Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
What's the reason double-slit experiment can't be explained by edge effects rather than quantum interference? Say we had exactly this...
But instead, it was a PING PONG GUN (imagine as table tennis players use to train),
throwing out PING PONG BALLS. The two slits are say 20 cm wide, and the observing screen is say 5... | Nope. The important thing about the double slit experiment isn't that you find a wavy pattern on the screen, it's that the output on the screen is not equal to the output you get with only one slit open, plus the output you get with only the other slit open. The particular pattern that one slit makes by itself doesn't ... | {
"language": "en",
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What does diagonalization mean here? In a gravity theory in spacetime, the metric has signature $− + +· · ·+$. Concretely this means that the metric tensor $g_{μν}$ may be diagonalized by an orthogonal transformation, i.e. $$(O^{-1})_{μ}^{\;a} = O^a_{\;μ}$$and $$g_{μν} = O^a_{\;μ}D_{ab}O^b_{\;ν}$$ with positive eigenva... | Let's go step by step as it seems you're missing some fundamentals.
We know from (linear) algebra, that a symmetric bilinear form can be transformed to a diagonal matrix with elements $e$ on the main diagonal $e\in \{0,1,-1\}$. The tripel counting the amount of times each number appears is called signature. If you did... | {
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Classical gravitational wave Consider the electric field surrounding an electron. The electron moves. A wave propagates at $c$ reflecting the electron's movement. This is an electromagnetic wave. Consider a mass. Classically surrounded by a gravitational field. The mass moves. The field deforms and it propagates at $c$... | You're not wrong! Gravitational waves do occur classically, and that's because general relativity is a classical theory! In Newtonian gravity, the gravity field doesn't "deform," nor does it have a speed - it's just a force that acts at a distance, instantaneously. So you're entirely correct - that is one way that grav... | {
"language": "en",
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Electric Displacement Vector How do I interpret what electric displacement vector is? I know that it exists and I know it's an equation but I'm not able to really understand or interpret what it is.
$$\oint_A \mathbf{D} \cdot \mathrm{d}\mathbf{A} = Q_\text{free}$$
| Well think that in vacuum, the electric field vector $\bf E$ is a good measure of the strength of the electrostatic field.
But if you are measuring inside a medium, say there are some charges around where you are measuring, now the presence of those charges affects the value of $\bf E$ from the external electric field... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Functions of state and definition of equilibrium Consider this -
A function of state is any physical quantity that has a well-defined value for each equilibrium state of the system.
(Blundell and Blundell - Thermal Physics)
My question is - Are equilibrium states not defined by the function of state variables themselve... | Whenever i say that the pressure of a gas is 2 atm then it is known that at the time i say this, the gas is in equilibrium. Well what i want to say that we define a state of any system when it is in equilibrium and by equilibrium i mean thermal equilibrium which means every knid of equilibrium like mechanical, chemica... | {
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Why are four-legged chairs so common? Four-legged chairs are by far the most common form of chair. However, only three legs are necessary to maintain stability whilst sitting on the chair. If the chair were to tilt, then with both a four-legged and three-legged chair, there is only one direction in which the chair can ... | This should be more of an engineering/economics question than a physics one. The statement "... the simpler, cheaper, three-legged chair?" Is false. 120 degree angles have historically much harder to manufacture than 90 degree angles, the small cost of an extra leg is easily offset by the increase in manufacturing cost... | {
"language": "en",
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Do we know why there is a speed limit in our universe? This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is?
I'm not asking why the speed limit is equal to $c$ and not something else, but why there is a limit at all.
EDIT: A... | $\hspace{50px}$The above picture I drew to expand on Kostya's wonderful answer.
Basically, imagine people who measure height of buildings in degrees of angle of the buildings' visibility from the certain fixed distance. This is not at all unreasonable if you fix the distance C large enough compared to the building heig... | {
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Why do most office chairs have 5 wheels? (Inspired by Why are four legged chairs so common?)
I've been wondering for a while... Why do most wheeled office chairs have 5 wheels?
My guess would be that while stability vs. simplicity results in 4 legs, adding mobility to the equation may result in the need for 5 wheels.
... | I don't know the history, I am just speculating. The chair in the picture has features that you don't find on a typical 4 point chair: It has casters, it has a swivel seat, and it has a springy reclining back. All of these make the chair more likely to tip over without warning. Lets assume that you roll the chair f... | {
"language": "en",
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Where does the energy go if a ball hits the ground WITHOUT bouncing? so if a very un-bouncy object hits the floor and crashes into it rather than bouncing, where does most of the energy go? Does it simply turn into heat from friction? Or does it go into the Earth's movement by an imperceptible amount?
| The majority of the energy is dissipated as mechanical deformation (as Jon Custer has stated).
Visualizing the situation can help a lot. All matter is somewhat elastic - there's no such thing as infinite elasticity. When an object collides with something, the force of the collision takes time to spread out across the o... | {
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Why do some materials shrink when their temperature increases? In my vision it would seem quite logical that all materials expand when temperature rises. Because the particles get more energy and travel larger distances when moving. But apparently there are some materials that tend to shrink as temperature increases.... | I imagine alot of negative expansion coefficients occour at phase transitions. Probably the temperature increase causes the material to overcome an activation energy and a change in lattice structure occours. In these cases one would not expect these processes to be reversible.
| {
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Chern-Simons theory The Chern-Simons 3-form is given by
$\omega_3={\rm Tr} \left[ A\wedge dA+\frac{2}{3}A\wedge A\wedge A\right]$
where $A$ is a connection one-form in the adjoint representation of a non-Abelian gauge group.
My differential geometry is rather rusty (and this is new to me too) hence my questions;
$A$ i... | This is an example of lie-algebra valued 1-forms. Actually you may write explicitly, $ A = A_{\mu} ^a T^a dx^{\mu}$. Since the generators also anti-commute so we get the result. And for the same reason sometimes you will find expressions like $[A,A]$ in literature for your term.
| {
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A mass in a rotating tube A mass rotates on a horizontal surface inside a frictionless hollow tube with a angular velocity omega. The only force acting on it is a force $N$ with which the tube pushes the mass.
It is expected that the mass would move away from the center of rotation due to centrifugal force, which is a... | In the absence of external forces a mass would move in a straight line. To maintain a circular motion the mass needs to be "constrained" and this is done by the contact normal force $N$ acting by the tube to the mass. This force is real.
The magnitude of this force is found by the difference between the straight line p... | {
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Charge inside a charged spherical shell
*
*If I were to put a negative charge inside a negatively charged spherical shell, will it move to the center?
*Electric field inside the shell due to the shell is zero (Gauss's Law), would that mean the charge inside the sphere faces no force?
