Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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Principle of least action and greedy algorithm Is the principle of least action sort of a greedy algorithm that all mechanical systems follow?, sometimes to minimise and sometimes to maximise the quantity we call action, at each individual step.
| No, the principle of least action (more properly, the principle of stationary action) is not an algorithm. In particular, it doesn't have steps.
It just states that the physically realized path of any system is a critical point of the action functional, which equivalently means its Euler-Lagrange equations are the equ... | {
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What is the energy source if a tall water tank is used to transfer floating objects upwards instead of cables with motors? Suppose I want to transport some logs from the ground to the roof of a tower. Originally I can use a lift, or some cables, or even move the logs upwards manually; then the energy is converted to th... | The work you need to do (to insert the log) against the pressure of the fluid at that depth is equal to the work done by the fluid to get the log up to the height you desire.
If you consider a log of volume $V$ and a tank of depth $h$, the pressure at that depth would be $\rho gh$, where $\rho$ is the density of the f... | {
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Can you compress pure carbon into diamonds? I'm doing a science project, and we're wondering if it is possible to compress pure carbon (C) to the point where it becomes diamonds? What would the process have to be and how much energy would this take? Has this been done and is this feasible?
| This is not entirely serious, but if you're looking for a really great project, you can't do much better than this. Are you aware that you can make diamonds with high explosives? And since the starter mixture is not well-known, you might have to make multiple attempts.
| {
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Comparing relative velocities and kinetic energy If object one is moving $0\frac{m}{s}$ relative to the Earth and object two is moving $1\frac{m}{s}$ and they both have $1kg$ of mass, then object one has no kinetic translational energy (relative to the Earth?) but object two has $\frac12$ joules of kinetic translationa... | The initial calculations of kinetic energy are correct in the local rest frame (the Earth). However the relative energy differences are not the same in other rest frames.
One way of thinking about this is to remember that work is "force times distance". Imagine you want to "push" two objects to accelerate them each ... | {
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Electrostatic induction, induced charges Is it true that if a conducting object is not grounded, the nearby charge will induce equal and opposite charges in the conducting object?
It is mentioned on Wikipedia (electrostatic induction) but it is also mentioned that charges will appear such that the total electric field... | The charge is induced inside an conductor so as to balance the electric field of the external charge.In steady state condition electric field inside the conductor is always zero,external electric fiels is cancelled by the induced charges on the surface of the conductors.
| {
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How do I figure out the totally airborne height for a given machine? Technically "airborne" can just mean to move through the air, but I would like to know how high you have to be before you are entirely supported by air in a helicopter-like machine, as opposed to benefiting from the reaction from the earth (or whateve... | I wasn't able to find a well-sourced table, but this page has a chart that shows a helicopter has almost zero ground effect advantage when hovering at a height greater than 1.25 times the rotor diameter.
That fits well with this page from a testing company that states that ground effect on a fixed wing craft reduces dr... | {
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Origin of radio waves In the same way as the origin of X rays is the excitation of electrons, what is the origin of radio and infrared radiations in this respect?
| Indeed, Xrays and infrared are just photons, of different energy. However, since they correspond to different frequencies and thus to different energies, they can be linked to other characterictic phenomena. For example while Xrays are commonly linked to their occurence in electronic spectra (the "Characteristic Xrays"... | {
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Hot Object In a Cooler Space How would you calculate the heat given by an object that is hotter than its surroundings? I know there's Newton's Cooling Law, but what about any heat given off by radiation? Are these additive quantities?
| It comes down to the calculated load to heat the total area in cubic feet of the space to 72 degrees. Then add the additional heat to overcome heat loss from walls, windows, doors, floors etc. Then minus the heat gain from people, equipment, solar etc. Finally you design for peak condition like the coldest day of the y... | {
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How to calculate relativistic mass versus time? I know that relativistic mass is calculated as:
But when mass m0 gets bigger, also mass of the accelerating body mass m gets bigger so m0 gets bigger and so on.
So if body accelerates with acceleration a in time t and original mass m, how big will be the m0?
If the formu... | The mass $m_0$ is the rest mass, also known as the invariant mass. It is a constant and is the same for all observers regardless of how they are moving. So you are overcomplicating your calculation - just hold $m_0$ constant and calculate $m$ by putting in the velocity $v$.
However, as you will find if you study physic... | {
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Does Quantum Mechanics say that anything is possible? I may be incorrect in this, but doesn't Quantum Mechanics state that everything has a probability of occurring (with some probabilities so low that it will essentially never happen)?
And if I am correct in my thinking, could it mean that, quite literally, anything h... | Quantum Mechanics does not say that everything is possible. In fact, it says that certain things are impossible. For example, a bound electron orbiting a hydrogen atom can only be measured to have certain discrete energy values with no possibility of measuring things in-between.
Although your example is right, quantum ... | {
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Why didn't the glass break? In the figure below, a needle has been placed in each end of a broomstick, the tips of the needles resting on the edges of filled wine glasses. The experimenter strikes the broomstick a swift and sturdy blow with a stout rod. The broomstick breaks and falls to the floor but the wine glasses ... | when a person hits a broomstick, the direction of the force F applied is vertically downwards. according to 3rd law of Newton, there would be an equal and opposite reaction. so in this case the broomstick breaks into two parts, each part having a torque equal to the force F and moment arm (1/2 length of the broomstick)... | {
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Why can't we define a potential energy for a non-conservative force? We could define potential energies for non-conservative forces too and then we could conserve it with kinetic and potential energy as we know it. But no one does that. Why is this? Please explain. Any help would be great.
| A system with friction is a simple example of a non-conservative force field. Let's assume that in this system, an object has potential energy E. Now let's make the object make a small excursion - up, left, down, right. It returns to the same point, but it had to do work in order to overcome the friction. If that work ... | {
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Is there any physical interpretation for $\nabla\cdot(\nabla \times F)=0$? It is well known that the divergence of the curl is always 0. Mathematically I understand why this happens ($d^2=0$ where $d$ is the exterior derivative) but today I was wondering what is the physical meaning of this.
The divergence represents t... | I don't think there is any physical meaning. There is geometric meaning, however, which is almost as good, in this case.
