Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
How does one exert greater force on the ground by jumping? When one jumps, how does he/she manage to exert greater force on their ground than their weight?
Also, what is normal force and the reaction force (are they the same thing?) and by newton's third law, shouldn't the reaction(weight) when we are standing on the ... | In a standing state, your legs exert a force on the ground that correlates to your weight. Any motion or acceleration caused by the movement of your legs is added to this.
Think of jumping as generating upward momentum in your body. Your legs are causing you to accelerate upwards and gravity is causing you to acceler... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/103473",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 6,
"answer_id": 1
} |
Derivation of the general Lorentz transformation The standard Lorentz transformation or boost with velocity $u$ is given by
$$\left(\begin{matrix} ct \\ x \\ y \\ z \end{matrix}\right) = \left(\begin{matrix}
\gamma & \gamma u/c & 0 & 0 \\
\gamma u/c & \gamma & 0 & 0 \\
0 & 0 & 1 & 0 \\
0 & 0 & 0 & 1
\end{matrix}\righ... | It's actually very simple. The general Lorentz transformation can be rewritten as
$$\left(\begin{matrix} 1 & 0 \\ 0 & H^\textrm{t} \end{matrix}\right)\,\left(\begin{matrix} ct \\ x \\ y \\ z \end{matrix}\right) = L_u \,\left(\begin{matrix} 1 & 0 \\ 0 & K^\textrm{t} \end{matrix}\right) \,\left(\begin{matrix} ct^\prime ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/103560",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
Quantum Excitations In the context of quantum mechanics, is the Schrödinger equation actually describing some sort of an actual wave in some field like light in EM field ? So all particles are actually waves in their respective fields ?
| In QM the Schrödinger equation, is the equivalent of Newton's law in Classical Mechanics. The Schrödinger equation describes the state of a quantum system (i.e. atoms, subatomic particles etc.), and how the quantum system changes over time. I think you are getting confused because there are two main places where the te... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/103622",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Aufbau principle in modern quantum theory What is the rigorous definition of the Aufbau principle and the mathematical model used for its description?
From Wikipedia, we have that the principle postulates a hypothetical process in which an atom is "built up" by progressively adding electrons. For modeling the postulat... | The Aufbau principle isn't rigorous because it's based upon the approximation that the electron-electron interaction can be averaged into a mean field. This is called the Hartree–Fock or self-consistent field method. The centrally-symmetric mean field results in a set of atomic orbitals that you can populate 2 electron... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/103718",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Should theory be the appropriate term? Should theory be the appropriate term? I mean, for example, because of the quantum field theory we have been able to find the subatomic particles that it theorized and make the Standard Model. Why then is is labeled as a theory? Also wave-particle duality is widely accepted fact y... | Terminology doesn't matter much as long as people understand the ideas involved. You should call and idea whatever you want to call it as long as you are clear about the substance of your ideas.
Some commentators have stated that the Standard Model or other scientific results are well-grounded. So far, these ideas have... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/103813",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Infinitely many points of spacetime? Please can someone provided me with academic literature (Journals/Books, titles & links) which discuss the current view on spacetime i.e. that there is not Infinitely many points of spacetime?
| This is a nice introduction to Loop Quantum Theory in which both space and time are the result of weaving together fundamental quantum of space itself. http://arxiv.org/pdf/1001.1330v1.pdf. I am sending you too a page which contains several links and options for the study of this theory. http://cosmicposts.wordpress.co... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/103864",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Creating electricity from mains water pressure. Could someone cleverer than me help me out?
I had a crazy thought going through my head the other day and I can't lay my mind to rest until I get an answer.
Q. How much energy could be produced by using mains water pressure to turn a generator? And would it be feasible ... | A generator converts mechanical power to electrical power; pressure alone is insufficient.
Assuming the flow in = flow out and a constant flow, the power output of the generator would then be proportional to the pressure difference between the inlet and outlet.
Thus, subtract the minimum pressure required by the buildi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/103949",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 5,
"answer_id": 1
} |
BICEP2 and e-foldings during inflation After the BICEP2 results, we now know that $n_s = 0.96$ and $r = 0.2$.
From what I understand, this fits extremely well with the basic chaotic inflation model given by $V(\Phi) = \lambda \Phi^4$.
We also know that amplitude of density fluctuations is $\approx 10^{-5}$ and
Energy s... | $r = 8(1-n_s)-\frac{8}{N_*}$ for monomial potential inflation models, equation 206 of this reference: http://lesgourg.web.cern.ch/lesgourg/Inflation_EPFL.pdf
where $N_*$ is "the number of e-folds between horizon crossing for observables scales and the end of inflation" (basically the observable number of e-folds).
so f... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104016",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 1,
"answer_id": 0
} |
Why do the planets orbit the Sun? Why do the planets orbit the Sun? If it's because the gravity of Sun then why don't they just fall in and burn up?
| The planets are attracted towards the sun, as you would expect from the gravitational force. The planets don't fall into the sun, though, because their velocities are at right angles to that force. The planets end up being pulled by the sun into a circle. A planet's speed is constant, but its direction changes. I think... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104237",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Size of Universe after inflation I read in some website that during the period of inflation, the expansion of the universe underwent incredibly fast, and its size increased by a factor of $10 ^{50}$, see this link
In this field, I think, there is nothing for sure, but if there was really inflation, what does it mean th... | The best way to think about inflation (according to my University's cosmology expert) is to think as if you were measuring the universe with an ever-shrinking ruler.
So if you were measuring 1 meter, then your two reference points would now be separated by $10^{50}$ meters, because your meter is shrinking. If you were ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104302",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
How does particles gain electrical charges and repel each others? (electrostatic stabilization) When I study electrostatic stabilization, I understand that the particles have same charge and thus repel others, this is how colloid is stabilize. But how does particles gain electrical charges and repel each other in the f... | John Rennie will probably have more details on the matter, but in general colloids (such as oil dispersed into soap water) are not so much stabilized by a net total charge of the mixture, but rather are stabilized by repulsions from separated charges. For example:
This is a cartoon representation of what an oil drople... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104446",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Which is the smallest known particle that scientists have actually *seen with their eyes*? Which is the smallest particle that has been actually seen by the scientists? When I say "actually seen", (may be using some ultra advanced microscope or any other man made eye, using any wavelength or phenomena) I really mean i... | Obviously, the smallest particle that scientists have ever seen directly is a photon.
The question is a bit silly because it tries to eliminate any simple device like a photographic plate. But the human eye, its nerves and the visual cortex together are far more complicated.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104523",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 2,
"answer_id": 0
} |
In the B mode power spectrum, what is the relationship between the multipole number and the wavelength of the seed gravitational waves? One of the key datasets of the recent BICEP2 results is the B mode power spectrum shown below. The existence of these B modes implies the existence of gravitational waves prior to i... | Wayne Hu (professor, Univ. Chicago) has a CMB website
that seems to answer this question.
It is stated that "The one-to-one mapping between wavenumber and multipole moment described in [a previous section] is only approximately true and comes from the fact that the spherical Bessel function is strongly peaked at $kD... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104598",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 1,
"answer_id": 0
} |
Help understanding proof in simultaneous diagonalization The proof is from Principles of Quantum Mechanics by Shankar. The theorem is:
If $\Omega$ and $\Lambda$ are two commuting Hermitian operators, there exists (at least) a basis of common eigenvectors that diagonalizes them both.
