Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Can electrical forces be reflected by non charged objects? If there is a charge in the center of a closed room, with just walls that are not charged: the electrical forces can traverse the walls, are they absorbed by the non charged walls, or are reflected back?
| Objects that are overall electrically neutral, but are made of positive and negative charges, can still interact electromagnetically. This is how wires work; even though the wire itself is not charged, it still carries current because the electrons in the wire are moving.
In the case of a wall, electromagnetic radiatio... | {
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How does an object in space travelling at constant velocity have a net force of zero acting upon it? If the definition of balanced forces is "two opposing forces that are equal" and an object with a net force of zero acting upon it means that the forces are balanced on the object, then it should follow that an object t... | You are right. If we have an object at rest and then we want it to start moving, we apply a force to the object. While the force is being applied, the object accelerates according to $F=ma$.
Now let's say we stop applying this force. Then there is no longer a net force acting on the object. Therefore, the acceleration... | {
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Why are RG flow fixed points associated with different phases? Why are RG flow fixed points associated with different phases? I thought the RG makes only statements about behavior near to critical points... a definite phase is far away from the critical point, right?
| RG fixed points themselves are not associated with phases. Rather, RG fixed points (and their basins of attraction) describe systems at a phase transition. You are correct that taking the limit as a system approaches the critical surface only allows us to make statements about the system near the transition, but the RG... | {
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Can we enhance evaporation rate with a vacuum pump? First, consider having water at 100C and 1atm and a heat source. If more heat added to water, we have more evaporation rate according to the following formula:
$$Q_{in}=m_{vapor}*h_{fg}$$
THEN
Consider having water at 100C and 1atm and a vacuum pump.
Can we change the... | As you drop the pressure, the boiling temperature drops. As the water boils the heat of vaporization comes from the water and the temperature drops. This should continue until the remaining water is frozen. You may need to find out if the heat of vaporization is a function of temperature.
| {
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Use of generating function in canonical transformation In the theory of Canonical transformations, initially we use the fact that the new and the old system of $(q_i, p_i)$ with the Hamiltonian $H$ satisfy the modified Hamilton's principle. Now here, the use of the theory of generating functions makes sense. But after ... | *
*For various notions of canonical transformations (CTs), see this Phys.SE post.
*Presumably what OP calls "Poisson bracket formalism of CTs" refers to symplectomorphisms (at least if there is no explicit time dependence).
If we furthermore assume that the $2n$ new coordinates $(Q^1,\ldots, Q^n, P_1, \ldots, P_n)$ ... | {
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Negativity for a diagonal reduced density matrix Suppose one has a tripartite system A,B,C with density matrix $\rho$ , and with reduced density matrix $\rho_{BC}=\text{Tr}_A\ \rho$. Suppose $\rho_{BC}$ is a diagonal matrix.
As the partial transpose of a diagonal matrix corresponds to the same matrix, and since all t... | A state with a diagonal density matrix is always separable (=not entangled): If
$$
\rho= \sum p_{ij} |ij\rangle\langle ij|\ ,
$$
then a separable decomposition is given by
$$
\rho = \sum p_{ij} \sigma_{ij}^A\otimes\sigma_{ij}^B\ ,
$$
with $\sigma_{ij}^A=|i\rangle\langle i|$ and $\sigma_{ij}^B=|j\rangle\langle j|$.
| {
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Is the randomized motion of particles produce heat? In the show on factomania from gregg foot I've heard that when sun light comes then some gas particles of air get more energy and velocity and as the average kinetic energy of molecules is Temperature the temperature increases for the same region . Now if thats true e... | Heat is energy transfer from one thing to another due solely to there being a temperature difference between the two. Things do not "contain" heat. The earth's atmosphere does not contain heat. Temperature is a measure of average translational kinetic energy component of a substances internal energy.
The three basic ty... | {
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Earth bulge and axis precession We know that earth is not spherical because it spins. Does every object that spins bulge? for example a rotating wheel bulges? Also how moon and sun create the torque that shifts the axis of rotation (precession) if gravitational force acts on center of mass therefore no torque?
| Every object that spins must be accelerating. This means different forces (and usually stresses) from the same object at rest. Whether it bulges or not depends on the material and its response to that stress.
If you have a steel marble spinning at 1 revolution per minute, the rotational forces on it are tiny. Given ... | {
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Why gas molecules move with different speed at a given tempreture? As per my understanding we know that molecules of an ideal gas are identical in all aspects (size, shape, mass). Since collisions are elastic in nature, they don't lose their kinetic energy. That means that kinetic energy of each molecule doesn't change... | That's a good question. Elastic collisions between isolated particles will indeed conserve energy and momentum. But, consider this: Suppose the particles' momenta before the collision are uncertain: they are only known within some range. Think about it a while and you'll realize that the uncertainty grows with each... | {
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Why does the warm air rises up? Warm air has more energy than cold air. This means that according to the Einstein equation $E = mc^2$ the warmer air has a greater mass than the cold one. Why is the warm air rising, if it has a greater mass, which means that the attraction of gravity between the Earth and the warm air i... | As other answers point out, the reason is buoyancy. This post is to show just how small the opposing (relativistic) effect of increased gravitational force is.
The increase in the gravitational attraction associated with kinetic energy is proportional to the Lorentz factor, $\gamma = \frac{1}{\sqrt{1-v^2/c^2}}$.
We c... | {
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What is the most efficient way to pour a liquid through a funnel? Be it in cooking or an experiment, when transferring liquid from one container to another using a funnel should you:
(a) pour it all straight into the funnel, filling it up and waiting for the liquid to drain through
(b) pour it such that the end of the ... | The Bernoulli equation for a non-turbulent fluid with density $\rho$ is
$$
P_\text{fluid} + \frac12 \rho v^2 + \rho g h = \text{constant}.
$$
At the top and the bottom of the funnel, where the fluid is exposed to the atmosphere, the fluid surface will move until the fluid pressure and the atmospheric pressure are the s... | {
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How to obtain $Y$ rotation with only $X$ and $Z$ rotation gates on the Bloch sphere? Let's say you have a system with which you can perform arbitrary rotations around the $X$ and $Z$ axis. How would you then be able to use these rotations to obtain an arbitrary rotation around the $Y$ axis?
I have seen somewhere that r... | Here it is in words, more details below.
*
*Rotate by $\alpha=-90^\circ$ about $z$ so as to bring your $y$ axis around to point along the $x$ axis.
*Rotate by desired angle $\beta=\theta$ about $x$.
*Rotate by $\gamma=90^\circ$ about $z$ to put the $y$ axis back where it was.