Consider the integral theorems, namely the Kelvin-Stokes-theorem: $$ \int_\Sigma \nabla\times \mathbf{V}\cdot d\mathbf{A}=\int_{\partial\Sigma}\mathbf{V}\cdot d\mathbf{l}, $$ and the Gauss-theorem: $... | {
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How exactly can a speaker produce the huge number of frequencies in musi at he same time? I know the basics of how a speaker works, but when I think about the fact that just one sound from an instrument has many frequencies happening at once and there are other sounds such as the guitar pick, echo in he chambers, etc. ... | it's superposition. all the waves add together, and that makes one complicated motion that the speaker vibrates. if it weren't for superposition of waves, none of this could happen. perhaps you would like to study the wave equation, and more about adding sine and cosine waves.
| {
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What causes an emulsion to be stable or unstable? The other day I made a salad dressing based on oil and vinegar. To my understanding, there is a positive energy associated with the surface between the oil and vinegar. The most stable state is also the state of minimum energy, hence the mixture tends to separate into t... | With the exception of some microemulsions an emulsion is always thermodynamically unstable. That is because the interfacial tension is always greater than zero so it always costs energy to increase the interfacial area.
Emulsions exist only because they are kinetically stable. If you calculate the energy as two emulsio... | {
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Thermodynamics equation that involves time? There are principles in thermodynamics—e.g., that heat transfers from hot to cold—and equations like $$Q=mc\Delta T,$$ but is there any equation in thermodynamics related with time?
| The area of Transport Phenomena deals with momentum transfer in solids and liquids (fluid- and solid mechanics), heat transfer, and mass transfer (diffusional). In freshman physics, you learned about the rate of conductive heat transfer along a rod from the hot end to the cold end, and learned it is related to the the... | {
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Motion of an object in rotating frame Yesterday I was looking at an old sloan video that describes motion in inertial and non-inertial frame. An experiment was actually like this. Two persons are sitting on the opposite side of a table fixed to a turning platform. The platform is rotating in uniform circular motion. N... | The ball will move in a straight line according to an observer outside. But the observer is rotating, he will see ball deflecting away. This is called $\textbf{coriolis effect}$. Watch this video to understand properly.
coriolis effect
| {
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Is spin-orbit coupling really necessary for topological insulators I have heard that for an insulator to be non-trivial, large spin-orbit coupling is necessary.
However, I have read the definition of $Z_2$ topological invariant and chern number. In no way can I recognize what role spin-orbit coupling plays in topologic... | Usually a topological insulator means a symmetry-protected topological state with $U(1)$ and time reversal symmetries. I believe you are using the same definition, since you are talking about the $Z_2$ topological invariant. In this case, spin-orbit coupling is necessary in the sense that it is responsible for breaking... | {
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How can light emit three kinds of spectra? In case of black-body radiation, radiation gives continuous spectra. Molecular spectra is an example of band spectra. Similarly, there's also the atomic spectra. Why are the spectra of light not same in all cases?
|
In case of black body radiations , radiations give continuous spectra
No. The radiation is still quantized, it just can't be identified with much ease as in the case of an atom or molecule. Because there are an enormous number of particles on a black body and there are all possible states you can think of. Since the ... | {
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How accurate is Newtonian mechanics? How accurate is (theoretical, i.e. the mathematical formulation basing on "formalised" physical constants, such as material density) Newtonian mechanics?
What does the accuracy depend on (material properties, environmental variations)?And how (well) can these be "coupled" into calcu... | The question is very vague and is perhaps best answered by reading about the failings of Classical Mechanics - for instance here.
Newtonian Mechanics is accurate enough for day-to-day calculations.
Where Newtonian mechanics becomes inaccurate is in the very small, Quantum world or the very fast, where Special Relativi... | {
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Stark broadening and Voigt fitting I have LIBS spectral data acquired with a CT spectrometer of resolution 0.4nm. I fitted the Voigt profile into the spectral peak at $\lambda_0$. The lorentz $\Delta \lambda_L$ and the gaussian $\Delta \lambda_G$ were calculated. I also used a UV lamp to obtain the instrumental parame... | There are several broadening mechanisms, and you have to know how they "add" together. Since a Voigt profile is the convolution of a Gaussian and Lorentzian profile, you rightly calculated both widths rather than just the overall width of the Voigt peak. For the Lorentzian portion, the width is the arithmetic sum of ... | {
"language": "en",
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If gravity dropped off with the cube of distance If gravity, for instance, dropped off with the cube instead of the square of distance from the Sun, would the planets still follow elliptical paths?
| The short answer is no, if you require that the orbits be stable and closed.
Whereas any central potential can produce "circular" orbits given the correct initial conditions, it has been shown (by Bertrand as mentioned in the comments) that there are only two potentials that can possibly produce stable and perfectly cl... | {
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Conservation of angular momentum during rolling
A disk having initial angular velocity $\omega$ is gently placed on a rough horizontal surface. What is the angular velocity of rotation when pure rolling starts?
I've tried applying conservation of angular momentum about the axis passing through the point of contact of... | because i think since there is no fixed reference axis about which angular momentum can be conserved.. where to choose axis is the problem.. if u choose axis at the point of contact the point itself is moving. if any external frame is asked to come in picture the value of r*v {rcrossv} (where r is the radius vector fro... | {
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Can we tell a wormhole from a black hole by observation? I'm assuming stable wormhole really exist (naturally/microscopic or not) how can we distinguish between a wormhole and a black hole since we can't probe their "insides"?
| This is a from Cornell University Library, hopefully you can delve more into it for your answer.
Distinguishing black holes and wormholes with orbiting hot spots
Zilong Li, Cosimo Bambi
(Submitted on 8 May 2014 (v1), last revised 25 Jul 2014 (this version, v2))
The supermassive black hole candidates at the center of e... | {
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Physics after a Theory of Everything There is a lot of controversy over the existence of a Theory of Everything (ToE), and as far as I know, we are a long way from having a possible candidate. But what interests me is, what after that? If a Theory of Everything is truly found, will there be anything left for physics to... | No, I think. There are many open problems in condensed matter physics. For example, I heard from my professors that the mechanism behind new superconductors is still a mystery.
Also. To understand the underlying rules does not always explain the system as a whole. There are complex systems that obeys simple underlying ... | {
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How does calblock (water softener) work? This "calblock" device claims to prevent limescale accumulation on water heaters in a washing machine. It sits between the water faucet and the washing machine; the water passes through it.
