the ink at the surface to harden after a while, hence, producing more friction. However, upon writing, this hardened layer slowly goes away, and liquid ink begins to flow out, producing less friction... | {
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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... | Physical systems are either proper or strict proper. This means, they either show low-pass properties or a 'constant' one (imagine the two end-points of a bar that can move in one direction and can't rotate: every movement, you apply to one end, will immediately be applied at the other end).
If a system's transfer func... | {
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How to interpret $\langle\pi|J|\pi\rangle$? The decay constant $f$ of a hadron with momentum $P$ or its in-hadron condensate $\kappa$ (See this paper, Eq. (8)) are given by terms like
$
\begin{align*}
f_\pi P^\mu &\sim \langle 0|J_\text{axial}^\mu (0) |\pi(P)\rangle,\tag{1}\\
\kappa_\pi &\sim \langle 0|J_\text{pseudosc... | Are you asking about the meaning of these expressions or their relevance to physics? I'll address the former and mention the latter in brief in closing.
Injection of a particular axial current operator onto a pion line gives rise to its decay. This is represented by the matrix element $\langle 0|J^{\mu}_{\text{axial}}|... | {
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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... | A simple physical argument should be whether the magnetic field will be able to polarize that medium in between. If that is possible then field intensity will be transported through polarization.
| {
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Confusion on quantum numbers So, I've known for a long time the famous quantum numbers $n, l, m, s$ and I thought these were all of the quantum numbers, and then when applying the Schrödinger equation to orbital angular momentum and getting the spherical harmonics, with their numbers $l$ and $m$, I thought, okay here t... | Hydrogen atom with and without Coulomb potential
For H-atom with Coulomb potential and no perturbations such as spin-orbit interaction, relativistic correction etc, $n,l,m_l,m_s$ are good (conserved) quantum numbers because the operators $\textbf{L}^2,L_z,\textbf{S}^2,S_z$ commute with the Hamiltonian, and hence can be... | {
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Does reversing time give parity reversed antimatter or just antimatter? Feynman's idea states that matter going backwards in time seems like antimatter.
But, since nature is $CPT$ symmetric, reversing time ($T$) is equivalent to $CP$ operation. So, reversing time gives parity reversed antimatter, not just antimatter.
... | Positrons have equal and opposite charge and parity to electrons. Hence when combined, they can produce a neutral gamma ray with no parity.
https://en.m.wikipedia.org/wiki/T-symmetry
| {
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What is the difference between a deuterium nucleus and a sexaquark? Assume a sexaquark contains 3 up and 3 down quarks.
What is the difference between this and a deuterium nucleus containing a proton bound to a neutron?
Is there any difference?
| My advisor posed me the same riddle ages ago, and it drove me stark raving mad.
To expand on Ben Crowell's comment ...
Deuterons have exceptionally weak binding energies, out of line with heavier nuclei, so they may be atypical. Alpha particles seem more representative of differences between nuclear matter and quark... | {
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Would we be able to be consistent in our theory if we were to only assume the particle nature of matter with the uncertainty principle As far as I understood, the uncertainty principle is direct consequence of the wave - particle duality of matter; however, would we be able to be consistent in our theory if we were to ... | In the mainstream quantum mechanical formalism the Heiseneberg uncertainty principle comes from the commutation relations of operators .
For non-relativistic energies there exists a theory, called Bohmian mechanics which reproduces the mathematics and the probabilistic values having particles accompanied by "pilot ... | {
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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... | Apply the definition of optical path.
| {
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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... | You have a great theoretical answer from CMS.
The work required stop is proportional to mass. Distance to stop is proportional to the force.
There are two coefficient of frictioncoefficient of friction: static and kinetic. Static is greater and is when you are not skidding. Max braking is to apply enough pressure to... | {
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Randomness of radioactive decay I know there are similar questions, but I don't think they address exactly this question.
Radioactive decay is a truly random process, according to physics. We believe that there is no hidden particle or cause that we have not discovered yet that governs particle decay.
On the contrary, ... | The theoretically derived statistical distribution for the radioactive decay, using the formalism of Quantum Mechanics (QM), which is intrinsically probabilistic, agrees well with empirical results.
