Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Difference between projection and component of a vector in product? This is a very basic question about dot products or scalar products:-
If I want to move a block and I apply a force parallel to displacement, the block will move and some work will be done. So in the formula will be $W= F\cdot S$, here we won't calcula... | As pointed out, the projection and component actually refers to the same thing. To solve a problem like this it useful to introduce a coordinate system, as you mentioned yourself you project onto the x-axis. As soon as you introduce a coordinate system you can talk about the $\textit{components}$ of some vector. E.g $\... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/537671",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Rotation in molecules I am a bit confused about the rotational motion in molecules. Assuming the bond length is constant, the motion can be described as a rigid rotor. In the center of mass frame the energies are given by BJ(J+1) and the wavefunctions are spherical harmonics. However when we measure the energies or the... | The molecule is not stationary in the center-of-mass frame. Any rigid body dynamics can be separated into motion of the center of mass, and motion about center of mass. The second includes rotational motion, which is what you are considering. Only translational motion is absent in the CM frame. The question of whether ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/537773",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Can a wormhole be created if it has not always existed? I know there are solutions to Einstein's field equations that give a wormhole geometry. But they are time independent. They are static. Is there a process where empty flat spacetime can evolve into a wormhole by an appropriate flow of matter and energy and negativ... | Unfortunately the answer is that we don't know, and most likely a classical theory like General Relativity that concerns itself mostly with local properties of geometry is inherently unfit to aboard the problem of global topology
The clues we get from General Relativity is that some static adiabatic (i.e. reversible) s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/537994",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "36",
"answer_count": 3,
"answer_id": 1
} |
Why are eggshells so strong? The usual explanation that someone can find on internet is that it is shaped like an arch, but it is not exactly an arch.
Does anybody knows something more on this?
| It is the same principle as applies to an arch. The curve of an arch is such that forces are transmitted along the curve of the arch. Ideal curves for this are the catenary arch, and for a bridge supporting weight, the cycloid, most famously used in the "New" London Bridge (1831–1967)
The difference with an eggshell is... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/538243",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why do antiferromagnets occur at lower temperature than ferromagnets? The minimal model for describing magnets is the Heisenberg Hamiltonian
$$H = -\frac{1}{2}J\sum_{i,j} \mathbf{S}_i \cdot \mathbf{S}_j$$
Where $i,j$ are nearest neighbors and the factor of $1/2$ is for double counting.
If $J$ is positive, spins will w... | It is not correct that AF have low Neel temperatures universally.
One recent popular example is Mn2Au, whose (anticipated) Neel temperature is so high that it can't be reached before the material decomposes. I've heard of estimates in the ballpark of a 1000 K.
There is also the widely commercially applied IrMn, which h... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/538480",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 0
} |
Maxwell equations and continuity equation I want to show the following equation with the maxwell equations:
$$\frac{\partial}{\partial t}W+\vec{\nabla} \cdot \vec{S} = 0 $$
The problem is that I'm not understanding why I can do the following step:
$$\partial_t W=\partial_t \frac{1}{8\pi}(\vec{E}^2+\vec{B}^2)=\frac{1}{4... | \begin{align}
\frac{\partial}{\partial t} \left( \vec{E} \cdot \vec{E} \right)
=\; &
\frac{\partial}{\partial t} \left( \sum_{i=1}^3 E_i^2 \right)
\\ = &
\sum_{i=1}^3 \frac{\partial}{\partial t} E_i^2
\\ = &
\sum_{i=1}^3 2E_i \frac{\partial}{\partial t}E_i
\\ = &
\;2 \vec{E} \cdot \frac{\partial}{\partial t} \ve... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/538621",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why are some energies dependent on reference frame, and some are not? And why is transfer between them possible? For example the chemical energy of a kilogram of gasoline is 44-46 MJ/kg. It is only dependent on its chemical structure, which stays the same, whether the gas tank moves or stays still relative to the obser... | Many of the answers are right for the most part, but I think they have too much detail. The simple answer is that velocities are reference frame dependent, while lengths are not. Kinetic energy depends on velocity, so it is reference frame dependent. Potential energies depend on lengths between two objects (or from som... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/538791",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 5,
"answer_id": 1
} |
The electric field should be in circular coil. But why do current flows in whole circuit? We know changing magnetic flux induces electric field which makes current to flow .here in the below picture the flux is changing through only circular coil bout not through the rectangular part.so what induces electric field in r... | The field induced in the circular part of the circuit causes the free carriers (electrons) in the wire of that part of the circuit to move slightly. The the movement of the electrons from one end of the loop to the other creates the field in the other part of the circuit.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/538903",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Energy conservation in reflection of light from a perfect mirror I came across a question where a light source is shined on a mirror attached to a spring which is attached to a rigid support. The question goes:
A perfectly reflecting mirror of mass $M$ mounted on a spring constitutes a spring-mass system of angular f... | Let's calculate the energy change of the mirror of mass $M$ from a photon with energy $\hbar \omega$
The photon has momentum $p = \hbar \omega/c$, and so the total change in momentum is $\Delta p =-2\hbar \omega/c$
The mirror on the other hand gains momentum $-\Delta p$ by momentum conservation. This change in momentum... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/539983",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "18",
"answer_count": 9,
"answer_id": 7
} |
How many optical and acoustical branches are in a primitive cell? I am reading Introduction to Solid-State Physics (by Kittel) and I don't understand how he counts the optical and acoustical branches in a primitive cell.
It says that if there are $p$ atoms in a primitive cell then we have $3p$ branches, 3 acoustical b... | For a primitive cell with p atoms in 3d space, there are 3p degrees of freedom, say x, y, and z for each of the p atoms. This leads to 3p total harmonic modes and hence there are a total of 3p branches.
For an acoustic mode, the atoms within the primitive cell need to move exactly in phase, giving a dispersion relation... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540050",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 1
} |
Is De Broglie's formula $\textbf{p}=\hbar \textbf{k}$ applicable to a discrete wave number system? I don't know if my question has sense at all but while doing my homework there appeared in my mind this question.
Say, for a particle in a box, the confinement makes that the wave number k is discrete, depending on integ... | Yes. But remember, you can think of the wavefunction in quantum physics like a wave in the configuration space of the system, that is isomporphic to the physical space only for a single particle case. Momentum is discrete. Actually, 1 of the 1st quantum observed effects was the discreteness of the energy spectrum, that... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540099",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Why are there no even harmonics in a closed pipe? I have seen a diagram on sites such as hyperphysics.com that show that there is a missing bit every time so that it makes every harmonic odd. I was hoping I could get a more intuitive explanation. We recently got introduced to harmonics and standing waves in class and ... | I dowwnloaded a trumpet A sharp and did the Fourier Transform in Matlab. I found both odd and even multiples.
Have not yet found an actual recording that yields only odd harmonics.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540335",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Moment of inertia tensor and symmetry of the object What information does the moment of inertia tensor give on the structure of an item.
I was told that its eigenvectors give the principal axes of the object.
Do you know more about this?
| The inertia tensor is a bit more descriptive in the spherical tensor basis (so instead of having nine basis dyads made from combinations like $\hat x \hat y$, you have 9 basis tensors that transform like the nine $Y_l^m$ for $l \in \{0,1,2\}$).
