Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Length contraction and simultaneous length measurements I am just working through an argument from Halliday Resnick to derive the Lorentz contraction (see quote below).
Some paragraphs before this, the authors note that:
If the rod is moving, however, you must note the positions of the end
points simultaneously (in... | I think in the derivation above length was not measured by measuring both ends of the moving rod (platform) simultaneously.
Einstein gives an example of a simultaneous measurement in his popular book explaining special relativity:
| {
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How was Newton able to guess that gravitational force is inversely proportional to distance squared? This question is puzzling me since I learnt about the gravitation law in school. Why did Newton guess/assume that gravitational force is inversely proportional to the square of distance?
Did he verify that experimentall... | Well, for one, if $F$ goes like $r^{-4}$, all orbits except the unstable circular orbit will fly out or will collapse into the center! So that's a no-no. (I know that because Newton's laws in a central force field give conservation of angular momentum, and in that case, if $U$ is your potential, you can get a different... | {
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Is there a substance that doesn't reflect OR absorb light from the visible light spectrum? Is there a substance that doesn't reflect or absorb visible light but may reflect light from another spectrum? Is there a theoretical substance that would have these properties?
EDIT:
Sorry I wasn't quite clear with my original q... | It's surprisingly difficult to get a material that absorbs almost no light across the whole visible range but one candidate would be black silicon. This has a textured surface created by etching, and the texture means light hitting the surface is multiply reflected sideways before it gets a chance to be reflected back ... | {
"language": "en",
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Why are magnetic fields so much weaker than electric? In EM radiation, the magnetic field is $ 3*10^8$ times smaller than the electric field, but is it valid to say it's "weaker". These fields have different units, so I don't think you can compare them, but even so it seems like we only interact with the electric field... | As you already indicated, physical units need to be considered. When working in SI units, the ratio of electric field strength over magnetic field strength in EM radiation equals 299 792 458 m/s, the speed of light $c$.
However, the numerical value for $c$ depends on the units used. When working in units in which the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/128512",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
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Higher-dimensional metrics in (hyper)-spherical coordinates I want to compute the components of the Riemann curvature tensor (for a case similar to the Schwarzschild solution) in 4 + 1 dimensions, but I want to use a higher-dimensional analogue of spherical coordinates. I first want to investigate a metric for the Eucl... | Aren't you basically asking to the formula of higher dimensional vectors to compute yourself being in the sphere at the center with appropriate lines to all vector points?
I think I get what your asking for I asked the same questions when I was younger
Anyways here's a hyperlink to 6th dimensional operation. The operat... | {
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Does Free Will Theorem imply that quantum mechanics plays crucial role in our brain’s functioning (consciousness)?
*
*Does Free Will Theorem imply that quantum mechanics plays crucial role in our brain’s functioning (consciousness)?
*Is opposite statement of Free Will Theorem right: If elementary particles have a ce... | I would bet that consciousness is an emergent property of our neural network and not a quantum mechanical effect. Quantum effects are most probably washed away very quickly by the thermal bath in our brain.
| {
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"source": "stackexchange",
"question_score": "6",
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What is the relation between photoelectric current and frequency of incident light? I googled it a bit and found that photoelectric current is independent of frequency(of incident light). Some further look revealed that actually "saturation current" is independent of frequency.I could not find about the instantaneous c... | If we increase the frequency..the current will not increase because frequency is the power by which the photon is hitting the electron...the electron will move fast towards the anode but..the number of electrons will not increase therefore the current is not increasing..
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
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Maximum Density that We Can Store Information at? I was informed that:
There is a maximum density at which we can store information. For a sphere with surface area A, the maximum information that can be contained
within is equivalent to the maximum entropy of a sphere of size A, which is given by $$S_{max} = \dfrac... | Yes, it is true.
The Planck length is defined as
$$ L = \sqrt{\frac{G\hbar}{c^3}} $$
which, in the real world, happens to be equal to $1.616\times 10^{-35}\,{\rm m}$ (meters). In everyday life, we use units like the SI units – based on kilograms, meters, second, kelvins etc.
But adult theoretical physicists often use ... | {
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When Fire meets Water To preface, I am not a scientific mind, but a writer looking for some validity to a possible scene. That being said, please forgive me!
In my scene, huge masses of fire are raining from the sky and crashing into a salt-water ocean.
[Edit]: I would imagine the buring substance as some sort of 'nap... | It matters what's on fire.
*
*If it's a flammable material that floats, like oil or gasoline, some fraction of it will remain on or return to the surface and you may have flames on the water. (This is basically the only thing that I remember from watching Black Beauty as a kid.)
*If it's a flammable material that ... | {
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Can Minkowski spacetime be redefined as a non-flat riemannian manifold? Minkowski space time is defined in terms of a flat pseudo-Riemannian manifold. I have wondered if it can be redefined as Riamannian manifold and in the case what type of curvature would there appear.
Formally:
Let M be a semi-Riemannian manifold of... | As mentioned by Qmechanic, the answer to your questions is no.
However, assuming space-time is oriented, we have the following:
For any pseudo-Riemannian metric $g$, there exists a normalized time-like one-form $h^0$ and a Riemannian metric $g^R$ so that
$$
g = 2h^0\otimes h^0 - g^R
$$
This yields a locally Euclidean t... | {
"language": "en",
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Normal modes of the 2D double pendulum I'm performing an experiment with a 2D double pendulum, and in part of it I want to investigate the normal modes of the double pendulum, where the pendula are not of equal length or of equal mass. My question is - how will I actually know when I've successfully excited a normal mo... | When you have excited a normal mode ,both of the pendulumns will oscillate with the same frequency and will be either in phase or out of phase.I think a better method to see the normal mode will be to oscillate the point of suspension with one of the normal mode frequencies.
| {
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"source": "stackexchange",
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Symmetric eigenfunctions? So a symmetric eigenfunction / wavefunction is defined as:
$$P_{ij} ψ_a (r_1,r_2,…,r_i,…,r_j,…,r_N )=ψ_a(r_1,r_2,…,r_i,…,r_j,…,r_N )$$
But for it to be symmetric does this have to be true for all $ij$ combinations or only one? (note that $P_{ij}$ is the exchanges the element $r_i$ for $r_j$)
| Well, it's just terminology:
A (wave)function $\psi(x_1,\dots,x_N)$ is
*
*symmetric in $i,j$ iff $\psi(x_1,\dots,x_i,\dots,x_j,\dots,x_N) = \psi(x_1,\dots,x_j,\dots,x_i,\dots,x_N)$
*antisymmetric in $i,j$ iff $\psi(x_1,\dots,x_i,\dots,x_j,\dots,x_N) = -\psi(x_1,\dots,x_j,\dots,x_i,\dots,x_N)$
*fully symmetric i... | {
"language": "en",
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Is there an analogous Gauss' law which is applicable for a gravitational field? Consider the Earth to be a flat infinite plane having linear mass density equal to the mass density of the actual earth.