So, first we have to acknowledge that QM is able to explain the radioactive decay observations, just as it is able to exp... | {
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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... | High-energy electron scattering gives a very direct measure of the density of the protons, and the density is found to be fairly constant.
Another example of the type of evidence that supports this is that measurements of the Coulomb barrier for nuclear fusion are consistent with a nuclear radius that varies as $A^{1/3... | {
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Does Power Law Inflation Lead to Eternal Inflation? If we have an inflation model with potential $V(\phi) = V_0 e^{-\sqrt{\frac{2}{\lambda}} \frac{\phi}{M_p}}$, where $V_0$ and $\lambda$ are free parameters, does this lead to eternal inflation for $\lambda > 1$?
The slow roll parameter $\epsilon_V(\phi) = \frac{M_p^2}{... | Yes, inflation does not end for single field inflation driven by a purely exponential potential. Either one interprets this form of the potential as approximating a different potential when observational scales exit the horizon, or, if taken to be exact, one must introduce some mechanism to end inflation. Non-canonic... | {
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Entangling two quantum systems that are separated by a distance If two quantum systems are entangled their measurements become correlated, even if they are separated by a distance.
But what if one has two different quantum systems at hand, initially un-entangled and separated, can one generate entanglement between the... | Usually it is impossible to entangle spatially separated systems $A$ and $B$, but you can evade this restriction by a trick called entanglement swapping. You entangle $A$ with a third system $C,$ and $B$ with a fourth system $D$. These operations must be performed in the usual way, by allowing $A$ $(B)$ to interact wit... | {
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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:
$$ \sigma_A \sigma_B = | \frac{1}{2} \langle \lbrace \hat{A}, \hat{B} \rbrace ... | In addition to the other good answers directly explaining the meaning, it would be constructive to see how it is derived:
The inequality is the generalized uncertainty principle, which is the most complete form of uncertainty principle.
Given Hermitian operators $A,B$, we define vectora $\left|q\right> \& \left|p\right... | {
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How does one calculate the stability of a ring cavity? When discussing whether or not a cavity is stable, i.e., if a beam stays inside the cavity, one can either employ the ABCD-matrix approach, in which case the cavity is stable if
$$
0 \leq \frac{A+D+2}{4} \leq 1
$$
In a cavity with two perfectly parallel planar mirr... | The general approach is that the magnitudes of the eigenvalues of the transfer matrix of the ring round trip must both be below unity. This translates to the physical statement that the beam width at any given point along the cavity shrinks with each round trip, i.e. that the whole beam stays within the cavity. If eith... | {
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How do physicists integrate? I've always thought that the integration notation in physics is weird, but I understood it nevertheless for a single variable, until I started reading Zee's QFT in a nutshell, where 4 dimensions are used. So I was wondering how the integration notation works.
For example, for a free theory ... | $$ J_a(k) = \int dx^0 e^{-ik^0x^0} \int d^3x e^{i\vec{k}\cdot\vec{x}}\delta^3(\vec{x}-\vec{x}_a) = e^{i\vec{k}\cdot\vec{x}_a}\int dx^0 e^{-ik^0x^0} $$
$$ J^{*}_a(k) = \int dy^0 e^{ik^0y^0} \int d^3y e^{-i\vec{k}\cdot\vec{y}}\delta^3(\vec{y}-\vec{x}_a) = e^{-i\vec{k}\cdot\vec{x}_a}\int dy^0 e^{ik^0y^0} $$
So you have
$$... | {
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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?
| The weight applies to the center of mass (CM) of the person. When the person stands perpendicular to the ground, the force goes downwards. The normal force compensates the weight, so nothing happens.
