Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Could one theoretically use the expansion of the universe to travel through it? At least in one direction? Could one theoretically use the expansion of the universe to travel through it? At least In one direction?
That’s it that’s my question.
I’m not a physicist but I do get ideas.
I also wonder if one could theoret... | *
*Yes, you are traveling the universe right now due to it's expansion, at a relative speed of 67.4 km/s per megaparsec, which is quite fast. Unfortunately there is no easy way you can change direction or speed, so you can just enjoy the ride.
*You can utilize magnetic fields to change your speed, but magnetic field... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/479302",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
Why are solar panels kept tilted? I have noticed that, in my country India, most of the solar panels are tilted southward at an angle of ${45}^{\circ} .$ Even on buildings with inverted V-shaped roofs, solar panels are still oriented southward on both the sides of roof.
Research
Many sites suggests that the tilt aids ... | Its because the sun is never quite in the same part of the sky during the year. Different tilts will generate more energy at different times of year - eg a report for the island of Sark gave 2 tilt options:
So 60 degree tilt gives more in winter, but less in summer, with a small loss of total efficiency. No doubt Indi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/479515",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "38",
"answer_count": 4,
"answer_id": 2
} |
How did Maxwell figure out the speed of light? The Wiki article is about 2 graduate years of physics beyond my understanding. What is a good high-school rendition of his thought process: regarding his use of the "distributed capacitance and inductance of the vacuum" to reach his conclusion?"
| Maxwell derived that the speed of propagation of electromagnetic waves was
$$c = \sqrt \frac{1}{\epsilon_0 \mu_0},$$
where epsilon and mu were known from experiment.
His result for $c$ was close to the experimental value of the speed of light in his days, so he could conclude that also light was an electromagnetic wav... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/479628",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Momentum anti-Hermitian in RQM? So in any book that discusses the Dirac equation in Relativistic Quantum Mechanics (For example you can check Bjorken and Drell, Greiner, or Ashok Das), whenever they take the dagger of the Dirac eq., they replace $i \frac{\partial}{\partial x}$ by $-i \frac{\partial}{\partial x}$, which... | $\hat{p}^\dagger = -\hat{p}$ is not correct. Momentum operator churns out the momentum of a particle/system which is a real observable. Hence, it has to be Hermitian, since the expectation value of any anti-Hermitian operator must be purely imaginary and hence can't be observed.
How do we see momentum is Hermitian:
*... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/479726",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Factorising a 4D Dirac delta function in a rest frame I'm working through a QFT problem and at one stage in the solutions we have this step:
$$\delta^{(4)}(p - q_1 - q_2) = \delta(E_1 +E_2 - M)\delta^{(3)}(\bf{q_1} - \bf{q_2}).$$
We are working in the rest frame of a meson with mass $M$ and the process is a decay to a... | We can quickly show this using a limit definition of the dirac delta, that is
\begin{eqnarray*}
\delta(x_1,\dots,x_n) &=& \lim_{\epsilon \to 0^+}\frac{1}{\epsilon^n}e^{-\pi (x_1^2+\dots + x_n^2) / \epsilon^2} \\
&=& \lim_{\epsilon \to 0^+}\frac{1}{\epsilon^n}e^{-\pi x_1^2 / \epsilon^2}\times\dots \times e^{-\pi x_n^2 ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/479857",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
What will happen to atmospheres of very large planets? Our earth can hold our atmosphere whereas Mars cannot. So the atmosphere retention mass must be between the masses of Mars and Earth, but if mass is to be considered then can an iron ball having the same mass of the earth hold an atmosphere of its own?
Also since l... | It must be enough to curve space-time around it significantly.But the more mass an object has doesnt mean the more atmosphere it will have.Neutron stars , which have 3 or 4 times the mass of the size have an atmosphere of 10 centimeters maximum . See :Neutron Stars - Kurzgesagt
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/479985",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Why do Candelas and Howard say that $\sum_{n=1}^\infty \cos\left( n \kappa \epsilon \right) \ = \ - \frac{1}{2}$? In the paper Vacuum $\langle \phi^2 \rangle$ in Schwarzschild Spacetime by Candalas and Howard, they say that for each non-zero $\epsilon$ it is true that
$$
\sum_{n=1}^\infty \cos\left( n \kappa \epsilon \... | Note that
$$III(x)~=~ \delta(x-\mathbb{Z})~=~ \sum_{m\in\mathbb{Z}} \delta(x-m)~=~\sum_{n\in\mathbb{Z}}e^{2\pi i xn}~=~1+2\sum_{n\in\mathbb{N}}\cos(2\pi xn)$$
is the Dirac comb/Shah distribution.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/480129",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why can't a low solidity wind turbine be used in high torque applications through gearing? From my understanding, low solidity wind turbines (such as the three blade type) are more efficient due to a higher tip speed ratio, giving a higher coefficient of performance. However, they are not well suited to high torque ap... | Wind turbines are application-designed, and adding gears (to address different applications) costs money. In addition, a many-bladed turbine is efficient enough as-is to run a water pump given that the required work comes for free. You just scale up the blade disc diameter a bit if you want more work output and live wi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/480315",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Pressure in irrigation water pipe I would like to know the pressure at the bottom of the pipe which I use for irrigation purpose. The land is step cultivated and the pipe goes slanting for the length of 140 meters and the top height would be 45 meters, water is pumped out from borewell using 7.5 hp submersible pump. I ... | It will be the pressure that you find at 45 meters depth in water, that is 543 kPa or equivalently 5.28 atm (my reference).
I think that when the flow of water in the pipe changes rapidly this value will change.
The result is a consequence of the fact that the gradient of pressure into a static liquid is proportional t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/480399",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Which side is convex in my plano convex lens? So, I am building an optical system and have got a Thorlabs plano-convex lens (part # LA1172-C) with a 400mm focal length. This makes the convex side of the lens so flat that it is difficult for me to discern (by naked eye) which side is convex on the lens. I need to know w... | In general in thorlabs lens there is a little > pointing the curved surface or written with a pencil or printed in the side of the lens.
If not, in general people place the lens under a long rectilign neon lamp and look at the reflexion on both surfaces. Is the neon reflexion seem curved on one of the surface? This is ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/480636",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Questions about BRST symmetry For a course about the standard model, I am writing a paper on BRST symmetry. For this I am mainly following the material developed in chapter 16.4 of Peskin and Schroeder. I am mostly done, however there are still two questions remaining that I cannot seem to answer.
*
*On page 518, t... | *
*Nilpotency follows from the fact that $c^cc^dc^e$ is cyclic in the indices $c$, $d$ & $e$, so that we may replace the structure constants $ff$' in the formula for $Q^2$ with the full Jacobi identity divided by 3.
*The Hermiticity of the BRST charge $Q$ is a first principle of BRST theory. In Yang-Mills theory, it ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/480835",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Liouville's integrability theorem: action-angle variables For classical dynamical systems, let $I_{\alpha}$ stand for independent constants of motion which commute with each other. 'Remark 11.12' on pg 443 of Fasano-Marmi's 'Analytical Mechanics' suggest that $I_{\alpha}$s can be taken as canonical coordinates.