*
*Can there be an analogous Gauss' law that can give the gravitational field for any point on a particular Gaussia... | Any inverse square law can be substituted by a Gauss law. In Gravitation, gravitational field
$$E_{g}(R)=\frac{GM}{R^{2}}$$
Think of a sphere of Radius $R$ around the object of mass $M$ (This can be generalized to any shape). This gravitational flux coming out of it is
$$\phi_{g}= E_{R}\times4\pi R^{2}=4\pi GM$$
So the... | {
"language": "en",
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Is there inductance to a DC circuit? When a DC circuit is carrying current, large amounts or small, is there induced-emf due to the inductance? Or is it only applied to AC circuits?
| Yes, there is Inductance in a DC circuit and if the Inductance changes which is the magnetic flux to current ratio then Inductance can and will change the original current flow.
if you have a continuous moving positive contact that changes the amount of inductance per the amount of current then one can control the curr... | {
"language": "en",
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What is $c + (-c)$? If object A is moving at velocity $v$ (normalized so that $c=1$) relative to a ground observer emits object B at velocity $w$ relative to A, the velocity of B relative to the ground observer is
$$ v \oplus w = \frac{v+w}{1+vw} $$
As expected, $v \oplus 1 = 1$, as "nothing can go faster than light"... | As already noted, Special Relativity cannot account for an observer moving at the speed of light.
It is also instructive to calculate the proper time for the object ${\bf A}$: as you approach the speed of light the proper time becomes zero. So it is even impossible to define in ${\bf A}$'s frame of reference the moment... | {
"language": "en",
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How does exciting an electron's surrounding electromagnetic field cause 'electron excitation'? In more meaningful words than the ones above, how does adding energy to the EM field cause the electron to to change orbitals or oscillate in a different pattern.
|
how does adding energy to the EM field cause the electron to to change orbitals or oscillate in a different pattern.
Here one is using two frameworks, the classical and the quantum mechanical. The classical electromagnetic field is composed by an enormous number of photons each with energy=h*nu, nu the frequency of ... | {
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Potential Energy Concept Imagine a book that we lift it with a force that is exactly equal to the force of gravity so the forces cancel out and the book moves with a constant velocity.
Consider the situation after the book has been lifted, and it has come to rest once again. According to the work and kinetic energy la... |
Imagine a book that we lift it with a force that is exactly equal to the force of gravity so the forces cancel out
Ok, so sum of the forces is 0 and the acceleration is zero.
and the book moves with a constant velocity.
Spooky. Was the book moving initially?
...after the book has been lifted, and it has come to ... | {
"language": "en",
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Diffusion in the standard map Consider the standard map (also known as Chirikov map):
$$
p_{n+1} = p_n + K \sin(\theta_n) \\
\theta_{n+1} = \theta_n + p_{n+1}
$$
I know that the diffusion coefficient according to the Einstein relation is defined through the relation:
$$
\sigma^2 = 2Dt
$$
being $\sigma$ the varianc... | The following is a passage from Diffusion in the standard map (pdf) by Itzhack Dana and Shmuel Fishman:
A major difficulty in the analysis of chaotic behavior of Hamiltonian systems is the proximity of chaotic and regular orbits on various scales [2]. Thus, for $K \approx 1$, the phase plane of (1) is an intricate mix... | {
"language": "en",
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When I move my arm forward in vacuum, will my body move backward? Let's say I stay at point $x=0$ in vacuum. When I move my arm forward such that it will have a positive $x$ position (say $x=5$) will the rest of my body move backward such that it will have a negative $x$ position (like $x=-1$)?
| Yes.
There are only internal forces of your body. Without external forces, the center of mass of your body cannot change position. As your center of mass did not move, the main body should move in the opposite direction.
| {
"language": "en",
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How to differentiate between rotating frame and linearly accelerating frame? Two friends, $A$ and $B$ are part of an experiment. $A$ is placed in a closed box and made to accelerate in free space at an acceleration $g$. $B$ is also placed in a closed box, but is made to rotate in a circle at uniform speed, such that th... | A simple pendulum would be a good experiment to detect both non-inertial frames - rotating and linearly accelerating:
If you know weight of the pendulum in inertial frame, in rotating frame, its weight would decrease because of radially outward centrifugal force acting on it. (If earth stops rotating, our weight would ... | {
"language": "en",
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How can gravity affect light? I understand that a black hole bends the fabric of space time to a point that no object can escape.
I understand that light travels in a straight line along spacetime unless distorted by gravity. If spacetime is being curved by gravity then light should follow that bend in spacetime.
In N... | If the mass of light is assumed to be strictly zero, Newton gravity would produce zero force. However the orbit of light is determined by acceleration, not force. For zero mass, acceleration is undefined. In the limit of photon mass going to zero the force goes to zero, but acceleration is of course independent of phot... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
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Which way does the scale tip? I found the problem described in the attached picture on the internet. In the comment sections there were two opposing solutions. So it made me wonder which of those would be the actual solution.
So basically the question would be the following. Assume we would have two identical beakers, ... | Here is a free body diagram of the balls:
… and one of the water volume:
The four balance equations are
$$ \begin{align}
B_1 - T_1 - m_1 g & =0 \\
B_2 + T_2 - m_2 g & = 0 \\
F_1 + T_1 - B_1 - M g & = 0 \\
F_2 - B_2 - M g & = 0
\end{align} $$
where $\color{magenta}{B_1}$,$\color{magenta}{B_2}$ are the buoyancy forces,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130688",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Some objects seem to have the same color whether the light that we perceive is emitted or reflected Is color only a property of perception, considering these two examples:
*
*The glass used in a green traffic light looks green no matter how it is illuminated, either by a white light bulb behind it or a white light b... | "Color" is a property of perception. "Optical spectrum" is a physical property of light and objects that interact with it. There is no great mystery behind the (scientific) definitions of the words "color" and "spectrum", which refer to very different aspects of light.
When a physicist talks about "red light", it onl... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/130876",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What exactly are crystal planes and how do they reflect x-rays?
*
*What exactly are crystal planes and how do they reflect x-rays?
*Are crystal planes real physical planes or just an abstract concept?
*What are these planes made of?
*If they are an abstraction, what do the x-rays hit and get reflected by? Indivi... | *
*and 2. Crystal planes contain sets of atoms, which occupy identical positions in the primitive cell.
*So to say, the planes are made out of atoms (and nothing in between). Or if you want a more quantum mechanical picture, you would have electron orbitals in between, which are responsible for the chemical bonding.
... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Will the Universe eventually stop expanding Sorry if this is a naive question, not being even a part qualified physicist in any way shape or form.
I've read that the universe is expanding and the rate of expansion is increasing. The assumption being that it will continue expanding indefinitely.