I cannot imagine how it could work. This video, made by a supplier, describes its functio... | I found this from an article, the writer was skeptical that it truly worked. The writer in the article says this has been a marketing scam since the 1930's.
Another common claim is that MWT causes calcium carbonate to precipitate as aragonite, a crystalline modification of CaCO3 that is slightly less stable (and more ... | {
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Effect of Particle Mass on Thermal Conductivity Fouriers law of thermal conductivity is
$$ \vec{q} = -k\nabla T $$
where $q$ is the heat flux, $k$ is the thermal conductivity.
Mass does not seem to appear in the equation. So I'm wondering what if keeping all else constant, but if we change the particle mass (i.e. incr... | The expression for thermal conductivity is given by
$$ K = 1/3 Cvl$$
where $C$ is the heat capacity per unit volume, $v$ is the average velocity of the thermal conducting particles and $l$ is the mean free path of the thermal conducting particles. In a solid, the main contribution of thermal conductivity comes from pho... | {
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Conservation of angular momentum while sitting on a spinning chair Today my friend was sitting on a spinning char. By moving his top part of the body left to right and his bottom part of the body the opposite he managed to spin. As I understand Conservation of angular momentum if he was just sitting still he should not... | The angular momentum is conserved only if the net torque on a system is zero. By performing his body motions, your friend was able to let the ground generate a torque on him, by the virtue of the frictional force acting on the chair due to sideways movement. If no frictional force were there, your friend would never sp... | {
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Distinguishing real forces and fictitious/pseudo forces in Newtonian mechanics In understanding the law of inertia I had to consider the motion of bodies screened from the so called "real forces".
*
*What characterises these real forces?
*What makes us call them real?
*Or what is separating the forces called tru... | A real force has a reaction force. A pseudo force has no reaction force.
| {
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Does quantum mechanics play a role in the brain? I'm interested in whether the scale of processes that occur in the brain is small enough to be affected by quantum mechanics. For instance, we ignore quantum mechanics when we analyze a game of tennis because a tennis ball is much too large to be affected by quantum mech... | Brain as a biocomputer is far to complex to expect an fully descriptive answer in a form of a forum post.
Don't worry, the situation is even more riddled! :) The opinions on the topic of quantum behaviour affecting the perception of reality (and accordingly creation of "reality", but I am touching very metaphysical vie... | {
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When an object is moving at a constant velocity,would the normal force and $mg$ be equal? Does the object's normal force and $mg$ cancel out, resulting in the two force becoming equal, or would one force be greater than the other? Thank you!
Edit: Also would the $mg$ be considered weight or would it be more correct to ... | If the object is moving on a horizontal frictionless surface, then the normal force equals $mg$.
Yes, $mg$ is the weight of the object.
| {
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What does the Dirac delta function physically do while deriving Gauss Law form Coulomb's law? While doing this derivation, the the source coordinates are mentioned as "$s$" and the coordinate of the point at which field is to be calculated is mentioned as "$r$". Kindly follow this Wikipedia link and click on the "Outli... | $\rho(\vec{s})$ is function of $\vec{s}$ where $\vec{s}$ is position vector of the location of charge density.Whereas $\vec{r}$ is position at which you want to calculate $E(\vec{r})$. So what you are essentially doing is calculating electric field due to some elementary volume $d^3s$ located at the position $\vec{s}$ ... | {
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Isn't LIGO basically measuring the luminiferous aether? I am bit confused about this one. I am not very acknowledgeable about gravitational waves and LIGO. But if it is basically a Michelson interferometer and can detect shifts in vacuum, doesn't this means that we detected the luminiferous aether and if not, why? Is t... | Here is my less-than-scholarly answer. Please tell me its weaknesses: The luminiferous aether was postulated as a medium necessary to the transmission of light-waves. If the light-waves were merely a perturbation of a medium, then the motion of an observer through that medium would change the apparent speed of the pe... | {
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Independent canonical coordinate variables? In Goldstein's Classical Mechanics (2nd ed.) on section 9-1 page 382, there is a discussion about finding a canonical transformation $(q_i,p_i)\rightarrow (Q_j(q_i,p_i,t),P_j(q_i,p_i,t))$ from a given generating function $F=F_1(q_i,Q_i,t)$. The following is written on the pag... | Let us suppress $t$-dependence in this answer for simplicity. A canonical transformation$^1$ $$(q,p)\quad\longrightarrow\quad (Q(q,p),P(q,p))$$ can be viewed as a graph, thereby
yielding a $2n$-dimensional submanifold $M$ embedded inside a $4n$-dimensional total manifold $N$. The total manifold $N$ has $4n$ local coor... | {
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visible light spectrum Why do we see black objects? Colors of objects are formed when the spectrum of that color is reflected. Example Green objects are green because they reflect the green spectrum of light, red objects are red because they reflect red spectrum of the visible light and white objects because they refle... | Black is entirely a construct of your brain, created internally to fill in a spatial region in which you eyes detect no appreciable level of photon emissions. That may sound a bit surprising, but try this: Imagine red, then green, then blue. You could do it, right? Well, no photons were involved there either. Black is ... | {
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"timestamp": "2023-03-29T00:00:00",
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"question_score": "3",
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How to calculate drag forces on an object Previously, I theoretically calculated the velocity of a bb, accelerated by air pressure, when it exits a barrel. In short, I calculated my velocity to be about 150m/s. However, I wanted a more realistic speed. I looked up the drag equation and tried to apply it to get a more r... | You have a different situation when the bb is inside the barrel of the bb gun. Assuming that the bb is a tight fit in the barrel (and it should be), there is pressurized air pushing it. The air is doing expansion work on the bb as it does so. Due to this, you need to use the thermodynamic relationship for the proces... | {
"language": "en",
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Can the Heisenberg Uncertainty Principle be explained intuitively? I have heard several pseudoscientific explanations about the Heisenberg Uncertainty Principle and find them hard to believe.
As a mathematician mainly focusing on functional analysis, I have a considerable interest in this matter. Although I still have ... | Non-scientific joking answer may be like that:
The Product Release Uncertainty Principle says that you may know what your product will do or when it will be released - but not both things together.