For a c... | In the Hamiton-Jacobi (HJ) approach, the Hamiltonian does not stay the same. It changes via (Eq. 9.17-c of Goldstein)
$$
K = H + \frac{\partial F_2}{\partial t},
$$
where $K$ is the transformed Hamiltonian. In HJ approach, we tune $F_2$ in such a way that $K=0$ (Eq. 10.2 of Goldstein). The above question assumes that $... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/480959",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
} |
Binding energy and strong force If binding energy is responsible for holding nucleons together than what is meaning of strong force?
| Whenever there is a force, of any kind, there can be energy associated with what the force does. For example, if the force is attractive then it can hold two things together. In this case to pull the things apart you would have to apply a counter-force, and as the things moved apart, pulled by such a counter-force, ene... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/481061",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Does light take the path of least time because it travels in straight lines or vice versa? My question is which of these two feats is a consequence of the other?
Light travels in straight lines, mostly. Does it do that as a result of Fermat's principle of least time? and if so, is there a reason as to why it follows th... | Feynman used to say that a good physicist knows many different ways to arrive at the same result. If you assume Fermat's Principle, you will get that light travels at straight line. If you assume light travels in straight lines and snell's law, you will get Fermat's Principle. This equivalence makes it hard to decide w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/481156",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
What is the wavelength at the classical turning points using WKB Approximation? According to what I know is that a classical turning point in Newtonian Mechanics is a point where a particle has a zero kinetic energy (Total energy is equal to potential energy) and must be instantaneously at rest. This means it stop its ... | *
*The WKB approximation is valid in regions where
$$\left| \frac{d\lambda(x)}{dx} \right|~\ll~2\pi, \tag{46.6}$$
i.e. the WKB approximation breaks down near a turning point $x$, cf. Ref. 1. Generically, the velocity/momentum behaves as $\sim |x-x_0|^{1/2}$, so that the de Broglie wavelength has a singularity $\sim |x... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/481274",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why does thermodynamics use the negative of the Legendre transform? So I see how the negative Legendre transform is very helpful in interchanging dependencies and giving us the four different major thermodynamic potentials, from internal energy to Helmholtz, Gibbs, and enthalpy.
But what I'm unclear on is why we use th... | The choice of the sign of the Legendre transform is purely conventional. There is no obliged way of defining it. The mathematical definition, used in physics in the case of the passage from Lagrangian to Hamiltonian function, has some advantages with respect to the opposite sign convention used in thermodynamics. Proba... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/481932",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 0
} |
Thermodynamic description of few-body systems How large should a system be to become thermal?
Thermodynamic description is well-established for systems with large numbers (say, of order of $N_A\sim 10^{23}$) of constituents. Is there a "lower bound" of sorts, for the number of degrees of freedom $N$ in a system, for wh... | Check this link out:
Statistical mechanics of Henon-Heiles oscillator
It is shown that although the system has only 2 degrees of freedom, due to the non-linearity the system exhibits ergodic character, which is endemic of systems with large DoF.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/482048",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Origin of the energy distribution of synchrotron radiation Is there an easy explanation why synchrotron radiation from a bending magnet (e.g. in an electron storage ring) has an energy distribution? In other words, given a specific magnet with a defined magnetic field and electrons with a specific kinetic energy, why i... | It’s a relativistic effect! For non-relativistic cyclotron radiation, the spectrum is mostly monochromatic at the gyro-frequency and the angular distribution is basically over all angles. But for highly relativistic synchrotron radiation, the radiation from the electrons is strongly “beamed”, like a headlight, in a ver... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/482157",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Angular velocity of rotating rod Consider the following system:
Newton's second law for rotational motion:
\begin{equation}\tau=I\alpha \Leftrightarrow rF=\frac{1}{3}mr^{2}\alpha \Leftrightarrow \frac{d\omega}{dt}=\frac{3F}{mr}\end{equation}
Considering RHS constant, we get $\omega=\frac{3F}{mr}t.$
I'm not sure if the... |
I'm not sure if the angular velocity whould be inverse proportional to the radius (from natural experience I know that pushing farther requiers lower force).
You are forgetting that you are also using $r$ as the length of the rod. So if you increase $r$ you are increasing $\tau$ that scales linearly with $r$, but the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/482302",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
If an object is travelling near light speed, would it's actions seem to be in slow motion? Hypothetically if we were observing a clock travelling near light speed relative to us, we would see the clock ticking at a much slower speed than us. If that is true, then would all actions that are at rest relative to the clock... | Due to time dilation , any system that would be moving with speeds near to the speed of light, would be slower. All actions would take longer time.
But if you were inside that system, ie also moving with speeds comparable to that of light, you will observe everything normal in your spaceship but the people on earth wil... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/482427",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Can a wire having a $610$-$670$ THz (frequency of blue light) AC frequency supply, generate blue light? We know that when we give alternating current across a wire then it will generate an electromagnetic wave which propagates outward.
But if we have a supply which can generate 610 to 670 terahertz of alternating curre... | Yeah definitely . You can create light corresponding on any frequency by this method. It’s just that creating this circuit will be very challenging. To give you a perspective, the highest frequency that we have obtained with modern electronic circuits is around 10^11 Hz.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/482541",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "33",
"answer_count": 6,
"answer_id": 5
} |
Direction of emission of Photoelectrons Where does the information about the direction of the emission of the Photoelectron come from? Does it get it from the incoming Photon?
I have seen a picture on wikipedia-page of the photoelectric effect, where it almost looked like if the angle of the emission of the Photoelectr... | The direction of emission of photoelectrons during the photoelectric effect is random. It is as per QM, all about probabilities.
When you are comparing it to a mirror image, that is not correct. A mirror image is caused by elastic scattering.
When a photon interacts with an atom, three things can happen:
*
*elastic ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/482798",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Why electric potential requires absence of acceleration? As of 28/05/2019, Electric Potential is defined in Wikipedia in the article with the same name as:
The amount of work needed to move a unit of positive charge from a
reference point to a specific point inside the field without producing
an acceleration
What... | The total energy of a body is the sum of kinetic energy and potential energy.
Kinetic energy depends on speed and potential energy depends on position.
The wiki text is a special case of the fact that if we move a body from A to B without altering the kinetic energy then its energy difference is only potential.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/482994",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 7,
"answer_id": 0
} |
Unorthodox way of solving Einstein field equations Usually when we solve field equations, we start with a stress energy tensor and then solve for the Einstein tensor and then eventually the metric. What if we specify a desired geometry first? That is, write down a metric and then solve for the resulting stress energy t... | In most situations the distinction “matter first, geometry second” or “geometry first, matter second” is not that clear cut. Often assumptions are made that constrain both the geometry and stress energy tensor.
