Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
GR Verification for a Charged Black Hole For a charged ($Q$) rotating ($L$) mass ($M$), the Kerr-Newman equations give the angular deflection of light. Has there been observational verification (I would prefer to use only the angular deflection of light) that all three $QLM$ parameters are needed?
I am wondering if all... | I don't think there has even been observational evidence of Kerr-specific effects. While gravitational lensing is well known these days I don't think any of the objects studied have been rotating fast enough for the difference between the Kerr and Schwarzschild metrics to be apparent. Well, not in lensing anyway - Grav... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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What is meant by rest in rest-mass? As far as I know only photons are considered to have no rest-mass. In common words when it doesn't move it 'disappears'.
*
*Electrons and quarks should have a rest-mass. But are they really at rest?
In atoms and molecules is always a kind of zero-point energy left which implie... | The term rest mass is a poor one because it implies it's the mass measured in the rest frame. But photons have no rest frame, and indeed any particle subject to some form of confinement has a $\Delta p\gt 0$ so its rest frame is somewhat poorly defined.
The modern term is invariant mass, which is simply the mass in the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/261400",
"timestamp": "2023-03-29T00:00:00",
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What are some resources for learning about x-ray powder diffraction? I am looking for up-to-date, applied resources to learn about x-ray powder diffraction. There is a lot of jargon with which I am unfamiliar. I am trying to refine theoretical curves to collected data using the program Maud using the Rietveld Method b... | As with any new thing one the best ways to do it is to look for publications. If you don't understand the jargon/notation, go to their references and look at those papers and keep recursively applying that method until you go back far enough to where you understand, then you can read back up to the first one.
Edit: I u... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Is it possible to have solid light? Is it possible to have solid light?
If so, what would it be like?
| This depends upon how you want light to solid. If you want light to be solid in the way the "Star Wars" movies have light sabers, I would say no. There are however materials that trap photons so they have zero velocity. Photons are in a sense trapped, and these are sometimes called artificial black holes. The energy of... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/261832",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
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What is the color of a group of trillions of electrons, protons, and neutrons Since an electron is smaller than visible light, then what what color would a group of electrons (trillions of electrons) be if there were enough of them to be seen by the eye? What color would a group of trillions of protons be? Color of tri... | The 'color' would be an ultra-bright burst of gamma rays as the trillions of electrons rush apart, frying both you and your eyes to a crisp.
More seriously, if you confined the cloud of electrons, it wouldn't emit any particular color on its own -- for instance, there could be no optical transitions since there are no ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/261946",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Uncertainty in the distance between Sun and other planets I have read about the orbit distances between Sun and the planets and have come to know for example: Earth is around 150 million km away from the Sun.
However I have seen that tht value is only an average radius of the orbit.
I however cannot find the uncertain... | The variation in the orbital distance between aphelion and perihelion is not an "uncertainty" - it is certainly variable.
The ratio between aphelion distance and perihelion distance is $(1+e)/(1-e)$.
The NASA fact sheet http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html gives the eccentricity $e$ to 8 decima... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/262149",
"timestamp": "2023-03-29T00:00:00",
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Does the charge leakage of capacitors lead to photon emission? Capacitors will leak charge over time. This charge is basically electrical current leaking through insulating layer of the capacitor. I am wondering, if there is some emission of photons as there is is a potential acceleration of the electrons that are part... | In principle any acceleration of an electron causes some radiation, and an electron has to accelerate in order to leak from one plate to the other. However:
*
*the velocities, and therefore the accelerations, of electrons in electrical circuits are small. Calculating the electron drift velocity is an exercise routin... | {
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Growth and Decay, Law or not? The differential equation for decay that applies to radioactive decay is:
$$dN/dt=-kN$$
for a positive constant k and number of particles N. My question is: is this, strictly speaking, a "Law"? I have seen this differential equation refered to as a "Law" and sometimes not, so what is the ... | Physics is about quantified observations of nature, modeled mathematically. The mathematical models are rigorous and self consistent but in order to connect to measurements extra postulates are needed, which define the connection of the mathematical formulae to data. Laws are parts of these postulates. In the same way ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/262833",
"timestamp": "2023-03-29T00:00:00",
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Fluid mechanics paradox: The force needed to 'balance' a fluid using a piston We know that the pressure in a fluid (like water) is dependent on the depth. Consider this example:
For the first setup, solving the pressure at the bottom yields
$P_1 = \rho gh = 1000kg/m^3 \bullet 9.8m/s \bullet 1m = 9800Pa$
And from the d... | The quick answer is that a long as the surface area of the piston, and the volume/mass of the remain unchanged, the pressure stayed the same.
So the erreanous step is to assume that $ P_2 = \rho g h $. This invalid as the cross section area changes with height.
| {
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Why are ceramics good electric insulators? I know it depends a lot on the composition, so not all are great electric insulators. So what makes it good or bad? And is it different from what makes them good thermal insulators?
Power line insulators are ceramic and they have to stave off huge voltages. That's what got me... | Most ceramics are ionic compounds, in which electrons are immobile. This is different to metal, in which the atoms are in a "sea of electrons" that are free to move.
Note that ceramics have some kind of conductivity, it's just extremely low. The conductivity of copper, for example, is ~6×107 S/m. Most ceramics have con... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/263574",
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The analytical result for free massless fermion propagator For massless fermion, the free propagator in quantum field theory
is
\begin{eqnarray*}
& & \langle0|T\psi(x)\bar{\psi}(y)|0\rangle=\int\frac{d^{4}k}{(2\pi)^{4}}\frac{i\gamma\cdot k}{k^{2}+i\epsilon}e^{-ik\cdot(x-y)}.
\end{eqnarray*}
In Peskin & Schroeder's bo... | Yes it is correct. The derivation in P&S is straightforward but I will expand on it a bit. The key observation is that
\begin{equation}
\int\frac{d^4k}{(2\pi)^4}e^{-ik\cdot(y-z)}\frac{i\gamma^{\mu}k_{\mu}}{k^2+i\epsilon}
=-\gamma^{\mu}\partial_{\mu}\int\frac{d^4k}{(2\pi)^4}\frac{1}{k^2+i\epsilon}e^{-ik\cdot(y-z)},
\end... | {
"language": "en",
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Can energy conservation be derived from Newton's laws? Can the idea of energy conservation be derived from Newtons's laws? From inspection of his laws you can vaguely discern a relationship but I want to know if you can manipulate his laws to prove it.
If not, what else did it take in history? What other assumptions ab... | The answer is not. Newton's laws are equivalent to linear momentum conservation, but it does not implies mathematically energy conservation.
The elementary proof that is usually given assumes that involved forces derive from a potential energy. In that case the answer is yes, taking into account potential energy.