Quick explanation: the company will never have enough "resources" to do the full testing in a certain amount of time. In ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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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 addition to the other answers, let me add an intuitive one:
Take the case of a body moving a bit too fast for circular orbit. As you say, it will move outward. But as it moves outward, it’s climbing against gravity. It’s kinetic energy goes down. It’s speed goes down. Eventually, the speed reduces to less than that ... | {
"language": "en",
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Homework - Young's Double Slit Experiment
I have attached the question as a picture. I simply don't understand where to begin this question. It is given that $l \gg d$. Why shouldn't the phase difference at $O$ be zero? What exactly is different about this set-up as compared to the 'normal' set-up of Young's Double Sl... | The text states that the source $S$ is placed just behind the slit $S_1$, so it looks to me that the sources $S_1$ and $S_2$ already are out of phase by a factor $\exp(i k d)$. Thus the phase difference in $O$ is $\Phi_O = k d$.
This is different from the usual experiment where $S$ is placed at equal distance from $S_1... | {
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Thermodynamics and Newton's second law Is it correct to say, that the Newton's laws (or a Newtonian system) is reversible if the friction isn't considered (the fact that the time is of second order $\frac{d^2x}{dt^2}$) and an isolated thermodynamic system is irreversible due to the second law ($\frac{dS}{dt}\ge 0$)?
Ca... | You have to be a bit careful about what you mean when you ask about Newtonian mechanics being reversible or not. As stated in one of the comments newtons mechanics is only a set of rules that tell you how objects accelerate if they are subject to some sets of forces. It does not necessarily tell you the nature of these... | {
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Where does the factor of half appear from in the Klein-Gordon Lagrangian for a real scalar field? The lagangian density of a scalar field or a Klein-Gordon field has the form of
$$\begin{align}
\mathcal{L} = \frac{1}{2} \partial_\mu \phi \partial^\mu \phi - \frac{1}{2} m^2 \phi^2.
\end{align}$$
Why is there a factor of... | This normalization is convenient when you work in Fourier space, since it implies that
$$ \langle \phi(p) \phi(q) \rangle = \frac{1}{p^2 + m^2} \delta(p+q)$$
which is easy to remember. You normally derive this using path integrals/functional integration, and in that way the factor of $1/2$ comes from a Gaussian integr... | {
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If I walk forward and stop was there work done? If I'm on a flat surface and I start walking and go to a constant velocity, then I slow myself back down to a stop, was there work done?
I would think the answer is no because $work = force * distance$ and when I walk up to constant velocity I've applied a force times dis... | Work is a scalar quantity, which means it doesn't have a direction, unlike velocity or force. So you did work in speeding up and more in slowing down.. I.e the works adds together and doesn't cancel itself out.
| {
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Why is the potential energy minimal when the repulsion and attraction force between molecules is 0? According to this potentialcurve and the other curve(?) the potential energy is minimal when the intermolecular forces equal zero.
Why? How should I interprete negative potential energy in this context?
| Remember that potential energy is defined up to a constant. Here it's defined so that when $r \to \infty$ the potential energy is worth 0, for convenience. So that there's nothing special about a negative potential energy per se.
What really matters here is that there's a "well", i.e. a region for $r$ where the potenti... | {
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Classical gravitational wave Consider the electric field surrounding an electron. The electron moves. A wave propagates at $c$ reflecting the electron's movement. This is an electromagnetic wave. Consider a mass. Classically surrounded by a gravitational field. The mass moves. The field deforms and it propagates at $c$... | Newtonian gravity doesn't predict anything traveling at speed $c$ as neither the constant $c$ nor any constants you can use to make the constant $c$ even appear in Newtonian Gravity.
Maxwell has $\epsilon_0$ and $\mu_0$ from which you can make $c$. And General Relativity and Special Relativity have $c$ directly as a co... | {
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How to calculate the frequency of oscillation of superposition states Been working on this question for a while and I'm not sure how to go about it. Could someone point me in the right direction, particularly for the frequency question.
The question is as follows:
A wavefunction is described by the superposition state ... | Yes, you multiply each of the super-positioned states (energy eigenstates) with their time evolution as
$$ \Phi(x,t) = \phi_n(x)e^{i E_n t/\hbar} + \phi_{n+1}(x)e^{i E_{n+1} t/\hbar} $$
here $\phi_n(x)$ is the first state and $\phi_{n+1}(x)$ is the second.
As for the frequency of this state, try to write it as
$$ \Phi... | {
"language": "en",
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Why bar magnet can produce magnetic field? Say I wrapped a piece of wire around an iron bar in a closed circuit connected to a DC power supply, the electrons starts flowing and moving charge produce magnetic field. Yet a bar magnet can produce a magnetic field without any charged particles moving around, are the magnet... | There is only one type of magnetic field, but there are multiple ways to generate a magnetic field. A bar magnet has a permanent magnetic field while a electromagnetic has a magnetic field only when the current is applied.
Another way to get a magnetic field is to induce one. So you place an iron bar near one pole of ... | {
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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 ... | Stability is a valid point, but probably not really the real reason.
People think and design in 90 degree angles. Everything around us is squarish. Rooms, houses, windows, doors, even books and screens. Orthogonality makes calculations and manufacturing simple. It's all about parallel and perpendicular lines, easy cutt... | {
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Why do most office chairs have 5 wheels? (Inspired by Why are four legged chairs so common?)
I've been wondering for a while... Why do most wheeled office chairs have 5 wheels?
My guess would be that while stability vs. simplicity results in 4 legs, adding mobility to the equation may result in the need for 5 wheels.
... | i'm pretty sure that if you check the OSHA (the U.S. Occupational Safety and Health Administration) regulations for chairs found at https://healthfully.com/osha-regulations-workplace-chairs-5916239.html you will find the ultimate answer to this question.
in addition to specifications pertaining to the back, seat and ar... | {
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Where does the energy go if a ball hits the ground WITHOUT bouncing? so if a very un-bouncy object hits the floor and crashes into it rather than bouncing, where does most of the energy go? Does it simply turn into heat from friction? Or does it go into the Earth's movement by an imperceptible amount?
| The energy has several options to get dissipated into:
*
*Major part of it is turned into heat as a result of friction.
*Some part gets transmitted into sound energy, causing the sound we hear when the object falls.
*A very feeble amount gets transformed into light energy.