But, that doesn't make intuitiv... | Since there are no charges inside a charged spherical shell . This means the net charge is equal to zero. So magnitude of electric field $E$=0. So there is no net force. So magnitude of net force =0. No movement towards centre.
| {
"language": "en",
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Same equation, different meanings I went into a physics classroom today and saw this equation written on the board:
$$
E = \frac \sigma \epsilon
$$
At first I thought it referred to the electric field $ E $ between 2 parallel plates of charge density $\sigma$ separated by a material of permittivity $\epsilon$. However,... | Coincidence, nothing deep I'd say. Note that the equation representing the electric field modulus depends on the units you've picked and as such putting so much emphasis on the exact characters appearing in the eq. is senseless. Note that it's possible to form many physics equalities and equations involving 3 character... | {
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Dependence of average speed of molecules of gaseous mixture We know that the average speed of gases in a single gas chamber is given by $\sqrt{8RT/\pi M}$
where R is universal gas constant,T is temperature,M is molar mass of gas.
But what if we mix two gases in any ratio say 1:1 and then try to find average speed of an... | If the two gasses are mixed then they will reach the same temperature, with the lighter molecules moving faster than heavier ones.
In the initial equation, 8, R, T and pi would all be the same. Since the gas molecules have different masses the speeds must therefore be different.
If a tank hits my car side on, my car go... | {
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Backyard experiments to falsify the Flat Earth theory I recently became aware that the flat Earth theory still exists in the 21st century, and has colored the views of a friend of mine. Roughly speaking, the tenets are:
*
*The Earth is a flat disk, with the south pole blown up into a circular "ice wall" where one wo... | Related to the Andrea di Biagio answer. Here is a typical flight path for the Beunos Aires-Auckland route. The distance is approximately 10,300 km by the shortest route along a sphere. Direct flights are offered by Air New Zealand and take 11h40m - an average speed of 882 km/h.
Looking at the map you present, the dista... | {
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Why does Diffracted Light cone diameter change in relation to angle of light beam? I have a question about light diffraction. Take a look at these images of the Pantheon oculus.
Now what I don't understand, in the first picture, the light is coming in from overhead and forms a really huge light diffracted light cone ... | Neither image displays diffraction. They both illustrate shadowing.
The second photo was apparently shot on a clear sunny day. All of the rays come from the sun. Some are blocked, those that aren't go straight down to the floor.
The second was probably taken on a hazy day. Rays originate from the haze; they come ... | {
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The many faces of electromagnetic waves In my waves and optics class, we have learned several ways to treat electromagnetic waves: light rays (geometric optics), electromagnetic plane waves, spherical waves, cylindrical waves (2D). One thing still confuses me: How can one determine which method to use when approaching ... | I will answer this question in two parts:
*
*first comparing plane waves, spherical waves and cylindrical waves (which are really the same thing as I will explain).
*how this relates to ray optics (which is completely different).
Plane waves, spherical waves and cylindrical waves are 3 different examples of doing... | {
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If there are long-lived elements in the Island of stability, why are they not present in Nature? To my understanding, some (but not many) physicists speculate that the Island of stability may contain long-lived elements, as in a billion or so years. But couldn't we rule that out just by the nonexistence of such element... | Because no rational process can make them. I've been over the tables, and there are only a couple of possible reactions to get there for any nuclei, and they require two rare ones. Alpha particle capture just isn't going to cut it. Look at the curve; you need more neutrons.
We remember that all elements heavier than ir... | {
"language": "en",
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Normal ordering in string theory: Polchinski vs. all others Polchinski defines normal ordering in string theory as:
$$:X^\mu(z,\bar z)X^\nu(w,\bar w): = X^\mu(z,\bar z) X^\nu(w, \bar w) + \frac{\alpha'}{2} \eta^{\mu\nu} \log |z-w|^2$$
and for more complicated expressions one obtains the normal ordered expression via Wi... | The usual definition of normal ordered product is:
$$:X^\mu(z,\bar z)X^\nu(w,\bar w): = X^\mu(z,\bar z) X^\nu(w, \bar w) - \langle X^\mu(z,\bar z) X^\nu(w, \bar w) \rangle $$
As you said, this is the regular part of the OPE, since only the divergent part of two operators gives non vanishing contribution to the correla... | {
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Why are $S = -k_B\sum_i P_i \ln P_i$ and $S = k_B \ln\Omega$ equivalent? This might be a silly question, but I don't see the equivalence relation between these two equations. Could somebody explain to me how to derive one from the other? Thanks in advance!
| Citing Wikipedia here,
In what has been called the fundamental assumption of statistical thermodynamics or the fundamental postulate in statistical mechanics, the occupation of any microstate is assumed to be equally probable (i.e. Pi = 1/Ω, where Ω is the number of microstates); this assumption is usually justified ... | {
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Helmholtz decomposition allows incompressible flow with an irrotational component? A vector field can be written in terms of irrotational and a divergence-free components. Using a 2D velocity field as an example,
$ \vec v = -\nabla \phi + \nabla \times \vec \Psi$
Where $\vec \Psi$ is a vector potential, which in fluid ... | You can use a trick to solve that: Instead to take the divergence, take the curl as follows:
$\nabla \times \vec v = \nabla \times (-\nabla \phi + \nabla \times \vec \Psi)$
but $ \nabla \times \nabla \phi=0 $, so
$\nabla \times \vec v = \nabla \times (\nabla \times \vec \Psi) = \nabla(\nabla \cdot \vec \Psi)-\nabl... | {
"language": "en",
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Coefficient of friction and practical experience of sliding The classical model of friction has a coefficient of friction depend only on the materials, but not area, and the force proportional to the normal force and coefficient of friction. So a given object on the same surface has the same friction whether it is supp... | When the child lies down there is less deformation of the slide's surface than when he is sitting. Whether the deformation is relatively deep and short in length, or more shallow and extends far beyond the child, it is always nonzero. The force required to move the trough of this deformaton down the slide opposes th... | {
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Conditions for the tension to vary in the rope What are the conditions for the tension to vary in the rope. I have read below conditions
1. rope has to have some mass
2. rope is accelerating
I get the 1st one, but I am not sure if I get the 2nd condition.
If a $10\ \text{N}$ force accelerates a mass-less role, what wi... | It also depends on whether or not the rope is inextensible. Assume it isn't. You tie one end to a rigid wall and pull on the other end with a constant tension $T$. Then at some time $t_0$, you start to pull with a slightly higher tension $T_1 > T$. It will take some time $\tau$ of order $\frac{L}{c}$, $c$ being the typ... | {
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What will be final velocity of three charges $q$, $q$, $2q$? What will be final velocity of three charges q, q and 2q kept along an equilateral triangle of side r at infinite distance.
All three masses are equal.
