Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
What size particle would be attracted to this 340 ton sculpture's gravitational field? I know that gravity is a weak force, but this rock has mass, so it has gravity. The Levitated Mass sculpture is big enough to attract something. What is the largest particle that would "stick" to the underbelly of this sculpture beca... | As many here have noted, Earth poses a problem for your experiment.
So, let's remove it$^\dagger$.
You now have a freely floating rock in space of mass $M = 3.4\times10^8\,\mathrm{g}$ and radius $R = 330\,\mathrm{cm}$.
The escape velocity of the rock is
$$v_\mathrm{esc} = \sqrt{\frac{2GM}{r}},$$
where $r$ is the distan... | {
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"timestamp": "2023-03-29T00:00:00",
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What is the metric tensor for? I am wondering how to use the metric tensor, in practice? I read the book and done the exercises in A student's guide to vectors and tensors by Dan Fleisch. The concept of a tensor and their applications are well defined.
In that book, is explained how to get the metric tensor for coordin... | The metric is an important concept in general relativity.
In GR, vectors correspond to weighted directions in spacetime (by "weighted", I mean any scalar multiple of a vector corresponds to the same direction, but weighted differently). The metric tensor can then tell us about the angle between two directions or the m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/229108",
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How are orbits made stable? I understand the concept of object staying in some orbit due to centrifugal force and gravity.
However I do not understand how is orbit of a body like satellite or planet has perfect balance between gravitational pull and centrifugal force of revolution?
because if the angular velocity is ... | In 2 dimensions, in any uniformly rotating frame of reference, there are two fictitous forces, the centrifugal force and the Coriolis force. The centrifugal force pulls directly away from the point of rotation is has the magnitude of $mr\omega^2$ where $m$ is the mass of the object; $r$ is the distance away from the ce... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/229249",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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What came first, the Universe or the Physical laws that govern the Universe? This sounds like the Egg and the Hen question but I am curious about this. If universe came first and created physical laws for itself, then what created the law or the principle as a consequence of which the universe came into existence in th... | This is a metaphysical/philosophical question, imo.
There is the platonic ideals school, in this case read for ideals=mathematics, which postulated that ideals existed and nature fell into their form. I have seen a number of theoretically inclined people who are really of that school. One does not have to think of the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/229389",
"timestamp": "2023-03-29T00:00:00",
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Kinetic energy of a rotating object in an exercise, a linear molecule is being subject to a force applied on the edge in its axis. Then $K_1=\frac{1}{2}mv^2$, all is well.
Then in the second point of the exercise, the force is applied on the same edge but in an orthogonal direction to its axis. Then the molecule begins... | The work done by the force in the first case is just translational, i.e. integral of the force over some distance and this gives rise to the translational kinetic energy.
In the second case, there are two types of work done on the system, first being the usual translational and second is rotational. This is due to the ... | {
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"url": "https://physics.stackexchange.com/questions/229438",
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Heisenberg's uncertainity principle In the Heisenberg uncertainty principle,
$$\Delta x \cdot \Delta p \geq \frac{h}{4\pi}$$
The values of $\Delta x$ and $\Delta p$ are the standard deviations which we get from the probability distribution function of the particle and I heard that it has nothing to do with the measurin... |
Actually while measuring, the Probability Distribution function of a particle also changes, Does this means that the measuring instrument has some effect ?
The measuring process may change the boundary conditions of the solutions of the quantum mechanical equations of the system under measurement, so the complex co... | {
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What's the reason double-slit experiment can't be explained by edge effects rather than quantum interference? Say we had exactly this...
But instead, it was a PING PONG GUN (imagine as table tennis players use to train),
throwing out PING PONG BALLS. The two slits are say 20 cm wide, and the observing screen is say 5... | We can't explain it like you want, because try closing one of the slits first. Then do the experiment. Then do the same for the other and do the experiment. Classically, you'd expect, that both slits will function independently, hence you won't receive an interference pattern but instead a summation of intensities from... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/229760",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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What would be the view like from inside a black hole looking towards the event horizon? Ignoring the fact that we would be torn apart by gravitational gradient and assuming we get some time to make some observations before hitting singularity, what would we see looking towards the event horizon or in any other directio... | The answer to this question is covered in the book "Exploring black holes: Introduction to General Relativity" by Taylor & Wheeler (2000), within the framework of classical General Relativity.
If we are talking about a supermassive black hole, such that a free-falling observer can survive tidal forces as they approach ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/229868",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
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What would it be like if the supernova ASAS-SN-15lh was in the Milky way? I'm simply wondering what it would be like if the super nova ASAS-SN-15lh (http://www.sciencemag.org/news/2016/01/universe-s-most-luminous-supernova-was-50-times-brighter-milky-way) was in our milky way, Would it bring to earth as much light as t... | From Wikipedia, from New Scientist
According to Krzysztof Stanek of Ohio State University, one of the
principal investigators at ASAS-SN, "If it was in our own galaxy, it
would shine brighter than the full moon; there would be no night, and
it would be easily seen during the day."[6]
[6] https://www.newscientis... | {
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What does diagonalization mean here? In a gravity theory in spacetime, the metric has signature $− + +· · ·+$. Concretely this means that the metric tensor $g_{μν}$ may be diagonalized by an orthogonal transformation, i.e. $$(O^{-1})_{μ}^{\;a} = O^a_{\;μ}$$and $$g_{μν} = O^a_{\;μ}D_{ab}O^b_{\;ν}$$ with positive eigenva... |
In a gravity theory in spacetime, the metric has signature $− + +· · ·+$.
That's a convention. Other conventions are that it has signature $+ - - -$.
Concretely this means that the metric tensor $g_{μν}$ may be diagonalized
The signature doesn't tell you that it is diagonalizable. The fact that $g_{\mu\nu}=g_{\nu\m... | {
"language": "en",
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Can focused light be treated as a point source? Imagine there is a uniform, collimated beam coming from a distant light source. This beam passes through a lens and is focused to a point at the focal length. Can this "point" be treated as a point source of light as you move away from the focal point in the same directio... | If, indeed, the light is collimated (note, there are limits to how well collimated a beam of light can be because of diffraction) and the lens that focuses it is so-called "diffraction limited" (ie. the lens doesn't abberate the light) then the spot created at the focus does obey the inverse square law. However, keep ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why are four-legged chairs so common? Four-legged chairs are by far the most common form of chair. However, only three legs are necessary to maintain stability whilst sitting on the chair. If the chair were to tilt, then with both a four-legged and three-legged chair, there is only one direction in which the chair can ... | Because the seat is usually square.
If it was round (or another shape) it would be harder to make and harder to attach the back (or extend the back legs to make a back).
If it had no back, it would be a stool. Stools can have three legs.