Take for example the Schwarzschild metric. We derive it by writing down the most general metric compatible wi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/483259",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 4,
"answer_id": 3
} |
Is my understanding of vectors correct? I recently learned that a vector in mathematics (an element of vector space) is not necessarily a vector in physics. In physics, we also need that the components of the vector on a coordinate transformation as the components of the displacement vector change. So, if my understand... | Yes, your statement is correct, however... I haven't seen your use of "component" in a long time. Your use is the strictly correct meaning of component. That is, components are vectors. But the term is often used to mean the "coordinates" of a vector. That is in $\vec{v}=v_{x}\hat{i}+v_{y}\hat{j}+v_{z}\hat{k}$ the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/483477",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 1
} |
Which one of them is the time-reversed wave-function, $\psi^{\ast }\left( x,t\right) $ or $\psi^{\ast}\left( x,-t\right) $? If the wave function $\psi\left( x,t\right) $ is a solution of the spinless
time-independent Schr$\ddot{\mathrm{o}}$dinger equation,
$$
i\hbar\frac{\partial}{\partial t}\psi\left( x,t\right) ... | As per your reference, it seems that you have mistaken anti-unitary operators for the time reversal operator. The time reversal operator is a kind of anti-unitary operator. The general expression for an anti-unitary operator is, as you had mentioned, on page 269 equation 4.4.14 of J.J Sakurai's book:
$$
\theta = U K
$$... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/483605",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 0
} |
Why does the Schrödinger equation work so well for the hydrogen atom despite the relativistic boundary at the nucleus? I have been taught that the boundary conditions are just as important as the differential equation itself when solving real, physical problems.
When the Schrödinger equation is applied to the idealized... | In solving the Schroedinger radial equation there is no boundary condition applied at $r=0$. At $r=\infty$ yes, $R(r)$ must tend to zero - so we reject the positive exponential solution; any change in that would have massive consequences. But there is no constraint laid on $R(r)$ or indeed $R'(r)$ as $r \to 0$.
So t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/483702",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "34",
"answer_count": 4,
"answer_id": 2
} |
Can Hydraulic System work on a Moon Robot? Since the Moon has no atmosphere and the temperatures reach maximum 123 C and min minus 153 C, how feasible is it to use hydraulic actuators to move the Robot legs?
Since my assistant Professor insists on going forward with the idea of building a smaller scaled model using the... | yes hydraulics could be made to work, the lack of air pressure is no big problem since hydraulic pressures will be fairly high, a few missing psi externally wouldn't really matter. The biggest problem would be having a hydraulic fluid with a multi-viscosity to handle the extreme temperature variations. Also any hoses o... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/483866",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Can I contract index in this expression? I'm reading Carrol text on general relativity, on page 96 they arrive to the term
\begin{equation}
\frac{\partial x^{\mu}}{\partial x^{\mu '}}\frac{\partial x^{\lambda}}{\partial x^{\lambda '}}\frac{\partial^2 x^{\nu '}}{\partial x^{\mu}\partial x^{\lambda}}.\tag{1}
\end{equatio... | No, because $$\color{red}{\sum_{\mu,\lambda}}\frac{\partial x^{\color{red}{\mu}}}{\partial x^{\mu '}}\frac{\partial x^{\color{red}{\lambda}}}{\partial x^{\lambda '}}\frac{\partial^2 x^{\nu '}}{\partial x^{\color{red}{\mu}}\partial x^{\color{red}{\lambda}}}$$already has a double summation implied in $\mu$ and $\lambda$,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/483984",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
What is so special about Paschen, Balmer and Lyman series? I'm reading McIntyre's QM book and I see the same diagram ever so often:
He goes on to say:
I don't understand what he is trying to say with the last sentence. It seems he is saying, that transitions to higher-levels higher than n = 5 requires absorption of p... | 1 He is saying that, just as in the three series described above, the absorption of photons holds for any n.
2-3 The significance of these series is mainly historical, because they are of course related to the first energy levels. There is nothing particularly special about any of these series other than the experiment... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/484290",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Monte Carlo Closure Test in Particle Physics What is the principle behind a MC Closure Test employed while analysing data in particle physics?
So, what I understand here is that this test checks if one's code is working properly or not depending on whether the (data-mimicking) MC, after being analysed, matches with th... | It is not a widely discussed subject , and the concept was certainly not used in my time ( twenty years ago) . I found this definition:
When using pseudo-data, generated with the help of Monte Carlo simulations, the truth distribution $x^{truth}$ is known, so the unfolding result $ˆx$ may be directly compared to it.... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/484396",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why does steam in Chernobyl reactors speed up nuclear reaction? After watching the TV series Chernobyl I am struggling to understand why steam in a reactor core would increase the rate of the nuclear reaction. My first guess would be that liquid water would accelerate the reaction as it is used as a moderator in other ... | The reactor type installed in Chernobyl (Wikipedia: RBMK) uses graphite as moderator. The water also moderates, but its effect is proportionally much less important than in a water-moderated reactor. This means a steam bubble forming results in only a small reduction in moderator efficiency but a large reduction in neu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/484492",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 0
} |
Tension in a string moving in elevator A lift can move in $x$ axis and also in $y$ axis. A bob of mass m is suspended with inextensible thread inside the ceiling of elevator. Determine tension $T$ in the string when elevator moves in $x$ direction.
Analysing the motion w.r.t. lift (considering that lift moves with acce... | Your free body diagram holds only for $t=0$.
Observe that at $t=0$, due to the force $ma$, there exists a torque on the bob, $\tau=ma\ell$ where $\ell$ is the length of the string. This causes the bob to rotate. If $a$ is directed to the left for the elevator, the inertial pseudo-force on the bob will be directed to t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/484729",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How to prove a 4D vector is a 4-Vector? This is a fairly open ended question.
Given a set of 4 Components, that is, a 4D Vector, what is the process for determining rather or not it is a "4-Vector" as defined in special relativity? I want to know the general method for answering this question. It is still unclear to me... | A set of 4 components, that is a 4-dimensional vector, is a 4-vector if it is transformed as the position 4-vector between reference frames.
Related : Transformation of 4−velocity.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/484876",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 0
} |
If the pressure inside and outside a balloon balance, then why does air leave when it pops? Sorry for the primitive question but when we inflate a rubber balloon and tie the end, its volume increases until its inner pressure equals atmospheric pressure.
But after that equality is obtained why does the air goes out when... | The pressure inside is greater until it balances with external pressure. The rubber is the only thing maintaining the balance. If it were any greater inside, it would continue to expand.
It takes very little pressure to lift large weight when applied to large surface areas. Since this minor pressure it applied to the e... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/485008",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "26",
"answer_count": 6,
"answer_id": 5
} |
Would setting the ideal gas constant to $1$ yield an attractive natural temperature scale? In this recent question, there was a comment 'The "zero point" of Kelvin is natural, but the scale is not'. This led me to wonder whether setting $R = 1$ in the ideal gas law would be an attractive and more natural temperature s... |
Would setting the ideal gas constant to 1 yield an attractive natural temperature scale?
Not really. The universal gas constant involves two arbitrary units: energy and temperature, and also the unitless mole. Getting rid of the concept of moles results in Boltzmann's constant $k_b$. This is the value that is set to ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/485182",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
If a black colored body absorbs all colors, why does the spectrum of light appear on a black shoe? Few days back in a 10 grade school practical, we were shown the dispersion of light by a prism to create spectrum.
Then we went into the open sun and performed it under a linear building roof and same results were obtain... | Black object "absorb the light falling on them". Sure. But how much of the light is actually absorbed?
Your visual system works mostly on difference between stimulii, so "black" is usually incontrast to things that are less black. This is the origin of the Chubb illusion and the Checker shadow illusion.
Even very black... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/485325",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Tearing paper by creasing/folding Why does folding/creasing loosen the fibre-fibre bonding in paper?