*Another miniscule portion is utilised i... | {
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Why do some materials shrink when their temperature increases? In my vision it would seem quite logical that all materials expand when temperature rises. Because the particles get more energy and travel larger distances when moving. But apparently there are some materials that tend to shrink as temperature increases.... | Water has a very narrow range of temperatures over which it expands when cooling rather than contracting (IIRC +2 to 0 Celsius). This occurs due to the way the highly polar molecules "line up" with each other near the freezing point.
For a more interesting example, consider a number of rubber compounds which shrink as... | {
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Why during free-expansion, temperature changes for real-gas while the same doesn't happen so for ideal gas? While reading Free-expansion, I got this:
Real gases experience a temperature change during free expansion. For an ideal gas, the temperature doesn't change, [...]
I know that temperature doesn't change for fre... | The internal energy of a real gas depends on both temperature and pressure. So, if U remains constant and pressure changes, the temperature must change. In the ideal gas limit of very low pressures, the pressure dependence of real gases weakens and approaches zero.
| {
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"source": "stackexchange",
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Chern-Simons theory The Chern-Simons 3-form is given by
$\omega_3={\rm Tr} \left[ A\wedge dA+\frac{2}{3}A\wedge A\wedge A\right]$
where $A$ is a connection one-form in the adjoint representation of a non-Abelian gauge group.
My differential geometry is rather rusty (and this is new to me too) hence my questions;
$A$ i... | That's because you are forgetting that $A$ has a Yang-Mills index. You better write this in components, which reads
$\epsilon^{\mu\nu\rho} g_{IJ} \Big( A^I_\mu \partial_\nu A_\rho^J + \frac{1}{3} f^J{}_{KL} A^I_\mu A^K_\nu A^L_\rho \Big) $
This component notation also answers your second question.
| {
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Does Type IIB string theory feature D(-1) and D9 branes? I'm slightly confused as to how one deduces the D-brane content of a string theory by studying the p-form fields in the effective field theory.
In type IIB supergravity one has:
The Kalb-Ramond 2-form potential, which couples to 1-dimensional objects, namely just... | The best way to systematically determine the "fundamental" brane content is to classify the relevant cocycles on extended supersymmetry algebras that serve as the WZW-terms ($\kappa$-symmetry) terms for the relevant Green-Schwarz sigma-models for these branes.
For type IIB this was done in:
Makoto Sakaguchi, "IIB-Brane... | {
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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... | It would experience no net force inside shell. It is free to move.
| {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Same equation, different meanings I went into a physics classroom today and saw this equation written on the board:
$$
E = \frac \sigma \epsilon
$$
At first I thought it referred to the electric field $ E $ between 2 parallel plates of charge density $\sigma$ separated by a material of permittivity $\epsilon$. However,... | Engineers created that problem. ;) (probably not) Many physics books use $Y$ for Young's modulus (Symon, Knight, Young & Freedman). Taylor's Classical Mechanics uses YM. Halliday, Resnick & -fill-in-the-blank- state that engineers use $E$.
I suspect that physicists started using $Y$ for exactly this reason: to highlig... | {
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Is there a tool to measure the chemical potential of a system? Let's suppose I put you inside a room filled with a gas. You can measure its pressure directly with a barometer. You can measure its temperature directly with a thermometer. Can you measure its chemical potential directly with a "chemical potentiometer?"
To... | This is a very interesting question, I'll try.
First let's compare to pressure measurement. Let's forget our microscopical knowledge that pressure is also force over area. Let's focus just on what thermodynamics tells us.
That P is the thing that is equalized between two systems separated by a movable barrier. And volu... | {
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Exact differentials and state functions I was reading a Wiki article on the relationships between heat capacities
And during the derivation I came across this formula (and others like it):
This equation was used as a tool in a derivation, and came about independently.
*
*Where does this come from (rigorously)? I s... | The equilibrium thermodynamic state of a single phase substance of constant composition is determined by specifying two intensive properties. Note that, from the equation of state, V and T uniquely determine P (where V is the specific volume).
| {
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Backyard experiments to falsify the Flat Earth theory I recently became aware that the flat Earth theory still exists in the 21st century, and has colored the views of a friend of mine. Roughly speaking, the tenets are:
*
*The Earth is a flat disk, with the south pole blown up into a circular "ice wall" where one wo... | I live close to Lake Erie and often see scenes like in this picture. Note that the bottom of the cargo ship cannot be seen due to the curvature of the Earth.
| {
"language": "en",
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"source": "stackexchange",
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An impulse is given to a sphere out of the center? I'm wondering what will happen if there is an impulse $J$ given to a sphere mass $M$ out of its center?
I'm sure that it will rotate about the center, but what is its translational motion? It will move with the speed $J/M$ with the center, or with the point where the i... | The impulse is the change in total momentum of the body of mass. The momentum $P$ of a rigid body is the product of the velocity of the centre of mass $v$ and total mass $M$.
A free rigid body's centre of mass translates at constant velocity, while the body itself performs a rotation about the centre of mass.
Thus, aft... | {
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Green's Function in the Lippmann Schwinger Equation When deriving the scattering cross section using the Lippmann-Schwinger equation we need to calculate the Green's function defined by $$G(\mathbf{r},\mathbf{r'},E)=\langle\mathbf{r}|\frac{1}{E-H+i\epsilon}|\mathbf{r'}\rangle$$ where $H=\frac{p^2}{2m}$ represents the f... | I haven't work out the math but looks like expression you get is correct, you just need to use the following identity
$$
f(x)\delta^n(x)=f(0)\delta^n(x)
$$
Therefore
$$
(E-H)G=e^{ik|r-r'|}\delta(r-r')=\delta(r-r')
$$
| {
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Why must this boundary condition be met? (Electromagnetic wave at interface between two mediums) My textbook says that
The laws of Electromagnetic Theory (Section 3.1) lead to
certain requirements that must be met by the fields, and they
are referred to as the boundary conditions. Specifically, one of
these is ... | The continuity conditions for electromagnetic waves are really the boundary forms of the Maxwell equations, and they are obtained from the integral forms of the equations in a procedure similar to how the differential forms are derived $-$ by expressing the laws for a small amperian loop (or gaussian surface) whose siz... | {
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Introductory physics: first second of free fall While doing my physics homework I encountered an acceleration problem that I've been unable to solve.