However, when the runner leans downwards, the vector from the ground to his CM is not parallel to the weight force. That... | {
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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... | In short, the same way as before but assume $\Omega_{rad}$ is currently very small
$$\frac {\dot a}{a}=H_{0}\sqrt{\Omega_{m}a^{-3} + \Omega_{\Lambda}}$$
which has the solution
$$a(t)=(\Omega_{m}/\Omega_{\Lambda})^{1/3}\sinh^{2/3}(t/t_{\Lambda})$$
where $t_{\Lambda}=2/(3H_{0}\sqrt{\Omega_{\Lambda} })$
Set $a=1$ which gi... | {
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Non-integer $k$ value in Friedman-Robertson-Walker model? I understand that $k$ describes positive, negative, or no curvature. However, why can't there be, for example, +0.5 (semi-positive) curvature, etc?
| The continuum of curvatures does exist, but we find it more convenient to put it elsewhere. The crucial part of the metric that encodes the curvature is a factor $1-K r^2$, where $r$ is the radial coordinate (using any point as the origin), and $K$ is any real number, positive or negative. By dimensional analysis, ther... | {
"language": "en",
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Bogoliubov Transformation with Complex Hamiltonian Consider the following Hamiltonian:
$$H=\sum_k \begin{pmatrix}a_k^\dagger & b_k \end{pmatrix}
\begin{pmatrix}\omega_0 & \Omega f_k \\ \Omega f_k^* & \pm \omega_0\end{pmatrix} \begin{pmatrix}a_k \\\ b_k^\dagger\end{pmatrix}\tag{1}$$
for bosonic operators ($+$) or fermio... | There is always a bottom-line answer to this question: write the complex boson/fermion in terms of real boson/fermion ($a=a_R+i a_I$, etc), plug it in, and then diagonalize it by orthogonal matrices. This is probably the more natural way to do it for particle non-conserving systems.
If one insists on doing it in terms ... | {
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Why doesn't the universe collapse under its own gravity? Is the reason the universe doesn't collapse into itself due to gravity because there is an infinite amount of bodies in infinite space, therefore there is an infinite amount of gravitational pull on an infinite amount of objects so it all balances out?
| No, the reason the universe doesn't collapse is because it's not dense enough. This can be seen from the Friedmann Equations, the main equations of cosmology. If you work through the derivation you'll find that there's a so-called critical density,
$\rho_c = \frac{3H^2}{8\pi G}$
If the universe's average density is abo... | {
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Why does the Hamiltonian define symmetry/invariance? In Sakurai's Modern Quantum Mechanics, in Chapter 4, he effectively states that the operation of rotation or translation, represented by a unitary operator $U$, is customarily called a symmetry operator regardless of whether the physical system itself possesses the s... | In classical mechanics, a conserved quantity has vanishing Poisson bracket with the Hamiltonian. Such quantities become "good quantum numbers" in QM: they commute with $H$, so simultaneous eigenstates from a complete basis. The evolution operator $e^{-iHt/\hbar}$ also commutes with good quantum numbers, so their probab... | {
"language": "en",
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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... | Water is an excellent thermal conductor but a poor electrical conductor.
| {
"language": "en",
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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... | While I agree with other answers as to the physics of the problem, there is at least one practical area where harvesting energy from the orbital motion of satellites could be of practical utility: space debris removal system. As already mentioned, removing kinetic energy of orbiting body would result in this body falli... | {
"language": "en",
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If momentum and kinetic energy are related, how loss in energy doesn't cause loss in momentum? Kinetic energy and momentum are related to each other by the following equation:
$$K.E.=\frac{1}{2}\frac{\textbf{P}^2}{m} $$
In inelastic collisions the momentum is conserved but the energy isn't. How can this be correct in t... | For simplicity's sake, let's restrict ourselves to collisions in 1 dimension, where object 1 collides with object 2. They have momenta $p_1$ and $p_2$ before collision and $p_1'$ and $p_2'$ after collision. We then have
$$\begin{align}
p_1' + p_2' &= p_1 + p_2,\tag{1}\\
K' = \frac{p_1'^2}{2m_1} + \frac{p_2'^2}{2m_2} &=... | {
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Is there a local source of Hawking radiation? Suppose a black hole is formed at time $t_0$, and after that even more energy falls in, in what we are calling mass shells. I'm inclined to believe that the initial black hole starts radiating before it continues growing, and that each shell of mass falling does change tha... | The Schwarzschild solution is a vacuum solution of Einstein's equations. So a black holes consists of vacuum; its mass M is in the singularity. Therefor there are no "mass shells". Some people talk about shell observer, who are stationary at a constant r-coordinate outside the event horizon but such shells are definit... | {
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Population II star orbits Ok so they are usually found in globular clusters and can consider orbits in a static spherically symmetric gravitational field. The orbits are randomly scattered. Would these be considered to be collisional or collisionless orbits?