The information you need
for the mo... | {
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If the molecular collisions are elastic will there be any dissipation in a fluid? Viscosity arises due to collisions of the molecules of one layer of a fluid with another in contact. But viscosity is a dissipative element leading to heating and dissipation. Where does it heat come from? Does it come from the molecular ... | What is happening is that molecules from one layer collide with those of adjacent layers, transferring both momentum and kinetic energy. If there is an organized motion of the molecules (e.g., mean velocity gradient), the kinetic energy of the organized motion is continually converted to random kinetic energy as a res... | {
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Why are there so many different "colors" in the line spectra of simple elements? I understand that atoms emit photons of wavelength $\lambda$ when electrons transition to lower orbitals according to the equation $E = \frac{hc}{\lambda}$. Based on my understanding, those orbitals have fixed energies, and simple elements... | There are an infinite number of orbitals. Even for simple one-electron Hydrogen, there are an infinite number of energy levels, $E=-E_0/n^2$ for $n=1,2,3,...$ all the way to infinity.
So there are many possible transitions between energy levels, each corresponding to a spectral frequency. Some transitions have a much h... | {
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Is there is any way to prove the Faraday's law of induction? I want to know if there is any way to prove the Faraday's law or is it just an experimental observed phenomena?
More specifically, is there any reason why the proportionality constant is 1? How did Faraday discover it? I also heard that we can prove Faraday's... | Every proof starts with axioms — things that are merely assumed rather than proven. The axioms should be motivated by their applicability to a broad range of phenomena, but utlimately they are just axioms. We test them by comparing their predictions to experiment. Axioms that apply to a broader variety of experiments a... | {
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Why is $E=mc^2$ and not $E=m\frac{c^2}{2}$? Kinetic energy for a moving object is the integral of force with respect to distance, often given as:
$$E=m\frac{v^2}{2}.$$
This would imply that for mass moving at the speed of light, the kinetic energy would be:
$$E=m\frac{c^2}{2}.$$
This puts it off from the Einstein resul... | You cannot make this 1:1 correspondence between the classic kinetic energy of a particle and the rest energy of a relativistic particle.
$E = mc^2$ does not apply only to objects moving at the speed of light, but instead to all objects, moving or not. As such, it isn't a relativistic correction of the kinetic energy, b... | {
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How should I imagine a multi-particle state in a free QFT? It is reasonable to think of single-particle Focks states as of plane waves. Indeed, since $|p\rangle=a^\dagger_p|0\rangle$ and $\langle x|p\rangle\sim \operatorname{e}^{ipx}$, we conclude that the state $|p\rangle$ can be thought of as a plane wave in the posi... | Previous answers look right, but I hope I may be able to add clarity.
In a free-field theory, you can read $a^\dagger_{p_1}a^\dagger_{p_2}|0\rangle$ as 'the occupation number of mode $p_1$ is 1 and the occupation number of mode $p_2$ is 1'. If you want to go to the language of spinor-valued (i.e. not operator-valued) w... | {
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Do Maxwell's equations predict the speed of light exactly? I know that $\frac{1}{\sqrt{\mu_0\varepsilon_0}}$ is equal to the speed of light but is this prediction accurate? I mean is it 100 percent accurate?
| In the current SI system $\epsilon_0$ is defined as $\epsilon_0=1/c^2 \mu_0$ so in the current SI system $c=1/\sqrt{\epsilon_0 \mu_0}$ is clearly exact. Furthermore, $c$, $\epsilon_0$, and $\mu_0$ are themselves all exact defined quantities with no uncertainty individually.
In the new SI system starting next year we wi... | {
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Can we say that bosons attract each other? We know that bosons donot follow Pauli exclusion principle, thus they can occupy the same state. But is it equivalent to say that bosons attract each other?
| No. When we say that two particles "attract," we usually mean that there is some intermediary field causing the attraction, i.e. a "force carrier." You can easily make theoretical models of bosons which do not attract. (These would be called "free particles.") However, sometimes people say that bosons "like to be toget... | {
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Why can't we see images reflected on a piece of paper? Why can't you see a reflected image on a piece of paper? Say you put a pen in front of the paper, even when light rays are coming from other sources, hitting the pen, reflecting back, and hitting the paper, there is no reflection.
What's wrong with the following "... | A reflective or polished surface respects and preserves the angular proximity of incoming beams on its points. The less the angular proximity of incoming intensity is preserved in the outgoing intensity profile, the more diffuse and "matte" looks the surface
| {
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Negative Miller indices and parallel planes The integers are usually written in lowest terms, i.e. their greatest common divisor should be 1.
https://en.m.wikipedia.org/wiki/Miller_index
Does this mean, that parallel planes are generally equivalent, particularly does $(200)$ for example even exist or is this a wrong no... | I like Pister's fold-up crystal for this: http://www-bsac.eecs.berkeley.edu/~pister/crystal.pdf
One sees then that $(\bar{1}00)$ is on the other side of $(100)$ and that those are perpendicular to $(010)$. In a cubic crystal like silicon all six cubic planes $\{100\}$ are equivalent.
In non-centrosymmetric crystals ... | {
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Could quantum fluctuations spawn real matter? Would it be plausible for fluctuations in the QED vacuum to spawn actual matter (such as quarks, electrons the constituents of a hydrgen atom) given enough time and space?
| The answers above are correct. Kramer and Steane make reference to gravitation. This is where things get strange, for a quantum vacuum is only defined in a local region. A black hole for instance has local internal frames patched together by transition functions that under derivatives give connections terms that furthe... | {
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Does a rock use up energy to maintain its shape? A rock sitting on land, the ocean floor, or floating in space maintains its shape somehow. Gravity isn't keeping it together because it is too small, so I'm assuming it is chemical or nuclear bonds keeping it together as a solid. If not it would simply crumble apart. So,... | There are various mechanisms that keep solid things together, they all have one thing in common: They reduce energy to a minimum! When you want to break it apart, it costs you energy to do so!
Examples of bonds are:
Hydrogen-Bonds, which are very weak and come from an asymmetry of the electron around the proton, in suc... | {
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Two People Pushing Off of Eachother, Newton's Third Law, and Unbalanced Force Different versions of this question have come up all over the internet. Usually it deals with tension in a rope or two people pushing on each other with the same force. I am trying to understand 2 people pushing each other with different forc... |
Say the person on the left pushes with 100 N and the person on the right pushes with 70N. What happens to this system?
This is not possible. They can only push on each other with the same force.
Suppose the person on the left is a weightlifter and capable of pushing very forcefully, and suppose that the person on th... | {
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Explanation of formation of Newton's rings I was asked to do a lab report in my University about Newton's rings experiment that we made in lab. I understand that the aim of the experiment is to measure the wavelength of a light after the formation of Newton's rings and I understand the mathematical derivation of formul... | From what you've written, I deduce that you know that the path difference between light reflected back up from a point P on the lower surface of the lens and reflected back up from the point Q immediately below it on the plane surface is $\frac{r^2}{R}.$ Here, $R$ is the radius of curvature of the lower face of the len... | {
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What is the difference between enthalpy and internal energy? What is pv in enthalpy's equation? if we transfer energy to a system in the form of work does it reflect in its internal energy fully and if it does why do we need a term called enthalpy which seperates pv term in its equation?if pv is not included in interna... | Enthalpy is a state property and is defined as
$$H=U+pV$$
Where $H$ is the enthalpy, $U$ internal energy, and $pV$ is the product of pressure and volume. So enthalpy is a state property derived from internal energy and other properties, $p$ and $V$.