Here, is the data table of the objects motion. At 1 second, why does the "Dist-meters" slot of the table have a value of 4.9m instead of 9.8m? Shouldn't the distance be r... | Draw a graph with time along the horizontal and velocity up the vertical.
Let's start with an object in motion at constant velocity. Its motion on the graph will be represented by a horizontal line at some distance from the y=0 axis. After some period of time, it will have covered a distance equal to velocity x time. ... | {
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Potential across the plate of a capacitor So I came across this question:
Two capacitors are connected in series of given capacitance $5\ \mu\mathrm F$ and $10\ \mu\mathrm F$. The first plate of the $5\ \mu\mathrm F$ capacitor is given a potential of $100\ \mathrm V$ and the second plate of the $10\ \mu\mathrm F$ capa... | No, the circuit is not complete.
If it were complete Capacitors would discharge and 1st plate would have 0 potential.
Next, potential of capacitor and plate are different.
Capacitor has potential difference b/w two plates.
Since both are connected in series net Capacitance = 10/3 uF .
$$Q = CV = 10/3 *100 = 1000/3 uC$... | {
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Helmholtz decomposition allows incompressible flow with an irrotational component? A vector field can be written in terms of irrotational and a divergence-free components. Using a 2D velocity field as an example,
$ \vec v = -\nabla \phi + \nabla \times \vec \Psi$
Where $\vec \Psi$ is a vector potential, which in fluid ... | The key concept needed here is that the Hemholtz decomposition is not necessarily unique.
Non-uniqueness can occur because there exist nontrivial vector fields which are both irrotational and divergence-free. For example, the constant 2D velocity field $\vec v = (1,0)$ can be expressed as either $\vec v=-\nabla \phi$ w... | {
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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... | Clearly, if the tension in one part of a rope is different than in another part, there will be a gradient in tension - which in turn means that if you look at a particular part of the rope at location $x$ where there is a gradient in tension $\frac{dT}{dx}$, then there is a net force on an element of length $\Delta x$:... | {
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What will be final velocity of three charges $q$, $q$, $2q$? What will be final velocity of three charges q, q and 2q kept along an equilateral triangle of side r at infinite distance.
All three masses are equal.
I tried to conserve Total Energy
$$\frac{2kq^2}{r} + \frac{2kq^2}{r} + \frac{kq^2}{r} = \frac{mv^2}{2}+ \f... | You're only a half-step away. You listed conservation of energy and linear momentum, both of which are due to there being no external forces on the three-charge system. But with no external forces, you know that the center of mass of the system won't accelerate. Since the COM starts at rest, this means that the COM wil... | {
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Why is it cold on the sea floor if pressure heats things? I was reading this and it says that Microsoft put a server farm at the bottom of the ocean because it's cooler there. Particularly it seems to imply that it get's colder as you go deeper, "Since ocean water gets pretty cold toward the sea floor..." But I know th... | Colder water is denser until it reaches a temperature a couple degrees above freezing, then it gets lighter again. So the water at the bottom is at the specific temperature where it is densest: any heating makes it rise. Any further cooling makes it rise.
See Why does the ocean get colder at depth?
This further point... | {
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} |
how is a pendulum clock's time and the time period of the pendulum in it related? I'm working out how much time a pendulum clock will gain or loose due to change of the length of the pendulum due to temperature. so far I've got, new time period, $$T_2=T_1(1+\frac12\alpha\Delta T)$$
due to $\Delta T$ change in temperatu... | You can't get an exact number for this without some assumptions. However, you can develop a relationship. What we do know is that one full period corresponds to some amount of time (my niave guess would be 1 second but I believe I have heard faster clocks which might be half seconds). So we introduce some constant C... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/233532",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Momentum operator representation If $\hat{p}$ acts on position eigenstate, it is
$$\tag{1}\hat{p}\left|x\right\rangle=+i\hbar\frac{\partial }{\partial x}\left|x\right\rangle .$$
But in general $$\tag{2}\hat{p} = -i\hbar \frac{\partial }{\partial x}.$$
Where does the sign difference come from?
| may be you can think this way.
The wavefunction $|x\rangle=e^{i(kx-\omega t)}$
How do we extract the momentum $\hbar k$ out of it by some operator ?
$$
\frac{\partial}{\partial x}|x\rangle=ik|x\rangle
$$
to get $\hbar k$, you need to either multiply the operator with $-i\hbar$ or with $\frac{\hbar}{i}$.
So the momentum... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/233687",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Can an absorbed photon be emitted as two photons? I am taking an intro to astronomy class, and have touched upon absorption and emission lines and etc, the prof asked this question in class and got me thinking.
I would want to say no, because one photon should stay as one photon, mass cannot be created.
Could someone ... | Absolutely it can - and it happens all the time. If you excite an atom, it can go through various "stages" of decay back to the ground state - with each drop in energy resulting in an emission of radiation.
This happens during photosynthesis: see this page from which I copy this image:
As you can see, there are multip... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How does a point charge interact with a Gaussian surface?
A spherical Gaussian surface encloses a point charge $q$. The point
charge is moved to to a point away from the center of the sphere.
Does the electric field at a point on the surface change?
Does the total flux through the Gaussian surface change?
I am not... | Does the electric field at a point on the surface
change?
Yes, it change. Where distance is decreased Electric field increase.
Does the total flux through the Gaussian
surface change?
No, it remains same. Consider a spherical gaussian surface of radius $2m$ and a charge $q$ inside it.
Let this charge emit $10$ field li... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/234292",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Weight of mass falling through liquid
*
*If I place a container of fluid on a scale and drop a non-buoyant mass into the fluid, will the scale read less as long as the mass is in motion downward as compared to when the mass is at rest on the bottom of the container?
Part two:
*
*Would the shape of the container... | Very interesting question. The fall of the mass will have different phases. I will only answer the phase where the mass has attained a constant speed and is falling at that speed inside the liquid. During this time, the weight will be same as combined weight of the liquid, the container and the falling mass. It will be... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/234366",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
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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... | The separation of spots in the diffraction pattern $y$ goes as:
$y \sim \frac{\lambda L}{d}$
where $\lambda$ is the wavelength of light, $L$ is the distance from the slits to the screen the spots are shone on, and $d$ is the size of the slit.