But c... | {
"language": "en",
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Why do thin films need to be thin? No matter what thickness a piece of glass is wouldn't its optical thickness be close to an integer multiple of a wavelength such that it could create interference effects? I feel like I am missing something here.
|
Ignoring for simplicity's sake the usual refraction that takes place at the interfaces of the media, if:
$$|OA|+|AB|=D\big(\frac{1}{\cos \theta}+\tan \theta\big)=n\lambda$$
with $n$ an integer and $\lambda$ the wave length, then we have positive interference.
But as we increase $D$, the distance $\Delta$ also increase... | {
"language": "en",
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Is there any effect on mechanical waves by electromagnetic waves (and vise versa)? Do electromagnetic waves like light and gravitational waves (due to moon for instance) affect on mechanical waves like sound?
Can sound change the path of light?
| Any physical phenomenon is potentially capable to cause some change to any other phenomenon, more or less directly. If it was not the case, the physical world could be divided into completely independent realms; there would not be the one single world we call Nature.
Practically though, many if not most of the actually... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Projectile motion with air friction force/resistance
We have to find the x (the distance, if you didn't know that then I'm not sure if you should be doing this problem) that the projectile travel during the time in the air until the time it hits the ground. I can do this no problem without air resistance, but I have n... | I assume that air resistance force is parallel to the velocity vector but in opposite direction.
We have:
$$a_x=\large{\frac{10\cos\theta}m}$$
$$a_y=\large{\frac{10\sin\theta}m}-g\;\Longrightarrow\;a_y+g=\large{\frac{10\sin\theta}m}$$
$$\tan\theta=\large{\frac{v_y}{v_x}}$$
Then,
$$\large{\frac{v_y}{v_x}}=\large{\frac{... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/264523",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Third Brillouin zone for a quadratic 2D lattice As far as I understand, the construction of Brillouin zones stems from the relation$$
2 \vec{k}\cdot \vec{G} +G^2 = 0
\,,$$where $\vec{k}$ is the wave vector and and $\vec{G}$ is the reciprocal lattice vector. This condition is supposed to be fulfilled when $\vec{k}$ term... | One reason is that the volume of your third BZ is twice the volume of your first BZ.
To see it another way, notice that the black dot on the far right (the middle of the three vertical dots) is a BZ center, a $\Gamma$ point. The black dot directly above that one (far upper right corner) is also a $\Gamma$ point, bu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/264646",
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Is the energy needed for a current through a straight and a coiled wire different? When you add current to a straight piece of wire does it use less electricity than if it was coiled? The power wire on telephone pole's are curved while buried cables are pretty strait in comparison. Does the curve of the wire and proxim... | Okay I'll have a go at answering this, although it may be a make belief scenario.
Looking at where you got your inspiration from, he stated using a magnet as a core for an electromagnet, so your curiosity must have piqued from the idea that the wire carrying electric could itself be magnetic. Because of polarization an... | {
"language": "en",
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Prove that an electron in a hydrogen atom doesn't emit radiation According to electrodynamics, accelerating charged particles emit electromagnetic radiation.
I'm asking myself if the electron in an hydrogen atom emits such radiation. In How can one describe electron motion around hydrogen atom?, Murod Abdukhakimov says... | I believe some of the answer in the links are correct, others are less obvious and might even be confusing. I am not gonna repeat the arguments there, but to stress the following idea. You cannot demonstrate that using classical electrodynamics. The theory as is does not apply to quantum objects and thus it was modifie... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/264950",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
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The expression of the density in terms of molecular mass and the distribution function I am reading a book about the boltzmann equation, the author gives the expression of the fluid density $\rho$ as follows:
$$\rho(\mathbf r,t) = \int {M\,f(\mathbf r,\mathbf c,t) \, \mathrm d\mathbf c}$$
where:
*
*$\rho$ is the fl... | Density is the average amount of mass per unit volume $\rho(\vec{r},t) = \frac{M}{V_r}$.
Distribution function is defined as a number of particles per unit phase space volume $ f(\vec{r}, \vec{c},t) = \frac{N}{V_r V_c} $ (which is space volume times velocity volume). Each particle has mass $M$, so to get the total mas... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Physical meaning of enthalpy I've been reading about thermodynamics and reached the topic about enthalpy . I've understood its derivation but I don't understand its physical meaning ... Also I don't understand why they have divided by the mass of gas to get to the specific enthalpy equation . what's the use of it? I kn... | In physics one of the most fundamental concepts is the conservation of energy and in thermodynamics we systematize, in an ideal manner how to account for the energy and changes in energy in systems. So basically a means of categorical naming, bookkeeping.
The units of enthalpy are energy units such as Joules. And for a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/265198",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Are the partial derivatives of Lagrangian in the varied action functional derivatives? In particle mechanics Lagrangian $L$ depends upon position, velocity (and may be explicitly on time), whereas in field theory the Lagrangian density ${\cal L}$ similarly (or analogously) depends upon the field and its derivatives. Wh... | Yes, OP is right. In the field-theoretic case, the partial derivatives in OP's first formula (1) should be replaced with functional derivatives
$$ \delta S~=~\int_{t_1}^{t_2}\!\mathrm{d}t\left(\frac{\delta L}{\delta q}~\delta q+\left. \frac{\delta L}{\delta v}\right|_{v=\dot{q}}~\delta \dot{q}\right),\tag{1'}$$
where ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/265286",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Multipole expansion of the electromagnetic field In Jackson's Classical Electrodynamics, section 9.7, he develops the multipole expansion of the electromagnetic fields in terms of the vector spherical harmonics and the spherical Bessel and Hankel functions. His expansion is somewhat confusing, and I was wondering any o... | It is better to avoid vector spherical harmonics by expanding the electric and magnetic scalar potentials instead of the vector fields. The electric potential
is expanded in every EM textbook. The magnetic scalar potential is treated in Section 7.10 of "Classical Electromagnetism, 2nd Edition" by Jerrold Franklin
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Electric field dependence on distance How can it be proved that for a point charge, $E$ is proportional to $$1/r^2$$ using the concept of Electric field lines (or lines of force)? I tried to show that if field lines are close, then magnitude of Electric field is higher. But, I couldn't show the given dependence.
| As such there is no real theoretical proof to the inverse square dependence of the electric field in classical electrodynamics. It is an experimental fact famously known as the Coulomb's law. When combined with the superposition principle, it gives us the Gauss's law of classical electrodynamics:
$$\nabla \cdot\mathbf... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/265816",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Can an inhabitant of a spacetime region measure its curvature tensor? So, lets say that I am an ant living on a 2-D spherical surface that is stretching to the equator...like half a sphere. I can not describe this surface in terms of the outside coordinates only someone living in the outside world can do this. So, can ... | You could construct a circle of radius $r$ and measure the circumference $c$. In a flat space, $c=2\pi r$. However, in a positively (negatively) curved space, $c<2\pi r$ ($c>2\pi r$).