Enthalpy is simply a useful derived property for analyzing certain ty... | {
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What is the function of this complicated tensioning system? I saw this arrangement for tensioning overhead cables from my train window (schematic below). Why not just have one pulley wheel leading directly to the weights? What function do the additional pulleys serve? For that matter, what are the cables for? They're c... | You already got an answer to your main question - why are there pulleys? I will attempt to answer the second question - what is this wire?
There are two possible answers.
First, I found the following image at http://www.rail.co.uk/rail-news/ecml-suffers-another-failure/
The label tells us that this is a "along-track c... | {
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Why are green screens green? How/why do green screens work? What's so special about the color green that lets us seamlessly replace the background with another image and keep the human intact?
Are there other colors that work similarly?
| I may misunderstand the question, but the method of selecting the background based on colour you are asking for is called chroma keying.
In digital post-processing, all pixels which are sufficiently green are considered background and hence treated as transparent. What is "green" is configurable, often in HSV colorspac... | {
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How do we know that light cannot travel faster than it does? We assume the speed of light in vacuum is its maximum speed but can we not assume that it could be faster, or slower?
| We used to think it could. Experiments proved that wrong.
In the 19th century, it was widely believed that light, being wave-like, must be propagating through a medium that permeates the universe, just like sound waves propagate through air. This hypothetical medium was called "luminiferous aether".
Many experiments we... | {
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Why is torque defined as $r × F$ and not $F × r$? Is it merely due to popular convention or does it supply any special clarification regarding other physical quantities?
| I don't know the correct explanation but I think that $\boldsymbol{r}\times \boldsymbol{F}$ signifies that the $\boldsymbol{r}$ vector rotates because of the $\boldsymbol{F}$ vector. To produce the same result of a screw coming out when we apply a force- This is the direction of torque, whereas $\boldsymbol{F}\times \b... | {
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What determines the color of the light emitted in a Tokamak? We see images of Tokamak plasma with all sorts of colours from red to purple. Why do we see any light at all, since the plasma should be so hot to have dissociated all its electrons? It is all from contamination or unwanted cooling?
| As you correctly state, a plasma is composed by a certain density of charged particles (ions, electrons,...). Due to many different reasons, such as the presence of external and internal (self consistent) electromagnetic fields, these charged particles are moving under the action of various forces. It is possible to sh... | {
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In what context is enthalpy a convenient concept? Internal energy $U$ is clearly an important concept; the first law of thermodynamics states that for an isolated system internal energy is constant $(\Delta U=0)$ and that for a closed system the change in internal energy is the heat absorbed by the system $Q$ and work ... | In addition to what @Bob D and and others said about the use of enthalpy (primarily) in performing energy balances on continuous flow systems, enthalpy is also important in quantifying the temperature dependence of the equilibrium constant for chemical reactions (via the van't Hopf equation) and the temperature depende... | {
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Periods of non-circular Schwarzschild orbits I have been thinking about non-circular orbits in the Schwarzschild spacetime. How would you define a period of one orbit? I was thinking, in terms of proper time, for $r$, how long it takes to go from one apogee to another. For $\phi$, again in terms of $\tau$, how long it ... | $\let\a=\alpha \let\b=\beta \let\phi=\varphi \let\De=\Delta \def\D#1#2{{d#1\over d#2}} \def\dr{\dot r} \def\dt{\dot t}
\def\dx{\dot x} \def\dphi{\dot\phi} \def\half{{\textstyle {1 \over 2}}}$
If I could assume you can read Italian I would have an easy life - had
only to give a link. But since I find it unlikely, I'll... | {
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Can we ever "measure" a quantum field at a given point? In quantum field theory, all particles are "excitations" of their corresponding fields. Is it possible to somehow "measure" the "value" of such quantum fields at any point in the space (like what is possible for an electrical field), or the only thing we can obser... | In QFT, it's not possible to measure the value of quantum fields at any point in space. This is because quantum fields are not in spacetime (per the Copenhagen Interpretation, Transactional Interpretation, and others which include the concept of wave function collapse). They are calculated entities which we infer from ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/446492",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Please help identify this physics apparatus!
This was my grandfather’s and have no idea what it is only that it is some piece of physics equipment!
The main black cylinder doesn’t seem like it wants to rotate but not sure if it should?
| It is a spark radio transmitter.
The first working radios.
Video: https://www.youtube.com/watch?v=YSf93g0heUA
Pics: https://www.google.com/search?q=spark+radio+transmitter&source=lnms&tbm=isch&sa=X&ved=0ahUKEwi-68m5vJjfAhXMx1kKHVuUASQQ_AUIDygC&biw=1920&bih=930
This one looks awfully similar and might give you some... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/446618",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "28",
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Difference in spectrum of green laser and green LED In an experiment I conducted I used a spectrometer to find the spectrum of green laser and green led.
this is what I found:
LED spectrum:
Laser spectrum:
why is the spectral width of the LED is wide compared with the laser?
| Also there are green lasers that produce light directly from semi-conductor materials (InGaN -see wiki). The green LED uses the same material but probably less pure. The laser is a tiny chip ( very pure, very few crystal defects, very carefully grown) with mirrors at either end. Due the exact dimensions of the cryst... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/446858",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Falling object and the side wind I don't have a big knowledge on Physics so I'm sorry in advance if it doesn't make sense..
If a brick is falling from 100m tall-building when there's the side wind of 30m/s, is there any way I can find how far the brick would have traveled from the origin?
If there's any equation I can... | The usual, and only simple, way of calculating the sideways displacement $d$ of a projectile in a cross wind $W$ is
$$
d= W(T_{\rm air}-T_{\rm vacuum})
$$
Here $T_{\rm air} $ is time to target, (or ground in your case) taking into account air resistence, and $T_{\rm vacuum}$ is the time the projectile or brick would ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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What is the "lowest energy"? In many textbooks I come across the term lowest energy. For example in atomic structures, electrons are placed in orbitals in order for the atom to have the lowest energy. But what is this energy? Potential- or kinetic energy or the sum of the two?
| The lowest energy of a quantum system is the minimum eigenvalue of the Hamiltonian of the system. The Hamiltonian is the operator which corresponds to the total energy of the system, so it is the sum of the kinetic and potential energy. This is often also referred to as the ground state energy of the system.
but what ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Can a battleship float in a tiny amount of water? Given a battleship, suppose we construct a tub with exactly the same shape as the hull of the battleship, but 3 cm larger. We fill the tub with just enough water to equal the volume of space between the hull and the tub. Now, we very carefully lower the battleship into ... | Note that the water does not need to have been present - this calculation gives just the way to calculate the non water volume occupied by the floating object (that hence is unavailable for water).