(Not all answers have to do with physics!)
| {
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"timestamp": "2023-03-29T00:00:00",
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Total number of primary maxima in diffraction grating I am trying to determine the total number of primary maxima that can be observed when light of wavelength 500 nm is incident normally on a diffraction grating, with the third-order maximum of the diffraction pattern observed at 32.0 degrees.
Rearranging the diffra... | any angle over theta 90 will mean that the diffraction will be going behind the diffraction gratings which is impossible. so 90 is the maximum that you can get this is why you have to round down the decimal answer you will get.
| {
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Why do products of nuclear decay have a lower mass than the original nucleus, when the sum of the mass of its nucleons is larger? I've just started covering the topic of binding energy in Year 13 at school (final year before University).
The definition we've been given of binding energy is that it is the work done when... | The mass defect and the binding energy are not linear functions of the number of nucleons. They increase until the iron and then decrease.
See the Figure 31-5 of The Mass Defect of the Nucleus and Nuclear Binding Energy
The thorium Mass Defect for one Average Nucleon ( MDAN ) is higher than an uranium MDAN.
Uranium de... | {
"language": "en",
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Why are these equations valid despite seemingly inconsistent units? I am having quite a difficult time in trying to understand what units are used in this paper and how to convert things to SI. For example, look at equation (1):
$$T_M \approx 1500 \rho^{1/3}\ \mathrm{K}\tag{1}$$
It seems to be showing that temperature ... | Each equation contain a different arbitrary constant: 1500, 2800, and 5 E20.
It can be assumed that each arbitrary constant has exactly the right units to make everything come out right...
It is sloppy to not specify the units of these constants...
Edited for example:
I could conduct experiments on the dynamics of fa... | {
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Why don't simple circuits cause the electrical breakdown of air surrounding them? Electrical breakdown of air occurs when the electric field exerted by a charged object exceeds the electrical breakdown limit of air which is $3.10^6 V/m$. Since even 1 Coulomb of charge can exert an electric field whose magnitude can be ... | Wires in a circuit are electrically neutral. They have as many positive charges as negative charges, so the net charge is zero.
There is a spatial distribution of charges within a wire. The charge is not uniform across the cross section. But that variation in charge density is fairly small. Given the overall neutra... | {
"language": "en",
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A mass in a rotating tube A mass rotates on a horizontal surface inside a frictionless hollow tube with a angular velocity omega. The only force acting on it is a force $N$ with which the tube pushes the mass.
It is expected that the mass would move away from the center of rotation due to centrifugal force, which is a... | The mass will move under the influence of the normal force exerted on it by the tube walls. In order to find the motion of the mass that force would have to be specified in some way. One option would be to specify constant torque of a given magnitude. In your case, however, you seem to be specifying a kinematic conditi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/231417",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Would you save energy by heating the air in a shower stall so that you could use colder water? It is refreshing to take a cool shower in hot weather. And for the sake of discussion, lets assume that one should be "comfortable" with temeratures when taking a shower.
Considering that the vast majority of the heat from a ... | Consider that the specific heat of water is 4.147 kJ/kg. The specific heat of air is less than 1/4 of that, so for the air to heat you as much as the water, you would have to heat the air to a much higher temperature. That's one reason, along with conduction, that Arctic explorers say that water cools you 30 times fast... | {
"language": "en",
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Charge inside a charged spherical shell
*
*If I were to put a negative charge inside a negatively charged spherical shell, will it move to the center?
*Electric field inside the shell due to the shell is zero (Gauss's Law), would that mean the charge inside the sphere faces no force?
But, that doesn't make intuitiv... | So, this is an interesting property of the mathematics of a force that diminishes like $1/r^2$ in 3D-space: if you have a uniform charge distributed over a sphere, that charge exerts no forces inside the sphere; they all balance out. Furthermore the field outside the sphere behaves as if all of the charge on the sphere... | {
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Showing the annihilation of an electron-positron pair How to show that the annihilation of an electron-positron pair under emission of one real photon (mass zero) is forbidden by energy-momentum conservation, the emission of two photons is allowed?
| The electron-positron pair has a center-of-mass reference frame where the momentum is 0.
Obviously, there exists no one-photon system with positive energy which has 0 momentum, as the energy-momentum relation for a photon is $E = p c$.
| {
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Dependence of average speed of molecules of gaseous mixture We know that the average speed of gases in a single gas chamber is given by $\sqrt{8RT/\pi M}$
where R is universal gas constant,T is temperature,M is molar mass of gas.
But what if we mix two gases in any ratio say 1:1 and then try to find average speed of an... | The parameter which is important is the average kinetic energy of the molecules.
When the two gases mix they will move to a state where the average kinetic energy of all the molecules is the same.
So go for something like the total kinetic energy before mixing is equal to the total kinetic energy after mixing which wil... | {
"language": "en",
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Do wormholes only allow FTL travel in "folded" spacetime? Faster than light wormhole travel depicted in science fiction and in popular science articles seems to assume that rather than being flat (with dimples centered around large masses), spacetime is fairly lumpy in order to facilitate wormhole shortcuts.
Is there a... | There is nothing in physics that describes the sort of folding shown in your picture. I'm afraid it is an invention of the Science-Fiction community.
The best tool we currently have for describing spacetime is general relativity, but GR does not and cannot tell us anything about the global topological properties of spa... | {
"language": "en",
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Why aren't $\hat{x}$ and $\hat{p}$ considered functions of time in the expectation value? In Griffiths Intro to QM (2nd edition), he gives the equation
$$
\frac{d}{dt} \langle Q \rangle = \frac{i}{\hbar}\langle [\hat{H},\hat{Q}] \rangle + \left\langle \frac{\partial{\hat{Q}}}{\partial{t}} \right \rangle
\tag{3.71}
$$
... | $\frac{\partial\hat{Q}}{\partial t}$ denotes the partial derivative of time, which is nonvanishing only when $\hat{Q}$ manifestly depends on time. Every operator $\hat{Q}$ can be time dependent in an implicit way such as $\hat{x}\hat{p}$ which can be time dependent when $\hat{x}$ or $\hat{p}$ depends on time.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/232021",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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Maximum work obtained by mixing 2 gases
Two boxes containing the same number of moles of two ideal identical
gases with the same adiabatic index (this is given as $\gamma$), at
the same initial temperature $T_i$ but with different volumes, $V_1$
and $V_2$, are brought together. Find the maximum mechanical work t... | You have this doped out pretty well. To get the maximum work out of it, you can manually hold an adiabatic partition between them and allow the gases to move the partition very gradually until the pressures equalize. The work that the partition transmits to your hand will be the maximum work. This is the same as the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/232215",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
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Metric components transformation under change of coordinates I have been studying Lie derivatives and some applications. While searching the web I found a reference with the following statement:
For a general Riemannian manifold $M$, take a tangent vector field $k=k^\mu \partial _\mu$ and consider the infinitesimal co... | The metric being a rank $(0,2)$ tensor transforms under general coordinate transformations $x^\mu \to x'^\mu(x)$ as
$$
g'_{\mu\nu} (x') = \frac{ \partial x^\rho}{ \partial x'^\mu } \frac{ \partial x^\sigma }{ \partial x'^\nu } g_{\rho\sigma} (x)
$$
Now set $x'^\mu (x) = x^\mu + \alpha k^\mu(x)$ in the above expression... | {
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If there are long-lived elements in the Island of stability, why are they not present in Nature? To my understanding, some (but not many) physicists speculate that the Island of stability may contain long-lived elements, as in a billion or so years. But couldn't we rule that out just by the nonexistence of such element... | Producing ultra-heavy elements in nature is not easy. So their absence "in nature" does not mean they cannot exist or cannot be created given the right conditions.