Alternatively, you could pick three non-collinear points and measure the three angles that they form. In a flat space, the sum of angles... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/266336",
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How does isolating transformer protect from geting shocked From what i know, isolating transformer makes it that you would have to touch both ground and phase cables to get shocked, but why exactly is that.
There should be some potential difference between phase (transformer output) and ground (that we stand on), and a... | If you had one sweaty hand on the isolated circuit, and one bare foot on the wet Earth, there would be no path by which current could flow through your body, and return to the isolated circuit. A (probably) trivial amount of current could flow because of the AC voltage, and (probably) very weak capacitive coupling bet... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/266431",
"timestamp": "2023-03-29T00:00:00",
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Why can we see the moon when it is between the Earth and the Sun? A rather stupid question, why can we see the moon when it is between the Earth and the Sun?
| The diagram you drew is flat, but the solar system is not. The Moon's orbit is not in the same plane as the Earth's orbit. Wikipedia has a nice diagram:
Because of this, when the Moon is "in between" the Earth and the Sun, it is usually a little "above" or "below" the Sun as well. You can observe this for yourself:... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/266492",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
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Is potential difference the difference in electric potential energy or electric potential? Referencing the book Physics for Scientist and Engineers, Ninth Edition, the book says that "Potential Difference should not be confused with Difference in Potential Energy."
I also reviewed several internet sources that say "Po... | Technically "potential difference" is the difference in electrical potential, i.e. $\Delta V$, not the difference in electrical potential energy, $\Delta U$. Potential difference ($\Delta V$) is also called voltage, in certain contexts.
However, many people and sources are sloppy about their terminology, and they will ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/266671",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Interactions preserving separability Consider the interaction (described by a unitary matrix U) of two qubits initially in a separable state |ab⟩ = |a⟩ ⊗ |b⟩, such that after interaction the composite system is in state U|ab⟩.
Are there unitary matrices U for which U|ab⟩ is also separable, independent of |ab⟩?
If so: ... | Any linear operation $U$ which maps all product states $|a\rangle_A|b\rangle_B$ onto product states must be of the form
$$
U \equiv (U_A\otimes U_B)\mathbb F\ ,
$$
where $U_A$ and $U_B$ act independently on the two parts of the system, and $\mathbb F$ swaps the two parts, i.e.,
$$
\mathbb F:|a\rangle_A|b\rangle_B \maps... | {
"language": "en",
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Is there a prohibited region in $P-V$ plane? Polytropic process generalize the particular thermodynamic processes with
$$P V^{n}= \mathrm{constant}$$
Where, if $n$ changes, the curve on $P-V$ plane changes, as shown in the diagram.
The orange region is not touched by any curve, so there is no value of $n$ for which t... | It's simply inherent to the definition of polytropic processes that they don't allow the system to increase both its pressure and volume at the same time. That doesn't mean you can't increase a system's pressure and volume. You just need a non-polytropic process to do so. For example, it could be a compound process con... | {
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How did physicists know that only negative charges move? I have phrased similarly another question about how physicists knew that two charges exist, positive and negative. The purpose of the question is not necessarily to educate me historically. It's just that I wish to know about classical subjects without making the... | Physics's don't know that only negatively-charged particles move. We can create ion currents on demand in many environments. We do know that the current flowing in a metal wire is negatively charged particles in motion.
As for how to determine that, you do a Hall effect measurement.
The measurement works by subjecting... | {
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"url": "https://physics.stackexchange.com/questions/267036",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why don't high pressure gases stored in containers lose energy? Containers holding gas at a high pressure don't slowly lose the internal energy of the gas. It seems like the high speed particles would collide with the metal walls and slowly transfer their energy to the slower particles outside the container.
Even if th... | There are two ways you can change the internal energy of a gas, one is macroscopic, that is, performing work on or by the gas, if the gas either expands or contracts. The other is microscopically through heat. If the compressed gas is at the same temperature than the outside gas, these microscopic collisions will not r... | {
"language": "en",
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"source": "stackexchange",
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What Postulate States Entropy must be Positive? I've been reading the Postulates of Classical Thermodynamics, and I haven't found anywhere to be said that the Absolute Entropy of a system has to be a positive number.
The third one states that the Temperature, $dU/dS$, must be Positive, since $S$ is monotone increasing ... | A better definition of Entropy is given by the statistical mechanics definition of the Gibbs Entropy:
$ S = -k_B \sum{p_i \ln{p_i}}$
Where $S$ is the entropy, $k_B$ is Boltzman's constant and $p_i$ is the probability of the $\mathrm{i^{\mathrm{th}}}$ microstate to be occupied. For any probabilities less than 1, the qu... | {
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"url": "https://physics.stackexchange.com/questions/267218",
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Finding the coefficient of static friction on slope My Problem
1.0kg block is sitting on a horizontal incline plane. The plane is tilted into the block begins to slide. You notice that the plane is inclined at an angle of 32 degrees above the horizontal when the block begins to slide. Calculate the coefficient of stati... | You have found the critical angle $\theta_c$ at which the block begins to slide. That gives you the coefficient of static friction $\mu_s = \tan\theta_c$. Kinetic friction comes into consideration when the block is actually moving.
Before the block can move, the force $mg\sin\theta$ acting down the incline must be at ... | {
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If the universe stopped expanding, when would we realize it? Suppose in this moment the universe stopped expanding, how long would if take and how would we register the change?
Since the bodies are still and it is space that is expanding in between, we wouldn't notice the change even if the expansion would completely s... | A few million to about 30 million years from now. I.e., we'd be able to measure a change in the previously observed redshifts for galaxies a few to about 10 Mega parsecs away. A parsec is a little more than 3 light years.
The reason is that closer in galaxies are in our local group or cluster, and we are to a great ex... | {
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Problem on Emissivity and absorptivity I have read the radiation chapter, where I have been introduced with the terms emissivity and absorptivity. emissivity tells about the ability to emit heat energy as thermal radiation compare to a black body. and absorptivity is the amount of heat absobed by body devided by the in... | In a way, these are intrinsic properties of a material that depend on temperature. The temperature of the object dictates what a black body curve looks like and which wavelengths are emitted in what amounts. The emissivity of a material is not always uniform across all wavelengths, which means at different temperatures... | {
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What would be the optimal diameter of gold grains if you want to fill a pool with them and swim in it? I was wondering, if I were to fill a pool with gold/gold dust, what would be the optimal diameter of the grains to minimalize the friction to a point at which you would be able to swim in it ((and if swimming is not p... | It's not so much about a grainsize. As long as the grains are big enough to aloud air(gas) flow through them, you can make them liquid by"Fluidization";
"Fluidization (or fluidisation) is a process similar to liquefaction whereby a granular material is converted from a static solid-like state to a dynamic fluid-like ... | {
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Does the speed of light in vacuum define the universal speed limit?