So in your example, assume you put the ship into the container. If you filled it up to the same water level without the sh... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/448673",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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The water analogy seems to imply that power = current. Why is this incorrect? Many people think of the water analogy to try to explain how electromagnetic energy is delivered to a device in a circuit. Using that analogy, in a DC circuit, one could imagine the power-consuming device is like a water wheel being pushed by... | Power is defined as work done per unit time. So a mass of water moving from one potential to some other lower potetial can do work when it hits the wheel. How much work per unit time? It depends how much mass falls times height times gravitational constant g and all of that divided by time. Water current, on the other ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/448724",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Future pointing light cones in Black Hole in Schwarzschild Coordinates In examining black holes in Schwarzchild Coords (ie without resorting to other coords) the r coord becomes timeline within the event horizon and the t coord spacelike.
Therefore the light cone is tilted by 90 degrees. However, how do we say which di... | You can't expect an explanation if you stick to singular coordinates,
where there is no smooth way to go from without to within.
Shift to well-behaved coordinates (e.g. Kruskal-Szekeres). Then
light-cones behaviour is quite simple and you may easily see which
their relationship must be wrt to Schwarzschild coordinates.... | {
"language": "en",
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Physical Model for a Black Body I am learning about black body radiation and it was stated in a textbook that
The best model for a black body is a large cavity with a very small hole in it which absorbs all incident radiation. The radiation hits the walls and rattles around inside and comes to thermal equilibrium wit... | I think in this case the author is referring to the small hole as being a good model for a black body. As mentioned any light which hits the hole will travel inside the cavity and reflect on the walls of the cavity until it is absorbed. The larger the cavity the more likely it is to absorb since it's less likely to be ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/449121",
"timestamp": "2023-03-29T00:00:00",
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How is pressure an intensive property? I've seen this question asked before but I can't find an answer to the specific point I'm troubled with. From the kinetic theory of gases, pressure results from molecules colliding with the walls of a container enclosing a gas, imparting a force upon the wall. Now, if we split the... | Yet another way of looking at it: if the container is in equilibrium, then in aggregate the particles on one side the partition are the same as the ones on the other side. Every side a particle hits the partition on side A, that particle stays on side A, but would have moved to side B if it weren't for the partition. B... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/449201",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Intuitive explanation for the free field Lagrangian? The free field Lagrangian is
$$\mathcal{L}=\frac 1 2 \partial^\mu\phi\partial_\mu\phi-\frac 1 2m^2\phi^2$$
with sign convention $(+,-,-,-)$.
Plugging this into the Euler-Lagrange equations gives the KG equation.
In class we were given the semi-intuitive explanation t... | The Lagrangian for a real scalar field $\phi\left(\mathbf x,t\right)$ given by OP with the sign convention $(+,-,-,-)$ is expressed alternatively as
\begin{equation}
\mathcal{L}\left(\phi,\boldsymbol{\nabla}\phi,\overset{\;\centerdot}{\phi}\right)\boldsymbol{=}\frac12\overset{\;\centerdot}{\phi}{}^{\,2}\boldsymbol{-}\f... | {
"language": "en",
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How can two electrons repel if it's impossible for free electrons to absorb or emit energy? There is no acceptable/viable mechanism for a free electron to absorb or emit energy, without violating energy or momentum conservation. So its wavefunction cannot collapse into becoming a particle, right? How do 2 free electron... | Basically, because the process doesn't simultaneously conserve energy and momentum; this is why we say that it's mediated by a virtual photon.
In more technical language, this means that the photons that are exchanged between two interacting electrons are allowed to be "off shell", where the "shell" is the relationship... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/449431",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Violating Newtons First Law! Suppose you are inside a very large empty box in deep space , floating ( i.e not touching the box from anywhere initially).The box is at complete rest.
Now you push the box forward from inside.
Now you would go backwards but the box will move forward to conserve momentum.
However since you... | Internal forces can make things move. Nothing says they can't. It's just that any internal forces have an "equal and opposite force" that is also internal, so momentum is conserved, at least when considering internal forces. This has everything to do with Newton's third law and nothing to do with Newton's first law. Ne... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/449699",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How can we define energy other than the definition that it's a capability to do work? It is actually a property of energy that it can do some work not an actual mean to define it because we cannot define a thing on the basis of what it is doing or what it can do.
| "[…] we cannot define a thing on the basis of what it is doing or what it can do."
Why not? Try defining a progressive wave other than in terms of what it's doing!
In my opinion "The energy of a system is the amount of work it can do" is an excellent starting definition of energy. It enables one to derive the Newtonian... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/449793",
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Why is lattice QCD called non-perturbative? Like, if you are approximating a smooth structure with a discrete lattice, isn't this like a perturbation from smooth space-time?
If Feynman diagrams are a perturbative method, why are Feynamn diagrams on a lattice/grid called non-perturbative?
| In general, by a perturbative approach, we mean an approximation of the form,
$$f = f_0 + \epsilon f_1 + \epsilon^2 f_2 + \dots$$
where $\epsilon$ is the perturbation parameter, for some solution $f$. That is to say, one can approximate the behaviour of the solution by this series.
However, summing all the terms does n... | {
"language": "en",
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I have a question regarding the Painlevé-Gullstrand (PG) metric with factor 2 I have a question regarding the Painlevé-Gullstrand (PG) metric.
If we have the line element in a radial fall we get:
$$d\theta = d\phi = 0$$
$$ds^2 = -dT^2 + \left(dr+\sqrt{\frac{r_s}{r}}dT\right)^2.$$
Writing out the binomial formula we obt... | No, the off-diagonal metric components are $g_{rT}=g_{Tr}=\sqrt{\frac{r_s}{r}}$ without a factor of 2.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/449959",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Why are ropeway pillars tilted? While skiing I have noticed that ropeways pillars are usually tilted to be perpendicular to the slope (fig.1). If the gravity is pulling straight down, why aren't they vertical as they are supposed to support ropeway's weight? Is there something more they "do"?
Also there are pillars whi... | The cable can only put a force on the mast that is perpendicular to itself: It runs over rollers which do not allow it to transfer any force in the direction of the cable.
As such, the force on the mast is exactly given by the angle between the oncoming and the outgoing cable. This force is roughly perpendicular to the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/450078",
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Potential Difference of a battery - What does it mean? I have studied current electricity for a while now. When I look back at basic concepts, I am quite clear about what current, electron, resistance is. But I cannot imagine about the potential difference or voltage of a battery. Or in a circuit, it is said that poten... | I think it is a measure of how much energy it takes to move a charge over a certain path.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/450445",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Propagator for W boson I've found in different literature that some write the propagator for the W boson as $\frac{g_{\mu\nu}-\frac{k_\mu k_\nu}{M^2_W}}{k^2-M^2_W+iM_W\Gamma_W}$ and others like $\frac{g_{\mu\nu}-\frac{k_\mu k_\nu}{M^2_W}}{k^2-M^2_W}$, without the $iM_W\Gamma_W$ term in the denominator.