However isn't there anot... | Carroll (2004) introducing the standard Friedmann-Lemaitre-Robertson-Walker model, shows that this will become clear in a formula that sheds light on the fate of the cosmos:
To determine the dividing line between perpetual expansion and eventual recollapse, note
that collapse requires the Hubble parameter to pass t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131013",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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What is entropy really? On this site, change in entropy is defined as the amount of energy dispersed divided by the absolute temperature. But I want to know: What is the definition of entropy? Here, entropy is defined as average heat capacity averaged over the specific temperature. But I couldn't understand that defini... | The entropy of a system is the amount of information needed to specify the exact physical state of a system given its incomplete macroscopic specification. So, if a system can be in $\Omega$ possible states with equal probability then the number of bits needed to specify in exactly which one of these $\Omega$ states th... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "46",
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How is the integrand concluded to be identically zero? In expanding the classical Klein-Gordon field in Fourier space to write it in terms of $\phi(\mathbf{p})$ instead of $\phi(\mathbf{x})$, I reached the following result.
$$\int \mathrm{d}^3p\exp({i\mathbf{p}\cdot\mathbf{x}})\left[\frac{\partial^2}{\partial \mathrm{... | This is essentially the statement that the Fourier transform is injective and therefore invertible. In looser language, it states that the functions $\mathbf x\mapsto\exp(i\mathbf p\cdot\mathbf x)$ are linearly independent, so any sum of them (i.e. $\int\mathrm d^3\mathbf p$) that gives zero must have identically zero ... | {
"language": "en",
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Transformer ratios - 1:2 vs 50:100 I am only the equivalent of a high school student, so please, if possible, don't answer this question with anything too complex or really advanced university level. I am very happy to research new concepts anyone mentions, but can you please keep it reasonably simple.
With transformer... | I cannot possible improve on CuriousOne's excellent answer but I can try and respond to Carl Witthoft,s comment: "Consider the inductance as well as the coupling efficiency in each design."
When not loaded the primary coil of a transformer acts as a self inductance. The self inductance is proportional to the number of... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "14",
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Is there a difference in handwritten nabla $\vec{\nabla}$ with an overset arrow and typeset nabla $\nabla$? According to some physicist at KIT it is usual to write the following when using pen and paper:
whereas in typeset texts you write $\nabla$.
Is that true? Are there sources for this convention?
| Yes, the reason for this is that in maths $\nabla$ is often used as the vector differential opertator. This is a vector. When typesetting the convention to denote a vector is bold text e.g. $\bf{x}$. However for handwriting you can't really write bold font so other conventions are needed. Common ones are putting an arr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/131525",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Amplification of magnetic field can we by any means amplify magnetic signal as we can with electric signal. As both electric and magnetic field can be represented in the form of a wave the analogy seems to be natural.
I want the input and output as magnetic signal.
| I presume your idea revolves around permanent magnets, if so stacking decreasing sized magnets in a pyramid configuration grants you a powerful localized field at the smallest point. Halbach configurations allow for combined field manipulation which comes with the downside of slowly degrading the total field strength o... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Why is density an intensive property? I am still trying to understand what are intensive and extensive
properties. Possibly someone can give a pointer to a decent text (preferably on the web), as I
am not too happy (to say the least) with what I found so far on the web.
I already asked here one question on this, which ... | From the ideal gas law $ PV = nRT $ we can develop: $$PV = \frac {m}{M}RT \rightarrow PM= \frac {m} {V} RT \quad $$
and since $ \frac {m}{V}= \rho \quad $ where $\rho$ is the density of the gas and $M$ the molar mass then we have $$ PM = \rho RT \rightarrow \rho = \frac {PM}{RT}$$ So density is dependable only of inte... | {
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What makes us move in time? Time is considered to be a dimension, and we are moving at certain rate in one direction in time. What force makes us move in time? I mean it must be ether time moving or us moving in time so there has to be some force that 'pushes'/'pulls'? Was this 'time inertia' acquired during big bang, ... | Take a landscape. It can be modeled by a function f(x,y,z). If all the derivatives, df/dx, df/dy, df/dz are zero, the landscape is flat to infinity and nothing interesting exists in the landscape.
If one of the derivatives is different than zero, then we perceive a shape, and generally a landscape has a shape. As an ... | {
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Magnetic Force on a Ferromagenetic Material I am currently working on a project involving solenoids, and I needed a force(Newtons, not a measure of magnetic field strength) equation. What I came up with after some digging around on the internet, is the equation:
$$F = (NI)\mu_0\frac{\text{Area}}{2g^2}$$
Where $F$ is f... | I don't know where you got your formula from, but but I derived it this way:
Field inside the solenoid$=\mu_0ni \hat{z}$ (say)
Since the material is ferromagnetic, there is an induced, bound surface current $K\hat{\phi}$ (and $K=M$, where $M$ is magnetization). The magnetization is uniform, so bound current is zero,$$
... | {
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Optical signal filters Are there any optical filters which filter the signal's frequency and not based on the wavelength of the light? So what I mean is, if I have a modulated/pulsating light signal riding on a large DC offset, is there some way I can filter out the DC offset using optics alone? I've tried searching th... | You can use a reverse saturable absorber in such a way that the threshold of the RSA must be such that the RSA transmits only the AC part and acts as an opaque object for the DC signal.
| {
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why are the square of the components of velocity equal? In Principles of Physics by Resnick,Halliday,Walker, when evaluating the $v_{rms}$ ,they first found $p = \frac{nM(v_x)^2}{V}$ & then substiuted $(v_{avg})^2 = \frac{1}{3} .v^2$ where $v^2 = (v_x)^2 + (v_y)^2 + (v_z)^2 $ saying that there are many molecules and al... | It is just splitting of vectors component wise: $$v_{i}= v_{x}^{2}+v_{y}^{2}+v_{z}^{2} \,.$$ Like when you split the velocity of a projectile in to two components. We know from the molecular model of ideal gas: $$ v_{x}=v_{y}=v_{z} \,,$$
squaring all sides we get $$ v_{x}^{2}=v_{y}^{2}=v_{z}^{2} \,.$$ Thus $$
v_{i}= v_... | {
"language": "en",
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How can I calculate the force that is applied on a tube by another tube? Let's say there is two tubes(cylinders with no tops or bottoms) with charges $q_1$ and $q_2$, radii $b_1$ and $b_2$, lengths $l_1$ and $l_2$. These tubes are located along the axis of each other's surfaces like in this figure:
If the electric fie... | The answers already in here are good; unfortunately the integrals that arise are quite nasty, and don't have solutions in terms of elementary functions. Here is some more detail, in the special case when the tubes have zero length (so they are just charged circular loops), and further they have the same radius $b$, wit... | {
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Why do the space time get curved around a massive object?What problems do we face if we do not consider the space time to be curved? As far as I have the knowledge of GTR that a mass bends the space time around it.But why does this bend occur?The example from real life that when a mass is placed on a net then the net b... | In general relativity the fundamental equation is roughly $$\textrm{Curvature} = \textrm{Matter content}$$ .In the context of general relativity it does not make sense to ask "why" matter curves spacetime, because this is the most basic assumption in the theory.