*
*Is light the thing causing the universal speed limit to be $299\,792\,458\,\mathrm{m/s}$? So the universal speed limit would be different if light travelled faster or slower?
*Or, is $299\,792\,458\,\mathrm{m/s}$ the universal speed limit anyway ... | Above all, speed of light is the speed of propagation of fields through space. While light may be slowed down when crossing matter, fields (electromagnetic fields, gravity) are always propagated at c. One of the consequences is the "speed limit for causality" mentioned by DavidZ and the speed limit for transmission of ... | {
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How / why do infrared thermometers work? How can reading the intensity of infrared radiation coming from an object let you calculate its temperature? Could any wavelength be used to measure temperature - why infrared, and not, for instance, green, or low frequency radio waves? And would an infrared thermometer give ... | *
*Every object with nonzero temperature (so, every object) radiates EM radiation
*The spectrum of a black body (that is, a body that absorbs all light at all wavelengths) is given by Planck's law.
*Generally, nothing is completely black (except for empty space, which is why cosmic microwave background has a very ni... | {
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Simple analytic examples of Multi-scale Entanglement Renormalization Ansatz (MERA) I want to understand Multi-scale Entanglement Renormalization Ansatz (MERA) with very elementary examples. So far I could find references which are mostly based on numerics. It would be a great help if someone could give a very simple ex... | You may want to start from the paper Entanglement Renormalization: an introduction. In this work author describes basic consepts of MERA, things like isometries and disentanglers and why one needs to use them in order to produce MERA.
Article Algorithms for entanglement renormalization is more advanced and is focuse... | {
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Large-scale antimatter production From what I can find, presently the only known means of producing antimatter consist of directing particle accelerators at various targets, and only infrequently getting positrons or anti-protons as byproducts of particle interactions.
Assuming a sufficiently large source of energy: Ha... | This Centauri Dreams article claims that a 0.01% efficiency is possible with current technology if there was a dedicated production facility.
Having searched for information on this problem myself, I have found nothing more of significance.
If you can get past the google scholar paywalls, you may want to look through t... | {
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What happens to Pauli's argument (that says that there is no time operator) when applied to $X$ operator for some simple systems? An argument by Pauli is usually referred to in the literature when it is stated that there cannot be a time operator in quantum mechanics. This argument can be found as a footnote to P63 of ... | Commutation relations between position and momentum operators, in quantum mechanics, are valid only when the actions of either of the two operators is contained in the domain of the remaining one. In particular we have:
$$
[x,p]\psi = (xp)\psi-(px)\psi.
$$
In order for the above to be defined $\psi$ must be in the doma... | {
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GR says that time and space are aspects of the same thing, yet there is no observable for time in QM I understand that the topic of a time operator in quantum mechanics has come up more than a few times so forgive me if this is a repeat question but I couldn't find anything specific to my question.
My question is more ... | Because time itself is the driving force of motion, whereas space is not, space is expanding as is time, but space doesn't measure motion, it's a measure of the area we occupy. If I have 80 billion units of space but no measurable time then the energy of motion would not exist, therefore nothing could move and space wo... | {
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What is the lowest energy atomic transition ever detected and identified? In this concise and insightful answer to Which is larger, all known, bound nuclear energy levels, or known, bound atomic energy levels? it was pointed out that atoms can have an extremely large (possibly infinite) number of bound excited states.
... | The 21cm line from atomic hydrogen is pretty low. Caused by the transition from parallel to anti-parallel spins between the atom and the electron.
| {
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Principle of Least Action Question Let's say we have a particle with no forces on it. The path that this classical particle takes is the one that minimizes the integral
$$\frac{1}{2}m\int_{t_i}^{t_f}v^2dt.$$
So if we graph this for the actual path a particle takes it is a straight, horizontal line on the $(t,v^2)$ pl... | You're missing that Dirichlet boundary conditions
$$ x(t_i)~=~x_i \quad\text{and} \quad x(t_f)~=~x_i $$
are implicitly implied. The stationary action principle is not well-posed without boundary conditions.
| {
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How many photons are needed to make a light wave? What is the smallest number of photons needed to make a "light wave"?
In other words, how many (coherent?) photons start to exhibit classical behavior?
For example, how many photons are needed to get linear polarization? (Single photon has circular polarization.)
| Even though there is a single photon in a volume of your choice the light is still a wave.
An experiment was performed which proved this. In this experiment a Michelson interferometer was set up and the incident light is so weak that only one photon was in the whole setup at a time. A photographic plate was used to det... | {
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$(1/2, 0)$ representation of the Lorentz Group $SO(1,3)$ Let us consider the $(j, j') = \left(\frac{1}{2}, 0\right)$ representation of $SO(1, 3)\cong SU(2) \otimes SU(2)$.
$j = \frac{1}{2}$ corresponds to $SU(2)$ generated by
$$ \tag{1} N_i^+ = \frac{1}{2} \left(J_i + i K_i\right); \quad i =1, 2, 3. $$
$j' = 0$ corre... | This is what happens when physicists try to do group theory but don't bother introducing the proper mathematical notions.
*
*There is no isomorphism between $\mathrm{SO}(1,3)$ and $\mathrm{SU}(2)\times\mathrm{SU}(2)$, the former is non-compact, the latter is compact. More around this apparently confusing topic can b... | {
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Spring behaviour under high atmospheric pressure We have a spring inside a chamber. We compress the spring and then let it decompress freely. Will its decompression (its speed and displacement) be the same if the air pressure of the chamber is $1\;\mathrm{atm}$ or $3000\;\mathrm{atm}$? If not, how will it be affected?
| In 3000atm it's speed of decompression will be slower because it is facing greater air density and the expanding spring has to move it. There will also be less "resonance" as the denser air damps the spring movement.
| {
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Why is the resistance in a parallel circuit smaller than the resistance in a series circuit? So I was told in the physics class that the resistance in a parallel circuit is smaller than the resistance in a series circuit. Why does that happen?
Is this statement also true for circuits which have no resistors or resistan... | From the wording of the question I would assume the OP didn't want formulas or a very technical answer, so I'll attempt answering in layman's terms.
What does resistance do? It resists the flowing of current. Given the same voltage, the bigger the resistance, the smaller amount of current can flow.
Now, imagine that th... | {
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Why is current the same in a series circuit? So I am a 10th grade student and my teacher told me that the current is the same at every point in a series circuit. It does split up in parallel circuit but it then recombines and the current flowing out of the battery is the same as the current flowing back into it.