Is there a reaso... | Putting an $iM\Gamma$ in the denominator of a propagator for bosonic particle with mass $M$ makes the particle unstable, decaying with rate $\Gamma$. However, it also makes the propagator non-unitary, since the decaying particle doesn't turn into anything else; it just disappears, and probability is lost. So the prop... | {
"language": "en",
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Why did Einstein pick his definition of synchronous clocks? In his paper "On the Electrodynamics of Moving Bodies", Einstein defines synchronous clocks as clocks which satisfy $t_B-t_A=t'_A-t_B$. $A$ and $B$ are two points separated by empty space, and identical clocks are located at $A$ and $B$. A light signal is emit... | This method of synchronization works because both clocks can agree on the speed of light.
If we want to determine whether or not two distant events are simultaneous, or more generally the order in which events occur, then we need to factor in the time it takes for the signals to reach us. We can easily find this time i... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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It seems that the Euler equation in thermodynamics and The first law of thermodynamics are in contradiction The Euler equation in thermodynamics are as followed:
$U=TS-PV+\mu N$
But The first law of thermodynamics states that
$dU=TdS-PdV+\mu dN$
But I think that The Euler equation can be written by
$dU=TdS+SdT-PdV-VdP+... | This is the Gibbs–Duhem equation. It's related to extensiveness of energy, entropy, volume and number of particles.
The assumption is that the edge effects are much smaller than the volume effects, in particular, when combining two systems the total energy is the sum of energies of the smaller systems, without "interac... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/450978",
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Does the inverse-square law apply to linearly polarized light? It's a stupid question but: We did and experiment using linearly polarized microwave radiation generators and receivers. Our teacher asked to check experimentally if the receiver measurements are proportional to the intensity of the radiation I or to the in... | Nope. Point sources of waves have fields falling off as $1/r$ while intensities fall off as $1/r^2$. To see why, think about conservation of energy.
A point source is spherically symmetric, and so the intensity should only be a function of the distance $I(r)$. Intensity is defined as the area flux density of energy du... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/451100",
"timestamp": "2023-03-29T00:00:00",
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What is a good site to use for finding physical constants at times when NIST web resources are affected by government shutdowns? There are multiple resources for finding physical constants (say, particle data, atomic spectra, or even special-function identities) which are generally hosted by NIST, and which form essen... | The Particle Data Group is the only way to go.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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The reasoning behind doing series expansions and approximating functions in physics It is usual in physics, that when we have a variable that is very small or very large we do a power series expansion of the function of that variable, and eliminate the high order terms, but my question is, why do we usually make the ex... | The key reason is that we want to understand the behavior of the system in the neighborhood of the state rather than at the state itself.
Take the equation of motion for a simple pendulum, for example:
$$\ddot{\theta} = -\frac{g}{\ell}\sin(\theta)$$
If we take the limit where $\theta \rightarrow 0$, we find $\ddot{\the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/451588",
"timestamp": "2023-03-29T00:00:00",
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$Q$-factor for damped oscillator (not driven)? How would this be defined?
Some of the Q-factor definitions I have encountered include:
$$Q=2\pi\frac{\text{Energy stored}}{\text{Mean power per cycle}}\\Q=2\pi\frac{\text{Energy stored}}{\text{Energy lost per period of
oscillation}}\\Q=2\pi\frac{1}{\text{Fractional powe... | The Q-factor tells you something about the frequency response of a driven system to a constant amplitude driver when a steady state (constant amplitude of driven system) has been reached.
The driver supplies energy to the driven system which at steady state results in the energy stored in the driven system staying cons... | {
"language": "en",
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Projective measurement using two mode squeezed state? Let me define two mode squeezed states as $ \left | \xi \right>_n=\exp\left(\xi \hat{a}^\dagger \hat{b}^\dagger-\hat{a} \hat{b} \xi^\star\right)\left | n,0 \right>$ where $\left|n,0\right>$ is the number state with $n$ photons in mode 1 and none in mode 2. Now, let ... | If you are happy with destructive measurements, this can be done in principle in the lab. Let $\rho$ be the input state, and write $|\xi\rangle = S|n,0\rangle$.
*
*Undo the squeezing transformation $S$, i.e., transform $\rho'=S^\dagger\rho S$. Note that $S^\dagger$ is just another squeezing transformation which can ... | {
"language": "en",
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are there changing magnetic and electric fields that are not EM radiation? Let us consider these two Maxwell equations:
$$\frac{\partial \vec{B}}{\partial t}=-\vec{\nabla}\times \vec{E}$$ and
$$\frac{\partial \vec{E}}{\partial t}=\frac{1}{\epsilon_0}\left(-\vec{J}+\frac{1}{\mu_0}\vec{\nabla}\times \vec{B}\right).$$
Whe... | Regular induction in a coil satisfies Maxwell’s equation (usually the integral form is used) and it’s usually not considered radiation.
| {
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Where does it getting wrong , when using $v^2 - u^2 = 2as $ down the incline, for different object having different moment of inertia? Well, Consider a situation there is a sphere and a ring, of same mass $M$ and radius $R$. They both starts rolling down the inclined plane. We know moments of them as well, $$I_\text{sp... | What you have left in calculations is acceleration,, a≠g nor a=gsinα.
$$a= gsinα-F/M$$
where α is angle of incline, F is force of friction and M remains mass..
Since friction acting on both are different, their acceleration are different for same distance s.
Same goes for your second equation
v=u+at, here a... | {
"language": "en",
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Photons exerting force Since gravity affects photons, and forces always work in pairs.
Does this mean that photons have a resultant force.?
And would we be able to harness this resultant force to move objects using light?
|
Does this mean that photons have a resultant force.? And would we be able to harness this resultant force to move objects using light?
Photons are elementary particles in the standard model of particle physics . They have zero mass, energy=$h*ν$, and spin $1$.