I tried to conserve Total Energy
$$\frac{2kq^2}{r} + \frac{2kq^2}{r} + \frac{kq^2}{r} = \frac{mv^2}{2}+ \f... | This is actually a particular case of the three-body problem (https://en.wikipedia.org/wiki/Three-body_problem), but with repulsion, rather than attraction. There are very few exactly solvable cases of the three-body problem, among them - the Lagrange's case, where the three bodies are at the vertexes of an equilateral... | {
"language": "en",
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"source": "stackexchange",
"question_score": "5",
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Is the conservation of probability in the Schroedinger's equation unique? The Schroedinger's equation can be viewed as a diffusion equation with imaginary constants $a$ and $b$ satisfying,
$$\quad \Psi_t=a \cdot \Delta \Psi-b \cdot V(x,t) \cdot \Psi
\tag{1}
$$
However if $a$ and $b$ are positive real coefficients, we g... | You are misreading the celebrated analytic continuation to diffusion of the Schroedinger equation. This is probably possible by notational overkill induced by superfluous dimensions potentials, and absorbable constants that should not deny you any firm ground to stand on, but evidently do.
The crucial piece to appreci... | {
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publication ethics What does it mean if somebody publishes the same concept of my paper with some changes without citing it although the paper has been published 5 years before his, and the concept was clearly first introduced in my paper? It is quite frustrating to know that nobody cares about your research, especiall... | If you try to just "cope with the disappointing feeling", you are setting yourself up for the same thing to happen again and again. You need to stand up for your rights, if the situation allows this.
But it's unclear from your question, whether this was a case of deliberate plagiarism, or just parallel invention.
| {
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how is a pendulum clock's time and the time period of the pendulum in it related? I'm working out how much time a pendulum clock will gain or loose due to change of the length of the pendulum due to temperature. so far I've got, new time period, $$T_2=T_1(1+\frac12\alpha\Delta T)$$
due to $\Delta T$ change in temperatu... | Assume that each swing advances the second hand by one second on the dial. You have a pendulum which is supposed to have a period of exactly one second.
If the pendulum has got longer in length it will have a longer period and so it will take longer for the pendulum to advance the second hand by one second on the dia... | {
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Why is the Pythagorean Theorem used for error calculation? They say that if $A = X \times Y$, with $X$ statistically independent of $Y$, then
$$\frac{\Delta{A}}{A}=\sqrt{ \left(\frac{\Delta{X}}{X}\right)^2 + \left(\frac{\Delta{Y}}{Y}\right)^2 }$$
I can't understand why that is so geometrically.
If $X$ and $Y$ are inter... | The formula
$$\frac{\Delta{A}}{A} \approx \frac{\Delta{X}}{X} + \frac{\Delta{Y}}{Y} $$
is an approximation because you are ignoring $\Delta X$$\Delta Y$
A better approximation would be
$$\Delta A=\frac{\partial A}{\partial X}\Delta X+\frac{\partial A}{\partial Y}\Delta Y$$
Since errors always add we take the absolute ... | {
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Can an absorbed photon be emitted as two photons? I am taking an intro to astronomy class, and have touched upon absorption and emission lines and etc, the prof asked this question in class and got me thinking.
I would want to say no, because one photon should stay as one photon, mass cannot be created.
Could someone ... | Yes, absolutely! This is the phenomenon of so-called non-linear light scattering. These include an absorption of a photon and its remission as several photons, or as a photon of different frequency, or a combination of a photon and a phonon, etc. (You may want to look up Raman scattering and Mandelstam-Brillouin scatte... | {
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Does shaking a kettle whilst boiling increase the temperature faster than a statically-placed jug? Water molecules move faster at higher temperatures.
*
*Does shaking a boiling kettle whilst it is in the process of boiling water increase the rate of rising temperature ?
*Is it worthwhile to do so to save time ? ... | Coherent motion does not add to the temperature; so you would have to shake it violently, with random motions.
Consider sound in air - this is a coherent motion - when you can no longer make out the sounds in a closed room, the energy of the sound waves has been transformed into heat.
| {
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Action with self-dual field strength It is said that writing down an action in presence of a self-dual field strength is subtle and not known till date. The familiar example people give is that of type IIB super-gravity which has a self-dual 5-form field strength. Can someone elaborate on what exactly the subtlety is. ... | Suppose you have a self-dual five-form field strength $F_5=*F_5$. The kinetic term of this field strength is proportional to
$$
\int F_5\wedge*F_5=\int F_5\wedge F_5=-\int F_5\wedge F_5
$$
where I used $A\wedge B=(-1)^{pq}\, B\wedge A$ for $p$-form $A$ and $q$-form $B$ in the second equality. So you can conclude that
$... | {
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Wavelength for imaging an ultracold atom? I was reading Stamper-Kurn's article Experimental Methods in Ultracold Atomic Physics (link). In the imaging section (page 13), he mentions:
Cold atoms are conventionally probed by optical imaging. Probe light at a well defined optical frequency
is sent through the atomic gas... | The most common type of imaging in these systems is absorption imaging- you just shine light on the atoms that matches a strong electronic transition, so the atoms scatter as much light as possible, and then get an image from seeing how much light is scattered. Essentially, you are looking at the shadow cast by the ato... | {
"language": "en",
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Weight of mass falling through liquid
*
*If I place a container of fluid on a scale and drop a non-buoyant mass into the fluid, will the scale read less as long as the mass is in motion downward as compared to when the mass is at rest on the bottom of the container?
Part two:
*
*Would the shape of the container... | The scale's reading will jump up when the mass impacts the liquid, then gradually decrease to a value larger than the original as the mass decelerates. This means that the scale could read the combined weight of both the liquid and the mass before the mass reaches the bottom if the mass is slowed to a constant velocity... | {
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Density parameter (cosmology) as a function of redshift Hopefully I'm missing some very basic algebra for this question. In essence, I need to derive the following:
$$
\Omega=\Omega_{1}\frac{(1+z)}{1+\Omega_1{z}}
$$
Now, I proceeded from the Friedmann equations as follows for a matter dominated Universe (and for $\Lamb... | You almost have it. The last step is
$$
\Omega = \frac{\rho}{\rho_\text{c}} = \frac{\rho_0\,a^{-3}}{\rho_\text{c,0}}\frac{\rho_\text{c,0}}{\rho_\text{c}} = \Omega_m\,(1+z)^3\frac{H_0^2}{H^2},
$$
where we used the critical density
$$
\rho_\text{c} = \frac{3H^2}{8\pi G},\qquad \rho_\text{c,0} = \frac{3H_0^2}{8\pi G}.
$$
... | {
"language": "en",
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Can observation change entropy? I don't know whether this even makes any sense, but if 'observation' can be considered as 'recieving and reading information', can an act of observation (of a system) change (increase or decrease) its entropy?
| I’m going to go out on a limb here and “answer” instead of comment because I struggled with the concept of entropy for so long until I came to this simple conceptual definition: entropy is the measure of how far away a system is from equilibrium.