Clearly, the separation of the spots (and thus generally the precision that d... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
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How can a product of Bra and Ket be a scalar if they are matrices? I am trying to teach myself Quantum Mechanics and am currently on Complex Vector Space Arithmetic. According to Wikipedia, product of a Bra and Ket is a scalar (which, I think, means a complex number). But then, on the same page, it also says that both ... | This may seem naive, but as an engineer used to visualizing things, I always understand the multiplication of vectors and matrices as in this diagram:
In order to multiply two matrices AB, A has to have as many columns as B has rows,
and the result has as many rows as A, and as many columns as B.
You get each element ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 7,
"answer_id": 0
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work done in reversible and irreversible process According to maximum work theorem work done maximum work is done during reversible process . consider a reversible adiabatic and irreversible adiabatic process that occurs between two states of the system. Now work done in an irreversible adiabatic process is less. Chang... | If you carry out an irreversible adiabatic process starting out at state A and ending at state B, you will not be able to identify a reversible path between the same two states that does not involve an exchange of heat with the surroundings. In other words, there is no adiabatic reversible path between states A and B.... | {
"language": "en",
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"source": "stackexchange",
"question_score": "2",
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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... | Yes, quantum mechanics allows you to speak of clockwise or anti-clockwise motion, but it comes with the usual caveats of quantum mechanics. The tool that tells clockwise from anti-clockwise motion is the angular momentum. Motion is anti-clockwise around an axis, say the $z$-axis, if the component of angular momentum al... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 4,
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In the process of Bel-robisons tensor: identity for $R_{amnk} R_{b}^{\phantom{b}mnk} = \frac{1}{4} g_{ab} R_{dmnk} R^{dmnk}$ Recently, i an starting to study some subject related with Bel-Robinson tensor.
In the process of computing its some basic properties i noticed i need to prove following identity which holds on ... | Starting from
\begin{align}
R_{almn} R_{b}^{\phantom{\b} lmn} = g_{ab} A
\end{align}
This can be constructed by considering free index and symmetric properties.
Thus by contraction we obtain coefficients $A$
For $D=4$, $g_{ab} g^{ab}=4$, thus $A= \frac{1}{4} R_{dlmn}R^{dlmn}$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/234920",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Solar energy in a specific location How to find the amount of solar power in a specific location on Earth per month?
By solar power I mean the energy received from the sun in a squared meter.
| Many sites focused on solar energy production can provide you with the desired information.
One example is http://www.solarelectricityhandbook.com/solar-irradiance.html which provides you with the monthly solar insolation for a country and city of your choice. In addition to a surface lying flat on the ground, the val... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/235106",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
When is the spin of an electron changing? An electron can have a half spin up or down. The up spin can become a down spin to lose his weak charge. But when are electrons changing their spin?
| The spin of the electron is unique in the sense that the electron moving through a magnetic field is always deflected in the same way: If the thumb points in the direction of the magnetic field and the next finger points along the direction of the electron's movement, then the third finger (which is perpendicular to th... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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Why is the speed of light exactly $299,792.458$ m/s and not faster or slower? Why is the speed of light $299,792,458$ metres per second, and not faster or slower.
Why not $500$ trillion kilometers per second or $120$ miles per hour?
This has been 'bothering' me for a while. Googling it usually found answers about why i... | The speed of light in natural units is 1. The value in SI units depends on the reasons why we chose the units in the way we did. Now, SI units were chosen to match older units that were defined to match the scales appropriate for daily life purposes. So, the meter is of the order of our length, the second is a small ti... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/235302",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
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.
To prove that gravity uses gravitons requires experiments that have not yet been done, and further more are likely very hard to do.
The experiment discussed here is a proposal and it's still a large factor away from getting to any sort of quantum level.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/235413",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 4,
"answer_id": 1
} |
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... | I don't know the answer to the first 2 questions but I have an answer to the third one. Let's say you have a 2 dimensional hyperbolic plane tiled with heptagons and one of those heptagons you call the central one. Now let's say you start at the central one and then keep random walking in each step randomly picking one ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/235511",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "99",
"answer_count": 9,
"answer_id": 6
} |
Fermions, different species and (anti-)commutation rules My question is straightforward:
Do fermionic operators associated to different species commute or anticommute? Even if these operators have different quantum numbers? How can one prove this fact in a general QFT?
| Fermionic creation and annihilation operators always satisfy commutation relations with bosonic (or more generally even) operators and anticommutation relations with the fermionic creation and annihilation operators (or more generally odd operators). This follows from the properties of super Poisson brackets. See Pois... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 4,
"answer_id": 2
} |
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... | Suppose you have an infinite thin sheet of charge with a certain charge density. By symmetry we know the electric field is perpendicular to the sheet. You can calculate the field using this method:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elesht.html
$E = \dfrac{\sigma}{2\epsilon_0}$
This field is constant e... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Why gravitational waves are not produced by objects moving at constant velocity? Gravitational wave is produced by change in gravitational field, source. If something is moving away from me at constant speed, its gravitational field will vary. But why only accelerating bodies produce GW?
| I think the other answers are sufficient, but I would like to leave my two cents just to clarify a few possible concerns.
I can see you imagining that moving objects in your reference frame leave a gravitational "wake", like a boat moving in water, but gravity and relativity don't quite work this way.
To grossly simpli... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/235928",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 4,
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Does Birkhoff's theorem hold inside the event horizon? Can Birkhoff's theorem be used to say that the blackhole exterior and interior sections of Kruskal-Szekeres's solution (or coordinate transformations of it like Gullstrand–Painlevé coordinates, etc.) are unique all the way down to the singularity? Or are there diff... | There is more than one way of stating Birkhoff's theorem, but one statement is that spherically symmetric vacuum solutions are Schwarzschild. Staticity is not an assumption, nor does staticity come out as a result, since the Schwarzschild metric is not static inside the horizon. However, it is asymptotically static. Fo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/236003",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
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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... | If there is no friction, there is no torque to change the rotation of the marble.
Normally, the marble would use its rotational KE to "climb up" the side of the bowl, but in this instance, it has no grip.
When the linear motion of the marble stops, it is still spinning (slipping without friction). This means you can on... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/237174",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 1
} |
Why do we think Spin is angular momentum as opposed to some other quantity? What leads us to the conclusion that spin is angular momentum? Could it not be some other quantity? Sorry if this is a rookie level question.
| By treating spin as angular momentum a lot of phenomenon could be explained.