It makes more sense to ask why one would try to constru... | {
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Why should any physicist know, to some degree, experimental physics? I've been trying to design a list with reasons why a proper theoretical physicist should understand the methods and the difficulty of doing experimental physics. So far I've only thought of two points:
*
*Know how a theory can or cannot be verified... | Because otherwise you are a mathematician.
The point of Physics is to describe the nature using the language of maths, but the only ways to stay in contact with nature is to interact with it through experiments and observations.
If you completely lose the ability to grasp how a process starts and develops, how much it... | {
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Why isn't jumping against a wall an elastic collision? According to this calculator http://www.abecedarical.com/javascript/script_collision1d.html when low mass object hits high mass object it is reflected gaining opposite velocity almost the same as initial velocity.
If I jump onto the wall why my body is not reflect... | Changing shape could still be an elastic deformation (of a rigid body). So obviously there are also plastic deformations involved, when jumping against a wall.
| {
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Poynting vector plane wave I'm calculating the poynting vector for a plain wave and I have some doubt.
$$ \bar S = \frac 1 2 \bar E \times \bar H^* = ... = \frac {| \bar E|^2} {2 \zeta} \hat i_k $$
Now if I consider a cylindrical volume and apply the divergence theorem I get
$$ \int_{s_1} Re \,\, \bar S \,\,\hat i_n ... | You calculate the amount of energy the is radiated from a cylinder with no source, and only a plane wave going through it, and get 0... seems right to me...
By the way, Re seems superfluous in you equations, since S is already real.
| {
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Gravity doesn't seem to work the way it is supposed to This has been a bit of an awkward question that's been plaguing me ever since I started watching space documentaries on discovery about 10 years ago. I was saving this for the day I would ever meet Professor Brian Cox where I can point and say "HA!" see you can't e... | The external force is the centrifugal force, which is a pseudo-force (or virtual force), it arises due to the fact that the motion is taking place in an accelerating force of reference.
The force pulling you inwards when in an orbit is actually Gravity. A pseudo-force is called that because it isn't actually exerted. ... | {
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Direction of current in concentric cylinders Example 7.2 in David Griffiths E & M book (3rd edition) has a side view of 2 concentric cylinders, with smaller radius $a$ and larger radius $b$. The region in between $a$ and $b$ has conductivity $\sigma$. "If they are maintained at a potential difference $\textit{V}$, wh... | Yes - electrons flow from the negative to the positive, so in the opposite direction to the conventional direction of the electric field (which points from positive to negative). So if the E field points outwards, the electrons flow from the outer to the inner cylinder.
The direction does not affect the answer (the cal... | {
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Who does work while walking? While walking, the work done by friction is zero.
But who does the work, actually? How someone is getting displaced?
This situation also arises when someone climbs without slipping or is climbing a ladder.
| Let's try to look at it from two different perspectives:
*
*Internal work:
Like work requires to stretch a spring, you do work or spend energy while stretching and contracting the leg muscles and also (angle) bending your legs.
*External work:
2a. When you move a crate on a horizontal surface, you do work if y... | {
"language": "en",
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Ice and liquid water interacting across a boundary Imagine we have two thermodynamic systems, one a mass of ice and the other an equal mass of liquid water, with both at 273.16K. Each system is isolated, except that they can interact with each other across a boundary that permits the exchange of heat but not matter or ... | The original state is not in thermodynamic equilibrium. The liquid side has a lot more heat than the solid side due to the heat of fusion.
| {
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What's causing the effect of salt in voltage arcs? I just came across this video demonstrating that salt increases length of voltage arcs. There is no explanation which leaves me quite confused.
Does the salt decompose during the process?
| SPECULATION:
"Does the salt decompose during the process?"
I suspect that it does. The salt ionizes easily and the ions would migrate under the influence of the electric field. In so doing, they will further ionize the air they traverse, creating a stream of charge carriers. As the electrodes are pulled further apart... | {
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Potential difference between point on surface and point on axis of uniformly charged cylinder Question:
Charge is uniformly distributed with charge density $ρ$ inside a very long cylinder of radius $R$.
Find the potential difference between the surface and the axis of the cylinder.
Express your answer in terms of the v... | Actually, using a cylinder for your Gaussian surface is your best approach. The fact the area is infinite should not matter, if you expression the infinite length of the cylinder as a variable, say $l$. Noting that the Gaussian surface area, $A = 2\pi Rl$, and that $Q = \rho l$, the $l$ term should eventually cancel ou... | {
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Why photon has a wave nature? Wave theory does not account for the photon model, which was developed only to explain quantum effects like photoelectric effect.
Then why do we talk about a photon's reflection and rarefaction, as that would require it to have wave properties? This has been mentioned here: (http://en.m.w... | It's a matter of definition. For example, by photon, do you mean first or second quantization (the latter being the canonical treatment)?
Although there's no ambiguity in theoretical physics, the nature of photon has generated much debate in the community of applied physics.
See, for example, these articles:
http://arx... | {
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Why are some variables summed infinitesimally and others aren't? This is something that has been bothering me and I hope the title kind of makes sense. It may be a stupid question but please be gentle. My question is, let's say we have current:
$$I=\frac{dq}{dt}$$
And I understand that if we want to find the total char... | The current by definition is the flow of charge. Also, time and charge can be quantified infinetsimally at least when dealing with macrospcopic system. As such, a small amount of charge during a small time interval is what current it. As the system gets smaller, quantum level, current is quantized based on ballistic ch... | {
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Relating Quantum Mechanics to Classic Electromagnetism I've been directed to a few articles, and I am sure there is a related post, but can someone explain the procedure by which we can view classic electromagnetism through quantum mechanics? Indeed we need to be able to look at any field as an ensemble of particles (p... | I think the fundamental problem that many people have with quantum mechanics is, that is seems to be about particles, when, in reality, it is about quanta. A quantum is not the same thing as a particle!
A quantum is the exchanged amount of physical quantities between two parts of a physical system. That can be a quant... | {
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How can there be a voltage when there is no current? I'm told at school that the Electromotive Force (e.m.f) of a battery equals the potential difference between the terminals of the battery when there is no current.
How is that possible? How can there be a potential difference with no charge flowing?
| Another useful analogy, apart from the gravity one described by David Z, is temperature. You can think temperature as your potential, and the heat flow as your current. Two points of space may be at different temperature, but if they are correctly insulated, they won't exchange heat. The heat will flow only if they are... | {
"language": "en",
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If everything is relative to each other in this universe, why do we keep the Sun to be the reference point? and study the solar system and universe relative to it and why not relative to the Earth?
|
why not relative to the Earth?