Some details:
The valley of stability becomes increasingly n-rich, so neutron capture reactions are essential.
To get beyond lead requires rapid neutron cap... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/232615",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
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Build a device that can set voltage according to intensity of the light shined I want to build a device that uses photodiodes and photoresistors to set the voltage in the circuit that shines a bulb. The idea is simple. When the light is intense, the bulb should shine less, when the light is less intense the bulb should... | Assuming the bulb emits in the visible wavelength regime, then you can use a silicon photodiode. Depending upon the particulars of your "bulb" you could put the photodiode in series with a resistor and put that combination in parallel with the bulb. So as the photodiode receives more light, it's resistance will decre... | {
"language": "en",
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What is the physical intuition behind the Bragg peak? The wikipedia page says:
Energy lost by charged particles is inversely proportional to the
square of their velocity, which explains the peak occurring just
before the particle comes to a complete stop.
What is the physical reason why the energy loss goes inver... | See if this argument works - I am making this up on the spot so there is definitely space for argument...
Most of the interactions with the electrons will not be "head-on collisions" but rather electrostatic interactions. If we get to a certain distance of an electron, it will feel the force and undergo acceleration. I... | {
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time step in kinetic montecarlo simulation I have a doubt about how the time step is calculated in kinetic montecarlo simulations.
One state with index $i$ is connected to other $N$ states, indexed by $j=1...N$, by transitions that happen with rates $r_{ij}$ (form $i$ to $j$). At each iteration of the algorithm, one of... | In kinetic Monte Carlo, the idea is to describe a trajectory as a set of events, at which the system makes a transition from one state ($i$) to another ($j$). To generate such a trajectory, we need to randomly select both the states that are visited and the intervals between them.
The time interval $t$ between a pair o... | {
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Torque: The T-shaped stick problem This is a problem that I have been unable to solve for some time.
Imaging a T-shaped stick, as shown in below image, which do not deform in any appreciable way and has pivot point at the tail of the "T".
There are two questions in my problem, the first is: given that the "T" is symme... | If a force $\vec{F}=(F_x,F_y)$ is applied at a location $\vec{r}=(x,y)$ then the torque at the origin is
$$ \vec{\tau} = \vec{r} \times \vec{F} \\ \tau = x F_y - y F_x$$
All you need to do is sum up the torques at the pivot for the different situations in order to understand how this mechanism will move.
If you have tw... | {
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Coefficient of friction and practical experience of sliding The classical model of friction has a coefficient of friction depend only on the materials, but not area, and the force proportional to the normal force and coefficient of friction. So a given object on the same surface has the same friction whether it is supp... | My guess would be that if you're sitting, the surface of contact between you and the slide is smaller, thus the pressure is higher than if you were to lay on your back. Your jacket is a deformable material, its friction coefficient with steel might vary with applied pressure.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Conditions for the tension to vary in the rope What are the conditions for the tension to vary in the rope. I have read below conditions
1. rope has to have some mass
2. rope is accelerating
I get the 1st one, but I am not sure if I get the 2nd condition.
If a $10\ \text{N}$ force accelerates a mass-less role, what wi... | Since the LINE, a section of ROPE, has no attached mass or restraint and it seems it is accelerated as a whole, there is no tension. If one end is pulled to accelerate it then F=ma=0.0 N.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/233160",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Is the conservation of probability in the Schroedinger's equation unique? The Schroedinger's equation can be viewed as a diffusion equation with imaginary constants $a$ and $b$ satisfying,
$$\quad \Psi_t=a \cdot \Delta \Psi-b \cdot V(x,t) \cdot \Psi
\tag{1}
$$
However if $a$ and $b$ are positive real coefficients, we g... | For time independent potentials there is a field of mathematics studying this. The answer is no for physically reasonable potentials. Look for Schrodinger semigroup.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/233304",
"timestamp": "2023-03-29T00:00:00",
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Why can't electrons leave negatively charged objects in a vaccum? If an object is negatively charged, and electrons repel each other since they have like charges, why doesn't that negatively charged object "kick away" electrons?
Why doesn't diffusion occur? Why does there need to be a conductor for the electrons to go ... | Matter is full of positively charged protons, and those positively charged protons attract the negatively charged electrons. In general, if you take a chunk of neutral matter and add one extra electron then the attractive force due to the protons in the matter outweighs the repulsive force due to the electrons in the m... | {
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how is a pendulum clock's time and the time period of the pendulum in it related? I'm working out how much time a pendulum clock will gain or loose due to change of the length of the pendulum due to temperature. so far I've got, new time period, $$T_2=T_1(1+\frac12\alpha\Delta T)$$
due to $\Delta T$ change in temperatu... | You are on the right track.
For a mechanical pendulum, the relationship is linear. You don't need to know how many swings of the pendulum corresponds to how many seconds. If the pendulum is x% slower, it will report x% fewer seconds per day.
Now since length goes as $\ell = \ell_0(1+\alpha \Delta T)$ and period of pend... | {
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What would happen if I gathered stellar sized masses of iron? Lets say I had a bag that when turned upside would start pouring out iron shavings and never ever stop. Viola, there's my infinite source of iron. Now, lets say I just continued to dump this iron together until I had a ball of iron the size of earth, Jupiter... | Supernova happens when the core of a supermassive dying star starts fusing iron and heavier elements under massive gravitational pressure. The reaction is endothermic, unlike the fusion of lighter elements (iron is the peak) so the resulting outward radiation pressure stops apposing inward gravitational pull and the st... | {
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Why is the Pythagorean Theorem used for error calculation? They say that if $A = X \times Y$, with $X$ statistically independent of $Y$, then
$$\frac{\Delta{A}}{A}=\sqrt{ \left(\frac{\Delta{X}}{X}\right)^2 + \left(\frac{\Delta{Y}}{Y}\right)^2 }$$
I can't understand why that is so geometrically.