They are described by a four vector of special relativity... | {
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Would someone be able to see where they are headed once they crossed the event horizon? Let us say an explorer was studying a supermassive black hole and ended up blundering past the event horizon. From my understanding, which may be wrong, the only paths allowed after crossing the event horizon are paths that bring th... | If there is other infalling matter, then certainly they may be able to see it. A simple example is that if they hold their own hand in front of them, they will still be able to see their hand. This is an example of the equivalence principle, one form of which states that spacetime is always locally flat, so that on sma... | {
"language": "en",
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number density of photons My lecturer put up as a solution to the number density of photons to be $n_{o\gamma}=\tfrac{8\pi}{c^3}\int^{\infty}_0(\tfrac{kT}{h})^3\tfrac{x^2dx}{e^x-1}$. The integral on the right hand side being the Riemann zeta function with value which is approximately 1.2. But then he said this gives th... | Putting this exact formula into Wolfram Alpha actually gives me $400\text{ cm}^{-3}$, assuming that your evaluation of the integral is correct. We can check to see that the units here make sense by doing some dimensional analysis. The equation is
$$\frac{8\pi}{c^3}\left(\frac{kT}{h}\right)^3\int^{\infty}_0\frac{x^2dx}{... | {
"language": "en",
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What gives mass to black hole? I like to know when a dying star collapsed into a black hole, is there anything inside or on the event horizon that is interacting with higgs field?
| It is important to understand that energy can have mass as well. For example 99% of the mass of the proton is due to the binding energy of the quarks inside it. Only about 1% comes from the interaction of the quarks with the Higgs field. So only the masses of the elementary particles are due entirely to the Higgs field... | {
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Conceptual understanding of quantum harmonic oscillators The way I understand it is that we have the time-independent Schrödinger equation for a particle described by a wave function $\psi$ in a potential V(x)
$$-\frac{\hbar}{2m}\frac{d^2}{dx^2} \psi + V(x)\psi(x) = E\psi(x)\tag{1} \, .$$
If I were to approximate the p... | The same problem arises classically. If you have a mass on a spring we have a harmonic oscillator. But do we say the mass is a harmonic oscillator? Perhaps. If so then we would say for your question that the particle is a harmonic oscillator
But the spring is an important part of the system too. If we really want to be... | {
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Mass hanging from spring in free fall
Q: A mass $m$ hangs from a massless spring connected to the roof of a box of mass $M$. When the box is held stationary, the mass-spring system oscillates vertically with angular frequency $\omega$. If the box is dropped falls freely under gravity, why does the angular frequency in... | Despite the weird geometry of the problem, this is simply two masses connected by a spring, and we can solve that problem using Lagrangian Mechanics. Let's assume that the masses have mass $M$ and m, and position coordinates $x_M$ and $x_m$ respectivly. They are also separated by a distance $d$, and the spring constant... | {
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Explanation of Lenz's Law phenomena If we drop a magnet through a copper pipe (without it touching any of the sides), it would fall slower than it would if there were no pipe.
Having the pipe otherwise accelerate the magnet would be in violation of Lenz's law and conservation of energy. I agree that, if there is any in... | Yes it has interction.
Beacuse the top side of the pipe will act as a opposite polarity and repel
| {
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Will any charge oscillating in space create an EM wave? Would it be correct to say that any charge oscillating in space (regardless of the spacial amplitude) at a given frequency will emit an EM wave of the same frequency?
related: What change in an EM field is required to create an EM wave?
|
Will any charge oscillating in space create an EM wave?
Almost always yes, but a counterexample was given by G.A. Schott, "The Electromagnetic Field of a Moving Uniformly and Rigidly Electrified Sphere and its Radiationless Orbits," Phil Mag Suppl 15 (1933) 752. For a uniformly charged spherical shell of radius $b$, ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Water mixture at different temperatures Let's say that we mix homegeneously and instantly cold water at $t^\circ $ C and hot water at $T^\circ$ C (like in a water tap) in ratio $p:1$. My question is the following: What is the instant temperature of this mixture? Is there a law in that sense?
I am a mathematician, not ... | If the mixture is homogeneous and instantaneous, the temperature is simply determined by the ratio p. The law is the law of mixtures.
Newton's law of cooling is not applicable even if the instantaneous homogeneity is not assumed. That assumes heat loss form a body to the surrounding.
| {
"language": "en",
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"source": "stackexchange",
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What does the 'cosmological constant' represent? Newtons theory of gravity involves a gravitational constant $G$, however one does not refer to it directly, we speak instead of gravity or the force of gravity.
Now Einsteins introduced a cosmological constant in his equation for GR, and ever since it's been referred to... |
Does the cosmological constant then refer to a cosmological force?
The Cosmological Constant acts as “repelling” gravity. This can be seen from the Friedmann equations, here it works opposite to matter and radiation density due to the negative pressure it exerts.
This just means that the universe expands accelerated ... | {
"language": "en",
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Why does angular momentum being constant prove Kepler's first law? So I was watching this video and this video on Kepler's first law in order to understand the proof of Kepler's first law. He started off by saying that for an ellipse, the distance from a focus point to a point on the ellipse equals to $$r = \frac{a(1-\... | The planets equations of motion can be represented in polar coordinate $r$ and $\phi$
$$r\,\ddot{\phi}+2\,\dot{r}\dot{\phi}=0\tag 1$$
$$\ddot{r}-r\,\dot{\phi}^2=-\frac{\mu}{r^2}\tag 2 $$
where
$\mu=G\,M$
if we multiply equations (1) from the left with $r$ we get:
$\frac{d\left(r^2\,\dot{\phi}\right)}{dt}=0\quad \Right... | {
"language": "en",
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3D perfectly elastic collision between two points There is a high probability, I think, that this question is a duplicate of some other question ... but to may knowledge, it hasn't been posed in this exact manner:
Assume we have 2 points, $P_1$ and $P_2$, of mass $m_1$ and $m_2$ in a world coordinate system $(O, \vec{i... | This is an indeterminate problem, there is infinity of solutions. To make it determinate, one has to add more assumptions to the model.
For example, one can add the assumption that the particles are not points, but perfectly solid spheres. Then we get two more equations (due to the fact that change of momentum of both ... | {
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Why we neglect the $\hbar ω/2$ in the Hamiltonian of the the Electromagnetic Field? After the quantization of the electric and the magnetic field, we get the Hamiltonian of the electromagnetic field:
$$H= \hbar ω(a^{\dagger}a +1/2) .$$
with $\hbar$ the planck constant and $a^{\dagger}$ the creation operator.
Why can w... | In general the term cannot be neglected as it causes the Casimir-Polder effect. In most cases it is just a constant and then it can be ignored, except for the fact that its present theoretical value is something like $10^{123}$. This value is ludicrously large. ZPE is an open problem of theoretical physics.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Apparent frequency as function of distance So the Doppler effect says that the frequency of sound changes due to relative motion of source and observer. My question is if there any expression that tells how the apparent frequency changes in terms of the distance between the observer and source.
I know we have
$$f'=\f... | To a pretty good approximation, the frequency of a sound wave does not change as it travels. In a plane wave solution, all points along the wave are oscillating up and down with the same period; and so the number of cycles per second is the same at the source as at the observer, no matter where the observer is locate... | {
"language": "en",
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How does the heat capacity of an object come into play in thermal radiation? So say there's a cube in space acting as a blackbody.
Each side is 2 metres. Initial cube temperature is 400 Kelvin. Mass is 15 kg.