The proof is:
Consider first the ca... | I don't know exsactly what it means with scalar but remember that:
A vector $|\Psi\rangle$ is invariant up to a phase because a global phase is always ruled out when you calculate, for example, with the state $e^{i\theta}|\Psi\rangle$, the expected value of and observable $A$ using this state is
$$\langle A \rangle = ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104674",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 3
} |
Proving a step in this field-theoretic derivation of the Bogoliubov de Gennes (BdG) equations In derivation of the BdG mean field Hamiltonian as follows, I have a confusion here in the second step:
$H_{MF-eff} = \int d^{3}r\psi_{\uparrow}^{\dagger}(\mathbf{r})H_{E}(\mathbf{r})\psi_{\uparrow}(\mathbf{r})+\int d^{3}r\psi... | The negative sign comes from the anti-commutation of the fermion operators $\psi$ and $\psi^\dagger$. That is $\psi^\dagger(r)\psi(r') = - \psi(r')\psi^\dagger(r) + \delta(r-r')$. The delta function just gives the some constant that will be absorbed into the $const$ term. Maybe it is bothering you that this term is for... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104741",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 2
} |
How to show the invariant nature of some value by the group theory representations? Let's have Dirac spinor $\Psi (x)$. It transforms as $\left( \frac{1}{2}, 0 \right) \oplus \left( 0, \frac{1}{2} \right)$ representation of the Lorentz group:
$$
\Psi = \begin{pmatrix} \psi_{a} \\ \kappa^{\dot {a}}\end{pmatrix}, \quad ... | If we assume that
$$\Psi {'} = \hat {S}\Psi$$
and
$${\bar{\Psi}}{'} = \bar {\Psi} \hat {S}^{-1},$$
it follows that the product of the two transforms as
$$(\bar{\Psi}\Psi)'={\bar{\Psi}}{'}\Psi {'}=\bar {\Psi} \hat {S}^{-1}\hat {S}\Psi=\bar{\Psi}\Psi,$$
which is a consequence of
$$\hat {S}^{-1}\hat {S}=\mathbb{1}.$$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104822",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
Renormalizing with external momenta set to zero I've often seen in textbooks that authors renormalize diagrams by setting external momentum to zero. Under what conditions is this justified?
An example of this is done in Manohar and Wise's book on Heavy Quark Physics after they renormalize QED and then calculate the ope... | In general, derivative couplings lead to momentum-dependencies in scattering amplitudes. This can be seen from the fact that the Fourier transform of a derivative operator corresponds to a multiplication by the relevant momentum. A mass dependence is implicit through by having a momentum, since the momentum of a fermio... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/104899",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Does the transmission axis matter for sending polarized light through polarized glass? If I have polarized light and I send through only one polarized glass plane, does the transmission axis matter, or will the intensity be halved no matter what.
| The axes definitely matter. If you put light through a linearly polarized glass pane, the output light will be entirely polarized along the polarization axis of that pane. The intensity of the output light will be
$$I_{\textrm{out}} = I_{\textrm{in}}\cos(\theta)^2$$
where $\theta$ is the angle between the polarizations... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105018",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Free particle propagator - Evaluating Integral In path integral formalism, when evaluating the free particle propagator, we obtain the functional integral of the form,
$$ K_0 = \lim_{n\rightarrow\infty} \bigg( \frac{m}{2\pi i\tau}\bigg)^\frac{n}{2} \int \prod_{i=1}^{n-1}dx_i \; \exp\bigg(\frac{im\tau}{2}\sum_{j=0}^{n-1... | Note that all your integrals are Gaussians in differences of positions at successive instants $(x_k - x_{k-1})$ so implement a change of integration variable from $x_k \longrightarrow (x_k - x_{k-1})$. You will have $N-1$ (straightforward) integrations to perform with $x_0$ and $x_N$ held fixed.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105263",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
How big was the first transistor? The first working point-contact transistor made in 1947 by Bell Labs. I'm looking for specific dimensions, all I've been able to find is "Fits in the palm of your hand".
| From "The Transistor, A Semi-Conductor Triode", by J. Bardeen and W. H. Brattain, Phys Rev. 74(2), 230-231 (1948):
"The device consists of three electrodes placed on a block of germanium as shown schematically in Fig. 1. Two, called the emitter and collector, are of the point-contact rectifier type and are placed in c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105401",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Deriving $F = ma$ - Newton's Second Law of Motion Context:
In my textbook it is given: 'momentum' short for 'linear momentum':
Mass = $m$, momentum is $p=mv$. In time $\Delta t$, momentum changes by $\Delta p$, the rate of change of momentum is:
$$\frac{\Delta p}{\Delta t} = \frac{\Delta(mv)}{t} = m \frac{\Delta v}{\D... | *
*Yes. It should be:
$$\frac{dp}{dt}=\frac{d(mv)}{dt}$$
I'm using $d$ instead of $\Delta$ because I am thinking about the limit where the changes in $p$ and $t$ are very small. Then these are called infinitessimal changes, and denoted by a $d$.
*Usually, when one considers simple problems in Newtonian mechanics, wha... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105460",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 5,
"answer_id": 1
} |
Magnetism due to relativity? So I have been reading in some books that magnetism does not have to be assumed a priori, but can be obtained from the electric field + special relativity.
And I have seen how this leads to the common formula for the magnetic field of a current carrying wire.
Fine.
What about materials that... | Oliver Heaviside showed that magnetic fields are a result of moving charges, and that relativity is not really needed. He showed that if gravity moves at a finite speed, then a magnetic like effect arises.
Bohr's atom is non-relativistic, but still has the correct magnetic properties.
Relativity itself is due to the f... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105565",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Viewing glass from an oblique angle When I view most glass from the side it's green which I've found out is due to impurities in the glass specifically from iron oxide.
Why is it when I view the larger face from an oblique angle, it isn't nearly as green? I cannot personally notice any different on the piece I have nex... | Even when held at a glancing angle, the light within the glass follows a relatively short path. As light crosses the boundary between the air an the glass its angle changes (refraction). Light that leaves the glass at a glancing angle and enters your eye was traveling much closer to perpendicular to the surface when ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105693",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
The shape of the graph of the equation: $V= -r I + E$ I have recently collected data (for a school experiment) in order to measure the EMF and the internal resistance of a solar cell. The data complied with the equation: $V = -rI + E$, i.e. the voltage decreased as the current increased and I have a negative gradient. ... | Your reasoning is correct.
If the solar cell is modelled as a voltage source $E$ in series with an internal resistance $r$ and the cell is connected to a load resistance $R_L$, the series current is given by Ohm's law:
$$I = \frac{E}{r + R_L}$$
or
$$E = (r + R_L)\cdot I $$
The output voltage $V$ of the solar cell is th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105738",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Why do particles in high pressure air always flow to lower pressure? The title really says it all: Why is this case? A "Feynman type" answer would be really appreciated as I'm more of a layman that a physicist.
| You mention particles, so I'll provide a micro-scale analogy. In a gas or liquid, particles are always interchanging energy and momentum due to collisions with other particles. So we can treat them as if their velocity and direction is always random and changing.