If $X$ and $Y$ are inter... | The general formula for error propagation is:
$$\Delta f(x_1,x_2,\ldots)=\sqrt{(\frac{\partial f}{\partial x_1}\Delta x_1)^2 + (\frac{\partial f}{\partial x_2}\Delta x_2)^2 + \cdots}$$
where $\Delta m$ means "standard deviation of lots of repeated measurements of m".
Where does this come from? By calculus, when all th... | {
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Cause for spikes in Trinity nuclear bomb test In Richard Rhodes' book, The Making of the Atomic Bomb, I was reading about the Trinity nuclear test. High speed photos were taken and this one is from <1ms after the detonation. The book mentions the irregular spikes at the bottom of the image, but does not explain them. I... | The answer is in wikipedia
The photograph on the right shows two unusual phenomena: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball surface.
The surface of the fireball, with a temperature over 20,000 kelvin, emits huge amounts of visible light radiation (m... | {
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Using Steam to lift a mass? I have a task set to create a system to produce the largest values for potential energy, mgh.
I am given 1 litre of boiling water and with this use the steam produced to lift the mass of weight that i can select. I am not aloud to add any energy to the system. I am not aloud to use pre-mad... | The maximum potential energy is limited by the energy content released by the one liter of boiling water. you are able to boil away all of the water - this gives you the total energy released.
The obvious answer is to follow Watt and build a steam engine - the height of the piston is the measure of the potential energ... | {
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Wavelength for imaging an ultracold atom? I was reading Stamper-Kurn's article Experimental Methods in Ultracold Atomic Physics (link). In the imaging section (page 13), he mentions:
Cold atoms are conventionally probed by optical imaging. Probe light at a well defined optical frequency
is sent through the atomic gas... | A good reference for choosing appropriate laser beam for a given type of atom, plz read Michal J Martin's PhD thesis at jila, Prof. Ye Jun's group site. Some moderator would not be happy with links so I delete it. Chapter 2 & 4 all provides condensed materials about laser wavelength.
| {
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"source": "stackexchange",
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How can "...electrons flow in metals, but not in the ground..." explain grounding rods? I really enjoyed Why is the charge naming convention wrong? But, in the comments at the very end, the statement that "...electrons flow in metals, but not in the ground..." left me uneasy.
I was taught that the physical process of "... | Ground is at 0 potential,so,it accepts electron from negative terminal.
And at very far place any positively charged electrode accepts electron from ground and current flow.
| {
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So how much fuel does a hypothetical fusion plant need? Reading about potential fusion plants it's easy to get the impression that we have nearly infinite access to the fuels required. But looking aside form how much we have available, I am interested in something I have a hard time finding information about; how much... | Factually we have next to no fuel for fusion reactors, neither hypothetically nor practically. All of the tritium that would be required will have to be bred either in fission reactors or the fusion reactors themselves. Since a fusion reactor does not have as many excess neutrons, special neutron multiplier enriched bl... | {
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Single slit diffraction - choosing a wavelength? For the classic experiment of determining the slit width of a single slit. If we assume the rough order of magnitude of the width is known. What factors determine the choice of wavelength?
(Clearly we want $\lambda<w$ where $w$ is the slit width, but what other factors c... | To answer your specific question
"I heard from someone that we should make the wavelength on the order of the desired resolution, I don't really understand how resolution comes into this as we are using monochromatic light. Can someone explain if and why this comment makes sense?"
In order to maintain Resolution for v... | {
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How do I remove the negative sign from this derivation? A homework problem required me to show that the first equation below can be written in the form of the second equation. It was all fairly simple except for the negative sign. I'm not sure how this is supposed to cancel out. Might there be some conceptual way that ... | There should be a negative sign in Equation 2. The exercise is a very simple one in substitution and does not require any sign cancelling.
| {
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Do electrons in an atom always have the same 'direction'? Perhaps speaking of direction of an electron isn't quite correct. But does QM indicates a kind of way whether all electrons are going e.g. 'clockwise' or not? Of course QM just gives a probability where the electrons are, but can you emerge whether they are goin... | In my opinion, in stable atoms part of its electrons should rotate in opposite direction of other electrons to maintain stable conditions.
This means that if it loose an electron or gain the atom charge will change and called ion.
In our old understanding only losing and gaining is from out orbit. I believe this is com... | {
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Can observation change entropy? I don't know whether this even makes any sense, but if 'observation' can be considered as 'recieving and reading information', can an act of observation (of a system) change (increase or decrease) its entropy?
| In my mind, to unravel this question, we need to consider the simplest scenario possible. So, let us disregard quantum systems and small systems and consider only statistical mechanics of big classical systems.
Entropy is defined as the log of the number of microstates at a given energy E (up to the Botlzmann constant... | {
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In a 2D CFT, is the free boson $X$ a primary field? A primary field $\mathcal{O}(w,\bar{w})$ with weight $(h,\bar{h})$ is defined by having the following OPEs with the stress tensor:
$$T(z)\mathcal{O}(w,\bar{w})=\frac{h\mathcal{O}}{(z-w)^2}+\frac{\partial \mathcal{O}}{z-w},\qquad \bar{T}(\bar{z})\mathcal{O}(w,\bar{w})=... | To see why the descendants are primary, you can use
$$
\partial\left(T(z)X(w,\overline{w})\right) = T(z)\partial X(w,\overline{w}) = \frac{\partial^2 X}{z-w} + \frac{\partial X}{(z-w)^2}
$$
And see that it is a primary field of weight $h = 1$, $\overline{h} = 0$, and similarly for the other field....
| {
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Evidence for quantum gravity from gravitational waves The rumor spreads that physicists will make their big gravitational wave announcement this thursday.
I am far from being an experimentalist, but I want to know if there is any chance that the mentioned observations will reveal any experimental evidence for quantum g... | Formula for the metric tensor of the Schwarzschild solution
$$g_{00}=-1+r_g/r:g_{rr}=\frac{1}{1-r_g/r},g_{\theta \theta}=r^2,g_{\phi \phi}=r^2sin^2\theta$$
Formula for the changing part of the pseudo-tensor energy density for the non-relativistic Schwarzschild solution
$$\Delta t_{00}=\frac{11}{2r_1^2}+\frac{15}{8}(\f... | {
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volume of the air bubble in the water How does the depth affect the volume (the radius) of an air bubble in the water, if the temperature and density of the water are constant. Is there any relation combining this?