From a Physics 101 point of view, observation cannot change this. If a b... | {
"language": "en",
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volume of the air bubble in the water How does the depth affect the volume (the radius) of an air bubble in the water, if the temperature and density of the water are constant. Is there any relation combining this?
Can I say that $dh/dt=dr/dt$?
| Using the ideal gas law:
$P(h) = P_0 + \rho_w g h$, ${V_o P_0} = {V(h) P(h)}$ which gives
$V(h) = {V_o P_0}{1 \over P_0 + \rho_w g h}$.
| {
"language": "en",
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When is the spin of an electron changing? An electron can have a half spin up or down. The up spin can become a down spin to lose his weak charge. But when are electrons changing their spin?
| If initially you had the spin projection in a certin (eigen)state and then you apply a spin-dependent interaction, the spin wave function becomes a superposition of up and down eigenstates. Measurements will find the up-state with a probability $P_{\text{up}}$ and the down-state with the probability $P_{\text{down}}=1-... | {
"language": "en",
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Does the existence (now proved) of gravitational waves imply the existence of Gravitons? I studied the theoretical part about the Gravitational waves in General Relativity (linearization of gravity and small perturbations of the metric and so on).
But I was wondering about: since electromagnetic radiation is composed/c... | While the detection of gravitational waves does not directly imply that gravitational force exists in the realm of the particle-wave duality, it does provide definitive link to the existence of the graviton. I sat in the assembly hall at UMCP during the celebration of the gravitational wave discovery and this very ques... | {
"language": "en",
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"source": "stackexchange",
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Fermions, different species and (anti-)commutation rules My question is straightforward:
Do fermionic operators associated to different species commute or anticommute? Even if these operators have different quantum numbers? How can one prove this fact in a general QFT?
| Fermions $f_i,\,f_j$ with respective momenta $\pi_i,\,\pi_j$ satisfy the equal-time canonical anticommutation relations $$\left\{\ f_i,\,f_j \right\} = \left\{\ \pi_i,\,\pi_j \right\} = 0,\,\left\{\ f_i\left(t,\,\mathbf{x}\right),\,\pi_j \left(t,\,\mathbf{x'}\right)\right\} = i\hbar \delta_{ij} \delta \left(\mathbf{x},... | {
"language": "en",
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"source": "stackexchange",
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Proving charge on outer surface of parallel plate capacitor must be zero If we have two conducting plates, with charge $Q$ and $-Q$, why is the charge on the outer surfaces of each conductor zero?
I've been trying to wrap my head around the problem. Firstly, don't excess charges on a conductor spread out towards the su... | I may possibly be very wrong, but wouldn't it be the same as when you get an electron and mesh it with a proton getting a neutron? If you have the same amount of an opposite charge, the charge will disappear..
| {
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Domain structure and ferromagnetism and reversal of polarity According to the domain theory, ferromagnetic substance when kept in the external strong magnetic field, it undergoes magnetisation by rotation and becomes a permanent magnet thereby even after removing the external magnetic field. I got several questions reg... | As far as I know, once the substance becomes a permanent magnet, it does what all magnets placed in an external magnetic field would do; which is: The north pole of the magnet rotates towards the south pole of the field and vice versa. And since the magnet being made out of substance having free electrons, it does heat... | {
"language": "en",
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Does Birkhoff's theorem hold inside the event horizon? Can Birkhoff's theorem be used to say that the blackhole exterior and interior sections of Kruskal-Szekeres's solution (or coordinate transformations of it like Gullstrand–Painlevé coordinates, etc.) are unique all the way down to the singularity? Or are there diff... | Ben Crowell's answer is right. I hope I can clarify a little. If one starts from the following two assumptions:
*
*$R_{ab} = 0\;$ (i.e. field equation in vacuum)
*The metric can be written in the form
$$
{\rm d}s^2 = g_{uu}(u,v) {\rm d}u^2 + 2 g_{uv}(u,v) {\rm d}u {\rm d}v + g_{vv}(u,v) {\rm d}v^2 + f^2(u,v) \left... | {
"language": "en",
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Doesn't rotational KE of a rolling marble change if there is no friction to provide torque? The question arise from the following situation:
A marble at the border of a uniform bowl begins rolling within it from rest. There is enough friction in the first half the bowl for the marble to not slip, but there's no frictio... | The marble begins to slip instead of roll; thus it continues to move upwards until gravity converts all of the kinetic energy the linear motion into potential energy - then it will begin to reverse course.
| {
"language": "en",
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Is the net force conventionally shown in a free body diagram? Is it standard convention to display the net force vector on a free body diagram? Internet searches seem to give mixed results.
| I'm not aware of any such convention. You can always show the net force vector acting on a free body as long as it is clearly labeled as such, to avoid confusion with any other applied forces.
I personally wouldn't include a net force vector unless there was a good reason to, like to illustrate some accompanying discu... | {
"language": "en",
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When I open a window to air out the room, how does the smell disperse? Let's say I'm in a room with some kind of noxious stink, possibly of flatulent nature. The quickest way to right the world that comes to mind is to open a window. When I open a window, how do the stank particles leave the room?
| Gases diffuse from higher concentration to lower concentration because this process of diffusion increases the gas's entropy.
Every thing in the universe, in general, aims for two things : Minimum energy and maximum entropy.
Entropy is the degree of randomness of a substance. A gas in a container has low entropy becaus... | {
"language": "en",
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On geometry of light's path in the Universe Is it possible for us to see our own galaxy from different perspective, as path of light emitted from our galaxy is curved by any possible ways and travelled back to our eyes even if it take much time?
| I know the context is bending of space by heavy bodies, but that is lensing effect and we know lenses do not reflect, however small their focal length is.
Even if it was possible to reflect/turn back good amount of light somehow, it would be next to impossible to see a meaningful image.
When moon reflects sunlight, we ... | {
"language": "en",
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How do you tell whether a force acting on an inclined plane is going up or down in its perpendicular component to the plane?
I'm practicing mechanics, and I had to resolve the following forces perpendicularly to the inclined plane in order to work out the reaction force (plus the weight of the ball)
But I cannot tell ... | Definition: The "tail" of a vector is the end that is straight (no arrow).
Definition: The "head" of a vector is the end with the arrowhead.
For each force, draw a right triangle such that:
*
*The original force line is the hypotenuse
*Draw a vector with the tail starting at the tail of your original force, point... | {
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The sign of the current flowing in a circuit I was doing the following problem:
And I was asked to find Iy.
I found Iy to be 2.64 using KCL. However, the right answer was negative 2.64.