Imagine a line in the middle of a field, with 10 people... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105848",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
How can point-like particles in an ideal gas reach thermodynamical equilibrium? Having learned that the particles of an ideal gas must be point-like (for the gas to be ideal) I wonder how they can reach thermodynamical equilibrium (by "partially" exchanging momentum and energy). First the probability of two point-like ... | Ideal gasses do not exist. In real gasses, molecules do take up volume. This does affect the pressure-volume curve.
Still, an ideal gas model is close enough to reality to be useful. It is more useful if not taken so literally as to include its most unrealistic features.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105918",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 5,
"answer_id": 1
} |
Is speed of light ( Sun-to-Earth ) related to the movement speed of solar system? The speed of light has been measured to be 299 792 458 m/s.
Now, the Solar System is traveling at an average speed of 828,000 km/h (230 000 m/s). Summing up the numbers we get close to 300 000 000 m/s
Does it mean, that the speed of light... | No the speed of light in vaccuum is an absolute constant $c$ = 299 792 458 m/s
The way to add up relativistic speeds is:
$u' = \frac{u-v}{1-\frac{uv}{c^2}}$ to account for the constancy of the speed of light
You cannot simply add them up.
Edit: This also applies to normal everyday speeds. The reason we don't use this f... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/105974",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Requirements for hydrogen fusion What are the requirements for hydrogen atoms to go through fusion? Is it a ratio of heat to pressure or are there specific heat and pressure values that must be met (per atom or per mole?) Are there other requirements?
| The Wikipedia article answers most of your questions.
What are the requirements for hydrogen atoms to go through fusion?
Two atoms must overcome the coulomb barrier, which can be done by forcing two atoms very close together, or by leaving them moderately close for long periods of time, which allows them to tunnel t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106053",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 0
} |
State of constant motion Why does an object remains in its state of constant motion if there are no forces acting on that object?
My understanding is that all the energy of the motion will be kept inside and a change in the speed needs a change in the amount of the energy stored by that object. But how is that energy s... | the main reason is that the space is uniform, and that there is nor absolute reference point in the universe.
basically, what appears to be moving at constant speed to you, will be moving at a different constant speed or even not moving at all to another observer who is moving at constant speed in reference to you. sin... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106158",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Does mass affect velocity when travelling through frictionless medium? I found the following question on an standardized test, and was debating with some friends what the answer would be:
A car of mass M is travelling with a constant velocity through a plane in which friction is non-existent. An object of mass m (m = M... | Your friends are correct.
If there is no force in the left-right direction, then linear momentum will be conserved in that direction. Because the new composite object has more mass than the original object, it will have a lower speed to the right.
What about energy?
Kinetic energy is not conserved in this case, because... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106236",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
How can we detect cosmic background radiation? From what I understand, CMB is the left over radiation from the Big Bang. As all matter, including the Earth, was made during the Big Bang and then as the universe expanded that matter/energy got further and further apart, but as that matter can't move faster than the spee... | You are assuming the Big Bang happened at a point, so the CMB is a shell of radiation expanding outwards from that point. However the Big Bang happened everywhere so every point in the universe is a source of the CMB. The CMB radiation we are detecting today comes from regions of the universe that were about 13.8 billi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106311",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 0
} |
Orthogonality of summed wave functions Problem. I know that the two wave functions $\Psi_1$ and $\Psi_2$ are all normalized and orthogonal. I now want to prove that this implies that $\Psi_3=\Psi_1+\Psi_2$ is orthogonal to $\Psi_4=\Psi_1-\Psi_2$.
My naive solution. From the premises, we know that
$$\int_{-\infty}^\in... | This problem could be done more simply through the application of linear algebra. You want to prove that
$$\langle \psi_1 - \psi_2 | \psi_1 + \psi_2 \rangle = 0$$
The inner product is analogous to the dot product of linear algebra, and it is distributive. Distributing, we find that
$$\begin{aligned}
\langle \psi_1 - \p... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106382",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Electron distribution around atom when moving I do not have much experience on this but if an atom has some electrons around nucleus and the atom itself it is moving at some speed does that affect the distribution of electrons around?
I am presuming that the interaction between the nucleus and electrons has a constant... | The speed only affects if there is an acceleration. If the atom is moving at a constant speed, you can do a galilean transformation, move with the atom, and it will be at rest.
The two simplest reasons for an atom to accelerate are collision with other atoms and bombardment.
The first one happens everytime you have a g... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106526",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
The other side of the lever If I have a lever, but I can see only up to the hinge and not the other half, can I know whether the other half is 1 m long with a weight of 3 kg on it, or 3 m long with a weight of 1 kg on it?
| Yes, you can. The moment of inertia of the lever will be different in each of the two situations. Let us assume that the lever is massless and the weight is a point mass. In the first situation, $I = mr^2 = (3)(1)^2 = 3 \text{ N} \cdot \text{m}$. In the second situation, $I = mr^2 = (1)(3)^2 = 9 \text{ N} \cdot \text{m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106566",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 2,
"answer_id": 0
} |
Can a laser be designed to ionize muonic atoms so as to prevent a-sticking? Muon catalyzed fusion is currently little more than a lab curiosity today in part because of how many hydrogen nuclei can be fused before the muon is carried away by an alpha particle. Deuterium+deuterium reactions are ten times more likely tha... | Muon mean lifetime is 2.2 µs. There's your problem. Muons mass 105.7 MeV/c2, about 200 times that of the electron. If you wanted to ionize a hydrogen atom, you would need 13.6 eV. If you wanted to ionize a muonic hydrogen atom, you would need about 2813 eV or about a 0.441 nm photon. Start building your laser.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106644",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Can magnetic flux be negative I am studying magnetic flux linkage in an ac generator and it appears to be that magnetic flux linkage is negative half the time, how can this be?? Also with lenz's law why is emf defined as negative when magnetic flux is increasing and how does this relate to the direction of the current?... | Magnetic flux is a scalar quantity and its positive/negative sign indicates the direction of the magnetic field.
And the Faraday's law of induction is a quantitative version of Lenz's law, which may help your understanding:
$\oint_{\partial \Sigma} \mathbf{E} \cdot \mathrm{d}\boldsymbol{\ell} = - \frac{d}{dt} \iint_{\... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/106949",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
The degeneracy of the ground state I wonder how can i know the degeneracy of the ground state of certain elements?
I'm doing Boltzmann distribution problems, and I'm not sure what to do. I have to calculate ratio of ions in 3p excited state, regarding ground state for Na$^+$ and Mg$^{+2}$ ions. I think that 3p degener... | Each s sublevel (l=0) can be occupied by 2 electrons.
Each p sublevel (l=1) can be occupied by 6 electrons.
Each d sublevel (l=2) can be occupied by 10 electrons.
Each f sublevel (l=3) can be occupied by 14 electrons.
Each sublevel can be occupied by 4l + 2 electrons.
But are you sure that it is Na+ and Mg2+ (not Na a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107114",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why do rocket engines have a throat? Diagrams of rocket engines like this one,
(source)
always seem to show a combustion chamber with a throat, followed by a nozzle.
Why is there a throat? Wouldn't the thrust be the same if the whole engine was a U-shaped combustion chamber with a nozzle?
| As addition to @tpg2114 answer, I suggest also to read about de Laval nozzle and Rocket engine nozzle on wikipedia.