Creasing makes tearing paper easier because it weakens the fibre-fibre bonds or makes the strong fibres easier to tear, it is said. But why would the creasing make the bond easier to break?
Some sources say that a certa... | Paper consists of cellulose fibers held together with a small amount of binder (a glue-like substance). Creasing a piece of paper back and forth applies nonuniform shear and tensile forces to the fibers which tends to break the mechanical bonds between adjacent cellulose fibers. This then allows them to tear free of on... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/485431",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How do phone loudspeakers work? From what i understand, loudspeakers require AC signals to cause an electromagnet to oscillate due to changes in current direction, thus force direction. How can this happen with a phone's battery/cell? shouldn't the battery only be capable of producing direct current?
| Starting out from a speaker assembly, we need to generate a voltage that varies at the same rate as we want our membrane to vibrate and thus create sound. This problem also exists in AC powered devices, as the AC comes in at strictly a single frequency, which could generate only a single frequency of sound.
To do this,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/485751",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Stokes's theorem in tensor field On pg 73 of "Tensors, Relativity and Cosmology"
The generalized Stokes's theorem in arbitrary $N$-dimensional space is given by:
$$\int_c A_mdx^m=\frac{1}{2}\int_S F_{mn}dS^{mn} \tag{1}$$
where $F_{mn}$ is the curl tensor of the vector $A_m$, $F_{mn}=A_{n,m}-A_{m,n}$ (, denotes cova... | Here, you still have the curl in antisymmetric tensor form, not a vector form. Once you are in three dimensions and there are no peculiarities with the metric, you have the correspondence:
$$(\operatorname{curl}{\bf A})_i=\frac12\epsilon_{ijk} \left(\frac{\partial A_{j}}{\partial x^k}-\frac{\partial A_{k}}{\partial x^j... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/485876",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Which particle mediates the Aharonov-Bohm effect? BACKGROUND
The Aharonov-Bohm (AB) effect induces phase shifts between the two paths that an electron could take around an enclosed magnetic field. In radial coordinates, assume that the magnetic field is localized around the origin and that the two paths traced by the e... |
At the moment the magnetic field is switched on, which particle
travels outward from the origin towards the electrons
You have already answered yourself: magnetic fields are mediated by photons.
And at what speed?
Photons generally travel at the speed of light :-)
the magnetic field is restricted to the origin ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/486203",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
Swinging Atwood and Hoop And Pulley Lagrangian
The picture is showing the swinging atwood and a hoop and pulley.
I know the lagrangian for both two, I have no problem with the kinetic energy of both but i couldn't convince myself that for the swinging atwood, the potential energy is :
$$V = Mgr - mgr \cos\theta$$
and... | In the first case we have $Mgr$ because as $r$ increases the potential energy increases for the block of mass $M$. In the second case we have $-mgR\theta$ because when $\theta$ increases the potential energy decreases for the block of mass $m$.
In general you pick the sign so that the potential energy function behaves ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/486302",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Shooting someone's past self using special relativity Suppose A and B are a long distance apart initially. B takes off in a spacecraft in A's direction at a really high speed. Both were aged 0 when B took off. When B about to cross paths with A, A observes him to be 30 years old (while A is 60). At this point, 60 year ... | Case I: When B takes off, A and B are both aged 0 in A's frame.
1) When B passes A, B is 30 (given in your setup).
2) But B's clocks run at half-speed according to A, so A says that B has been traveling 60 years.
3) Therefore A is 60.
4) But B says A's clocks run at half speed, so B says A was born 120 years ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/486760",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
How does a Leyden Jar work? When electricity goes inside the Leyden jar, the inside of the jar is negatively charged. Then the aluminum foil on the outer layer of the jar is positively charged. Does the inside of the jar want to cancel out the aluminum foil, but it can't because of the insulator between the two conduct... | All you have in a Leyden jar are two thin tin foils on the interior and exterior of the jar.When you pass a current like you said one plate is going to have -q amount of charge and the other will have +q amount of charge.But they cant cancel out each other since the have a insulator in between that doesn't allow the ch... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/486839",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Is there a maximum distance from a planet that a moon can orbit? Given a planet that orbits a star, and a moon that orbits that planet, is it possible to define a maximum orbital radius of that moon, beyond which the moon would no longer orbit the planet, but the star instead?
I initially (naively) thought this point w... | What you want to look for is the Hill Sphere. The resultant distance is about 1.5 million km.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487011",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "33",
"answer_count": 3,
"answer_id": 0
} |
Is it possible writing conservation of relativistic energy in this naive way? Conservation of charge or rest mass can be written in this way and it is Lorentz invariant
$$
\nabla \cdot (\rho \mathbf{u}) + \frac{\partial \rho}{\partial t} = 0
$$
So we could be tempted to naively write conservation of energy in this way ... | We can't just insert $\gamma$ into the equation of continuity, which in this case is a formulation of conservation of rest mass for a free particle:
$$
\frac{\partial \rho}{\partial t} + \nabla \cdot (\rho \mathbf v) = 0,
$$
and expect the resulting equation will still be valid.
Also, although the above equation has th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487168",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
Why is the drag force proportional to $v^2$ and defined with a factor of $1/2$? $$Drag = \frac{1}{2}C_d \rho Av^2$$
I understand that the strength of the drag depends on the density of the fluid the body passes through, the reference area of the body, the drag coefficient, and the velocity of the object.
I don't, howev... | It's similar question if you would ask - Why kinetic energy is defined as $E_k = 1/2~m v^2$. The answer is below.
Elementary work is :
$$
\begin{align}
dW &= F \cdot dr
\\&= F\frac {dr}{dt} dt
\\&=Fv~dt
\\&=m\frac {dv}{dt}v~dt
\\&=mv~dv
\end{align}
$$
Now integrate both sides :
$$ \int dW = m \int v~dv $$
Which give... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487220",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 4,
"answer_id": 3
} |
Partition function of Hamiltonian without momentum dependence Considering an ideal gas of $N$ diatomic molecules with dipole momenta $\vec{p}$. Given the Hamiltonian of one molecule
$$ H = -\vec{p} \cdot \vec{E} = -pE\cos(\theta)$$
when calculating the partition function we "ignore" the p-dependence by not integrating ... | Yes, we cannot ignore the momentum dependence of the system and must integrate over all of state space. However as @d_b pointed out, this is not the momentum, it is the dipole moment. So, this is not the same $\theta$ associated with the position of the centre of mass of the diatomic molecule but rather the angle betwe... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487511",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What makes north pole of a magnet north pole in the first place? This question might seem absurd and illogical to many. But it just popped out in my mind while I was reading about magnetism.
-Like in case of charges, positive and negative charge on an atom means absence and presence of extra electrons respectively. So... | To answer that you need to know that in a classical mechanic view elementary quantities of magnetic field are generated by small loops of current call magnetic dipoles. Macroscopic magnet are only an assembly of elementary magnetic dipoles in macroscopic dipole. Although the magnetic field generated by permanent magnet... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487584",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 1
} |
Magnetic field of an infinite hollow cylinder (with volume current) Consider an infinite hollow cylinder with inner radius $a$ and outer radius $b$. The volume current density flows anti-clockwise across the surface of the cylinder ($\vec{J} = J\hat{\phi}$). The charge density is $0$ everywhere.