Is it negative only because there is a dependant voltage source with "+ -" ? And why must it be negative? Does "-" in the final ans... | Kirchoff's current law (KCL) states that "The algebraic sum of all currents entering and exiting a node must equal zero". By algebraic care must be taken to choose a convention whereby you set currents flowing into the node as positive (+ve) and subsequently those flowing out of the node as negative (-ve) or vice versa... | {
"language": "en",
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Good reference on the parametrization of $SU(3)$ and $SU(N)$ For the 2-dimensional $SU(2)$ matrices, there is a fairly general parametrization formulation:
$s_2=\begin{bmatrix}
e^{i\alpha}\cos(\theta) & -e^{-i\beta}\sin(\theta) \\
e^{i\beta}\sin(\theta) & e^{-i\alpha}\cos(\theta)
\end{bmatrix}$
For the 3-dimens... | There's this article by Adam Bincer, published in Journal of Mathematical Physics, vol 31 (1990). It is titled Parametrisation of SU(n) with n-1 orthonormal vectors and it's abstract states:
A generalisation to SU(n) of a well-known relation to SU(2) is proposed. It relies on the observation that an element of SU(n) ... | {
"language": "en",
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"source": "stackexchange",
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Effect of Gravitational Waves on light? We all know about the gravitational lensing effect. From the analogy of fabric of space time used to explain this concept to laymen like me, I understand that light follows the curvature of spacetime.
Following on that same line of thought process, gravitational waves would caus... | Yes, without going into calculation light path should be affected since gravitational wave is a perturbation in the spacetime metric. If metric is perturbed, then geodesic equation is affected (which governs motion of all particles and photons in spacetime).
The only tricky thing is the fact that GW propagates at the s... | {
"language": "en",
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Is the universe bounded? As I understand it nobody can pinpoint an objective "center" of the universe nor "where" the Big Bang happened. It seems the observable universe is limited by our event horizon at some 14 billion light years and my question is simply: If an astronomer was placed at one of the outermost visible ... | The astronomer would see mostly what we see. That does not mean that the universe is unbounded, it just means that the universe is much, much larger than the observable universe -- which is indeed a consequence of current cosmologies which contain cosmic inflation; in these the universe goes rapidly from being in therm... | {
"language": "en",
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Force on sides of a pool I am going to create a large pool of water for a stage production. The volume of water will be 6m x 5.2m x .15m deep. I want to check how much pressure or force will be exerted on the surrounding wooden frame, so I can check the tensile strength of the wood is sufficiently capable of holding th... | i'm gonna try here, but i'm kinda short of information; such as how much free space do you have surrounding the pool and how thick is the wood on the perimeter. i'm american, so bear with me, i think in SAE terms.
your answer is not very much. each cm of length will have 120 grams of pressure exerted upon it. so eac... | {
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Ladder operators - commutation relations and their properties At the beginning of Fetter, Walecka "Many body quantum mechanics" there is a statement, that every property of creation and annihilation operators comes from their commutation relation (I'm translating from my translation back to english, so it's not literal... | Actually the commutation relation must be accompanied by the assumption of a vacuum state $|0\rangle$ for which $a|0\rangle = 0$ ($\langle 0| 0\rangle = 1$).
Then the commutation relation $aa^\dagger - a^\dagger a = 1$ implies also
$$
a(a^\dagger)^2 - a^\dagger a a^\dagger = a^\dagger\\
a(a^\dagger)^2 - a^\dagger a a^... | {
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Is it wrong to say that an electron can be a wave? In QM it is sometimes said that electrons are not waves but they behave like waves or that waves are a property of electrons. Perhaps it is better to speak of a wave function representing a particular quantum state.
But in the slit experiment it is obvious to see that... | Electrons are neither particles nor waves - they are electrons.
We say they behave as particles or waves because we are familiar with macroscopic objects having these properties and want to provide a kind of "feel" for what they are in terms we can easily understand. We are the ones that select the experiment that show... | {
"language": "en",
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"source": "stackexchange",
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Force on current carrying square loop I'm asked to find force on square loop (side a) carrying current $I$, flowing counter clockwise, when we look down x-axis, lying in yz plane. the loop is centered at the origin. The magnetic field is given as:
$\vec{B} = kz\hat{x}$
Its solution states that force on left an right ca... | I'm not sure I understand how the loop is set but your case must be similar to the left and right side of the loop below.Since the current is flowing in a loop, on the left side it will flow the opposite direction of the way it does on the other side.Using the right-hand rule you will now see that the forces are equal ... | {
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Are there alternatives to steam in thermal power stations? 'A thermal power station is a power plant in which heat energy is converted to electric power. In most of the world the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator.'
Why is H2O ch... | Typically, modern gas powerplants use combined cycle and are projected to achieve over to 61 % thermal efficiency, which is to my knowledge far highest of any contemporary technology. The first stage relies of thermal gas expansion similar to a jet engine, the second stage operating at lower temperature is the classica... | {
"language": "en",
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How precise must the energies match for absorption of photons? According to Quantum Mechanics, in order for an atom to absorb a photon the energy of the photon must be precisely that of a "jump" between energy states of the atom.
How precise must it be?
If I create a photon with an energy within an error of 0.0001% of... | In atoms the energy levels do not have a precise energy. When you solve Schrodinger's equation for an atom the results are the energy eigenfunctions. However these are functions that are time independent, and they have an exact energy only because they are time independent.
At the risk of oversimplifying, you can regar... | {
"language": "en",
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Finding the Force of two objects - by using Acceleration but only ONE of the given masses? I came across the following question in my physics textbook and wanted to try to solve it:
A 1700 kg car is towing a larger vehicle with mass 2400 kg. The two vehicles accelerate uniformly from a stoplight, reaching a speed of ... | Forget about the towing vehicle, and focus just on the rope connected to the truck. If you were looking at the scene through a small window, and all you could see was the rope and the truck, what force should there be on that rope in order to accelerate that truck? You don't have to know what is on the other end of the... | {
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From what wavelength can radiation go through a human body without very much changing? Gamma-rays can go through a body but they will ionize a lot of atoms (I don't know whether some of the gamma photons will go through without any interaction at all?). The same for X-rays. Visible light and infrared (till $1mm$) proba... | There are two effects of electromagnetic waves on human body.
*
*Heating effects, occurring mostly around 2.45 GHz (which is the frequency used in microwave ovens)
*Ionizing effects, which can damage human body and cells
Here is a picture of how the radiowaves penetrate the human body according to the frequency.... | {
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Fraunhofer diffraction contradiction Assume two obstacles are complementary. For example, a circular aperture and a circular stop of same radii. My theoretical analysis leads to contradictory conclusions about such situations.
1) Assuming incident light is a plane wave consisting of N sources of Huygens wavelets, obsta... | You're mostly correct, about how you expect both patterns together to produce uniform intensity corresponding to a plane wave (this is described as Babinet's principle. What you're forgetting is that it's the sum of the amplitude, including a phase. A circular hole and the complementary shape will produce identical pat... | {
"language": "en",
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How does the drift velocity of electrons in a conductor depend on the temperature? How does the drift velocity of electrons in a conductor depend on the temperature?