Can I say that $dh/dt=dr/dt$?
| As you go deeper, the pressure increases, decreasing the volume of the gas in the bubble. In water, the pressure is appoximately $14.7(1+\frac d{33})$ psi where $d$ is the depth is measured in feet. The volume goes as the inverse of the pressure.
| {
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Why do we assert Hulse–Taylor binary system's orbital decay to gravitational waves and not radiation? From this link
The Hulse–Taylor system's orbit has decayed since the binary system
was initially discovered, in precise agreement with the loss of energy
due to gravitational waves. The ratio of observed to predic... | The emission of gravitational waves causes the separation $r$ between the two binary components to decrease. As they do so, the power emitted in gravitational waves increases as $r^{-5}$. Thus the rate of change of the orbital period is very non-linear, with $dr/dt \propto r^{-3}$.
Now, if the mechanism responsible for... | {
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So Black Holes Actually Merge! In 1/5th of a Second - How? I've read a lot of conflicting answers in these forums. However, today saw the awesome announcement of gravitational waves. Two black holes merged: http://www.slate.com/blogs/bad_astronomy/2016/02/11/gravitational_waves_finally_detected_at_ligo.html
Not only th... | Just like photons don't age but still move, black hole horizons don't age but move and can thus merge with other black holes.
The formation process of a black hole takes an infinite amount of time according to external observers, but that doesn't mean it needs to remain in its initial position. The frozen horizon moves... | {
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Does the existence (now proved) of gravitational waves imply the existence of Gravitons? I studied the theoretical part about the Gravitational waves in General Relativity (linearization of gravity and small perturbations of the metric and so on).
But I was wondering about: since electromagnetic radiation is composed/c... | No, think of gravitational waves as normal ocean waves. When you see a wave gliding across the sea, it's merely the result of a Force acting on that sea, not the seas creating the wave. There may be Gravitins, but they can't create a wave within themselves.
| {
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What is time, does it flow, and if so what defines its direction? This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address issues such as:
*
*What do physicists mean by time... | What is time? Ask ten different people and you'll get ten different answers. This is how I understand it. The word time is a term describing temporal motion. Time is what we do. Time doesn't flow, we time. We don't move through the time dimension, we time through the temporal dimension. Time is a verb.
It's our motion ... | {
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Why do detectors for gravitational waves have only two perpendicular arms, not three? I wonder why detectors for gravitational waves have only two perpendicular arms, not three. Having three arms appears to allow for better detection of direction, or may even increase sensitivity (I may be wrong). So far I came up with... | The basic idea is that you have destructive interference. This is hard(-er) to achieve with three photons as well as I don't see a trivial way to do the beam-splitting.
| {
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Proving charge on outer surface of parallel plate capacitor must be zero If we have two conducting plates, with charge $Q$ and $-Q$, why is the charge on the outer surfaces of each conductor zero?
I've been trying to wrap my head around the problem. Firstly, don't excess charges on a conductor spread out towards the su... | See if there will be charge on outer faces, then electric field inside plate will not be zero, which is not possible. So all the charges will be on the inner faces ( facing each other of parallel plate capacitor)
| {
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Gravitational waves in other dimensions I know this question is purely speculative, as we don't know if more dimensions do exist and also we do not know if gravity is indeed stronger in other dimensions (if they were to exist). But, one of the possible explanations of why gravity is so weak compared to other forces is ... | The other dimensions in those examples are quite small.
Imagine a long very unwide piece of paper. Now attached the long ends together so you get a tall narrow cylinder. It's like you have one direction where if you go that way you very quickly end up back where you started and one direction where you can walk very far... | {
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Doesn't rotational KE of a rolling marble change if there is no friction to provide torque? The question arise from the following situation:
A marble at the border of a uniform bowl begins rolling within it from rest. There is enough friction in the first half the bowl for the marble to not slip, but there's no frictio... | By suggesting that the RKE at the top is zero, you are suggesting that there is a change in RKE from the bottom and the top. However, just like you need a net force to change kinetic energy (i.e. do Work) you need a net torque in order to change the rotational kinetic energy. In this situation, because this part of the... | {
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Is the net force conventionally shown in a free body diagram? Is it standard convention to display the net force vector on a free body diagram? Internet searches seem to give mixed results.
| The method that I usually follow (it works perfect and is easy to understand and also to find errors if any):-
*
*Choose an observer (in most problems specially with pseudo forces) place your observer at a place which makes the motion of other bodies comparatively easier to consider. [PHYSICS is based on how you obs... | {
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When I open a window to air out the room, how does the smell disperse? Let's say I'm in a room with some kind of noxious stink, possibly of flatulent nature. The quickest way to right the world that comes to mind is to open a window. When I open a window, how do the stank particles leave the room?
| They leave slowly because of the air exchange via window, depend upon rate of exchange.
On a lighter note, even if you do not open the window, the smell will seem to disappear in a little bit :)
| {
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How is a potential divider able to reduce current to zero? For example, a filament lamp, in series with a uniform resistive wire, can have its voltage varied by moving the sliding contact, e.g., a rotatable wheel.
However, why is a potential divider able to reduce the current to zero, but a variable resistor in series... | A standard sort of potential divider has a slider which can travel from one end and produce a maximum potential across its output terminals and hence to the circuit under investigation and then when the slider is at the other end the voltage across its output terminals is zero and hence as no voltage is supplied to the... | {
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How do you tell whether a force acting on an inclined plane is going up or down in its perpendicular component to the plane?
I'm practicing mechanics, and I had to resolve the following forces perpendicularly to the inclined plane in order to work out the reaction force (plus the weight of the ball)
But I cannot tell ... | There is a trick that every mechanical engineer uses to solve the problems on mechanics
firstly draw a perpendicular at the point of application of the force on the plane.
then make an arrow along this line which you have constructed and along the plane. this arrow will point to the gross direction of the vector .
l... | {
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Spin $1$ vs. spin $1/2$
A crystal contains $N$ atoms which possess spin $1$ and magnetic moment $\mu$. Placed in a uniform magnetic field $B$ the atoms can orient themselves in three directions: parallel, perpendicular, and antiparallel to the field. If the crystal is in thermal equilibrium at temperature $T$ find an ... | A atom is constituted of fermions(proton, neutron and electron). But whenever the atom an atom has even no of constituents it behaves like a boson. This is very easy to understand. All fermions have spin 1/2. So even no of fermions will have integer spin and therefor behave like bosons. Thus atom with spin 1 is possibl... | {
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Is gravitational wave affected by gravitational field Does gravitational wave show gravitational lensing?
Does it bend around mass?
| At the moment only theoretical calculations can be attempted, as here
Strong gravitational lensing of gravitational waves in Einstein Telescope
Gravitational wave experiments have entered a new stage which gets us closer to the opening a new observational window on the Universe. In particular, the Einstein Telescope (... | {
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Effect of Gravitational Waves on light? We all know about the gravitational lensing effect. From the analogy of fabric of space time used to explain this concept to laymen like me, I understand that light follows the curvature of spacetime.