| They are pretty collisionless. As a rough estimate, the time to a close encounter within radius $r$ is $\tau \approx 1/(\pi r^2 v \rho)$ where $v$ is the average velocity ($\approx$ 20 km/s) and $\rho$ the average number density ($\approx$ 0.4 per parsec on average, 100-1000 times more in the core). So for <1 AU encoun... | {
"language": "en",
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Zero-level of combination of $1/r$ and $r^2$ potential I am solving a problem which involves a central big mass $M$ and around it a spherically symmetrically distributed mass of constant density $\rho$.
The force on a mass a distance $r$ from the centre can be shown to be:
$$ F = \frac{-GMm}{r^2} - \frac{4\pi\rho Gmr}... | The choice of the zero point for the potential energy is entirely arbitrary. In practice we choose it in a way that makes out calculation simple.
In this case your test mass is going to oscillate around the centre of the mass distribution so it will start at rest at some distance $r_\text{max}$, fall through the centre... | {
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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... | To get something to turn you need to apply a minimum amount of torque to overcome friction. The impulse of a collision (eg swinging a hammer at the lever) enables you to apply a high torque for a short time, when a constant push with the maximum stationary force you can provide is less than the required minimum torque.... | {
"language": "en",
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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... | It seems you completely understand this problem, except between firing and impact, when:
$$ \vec p_{bullet} = -(\vec p_{gun} + \vec p_{box}) $$
so the sum of all three remains $\vec 0$.
| {
"language": "en",
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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... | You cannot simply divide by 4, no. If you set up that formula for each wheel, then you'll have to take into account that the speeds and distances are different.
The one thing all four wheels have in common is angular velocity $\omega$. Even at a 90 degree turn, the rear and front wheels spin equally fast (degrees per s... | {
"language": "en",
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How can we make coherent laser? When im studying about fundamental physics, book says that induced emission makes two coherent photons so whole family of that photons will be coherence so laser can be configured by coherent photons.
But if there is two 'first photon' which is not coherence each other, then after some a... | The laser action is due to stimulated emissions. The atom in the ground state absorb energy and jump to higher state. If the state of atom is a metastable state then the scattered photon and the emitted photon have the same phase always. So the emitted light is coherent and have longest wavelength as well. If two phot... | {
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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?
| In a semi-classical description, if we think of the temperature as being related to the kinetic energy and therefore velocity of the electrons, the Pauli exclusion principle disallows the electrons to all be in the same state, meaning that the vast majority of electrons must have nonzero kinetic energy (since they can'... | {
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Finding translational acceleration
Given
$\alpha = 2.44 $ rad/$s^2$
$\omega = 2.44t+8.35$ rad/s
$\theta = 1.22t^2+8.35t$ rads
Find an expression for the magnitude of translational acceleration at $t=1.82s$, given that the radius of the circle is r.
What I did was use the fact that $a = r\omega^2 = r[(2.44)(1.82)+8.35... | Neither result is correct, but both are part of the solution!
The angular motion is given by ($r$ is the radius and $\theta$ is a function of time):
\begin{align*}
\vec r &= r \begin{pmatrix} \cos(\theta) \\ \sin(\theta) \end{pmatrix}
\end{align*}
From this we define $\omega = \partial_t \theta$, $\alpha = \partial_t... | {
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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... | If you use a polarizing filter for ultraviolet light you could see that visible light will be dimmed more as if you use a polarising filter for visible light. The ratio of reflected and absorbed light to the light which is going through the filter depends from the slits width.
If 50% of monochromatic light goes through... | {
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Ginzburg-Landau theory for first-order phase transitions? In AlQuemist's answer to this PSE question:223892 and Thomas' recent answer to one of my questions. There is a mention of the application of the Ginzburg criterion and in general the Ginzburg-Landau theory restricted to second order-phase transitions. This appea... | The usual Landau-Ginzburg potential can be slightly generalized to
$$W(\phi) = t \phi^2 + a \phi^4 + \phi^6.$$
The phase transition is at $t = 0$, and it is continuous if $a > 0$ and discontinuous if $a<0$. $a = 0, t = 0$ is a multicritical point.