Scientists do express things relative to the Earth, where that makes sense. I couldn't imagine trying to forecast the weather or model the global circulation of the Earth's atmosphere from the perspective of a non-rotating frame with it's origin at the solar system barycenter. Astronome... | {
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How to prove the Levi-Civita contraction? I want to prove the following relation
\begin{align}
\epsilon_{ijk}\epsilon^{pqk}
=
\delta_{i}^{p}\delta_{j}^{q}-\delta_{i}^{q}\delta_{j}^{p}
\end{align}
I tried expanding the sum
\begin{align}
\epsilon_{ijk}\epsilon^{pqk}
&=&
\epsilon_{ij1}\epsilon^{pq1}
+
\epsilon_{ij2}\eps... | One way to see this is to consider the fact that the vector space of rank (3,3) completely antisymmetric tensors ($ \Lambda_3^3(R^3) $) has dimension one (it's just a linear algebra exercise). Then define the tensor:
$$
M_{ijk}^{lmn} = \delta_i^{[l} \, \delta_j^m \, \delta_k^{n]} = \frac{1}{3!} \sum_{\sigma \in S_3} sg... | {
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How many molecules rub off when I press a key? I have a lot of questions below but my overarching questions are: Do surfaces rubbing lightly together always strip molecules off of each other? and How can we model that?
Clearly the answer to the first question is yes in general. We've all seen worn out keyboards and the... | An answer wrote by user Enthalpy found on https://www.chemicalforums.com/index.php?topic=103339.msg363521#msg363521:
This question is still debated in mechanical engineering where unsound loads on ball bearings serve to measure a life expectancy which is then extrapolated to normal load. Books give formulas for that, b... | {
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Special Relativity, 2nd Postulate -- Why? As a lowly physics undergrad who has been chewing on this 2nd postulate of special relativity for a year or more, I simply can't wrap my head around reasons why it is true or how Einstein might have been convinced enough to propose this postulate.
Consider Alfred who is riding ... | Maxwell's theory had predicted that the speed of light varies with the speed of the observer. Initially (prior to Fitzgerald and Lorentz advancing the ad hoc length contraction hypothesis) the Michelson-Morley experiment was compatible with the assumption that the speed of light varies with the speed of the light sourc... | {
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Converting two component product to four component notation Consider the product of two left Weyl spinors in the notation commonly found in supersymmetry,
\begin{equation}
\chi ^\alpha\eta_\alpha = \chi ^\alpha \epsilon _{ \alpha \beta } \eta ^\beta
\end{equation}
This is equal to,
\begin{equation}
\left( \begin{... | Just realize that you can form ordinary Dirac spinors from 2-spinors by using charge conjugation, $i\sigma_2\eta^*$, that gives a right- handed field that can fit in the right-handed slot (forming a 4 component Majorana field)
$$
\Psi_1=\left(\begin{array}{c}\eta \\ i\sigma_2\eta^*\end{array}\right)
$$
And analogous fo... | {
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Difference between heat capacity and entropy? Heat capacity $C$ of an object is the proportionality constant between the heat $Q$ that the object absorbs or loses & the resulting temperature change $\delta T$ of the object. Entropy change is the amount of energy dispersed reversibly at a specific temperature. But they ... | It is bit difficult to see that entropy and heat capacities are entire different concepts. They are both related to the filling of energy "destinations" in a system as temperature increases. They are different ways of seeing the same thing.
As a starting point we can clearly see that the two are related:
$C=T\left(... | {
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Why isn't the quark charge taken as primitive? Why are electrons taken implicitly to be the elementary charge? It would save a lot of fractions in particle physics problems.
| It's a matter of history. When George Stoney developed Stoney units in 1881, or when Robert Millikan performed the oil drop experiment in 1909, it wasn't yet known that it was possible for anything to have a charge smaller in magnitude than the charge of an electron. By the time the quark model was proposed, in 1964, ... | {
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Is the fine structure constant a rational number? Since the fine structure constant (denoted alpha) is a pure real number, it just occured to me to ask if it is a rational number or not.
| As other people have stated, we are currently getting this quantity experimentally, so thus far this is not an appropriate question.
The origin of the value ~1/137 remains unexplained; There is currently no theoretical explanation for why is has the value that is has. In the future, physicists might be able to explain ... | {
"language": "en",
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Why do we need $2^\text{nd}$ quantization of the Dirac equation As a Mathematician reading about the Dirac equation on the internet, leaves me with a great deal of confusion about it. So let me start with its definition:
The Dirac equation is given by,
$$
i \hbar \gamma^\mu \partial_\mu \psi = m c\cdot \psi
$$
where th... | Solutions of the Dirac equation were originally interpreted as multi-dimensional wave functions or states. Each component is similar to good ol' non-relativistic quantum mechanics. This non-operator theory is sometimes called relativistic quantum mechanical spinor theory.
Yes, second quantization is a method that, afte... | {
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Hydrogen Bomb Mass to Energy? How much mass is converted to energy when a hydrogen bomb explodes? I remember an eighth grade chemistry class where, by going through the nuclear processes, my teacher estimated that roughly 2g of matter was converted in a fission bomb.This is a surprisingly small amount of mass! I have... | The most powerful hydrogen bomb ever exploded had a TNT equivalent of 50 Mt TNT, if I remember correctly, TNT energy equivalent is 4184 MJ/kg, that gives a mass loss of about 2.3 kg, if my calculations are correct.
| {
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Quantum mechanics threshold First of all I beg your forgiveness as I am not a physicist and the question I am going to ask may sound silly.
I am aware that beyond a certain threshold in the hierarchy of building blocks of matter (electrons, atoms, etc.) the 'standard' laws of physics (e.g. Newtonian physics) do not app... | Newtonian physics is generally a good approximation in a problem as long as any significant differences in the action involved in the problem are much larger than Planck's constant (if not, quantum mechanics will be needed), the speeds involved in the problem are much less than the speed of light (if not, special relat... | {
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Work done against gravity The work done against gravity is $mgh$, well at least that's what my textbook says.
I have a question:
I can apply a force say 50N, so total work done = $mgh + mah$. Where $ma$ = Force.
But the truth is irrespective of the force applied, the work done against gravity is always $mgh$. Why?
For ... | If I take a mass $m$ and apply a force $F$ (greater than $mg$) to it for a distance $h$ upwards then I will do work of:
$$ W = Fh \tag{1} $$
The force $F$ has to be greater than the force due to gravity, $mg$, or the object won't move upwards, so let's write the force I apply to the mass as:
$$F = mg + F'$$
then equati... | {
"language": "en",
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"source": "stackexchange",
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The purpose of auxiliary lens in microscope Can someone explain me the purpose of auxiliary lens in microscope.