Some typical values of the exhaust gas velocity
for rocket engines burning various propellants are:
1.7 to 2.9 km/s (3800 to 6500 mi/h) for liquid monopropellants
2.9 to 4.5 km/s (6500 to 10100 mi/h) for... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107191",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "52",
"answer_count": 4,
"answer_id": 2
} |
What happens with a tunneling particle when its momentum is imaginary in QM? In classical mechanics the motion of a particle is bounded if it is trapped in a potential well. In quantum mechanics this is no longer the case and there is a non zero probability of the particle to escape the potential through a process call... | One of the strangenesses concerning the behaviour of particles crossing a tunnel barrier is that one : crossing the tunnel barrier at superluminal group velocity is theoretically possible (see Wigner time and Hartman effect).
This has been experimentally confirmed by experiments conducted by R. Chiao with the participa... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107261",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
How can one reasonably theoretically model polycrystalline materials? Many techniques are taught in advanced solid state courses but they are almost all derived for perfectly crystalline materials. For example, band structure really only appears theoretically when you look at periodic potentials that are pretty big in ... | I can't give a general answer, but let me give an example. In graphene, grain boundaries have been modeled as dislocations. Their effect on transport properties can be estimated from an (analytic) Dirac equation model, and these estimates can be verified computationally, in this case using nonequilibrium Green's func... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107310",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Sum of acceleration vectors If a point mass has some accelerations $\mathbf{a_1} $ and $\mathbf{a_2} $, why is mathematically true that the "total" acceleration is $\mathbf{a}= \mathbf {a_1}+\mathbf {a_2}$?
| The expression "Total accelration" does not fit if the accelrations have different directions. The vector resultant is actually the "net accelration", or the combined effect of these two accelrations, or equivalently, forces. The vector resultant makes sure that only the effective components are added, and the opposing... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107383",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 3
} |
What's the difference between Raman scattering and second harmonic generation in crystal? As far as I know, the Raman scattering is from the stokes and anti-stokes scattering that the laser light interact with molecular vibrations. So we know that ""laser light interact with molecular vibrations"" in crystal can caus... |
I want to know exactly the physical model that what's happening in the crystal which create second harmonic
One physically intuitive model for thinking about light-matter interactions is in terms of an energy level picture. In this picture, light propagating through a material can be thought of as series of absorptio... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107501",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 0
} |
Can doppler shift be used to find the MH370 black boxes? The Australian ship Ocean Shield has detected multiple pings from the black boxes onboard the missing Malaysia Airline Flight 370, specifically on 4 lines of bearing according to this article. The same article also states that they need a few more lines of bearin... | Doppler shift occurs only when the sender, the receiver or both are moving relatively to each other. As the black boxes rest at the bottom of the ocean and the search ships move relatively slow, there won't be any significant Doppler shift. However, if the Ocean Shield receives several signals at different locations (t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107576",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Evolution of minimization of surface tension What are governing equations (or/and variational principles) for evolution of a simply connected body of water in vacuum?
Initial state - for time $t=0$ we have a bounded simply connected set $\Omega$ containing water, for simplicity - macroscopic velocity is zero and macros... | I'll assume that you have a well-defined interfacial energy $\gamma$ between water and "vacuum".
If there's only mechanics involved, Navier-Stokes equations with a boundary condition that the total stress along the normal $\vec n$ of the boundary (left hand side below) is normal and equal to the product of mean curvatu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107688",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
In calculating work done by a constant force over a constant distance, why doesn't the subject's initial velocity matter? Assume a point-mass $m$ is travelling in a straight line, and a force $F$ will act on $m$ (in the same direction as $m$'s velocity) over a constant distance $d$; why doesn't $m$'s velocity matter to... | Well, the reason it doesn't matter is that work is defined as
$$W = \int\vec{F}\cdot\mathrm{d}\vec{s}$$
so if you keep the force the same and the distance the same, this remains the same, regardless of what you do with the initial velocity.
Of course, that definition probably isn't particularly satisfying. So consider ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107845",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 7,
"answer_id": 1
} |
If the MH370 black box did sink to 15000 ft, how long would it have taken? I have been following MH370 on the news just as everyone and latest reports seem to indicate that the black-box could be found. A recent info-graphic http://t.co/lyBBE9C2hF shows the insurmountable depth of the oceans and how the black-box could... | The relevant equation is the kinematics with linear drag. In this case, there is a resistant force that acts opposite gravity (i.e., upwards) and is linear to the velocity at which it travels:
$$
\mathbf F_D=-b\mathbf v
$$
where $b$ is some fluid- and object-dependent constant.
Using Newton's 2nd law,
$$
m\ddot{\mathbf... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/107997",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Projection operators and their subspaces (of Hilbert space) I've been watching Susskind's lectures on Quantum Entanglement, and something he said regarding (non-)commuting projection operators confused me.
Consider two subspaces {$|a\rangle$} and {$|b\rangle$} of Hilbert space, with operators $K$ and $L$ for which:
*
... | "projection operators commute → they're the same"
Are you sure he said this predicate ? or it is your conclusion?
Anyway, it is not true ! Consider two dimensional X-Projector And Y-Projector , they commute but they are not the same!
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108077",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Why is the divergence of a magnetic field equal to zero? We know due to Maxwell's equations that:
$$\vec{\nabla} \cdot \vec{B}=0$$
But if we get far from the magnetic field, shouldn't it be weaker?
Shouldn't the divergence of the field be positive?
If we define the vector field as a function of distance, then if the d... | Divergence means the field is either converging to a point/source or diverging from it.
Divergence of magnetic field is zero everywhere because if it is not it would mean that a monopole is there since field can converge to or diverge from monopole. But magnetic monopole doesn't exist in space. So its divergence is ze... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108224",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 2,
"answer_id": 0
} |
Is there a known equation for evolution of classical particle probability density? Suppose we have some very imprecise knowledge of classical particle's coordinates and momentum: what we can only tell is the probability density to find it in some point of phase space. This is (almost?) all what is usually known by quan... | It seems to me that you're looking for the Boltzmann transport equation:
$$
\frac{\partial f}{\partial t}+\frac{\mathbf p}{m}\cdot\nabla f+\mathbf F\cdot\frac{\partial f}{\partial\mathbf p}=Q+\left(\frac{df}{dt}\right)_{\rm coll}
$$
with $f$ the distribution in phase-space, $\mathbf p$ the particle momentum, $Q$ some s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108361",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
How does the frequency of a particle manifest itself? In terms of wave-particle duality for, let's say a photon; how would the frequency practically manifest/demonstrate itself? Like, i understand that the frequency is related to the energy a particle has, but frequency in my mind suggests oscillation about a point. Is... | The easiest way to see frequencies is in interference. Imagine you have waves coming towards a wall. Imagine too that the frequency of the waves is way higher that what you can see. You cannot directly observe the waves, but you will see that the wall is wet a few centimetres over the surface. Now, instead of one wave,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108471",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 2,
"answer_id": 1
} |
2D Gauss law vs residue theorem I used to have a vague feeling that the residue theorem is a close analogy to 2D electrostatics in which the residues themselves play a role of point charges. However, the equations don't seem to add up. If we start from 2D electrostatics given by $$\frac{\partial E_x}{\partial x} + \fra... | The analogy follows with the right definitions. The "flux" of the "vector" $E(z)$ through a contour $\Gamma$ is $\mathrm{Re}\left(\int_\Gamma E(z)^*\,\mathrm{d} z\right)$.