Since $I_{ij}$ is antisymmetric, all $l=1$ spherical tensors are zero. The ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540586",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Why does frequency remain the same when waves travel from one medium to another? I was reading about reflection and refraction on BBC Bitesize and I can't understand why frequency is a constant in the wave speed equation. I can't visualise the idea of it. I know that wave speed and wavelength are proportional to each o... | Loosely speaking, you can think of it this way: a wave propagates in space and time, but it encounters a change in the spatial properties of the medium at the interface. What should the time part change?
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540685",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
Why can't a wave travel in a non-elastic medium? Why a wave cannot propagate in a non-elastic medium
We know that wave is a distrubance and carries energy. In this sense let imagine fall of dominoes, which carries disturbance and energy. Here fall of dominoes is non-elastic and we can see that wave propagates.
Can I c... | Waves can propagate in an inelastic medium, and the fall of dominoes is a wave. So when people say that a wave can't travel in a non-elastic medium can't, this statement applies to particular kind of waves.
When talking about waves one often means perturbations that are periodic in space and periodic in time at every p... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540791",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Phase Difference in mutual coupling I was reading about oscillator circuit then in my textbook it was written that we get phase change of π at mutual coupling.But, when I workout through mathematics of situation (which I have shown below) I get phase difference of π/2.(M is mutual inductance,i is amplitude of current i... | I think that you are mistaken in the language that book uses
That $\epsilon$(in your question) is the emf developed in second coil due to flux change in first, But what is the emf in the first coil that drives the current $I= i \sin(\omega t)$?Assuming pure inductive circuit(with no resistance) you can work out it to b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540864",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Why do we need Gauss' laws for electricity and magnetism? The source of an electromagnetic field is a distribution of electric charge, $\rho$, and a current, with current density $\mathbf{J}$. Considering only Faraday's law and Ampere-Maxwell's law:
$$
\nabla\times\mathbf{E}=-\frac{\partial\mathbf{B}}{\partial t}\qquad... | I don't agree that you get that you obtain the Gauss law using the method proposed. What you obtain instead is
$$\frac{\partial\nabla\cdot\mathbf{B}}{\partial t} = 0,\\
\frac{1}{c^2}\frac{\partial\nabla\cdot\mathbf{E}}{\partial t} + \mu_0\nabla\cdot\mathbf{J}=
\frac{1}{c^2}\frac{\partial\nabla\cdot\mathbf{E}}{\partia... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/540991",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 3,
"answer_id": 0
} |
Why does the dielectric field not cancel out the capacitor's field? When a conductor is in a region with electric field, free charges will move until they balance out the external electric field. However in dielectrics this does not happen. I know that charges are bounded to the atoms, and there is only a small portion... | The charges in the dielectric to rearrange and lessen the field inside of the capacitor, the field just isn't completely canceled. This is because, as you said, we are dealing with bound charges.
If you compare a vacuum filled capacitor with charge $\pm Q$ on its plates to a capacitor with the same charge filled with a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/541061",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
Why a ball doesn't stop when it collides with a wall? If a ball collides with a ball of same mass the first ball stops and the second ball gets the velocity of first ball.The first ball stops due to the reaction force acting on it. But when a ball collides with a wall why doesn't it stop due to the reaction force?
| Try considering momentum conservation. The wall won’t be moving before or after the collision with the ball, and presuming the collision is elastic (no energy lost to heat, sound etc.) then the sum of the momenta will be conserved before and after the collision and you should find that the ball must have the same momen... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/541318",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Newton 3rd law of motion If I took both hands and hold as I am greeting someonei.e.namaste. Now, I push both hands with same force say 10N , there is no change in position.But Newton 3rd law has said,equal and opposite forces are on different bodies and they never cancel each other (action reaction). So, why they don't... | On each hand, you really have two forces: one from the other hand (10N, pushing out) and one from the arm (10N, pushing in).
These are the equal and opposite forces, hence your hands do not move.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/541557",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
Tree Level Scattering Amplitudes in Spinor QED I am currently trying to calculate scattering amplitudes of tree level QED processes and i am a bit confused due to the ordering of the factors yielded by the feynman spinor QED rules. Especially in processes having photons in the initial or final states.
Could someone wri... | Let't take a simple process for example $$e^-+e^+ \to \mu^-+\mu^+$$ The only tree level diagram for this process is the following
To build up the matrix element of this process you just have to follow the fermion lines backwards, that's all. Beginning from the end of the process you have then $$(\mu^- \to \text{verte... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/541768",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why the maximum number of supercharges in supergravity must be $Q=32$? For a supergravity theory not to have particles with spin greater than 2, all books state that $Q\leq 32$, where $Q$ is the number of fermionic supercharges and for a given dimension $D$ it's related to the number of supersymmetries $\mathcal{N}$ th... | You have to first understand the contruction of massless multiplets which is found in the beginning of every introduction to supersymmetry, so I won't repeat it here. Then the argument goes like this:
*
*Given $Q$ supercharges, half of them will be zero for the massless case, thus leaving you with $Q/2$ non-zero sup... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/542151",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Massless string vs massless spring in a mass-spring system
Two masses connected by a massless spring, on a frictionless surface , and a force of $60$N is applied to the 15kg mass such that it accelerates at 2 $\frac{m}{s^2}$. What is the acceleration of the $10kg$ mass?
I came across this question. I first thought ... | The thing is, an 'inelastic' massless string ensures constrained motion because it has a definite length.
If x is the distance moved by the first block, then the second block is also constrained to move so that the net extension of the string is 0.
But, a spring can become compressed or stretched. So, if block 1 cover... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/542333",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Manipulation of the diffusive term in MHD induction equation I am trying to solve the magnetohydrodynamic (MHD) equations with a spatially varying resistivity, $\eta$. To remove some of the numerical stiffness from my finite volume approach, I am trying to get rid of these curl expressions with some vector calculus ide... | There are several vector/tensor calculus rules that will come in handy, so I will defined them here first (in no particular order):
$$
\begin{align}
\nabla \cdot \left[ \nabla \mathbf{A} - \left( \nabla \mathbf{A} \right)^{T} \right] & = \nabla \times \left( \nabla \times \mathbf{A} \right) \tag{0a} \\
\nabla \cdot... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/542898",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
WKB method to calculate the ground eigenvalue of a Quartic Potential I've come across a problem while studying the WKB method. I want to calculate the eigenvalues of a symmetric quartic double well potential. It could be any potential. I chose it to be $$V(x) = x^4 - 4x^2 +4$$
The hamiltonian with $\hbar$ = $m$ = $1$ ... | You can do this in Mathematica using NIntegrate to do numerical integration and either NSolve or FindRoot to find the energy that satisfies equation (1).