How can I compute th... | Compare the scenario with that of an infinite solenoid.
(Image source, note that $B \neq \mu nI$ in this case.)
Since you know the volume current density, you can calculate the current enclosed by the loop.
Also, the magnetic field outside (e.g. at point P) the infinite coil is zero, due to the following reason.
(ima... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487690",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Problem while constructing langmuir probe I am working on DC glow discharges and want to construct a langmuir probe. Circuit I am using is as shown in the picture
Also, I am applying ~1KV across cathode and anode.
Problem I am facing is that I am getting discharge btw probe and cathode. Which isn't unexpected, but des... | you probably need a current limiting resistor in the part of the circuit containing the probe, either that or a way of isolating the probe ground from the power supply ground.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487791",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
What is the change of entropy for a resistor at constant temperature? A 10 Ω resistor is held at a temperature of 300 K. A current of 5 A is passed through the resistor for 2 minutes. Ignoring changes in the source of the current, what is the change of entropy in (a) the resistor and (b) the Universe?
My attempt:
... | In this problem, it would be incorrect to say that the change in entropy of the resistor between its initial and final states is anything other than zero. Entropy is a function of state, and the initial state of the resistor (300K) is exactly the same as its final state (300K).
The fallacy in applying the expression... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/487862",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Understanding the rolling constraint for one cylinder rolling inside another cylinder This is the problem to find the equation of motion of 2 cylinders in which 1 cylinder is placed inside another cylinder with larger radius as shown in figure. The condition is that both are rolling. Now, in the figure they say:
The r... | The $r_2 \phi_2$ is simply the length of the red line on the inner cylinder in your picture. That must be equal to the length of the red line on the outer cylinder, since this is the line that the inner cylinder tracked on the inner surface of the outer cylinder. That length is given according to the picture by $r_1(\... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/488095",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Friction due to pure rolling on an inclined plane When a body is executing pure rolling we know that the point of contact of the body with the ground is at rest with respect to the ground. If that's the case no friction should act as it is stationary.So when a body is rolling down an inclined plane its point of contact... | False premise. Static friction only acts when there is no motion between the two objects being considered in the system. For instance, a stationary block on an inclined plane is held there by static friction which equals the component of its weight down the plane, $mg\sin{\theta}$. Under such scenarios, the force of st... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/488195",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Anomalous dimension of double-trace operators Is it true that if a single-trace operator, say, $O$ acquires an anomalous dimension $\gamma_o$, then the anomalous dimension of the double-trace operator $O^2$ is $2\gamma_o$? If no, can anyone please provide counter-examples?
| A correct statement is that in the large-N limit of a theory, if you have an operator $O$ with scaling dimension $\Delta_O$, the theory will also contain an operator $O^2$ with scaling dimension $2\Delta_O$.
But be aware that:
1) This is a statement about scaling dimensions, not anomalous dimensions.
2) If you include ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/488331",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Does friction always oppose motion? Recently I had the following misconceptions:
*
*Static friction always opposes the motion of body.
*The force of friction cannot initiate motion in a body.
Now I came to know that my understanding was wrong and that friction indeed can cause motion in bodies, and that static fri... | The top answer given is correct, but I wanted to slightly extend this:
how can a force which has always been taught to us to oppose motion, oppose points 1) and 2)?
Imagine a table, I slide a very heavy hockey puck over the table. Without friction, the puck keeps sliding. But because there is friction, the puck is sl... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/488562",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 5,
"answer_id": 0
} |
Why is it that when a chalk board gets cleaned, the area that used to have chalk is the cleanest? Why is it that when you erase a chalk board, the area where the chalk used to be becomes the cleanest? By that I mean that when you erase a chalk drawing, the board gets smeared with chalk dust, but the area where the draw... | My guess.
The abrasive character of the chalk itself results in a higher coefficient of kinetic friction between the chalk and the board, than between the soft material eraser and the board, whereas the normal force applied to the board is the same in each case. The greater friction force takes away more material from ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/488780",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 4,
"answer_id": 3
} |
Where the derivative corrections come from in Wilson renormalization? I known that in the Wilson renormalization process fast modes are integrated out in order to define an effective action for the low modes field. Considering phi to the fourth theory it's easy to see how the quadratic and quartic terms are corrected ... | Derivative corrections appear in just the same way as usual. For example, quadratic terms with derivatives can be written as $\phi f(\partial) \phi$ where $f(\partial)$ is some combination of partial derivatives, such as $\partial^2 + m^2$ in free field theory. The corresponding propagator is $1/f(ip)$.
So derivative ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/489127",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
How do I transform flux density into temperature? I have flux density data from Planck mission (in Jy) - measure in a certain frequency - and I would like to transform it to temperature data (Kelvin). I'm not sure if I should use Stefan Boltzmann's Law or not because the units don't match.
| We start from the Planck's law, describing the spectral density of electromagnetic radiation:
\begin{equation}
B_{\nu}(T) = \dfrac{2 h \nu^3}{c^2} \dfrac{1}{\mathrm{exp} \left( \dfrac{h \nu}{k T} \right) -1}
\end{equation}
Where $k$ is the Boltzmann constant, $\nu$ the frequency of the signal and $c$ the speed of light... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/489222",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Will a free neutron radiate if it is decelerated? In this answer it is said (and I fully agree):
Yes, a ... photon can accelerate a lone neutron. The kinetic energy imparted to the neutron reduces the photon's wavelength (redshifts it) by the same amount, so the total energy of the system remains the same.
In turn, t... | We take a charged ball and shake it. There must be electromagnetic radiation from the shake. When we shake an electrically neutral object, it does not emit electromagnetic radiation from the shaking.
This situation cannot be explained by photons. Otherwise, an electrically neutral object will radiate when it is shaken... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/489412",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 2,
"answer_id": 1
} |
Proving that motion of an $n$ dimensional oscillator can be written as a linear combination of "sine waves" Here is a related question which might provide some context: LINK.
Let's consider an oscillator with equation of motion in $n$ dimensions:
$$
\frac{d^2}{dt^2} \vec{x} = K \vec{x}.
$$
Given that $\vec x=0$ is a st... | In the case that $K\vec{x}$ is exact rather than an approximation, note that if the equilibrium is stable, work done of a virtual displacement $\delta \mathbf x$ must be negative. So,
$$
\text{work done}=F.d\propto (K\delta \mathbf x).\delta \mathbf x=\delta \mathbf x^T K\delta \mathbf x<0.
$$
So $K$ is negative defin... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/489728",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
In a circuit having one resistor why do the electrons lose all their potential energy across that resistor and not do so if there are many resistors In a simple circuit which consists of a battery and one resistor, why do electrons lose all their potential energy across this one resistor regardless of the magnitude of ... | It's simply due to energy conservation.
What does a battery? It uses its own internal chemical stored energy to create potential difference across its terminals and when it's terminals connected to the circuit it potential energy goes to electrons and if there is a resistor attached to the circuit.
then firs... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/490109",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 5,
"answer_id": 4
} |
Can there be interference between a proton and an electron? For example, we know that we can interfere two different electrons or two different protons by employing them in a double-slit experiment.