Say the heat capacity is 500 J/kgK. How would that affect thermal radiation? Or is it not a factor? How does i... | We can answer your question by considering the first law of thermodynamics on a body with uniform temperature $T$, which is only losing thermal energy via radiation:
$$ \frac{dU}{dt}=\dot{Q} - \dot{W}$$
where
$$ U = \rho c_p T = \frac{m}{V}c_pT$$
$$ \dot {Q} = -\epsilon \sigma T ^4$$
$$ \dot{W} =0 $$
where $ \dot {Q}... | {
"language": "en",
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Young tableaus for $SO(n)$ I know how to use young tableaus to find irreducible representations and their dimensions of $SU(n)$. Are there similar rules for $SO(n)$?
| This is not a complete answer to this question
and is adapted from my answer to Irrep decompositions for $SO(N)$ tensors for $N>3$ as it also seems to apply here. Indeed you can do decompositions and find the dimension of irreps using young tableau also for SO(n). The young tableaux describe permutation of indices and ... | {
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How does the negative energy solution to the Dirac equation predict the antielectron? Please, can someone explain how the negative energy solution can be used to predict the existence of the antielectron?
| If you have a complex field, you can write it as a sum of two real fields. Now if you construct the observables for both fields, you'll find that are all identical except the charge operator, that gives a negative charge to one field and positive for the other. Then you have identical particles except for the charge. Y... | {
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Derivation of the work-energy theorem We state the following version of work-energy theorem :
$$ K_2-K_1=Fd=W $$
Where acceleration is assumed to be constant, so is the force $F$.
Then the physicists proceed by writing
$K_2-K_1=F[x(t_2)-x(t_1)]$
$$=F(x_2-x_1)$$
Notice, $x(t)$ was a polynomial of time $t$ with highest... | The force is constant in the result because you start out by assuming it constant from the very beginning. The expression $$Fd=W \qquad \text{(or }\; F(x_2-x_1)=W\text{)}$$ only holds true for constant force. The general expression is $$\int_{x_1}^{x_2} F \;\mathrm dx=W$$ If you start out with that and redo the whole d... | {
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Rotational work done So the formula for rotation work is
$$\mathrm dW=T\cdot\mathrm d\theta$$
where T is torque.
While solving a question like in case of a body rolling down an incline (pure rolling), we usully equate the change in kinetic energy equal to work done by gravity (just the translational work). Why do we no... | First, for something like a ball rolling down an incline, gravity has no torque about the center of the ball. The force that causes the ball to start rolling is friction. On a frictionless incline the ball would just slide down the incline without rolling.
Now, with that out of the way, it turns out that we do take int... | {
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Why does sound behave differently in water than in air? I noticed in some experiments at home that sound does not behave the same in water than in air. Is there a good scientific explanation to this?
I noticed that the sound sounded distorted in water but not in air.
I also used a software that I could use to hear the... | When any wave enters a different medium, the wavelength and direction can change (from the refractive index of the medium). For example, there is a 33% change in refraction between water and air (for light).
Sound waves are less affected than waves at light speeds, but there is still an effect.
http://hyperphysics.phy... | {
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Torque due to friction on a series of disks Recently I was looking at two situations involving friction and torque.
The first situation seemed pretty straightforward at first. A disk of mass $m$ and radius $r$, with a coefficient of static friction $\mu _s$ with the ground, is given a force $F$ originating at its cente... |
Lets write the equations for a free body diagram and see if we can determined the force $F_r$
Left disk
$$-m\,a_1+F-F_c-\mu\,N_1=0\tag 1$$
$$-m\,g-F_r-N_1=0\tag 2$$
$$-m\,r^2\,\alpha_1+\mu\,N_1\,r+F_r\,r=0\tag3$$
right disk
$$-m\,a_2+F_c-\mu\,N_2=0\tag 4$$
$$m\,g+F_r-N_2=0\tag 5$$
$$-m\,r^2\,\alpha_2+\mu\,N_2\,r-F_r... | {
"language": "en",
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Charge $Q$ in cavity inside conductor A point charge Q is inside a cavity in an uncharged conductor. Is the force on Q necessarily zero?
The explanation in the answer goes as
No. For example, if it is very close to the wall, it will induce a charge of the opposite sign on the wall, and it will be attracted.
This sou... |
First of all, it says if Q is very close to the wall, it will induce a charge of the opposite sign on the wall but opposite sign shall develop every time on the inner surface of cavity irrespective of the particle's position inside the cavity.
Yes, the presence of the charge Q will always induce charges of the opposi... | {
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Conceptual understanding of operators in QM Do operators in QM represent in some fashion the action of the measurement apparatus on a state being measured? Usually operators in QM are introduced as abstract transformations whose eigenvectors/eigenvalues are axiomatically the possible results of measurement, with an exp... | This is the essence of the decoherence-based understanding of measurement. The idea is that the operators in the Hamiltonian act upon the system's state, causing it to decohere into a probability distribution at a certain basis, called a "pointer basis". For example, because physical interactions are generally based on... | {
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Dot product in E&M
I'm learning graduate level E&M. Textbook is a famous Jackson book. What I would talk now is about pp.295-298 in 3rd ed. I attached the photo of p.298.
It says (paragraph above eq.(7.15) and footnote in the photo) that $\vec{n}\cdot \vec{n}=1$ doesn't mean n is unit vector if n is complex vector. An... | The vector $\hat{n}$ is meant to have real components, so the definitions are equivalent.
The conjugation is applied to calculate the Poynting's vector only because we like to work with complex exponentials, but you should only care about the real part. Consequently, you can either use $S\propto\Re e(\vec{E})\times\Re ... | {
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Friction acting as an internal force I was solving this problem in my assignment:
Assuming a frictional force F acts on the block of mass m, a force -F will act on plank of mass M. Hence, the net work done by frictional force should be zero, as friction is an internal force , but option D is given incorrect. What's t... | Assume the unit vector $\hat i$ points to the right in your diagram, the bottom block is on a frictionless surface, block $M$ starts from rest and block $m$ has an initial velocity $v_{\rm i} \,\hat i$.
The magnitude of the frictional force on both blocks is $F$ and their directions are opposite (Newton's third law).... | {
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Different expressions for distance & displacement : $\int$$d$$|\vec r|$, $\int$$|$$d$$\vec r$|, and $|$$\int$$d$$\vec r|$ I came across these expressions in my book. And the book says that all these are different from each other.
The expressions are : $\int$$d$$|\vec r|$, $\int$$|$$d$$\vec r$|, and $|$$\int$$d$$\vec r... | It is the radial part of the total distance along some path. For example, for a circle there is an angular part of the total distance ($2\pi r$), but there is no any radial part contribution: $dr=0$.
| {
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Why is the far point of human eye infinite? In my exams, the presence of this question, which unfortunately I couldn't answer, made me wonder why is the far point of an eye infinite?
First thing that came into my mind was that how come we can see till infinity?
Far point of eye is sometimes described as the farthest po... | Seeing an object is equivalent to seeing the light reflecting off this object. Without outside interference, light traveling from a far away object is identical to light traveling from a close object.