In this way I found the $n=0$ energies $E_0^\text{even}\approx 1.74646$ and $E_0^\text{odd}\approx 2.07823$ with a few lines of code. For the former, $\theta\approx 1... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/542987",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How can a photon collide with an electron? Whenever I study the photoelectric effect and the Compton effect, I have always had a question about how a photon can possibly collide with an electron given their unmeasureably small size. Every textbook I've read says that the photo-electrons are emitted because the photons ... | Indeed, the picture that one gets from a typical discussion of the Compton effect is far from realistic. Here are a few points to consider, if you are to do an actual experiment (the list is by now means exhaustive).
*
*Detecting a single photon or a single electron is hard (if not impossible) even with the modern e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/543056",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "38",
"answer_count": 9,
"answer_id": 4
} |
What's the meaning of a continuity equation with $\nabla^2 \rho$ on the right-hand side? I stumbled upon a continuity equation with a $\nabla^2$ term on the right-hand side:
$$ \partial_t \rho + \nabla (\vec b \rho) = D \nabla^2 \rho , $$
where $b$ denotes the forward velocity and $D$ is a constant.
What's the mean... | It looks like a diffusion equation with advection. Such an equation would be relevent for heat transport in a fluid moving at velocty ${\bf b}$. If $D=0$ the LHS says that the stuff whose density is $\rho$ is being moved about the flow. If ${\bf b}=0$ the suff is just diffusing. With both ${\bf b}$ and $D$ non zero, y... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/543341",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
What are the optical properties that cause the glossy look of wood varnish? Applying varnish to a painting or wood makes the colors more vibrant. Why?
| Disclaimer: Since I am not an expert in this field, this is in no way a complete answer. The only intention is to provide some insights into some of the notable developments in the area, from which the readers can set out to explore.
What an interesting observation! Wooden surfaces offer an incredibly uneven surface fo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/543465",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Calculating work of an object moving up a slope If an object is being pushed across a horizontal surface, the equation for the work done is $W = F s$, where $s$ is the horizontal displacement.
If an object is being lifted to a height of $h$, the equation for the work done is $W = F h$, where $h$ is the vertical displac... | You have to consider the forces acting on that body.
If the body moving on a slope experiences no horizontal force i.e. its velocity in horizontal direction is zero, no work will be done in horizontal direction. But, if you consider a force acting in vertical direction then work will be done by the body to move in vert... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/543766",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
} |
“Bananagrams” under black light? There is a game called “Bananagrams” which includes a bunch of pieces with a letter on each. It seems when I shine a black light flashlight on the letters, the “M” letters glow, but no other pieces do.
All the pieces appear the same under normal lights (except of course the letters on e... | The blocks with the M came from a different batch than the others. There is no way to know why without more information.
It could have been that a previous batch of M blocks was defective (e.g. incomplete letters due to a defective M die) and had to be replaced, but that is pure speculation and there could be many oth... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/543905",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Is there anyway to know when the wave function collapse? If we get 2 entangled particles and move them away from each other, is there a way to put one of them is some kind of "sensor" that would tell if the entangled particle have been measured?
If yes, how does one function?
If not, how do we know that the measure of ... | In the case of entangled particles, they are usually separated so that a signal cannot pass from one to the other.
The wave function is an expression of the probability for the result of a measurement. Probability is a human assessment of likelihood. Since Alice and Bob have different information, it is natural that th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/544042",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Power loss in power cable contradiction To minimize the power loss in long-distance power cables it is best to minimize the current and maximize the voltage. This is because the power loss in the cable is calculated by $P=VI$, which we can also express as $I^2R$ by use of Ohm’s law. But by the same reasoning the power ... | Power loss in cables (Joule heating) cannot be calculated as $P=VI$ where $V$ is voltage between the lines. The proper voltage to use for power loss over some wire segment is voltage drop over this segment, which is much smaller than $V$ and is given by Ohm's law (the wire used to transfer current over long distances i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/544187",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How does a string store information during wave superposition? Given we have two Guassian wave pulses in the same medium (string) but in opposite directions. The principle of superposition states that they should pass through each other without being disturbed, and that the net displacement is the vector sum of the ind... | Wave superposition happens because each point in a string of superimposed state, gets a pair of velocities, from a first and second impulse :
Actually, it's this individual point speed addition rule that makes wave interference work.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/544315",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 1
} |
Fraunhofer line width What sets the width of Fraunhofer lines on the solar spectrum ?
I first thought of Doppler broadening, but numerical applications result in much too high temperatures. For instance, using these data, I find a $\Delta \lambda =$ 0.01nm line width on the 630.25nm line of iron, corresponding to a te... | To begin with, your reasoning assumes near transparency of the solar atmosphere. This is generally not true. The solar atmosphere is “optically thick” (highly absorbing in the vicinity of a spectral line) as opposed to “optically thin” (nearly transparent). The shape of such a spectral line generally requires radiat... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/544554",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 3
} |
Is there any operator in quantum mechanics that measure an observable with non-zero uncertainty? What does a measurement do?
The answer is: If the detector is designed to measure some observable O, it will leave the measured object, at least for an instant, in a zero-uncertainty state.
I want to know, in the context of... | Defining an observable $M = \sum_m m P_m$, with $m$ the eigenvalues and $P_m$ the eigenspace projectors, is equivalent to defining a set of projective measurements $\{P_m\}$ such that $\sum_m P_m = I$ and $P_m P_{m'} = \delta_{mm'} P_m$
Since the operators $P_m$ are idempotent ($P_m^2 = P_m$), two consecutive measureme... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/544836",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Are antileptons and antibaryons linked? The recent news about the T2K experiment got me thinking: is there any linkage in the Standard Model between the matter and antimatter categories across the families of Standard Model particles? Are antileptons necessarily linked to antibaryons?
As a specific example: In our uni... | Protons and electrons are not composed of the same "type of matter". Electrons i.e. leptons are fundamental particles whereas protons are Hadrons which are compositions of quarks. Quarks and leptons are the fundamental particles.
Also, obviously opposite charges attract so an atom could not be made where you've combine... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/545014",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 1
} |
Wavefunction of a photon Does anyone have an explicit closed-form expression for the wavefunction of a single photon from a multipolar source propagating through free space? Any basis is acceptable as long as it is a single photon state.
A reference would also be appreciated, but not essential.
————————
A possible du... | I think you are asking for the wavefunction of a single photon in a state of sharp total angular momentum $j$. This is,
\begin{equation}
|jmk\lambda\rangle=\frac{(2j+1)}{4\pi}\int\sin{\beta}d\alpha d\beta \{D^{j}(\alpha,\beta,0)^{m}_{\ \lambda}\}^{*}|\vec{k},\lambda\rangle
\end{equation}
In this equation the 3-momentum... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/545184",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
If a charge comes into existence, and there is another charge 20 light years away(which existed for eternity), what will be the interaction? Our physics teacher just taught us a bit about electric fields, and why the concept was invented. He said that if a charge comes into existence, another charge won’t feel its effe... | If a cork is floating on a still pond and then you drop a brick into the pond several metres away, does the cork start bobbing about as soon as the brick touches the water? Or does it have to wait until the waves reach it?
So the remote charge has to wait on the electric field to propagate from the initial charge.
By t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/545508",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Graphical explanation for length contraction I can understand the mathematical explanation for the reason why there should be a length contraction, but I fail to understand it intuitively. That is why I tried to explain it using spacetime diagrams, but for some reason, I was unable to do so.