My que... | Current is a measure of how many electrons go past a particular point in the circuit every second. So there are electrons rushing into one side of the light bulb, and rushing out of the other side. The number rushing IN each second is equal to the number rushing OUT each second. If that wasn't the case, then there'd... | {
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Does the charge of a black hole affect its space-time geometry? Does a black-hole of a given mass and angular momentum have the same space-time geometry regardless of its charge? I'm pretty sure that an electric field can accelerate a charged particle but doesn't curve space-time so the only way an electric field can a... | So the answer is no.
An electric field has energy and energy generates a gravitational field, just like any mass.
See the charged black hole solution is the Wikipedia article https://en.m.wikipedia.org/wiki/Charged_black_hole
The charge of a black hole, if nonzero, changes the metric and solution to account for the c... | {
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Confusion in a trick in solving an energy eigenfunction Given a non-relativistic energy eigenfunction for a central potential $\left|\Phi \right>$
In solving relativistic hydrogen atom, one of the terms is
$$
\left<\Phi\middle|\frac{e^2}{r}\middle|\Phi\right> \tag{1}
$$
I have read a trick to solve it is:
$$
\left<\Ph... | The theory behind the trick is based on the Hellmann-Feynman (HF) theorem
$$ \frac{dE_{\lambda}}{d\lambda}~=~\langle \psi_{\lambda} | \frac{d\hat{H}_{\lambda}}{d\lambda}| \psi_{\lambda} \rangle,\tag{A}$$
which works with a single derivative, but not with a square of a derivative, cf. OP's failed calculation (5) for th... | {
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Does the force on the bottom piece of a hummingbird feeder depend on the height of the water in the reservoir? Intuitively I would think that the more nectar you put into the reservoir the more weight it will put on the bottom piece (the tray that holds the open pool of nectar that the birds can feed out of). While I w... | The force the fluid does on the bottom piece does not depend on the height of the water column of the reservoir. It does depend on the height $h$ of the water column in the plate.
This can be easily seen by the fact the water is static so the pressure at any horizontal plane is the same. The pressures in $a$ and $b$ a... | {
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Quick question: Curvature form of a connection on the trivial bundle Let $L=\mathbb{R}^2\times U(1)$ be the trivial $U(1)$-bundle over $\mathbb{R}^2$. Define a connection $\nabla=d+A$ where $A=fdx+gdy$ is an $\mathbb{R}$ valued $1$-form on $L$. That is, $\nabla$ gives a distribution $\mathcal{H}$ on $L$ - the horizonta... | $f$ and $g$ are defined on the base space to begin with. You would need to extend to functions on the total space in order to define taking $\partial_z$.
| {
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Could a radar pulse be considered particular in nature? I ask because I have a real problem with wave/particle duality (one of the biggest cop outs in the whole of physics, in my opinion)
A radar pulse is definitely a wave.
It is spatially bounded. (at least in one dimension!)
It has energy.
It is a 'quanta'.
Is this n... | $E=mc^2$ is true only for a particle at rest, the photon is never at rest.
The full equation is
$$E^2=p^2c^2+m^2c^4.$$
A photon has no mass, but two photons can meet to form an electron and a positron, see pair production.
| {
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Radius of centripetal acceleration Suppose you are moving in circle of radius $r$. So there should be centripetal acceleration towards the center. Now you want to decrease the radius of the circle, so someone should apply more centripetal acceleration in order to decrease your radius. But I had read that with the incre... | Let's use the classic example of a ball revolving around your fingers via a tensed string.
There is a tension in the string because the ball is travelling in a circular motion. The tension force is constantly causing the ball to accelerate toward your fingers. The ball, at all times, wants to travel in a tangentially s... | {
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What does "non-normal parity" mean? Nuclear physicists seem to use the term "non-normal parity" a fair bit. Googling for the term yields some 840 results, and the same search on Google Scholar indicates that about 430 of those are research papers.
Unfortunately, those searches seem to only yield research-level papers t... |
A state with parity of $(−1)^L$ is defined as a natural parity state, with $L$ being the total angular momentum of the system, and parity with value of $+1$ or $−1$ are called even or odd, respectively. A state with parity of $(−1)^{L+1}$ is called unnatural parity state.
Source: Ye Ning et al., Natural and Unnatural... | {
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Could all the energy in all the photons in the universe account for dark matter? I was hit with fridge logic, and I am curious:
Is it possible that the gravitational influence of photons inside of galaxies (And all throughout the universe) could account for dark matter?
Photons would be most concentrated close to the c... | The problem here is the contrasting effects the two forms of matter have.
Photons are massless; they create negligible gravitational fields.
Dark matter, on the other hand, is about 85-90% of all mass in the universe. It's responsible for holding galaxies at constant rotation rates past certain radii (see Galactic Ro... | {
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Are Newton's 1st and 3rd laws just consequences of the 2nd? Can Newton's 1st and 3rd laws be assumed given just $F=ma$. I know that the argument would be, "No, then there would only be 1 law". But I can't think of any situation where 1 and 3 aren't superfluous.
If you just told me $F=ma$:
I would assume nothing else c... | Newton's first law defines the inertial reference frames:
*
*There exist in the universe some very particular reference frames such that, in those very particular reference frames (and only in those and no more) a body not subject to external forces or interactions moves with constant velocity.
Newton's second law... | {
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Interference with a minus sign In Lectures on Physics, by Richard Feynman, pg 3-11, I found the following:
In case of electrons, the interfering amplitudes for exchange interfere with a negative sign.
I was unable to understand this. On reading further, it seems that electrons have spins unlike alpha particles and t... | This might be something related to particle statistics: Electrons have spin 1/2 and thus belong to the group of fermions. Exchanging fermions causes an additional minus sign for the state. Alpha particles are made up from two protons and two neutrons, all of them spin 1/2 particles. These spins can add up to either zer... | {
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Does time period of a simple pendulum vary if I heat its metallic bob? How does the time period of a simple pendulum with a metallic bob vary if we heat its metallic bob? The pendulum is assumed to be a simple one and air drag is taken to be negligible.
Please provide a proof if possible.
| Let's assume the length of the string remains constant, i.e. only the bob is affected by the heating. This appears to what the question implies. In that case to a first approximation there will be no effect on the period. The period is given by:
$$ \tau = 2\pi\sqrt{\frac{\ell}{g}} $$
and since neither $\ell$ nor $g$ ar... | {
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Wet metal disks stuck together - Casimir Effect or surface tension? I work in a processing plant where round steel cutting blades are used, e.g. 1-2 mm thick and 15-30 cm in diameter. During normal plant operations, these blades are mounted on machinery and rotated at circa 1000 RPM -- thus they are perfectly flat who... | Surface tension is part of it, but probably it's straight adhesion. Glue. A film of water on (for instance) glass can be wiped off, but usually that just thins the film, the last few microns of water film sticks hard to the surface.