I think you may have forgotten the conjugate in the relationship between the "Electric field" and the complex potential $\Omega$: $E(z) = -(\mathrm{... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108560",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 1
} |
What is an intuitive explanation using forces for the equatorial bulge? The earth is not a sphere, because it bulges at the equator.
I tried fiddling with centripetal force equations and gravity, but I couldn't derive why this bulge occurs.
Is there
(a) a mathematical explanation using forces (not energies) and
(b... | There is a wikipedia article which describes the effect http://en.wikipedia.org/wiki/Equatorial_bulge
Basically the bulge is caused by the rotation of the Earth. The centripetal force is given by $F=m\omega^2 r$. Therefore the poles feel a lesser force than the equator which wants to spin out into a disc. This is balan... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108640",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
In what sense do Goldstone bosons live in the coset? Goldstone's theorem says that if a group, $G$, is broken into its subgroup, $H$, then massless particles will appear. The number of massless particles are given by the dimension of the coset, $G/H$. It is then often said that the Goldstone boson's live in the coset. ... | I understand the statement in the following way:
Pions, which are pseudo-goldstone bosons of chiral symmetry breaking, are described by the introduction of a unitary matrix $U(x)$, defined as
$$U(x)=\text{exp}\left(2i\pi^a(x)T^af_\pi^{-1}\right),$$
where $\pi^a$ is the pion field, $f_\pi$ is the pion decay constant an... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108722",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "25",
"answer_count": 2,
"answer_id": 0
} |
Why can colors be mixed? We can combine colored light, creating other colors, at least in terms of visual perception.
But how it the result physically "a different color" - if it is at all?
Or is all this not a physical question to begin with - but only about our eye and brain?
To have an example, we
*
*have an i... | In my answer to this question: (What is the sun's spectral series?), I give a very detailed answer about why mixing colours of light produces other colours and how it is purely a result of biology and evolution. I also delve a bit into the structure of the human eye and why, in fact, only three colours are necessary to... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108783",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Finding the vacuum which breaks a symmetry I will start with an example. Consider a symmetry breaking pattern like $SU(4)\rightarrow Sp(4)$. We know that in $SU(4)$ there is the Standard Model (SM) symmetry $SU(2)_L\times U(1)_Y$ but depending on which vacuum we use to break this symmetry, in a case you can totally bre... | *
*As your QFT theory text should tell you, for an action invariant under G, addition of a Higgs potential only invariant under its subgroup H will spontaneously break the generators in G/H. You ought to do due diligence and study and understand and reproduce all examples of elementary classics such as Ling-Fong Li, P... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/108930",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 2,
"answer_id": 0
} |
Studying Quantum Electrodynamics? As an electrical/computer engineer, I already have a relatively thorough understanding of classical electromagnetism.
From what I understand though, classical EM is only an approximation to quantum electrodynamics. I'm very curious about how it all really works though.
So as an ECE en... | In order to gauge your level of previous knowledge, you could try Engineering quantum electrodynamics, by Dietrich Marcuse ( https://books.google.com.br/books?id=SGd5AAAAIAAJ&dq=editions:STANFORD36105030208412&hl=pt-BR&redir_esc=yEngineering quantum electrodynamics )
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109018",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Can molten metal be suspended in air? I wondered if magnets could be used to hold a drop of molten liquid metal in air (not for any particular reason just because it could be done), but was disappointed when a quick Google search showed the metal would lose its magnetic traits before it melted.
Are there any other forc... | Yes, it is possible to magnetically levitate molten metal. This is not due to ferromagnetism however. As seen in the below references, the metal sample is placed within a tapered conducting coil, which carries alternating electric current in the ~400kilohertz range. This sets up a magnetic field gradient inside the c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109163",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 2,
"answer_id": 0
} |
Is the molecule of hot water heavier than that of cold water? We know that the molecule of hot water($H_2O$) has more energy than that of cold water (temperature = energy)
and according to Einstein relation $E=mc^2$ ,this extra energy of the hot molecule has a mass.
Does that make the hot molecule heavier?
| I am going to take a different approach from DavePhD and Floris.
"Hot"ness or temperature more generally is a thermodynamic idea, and can't really be applied to an individual molecule. Dave and Floris have avoided the issue by simply comparing and energetic molecule with a less energetic one, and that is reasonable, bu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109353",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 2
} |
Why does the Fermi Surface cross the Brillouin zone boundary at right angles? I'm not sure why the fermi surface crosses the Brillouin zone boundary at right angles. I understand that this is normally the case, but not necessarily always.
I'm aware that the fermi surface is a constant energy surface up to the filling p... | This answer nothing to the OP's question, please don't vote up anymore; and anyone who know the answer to this question please share.
This is mainly due to the time reversal symmetry.
Consider the Bloch equation:
$$[-\frac{\hbar^2}{2m}\nabla^2+U(r)]\psi_{nk}=\epsilon_{nk}\psi_{nk}$$
Recall that $\psi_{nk}=e^{ik\cdot r... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109416",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "8",
"answer_count": 3,
"answer_id": 0
} |
Could an orbiting mass generate power? If an heavy object (e.g. 10 tons) orbiting around Earth at 370 miles high, is connected with a cable back to Earth, we assumed either Earth is going to pull the mass or vice versa (or it will fall back to Earth). Assuming correct pressure/release/pull can be applied from the groun... | This would be space elevator type of setup but it is too short to stay in orbit. It should extend beyond the geostationary orbit (~22,000 miles). International space station is a an altitude of ~200 miles and goes around the planet every 90 mins. Even if you build a proper space elevator there is no way to generate mo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109494",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
simple question about collimating lens Let me preface this by saying that I have a very limited knowledge of optics -- basically, I know enough to be dangerous. So I have a square Fresnel lens that measures about 10.5 inches on a side. The focal length is about 11.5 inches. I'm trying to find a presumably-smaller glass... | Since your diagram indicates a point source, you only need one lens to do this. If you place the Fresnel lens at a distance equal to its focal length from the light source you will get a parallel beam of light.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109612",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Interpretation of Conjugate Momentum in Field Theory The conjugate momentum density, following as a conserved quantity with Noethers Theorem, from invariance under displacement of the field itself, i.e. $\Phi \rightarrow \Phi'=\Phi + \epsilon$, is given by $\pi=\frac{\partial L}{\partial ( \dot{\Phi})}$.
On the other ... | The momentum you call $\Pi$ is the momentum in a certain spatial direction. The momentum you call $\pi$ is the momentum in a direction in field space.
For example, consider vertical waves on a horizontal string. The "field" $y(x, t)$ represents the vertical displacement of the string at point $x$. Then $\Pi$ is the ord... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109707",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 0
} |
What is the Physical Significance of Tr(A) w.r.t. Matrix Representations in Group Theory I've seen the post on mathoverflow.SE asking almost the same question, and I have indeed flipped through said answers, but most are in a more general context ie quantum mechanics and do not provide a conceptual answer with physical... | In physics one tends to write (for a Yang-Mills field), $A_{\mu}^i$, where $\mu$ is the spacetime index and $i$ is the `group' index. To be more specific, it means that $A_{\mu}$ take values on (i.e., is contracted with the generators of) a Lie algebra, $$A_{\mu} = A_{\mu}^i T^i = A_{\mu}^i (T^i)_{mn}, $$ where in the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109768",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Does electricity flow on the surface of a wire or in the interior? I was having a conversation with my father and father-in-law, both of whom are in electric related work, and we came to a point where none of us knew how to proceed. I was under the impression that electricity travels on the surface while they thought ... | The short answer is the surface. Being in a car during a lightening strike or high voltage line drop would kill you. Also think of the Tesla videos where someone is wearing a suit of armor and doesn't die from the arcs of electricity hitting him in the head; the difference in potential from his head to his feet, alth... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109897",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "64",
"answer_count": 9,
"answer_id": 7
} |
Feynman diagram for annihilation What is the difference between these two Feynman diagrams?