Now suppose, we mix protons and electrons and shoot them simultaneously for a double -slit experiment. Will the protons ... | The root of this confusion is the visualization that is very commonly given to explain wave particle duality. When we say that particles behave like waves, we do not mean that in the sense of a wave in a puddle. In quantum mechanics (and field theory), we say we have an electron wavefunction (in QFT, we say we have an ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/490828",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 1
} |
Newton's third law in magnetic fields Say I have a charged particle moving through a magnetic field perpendicular to it. It will experience a force, but according to Newton third law
Every force has an equal and opposite reaction.
So what is the opposite reaction/force of this magnetic force.
Which body experiences... | The answer by user224659 is correct but I want to focus on the second part and I hope bring some added clarity.
The magnetic force is an interaction between a charged particle and an electromagnetic field. The rate of change of the momentum of the charged particle is
$$
\frac{d{\bf p}}{dt} = q {\bf v} \times {\bf B}
$$... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/490909",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Why is dry soil hydrophobic? Bad gardener paradox When I forget to water my plants, and their soil becomes very dry, during the next watering I can see that the soil becomes hydrophobic. I can even see pockets of air between the repelled blob of water and the soil.
On the contrary, when the soil is moist, it very quick... | Diffusion and adhesion are different phenomena. Diffusion happens due to a concentration gradient, and this is how the water is slowly absorbed by the soil. This happens faster in dry soil per unit area per unit time than damp soil. On the other hand, adhesion is the ability of one material to stick to the other. Damp ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/491058",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "70",
"answer_count": 3,
"answer_id": 0
} |
Is $X\otimes X$ not the simultaneous position operator? I had thought that $X\otimes X$ would be the operator on $H_1\otimes H_2$ to simultaneously measure the x-positions of two particles.
But there seems to be something wrong with this -- for a given eigenvalue $z$, there is an entire subspace $\mathrm{Span}\left(|x... | Let me first point out that this is not a conceptual issue but a "quirk" in the old formalism of quantum theory. Ask yourself this: if real numbers $x$ are the eigenvalues of position operator of a single particle then how the eigenvalues of a position operator of two particles should look like? If your answer is some... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/491383",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 2
} |
The solution to the non-linear convection equation The non-linear convection equation
$$u_{t} +uu_{x}=0$$ admits implicit solutions of the form $$u=f(x-ut).$$
How does one interpret this solution intuitively? Is there an example of a solution of this equation that can be written in an explicit form? How does an initia... | *
*This quasi-linear 1st-order PDE
$$\left(\frac{\partial u}{\partial t}\right)_{\!x} + u\left(\frac{\partial u}{\partial x}\right)_{\!t}~=~0\tag{1}$$
is the inviscid Burgers' equation.
*It can be solved via the method of characteristics. The ODE IVP
$$ \frac{dx}{dt}~=~u, \qquad x(t\!=\!0)~=~\xi, \tag{2}$$
(where $u... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/491696",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Abelian and non-Abelian holonomies I read the article Geometric Manipulation of Trapped Ions for Quantum Computation, and it mentioned “Abelian and non-Abelian geometric operations (holonomies)”. I know what is holonomy, and what is Abelian, but I didn't understand what are Abelian/non-Abelian holonomies.
I tried readi... | A "non-Abelian holonomy" is the holonomy of a principal connection with the group of the principal bundle being non-Abelian. (In more physics-y parlance, it's the Wilson lines of a non-Abelian gauge theory)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/491812",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How does alternating current provide energy? In my head, direct current makes complete sense; the electrons carry energy around the circuit to something being powered losing its potential and then return to the battery or whatnot to have their potential raised again.
This is probably wrong, so I would like an explanati... | Here is an analogy which might help.
Imagine you are cutting a board with a saw. You push the saw through the board, and it cuts. Then you lift the saw up out of the board, pull it back, set it back into the board, and push it through again, always in the same direction. Eventually, the board is cut. Or...
You can pus... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/492136",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Do air conditioner power ratings violate the 2nd law of thermodynamics? I just got a new AC rated at 6000 BTU and wanted to determine its power consumption. Some research on AC conventions quickly reveals that 6000 BTU really means 6000 BTU/h, where BTU is a measure of energy (British thermal unit). This is supposedly ... | Never mind, I'm pretty sure my statement that the power required to operate an air conditioner (or any heat pump) must be greater than its rate of pumping heat is wrong. I.e., the second law doesn't forbid the coefficient of performance (heat pumped / energy consumed) from being greater than one.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/492244",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 3
} |
Why do jet engines sound louder on the ground than inside the aircraft? Everyone is familiar with the whirring sound of jet engines when seeing an aircraft taking off from a nearby airport. It is distinctly very loud on the ground and one can hear it even when the airplane is miles away.
Although one can hear a 'white... | First, the cabin is quieter because the fuselage walls are designed to limit the transmission of sound from the engines.
Second, on most commercial aircraft, the engines are suspended beneath the wings, which block the noise from the engines before it can strike the fuselage.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/492353",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "20",
"answer_count": 4,
"answer_id": 2
} |
Common potential in Capacitors If two isolated charged capacitors (of different capacitance) are connected in parallel to each other they acquire a common potential. But suppose if i connect positive plate of one capacitor to negative plate of another capacitor will they still acquire a common potential or will the cha... | If positive plate is connected to negative still the process remains the same charge will flow untill two capacitor are at same potential.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/492614",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Intensity mapping of the 21 cm line I'm currently reading Astrophysics for physicists by A.R. Choudhuri. Section 6.5 of his book he presents the following plot:
and tries to explain how it was generated. I'm having a hard time understanding how we can differentiate between the different velocities. As far as I underst... | Actually the velocity can be measured by utilizing
the Doppler effect.
The frequency of the 21-centimeter hydrogen radiation
is known with high precision: $f_0 = 1.420405752\, \mathrm{GHz}$.
We also know the speed of light: $c = 2.998 \cdot 10^8 \mathrm{m/s}$.
The Doppler effect is the following. When the radiating sou... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/492714",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What is the difference between uniform velocity and constant velocity? I think that uniform velocity implies constant speed but not constant direction. while constant velocity implies constant speed without any changes in direction.
Both tell us that there's no acceleration (since magnitude of velocity is constant).
Th... | Regardless of what you call it, in order for an object to not be accelerating both the magnitude of its velocity (speed) has to be constant AND its direction (path) needs to be in a straight line (aka rectilinear motion).
In order for the direction of an object to change it must experience a net force and thus an accel... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/492797",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 4,
"answer_id": 1
} |
Potential and potential energy I know when a negative charge moves in the direction of a uniform electric field its potential energy increases and its potential decreases. For example, its potential energy changes from $0.9\ \rm{mJ}$ to $1.2\ \rm{mJ}$, but its potential changes from $-90\ \rm V$ to $-120\ \rm V$. Wher... | Electric potential is just the electric potential energy per charge. In other words, electric potential just depends on the charge distribution around you, where as if you were looking at a charge in the field caused by the charge distribution you could then say it has an associated potential energy in that configurati... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/493068",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
EM-Wave: Calculate magnetic field $H$ from electric field $E$ In an exercise I am supposed to calculate the magnetic field from the electric field for a plane, harmonic wave in vacuum.
$$\vec{E} = - E_0 \cdot \sin(\omega t - k z) \cdot \vec{e_y}$$
Using the law of induction
$$\operatorname{rot} \vec{E} = -\mu \dfrac{\p... | As my2cts mentions in the comments, the magnetic field in the presence of currents is only partially determined by the electric field. However, in vacuum they do determine each other fully.