If two flats have a thick layer of water between them, it's a lubricant (you get str... | {
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Difference between analog signal and digital signal There is no perfect generator of a true digital signal in the real world natural phenomenon. Most of the physical signals produced by humans and animals like roaring, speaking, playing, clapping, electrostatic charging, drumming are all analog in nature. Even physical... | You are right. Everything in nature is analog (including man-made "digital" signals). The digitization of signals is a man-made abstraction. Nature just evolves as it does and humans then categorize observed phenomena. The assigning of one signal as a "1" and the other signal as a "0" is a categorization.
It is possibl... | {
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How is potential energy lost when a water droplet is dropping down slowly on a wall? When a water droplet is on a vertical wall, it usually drops slowly, which is different from free falling. As the dropping speed is slower than free drawing, so I guess some energy must be lost.
I guess it is lost as internal energy, ... | You are not dealing with dry friction here. What is involved is viscous flow within the water droplet. There is a viscous circulation pattern set up within the water droplet, and the flow results in viscous dissipation of the (mechanical) potential energy to internal energy. So, if the water droplet where somehow ins... | {
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Why the deflection angle is important for verifying GR? Why the deflection angle is important for verifying GR? What about redshift?
But in particular deflection angle, because we have it in Newtonian Gravity as well.
Please guide me with your answer.
| Newtonion theory did predict the gravitational deflection, but gave an estimation of half of that which GR provided. Experimental evidence showed that GR was a more accurate model of gravitation than the previously universally accepted Newtonion theory.
The details below are a quick summary of this article Gravitation... | {
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What is the shape of a deuterium nucleus? What is the shape of a deuterium nucleus?
I can think of two obvious extremes.
A positive proton end intersecting with a neutral neutron end.
Or a cylinder with spherical caps on the ends that is positive on one end and neutral on the other.
| The deuteron has an r.m.s. charge radius $r=2.14$ fm and a static electric quadrupole moment $Q_0=0.2859\ e\cdot\text{fm}^2$. This is related to the nuclear deformation parameter $\beta$, describing prolate approximately ellipsoidal deformations, by
$$\beta \approx 0.080\frac{Q_0}{e\cdot r^2} \approx 0.050.$$
The nucle... | {
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Spring force on both sides of spring I am a little confused about springs. I just wanted to know that if I pull an ideal spring of spring constant $k$ such that the spring has been symmetrically pulled and its elongation (total) comes out to be $x$ then would the force on one side by $$F=kx$$ or $$F=kx/2$$ I am a litt... |
An ideal spring - Three loading cases : free,compressed,stretched.
| {
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Heat pump intuition What is an intuitive explanation for the concept of heat pumps? I know that it is basically a reversed Carnot process. We can for example take an amount of heat $Q_1$ out of a warmer system and transform part of it into work W. The rest goes to the colder system. If we now reverse that process we ne... | You describe the process. For an example, you might think of how a gas refrigerator works. You take a gas and expose it to the cold area, which cools it to the cold temperature. Then you isolate it from the cold area, which costs no energy. You compress it, raising the temperature above that of the hot reservoir, a... | {
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Angular Momentum of a Body Constantly Losing Mass My friend has a question: assume that a body such as a star loses mass as it orbits the galaxy. If there is conservation of angular momentum, how is this explained? What happens to its orbital velocity and orbital radius? Thanks.
| Well, the angular momentum is conserved when net torque on a body(or a system) is zero.
Now in your case, the torque produced by the gravitational force between the galaxy and the star is zero as the line of force passes through the axis of rotation.
For simplicity, i am neglecting any effect produced by other celestia... | {
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What is the difference between these two ways to calculate average velocity? Average velocity:
$$v_{\rm avg,1}=\frac{v_{\rm final}+v_{\rm initial}}{2}$$
and average velocity:
$$v_{\rm avg,2} =\frac{\rm total\;displacement}{\rm time \;taken}=\frac{\Delta x}{\Delta t} $$
What is the difference between them and when do ... | Your first way of calculating an average velocity is inaccurate and really should be avoided.
Only the second method is accurate. This is a consequence of the underlying calculus of kinematics.
When a object travels (e.g. but not necessarily on a straight line) its velocity is not required to be constant. In fact for t... | {
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Boltzmann formulation of entropy I could be doing something quite stupid here; this is a tad over my head, so I apologize if this is a stupid question, and if requested to delete it, I will do so.
I know that Boltzmann's formulation of entropy is $S = k \log W$ where $k$ is Boltzmann's constant, $S$ is the entropy of ... | The expression $W^k$, where $k$ is Boltzmann's constant, does not make sense mathematically because $k$ is not a number but has dimension of Energy/Temperature. So its numerical value depends on the units you choose. For example if you measure it in Joule/Kelvin then its value is about $1.38 \times 10^{-23}$ but if you... | {
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Kinetic energy of photoelectrons I came across the following question:
A photon of energy $h\nu$ is absorbed by a free electron of a metal having work function $W<h\nu$. Then:
*
*The electron is sure to come out
*The electron is sure to come out with kinetic energy $h\nu-W$
*Either the electron doesn't come out ... | The work function $W$ is the minimum energy of photon that is required to eject the electron from the metal in photoelectric interaction i.e. the energy of least tightly bound electron in the lattice. However if the energy of the photon is higher than W then it can eject the electrons that are more tightly bound in the... | {
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Why is 3D stress tensor acting only on three surfaces? I'm trying to learn about the stress tensor (in 3D)
The tensors are said to have directions (the first subindex $i$ in $\sigma_{ij}$) and specify the surface upon which they act (the second subindex $j$ in $\sigma_{ij}$).
What confuses me is why is it defined only... | That diagram shows only that set of three arrows for clarity: it is the smallest set that displays all the information necessary, but adding arrows to all six faces would make the diagram too crowded.
In general, if you have a material with a given stress tensor $\sigma_{ij}$, and you look at one small internal surface... | {
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How to predict length of conductor? A long round conductor of cross sectional area $S$ is made of material whose resistivity depends only on a distance $r$ from axis of the conductor $\rho=\frac{\alpha}{r^2}$, where $\alpha $ is constant .Find the resistance per unit length of such conductor .
My work
$$dR=\frac{\rho... | I think you have set out the problem incorrectly from the start: Instead of working with the resistance of the shell between $r$ and $r+dr$ and length of material $l$, first work with the conductance, $G(r)$, which is the reciprocal of the resistance. Because the shells are "in parallel" with each other, conductance's ... | {
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Does a higher water volume increase pressure? I am constructing a gravity flow water system. I have 100ft point where I can put my tank. My question is does the size of my tank matter? I am using a 1" pipe. Will I get more pressure if I use a bigger tank? For example what is the difference in pressure if I use a 10 gal... | If you have a tank with an outlet at the bottom then the pressure in the outlet is the same as the pressure in the water at the bottom of the tank.