They should both describe the same physical process, annihilation between an electron and a positron.
| The first process corresponds to $e^{-}e^{+}\to e^{-}e^{+}$ (Bhabha scattering), where the final and initial states are the same, consisting of an electron and positron. However, the second process is $e^{-}e^{+}\to \gamma \gamma$, where instead the final state is that of two photons. The scattering amplitudes will be ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/109979",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
Is refraction sharp or smooth? Refraction: light changes direction of propagation when entering a material with a different refractive index.
Does the direction of propagation of light change sharply and almost instantaneously (as shown in the diagram) or smoothly?
| Generally speaking, it's impossible to physically observe a strictly "sharp" phenomenon in physics (there still some exceptions of course). However one phenomemon can have some characteristic constants which vary much more faster compared to others phenomema.
Here, in the geometrical optics approximation, since you are... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110119",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Higgs boson production via positron-electron collision One of the suggested diagrams for the Higgs production is the following:
so basically an electron-positron pair annihilates and forms an (excited?) Z boson, which then decays into another (less excited?) Z boson and a Higgs boson.
Why can't the electron-positron pa... | The electron-positron pair can produce directly a Higgs boson, but this process is very suppressed, because the coupling between the leptons and the Higgs is proportional to the tiny mass $m_e$:
$$g_{\rm Hee}=-i\frac{ m_e}{v},$$
where $v\approx 246 \,\rm{GeV}.$
On the other hand, the process $e^+ e^-\to H Z$ is more l... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110241",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 2,
"answer_id": 1
} |
Deriving formula for time dilation Last week in class we derived the formula for time dilation using light clocks and got $$t=\gamma t_0\quad \gamma=\left(1+\left(\frac vc\right)^2\right)^{-1/2}$$
So far so good. However, after class I was thinking of an alternate proof, but for some reason I got another result and I c... | Light always travels with $c$. That is why Michelson-Morley's experiment is so famous and groundbreaking.
If you send a beam of light from point A to point B, and it gets reflected and comes back to B than it doesn't matter, if A is moving relative to B with velocity v or not. You still get the same total velocity for ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110316",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Is frequency quantized in the black body spectrum? I'm aware that there're some questions posted here with respect to this subject on this site, but I still want to make sure, is frequency quantized? Do very fine discontinuities exist in a continuous spectrum like the black body spectrum?
The quantization of photon ene... | Frequency is not quantized, and has a continuous spectrum. As such, a photon can have any energy, as $E=\hbar\omega$. However, quantum mechanically, if a particle is restricted by a potential, i.e.
$$\hat{H}=-\frac{\hbar^2}{2m}\nabla^2 + \hat{V}$$
for $V\neq 0$, the energy spectrum is discrete. For example, in the case... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110463",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 3,
"answer_id": 1
} |
Why can't an excess of electrons or holes by themselves cause current flow? I am a beginner in electrical engineering. Often times (most cases actually), the underlying physics aren't really explained to us and we are just left to assume that it works "because it works." This is never enough for me in classes etc and I... | If you connect a piece of wire to just one terminal of a battery, the whole wire tends to gain the same potential as that of the terminal it is connected to, this requires an instantaneous current and is achieved very quickly.
The need of connecting the wire to both terminal, is that the internal chemical mechanism of ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110549",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 1
} |
Exact diagonalization to resolve ground state degeneracies I am studying a perturbed Toric Code model that is not analytically solvable.
On a torus the ground state degeneracy of the unperturbed model is 4.
Once we turn on the perturbation there is a change in the ground state degeneracy.
I would like to detect this... | What implementations of the two algorithms are you using? In my understanding, the Jacobi-Davidson algorithm is supposed to be good at dealing with degeneracy.
In my personal experience, using the ARPACK package is usually good enough even for highly degenerate eigenvalue problems (~10 fold or even more), as long as a ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110626",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 2,
"answer_id": 0
} |
Why does squeezing a water bottle make the water come out? This seems natural, but I can't wrap my head around it when I think about it.
When I squeeze an open bottle filled with water, the water will spill out. When I squeeze a bottle, the material collapses where I squeeze it, but expands in other areas, resulting in... | The surface area of the bottle is conserved, but the volume is not. Squeezing the bottle deforms it into a shape whose volume to surface area ratio is lower than it was previously.
As an example consider a bottle whose cross-section is initially a circle. The volume of the bottle will be $V_0=\pi r^2h$ where $h$ is... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110747",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 2
} |
Why electrons have less energy than photons with the same wavelength? I am studying quantum physics and I have a question: what is the physical explanation for electrons having less energy than photons with the same wavelength?
Energy of a photon : $E = h ... | A slightly different viewpoint is the following. First, note that the formulae given by the OP do not in themselves exclude the possibility that the electron has the same or greater energy than a photon of the same wavelength. Indeed, for small enough $\lambda$ the non-relativistic formula $E = h^2/(2m \lambda^2)$ pred... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/110844",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 3,
"answer_id": 2
} |
Effect of wavelength on photon detection When some photon detector detects a photon, is it an instantaneous process (because a photon can be thought of as a point particle), or does the detection require a finite amount of time depending on the wavelength of the photon?
EDIT: I guess what I am wondering is if a photon ... | Well I would argue a much simpler and shorter explanation:
That the measurement of a photon will collapse the wave function of the photon and therefore the system will no longer be a quantum mechanical one but classical. Before the measurement there will be a error due to the uncertainty principle, see it like the phot... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/111231",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 7,
"answer_id": 5
} |
How are complex sound waves combined? Audio is often explained by single frequencies. Typically this is a sound wave:
plot sin(x) * 2 from 0 to 10
However we usually deal with more complex sounds, more specific various frequencies and amplitudes. Varying amplitudes within same frequency etc. in the same time frame.
H... | Yes, you have the right idea. You will want to learn about Fourier analysis, which lets you take a complicated-looking waveform like your third figure and analyze it to say "this is two sine waves, frequencies 1 and 5, equal amplitudes, zero relative phase."
I like to think of a piano as an inverse Fourier transform m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/111266",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Why fermions have a first order (Dirac) equation and bosons a second order one? Is there a deep reason for a fermion to have a first order equation in the derivative while the bosons have a second order one? Does this imply deep theoretical differences (like space phase dimesion etc)?
I understand that for a fermion, w... | Steven Weinberg starts with symmetries of relativity, and quantum framework, and arrives in chapter 5 of volume one at Dirac equation. He first gets the quantum field for spin 1/2 particles (without reference to any Lagrangian or wave equation), then he constructs vacuum expectation value of the field and its
adjoint,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/111401",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "46",
"answer_count": 7,
"answer_id": 5
} |
How to find the electric field due to a point charge in 3 dimensions? A point charge with charge $+q$ is situated at $(x,y,z)$. How do I find the electric field at $(p,q,r)$? $E=k\frac{q}{r^2}$, right? So why isn't the electric field, $E=k\frac{q}{(x-p)^2+(y-q)^2+(z-r)^2}$ ???