The specification says:
Eyepiece: Extra wide field 10x Eyepiece w Spectacle Correction 30mm Ocular
Objective 0.7-45x , Auxiliary 2x
And the manufacturer claims that it is having magnification of 90X. Is it... | This is likely to be a standard Barlow Lens (see http://en.wikipedia.org/wiki/Barlow_lens) which, when attached to one of the normal eyepieces, decreases the focal length by half and thus increases the magnification by 2X. The previous answer describes a more complicated type of Barlow lens that turns any eyepiece int... | {
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Only transverse photons are gauge-invariant (Peskin page 298) Seven lines down from the top of page 298 of P & S, it says "Single particle states containing one electron, one positron, or one transversely polarized photon are gauge-invariant, while states with timelike and longitudinal photon polarizations transform un... | Just consider the gauge transformation after Fourier transforming everything. A Fourier transform turns derivatives into momenta, such that we get
\begin{equation}
\tilde A_\mu \rightarrow \tilde A_\mu - \frac1e k_\mu \tilde\alpha \;.
\end{equation}
This mean that only the component parallel to $k_\mu$ (the longitudina... | {
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$\rm Lux$ and $W/m^2$ relationship? I am reading a bit about solar energy, and for my own curiosity, I would really like to know the insolation on my balcony. That could tell me how much a solar panel could produce.
Now, I don't have any equipment, but I do have a smartphone, and an app called Light Meter, which tells... | There is no simple conversion, it depends on the wavelength or color of the light.
However, for the sun there is an approximate conversion of $0.0079 \, \text{W/m}^2$ per Lux.
To plug in numbers as an example: if we read 75,000 Lux on a light sensor, we convert that reading to $\text{W/m}^2$ as follows:
$$75,000 \times... | {
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Dirac Notation Question Appearing In a Projection So I have a part of the energy eigenvalue equation that look like this:
$$
\delta(\hat{x})|n\rangle
$$
Where n is the energy basis of the Hamiltonian I'm considering. To deal with this, I tried projecting onto a complete set of position states and I obtained:
$$
\int^{\... | As Alfred Centauri said, I don't know that there's much more to do here than to say
$$
\int^{\infty}_{-\infty} dx \ \delta(x) \phi_n(x) |x\rangle = \phi_n(0) |x=0\rangle
$$
You could instead write this as
$$
\sum_m \phi_n(0) |m\rangle \langle m |x=0\rangle = \phi_n(0) \sum_m \phi^*_m(0) |m\rangle
$$
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What technology can result from such expensive experiment as undertaken in CERN? I wonder what technology can be obtained from such very expensive experiments/institutes as e.g. undertaken in CERN?
I understand that e.g. the discovery of the Higgs Boson confirms our understanding matter. However, what can result form t... | Places like CERN are a huge forcing function for computer science - think high performance computing, networking, data storage, etc. If my memory is correct, Tim Berners-Lee was at CERN when he started developing the WWW...
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/135764",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "49",
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Directions and magnitudes of static and kinetic friction between stacked blocks
Say you have block1 on top of block 2 and the whole system is accelerating toward the right at a certain acceleration. Due to inertia block 1 wants to move backward to the left so there has to be a force of static friction on it that acts ... | *
*No, block 1 would move to the right but with smaller acceleration than block 2. You could say it would move to the left relative to the block 2.
*Yes it would, since block 1 still 'wants' to move to the left relative to the block 2 (the 'relative' part is very important)
*No it wouldn't, it would be smaller. Bloc... | {
"language": "en",
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Tensor product notation convention? For two particle state, the Dirac ket is writren as
$$\lvert\textbf{r}_1\rangle \otimes \lvert\textbf{r}_2 \rangle. $$
Then how do we write its bra vector,
$$\langle\textbf{r}_1\rvert \otimes \langle\textbf{r}_2\rvert ~~\text{or}~~\langle\textbf{r}_2\rvert \otimes \langle\textbf{r}_1... | The way I imagine it is that the left side of the direct product is exclusively reserved for hilbert space 1 and the right side is for Hilbert space 2.
So that the total hilbert space you are working in is written as:
$$
H=H_1⊗H_2
$$
And so when you have a wavefunction in H you write:
$$|\psi\rangle = |r_1\rangle \otim... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
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Does a pulled rubber band contain as much energy as a twisted rubber band? lets say I take two similar rubber bands. One of them I pull until it almost reaches its breaking point. The other I twist until it almost reached its breaking point. Do both of these rubber bands contain (roughly) the same amount of energy? ... | Yes, I believe they do possess the same amounts of elastic potential energy. By stretching both rubber bands to breaking points, this means that both are stretched for equal distance, only that one loops around itself when twisted, while the other gets stretched far apart. In the end, they will possess the same amoun... | {
"language": "en",
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Relationship between two viewpoints on Thermodynamics I've always seem the standard viewpoint on Thermodynamics that it is all about studying phenomena related to a property of systems called temperature. Then we have the zeroth law which allows us to give means to measure temperature and so on. Every basic Physics cou... | Regarding the first viewpoint, it has also been long emphasized that thermodynamics is about studying phenomena related to a property of equilibrium systems called temperature. The equilibrium is always important in all thermodynamics --- although you can make up some measure of temperature for a nonequilibrium system,... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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"Magic" Speed to Drive Over a Speed Bump: Myth or Reality? A somewhat controversial aspect of speed bumps (sleeping policemen for those in the UK) is that they arguably cause more loss of life than they prevent by acting as a hindrance to emergency response vehicles. In thinking of ways to minimize this adverse impact,... | Yes it's true. I had a 1990 Toyota Camry and my friend had a 1987 Honda Accord. If we drove 30mph over 15mph speed bumps it glide right over and when hit 50mph over our many 25mph speed bumps on Ramblewood Drive Columbus Ohio 43235 lived on Maleka Ct 43235 from 1986/87 to 1998/parents moved 2002. I'm not sure if we had... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136102",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Does air conduct heat better than saturated steam? This engineering toolbox table shows thermal conductivity of steam at 0.016. I understand that water is better in conducting heat than air, but if I read this correctly, steam is worse in conducting heat than water?
Would there be any difference of steam in a vacuum? ... | If you got vaccum inside a closed container, you would get the water inside or(some water) goes to vapor directly without any heat source till you get some equilibrium.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136178",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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DC Generator with magnet as rotor DC generators convert the AC current in them by split ring commutators right and the graph of the current will be like this
but the question is how would be the graph if the magnet is the rotor and not the armature? Me and a part of my friends are on the opinion that the the graph wil... | Yes is correct. The rotation of the armature commutes the connection to the circuit so current in the circuit flows in the same direction regardless of the direction in the armature. If the armature is fixed, it will only couple to the circuit in a unique way, and the circuit will experience the current in the same sen... | {
"language": "en",
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"source": "stackexchange",
"question_score": "1",
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Advanced kinematics problem regarding $F = ma$ I really need help with this problem. Thank you for your time.