Let us use the following ... | Let $S'$ be a reference frame fixed on the rod and let us measure the positions of both ends of the rod (at the same time) when one end is at $x = 0$ wrt (with respect to) $S$.
We have two events
$$e_1: \quad (t, x_1) = (0, 0)$$
$$e_2. \quad (t, x_2) = (0, L) \quad and \quad (t_2', x_2') = (t_2, L_0))$$
Using the Lore... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/545736",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 3
} |
Where did the work done by smaller force go? Suppose I have a spring of spring constant 150 N/m.One person pulls with it a force of 15 N. The extension produced is 0.1 m. Now another person comes and pulls with a force of 30 N (The first person is still there). The final extension is 0.3 m.The initial and final potenti... | When both are pulling it together, the total fore is $45\ \mathrm N$ and the extension is $0.3\ \mathrm m$ and the work-done is still $6.75\ \mathrm J$. When they pull individually the work done is $6\ \mathrm J$ and $0.75\ \mathrm J$ respectively irrespective of who pulls first. It doesn't matter how they are being pu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/546063",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Matsubara frequencies and analytic continuation - Domain and uniqueness In the Matsubara formalism, there is a commonly-made statement that an imaginary-time correlation function is related to a retarded correlation function via the replacement $i\omega_n \rightarrow \omega + i0^+$, also called analytic continuation.
I... | I hope am not confusing something, the issue is that the retarted propagator $G^R$ is regular in the upper half plane, so by the second statement from your question, two functions equal on the set, with an accumulation point at infinity are equal at every point in the domain of analycity. The same statement also holds ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/546408",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 1,
"answer_id": 0
} |
Heat transfer in glass top ranges
*
*The "glass" in glass top ranges apparently has quite low heat
conductivity as areas just a short distance from the heating element
don't get so hot - especially compared to a metal surface which has
excellent heat conductivity.
*The glass does not get red-hot.
*Presume the glas... | You are correct that the primary mechanism of heat transfer to the cooking utensils on a glass ceramic cook top is infrared radiation from the hot metal coil just below the surface. The low thermal conductivity of glass ceramic helps keep the conductive heating of the glass localized below the utensil.
Hope this helps ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/546513",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why doesn't a backward wave exist? (Huygens principle) Since every point on a wavefront act as a source of secondary wave (wavelets) then why do we get only forward wavefront not backward.
Huygens principal says that amplitude of the backward wave is zero, but why and how it happens?
| Huygens original description of wave propagation did not adequately explain the backward wave. Elimination of the backward wave is the reason for the 'obliquity factor' or 'inclination factor' that was added by Fresnel/Kirchhoff. It adjusts the strength of the Huygen's wavelets as a function of the direction of propag... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/546665",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Is the net torque of an only translating rigid body zero, independent of the point chosen? I know that the Newton-Euler equations can be proven using the center of mass as reference, but I was wondering if this is a special case, or if you can provide a counter-example. We know that when a rigid body is only translatin... | No, it is not true; for example, consider a uniform-mass ball of radius $r$, which has a force $\boldsymbol{F}$ acting horizontally through its centre, such that it translates without rolling. The torque about its centre of mass is zero (because it is not rotating), but the torque calculated about its contact point wit... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/546847",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Is the force of gravity always directed towards the center of mass? This is a pretty basic question, but I haven't had to think about orbital mechanics since high school. So just to check - suppose a [classical] system of two massive objects in a vacuum.
If the density of either object is the same at a given distance f... | No. It is not correct.
Consider this ridiculously contrived counter-example... Three spherically symmetric bodies (or point masses if you can tolerate this) are at the three vertices of a 45°, 90°, 45° triangle, ABC. The masses of the bodies are: $m_\text{A}=m,\ \ m_\text{B}=M,\ \ m_\text{C}=2M$. Regard the bodies at B... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/547505",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "25",
"answer_count": 6,
"answer_id": 0
} |
Can massless particle have effective mass? The effective potential was probably very familiar in many concepts.
However, what about effective mass?
Suppose a massless particle. For simplicity, suppose it's not some superficial particle, i.e. it has observable effect.
Is it possible for such massless particle to gain... | The answer is no, a particle that has zero mass, as the photon, is a relativistic particle and the algebra of the Lorentz transformations does not allow it because of energy and momentum conservation laws. The mass is the length of the energy-momentum fourvector.
For example, a photon could well obtain a $e^-\sim e^... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/547610",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 1
} |
If I apply a constant force to an object until I've reversed its starting velocity, does its final position remain unchanged?
A body of mass $m$ has initial velocity $v_0$ in the positive $x$-direction. It is acted on by a constant force $F$ for time $t$ until the velocity becomes zero; the force continues to act on t... | The textbook's answer reflects the total distance traveled, i.e.
$$\vert s(t') - s(0) \vert + \vert s(2t') - s(t') \vert = \vert \frac{1}{2} v_0 t' - 0 \vert + \vert 0 - \frac{1}{2} v_0 t' \vert = v_0 t' = -\frac{\vec F}{m}(t')^2 = \frac{F}{m}(t')^2$$
I prefer this way of organizing it because it lets me treat the disp... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/547916",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Is the potential energy stored in a spring proportional to the displacement or the square of it? Suppose a mass of $M$ kg is hanging from a spring in earth. The mass will stretch the spring about $x$ m. So the change in the gravitational potential energy is $mgx$ J (supposing $x$ to be very small compared to the radius... | You are correct that
$$
\text{change of gravitational energy} = mgx \ \ \ \ (= \text{potential energy stored in the spring}).
$$
No mistake there. That only gets you halfway there though. It is ALSO true that
$$
\text{potential energy stored in the spring} = \frac{1}{2}kx^2 \ \ \ \ (= \text{change of potential energy}... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/548021",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 3
} |
How does cutting a spring increase spring constant? I know that on cutting a spring into n equal pieces, spring constant becomes n times.
But I have no idea why this happens.
Please clarify the reasons
| Let us consider that you join these n pieces of springs in series. Now you know that you have got the original spring whose spring constant is $k$ (say). Now joining springs in series is like joining resistors in parallel (identical formulae) which can easily proven by balancing forces. Hence,
$$\frac{1}{k} = \frac{1}{... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/548186",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 6,
"answer_id": 3
} |
Massive spin-1 field and Proca Lagrangian In his book Quantum Field Theory and the Standard Model, Matthew D. Schwartz derives the Lagrangian for the massive spin 1 field (section 8.2.2).
In eq. (8.23) he finds this to be
\begin{align}
\mathcal L&=\frac{1}{2}A_\mu\square A^\mu-\frac{1}{2}A_\mu\partial^\mu\partial_\nu A... | This somewhat sloppy statement means that the two Lagrangian expressions are the same up to a total derivative. Such a total derivative does not contribute to the action $A=\int d^4x \cal L$, for suitable conditions at the edge of the integration domain, and therefore is considered to be without physical consequence.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/548314",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Mathematically prove that a round wheel roll faster than a square wheel Let's say I have these equal size objects (for now thinking in 2D) on a flat surface.