Question:
A charge $q=1\,\mu C$ is placed ... | The difference between your (correct) answer and the incorrect given answer is a factor of 6.16, which is the length of $(x,y,z)-(p,q,r)$ in meters. Your textbook answer is the value of the potential at $(p,q,r)$, in volts.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/111615",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why doesn't this model plane fly? I have been designing a model plane for Design Technology for the past month or so, and today I laser cut my final design and assembled, it then tested it. Upon testing the plane does not get any lift, whereas the previous testing model which was virtually the same did.
The plane is bu... | Most model planes don't have wings aerodynamic enough to lift it by those means. Honestly, put a rotor strong enough on one side of a plain board and it will fly also.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/111746",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Nonzero ground state energy of the quantum harmonic oscillator Since $\frac{1}{2}\hbar \omega$ is the zero point energy of the ground state of the harmonic oscillator, then there is no way to extract this energy.
Therefore, in what way is this value different from zero? Is not it just about where we decide to choose th... | You are correct in that for any given harmonic oscillator we can define the zero of the energy so that the ground state has zero energy. However, there are two things to point out.
*
*Coming from a classical perspective, it's still a curiosity: The harmonic potential itself, $\frac{1}{2} m \omega^2 x^2$ has $0$ as i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112011",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Twin Paradox: Still a Paradox? Alright, so David Griffiths in his "Introduction to Electrodynamics" states that the Twin Paradox is not a paradox at all since the traveling twin returns to Earth. By returning to Earth, the twin had to reverse direction, thus undergoes acceleration, and therefore cannot claim to be a st... | No absolute relativity as long as a constant reference exists this reference is speed of light (this reference will be responsible for defining the moving body
Earth twin will see that other twin's clock is slow because it is really slow
But space twin will not see that other twin's clock is slow because his time
Per... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112158",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 10,
"answer_id": 6
} |
A question about Feynman diagram and symmetry factor Consider a $\varphi^3$ theory:
$$
Z_1(J) \propto \exp\left[\frac{i}{6} Z_g g\int \mathrm{d}^4 x \left(\frac{1}{i}\frac{\delta}{\delta J}\right)^3\right] Z_0(J),
$$
where
$$
Z_0(J) = \exp\left[\frac{i}{2} \int \mathrm{d}^4 x \mathrm{d}^4 x' J(x)\Delta(x-x')J(x')\rig... | This is exactly the point of the symmetry factor.
Let's call the term in $Z$ that we're considering $T$.
Without considering the symmetric exchanges that produce the symmetry factor, the contribution of each diagram to $T$ is simply its associated term without any numerical factor in front (a factor of 1). This is beca... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112236",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 1,
"answer_id": 0
} |
Gauss’s Law inside the hollow of charged spherical shell Use Gauss’s Law to prove that the electric field anywhere inside the hollow of a charged
spherical shell must be zero.
My attempt:
$$\int \mathbf{E}\cdot \mathbf{dA} = \frac{q_{net}}{e}$$
$$\int E \ dAcos\theta = \frac{q_{net}}{e}$$
$$E \int dA = \frac{q_{net}}{... | You may have forgotten to consider the case where $\vec E \perp\vec A$. Then, also flux is zero. But, it is easy to tell using symmetry that then $\vec E$ would form closed loops which is not permissible. Hence, $E$ has to be zero units.
And yes, your reasoning is correct. You can show this for any (imaginary) shell in... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112315",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Noether's theorem and gauge symmetry I'm confused about Noether's theorem applied to gauge symmetry. Say we have
$$\mathcal L=-\frac14F_{ab}F^{ab}.$$
Then it's invariant under
$A_a\rightarrow A_a+\partial_a\Lambda.$
But can I say that the conserved current here is
$$J^a=\frac{\partial\mathcal L}{\partial(\partial_aA_b)... | The current
$$J^a=\frac{\partial\mathcal L}{\partial(\partial_aA_b)}\delta A_b=-\frac12 F^{ab}\partial_b\Lambda$$
is a conserved current. It is indeed a direct consequence of the Noether theorem. However, this current does not represent any physical observables since it is not gauge invariant.
This is explained in gr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112367",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "39",
"answer_count": 3,
"answer_id": 1
} |
What is the symbol Å? I saw this symbol like:
$$\lambda=3000\overset{\circ}{\text{A}}$$
and I don't know what this means. Is it a frequency? (since $\lambda$ is usually used for frequency)
| The angstrom was intended to be a tenth-metre, but because the definition being used was more exact than the metre at the time, and based on a mis-estimation of the Stockholm prototype, a different name is used. In a similar vein, the X-Unit is defined as a thirteenth-metre, but also is out by some factor, so is not s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112443",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Will glass always break in the same way? This question has had me thinking for a while. If I have two large panes of glass and a rock or similar item is thrown in exactly the same place on the glass, would the two panes break in the same way. Does the shattering of glass follow any rules or is it always random and subj... | I believe the breaking itself will be fairly deterministic. Since I would expect that quantum mechanical uncertainty will play only a very small roll at those scales.
However I do suspect that the results will be different. But this would be due to the structure of the glass. By this I mean the imperfections within it.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112543",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "11",
"answer_count": 2,
"answer_id": 1
} |
Which clock is the fastest inside an accelerating body? The picture shows an accelerating spaceship with two clocks inside it. It is so far away from all other bodys that gravity is of no importance.
Will the bottommost clock be slower than the topmost one? Or will both clocks have the same speed?
| The bottom clock will run slower than the top clock.
The simple way to explain this is to appeal to the equivalence principle. This tells us that locally an acceleration is equivalent to a gravitational field. So if the rocket is accelerating at some acceleration $a$ this is equivalent to two clocks sitting stationary ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112645",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 4,
"answer_id": 1
} |
How can one (formally) determine the particle content of a free field theory? Here's my question: Suppose I'm given a free field theory, where my fields are functions $\phi:\mathbb{R}^4 \rightarrow V$, and the equations of motion are a system of linear Lorentz-invariant differential equations (Given an action of Lorent... | Yes, the solutions to your linear equations of motion will furnish linear representations of the Poincare group. Particles will correspond to the irreducible representations present. The unitary representations of the Poincare group were classified by Wigner using the method of induced representations [see Weinberg Ch.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112692",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 1,
"answer_id": 0
} |
If I'm floating in space and I turn on a flashlight, will I accelerate? Photons have no mass but they can push things, as evidenced by laser propulsion.
Can photons push the source which is emitting them? If yes, will a more intense flashlight accelerate me more? Does the wavelength of the light matter? Is this practi... | See Solar Sails http://en.wikipedia.org/wiki/Solar_sail.
As other people have pointed out, this is extremely inefficient energy-wise, but has the advantage of being purely passive - no need to carry an energy source, and few or no moving parts to fail.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/112866",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "60",
"answer_count": 4,
"answer_id": 3
} |
Which side of wave-particle duality to choose in a given situation How does one know whether, in treating a certain problem, one should consider particles as waves or as point-like objects? Are there certain guidelines regarding this?
| As a general rule the wave model is most useful when you're looking at the propagation of light and the particle model is most useful when you're looking at the light wave exchanging energy with something else.