I have two contradicting views for this.
*
*First, we can say that increasing the temperature of the conductor will increase the kinetic energy of the... | Drift Velocity - It is defined as the velocity gained by free electron of conductor in the opposite direction of applied electric field
We know that:-
1- E = F/q = F/e (here, the charge is electron)
Where, E=electric field, F=Force applied, e=charge on the electron
2-F= mass × acceleration = m × a
Now,
F= m × a
here,... | {
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Spin 3/2 matrices in terms of Pauli matrices Let $\sigma_i (\frac{3}{2})$ be the three generators of the irreducible spin 3/2 representation of $SU(2)$ (see http://easyspin.org/documentation/spinoperators.html for their explicit forms). Similarly, $\sigma_i (\frac{1}{2})$ are the usual Pauli matrices.
I have reasons to... | I can't claim this is a complete argument, but you might consider it...
This is definitely not the standard coproduct, which, coincidentally, I have given to my students as a homework problem in the past.
Consider what Lie algebra both sides of your equation would satisfy: take the commutator of the left hand side, for... | {
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Difference between a wavevector and wavefunction I often see both terms used in textbooks, but I am not sure whether I understand the difference between them. Both describe the state of a system, however, they seem different in some ways. From what I have found, what is important in the wavefunction is its direction. W... | A wave function can be thought to be analogous to a vector in the sense that the set of all functions on a domain forms a vector space. Therefore, each function can be thought of as a vector in the vector space of all continuous functions on that domain. The reason you might have seen these differently is that matrix f... | {
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How do I find a stream function given a volumetric flow rate? How do I find a stream function given a volumetric flow rate?
The flow only occurs in one direction, between 2 plates, and I have no knowledge of velocity.
I know that volumetric flow rate = change in stream function between two points but I have no idea ho... | If you have two stationary plates h apart and know Q. You know that the fluid is not flowing at the boundary of the plates (right up against the plates, this is called the no-slip boundary condition), so at y= 0 and y= h, the velocity is zero.
You also know that the flow is symmetrical about the height y= h/2 because t... | {
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Lorentz Force and Apparent Conservation of Momentum Violation Useful for Unidirectional Force? My understanding is that the apparent violation of Newton's Third Law by the Lorentz Force necessitates a description of the system that describes the "missing" momentum as being absorbed/carried by the magnetic field itself.... | The moment you rigidly connect the magnet (or two magnets in your example) to the rod, they become one body.
Therefore a force between them should be treated as an internal force, which means that it cannot move the body as a whole, due to conservation of momentum.
| {
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What is the reason behind why energy must always be conserved, apart from observation? I know that we see in experiments (physical and thought) that energy is always transformed into something else, but what propels our universe to behave this way? What is happening at small levels that only allows conservation as a po... | The answer saying that energy was mass was incorrect. And neither is conserved, and neither is even additive.
In General Relativity you have a Stress-Energy energy tensor. It has ten independent components in any frame. And you can try to extract one of them to be the energy density and three others to give you the com... | {
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How do people go about looking for asymptotic safety in quantum gravity? Do we have (proposed?) methods to look for fixed points in the renormalization group flow of the Einstein-Hilbert action? My understanding of the RG is still somewhat sketchy at this point and I am having trouble understanding how one would go abo... | Here are two papers on the subject:
https://arxiv.org/abs/0805.2909
Investigating the Ultraviolet Properties of Gravity with a Wilsonian Renormalization Group Equation
From the abstract:
We review and extend in several directions recent results on the asymptotic safety approach to quantum gravity. The central issue in ... | {
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Why do we take $h$ as "height from surface to bottom" when calculating liquid pressure? In the following image, pressures of points x, y and z are $P_{x}, P_{y}$ and $P_{z}$ respectively, and they all are equal. My question is, why?
The amount of matter on x is much more than amount of matter on y. Why do they still ha... | Weight is a downward-directed force; if the question were about weight, there might be differences in the X, Y, Z positions. But, the question is about pressure, which is without direction; pressure is the work required, per unit of volume, to displace the liquid (like making a bubble).
Regardless of position, an inje... | {
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Three questions about time Since I don't want to abuse to the ASK QUESTION form, and since those questions are all about time, I decided to write them all here. Hope it's ok.
First Question
Could it be possible (how?) to create some regions, in the Universe, in which the times seems like to stop (for a little while)? I... | Question 1:
If a region of space contained no energy (and thus no matter or information), it would not be possible to measure the passage of time in that region. It's important to mention however, that such regions do not exist as there will always be quantum fluctuations.
Question 2:
According to basically all accepte... | {
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Relative velocity of light in a medium The speed of light in a medium is independent of motion of source relative to medium but it depends on the motion of observer relative to the medium.
I don't understand why it is so.
| The speed of light with respect to an observer in a medium depends on its refractive index only, and therefore has no reason to change if the source is moving or stationary (although the refractive index depends on wavelength of light, which can in turn change the speed of light in the medium, we ignore this aspect ent... | {
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How to prove that gravitational potential energy of a body of mass $m$ at a height $h$ is $mgh$? Many introductory physics books just write that potential energy of a body of mass $m$ at a height $h$ as $U_\text{g}=mgh$. However, they never show how this was derived. I'm interested in knowing this derivation – if poss... | The form $U=mgh$ is simply an approximation to allow people to quickly calculate small changes in the gravitational potential energy of the system. The actual value of $U_g$ is usually not important in classical mechanics. $\Delta U_g$ is the important concept.
On a planet, small changes in height change $U_g$ in a spa... | {
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Continuing on voltage drop across a resistor What exact wire diameter allows approximately $6.25\cdot 10^{18}$ electrons to pass in 1 second? Will a thinner diameter or a wider diameter wire allow the same approximate number of electrons in a second?
If the same approximate number of electrons enter and exit the resist... | Just think of it as a hose pipe carrying water. The higher the pressure (voltage) the more water passes through it (charge). However, the pressure drops along the pipe. The current is the amount of water per second flowing. The pipe offers a resistance to flow. The thinner the pipe, the higher pressure is needed to get... | {
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Get a ball to keep jumping using momentum, and force if necessary To make a ball keep bouncing with momentum, or with force. When a bouncy ball hits the ground, the ground takes some of the momentum. But it doesn't disappear. Would you be able to make the ball keep bouncing with use of a bass speaker. To make the bass ... | The problem is that the collision will not be elastic and so the kinetic energy of the object after the collision will be smaller than that before the collision. The will also be a loss of kinetic energy due to air resistance.
So you could get you bass speaker connected to a suitable oscillator to be vibrating at such... | {
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Number of e-foldings in cosmological inflationary theory It seems that in a lot of lecture courses and notes, including mine and those online, seem to state that the number of e-foldings required is of the order of 50-60.