Following on that same line of thought process, gravitational waves would caus... | Probabily, it would be beneficial to first compare gravity, and gravitational waves (GW) as they pass earth's gravitational field.
To compare gravity, and GW, you can consider gravity as a permanent dip in space, while GW is a moving ripple in space.
Wherever this ripple passes, it minutely, and temporarily, changes t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/238226",
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Is the universe bounded? As I understand it nobody can pinpoint an objective "center" of the universe nor "where" the Big Bang happened. It seems the observable universe is limited by our event horizon at some 14 billion light years and my question is simply: If an astronomer was placed at one of the outermost visible ... | As the CuriousOne commented, the idea of the Big Bang is that it happened everywhere. It was not an explosion from one point in a spatial coordinate system at t=0, the spatial coordinates just started expanding (straining by the factor a(t)). Therefore all space points in our universe have similar histories in time. ... | {
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Is it wrong to say that an electron can be a wave? In QM it is sometimes said that electrons are not waves but they behave like waves or that waves are a property of electrons. Perhaps it is better to speak of a wave function representing a particular quantum state.
But in the slit experiment it is obvious to see that... | In the micro world particles like electron has dual nature .In some experiments it behaves like waves such as diffraction of electrons by a single slit but in other experiments like compton scattering or photoelectricity it behaves like particle.
In wavelike representation of electrons by a quantum mechanica... | {
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Force on current carrying square loop I'm asked to find force on square loop (side a) carrying current $I$, flowing counter clockwise, when we look down x-axis, lying in yz plane. the loop is centered at the origin. The magnetic field is given as:
$\vec{B} = kz\hat{x}$
Its solution states that force on left an right ca... | The minus is coming from the value of z. Note that the loop is centered at the origin on the yz plane, and the value of B is dependent on the value of z.
The forces on the left and right arms of the loop cancel out, the forces on the top and bottom arm are also opposite but due to the nature of the B field (dependence ... | {
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Is the local Lorentz transformation a general coordinate transformation? There is a saying in Nakahara's Geometry, Topology and Physics P371 about principal bundles and associated vector bundles:
In general relativity, the right action corresponds to the local Lorentz transformation while the left action corresponds t... | You did not understand correctly, although Nakahara's statement is slightly wrong. I looked into the book, because your quote is insufficient to determine what you or Nakahara are talking about.
Nowhere is Nakahara talking about a local Lorentz transformation being a general coordinate transformation. In the context of... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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How precise must the energies match for absorption of photons? According to Quantum Mechanics, in order for an atom to absorb a photon the energy of the photon must be precisely that of a "jump" between energy states of the atom.
How precise must it be?
If I create a photon with an energy within an error of 0.0001% of... | Agree with the above, but also if the atom, or collection of atoms, are in thermal equilibrium, then there is another broadening mechanism, besides lifetime broadening, called Doppler broadening that accounts for the motion of the atom(s). This has the effect to substantially widen the effective line width depending u... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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Finding the Force of two objects - by using Acceleration but only ONE of the given masses? I came across the following question in my physics textbook and wanted to try to solve it:
A 1700 kg car is towing a larger vehicle with mass 2400 kg. The two vehicles accelerate uniformly from a stoplight, reaching a speed of ... | "Force" is poorly defined here. The force the tow truck will need to exert to accelerate itself and the towed car with the desired acceleration will obviously depend on the sum of the two masses, but the force exerted by the rope on the second car (and hence the minimum required tensile strength of the rope) will only ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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From what wavelength can radiation go through a human body without very much changing? Gamma-rays can go through a body but they will ionize a lot of atoms (I don't know whether some of the gamma photons will go through without any interaction at all?). The same for X-rays. Visible light and infrared (till $1mm$) proba... | There's no uniform relationship between wavelength and extent of penetration/absorption. It's a quite complex function of both wavelength and material.
There are examples of low as well as high frequencies which penetrate, as well as some middle ones that don't. Also, from among the penetrating ones, some are also abso... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/239494",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How does the drift velocity of electrons in a conductor depend on the temperature? How does the drift velocity of electrons in a conductor depend on the temperature?
I have two contradicting views for this.
*
*First, we can say that increasing the temperature of the conductor will increase the kinetic energy of the... | You can think of it in simple terms.
The average kinetic energy of the lattice ions increases as the temperature increases.
Between "collisions" with the metal lattice ions the free electrons are accelerated by the electric field and so increase their velocity along the electric field direction.
However because the lat... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/239719",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why does wind sound the way it does? You can obviously hear wind. From everyday experience, the stronger the wind, the louder its sound. But this sound is always similar, and quite distinctive, noise-like. Can one compute the spectrum of the noise generated by the wind?
There is an inkling of an answer provided here... | I would recommend you to study Howe's Theory of Vortex Sound. Generally, an appearance of vorticity $\omega$ leads to conversion of a tiny part of flow kinetic energy in a pressure wave (really small part: proportional to $M^5$ in a free field). The main free field source is the divergence of the Lamb vector:
$$
\frac{... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Could IR and Raman be measured simultaneously? IR and Raman are measured using two different machines. Looking at the nature of excitation and at the way data is collected, it seems that these two measurements could be done simultaneously (or with some short time separation). Is that so?
(source)
| Assuming you are trying to do vibrational Raman and IR then these two experiments are done in completely different frequency regions. This can be seen in your diagram where the arrows in the Raman experiment are much longer (higher frequency) than the arrows in the IR experiment. Because of this you would need two diff... | {
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Free body diagram of pulley Is there any difference between the free body diagram of fixed pulley and movable pulley? I've read that both of the rope of fixed pulley and movable pulley have the same direction (both upwards or downwards). But, one thing that confused me: is it true that fixed pulley has T1 and T2, but m... |
Is there any difference between the free body diagram of fixed pulley and movable pulley?