Question:
Look at the figure below. Derive the formula for the magnitude of the force F⃗ exerted on the large block (mC) in the figure (Figure 1) such that the mass mA does not move relative to mC. Ignore all... | "I found out that F applied to mC must equal to the tension exerted on mA" is wrong
F=(m_a+m_b+m_c)*a
T=m_a*a
T_x=m_b * a
T_y=m_b *g
T^2=T_x^2+T_y^2
Replace and get
F=[(m_a+m_b+m_c)*m_b*g]/sqrt(m_a^2-m_b^2)
| {
"language": "en",
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Why does Energy-Momentum have a special case? I was reading Energy-momentum, and I came across this simplified equation:
$$E^2 = (mc^2)^2 + (pc)^2$$
where $m$ is the mass and $p$ is momentum of the object. That said, the equation is pretty fundamental and nothing is wrong when looked upon, I similarly also believed thi... | First, your findings are correct and are found by Einstein. However the fact that a new term appears is only showing that our previous knowledge was incomplete. The new term expresses the Energy related to the rest mass of the body which was not considered before. And it makes sense because before, in equating the Ener... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/136407",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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How can I calculate the speed of an object knowing its horizontal and vertical velocity components? Let's say a ball is thrown and it experiences typical projectile motion (moves in a parabolic arc etc.) and the only information we know are the equations for the horizontal and vertical components of its velocity for it... | Yes, of course. Velocity is just a vector, and its norm is $\sqrt{V \cdot V}$,
i.e., $\sqrt{\sum_i V_i ^2}$
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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How to carry out the perturbation expansion of an anharmonic oscillator to high orders? I think this is a standard problem in quantum mechanics. Consider the anharmonic oscillator
$E \psi = \left(- \frac{1}{2} \frac{\partial^2}{\partial^2 x } + \frac{1}{2}x^2 + \epsilon x^4 \right) \psi$.
Formally, the ground state en... | As mentioned in the comments by Bubble, this is answered in
Ground State Energy Calculations for the Quartic Anharmonic Oscillator, Robert Smith. Notes for Math 4901, University of Minnesota, Morris (2013).
but as the document is not crawlable by the Wayback Machine I'll summarize it here.
Smith considers hamiltonian... | {
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Loads in decelerating a falling object I remove trees in confined spaces setting up rigging systems, I am keen to look at the science behind it in more depth so far the formulas that I have found don't take into effect the ongoing effect of gravity once the object or log begins decelerating.
So far,
If a $300\:kg$ log... | It may help to decompose the force required into two parts.
First, notice that if you provide mg, or 300 x 9.8 newtons in opposition, the log will not accelerate. If stationary it will remain so, and if moving at some velocity it will continue to move at that velocity.
Now look at the 1 meter fall. This will provide a... | {
"language": "en",
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What will happen in principle if one tries to push a neutron star? When we push an object it moves due to the mutual repulsion between the electrons in our hand and the electrons in that object. Since a neutron star contains only neutrons, what will happen in principle if one tries to push it?
| Repulsion happens with any fermion particles, not just electrons. If neutrons would not repel each other we wouldn't have neutron stars. All of them would always collapse into black holes instead.
So neutrons in our atoms still repel neutron star.
However what would really happen is, neutron star has really massive gra... | {
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Why is cesium used to measure time in atomic clocks? Seconds are measured by the frequency emission of cesium. Why is a frequency from the emission spectrum of cesium used as the standard in defining a second? Why particularly cesium?
| Rather than write something unintelligible, I'll quote from a page on cesium clocks.
According to quantum theory, atoms can only exist in certain discrete ("quantized") energy states depending on what orbits about their nuclei are occupied by their electrons. Different transitions are possible; those in question refe... | {
"language": "en",
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Natural frequency of spring-mass system We can found on internet videos or articles about resonance. For this they for e.g. take a system of two spring and mass in between. When they set force frequency to close to natural frequency. But from where they know the natural frequency of the system?
| You can find the natural frequency of such a system theoretically, if you know the stiffness of the springs. Refer:this example on wolfram.
How do you find the stiffness of the springs? Well... do experiments on simple pendulum, find the natural frequency from time period of oscillation, reverse calculate the stiffness... | {
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The effect of windspeed on a car I've worked problems in the past in trig class concerning the effect of wind on the speed of a plane and it's flight path and was wondering if a similar thing occurs with a car.
First off, I'm pretty sure that if the speedometer reads 60 mph, even if the wind is blowing 15 mph in the s... | Yes indeed. The wind does do some work in increasing the speed of the car. But in everyday situations, the speed of the wind and therefore the force exerted by it is just too less to do any work on the car. To get an idea as to how much work the wind is doing while the car moves, look at how much the car moves only due... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Will 5 pizzas in the same Hot Bag stay warmer than 5 pizzas in 5 separate Hot Bags? For example, say I am delivering 5 pepperoni pizzas to 5 different addresses. In one scenario, I Keep all 5 in the same insulated Hot Bag, I carry that bag to the door, and I quickly remove one of the pizzas from the bag to give to the ... | My guess is it's best to put each pizza into its own insulator, but then stack them for transport so that the stack has the same size and surface area a larger insulating container would have.
Stacking the boxes in transport minimizes heat loss since it's proportional to exposed outside area. Opening a container would... | {
"language": "en",
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When is the displacement current equal to conduction current in case of a parallel plate capacitor being charged? I came across a text - "Whenever a conduction current is oscillating in time, the displacement current is equal to the conduction current in case of parallel plate capacitor."
I am not sure what's oscillati... | Assuming that we have a uniform electric field between the plates of a capacitor, we can find its intensity to be (using Gauss' Law) $E(t)=\frac{\sigma(t)}{\epsilon_0}=\frac{q(t)}{\epsilon_0 A}$, where $q(t)$ is the absolute value of the charge on each plate, and which is not constant in time since the capacitor is bei... | {
"language": "en",
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Why Earth's atmosphere behaves like an ideal gas? In every book I found this sentence like an assumption, without explananions, somebody can help me understand it better?
| The Earth's atmosphere is mostly nitrogen and oxygen, both of whose behaviors are very close to ideal at the temperatures and pressures found in the atmosphere. Nitrogen, the dominant gas in the atmosphere, comes particularly close to exhibiting ideal behavior. Gaseous oxygen exhibits about a 3% departure at 20 atmosph... | {
"language": "en",
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When referring to weights and mass of weights in a physics laboratory, do we use the term mass or weights? What terminology is used to refer to weights/ mass/ weight of mass/ mass of weights when referring to the mass of weights in a physics report? My question is more of the weights that we use in the physics laborato... | If you care about the inertia you use "mass". When you are considering the force of gravity you use "weight".
So when you do calculations about the force in the cables due to acceleration of an elevator car you need to know both it's mass and it's weight...
The (calibrated) object you place on a scale is called a "weig... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/137538",
"timestamp": "2023-03-29T00:00:00",
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What is the difference between diffraction and interference of light? I know these two phenomena but I want to know a little deep explanation. What type of fringes are obtained in these phenomena?
| Feynman has come from heaven to answer your question! Listen to him:
No one has ever been able to define the difference between interference and diffraction satisfactorily. It is just a quest of usage, and there is no specific, important physical difference between them. The best we can do is, roughly speaking, is t... | {
"language": "en",
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Will an un-scattered photon go to the edge of the universe? Will an unhindered (un-scattered) photon go to the edge of the universe?
|
Will an unhindered (un-scattered) photon go to the edge of the universe?