At the center of those objects I add equal positive angular torque (just enough to make the square tire to move forward).
Of course the round tire will move fast... | Circular objects are not the fastest.
Any other smooth convex shape can roll faster than a circle. As a random example, this shape (picture found on wikimedia) can roll faster than a circle:
Start it in the orientation shown. This is the orientation where its center of mass is highest. Then it will generally be rol... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/548519",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "64",
"answer_count": 12,
"answer_id": 8
} |
What is the range of Pauli's exclusion principle？ In many introductions to the pauli's exclusion principle, it only said that two identical fermions cannot be in the same quantum state, but it seems that there is no explanation of the range of those two fermions. What is the scope of application of the principle of exc... | The most common way to visualize the range of the exclusion principle comes to us from the study of ultra-dense objects like white dwarf stars and neutron stars. In a white dwarf, gravity squeezes the matter in it so hard that the wave functions of the electrons in it begin to overlap- and that's where the exclusion pr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/548642",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
"answer_count": 6,
"answer_id": 2
} |
Do things have colors because their electrons are getting excited when photons hit them? Atomic electron transitions can be caused by absorbing a photon with a certain wavelength. An electron jumps to an higher energy level, then it falls back and a photon is emitted. The perceived color of the photon depends on the en... |
Could we say that electrons in the atoms of different objects are excited when white light hits them, and they release photons which in turn causes the object have a color?
What you describe there is known as fluorescence but that's not what happens when white light hits a coloured object, at least not in most cases.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/548900",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
How to model a quantum circuit Let's say we have a system of 2 qubits, which are entangled in an unknown Bell basis configuration. Since the qubits are in a Bell configuration, each state is orthogonal to every other state, and thus must be distinguishable from each other.
My understanding is that there are 4 measureme... |
On a related note, let's say I have an arbitrary unitary matrix. How does one map that to a quantum gate?
There is a several quantum gates you can use for construction other more complex gates. See list of gates on Wikipedia.
Please also find other technique how to decompose arbitrary unitary gate in article Elementa... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/548963",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Is 1 Joule the work done to lift ~100g through a distance of 1m? I am seeing many videos saying that 1 Joule is the work done to lift ~100g through a distance of 1m (like this one https://youtu.be/BYpZSdSEk4A?t=348).
The idea is that 100g has a gravitational force downward of 1 Newton. So lifting it means applying 1 Ne... | Joule is the work done by a force of 1N (1 Newton) when moving the object by 1m (1 meter). It is one of the SI units, that is: it is defined in terms of the basic units of kilogram, meter and second.
*
*Given the free fall acceleration of $9.8m/s^2\approx 10m/s^2$ the gravity force acting on an object of mass 100g=0... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/549135",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Gravity at light speed my question in regards to light. Since gravity warps space time, and we have discovered gravitational waves. Would the light at the front of the wave, be traveling faster than lightspeed? Essentially like a surf board on a wave?
Also, does gravity behave differently at light-speed? Would faster t... | Gravitational waves always move at the speed of light as you say. Light also moves at the speed of light. For the question does light move faster than the speed of light in a gravitational wave. Well the answer is no. This is because light can never move faster than the speed of light. This is because the maximum speed... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/549270",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
How do gravitons and photons interact? First of all, I am a noob in physics (I‘m a computer scientist) and started reading Hawking‘s „A brief history of time“. In Chapter 6 he says that “electromagnetic force [...] interacts with electrically charged particles like electrons and quarks, but not with uncharged particles... | Gravity couples to energy, not just mass as in Newtonian theory (really it couples to energy density, momentum, and stress). Since photons have energy, they feel gravity.
As a classical phenomenon, lensing is generally thought about as light interacting with the curvature of space rather than gravitons. A physical proc... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/549411",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
Why does the ideal gas law exactly match the van't Hoff law for osmotic pressure? The van't Hoff law for osmotic pressure $\Pi$ is
$$\Pi V=nRT$$
which looks similar to the ideal gas law
$$PV = nRT.$$
Why is this? Also, in biology textbooks, the van't Hoff law is usually instead written as
$$\Pi=CRT =\frac{NC_m RT}M$$
... | The law $PV = n RT$ gives the pressure $P$ of $n$ moles of ideal gas in volume $V$. Meanwhile, the law $\Pi V = n R T$ describes the osmotic pressure $\Pi$ due to $n$ moles of solute in volume $V$.
These are qualitatively very different situations, but there's a simple fundamental reason that they end up looking the s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/549522",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "20",
"answer_count": 2,
"answer_id": 1
} |
Biomechanics statics of an push-up So I am really struggling with a statics problem about calculating forces and torques of an exercise.
So I need to calculate the forces acting on the feet and hands, as well as the torques, for 2 static push-up positions. I've linked the 2 static push-up positions at the bottom.
How d... | Hint: There are two equations for static equilibrium, (1) the sum of the forces equals zero and (2) the sum of the moments (torques) equals zero.
For the first equation you only need the persons weight. For the second equation you need the location of the person's center of mass. You will need to apply the second equa... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/550078",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Can a very small piece of material be superconducting? The existing theory of superconducting seems to be based on statistical mechanics.
Can an ultrasmall piece of material, like a quantum dot with very few atoms (like a small molecule), be superconducting?
For example, can a cubic of 3 * 3 * 3 = 27 copper atoms be su... | Tunneling of Cooper pairs through metallic islands has been a subject of research fir quite some time now. One does not call these quantum dots, since the QDs are usually understood in the context of semiconductors, but many ideas are the same - particularly relevant here is the Coulomb blockade, which permits counting... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/550244",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 0
} |
Why is the Force of Gravitational Attraction between two “Extended” bodies proportional to the product of their masses? Newton’s Law of gravitation states that force of attraction between two point masses is proportional to the product of the masses and inversely proportional to the square of the distance between them.... | It is not true that the gravitational force between extended mass distributions depends only on the product of the total masses. It is true that the time averaged total force integrated over each body is $$\vec F = Gm_1m_2 \frac{\vec r_{12}}{r_{12}^3} ~.$$ However, unless both mass distributions are spherical, the attr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/550387",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 9,
"answer_id": 4
} |
Actual meaning of refraction of light The definition of refraction which I found on wikipedia is
In physics, refraction is the change in direction of a wave passing from one medium to another or from a gradual change in the medium.
But in the below case, there is no change in direction of light. So, is this also ref... | I really feel like I genuinely understand your question. In essence you are asking "do we count it as refraction when there is no refraction in virtue of the angle of incidence being 90 degrees (or zero degrees, however you look at it).
The example you give is an example of something that is vacuously true. It's a bi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/550515",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Why does graphene break on mechanical exfoliation? We know that graphene is the strongest metal on earth. It has very high tensile strength. The inter-planar bonds are van der waals force but the covalent bond across the plane between C-C atoms is what gives them the strength. If the covalent bond is soo strong, then, ... | Graphene is strong per bond, but at the end, it's still a 2D material. The strength of most objects scale with surface area (proportional to the number of bonds), but graphene can only scale linearly.