*
*Conservation of angular momentum makes sense only if you include spin with the orbital angular momentum. Spin-orbit coupling will mix these quantities even if the system is isolated.
*The algebra of the spin operators (which can be rep... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/237486",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 1
} |
On geometry of light's path in the Universe Is it possible for us to see our own galaxy from different perspective, as path of light emitted from our galaxy is curved by any possible ways and travelled back to our eyes even if it take much time?
| Most of light emitted either reach other galaxies or escape to cosmological horizon. Our galaxies gravitational force is no enough to deflect it back.
Also, if it is deflected by some other black holes in a pattern,
To get a complete veiw of galaxy we need light rays to simultaneously deflect back to us.
Such possibil... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/237568",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
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Where do the energy and momentum come from when light is perfectly reflected on a surface? If you send a light beam to a surface which reflects the light perfectly I would say that on the surface is put a force of the light so the surface moves backwards. But where does that energy comes from? If it perfectly reflects ... | Electromagnetic radiation does exert a radiation pressure and so there must be momentum transfer between the incident radiation and the surface. So the reflector exerts a force on the incoming radiation. In the case of light being reflected from a mirror this force is not very large and so the recoil of the relevtor ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/237638",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
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The sign of the current flowing in a circuit I was doing the following problem:
And I was asked to find Iy.
I found Iy to be 2.64 using KCL. However, the right answer was negative 2.64.
Is it negative only because there is a dependant voltage source with "+ -" ? And why must it be negative? Does "-" in the final ans... | In first Question, $V_x$ is 13.2V and the current flowing in R1 is +4.4A. And the current flowing in R2 is +7.04A(=13.2*4/7.5). Using KCL, Iy must be -2.64A. The sum of currents flowing into that node is equal to the sum of currents flowing out of that node. In the circuit 4.4A is flowing into top node from R1 and 7.04... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/237788",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 2
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If the speed of light was infinite would elementary particles still have a property called spin? From what I understand the spin of elementary particles comes from combining relativity with quantum mechanics so if the speed of light was infinite would elementary particles still have spin?
| The property of spin is independant from relativity and quantum mechanics (well not quite). It is linked with the rotational invariance of space.
As per Noether's theorem, continuous symmetries are associated with invariant quantities. In the case of rotational symmetries, this is angular momentum. For a field that d... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/237917",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
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Good reference on the parametrization of $SU(3)$ and $SU(N)$ For the 2-dimensional $SU(2)$ matrices, there is a fairly general parametrization formulation:
$s_2=\begin{bmatrix}
e^{i\alpha}\cos(\theta) & -e^{-i\beta}\sin(\theta) \\
e^{i\beta}\sin(\theta) & e^{-i\alpha}\cos(\theta)
\end{bmatrix}$
For the 3-dimens... | A possible parametrization for a real transformation matrix is via the Cayley transform: an orthogonal (i.e., real unitary) matrix can be represented as
$$
Q=(I-A)(I+A)^{-1},
$$
where $I$ is the identity matrix and $A$ is skew-symmetric.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/237988",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 4,
"answer_id": 2
} |
Why does Triple point exist?
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.
Is the existence of Triple point a coincidence?
how does one explain existence of a three way junc... | The existence of a triple point is not a coincidence at all. It will be a necessary feature as soon as you have three different phases in your phase diagram that can all be reach from each other.
It's like when you have three countries that all share borders. By geometry alone there has to be a point were the borders i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/238097",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 3,
"answer_id": 1
} |
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... | Gravitational waves do affect the path followed by light. In fact, that's how LIGO works.
Gravitational waves cause space to stretch and shrink only in the directions transverse (perpendicular) to their motion. Lets say that LIGO's two arms are aligned to the $x$ and $y$ directions. A gravitational wave traveling in... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/238226",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
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 ... | We do know 'where' the big bang happened. It happened everywhere.
Imagine you are inside a big inflated balloon. Now ask where that balloon inflated from. The answer is, it inflated from everywhere.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/238498",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 5,
"answer_id": 1
} |
How is locality preserved in quantum mechanics? I was reading this post:
http://motls.blogspot.com/2015/06/locality-nonlocality-and-anti-quantum.html
Specifically here: "There is no nonlocality. There is no action at a distance. There is no doubt about this statement."
I am puzzled how locality is preserved. Suppose we... | A quantum system is described by a set of operators called observables. Those observables represent all of the possible outcomes of a particular measurement. When you measure an entangled system, all of the possible outcomes happen. The correlation is only established after the measurement results are 'compared', that ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/238744",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Converting Stokes Parameters to Jones Vector How do you convert a Stokes vector into a Jones vector? I am only concerned about fully polarised light, and I need to convert the Stokes parameters (or the azimuth and ellipticity angles) as measured from a polarimeter into Jones vectors for further analysis.
A trivial exam... | Stokes parameters are written in terms of intensities; Jones vectors use the electric field components. It's easy to go from Jones to Stokes, but the other way does not have a general method.
See < http://en.wikipedia.org/wiki/Mueller_calculus#Mueller_vs._Jones_calculi>
You can find tables of equivalents in the liter... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/238957",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
Are there alternatives to steam in thermal power stations? 'A thermal power station is a power plant in which heat energy is converted to electric power. In most of the world the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator.'
Why is H2O ch... | Expanding Liquid: the expansion per energy unit is tiny compared with the expansion at the phase transition from liquid to gas.
Expanding Solid: even worse than expanding liquid
Bimetallic strips: very little energy transfer capability and long cool-down cycle.
Ice: see "expanding solid" above.
The next best choice af... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/239045",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
How is work function related to oxidation? Low work function metals, such as Li and K, oxidize in ambient conditions, whereas high work function metals such as Au do not oxidize.
In chemistry there's activation energy and reaction rate constants, but how is the oxidation chemistry related to the work function? (they me... | Easily oxidizing atoms and ones with a low work function are correlated. In an atom that is easily oxidized, the valence electrons are very weakly bound to the nucleus of the atom for a number of reasons.
*
*In a large atom, the large number of electrons provides shielding from the nuclear charge. Therefore the vale... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/239416",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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"answer_id": 0
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