Perhaps I'm looking in the wrong places, or my understanding is a little off but this value often... | 60 e-folds is more or less what you need in order to solve the horizon problem, i.e. the fact that the universe appears to be extremely homogeneous despite the fact that different parts of the universe would not have been in causal contact under the usual Big Bang evolution.
Inflation allows for the whole of the observ... | {
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Condition for looping the loop Consider a ball tied to a string and it is imparted a velocity we have studied that condition for looping the loop is that tension at the uppermost point must be zero, but why is this condition imposed please explain?
If tension becomes zero at some point below the uppermost point won't t... | At highest point the body has minimum tension but the velocity of the body is greater compared to some point of the curve hence a body continued in its circular path.
| {
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What does well defined parity mean I'm reading a textbook (Physics of Quantum Mechanics by Binney) and it says that the ground state ket $\left\lvert 1 0 0 \right \rangle$ of the hydrogen atom has well defined (even) parity. What does this mean?
Does it mean that the wave function is even? The wave function for this is... | "Well-defined parity" here means it is an eigenstate of the parity operator that sends $\vec x \mapsto -\vec x$. Even wavefunctions are such eigenstates for the eigenvalue 1, odd wavefunctions are eigenstates for the eigenvalue -1.
| {
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Coefficient of Linear Expansion (Formula) Quoting directly from my textbook:
We define average coefficent of linear expansion in the temperature range $\Delta T$ as:
$$ \bar \alpha = \frac 1L \frac{\Delta L}{\Delta T} $$
The coefficient of linear expansion at temperature $T$ is the limit of average coefficient as ... | You obtain your formula assuming that $\alpha$ is constant. Now assume it is small. Then $\exp(\alpha\theta)\cong1+\alpha\theta$ and
\begin{equation}
1+\alpha\theta=\frac{L_\theta}{L_0}\iff\alpha=\frac{L_\theta-L_0}{L_0\theta}
\end{equation}
| {
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Help me solve a heat conduction/emission transfer problem. Mathematica has failed me My problem: A thin-walled tube (length $L$, diameter $D$ and wall thickness $t \ll D$) is in a vacuum. It is held on one end (at $x=0$) by a heat source at constant temperature $T(0)=T_0$. The only way it can dissipates heat is radiati... | You need to do a differential heat balance on a small segment of the tube between x and x + $\Delta x$.
Heat in at x = $-\pi Dtk\left(\frac{\partial T}{\partial x}\right)_x$
Heat in at x + $\Delta x$ = $+\pi Dtk\left(\frac{\partial T}{\partial x}\right)_{x+\Delta x}$
Heat lost due to radiation = $\pi D\Delta x\epsilo... | {
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Boiling as apparent violation of the second law of thermodynamics One of the statements of the second law is that no agency can be built whose sole effect is to convert some amount of heat entirely to work.
But in case of boiling, the temperature being constant, entire heat supplied is converted into work, namely the w... | When water boils, the heat mostly goes into breaking the bonds between water molecules. Suppose you boil mass $m$ of water under constant pressure $P$. The work done due to expansion is
$$
A=P(V_\text{gas}-V_\text{liquid}),
$$
and one can estimate
$$
PV_\text{gas}\simeq NkT,
$$
where $N$ is the number of molecules in m... | {
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Sliding blocks problem My first post here, so I apologize if this is duplicated elsewhere. It IS a "homework" problem, but it's public domain, a posted exam with answers...
http://www.mun.ca/physics/undergraduates/finals/P1020F06.pdf
Here is the diagram in question
And here is the FBD for each block.
(question a i... | The first thing first -The concept of friction
Friction come in play when there is any tendency of relative motion between 2 surfaces and it is in opposite direction of relative motion (Note- I used word relative motion not simply motion). In simple word friction try to reduce relative motion.
in 2 block problem and th... | {
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About the fluid friction along the wall in pipes I wonder how can the friction between a fluid and a wall in a cylindrical pipe can be calculated. Is there any theory that I can refer to? I also want to check if there's any relation between the diameter of the pipe, the velocity and the temperature of the fluid and fri... | We know that adhesive forces bound fluid particles at the surface of the pipe. The rest of the fluid will not be affected by this force. But force acting on the remaining fluid is viscous force. The below figure shows relationship between velocity profile and radius of pipe. Where n is coefficient of viscosity.
Even if... | {
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Do solid objects really behave like fluid matter on extremely long timescales? Freeman Dyson's fascinating paper Time without End: Physics and Biology in an Open Universe contains the following passage:
I next discuss a group of physical processes which occur in ordinary matter at zero temperature as a result of quant... | The number of years mentioned appears even larger than total anticipated life span of universe (gravity) itself. One of these - Heat death, Big Rip, Big crunch would probably happen before the time frame mentioned. Therefore not sure what the answer is as the conditions mentioned may no even exists that long. For examp... | {
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Could we curve the flight path of a photon? I was wondering about photon's interaction with matter, and why photons dont slow down. They seem to always bounce in a straight line at the same speed (I think), as if some force is charging them forward after the bounce. First, what is this? I've heard of experiments where ... | Adding to Anna's response,
In small scales, photons wont slow down because they are thought of as being relativisitc quantum fields in all their possible interaction channels. However, at large scales (cosmological distances) there is an indication of slowness (delay in arrival time)-- either due to a varying gravitati... | {
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Why do bubbles group when one pops? I was recently observing the way bubbles move as they pop and disappear. I noticed that when bubbles destabilize and pop, the remaining bubbles immediately surrounding it will move to fill its place. I was wondering what the driving cause is here.
At first, I figured that stickiness... | The blubbles exerce pressure forces with their neighboor through the shared membranes. Pressure is higher in blubbles than in free air, when within a field of bubbles it is balanced on each sides of bubble wall... as long as there is a neighbor. When this one pops, the rest of the pack around push towards the empty spa... | {
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Why is speed of light a constant while distance in space is not? Disclaimer: I asked this at Astronomy.SE, but got no answer whatsoever, so I am trying my luck here.
As you probably know current state-of-the art physics (i.e. gravitational waves, cosmic expansion) basically states that space itself is subject to expans... | When we observe very distant objects, the spectral lines of atoms seems to keep the same, and the physics (and the objects it permits) as well, despite c appears in many balances of the microphysics. So c does not seems to change in all the observable Universe. Beside, some pulsing phenomena seems to be slowered as pre... | {
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Why is it said that standing waves do not transfer energy? The author of my physics textbook writes that standing waves, unlike travelling waves, do not transfer energy. He says that this is because a standing wave is composed of two travelling waves carrying energy in opposite directions. Is this explanation sufficien... | You may conclude this by thinking that the standing wave is actually the superposition of two travelling waves which were moving in opposite directions, so the energy transferred by a single wave in one direction is completely compensated by energy transfer in the opposite direction. This keeps the total energy (sum of... | {
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Mirage formation in deserts Why does a person sees water in the desert even then when no water is present there at that time?