Also, in the case of a pencil, that's not a pure crystal of graphite, it has defects. Most graphite don't have contiguo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/550769",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why is electric flux through a cube the same as electric flux through a spherical shell? If a point charge $q$ is placed inside a cube (at the center), the electric flux comes out to be $q/\varepsilon_0$, which is same as that if the charge $q$ was placed at the center of a spherical shell.
The area vector for each inf... | A good way to visualize the problem is to imagine first that the charge is enclosed by a sphere. Draw a small area on the surface of the sphere, ane draw lines from the charge through the small area. Those lines are the flux through the area. Now imagine a larger sphere concentric with the first one. The continued li... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/550820",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 7,
"answer_id": 6
} |
Can we have pressure with zero net force on a 2d plane? From Wikipedia:
Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.
Suppose we apply 2 equal and opposite forces on a 2d plane with area 'A' perpendicular to it ($F_a$ & $F_b = F$)
Will we ... | If you insert the plane into a container of gas at pressure P, the plane experiences equal pressure on both sides. That pressure is simple P. You are mixing up force and pressure. If pressure is only on one side of a plane, the net force on the plane is PxA (force per unit area, times area). Since the plane in this... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/550970",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Selections rules for spin what do we mean by the selection rule $\Delta S=0$?
Can you give me some example for hydrogen atom?
For example if I want to go from $1s$ to $2p$ how can I calculate $S$ for $1s$ or for $2p$?
| $\newcommand\bs\boldsymbol$
We note that the spin $S$ of an atom is the spin angular momentum sum of all the electrons (and nuclei) in the atom, as given by $$\bs{S} = \sum_i \bs{S}^{(i)} .$$
Most often, selection rules help us specify what happens to the atom upon interaction with a photon, which has spin angular mome... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/551123",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Why is there an electric potential drop in electric circuits? I know a battery creates a potential difference, making an electric field that exerts a force on the electrons, who start moving. But why is there a potential drop after a resistor for example? How does it go in hand with electric potential being a scalar as... | Think of it in terms of impedance to a wave. Say there is a rope in air with some portion in the middle of it being in water.
Now if we move one end of the rope continuously, the wave travels until it reaches the water-air interface. Here some of the wave reflects and other gets transmitted, constrained by energy cons... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/551242",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
How does focal length affect magnification? My answer would be the longer the focal length, higher the magnification will be, resulting in a larger image. But in a ray diagram, how does it look? I am searching for a comparison of ray diagram between short focal length and long focal length but didn't manage to get anyt... | NOTE: Lines are not straight and don't question the size of mirror and I have used concave mirror only.
In the image the position of the object is fixed (30cm) and two concave mirrors of focal length (20 and 10cm) are taken.
[
In the image it can be noticed that as focal length decreased the magnification also decreas... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/551638",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
What is meant with overdamped motion? I'm learning about Brownian motion. I use the approximation of overdamped motion. I read that the average acceleration is $0$ then, but I don't really understand the concept. So, what does overdamped exactly mean, especially in the context of Brownian motion, and why is the acceler... | Overdamped means that viscosity forces are much more "relevant" than inertia. When this is the case, essentially any movement will very quickly reach terminal velocity, so the acceleration will be $0$.
As a toy model, imagine trying to push an object through a viscous medium. If we apply some constant force $F$, then N... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/551769",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Equation of motion for a particle under a potential in special relativity The equation of motion of a particle in Newtonian mechanics in 3D under an arbitrary potential $U$, is written as $$m\frac{\mathrm{d}^2 \mathbf{r}}{\mathrm{d} t^2}=-\nabla U.$$ Now, my question is, how can this be generalised to Special relativit... | As you point out, if $K$ is the force 1-form and $v$ the velocity 4-vector, $K(v) = 0$. This means that we cannot hope to find a scalar field $\Psi$ on space-time that gives $K$ by exterior derivative, that is, $K=\text d\Psi=(\text d_0\Psi, \text d_{(3)}\Psi)$. To see this, assume that the spatial part of $K$ is $\tex... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/551866",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
What does CERN do with its electrons? So to get a proton beam for the LHC, CERN prob has to make a plasma and siphon off the moving protons with a magnet. Are the electrons stored somewhere? How? I don’t mean to sound stupid but when they turn off the LHC, all those protons are going to be looking for their electrons. ... | You're right that CERN gets its protons by ionizing matter and collecting them. But the number of electrons & protons CERN deals with is far smaller than you might think. They get about 600 million collisions a second at CERN. So call it 1.2 billion protons used per second. $1.2 \times 10^9$. That'd be a large nu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/551992",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "35",
"answer_count": 5,
"answer_id": 0
} |
Is three dimensional supergravity dynamical? So it is well known that standard $D = 3$ Einstein gravity is non-dynamical in the sense that the graviton has no on-shell degrees of freedom (d.o.f $= D(D-3)/2$ and the theory is topological).
However, I have also seen that there are various theories of supergravity with up... | No; which is to say, the 3D SUGRAS most closely analogous to pure Einstein gravity in three dimensions are also topological in the sense you mention.
In fact the original reference that showed 3D gravity can be formulated as a Chern-Simons theory actually treated 3D supergravity! (Achucarro & Townsend https://inspirehe... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/552496",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
What happens to an inductor if the stored energy does not find a path to discharge? Suppose an inductor is connected to a source and then the source is disconnected. The inductor will have energy stored in the form of magnetic field. But there is no way/path to ground to discharge this energy? What will happen to the s... |
But there is no way/path to discharge this energy? What will happen to the stored energy, current and voltage of the inductor in this case?
In that case it makes its own circuit with its own path to ground. Often, that is through dielectric breakdown at the switch itself, but the details are highly unpredictable and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/552611",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "33",
"answer_count": 10,
"answer_id": 6
} |
Notation for vector time derivatives So I am self-studying mechanics using Marion and, as many books, it uses the notation of the dot over the function to express a time derivative, as in
$$x = x(t)$$
$$\dot{x}= \frac{dx}{dt}(t) $$
The book also uses the bold notation for vectors, like for example the position vector i... | Consistent notation is needed for clear communication, but there is no one "correct" notation for anything.
If you can remember what things mean in your own work, you can do whatever you want. If you are writing something that others must read too, like a graded homework assignment, it is important to define any notat... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/552973",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Calculation of effective mass from bandstructure The effective mass is defined as
$$
\frac{1}{m_{ij}^*} = \frac{1}{\hbar^2} \frac{\partial^2\epsilon}{\partial k_i \partial k_j}
$$
where, $m_{ij}^*$ is the effective mass, $\hbar$ is the Planck's constant, $\epsilon$ is the energy and $k_i,\ k_j$ are reciprocal latttice ... | From your data, it seems like you can obtain the longitudinal effective mass in that direction. By the way, that line in the reciprocal space, the [111] direction, is often called the $\Lambda$ axis.
I suppose that if you represent the energy as a function of $|\vec{k}|$ in that direction there will be a minimum or a m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/553254",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Chemical potential in canonical partition function I'm a bit confused on the interpretation of the chemical potential in a canonical ensemble (a system which can only exchange energy with a reservoir but not particles).