The pressure $P$ at the bottom of the tank is $P=\rho g h$ where $\rho$ is the constant density of water, $g$ is the acceleration due to gravity, and $h$ is the height of y... | {
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When should you jump off a falling ladder? If you stand on the top of a falling ladder you will hit the ground at a higher speed (and therefore presumedly sustain more injury) if you hold on to the ladder than if you jump off it.
This was solved
here.
Where is the "break even" height on the ladder, from where you will ... | If one has the presence of mind, I vote for sliding down the ladder at any cost - gloves and shoes acting as a control brake. Shortening the diameter (height of your feet from ground) will surely reduce any impact. The risk is loss of control in the vertical decent. Gloves might be a recommended safety feature of an... | {
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Do uncertainty relations for charge or mass exist? Is there a uncertainty relation for charge $q$ of the form
$\Delta q \Delta? \geq \hbar$ in quantum mechanics? From checking the units ($[q] = A\cdot s$) I guess that $?$ would have to be the magnetic flux $[\Phi] = V \cdot s$, so we would have
$$
\Delta q \Delta \Phi... | Regarding electrical charge the answer is definitely negative: In Quantum Mechanics there exists a so-called superselection rule of the charge which requires that the charge is always definite in every quantum state of any quantum physical system carrying electrical charge. So $\Delta q_\psi =0$ in every state $\psi$ ... | {
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Deceptively simple mass-spring problem? This question is inspired by two other, similar, so far unanswered questions (posed by different OPs).
Mass $m_2$ sits on a incline with angle $\theta$ that provides just enough friction for it not to start sliding down. It is connected by a massless string $S$ and perfect sprin... | If $m_1$ is too small. The mass fall down and bounces back up. The force on the masses is highest at the lowest point of the bounce. A mass that is just large enough will also bounce back but at the lowest point of the bounce the force will just be large enough to move $m_2$. So we have to find an expression for the fo... | {
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The meaning of covariant but not manifestly covariant What is the most general meaning of the expression covariant, but not manifestly covariant? Suppose I have a general (local) change of coordinates, $x^{\prime} = f(x)$, on an $(n+1)$-dimensional smooth manifold on which some classical fields are defined, say $A_{\al... | In my experience, we usually call an expression manifestly covariant under some transformation if all the objects which appear in the transformation transform as tensors (or tensor fields) under the transformation.
For example, Maxwell's equations are not manifestly covariant under Lorentz transformations when written ... | {
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Is an EMF more/same/less in an insulator than in a conductor? Is an EMF (electromotive force) more/same/less in an insulator than in a conductor?
For example: A loop of copper and a loop of plastic in a changing magnetic field.
In which will the emf be the greatest?
| EMF induction depends on change of flux or condition of motional emf if the conditions are same then emf induction is same for all material as faraday's law is being followed irrespective of material medium.
| {
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Does gravitational time dilation happen due to height or difference in the strength of the field? The reason why you have to tune differently the atomic clocks in GPS is because the GPS is higher or because there is less gravity there, or both? In other words in a constant gravitational field which doesn't differ with ... | The time dilation is due to a difference in the gravitational potential energy, so it is due to the difference in height. It doesn't matter whether the strength of the gravitational field varies, or how much it varies, all that matters is that the two observers comparing their clocks have a different gravitational pote... | {
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Why my touchscreen doesn't respond to a pencil? I noticed my phone responds to my finger, but not to a plain graphite pencil.
Why is that, what is its physical reason?
How are styli able to approximate the human touch? What kind of conductive materials are being used there?
| Capacitive touch screens work by measuring capacitance at a number of locations on the screen, then interpolating between them to get a precise position. The simplest form of capacitor is two parallel conductive plates on opposite sides of an insulator, with the capacitance being proportional to the area of intersecti... | {
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Coupling constants in QFT equations of motion My question is about the coupling constants that appear in the quantum analogue to the equations of motion in a classical field theory (the Schwinger-Dyson equations). As a concrete example say we have the Lagrangian
$$ \mathcal{L} ~=~ \frac{1}{2} \left ( \partial_{\mu} \ph... | Yes, the field equations are ill-defined and must be renormalized. This has been done for simple theories like $\phi^4$ and QED in old work by Brandt and Zimmermann around 1970.
Hovever, their developments have been subsumed into the concept of operator product expansions (OPE), which are the modern analogue and exten... | {
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Are Black Holes Neutrino Factories? Are Black Holes Neutrino Factories? That is.. are they an intense source of neutrinos? If so, then the center of almost every galaxy would be a neutrino factory.
| In isolation, black holes probably won't generate reactions powerful enough to produce neutrinos. However, in cases where there is a ring of gas around the black hole (called an accretion disk, and likewise the system is now referred to as an accreting black hole), then they probably will produce neutrinos. Likewise, a... | {
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Kinetic energy of electron in metals Will it be correct to relate temperature of metal with kinetic energy of electron in metal just like as we do to find kinetic energy of gas molecules if we know the temperature by using the following relation:
$$E=\frac32\ k\ T$$
where $E$ = kinetic energy,
$k$ =Botzmann constant, a... | If you use the expression you mention you are implying a classical treatment. Use the expression you mention. You should be able to the wavelength larger than the separation implying a quantum mechanical treatment for the electrons in metals.
| {
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Does cosmic activity outside our observable universe affect us? I am not a scientist or physicist, but i was wondering if the cosmic activities or events which might be on a super massive scale happening outside our observable universe, also affect us?
Like the gravitational waves is due to black holes, which affects u... | The observable universe is named that way, because this is the universe where light has reached us, this is what we can see. Since everything we know of travels at a speed less or equal to the speed of light (including gravitational waves), it should follow that nothing can affect us that is not inside the observable u... | {
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Neutron-Antineutron Annihilation Is the process $n + \bar{n} \rightarrow \pi^{+} + \pi^{-} + \pi^{0}$ possible?
| Nice question. As anna pointed out if neutron and anti-neutron are throwing together with some MeV, no question they will splash into different particles and EM radiation.
But what means annihilation? Usually it means the attraction of charged particles from different sign, for example an electron and a positron. Durin... | {
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Why do clouds generally look flat at the bottom? Clouds on a still summers day generally look flat at the bottom and fluffy on top. Why? I was asked the question a couple of days ago and he ventured that it had something to do with the density gradient of air. Is he right? I suspect he is, but wanted a more informed op... | As hot moist air rises, there is a specific height where the gaseous water vapour begins to condense into a mist of tiny suspended liquid droplets.