To add to the answer of @RedAct : If you are thinking of the three dimensional part of the universe we observe now, it is an instantaneous universe, i.e. time t has one value. In this sense we are at the edge of the expanding universe, the right... | {
"language": "en",
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Why is cooling much harder than heating? I'm trying to invent a distillation apparatus that runs solely on electricity. Suddenly, I realized that cooling things is really hard, while heating them up is so easy.
Actually, it seems that there are just three ways to cool something down:
*
*Peltier modules (incredibly i... | It is because heating and cooling happen because of different physics. Consider the following equation:
$\partial T/\partial t =c \nabla^2 T + H(x,t, T)$
Where $H(x,t)$ is the external heating. This is the source term. Cooling would be a diffusive process but heating could be nonlinear. In many cases $H(x,t) >> c \nabl... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138011",
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"question_score": "20",
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$ε_0$ affects electric field intensity, but $μ_0$ doesn't affect magnetic field intensity? I'll be honest: this question is actually a homework problem. I've spent the past hour going through Google and several textbooks trying to answer the question "Why does $ϵ_0$ affect electric field intensity but $μ_0$ does not af... | What's probably happening here is the following: The fundamental or microscopic fields $\mathbf{E}$ and $\mathbf{B}$ are technically called the electric field strength and the magnetic induction, while $\mathbf{D}$ and $\mathbf{H}$, their macroscopic counterparts, are called the electric displacement and the magnetic f... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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how is an electron able to emit and absorb the same (virtual) photon? When calculating the lowest order self energy corrections for an electron for example, feynman diagrams involving the emission and re-absorption of a (virtual) photon need to be considered, (as here for example: http://quantummechanics.ucsd.edu/ph130... | Indeed, do not take Feynman diagrams as literal representations of what is happening in a particle picture. Only the external lines of a diagram correspond to real particles - the internal lines, though called virtual particles, are little more than artifacts of the perturbative expansion we do to calculate QFT amplitu... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Simple estimation of the critical temperature of water I'm trying to develop fermi estimation skills and I came up with a question for which I don't even know where to start from. Here goes:
Is it possible to estimate the critical temperature (say in Kelvin degrees) of water in a simple way using fermi estimation?
By c... | I haven't done a calculation yet, but I would use an extrapolation based on the Clausius-Clapeyron formula:
$$\frac{dP}{dT} = \frac{L}{T\Delta V}$$
You then take any two known thermodynamic quantities of water and water vapor and linearly extrapolate to that point where the difference is zero. A good choice could be th... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
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Drift velocity of electrons with changing area What would happen with the drift velocity of a cylindrical resistor's diameter increases, with a given voltage between its terminals? According to the expression:
\begin{align}
R&=\rho\frac{L}{A}\\
I&=neAv_d\\
\Delta V &= IR\\
v_d&=\frac{\Delta V}{\rho L n e}\\
\end{align}... | The drift velocity is the average velocity due to an applied electric field. In a conductor, electrons scatter around at the Fermi velocity but have a net zero average (i.e., equal scattering in all directions). When the electric field is applied, the electrons are given a small velocity in one direction. Thus, we can ... | {
"language": "en",
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Why rubber is incompressible material? Why rubber is incompressible material? I know its Poisson's ratio is nearing to 0.5. So I don't understand physically, what it means by 0.5 Poisson's ratio and incompressibility. When I tried searching it, I found that rubber (or similar polymers) conserve volume after deformation... | Bulk modulus of elasticity is given by : E/3(1-2v)
where E= strain (or linear deformation and in your case compressibility)
v= Poisson ratio ( for rubber it is 0.5)
and bulk modulus of elasticity basically tells us the capability of compressing of a material.
hence inputting the values we result in a zero denominator... | {
"language": "en",
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why non orthogonal states are indistinguishable? I want to know what does it mean by distinguishable quantum state from Mathematics perspective I mean mathematically. As a non physics background student could any one explain me why non orthogonal states are indistinguishable by using linear algebra only?
| Two states $\rho_0$ and $\rho_1$ are perfectly distinguishable if there exists a POVM measurement $\{P_0,P_1\}$ such that
$$\operatorname{Tr}(\rho_0P_0)=1$$
$$\operatorname{Tr}(\rho_0P_1)=0$$
$$\operatorname{Tr}(\rho_1P_0)=1$$
$$\operatorname{Tr}(\rho_1P_1)=0$$
and $P_0+P_1=I$, where $I$ is the identity matrix. You can... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Origin of quark masses Does all the mass of the quarks in the standard model come from the Higgs sector or is there also a contribution to quark masses due to QCD chiral symmetry breaking?
| In the standard model the mass for the quarks and other elementary particles comes from from the Higgs mechanism. In the case of the quarks, the masses given in the table of the link are calculated using convoluted theoretical models and data input from scattering experiments.4
At the moment chiral symmetry breaking do... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/138704",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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Gibbs free energy and maximum work I'm getting confused between two important results.
The Gibbs free energy is $G = H-TS$
where $H$ is the enthalpy and $S$ is the entropy.
When the temperature and pressure are constant the change in the Gibbs energy represents maximum net work available from the given change in system... | It means that there exists a third variable, say, $X$, describing the system. The corresponding thermodynamic relation should be
\begin{equation}
dG = VdP - SdT + \bigg(\frac{\partial G}{\partial X}\bigg)_{P,T} dX.
\end{equation}
If $X$ is not fixed, the system at constant $T$ and $P$ adjusts itself such that $G$ is mi... | {
"language": "en",
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If light is electromagnetic then can light produce electricity or attract metals? what I mean to say is that if light is electromagnetic in nature then shouldn't it show electric or magnetic properties on matters? Like if light falls on a metal it should produce current due to its electric nature but it doesn't. Seco... | I think the OP is refering to the classical nature of light (not the famous quantum effects associated to it). Like, EM radiation can stimulate antennas or induce electric currents on conductors - if light is EM radiation then why doesn't it create electrial currents on conductors?
This is because the wavelengths are s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139034",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 5,
"answer_id": 3
} |
How to choose the Correct Green's Function? In order to solve the Green’s function of the Helmholtz operator $$(\nabla^2+k^2)G(\vec r-\vec r’)=\delta^{(3)} (\vec r-\vec r’)$$ one can obtain four different Green’s functions corresponding to four different choices of avoiding the poles and choosing the contour. How to ch... | Consider the case of a free scalar field, governed by the usual Lagrangian,
$$\mathcal{L} = \frac{1}{2}\partial_\mu \phi \partial^\mu \phi - \frac{1}{2}m^2 \phi^2$$
The propagator, or equivalently Green's function for the theory is a function which can be though of as a response when we use a delta function as an input... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/139165",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
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