Here is what I think I know:
As far as I understand, when one deals with a system that can exchange... | By a "fixed" value of $N$ here we mean that for the system at equilibrium, the value of $N$ is not allowed to fluctuate. So yes, $N$ is fixed for a canonical ensemble.
If you allow particle number to change to new value so that the system finds a new equilibrium state with a new fixed value of $N$, the system has a ne... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/553425",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 1
} |
Why is surface tension measured in units of milliNewtons per meter? Rather than square meter(s)? Why is liquid surface tension written in units of mN/m, or milliNewtons per meter?
The related concept of surface energy for solids uses units of milliJoules per square meter.
| Both are equivalent, since $\text{mJ}=\text{mN}\cdot \text{m}$ where $\text{mJ}$ is millijoules, $\text{mN}$ is millinewtons and $\text{m}$ is metres. Thus
$$\text{mJ}\cdot \text{m}^{-2}\equiv \text{mN}\cdot \text{m}^{-1}$$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/553657",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 1
} |
Is the zero-point energy of helium stronger than other liquids to disfavour freezing? Under normal atmospheric pressures, liquid helium does not freeze even when cooled very close to absolute zero. This is attributed to the uncertainty principle or due to zero-point energy.
But the quantum uncertainty or zero-point en... | Helium, at pressures below 25 atm and absolute zero, does not freeze because it's zero point energy is high enough to stop it from going into a solid phase, and hence is stable as a liquid. Other gases in general do not have such high zero point energies, and thus transition from liquid to metal as the temperature plun... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/554096",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 1
} |
Examples of path integral where path of extremal action does not contribute the most? I have learnt that by doing a saddle point approximation in the path integral formulation of quantum mechanics, the classical action (extremal action where $\delta S=0$) is the one that contributes the most, hence seeing how classical... | In general it seems quite unlikely, unless there is some special symmetry that enforces this. I guess it could also sometimes happen in 0+1 dimensions because here the space of fields is much nicer.
But anyway, the closest example that I can think of is $\mathcal N=2$ supersymmetry in four dimensions. Here, due to supe... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/554165",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 2,
"answer_id": 0
} |
Lorentz Transformation Proof - Special Relativity This is from A.P. French Special Relativity book, Chapter 3 (page 78)
Setup of the proof: $S$ and $S'$ be inertial reference frame. $S'$ move to the right with respect to $S$ at velocity $v$.
Let co-ordinates in $S$ be $(x,t)$ and co-ordinates in $S'$ be $(x',t')$
Equat... | $b$ has to have the dimension of a velocity to be consistent. When the reference frame $S$ looks at the reference frame $S^\prime$, he sees it has moving to the right with velocity $+v$. But if you swap the frames, in the $S^\prime$ frame, if you look at the $S$ frame, you'll see it moving with the same velocity as bef... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/554413",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
How is it physically possible that the electric field of some charge distributions does not attenuate with the distance? Let's consider for instance an infinite plane sheet of charge: you know that its E-field is vertical and its Absolute value is $\sigma / 2 \epsilon _0$, which is not dependent on the observer positio... | First, by symmetry, you would expect the field to be perpendicular to the plane. All the sideways components cancel.
You can calculate the field at a point by adding up the field from all the charges in the plane. We start with a point a distance $z$ from the sheet. We can get the total field by integrating over rings... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/554594",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 6,
"answer_id": 2
} |
The 1D wave equation with gravity and the catenary I'm in the middle of writing a PDEs assignment and I thought I'd use the wave equation for a horizontal string with gravity. Easy I thought: $u_{tt} = c^2u_{xx} - g$. We solved it without gravity. Then added gravity: I set up the question on what the steady solution... | See Time Independent and Time Dependent Catenary Problem, a short paper that includes diagrams and derivations and behind some of what probably_someone posted.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/554698",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What means "Standard Sirens" associated to Gravitational Waves observable(s)? We know the Gravitational Waves (GW) provide a new way to observe the Universe. Now, we face a new era for cosmological and astrophysical researches. I know that it is possible to obtain the gravitational redshift from some astrophysical phen... | The meaning is, that by measuring the amplitude and frequency of the gravitational waves received from an inspiralling binary system, and by also measuring the rate of change of that frequency, we can determine both the"chirp mass" and the luminosity distance to the gravitational wave source.
In an analogous way, the o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/555033",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Acceleration as a function of position and time I know if you have an acceleration as a function of $t$, $a(t)$, to find the velocity you simply integrate $a(t)$ with respect to $t$. Moreover, if the acceleration was a function of position, $a(x)$, you use the fact that $a(x) = v(x) \cdot dv/dx$ and solve for $v(x)$. H... | This is a standard mechanics problem.
There definition of acceleration is
$$a=\frac{\text d^2x}{\text dt^2}$$
So if you know what $a(x,t)$ is, and you have initial conditions like $x(t_0)=x_0$ and $v(t_0)=v_0$, then you just need to solve the differential equation $$\frac{\text d^2x}{\text dt^2}=a(x,t)$$
From there it ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/555315",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Power consumption of phototubes A phototube1 (or "photoemissive cell") is a simple vacuum tube device that works by the photoelectric effect; it produces a current when light strikes the photocathode.
For it to work, a potential difference is generally applied between the cathode and the anode of 15V, I only need to kn... | An old fashion photo-tube (These days you might use a photo-transistor.), has a plate (the cathode) and an anode in a vacuum. A small fraction of the photons striking the plate cause electrons to be ejected into the vacuum. The applied voltage produces an electric field which accelerates these free electrons toward th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/555425",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Does the number of accessible microstates decrease overall when heat is transferred? We have two systems of ideal gas with different temperatures. $N$ & $V$ are being kept constant. The number of accessible microstates of each gas is thereby only influenced by a change in $E$.
The number of accessible microstates is:
... | I think the main confusion here is that you use $\Omega$ and the entropy interchangeably. $\Omega$ is not directly proportion to entropy. Rather $S$, the entropy, is proportional to $\log \Omega$. Let’s call the two systems you have $A$ and $B$. The entropy is additive $S_{tot} = S_A + S_B$. However the total number of... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/555561",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Does tangential acceleration change with radius? Do tangential velocity and tangential acceleration change with radius (change of radius on the same object)?
For example consider a spinning disk. Does the equation $$a_t = \alpha R$$ (where $a_t$ is the tangential acceleration, $\alpha$ is the angular acceleration and ... | To an extent, you are correct in your assumptions.
The formula $$\mathbf{\vec a = \vec \alpha \times \vec R \implies a=R \, \alpha }$$ when $\vec \alpha$ and $\vec R$ are perpendicular to each other, which is quite the general case.