There is not a specific limit to how far this misty air can be carried upward by air convection (producing billowy cloud tops), but if it falls below that specific height ... | {
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Why does image noise happen? Why are digital images shot with a high iso (which means the camera sensor is much more sensitive to light) so much grainer than pictures taken with a lower iso? Here is an example of the type of grain I'm talking about.
At first I thought this was a limit on the cameras precision, but the... | On the other hand, there are some forms of noise that are a property of the light.
For instance, if you have a detector that is just counting photons per pixel, and during the detection time you average $N$ photons, counting statistics says that number will vary by $\sqrt N$. (For Poisson distributed photons--there are... | {
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Volume charge density of an electric dipole
My attempt:
$$\rho(\overrightarrow r) = Q\delta^3 (\overrightarrow r)$$
$\overrightarrow r =$ separation vector from the origin to $\overrightarrow a$
$Q$ = the total charge of the electric dipole
Here's what I don't understand though. The total charge of the dipole is $-q +... | Note that whatever your answer is, when you integrate over a region of volume $V$ containing the charge $-q$, but excluding the positive charge, your total charge should be $-q$, since there is exactly that much charge in that volume. If there is a total charge $-q$ in a volume $V$, the charge density in that region is... | {
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Why does increasing resistance decrease the heat produced in an electric circuit? If $H=\frac{V^2}{R}{t}$ ,then increasing resistance means decreasing the heat produced.
But, isnt it that the heat in a circuit is produced due to the presence of resistors? Moreover metals with high resistances are used as heating elem... | You are just messing up with the equations. In order to state a proportionality relationship between a physical quantity and some other quantities, you need to be make sure that the quantities are all independent.
In your equation, the voltage and resistance are not independent quantities, but voltage is a function ... | {
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How does the law of conservation of energy explain magnetism? The law of the conservation of energy describes that energy cannot be created nor destroyed however in fact only changes form.
How does this law explain the energy transferred by magnetic fields?
| Magnetism is not explained by conservation of energy. But magnetism is consistent with conservation of energy.
It is commonly stated that the magnetic field does no work. It is true that the magnetic field does no work on charged particles through the Lorentz force law. The force delivered on a charge due to a magnetic... | {
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Formula for lens periscope I want to build a periscope.
To my great surprise even after intense googling I could find little relevant information about periscopes anywhere on the internet.
Let's say I want to build a periscope with height h, apparent height h1, diameter d and viewing angle v. What other variables shou... | What you are trying to do is build the combination is a terrestrial refracting telescope and a periscope.
As the design of the periscope is straight forward so you need to look up terrestrial refracting telescope to get some idea of the linear dimensions between lenses and the focal length of the lenses.
There is some... | {
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Origin of the force of gravity Where does exactly gravity come from when bodies exert this force? Does it come from the center of every object or it arises from their surface?
| It is unknown what physical mechanism "causes" gravity, and indeed the answer to that will depend a lot on the model of gravity that you choose. If you use Newtonian gravity, you assert "action at a distance" that is in some sense caused by the mass itself, but it is known that this is not a complete description, now ... | {
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Showing that pseudorapidity equals $\ln \left(\frac{\sqrt{p_z^2 + p_t^2}+p_z}{p_t}\right)$ I found one method to calculate pseudorapidity, $\eta$, using the formula
\begin{equation}
\eta = - \ln \tan \frac{\theta}{2}
\end{equation}
And I know how we can get to this point from Lecture 7 - Rapidity and Pseudorapidity i... | There is a trigonometric identity for the tangent of a half angle
$$\tan\frac{\theta}{2} = \frac{\sin\theta}{1 + \cos\theta}$$
which you can use to expand the first formula.
Then it's just geometry.
| {
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Why use 2 nonlinear crystals in conjunction for SPDC? I've seen a few articles (ex: 1, 2) that use either two BBO crystals consecutively or a BBO and a KDP crystal to create entangled photon pairs from Spontaneous Parametric Down-Conversion (SPDC). And from this answer, I understand that it's used as a trick because ty... | There are some good answers to related questions. So I probably don't need to go into too much detail. Spontaneous parametric down-conversion will usually produce a pair of entangled photons. The questions is, in terms of which degree of freedom are they entangled? Type I phase matching produces the two photons with th... | {
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How do we prove or disprove that a particle has no internal structure? In many pop physics books I have read that an electron has no internal structure.
How do we know that and how can we rigorously prove that it has no such structure at all?
| We study it using deep inelastic scattering. It was this type of experiment that first revealed the proton had an internal structure, and if the electron has an internal structure it will be this type of experiment that reveals it.
No experiment has yet discovered evidence for an internal structure in the electron, how... | {
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Why doesn't electric charge immediately leak off charged objects? I will focus my question with a particular example: a metal sphere, surrounded by vacuum, is given a negative charge. I know that when this charge is great enough, electrons will be emitted from the sphere, but why is the threshold for this so high? As I... | The threshold for electron emission from a metal is so high because at room temperature the work function of the metal of several 1eV (which acts as an emission barrier) is far above the thermal energy kT=0.026eV of the electrons in the metal. Therefore, at room temperature, only a minuscule fraction of electron can ov... | {
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How can the work done in bringing a charged particle from infinity to a grounded conductor be zero? Suppose we have a grounded conducting sphere (potential of sphere = 0) and we bring a charged particle to its surface.
Since work done by us would be equal to the change in potential energy of the charge, it will be equa... | The potential at a point is the work done in order to bring a unit positive charge from infinity to that point, without any acceleration. Electric potential is a location-dependent quantity that expresses the amount of potential energy per unit of charge at a specified location.
For a conservative field, we have by d... | {
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How can Noether's Theorem be used to prove that the probability density satisfies a continuity equation? How can I use Noether's Theorem to show that the probability density $\rho (x)=|\psi(x)|^2$ for a wave function $\psi(x)$ satisfies the continuity equation $\frac{\partial \rho}{\partial t}+\nabla \cdot\vec{j}=0$, ... | First note that Schrödinger's equation can be understood to come from an action. The Lagrangian is
$$L = \int~\mathrm d^3x \,\,\psi^†(x) \left(i \frac{\partial}{\partial t} - \frac{\nabla^2}{2m}\right)\psi(x) - \psi^†(x)\psi(x)V(x)$$
The Euler-Lagrange equation for $\psi^†(x)$ is exactly the Schrödinger equation. Since... | {
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If you are vacuuming your carpet and you wrap the cord around your body do you become a magnet? If you wrap an active electric cord around your body, do you become an electromagnet?
| Okay, accuse me of having too much time on my hands, but here's what I did:
If you can't tell from the pic. I wrapped the vacuum cord around a steel bar. I turned on the vacuum and tried to pick up the screw. Absolutely nothing not even a hint of attraction so maybe BowlofRed has a point.
In case the comment gets dele... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/279783",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "85",
"answer_count": 4,
"answer_id": 2
} |
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