This basic observation begins to explain why it's so hard in practice to... | {
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Is there something similar to Bernoulli effect with electricity? There are many parallels between fluid dynamics and electricity. Is there a thing similar to a Bernoulli effect with electricity? For example, would you see a decrease in voltage as the conductor narrows, and an increase as it widens?
| No, I don't think so. Electrons move differently through conductors than fluids move through a given volume, you cannot apply Bernoulli's principle.
In fact, a constriction in a conductor leads to an additional spreading resistance and a wider conductor of the same material will have a smaller resistance.
| {
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Acoustic, optical, ferromagnetic and antiferromagnetic spin-waves? In the context of spin-waves I have seen the following words as descriptors*:
*
*Acoustic
*Optical
*Ferromagnetic
*Antiferromagnetic
which I have seen used together e.g. "acoustic ferromagnetic spin waves" as well as individually e.g. "antiferr... | A ferromagnetic spin wave belongs to a ferromagnet, where there is a net magnetic moment, while an antiferromagnetic spin wave belongs to an antiferromagnet, where there is no net magnetic moment. Does this answer your question?
| {
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Can there be general relativity without special relativity? Can General Relativity be correct if Special Relativity is incorrect?
| Where gravity is weak the metric of spacetime is flat and SR holds.
For example,
As $r \to \infty,$
The Schwarzschild Metric of GR $\to $
The flat metric of SR in spherical coordinates:
$ds^2 = c^2 dt^2 − dr^2 − r^2 dθ^2 − r^2 sin^2 θdφ^2$
"GR without SR" $\to$ There is never no gravity or weak gravity.
| {
"language": "en",
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"source": "stackexchange",
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Christoffel symbol derivation in book by Wald In chapter 3 of Wald's General Relativity he starts by defining a covariant derivative $\nabla$ as a map on a manifold M from tensor fields $\mathscr{T}(k,l) \to \mathscr{T}(k,l+1)$ plus some required properties (linearity, Leibniz rule, etc.).
He then goes on to show that ... | Wald states, in eq. 3.1.14, that the difference between two distinct derivative operators is characterized exactly by the tensor field $C^c_{ab}$.
Schematically, he is saying that
$$
\nabla T = \tilde{\nabla}T + CT
$$
Where $\nabla$ and $\tilde{\nabla}$ are distinct derivative operators. He now chooses that one of the... | {
"language": "en",
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What is the difference between Quantum Dots and nanoparticles? As far as I understand, both quantum dots and nanoparticles are mainly characterised by the fact that all three dimensions are in the nanoscale. Quantum dots are always mentioned to be made from a semiconductor, while nanoparticles can be anything (dielectr... | There is no strict definition, although as you suspect, quantum dots are a subset of the more generic “nanoparticles”. The reason “quantum dot” is normally referring to a semiconducting system is that it evokes a certain group of physical properties used in a certain way: the band states being quantized, for example, w... | {
"language": "en",
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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?
| If an object is completely immersed in a liquid denser than it then the resulting buoyant force will exceed the weight of the object because the weight of the liquid displaced by the object is greater than weight of the object.As a result, the object cannot remain completely submerged and this causes the object to floa... | {
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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... |
... it very well rotates only through the center of mass.
This is not the case. In some applications, it's most convenient to describe the motion of a rigid body as a combination of translation by the center of mass and rotation about an axis passing through the center of mass. You appear to have fallen into the trap... | {
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Stern-Gerlach experiment with a magnetic field inbetween An experiment is set up so that a beam of spin-1/2 is prepared for $S_{z} = \hbar/2$, it then passes a constant magnetic field $\textbf{B} = B_{0}\textbf{e}_{x}$ with the velcity $v_{0}$ for a distance of $L$ before it passes an aditional Stern-Gerlach apparatus ... | HINT: your solution to Schrödinger's equation is wrong. Try it out in terms of its components, remembering that $\textbf{H}$ is a matrix.
| {
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How to properly calculate off-diagonal terms in covariance matrix for entangled Gaussian state? I would like to ask how to properly calculate the off-diagonal terms in covariance matrix for the entangled Gaussian state?