The aforementioned formula relates the tangential acceleration $\vec a$ of a point par... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/555649",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 0
} |
How to verify the Klein-Gordon field commutation relations, Peskin and Schroeder Equation (2.30) I am trying to verify the commutation relation given in Peskin and Schroeder. In particular, I don't know how to go between these two lines:
$$[\phi(\textbf{x}), \pi(\textbf{x}')] = \int \frac{d^3p d^3p'}{(2\pi)^6} \frac{-... | Well you're nearly done. That delta function $\delta^3(\mathbf{p}+\mathbf{p}^\prime)$ just sets $\mathbf{p}^\prime = -\mathbf{p}$ when you integrate in $\mathbf{p}^\prime$ (or the opposite if you integrate in $\mathbf{p}$, but it's the same). What you get at this point is what follows
$$\begin{align}
[\phi(\textbf{x})... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/555912",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 1,
"answer_id": 0
} |
Approaches to Model Building I often see references to 'model building' in the particle physics literature, presumably to refer to creating new QFTs which go beyond the Standard Model.
How exactly does this process of model building begin? Does one simply write down a Lagrangian which has the desired properties and th... | I am writing this in an answer because comments tend to be deleted, it is a sort of answer from an experimental physics physicist.
One does not start from mathematical impulses to generate models. In physics the process is data driven, there exist data that the standard model cannot fit. This starts a search for a ma... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/556030",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Does putting a thin metal plate beneath a heavy object reduce the pressure it would have applied without it My dad bought an earthen pot and he kept it on our glass table. Worried that the glass could break on filling the pot with water. I kept a metal plate beneath it. At first, it seemed like a good idea , but on fur... | Here's how to think about this case.
imagine a very small glass table, just wide enough so that the three legs of the pot fit inside the circumference of that small table.
For such a small table the force exerted by each of the three legs would not be a problem at all.
For comparison, imagine a table where the top is ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/556175",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 0
} |
Evaluating sum of torques for different choices of origin when solving equilibrium problem My textbook says,
When applying equilibrium conditions for a rigid body, we are free to choose any point as the origin of the reference frame.
(source)
I am trying to understand this by looking at the following picture (from an... | The friction force is proportional to the normal force, which depends on both gravity and the applied force. Letting the friction force $F_s$ stand on its own, the correct expression for the torque about the origin is
$$\sum \tau' = 0 = F_s$$
From this it follows that the friction force is, in fact, zero.
The same can ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/556508",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Applying the principle of Occam's Razor to Quantum Mechanics Wolfgang Demtröder writes this in his book on Experimental Physics,
The future destiny of a microparticle is no longer completely determined by its past. First of all, we only know its initial state (location and momentum) within limits set by the uncertain... | I don't know what Demtröder meant but the sentence
Furthermore, the final state of the system shows (even for accurate initial conditions) a probability distribution around a value predicted by classical physics.
Is wrong.
Aspect's famous experiment on Bell's inequality gave a result (as predicted by quantum mechani... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/556585",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 10,
"answer_id": 1
} |
Identical particle state with spin We construct identical particle state by symmetrizing or antisymmetrizing the tensor product of single partice states. When considering spin, a two fermions state should be $$|\psi\rangle=\frac{1}{\sqrt{2}}(|\psi_1\rangle_{\sigma_1}\otimes|\psi_2\rangle_{\sigma_2}-|\psi_2\rangle_{\sig... | Neither answer is right in general.
*
*The only actual requirement is that the state be [anti]symmetric with respect to exchange. That's it.
*The first state you give, $|\psi\rangle=\frac{1}{\sqrt{2}}(|\psi_1\rangle_{\sigma_1} \otimes|\psi_2\rangle_{\sigma_2}-|\psi_2\rangle_{\sigma_2} \otimes |\psi_1 \rangle_{ \sig... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/556901",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Do virtual electron-positron pairs have mass? When a photon produces an electron-positron pair, do both these particles have mass? Why or why not?
| A virtual pair is by definition "off shell". That is, it does not obey the usual mass relation for that kind of particle:
$$E^2 = m^2 + p^2$$
Rather, it can have any $E$ and $\vec{p}$ such that 4-momentum is conserved at vertices. That means that the meaning of the "mass" of an off-shell system is pretty nebulous. You ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/557037",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
How do you hang a bottle off a toothpick? I came across this video: https://www.youtube.com/watch?v=mHsVxNMFWwA
How is this possible? I'm guessing the vertical toothpick is exerting an upward force on the table toothpick to balance the torque. But how? I am confus
| Firstly, the bottle creates tension in the string which is then imparted on the tootpick. This creates a torque on the toothpick which is neutralized by the truss-like arrangement attached to both strings. The support beam (toothpick) which exerts a normal force on the vertical beam then carries over as an opposing tor... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/557125",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Does energy conservation imply time invariance? This is similar to this question: Is the converse of Noether's first theorem true: Every conservation law has a symmetry?. However, the answer given there is very technical and general. I am only interested in the specific case of energy conservation (mostly because dark ... | It is actually the other way around. Time translation symmetry refers to Energy conservation.
We define the Hamiltonian as
$$\normalsize {H} = \Large{\Sigma_i}\normalsize{p_i\overset{.}{q_i} - L} $$
This says that the Hamiltonian in other words the energy is conserved when the Lagrangian has no explicit time dependance... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/557331",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Why don't hovercrafts move West relative to the Earth Suppose that there is a hovercraft floating a few centimetres above the Earth's surface. As it is disconnected from the Earth, which is spinning from West to East, shouldn't it appear to move East to West to observers on the ground? Does this happen? If not, why not... | Let's make it even more interesting! If what you said were true, then all you'd need to do to travel large distances is jump up in the air. The Earth turns at a speed of around 500 m/s (at the equator) and so if you were up in the air for a second, you'd travel 500 metres! Well, obviously, you don't. (If you did, long-... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/557462",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
How does the reversibility of physics interact with nuclear fission? The laws of physics are reversible and quantum information is never destroyed.
Given this, how do I time reverse the $U_{235}$ fission reaction, n which
${}^1_0n + {}^{235}_{92}U \rightarrow {}^{141}_{56}Ba + {}^{92}_{36}Kr + 3 {}^1_0n + \gamma +$ 202... | The reverse reaction, in which six particles join to make two, is allowed by time-reversal symmetry but forbidden by entropy considerations arising from classical thermodynamics. You just plain can't get those six particles to meet each other "just so"; there are too many ways for them to miss.
The way that thermodynam... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/557614",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Why do peaks and troughs of a wave cancel each other out? And why peaks and peaks or troughs and troughs add up? I have thought that the cancellation of peaks and troughs is a consequence of Newton's third law of motion that equal and opposite forces cancel each other out.
Or it has something to do with conservation of... | If you flick a taught rope from both ends, you will see waves forming and moving.
*
*A rising peak happens because the particles were flicked upwards by an upwards force by the person, and this upwards force propagates from particle to particle.
*A lowering valley happens likewise due to a downwards initial force w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/557719",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Why doesn't water boil in the oven? I put a pot of water in the oven at $\mathrm{500^\circ F}$ ($\mathrm{260^\circ C}$ , $\mathrm{533 K}$). Over time most of the water evaporated away but it never boiled. Why doesn't it boil?
| The "roiling boil" is a mechanism for moving heat from the bottom of the pot to the top. You see it on the stovetop because most of the heat generally enters the liquid from a superheated surface below the pot. But in a convection oven, whether the heat enters from above, from below, or from both equally depends on ho... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/557812",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "139",
"answer_count": 7,
"answer_id": 3
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.