Not particularly. The main thing is that you can assume the fixed pulley isn't accelerating, so all forces on it must sum to zero. A movable pulley may or may not be accelerating.
is it true that fixed pulley has T1 and T2, b... | {
"language": "en",
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Cosmology: equation of motion for a scalar field in conformal time So, I've derived the equation of motion for a scalar field in "normal" time, $t$:
$$
\ddot{\phi}+3H\dot{\phi}+\frac{dV(\phi)}{d\phi}
$$
Then, using the expressions for the scalar field density, $\rho_{\phi}$, and pressure, $P_{\phi}$, but transformed in... | The continuity equation is
$$
\dot{\rho} = -3H(t)(\rho + p),
$$
or, in conformal time,
$$
\tag 1 \rho{'} = -3aH(\eta) (\rho + p)
$$
By using the espression for $\rho$, we have that
$$
\tag 2 \rho{'} = -\frac{a'}{a^{3}}(\varphi {'})^{2} + \frac{1}{a^{2}}\varphi{'}\varphi{''} + \frac{dV}{d\varphi}\varphi{'} = -\frac{H}{a... | {
"language": "en",
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"source": "stackexchange",
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How do integral representations of $\mathbf A$ and $\Phi$ satisfy Lorenz condition? The following are the integral solutions of the potentials, obtained from the retarded potentials (by a Fourier transform):
$$\mathbf A (\mathbf r) = \frac{\mu_0}{4\pi}\int_V \frac{\mathbf J (\mathbf r')e^{-jk|\mathbf r -\mathbf r'|}}{|... | Given your description, you've probably done the following already:
\begin{align*}
\nabla \cdot \mathbf A &= \frac{\mu_0}{4\pi}\int_V \nabla \cdot \left[ \mathbf J (\mathbf r') \frac{e^{-jk|\mathbf r -\mathbf r'|}}{|\mathbf r - \mathbf r'|} \right] \, \mathrm{d}^3\mathbf r' \\&= \frac{\mu_0}{4\pi}\int_V \mathbf J (\mat... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/240320",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
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How do people go about looking for asymptotic safety in quantum gravity? Do we have (proposed?) methods to look for fixed points in the renormalization group flow of the Einstein-Hilbert action? My understanding of the RG is still somewhat sketchy at this point and I am having trouble understanding how one would go abo... | You can use functional renormalization group method to renormalize Quantum Einstein Gravity.You can see how to renormalize it in arXiv:hep-th/9605030.
If you have problems with understanding this paper, see the paragraph 'Exact renormalization groups' in article Renormalization group in Wikipedia.
I wanted to post more... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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"answer_id": 1
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Where does the energy go when engine braking? If you're in gear in a car and not accelerating, the car slows down faster than it would from just air resistance and tire deformation. In normal braking, the energy is turned into heat from the brake pad rubbing on something connected to the tire. Where does your car's kin... | Kinetic energy is transformed into the thermal energy of the brakes. Check out a similar question/answer: https://www.physicsforums.com/threads/transforming-kinetic-energy-into-thermal-energy.634380/
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/240587",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 4
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Nesting in Fe-based superconductors Many studies about iron-based superconductors mention the nesting of Fermi pockets, such as here or here. As far as I understand it it represents some kind of interplay between different Fermi surfaces.
Does anyone have a clear explanation of what nesting is in the context of Fe-base... | There is a previous stackexchange post on nesting. I will try to briefly explain nesting in the context of the Terashima et al. paper. In Fig 3(C) of the paper there is an illustration of the Fermi surface of a material as measured using ARPES. There are two pockets to this Fermi surface, one pocket near $k=0$ (the $\G... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/240821",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
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} |
Why should the ray become parallel to base in a triangular prism at minimum deviation? In the case of minimum deviation, the refracting angles at two surfaces are equal. Then the ray inside the prism should be parallel to the base only if it is isosceles triangular prism. But everywhere I read they haven't mentioned th... | If the angle of the ray to each of the two surfaces of the prism is the same, it follows from symmetry that you should be able to flip the scenario left-right about an axis that bisects the angle connecting the surfaces, and have the same exact picture.
But this implies that the ray inside the prism must be perpendicul... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/241295",
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Is the cosmological horizon expanding faster than space itself? I read that the rate of expansion of the universe is still a mystery. But if it's actually decelerating, wouldn't it mean that our cosmological horizon is expanding faster than space, and that one day in the far far future, it will be possible for any two ... | The particle horizon grows faster than the scale factor (which it has to, because it shows the path of a light ray travelling through space with c, so it has to go farther than a regular comoving galaxy just sitting still on its coordinate in expanding space).
On the space-time-diagrams below the dashed lines are space... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/241397",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Photon emission between an electron and positron If I placed and electron and a positron at a certain distance apart in a vaccumm, they would attract each other and annihilate producing 2 or more gamma rays. But, what I would like to know is, does the electron and positron emit photons as they are attracted towards eac... | When an electron-positron pair approach they can form positronium, a very low mass and very temporary hydrogen-like system. It comes in two forms, ortho- and para-, depending on the relative orientation of the spins. With para-positronium they have opposite orientations; this is the lowest energy or ground state for p... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/241667",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Normalization of a wave function in quantum mechanics A more simple question, so I am watching a quantum mechanics lecture on potentials of free particles and am doing the general solution of schrodinger's stationary equation for a free particle when I was told to normalize the solution (which I can do all well and goo... | Born's rule: the probability density of finding a particle in a certain place is proportional to its square absolute value.
To change the "is proportional to" to "is", you multiply the wave function by a constant so that the absolute value squared integrates to 1, and so acts as a probability density function.
That's c... | {
"language": "en",
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"source": "stackexchange",
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What does well defined parity mean I'm reading a textbook (Physics of Quantum Mechanics by Binney) and it says that the ground state ket $\left\lvert 1 0 0 \right \rangle$ of the hydrogen atom has well defined (even) parity. What does this mean?
Does it mean that the wave function is even? The wave function for this is... | Answering your second question in the comments, whether the wavefunction has well defined even or odd parity when you take the expectation value
$$\langle \psi|f(x)|\psi\rangle = \int f(x) |\psi(x)|^2 d^3 x$$
the function $|\psi(x)|^2$ is an even function (the product of two odd functions is even).
But the functions x... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Propagator in Quantum Mechanics What does the propagator in Quantum Mechanics mean? I mean, except from the mathematics behind it, what does it tell us? Is it something that has to do with translations in time?
| The clarity offered by using Feynman diagrams to set up the calculations for a scattering or a decay process also gives an intuitive meaning to the propagator function. The Feynman diagrams are iconal representations with one to one mathematical rules of how to set up, order by order in a perturbative expansion,
the i... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Examples of Bernoulli Numbers, Euler-Mascheroni Integration, and the $\zeta(n)$ in physics In Arfken's Mathematical Methods for Physicists, there is a subsection of the "Infinite Series" chapter which covers the Bernoulli numbers, Euler-Mascheroni integration (or summation), and the connection these have with the Riema... | I do not remember ever seeing Bernoulli numbers in physics, but I have seen Riemann zeta function pop out in quantum statistical physics.
The integrals of the type
$$g_n(z)=\frac{1}{\Gamma (n)}\int _0^{+\infty }dx\frac{x^{n-1}}{z^{-1}e^x-1}\tag{1} $$
often occur in calculations involving ideal Bose gases. For example... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/242068",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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How does the propagation of gravity work for photons? As explained in the answers to this post, photons apparently exert a gravitational pull on other objects. It has also been explained on this site, that gravity propagates at the speed of light.