E.g. from https://arxiv.org/abs/0810.0534v1 we have a coherent Gaussian state in the following form
... | You need to take two independent summation indices for the ket and the bra vector in the density matrix (and thus in the computation of expectation values).
| {
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Is the wave model an approximation to the photon model at higher (or lower) frequencies? Certain models hold better in certain regimes. For example, Newtonian mechanics are more useful in the regime of speeds much slower than c. I was wondering, are there specific frequencies for light where the wave model breaks down ... | Rather than frequency, a better way to parametrize this is in terms of the quantum concentration. If you consider a radio wave of wavelength $\lambda$ and frequency $\nu$, the smallest volume to which such a wave can be localized is on the order of $\lambda^3$. If the energy density of the wave is $\rho$, then the numb... | {
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What would happen if Jupiter collided with the Sun? This question is inspired by a similar one asked on Quora. Let's say a wizard magicked Jupiter into the Sun, with or without high velocity. What happens?
The Quora question has two completely opposed answers: one saying "nothing much happens" and the other saying "the... | Simply calculating the amount of heat generated, and comparing it to the heat capacity of Jupiter (even ignoring, as @rob says, that Jupiter's core temperature is hotter than the surface of the sun) is fallacious reasoning. What mechanism would cause the entirety of the sun's energy output be directed towards heating u... | {
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"source": "stackexchange",
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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... | Coherence have a wide range of definitions from the simpler ones in the other answers to more complex ones based in resource theory and fisher information. Roughly they all try to quantify the ability of a state to display interference in various properties. This ability depends on what’s making changes(your Hamilton)... | {
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"source": "stackexchange",
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How are real particles created? The textbooks about quantum field theory I have seen so far say
that all talk in popular science literature
about particles being created spontaneously out of
vacuum is wrong. Instead, according to QFT those virtual particles are
unobservable and are just
a mathematical picture of the pe... | ''The textbooks about quantum field theory I have seen so far say that all talk in popular science literature about particles being created spontaneously out of vacuum is wrong.''
And they are right doing so. See also my essay
https://www.physicsforums.com/insights/physics-virtual-particles/
''How does QFT describe pa... | {
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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... | A, B and C produce a counter-clockwise torque. You can see how that will pevent the body from accelerating to the right with no slipping.
The only choice is D that gives the proper torque sense in this case. It is also the only one different from the other three and there has to be the correct answer by the principal t... | {
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Nuclear Physics Modeling Software I have a nuclear reactor design I would like to model. I would like to show the individual atoms and how they interact with each other in the reactor (specifically, I would like to model decay modes, interactions with photons). I was wondering if there was any software which would help... | Two codes used by the neutrino community to generate predictions of the anti-neutrino flux expected for our detector systems are DRAGON and MURE.
I'm not an expert on either code, but I know they produce slightly different level of detail. DRAGON is a parameterized code while MURE is a full particle-level Monte Carlo. ... | {
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"answer_id": 1
} |
Spectral lines in Franck-Hertz expreiment In a Franck-Hertz experiment in which mercury vapor has been replaced by atomic hydrogen, it is observed pronounced maximum in the current that circulates through the galvanometer for potential values of 10.2 V and 12.09 V; and if it analyzes spectroscopically the emergent ra... | In the Franck-Hertz experiment, most of the vapor is in the ground state (quantum number n=1) to begin with. The 10.2 V signal corresponds to the 10.2 eV transition from n=1 to n=2 in hydrogen. The 12.09 V signal corresponds to the 12.09 eV transition from n=1 to n=3.
The 3 spectroscopic lines in question are then fo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/402985",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Resistance and resistivity: which one is the intrinsic and which is the geometric property? Why? The electrical resistance $R$ and electrical resistivity $\rho$ of a metal wire are related by $$\rho=\frac{RA}{l}$$ where $l$ is the length and $A$ is the cross-sectional area of the wire. One could also have written $$R=\... | Resistivity is the resistance of a given material when the material is of unit length and unit area. So, resistivity is an intrinsic property. Resistance changes with the material geometry, for example, the resistance of the material is doubled when length of the material is doubled and halved when area of cross-secti... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/403100",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 0
} |
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