How does this happens?
| Ideal conditions for a mirage are layers of air in contact with ground that has been heated by the sun .
As one goes up from the ground the temperature slowly decreases. this gradient of temperature changes the density of air . The layer closer to the ground is rarer and upper layers are denser .
Thus refra... | {
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Free falling and bouncing back My confusion arises with free falling body.
*
*For a free falling body the displacement ~ time graph has a kink (at the time when the body hit the ground ). at a kink point, a function is not derivable by the rule of calculus. but we see in the free falling case the body has velocity... | Acceleration of an object, e.g. a free falling ball on earth, is $g=9.81 \frac{m}{s}$. This is a constant, as can be seen in your diagram. Look at the scale - yes acceleration is not zero.
Having this in mind we look at the moment before and after hitting ground. This way we omit looking of the elasticity of the ball. ... | {
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Force distribution on corner supported plane This question has been annoying me for a while. If you have a completely ridged rectangular plate of width and height x and y that is supported on each corner (A,B,C,D) and has force (F) directly in its center then I think the force on each corner support will be F/4.
What ... | I think I have a solution. Considering the corner forces $A$, $B$, $C$ and $D$ you have a system of 3 equations and 4 unknowns
$$\begin{align}
A + B + C + D & = F \\
\frac{y}{2} \left(C+D-A-B\right) &= 0 \\
\frac{x}{2} \left(A+D-B-C\right) & = 0
\end{align}$$
$$\begin{vmatrix}
1 & 1 & 1 & 1 \\
-\frac{y}{2} &... | {
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Has such experiment been performed before? Consider a charge of 1C kept a distance of $6*10^8$ m from a detector.
I find electric field due to this charge at detector.
Then, I suddenly earth that charge
and not the time it takes to be detected by detector.
It should be about 2 sec.
Has any similar experiment been perfo... | This has been already studied extensively in the electrodynamics formalism of James Clerk Maxwell and experimentally proved to be correct many times. So magnetic fields and electric fields are unified into a single formalism called Electromagnetism which propagates with constant speed of light in vacuum. This means tha... | {
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Newton's second law of motion and viscosity In a Coutte flow, the applied force on one of the plates can be expressed by $F = \eta A \frac{dv}{dz}$. F here can also be defined using Newton's second law of motion, $F = m \frac{dv}{dt}$.
The two forces are equal here. That said, there has to be a way to derive $m \frac{... | The proportional relation between force and velocity we sometimes refer to as a viscous friction factor is a highly idealized model that may fit actual physical behavior sometimes, but in general, not.
Although we might be able to fudge the units and attempt to wedge viscosity in, the result doesn't stand as any gene... | {
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Help! An 8 year old asked me how to build a nuclear power plant I would really like to give an explanation similar to this one.
Here's my current recipe:
(i) Mine uranium, for example take a rock from here (picture of uranium mine in Kazakhstan).
(ii) Put the rock in water. Then the water gets hot.
(iii) [Efficient way... | RTG
The described approach mirrors https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator devices used to generate power in, for example, space probes - simply take some radioactive material and extract energy from its decay, without needing to control a chain reaction or something like that.
| {
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How can a parallel circuit work? The electrons always takes the easiest way in a circuit, right?
So in a parallel circuit, why does the electrons flow through all parts of the circuit and not just the one with the least resistance?
| Ohm's law helps here.
When a current $I$ flows through a resistor $R$, it develops a voltage $V=I\cdot R$. If there is less current, there is less voltage.
When the voltage developed is equal to the voltage applied, you reach equilibrium. So when current can "choose" between a high resistance branch and a low resistanc... | {
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Cylinder rotating without slipping on an accelerating slab I am very confused by the following problem asked in my first year physics class:
Please let me know if you can assist in any way! I've spent hours and hours on this question and gained absolutely nothing. Everything I do seems to lead to a contradiction one w... | As this is a homework question I won't give you a full solution, only point you in the right direction.
On the lower block acts a second force, $F_F$, a friction force that causes torque and the angular acceleration $\alpha$ of the cylinder:
$$F_F R=-I\alpha$$
Where $I$ is the moment of inertia of the cylinder and $R$ ... | {
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Why do some stars have a negative parallax? I am constructing a Hertzsprung-Russell diagram for stars within some radius around Pleiades and have repeatedly come across stars that have negative parallaxes. For example, http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-info=XML&-out.add=.&-source=I/239/tyc_main&recno=161838
... | i think its the refraction effect.
if parallax is given by (angle_final-angle_initial). and if this quantity is -ve, (due to refraction effect of stars lying in the foreground of the stars under observation), then parallax can be -ve.
| {
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Does continuity equation hold if the flow is accelerated? I'm Studying the streamline flow, specifically the continuity equation Bernoulli's Principle.
Consider the following system where a liquid flows through the pipe of a uniform area of cross section A, from high pressure P2 to low pressure P1, both of which are m... |
The question is very good but the assumption you have made is wrong!
I agree that the water is moving because of the pressure difference between water and air
As I have shown point $P_1$ and $P_2$ in the diagram.
Now the water moves because of pressure difference between this $P_1$ and $P_2$ so as the water reaches t... | {
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The force felt by a pilot when a package is dropped? Here is a physics question that I wish to prove mathematically.
A 100,000 kg aircraft drops a 1000 kg package of supplies over an arctic research station. What approximate force is felt by the 100 kg pilot at the instant of the release?
Is there a reactive force on t... | Sounds like a homework question. Here is a partial answer.
Before the drop, gravity acts on the plane, the pilot, and the package. This is the weight of each. After the drop, gravity continues to act on each. The weight of the pilot does not change.
If the plane is in straight level flight, the upward force of the ai... | {
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"answer_id": 1
} |
Are electric field lines always conserved? Suppose we have a positive +q charge and a -6q charge at some separation. Then will every field line originate from the +q and end up to -6q or will there be some extra lines coming to -6q from infinity because of higher charge to get 6 times the number of field lines? That is... | In such ideal problems, it is always implicitly assumed that there is no other charges in the Universe. So, if our Universe consists of only these two charges and everything else is neutral, then we cannot assume that field lines start at some infinity and end at individual charges. In fact, lines start from one charge... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245018",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 5,
"answer_id": 4
} |
Why does the temperature of the gas in a container moving with constant velocity not change?
Systematic changes do not affect thermodynamic equilibrium.
What does this mean? And what kind of systematic changes are allowed?
The container with gas is stationary till some time then it's given a constant velocity and the... | It is simpler to think of the thermodynamic definition of temperature, to start with, the one on the left:
where N is the number of molecules, n the number of moles, R the gas constant, and k the Boltzmann constant.
For the expression on the right, randomness is implicit, Brownian motion after all led the molecular mo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245123",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 5,
"answer_id": 1
} |
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