Therefore the far point of the human eye isn't limited by distance, but by circumstances obscuring light beams from re... | {
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How can I recreate the greenhouse effect in my car? I am trying to preheat my car during winter mornings using the greenhouse effect. I understand the greenhouse effect in cars works by visible light passing through the glass, with most UV and infrared being blocked by the glass. The visible light is either absorbed by... | Greenhouses are viable because the Sun's irradiation is (1) free and (2) strong (maximum of about 1 kW/m²). These advantages aren't available with your current configuration.
To heat your car, you'd want a mechanism that offers at least 100% efficiency (e.g., a resistance heater or heat pump) that transfers energy dir... | {
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Does the resistance of the voltmeter affect the behavior of this circuit? I have this setup.
It consists of a battery of no internal resistance with voltage $V$ and a resistor with resistance $R$. It also consists of a voltmeter of some (not so large) resistance as good ones should have.
Now my question is, will th... | You have it backwards. The resistance of virtually every digital voltmeter is standardized at 10 megohms. The idea is the circuit should not "see" the meter. Almost no current flows through the meter. Conversely anmeters should have the lowest resistance possible so there is almost no voltage drop across them. The reas... | {
"language": "en",
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"source": "stackexchange",
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Are the net microstates of the universe increasing? In physics and chemistry we learn that entropy is given by
$$S=k\ln\Omega$$
where $S$ is entropy, $k$ is Boltzmann's constant, and $\Omega$ is the number of microstates in the system. We also learn by the 2nd Law of Thermodynamics, that the entropy of the universe al... | In statistical mechanics, the word entropy is used for $k\ln\Omega$ where $\Omega$ is the number of microstates that are compatible with specified conditions, such as a given total energy and/or a given total volume. If no conditions are specified, then it's the total number of possible microstates. Therefore, whenever... | {
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Can we know whether it’s a $1D$ or a $2D$ motion just by looking at the position-time relation? How do I know whether it is a $2D$ or a $1D$ motion, just by looking at position-time, or velocity-time, or acceleration-time equations?
Maybe the question is not very clear, I’m not sure I’m getting it across properly, so ... | My solution:
2D case
given the position vector $\vec {R}$ with the parameter $t$
$$\vec{R}=\left[ \begin {array}{c} x\\y\end {array} \right]=\left[ \begin {array}{c} f \left( t \right) \\ g
\left( t \right) \end {array} \right]
$$
Ansatz $y(x)=a\,x+b$
,the slope $a$ must be const.!, $\quad$ with $a$
$$a=\frac{\parti... | {
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Why magnetic field starts from north pole and ends in south pole? In magnets such as bar magnets the magnetic field lines are starting from North pole and ends in south pole..but I don't know what is the reason for it..why this happens.
| Because it is a fundamental property of nature. Magnetic field lines forms closed loops. Two bar magnets held over unlike poles (a north end and a south end) would attract. You can imagine a field line connecting the two ends. That is a loop by itself.
In fact, all about magnetic field lines that physicists know is ab... | {
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Why my cloud chamber won't work? let me briefly introduce myself, I'm a highschool student from Indonesia, I'm currently making a cloud chamber for my science fair project, I did what the videos on youtube told me, but for some reason my cloud chamber won't work. Here are some pictures of my setup
So, I glued felt on t... | The type of light one uses seems to make a difference.
When using a linear LED source, I saw nothing (not even the "snow"). Then I switched to an old-style (filament bulb) flashlight, I saw the "snow" AND a few tracks. Using a point-source LED (3rd attempt) saw nothing (again). All other parameters (chamber, temperatu... | {
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Length Contraction Scenario Suppose a space ship is traveling from star A to star B at some significant fraction of the speed of light. In the frame of the ship, the distance A to B is less than the distance in A and B's rest frame. Is it possible for the ship to quickly increase its speed so that in its frame the ship... | Results are somewhat different.
Suppose the observer aligns his x-axis passing through stars and Star A has some negative x-coordinate (-x1) and B has some positive (x2). Clearly observer stands at origin (0). We clearly quantify our definitions:
*
*Distance between A and B: x1+x2
*Distance between observer and A: ... | {
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Hamiltonian of a quantum heat bath I have seen the Hamiltonian for a heat bath written as:
$$ H_B = \hbar \int_0^\infty \omega b(\omega)^\dagger b(\omega) d\omega $$
I was hoping to understand this equation better. This suggests that the heat bath is written as a sum of harmonic oscillators with raising/lowering operat... | *
*Yes the raising operator $b(\omega)^\dagger$ corresponds to a mode of frequency omega and $n(\omega) = b(\omega)^\dagger b(\omega)$ count the number of excitations. You can see that the mode has a frequency of $\omega$ because that is the prefactor of the mode in the Hamiltonian, so one excitation of that mode incr... | {
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Is compressive strain equal to streching strain? The stress induced on a piece of material (like a rectangular beam) is defined as:
$$\sigma = E\epsilon$$
where $E$ is the elastic modulus and $\epsilon$ is the strain.
The strain on the other hand is defined as the ratio of the new length of the beam to the original len... | This is not correct. Stretch $\lambda$ is the ratio between length increments so for an undeformed part it would be close to 1, while the strain for an undeformed part would be close to zero. The stretch in both cases is:
$$\lambda_1 = \frac{\text{d}L_f}{\text{d}L_0} = 1.50, \qquad \lambda_2 = \frac{\text{d}L_f}{\text... | {
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Are all solutions of Maxwell's equation related by a gauge transformation? Consider Maxwell's equation (without source):
$$ \partial_\mu F^{\mu \nu} = 0 \implies \partial_\mu \partial^\mu A^\nu = \partial_\mu \partial^\nu A^\mu.$$
Can we find a pair of classical field configurations $A^\mu(x),A'^\mu(x)$ such that they ... | Of course there are many solutions that are unrelated by a gauge transformation, namely plane waves of any frequency, propagation direction, polarisation and phase and superposition thereof.
| {
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"answer_id": 0
} |
Why aren't particles constantly "measured" by the whole universe? Let's say we are doing the double slit experiment with electrons. We get an interference pattern, and if we put detectors at slits, then we get two piles pattern because we measure electrons' positions when going through slits. But an electron interacts ... | As long as these interactions are weak and do not distinguish between the two slits, they can be disregarded.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/460855",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "66",
"answer_count": 6,
"answer_id": 1
} |
Angular acceleration of a double compound pendulum How can I calculate the angular acceleration of a double compound pendulum? I'd like to know what the angular acceleration of each of the pendulum's center of mass will face at any point in time.
PS - Not a physics guy but love the subject would really like to learn. I... | The deflection angles of both pendulums are the degrees of freedom of the system.
First pendulum will have $\ddot{\theta}_1$ and second one will have $\ddot{\theta}_1+\ddot{\theta}_2$.
This is very trivial, maybe you are trying to ask something else?
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/460980",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
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