If you take the good old Young's slits experiment as an example, the wave model well describes how the light ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113021",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 0
} |
Feynman Diagram in $\phi^3$ theory I'm slightly befuddled by is what it means when I'm asked to
Draw the Feynman diagram in momentum space for the two point function of $\frac{\lambda}{3!}\phi^3$ theory for order $O(\lambda^2).$
I can draw Feynman diagrams, and I thought two-point function meant
$$\langle0\|\phi(x)... | Order $O(\lambda^2)$ means that your diagram includes two such $\lambda\phi^3/3!$ vertices. Since overall you would have 6 legs of which 2 are the external (you are calculating a two point function with just two external legs) you have to contract four of them. This gives you a loop diagram (Well, there is more than on... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113148",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Does the Standard Model require neutrinos to be massless? I am an undergraduate student in Physics, I have a basic understanding of Particle Physics and Quantum Mechanics but none whatsoever of Quantum Field Theory.
I know that Neutrino mixing requires neutrinos to be massive (but why? Physically, couldn't neutrinos mi... | As could be quickly found from a wikipedia search, the 'classical' Standard Model of particle physics indeed predicts massless neutrino's. Therefore, the experimental evidence of neutrino oscillations is a strong indication that the Standard Model is missing some important physics.
This is not a huge problem (it's an ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113242",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 3,
"answer_id": 2
} |
Query on an operator acting on a function I have a naive question about an operator acting on a well-behaved function. Let us say, we are talking about space translation operator acting
on a function $\psi(x)$:
$$\hat{T(a)}\psi(x)=\psi(\hat{T}(x))=\psi(x+a)$$
For a rotation operator $\hat{R}$, the equation becomes:
$... | I believe that the reason is to make the operator $\hat M$ unitary. The $\frac{a}{r}$ comes from the Jacobian of thee transformation $$\mathbf r \mapsto \frac{a^2}{r^2} \mathbf r.$$ This transformation is $$(r, \varphi, \theta)\mapsto (\frac{a^2}{r}, \varphi,\theta)$$
so clearly $r^2 \, dr = -a^2dr'$. This minus sign w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113369",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Is it possible to create nothing? Is it possible to create nothing?
Lets say you take a cube serving only as a a shell. Then expanded the cube.
Or a balloon, the size of really really small. Then expanded the balloon using outside forces to pull on the exterior. Could it be possible to create a shell that harnesses... ... | I have been thinking along the same lines.
Seems to me it could be the answer to a lot of questions.
If one could make a space of nothing (no gravity, no gas, no light etc )
One could then look at one partical or atom at a time to examine the properties without external force impacting on the results.
How one could mak... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113502",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
An error in Gravitation by Misner Thorne and Wheeler? I was studying on Gravitation the PPN formalism.
Since in equation (39.41) pag. 1087, the term
$1 + \dfrac{v^2}{2}+(2+\gamma)U = 1 + \dfrac{v^2}{2}+3U$ (the second in GR)
looked odd, I tried (several times) to derive the formula, but I always find:
$1 + \dfrac{v^2}{... | Equation 39.41 is quite different in my edition:
$$ A^0_0 = 1 + \frac{v^2}{2} + U + O(\epsilon^4) $$
This is from the hardcover edition published 12/31/1973 (ISBN 0-7167-0334-3). Prior to this the paperback version was published 9/15/1973 (ISBN 0-7167-0334-1).
If your edition has an ISBN number ending in 1 or 2, then t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113593",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Infinities in Newtons law of gravity (for point particles) Newtons law of gravity for two particles of mass $m_1$ and $m_2$ is:
$G\frac{m_1.m_2}{r^2}$.
Supposing that the particles are point particles then gravitional attraction will bring them closer together, and in fact infinitesimally closer together. Now in Ne... | Classical mechanics has only theoretical point masses. In the simplest case of taking the particle's dimensions to a point, the following argument would hold.
The mass of a particle would be given by its mass density times its volume.
Take the gravitational field:
$$
{\bf g}({\bf r}) = -G\frac{m_1}{|{\bf r}|^2}{\bf \ha... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113660",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 1
} |
Apparent dimensional mismatch after taking derivative Suppose I have a variable $x$ and a constant $a$, each having the dimension of length. That is $[x]=[a]=[L]$ where square brackets denote the dimension of the physical quantity contained within them.
Now, we wish to take the derivative of $u = log (\frac{x^2}{a^2})... | I think the second half of your derivative is wrong:
$ \frac{d}{dx} \log\left( \frac{a^2}{x^2}\right) = \frac{x^2}{a^2} \cdot \frac{d}{dx} \left(a^2 x^{-2}\right) = \frac {x^2} {a^2} \left(-3a^2\right) x^{-3} = \frac{-3 a^4}{x} $
which has the correct dimension.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/113715",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Is evaporative cooling more efficient with dry or moist air? I live in India, and in the summer season, the temperature can reach up to $45 \sideset{^\circ}{}{\mathrm{C}} .$ We use Split 1.5 Ton AC in our small office. The idea is to put an evaporative cooler on the inlet side of the heat exchanger of AC to give it mor... | This is what i have seen work well in my home town. If your climate us dry evaporator cooler will help. 1st make syre your condensing unit outside gas some shade cover then blow the evaporated air onto to the condensing unit outside. If you blow tge evaporated air inside it will raise the humidity inside your office,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/114156",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 2
} |
Force between two current carrying parallel wires, multiple proof? Having two current carrying (currents $I'$ and $I$) wires of length $a$ parallel to the $z$-axis, one with end points $(0,0,0)$ and $(0,0,a)$ and one from $(a,0,0)$ to $(a,0,a)$, I'm looking for the force on the second one due to the first one. Here is ... | Sorry for bad-diagram.
This is how I proceed.
Between two parallel current-carrying wires, the magnetic-force is given by
(mu*i1*i2*l)/(2*pi*r), where, mu=magnetic-permeability of medium, i1,i2: currents, l=length of wire on which force is to be found, r=perpendicular separation between the wires.
so, considering dia... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/114226",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Which Force would help find the mass? Two masses, A and B, are connected to a rope. A constant upward force 86.0N is applied to box A. Starting from rest, box B descends 12.1m in 4.70s . The tension in the rope connecting the two boxes is 32.0N.
What is the mass of B?
What is the mass of A?
My work and what I am stru... | The two possible equations you bring up only differ by the magnitude of the upper force. So you're asking: should you use $32\ \text{N}$ or $86\ \text{N}$ as the upward force on box B?
To help you arrive at your own answer: Ask yourself which object is actually exerting the force on box B. Is it whatever is exerting th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/114354",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Is darkness really light? According to this wikipedia article
"Consequently, most objects that absorb visible light reemit it as heat. So, although an object may appear dark, it is likely bright at a frequency that humans cannot perceive."
Can someone explain this to me in lay man's terms? Does this mean that darkne... | Mike Dunlavey's answer is accurate. But, I will try to add little more info on the topic.
Your cat or dog can see things that are invisible to you because of UV detecting ability, a python can see things that are invisible to you because of infrared detecting ability.
The cat, python, dog, most probably you will be thi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/114503",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "15",
"answer_count": 9,
"answer_id": 1
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.