Remember that the magnetic field is not only governed by Farday's law of induction
$$ \epsilon_0^{-1}\mathrm{rot} D = -\mu_0\parti... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/493294",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Is the only absolute difference between types of light frequency? Probably a bad question but for some reason, it seems too simple in my head that anyone at home could theoretically create anything from radio waves to gamma waves by generating electrical signals at different frequencies.
Say I had a electronic frequen... |
[F]or some reason, it seems too simple in my head that anyone at home could theoretically create anything from radio waves to gamma waves by generating electrical signals at different frequencies.
I don't see how to easily create a device that vibrates at 10¹² Hz, not even 10⁶ Hz. Technologically, this is not as simp... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/493394",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 2
} |
Relativistic beaming/ Aberration effect derivation I'm researching applications of relativistic beaming and I want to derive a formula for the aberration effect but I am stuck (I am off by a factor of 1/c). Here's what I have:
Consider a star in its rest frame, $S^\prime$, moving at speed $v$ that emits a photon at an ... | $$\cos(\theta)=\frac{x}{ct}\rightarrow x=ct\cos(\theta)$$
$$c\beta=v$$
$$x'=\frac{x-vt}{\sqrt{1-\beta^2}}=\frac{ct \cos(\theta)-c\beta t}{\sqrt{1-\beta^2}}=ct(\frac{ \cos(\theta)-\beta }{\sqrt{1-\beta^2}})$$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/493619",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Do photons exchange energy with objects according to their momentum? I've been thinking about the doppler effect, where electromagnetic radiation experiences a redshift if the radiation source is relatively moving away from the observation point, and a blueshift if it's moving closer.
If I have a light source, a sensor... | The collision between an object and a photon can be modelled like any other collision in physics; you take a reference frame (I recommend the rest frame of the object being struck) and give the photon an energy $h\nu$ and a momentum $\frac{h\nu}{c}$. If the photon is absorbed, then the object gains velocity $\frac{h\nu... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/493963",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Deriving Current density of a Moving Point Charge Using the Continuity-Equation Problem:
We know, a point charge at position $\mathbf{r}_q$ has the charge density
$$\rho_q(\mathbf{r})=q\delta(\mathbf{r}-\mathbf{r}_q) \tag{2}$$
if it moves with the velocity $\mathbf{v}$, we get the current-density:
$$\mathbf{j}_q(\mathb... | Note that $\mathbf{r}_q$ is a function of time, and thus so is $\rho_q$. We have
$$ \rho_q = q\delta(\mathbf{r} - \mathbf{r}_q(t)) $$
You can use the identity
$$\frac{\partial}{\partial t}f\left(\mathbf{A}(t)\right) = \frac{\partial\mathbf{A}}{\partial t} · \mathbf{\nabla}f(\mathbf{A}(t)) $$
to find
$$ \dot{\rho} = -q\... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/494217",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Does magnetic moment change under inversion symmetry? Since magnetic moment can be view as a small electric current circle. Pictorially, when apply inversion operation, the current direction is reversed, so I think the $\vec{m}\to -\vec{m}$ under inversion symmetry operation.
On the other hand, the formula for the mag... | $\vec m$ and $\vec B$ are indeed "axial vectors" and invariant under space inversion. Imagine a circular current. Under inversion the current changes direction but also the circle is inverted. This leaves $\vec m$ unchanged. (just noticed Ben Crowell's ear lier answer which is identical to mine.
$\vec E$ also is not a ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/494488",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Motion between two particles in a relative manner Suppose a particle A is travelling in east direction with velocity of x m/s and another particle B is travelling with velocity y m/s in the west direction. Why does the the particle B appears to move towards A with a velocity of x+y and not just y m/s?
|
given two vectors $\vec{v}_{01}$ and $\vec{v}_{02}$
thus:
$$\vec{v}_{12}=\vec{v}_{10}+\vec{v}_{02}\,,\quad\text{("zero cancel")}$$
where
$\vec{v}_{10}=-\vec{v}_{01}$
your example
$\vec{v}_{01}=x$
$\vec{v}_{02}=-y$
$\Rightarrow$
$\vec{v}_{12}=-x-y=-(x+y)\quad \surd$
$\vec{v}_{21}=-\vec{v}_{12}=(x+y)$
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/494630",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 2
} |
Why does an inductor oppose the change in current (magnetic field)? May I get a physical interpretation on this question? What is happening in the inductor when the current is running through it and what is physically happening when the current starts changing?
| Here is one way of looking at this.
We start with an inductor that has a steady current flowing through it from a power source. Because of this, there is a magnetic field extending into space surrounding the inductor.
Now we attempt to cut off the flow of current through the inductor, by switching off the source. At t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/494710",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Resistors are used to reduce current to prevent light bulbs from "exploding" but it's also said that "current remains same at all points in a circuit" Resistors are used to reduce current in order to prevent light bulbs and other electrical components from “exploding”, but it is also said that “current remains the same... | You misunderstood the phrase "current remains constant in a series circuit".
Consider the circuit below with a light-bulb and a resistor in series.
In this circuit the current through the light-bulb is the same
as the current through the resistor.
This is meant by "the current remains constant in a series circuit".
Th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/494828",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 1
} |
Does the law of conservation of momentum mean both linear and angular momentum? I was solving a problem where a bullet hit a rod hinged about one of its ends. The rod is standing vertically before the collision. We had to find the final angular velocity of the rod. The way we did it was to find the angular momentum of ... | The angular momentum conservation law is independent of the linear momentum conservation law. It is two laws, that means that they have been established by experiment, together with energy conservation that are always fulfilled in the mathematical theories we at presently have for physics, both quantum and classical.(... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/494951",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 5,
"answer_id": 2
} |
Spacetime - reasons for "unification" of space and time - why we imply some unity in the term spacetime? Why do we say spacetime, as though stressing that this is some kind of unity: space together with time?
I understand a little that mathematically we have 4 variables in the same (Einstein) equation(s) (3 for space, ... |
Why do we say spacetime, as though stressing that this is some kind of
unity: space together with time?
Special Theory of Relativity (STR) teaches that time is a coordinate rather than a universal parameter.
That is, when we transform (using the Lorentz transformation ) from one inertial coordinate system (ICS) to ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/495058",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Sequential Stern-Gerlach experiment Consider the following diagram:
(Sakurai)
In the first lecture of MIT OCW Quantum Physics 1, 2013 (https://www.youtube.com/watch?v=lZ3bPUKo5zc), Allan Adams implies that if we remove the barrier on the Sx- beam, thus allowing the Sx+ beam and Sx- beam to merge together and pass thro... | Consider an initial state
$$|1 \rangle = \frac{1}{\sqrt{2}} (|S_z + \rangle + |S_z -\rangle). $$
Now it passes through the first SG experiment, and all down spins are filtered out, so you have a collapsed state
$$|2 \rangle = |S_z + \rangle = \frac{1}{\sqrt{2}} (|S_x + \rangle + |S_x -\rangle). $$
If a measurement is m... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/495185",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Causality under relativity If simultaneity is relative; meaning that for each any events A and B and we can find a reference frame in which two events A and B occur in inverse order, how does our notion of 'causality' withholds?
| Spacetime causality holds within the light cone. Only events within the past light cone can affect the present event (the here-and-now at the vertex of the light cone). The present event can affect only the future light cone. Events outside the light cone can neither affect nor be affected by the present event.