I'm wondering, though, how do you reconcile these two facts? I'm trying ... | Photons travel along a gravitational compression wave of gravitational lines traveling in the same direction caused when the energy was released from an electron during a shell jump. Photons travel at the same speed as the wave. The reason they may not reach the same destination simultaneously is that photons collide w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/242164",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 2
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Boiling as apparent violation of the second law of thermodynamics One of the statements of the second law is that no agency can be built whose sole effect is to convert some amount of heat entirely to work.
But in case of boiling, the temperature being constant, entire heat supplied is converted into work, namely the w... | When things are heated electromagnetic rays are emitted, "black body radiation" I think, hence the reason why hot iron glows. As the radiation is emitted, the energy in the boiling device is dispersed into the surrounding, even if the surrounding is vacuum. (Also the reason why things in space cools down and freeze)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/242370",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
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Sliding blocks problem My first post here, so I apologize if this is duplicated elsewhere. It IS a "homework" problem, but it's public domain, a posted exam with answers...
http://www.mun.ca/physics/undergraduates/finals/P1020F06.pdf
Here is the diagram in question
And here is the FBD for each block.
(question a i... | You are right in all you have written. But a more fruitful sentence could be: Friction always tries to prevent sliding.
*
*(Kinetic friction) If two boxes are sliding over one another, the friction on the top block will pull in the lower block's direction, and friction on the lower block will pull in the top block'... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Derivation of Displacement current term in Ampere's Law I have a quick question:
In deriving the displacement current term for Ampere's Law, my book has the line:
$$\Phi_E= \int_S \mathbb{E} \cdot \hat{n} da= \int_S \frac{\sigma}{\epsilon_0} da = \frac{Q}{A \epsilon_0} \int_S da= \frac{Q}{\epsilon_0}$$
My question is:... | The flux is only has a nonzero part where there is a nonzero electric field. Next if we assume the $\sigma$ is the charge density on the plate, then when you replace the $\bf E$ integral with a $\sigma$ integral then the area there is just over the area of the capacitor $C$ because that is where the electric field is n... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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What is the relation between interaction range and the mass of gauge bosons? I have just started to read spontaneous symmetry breaking, where it is mentioned that EM fields are infinite in range, so the gauge boson has to be massless, while for the strong and weak interactions, they have to be massive.
Why is it that l... | Elementary particle interaction crossections and lifetimes are calculated in perturbative expansions that are set up using Feynman integrals.
The wavy line represents the exchanged boson , in this case a photon which has zero mass. The wavy line in the integral is the propagator of the electromagnetic interaction in ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/242659",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
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Why do bubbles group when one pops? I was recently observing the way bubbles move as they pop and disappear. I noticed that when bubbles destabilize and pop, the remaining bubbles immediately surrounding it will move to fill its place. I was wondering what the driving cause is here.
At first, I figured that stickiness... | The Nature is Smart; it prefers the state with the lowest energy (maximum entropy) in different manifestations.(surface tension in this case). Please refer to this link already answered. (Why does the nature always prefer low energy and maximum entropy?)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/242963",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
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Why is speed of light a constant while distance in space is not? Disclaimer: I asked this at Astronomy.SE, but got no answer whatsoever, so I am trying my luck here.
As you probably know current state-of-the art physics (i.e. gravitational waves, cosmic expansion) basically states that space itself is subject to expans... |
Is there any reason why the point of view of an expanding or contracting
space is preferred over, say, a reduction in the speed of light or an increase
in the "speed" of time
The postulate that the speed of light is the same in different reference frame led to a whole theory that can explain observations well and... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/243100",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
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Michelson Morley experiment?
*
*Its not that I question the conclusions reached concerning the Michelson–Morley experiment, however I would like to know how the following issue was addressed please?
If I could pass bob through a beam splitter, and have each copy of him pace out each leg of the interferometer at say... |
So the other bob who travelled at 2 km/hr the whole time, makes up
ground on the other bob on the return journey, and they arrive home at
the same time as each other.
You have missed an important feature of the experiment.
The escalator (ether wind) is in action for all Bob's movements; both up and down the escal... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/243325",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
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Free falling and bouncing back My confusion arises with free falling body.
*
*For a free falling body the displacement ~ time graph has a kink (at the time when the body hit the ground ). at a kink point, a function is not derivable by the rule of calculus. but we see in the free falling case the body has velocity... | When the ball makes contact with the ground, the ground exerts a very large (upward) force on the ball for a very short interval of time. This large force causes the ball velocity to change direction from downward to upward, and translates into a large upward acceleration of very short duration. So there is no incons... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/243514",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Differences between eigenstates, bound states and stationary states I am not very clear about the differences between eigenstates, bound states and stationary states.
|
*For any operator $\hat A$ an eigenstate $|\psi\rangle$ is one for which:
$$\hat A|\psi\rangle=\lambda |\psi\rangle$$
Where $\lambda$ is a constant, and is called the eigenvalue of that state. If $\hat A$ is an observable, then $\lambda$ will be real.
* A stationary state is an eigenstate of the Hamiltionain $\hat H$ ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/243719",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Has such experiment been performed before? Consider a charge of 1C kept a distance of $6*10^8$ m from a detector.
I find electric field due to this charge at detector.
Then, I suddenly earth that charge
and not the time it takes to be detected by detector.
It should be about 2 sec.
Has any similar experiment been perfo... | To earth the charged body does not mean that the electric field of this charges disappears.
To make such a charged body one has to separate some amount of electrons and as a result one get a negative charged body and a positive charged body side by side. So beside the weakness of the electric field of the charged body ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/243796",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
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How do pressured containers within pressured containers behave? If a container could hold 60 PSI of pressure and was placed into a larger container with the same strength, could you fill the large container to 50 psi and the smaller one to 100 psi, creating only 50 psi changes?
If so, could you do this infinitly, havin... | As you suggest, you could use a large number of nested containers to gradually increase the pressure of the innermost container to very large values. Generally it's only the pressure difference that matters. These could even be weak containers, like a balloon within a balloon within a balloon within...
But this can't... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/243910",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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Can you measure a motor torque using a load cell connected to the stator? how? I saw a picture here: https://measurementsensors.honeywell.com/techresources/appnotes/Pages/Ways_to_Measure_the_Force_Acting_on_a_Rotating_Shaft.aspx
It shows a motor casing connected to a load cell. I don't understand how it transmits the ... | If you are only considering steady state torque with accuracy, then yes. Newton's third law applies and so any torque seen by the motor shafts and armature sees a reaction torque on the motor frame, attached to the stator.
But during transient acceleration there are losses not necessarily seen in transient torque. Thes... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/244043",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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