Lorentz... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/495364",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
What's wrong with this argument that the potential energy of an arbitrarily heavy object at arbitrary height is $0$? Consider an object with mass such that there is a gravitational force downward of $1N$. Also assume the environment is a perfect vacuum. Now assume that we exert a force of $1+\epsilon $ Newton upward on... | The point is that when a mass is not moving in a gravitational field, the amount of potential energy it has depends on its position.
If you release a mass 1m above the ground and it falls freely, it has less kinetic energy when it hits the ground than if your released it from 2m above the ground.
That difference in ene... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/495510",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
How can we tell if the Earth is spinning without any external references? The rotation of the Earth about its axis makes it bulge at the equator and contract at the poles due to the centrifugal forces. How do we know, without any external references, that the Earth is spinning if there is nothing to compare it to? For ... | Earth rotation can be detected using a gyroscope (http://www.tkt.cs.tut.fi/research/nappo_files/Symposium_Gyro_Technology_2010_web.pdf). It looks like the equipment used in the article costs under $1000 (not including a computer).
EDIT (8/24/2019): Another approach was proposed by Compton (Science 23 May 1913:
Vol. 37... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/495614",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
"answer_count": 6,
"answer_id": 1
} |
Why does $k_\text{B} T \ll \hbar\sqrt{k_\text{Hooke}/m}$ imply that vibrational motion is negligible? I want to estimate the heat capacity of a diatomic molecule whose movement has been constrained to a 2-dimensional plane.
I can assume that
$$
k_\text{B} T ~\ll~ \hbar\sqrt{\frac{k_\text{Hooke}}{m}}
\tag{1}
\,,$$ wher... | To help understand what your assumption is saying, it might be easiest to start by defining what each side measures. What does $k_B T$ measure? What does $\hbar\sqrt{k/m}$ measure? What are the units of them?
And once you figure out the units and what each side represents, what does it mean that one side is much, much ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/495748",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Must an operator that preserves probability be unitary? One property of the unitary operator is that it preserves the norm of the state-vectors:
$$
\langle \Psi | U^\dagger U | \Psi \rangle = \langle \Psi | \Psi \rangle
$$
If $U$ is unitary.
Is the inverse statement also true? For an operator that will satisfy above ... | If $U:H\to H$, with $H$ a Hilbert space, is linear it turns out that it preserves the norm (in particular it is injective) if and only if it preserves the scalar product. The proof is based on the so-called polarization identity as Emilio suggested. However this does not mean that the operator is unitary, since the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/495900",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
What prevents ice from being an electret? My question is apparently simple: if we put water in a electrostatic field and leave it to freeze, while still in the strong electrostatic field, to make ice, why wouldn't this ice exhibit electret capabilities?
| In the structure of ordinary hexagonal ice at low pressure, each molecule is held in a sort of tetrahedral cage by its four nearest neighbors. It acts as donor to two hydrogen bonds, and acceptor to two more. You might get the wrong idea that it can have any of $\left( _{2}^{4} \right)=6$ possible orientations, but t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/496390",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
Are the electrons' orbitals the same for all atoms? Are the electronic orbitals of an atom always quantified in the same way (i.e. the same energy required to reach the next level), or does each atom have its own values for each level?
If the quantification is universal, then the creation of photons (due to the deexci... | The energy levels depend on two things:
*
*the electrostatic attraction between the electrons and the nucleus
*the electrostatic repulsion between the electrons
If you take a hydrogen atom, which is what your diagram shows, then there is a single electron and a single proton. The electron is attracted to the proton... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/496531",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
What's the point of dimensional regularization? I'm studying regularization of divergent integrals in QFT from Here: Roberto Soldati - Field Theory 2. Intermediate Quantum Field Theory (A Next-to-Basic Course for Primary Education)
I think I'm missing the big picture, let me explain.
At page 166 he says
In order to g... | The integral is absolutely convergent for $\epsilon >0$. The point is the following. Frequently in QFT we care about an amplitude $A$ which can be expressed as the sum of multiple integrals
$$A = I_1 + I_2 + \cdots + I_n.$$
Each integral, roughly, corresponds to a different Feynman diagram. The sum $A$ is the only phy... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/496650",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
What happens if a traveling sound wave encounters vacuum? Suppose a sound wave is emitted by an object in a medium like a gas so it travels in some direction. If the wave meets a rigid object, for example a wall, it reflects back as one should expect; and if it encounters another medium, like a denser one, it will be t... | The Sound wave would continue to propagate outward until it cannot continue, even while in a vacuum, and like many of us know, it would not be considered sound unless there are particles to be pushed against it and received by us through our sensation of hearing, and a Vacuum is a place devoid of said particles to carr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/496791",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
Current in RC circuit Why is current drawn in an RC circuit (in a circuit powered by DC voltage supply) independent of the capacitor used?
While the capacitor is charging current drawn from the battery only depends on resistor and on the value of capacitor.
| In a charging RC circuit where the capacitor is initially uncharged, the charges will move as if the capacitor is essentially absent. Therefore, the initial value of the current is just equal to $V/R$.
If the RC circuit starts with a fully charged capacitor and is discharging, then once the current starts the capacitor... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/497338",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Can anyone explain the Planck area? First of all, I am not an expert. I was reading about the holographic principle and came across the Planck area. It says that Planck area is the square of Planck length and there were some pictures like this:
source
I know that this Planck area is used for black holes. But doesn't ... | The Planck length is usually defined as
$$l_P=\sqrt\frac{\hbar G}{c^3}\approx 1.6\times 10^{-35}\,\text{m},$$
where $\hbar$ is the reduced Planck constant, $G$ is Newton’s gravitational constant, and $c$ is the speed of light.
The Planck area is usually defined as the square of the Planck length,
$$A_P=l_P^2=\frac{\hba... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/497475",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 0
} |
Total force on upper block in two block system If a block $m$ is placed on another block $M$ and a force $F$ is applied on bolck $M$. Then how many forces are acting on block $m$.(Friction is non zero)
The image is taken from this site.
Is pseudo force acting on block $m$ or not?
| Three forces act on block $m$. (1) the force of gravity acting downward (2) the normal reaction force that block $M$ exerts on $m$ acting upward and equal to the downward force of gravity and (3) the friction force, if block $M$ accelerates, that block $M$ exerts on block $m$ horizontally in the same direction as as ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/497577",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 2
} |
Schwarzschild Radius of a Galaxy If an ultra compact/dense Galaxy has a Schwarzschild radius same as it is own radius, how can it be observed from the outside of the Galaxy?
|
If an ultra compact/dense Galaxy has a Schwarzschild radius same as it is own radius
This is not possible. The Schwarzschild radius of a galaxy with say 200.000 solar masses is 590 km. Disregarding the fact that all stars would reach the singularity in very short time (for comparison 13 hours in case of 3 billion sol... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/497769",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.