Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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If we connect the two terminals of a battery with a wire and put nothing in the circuit, will the charge loose energy? Charges loose their energy within the circuit if they move through a resistor(like a bulb). Now, if I make a circuit with just wires and no resistor, will the charge loose its energy?
One reason of lo... | Conduction is not about batteries magically delivering charges and passing them through a circuit towards the other end of the battery like a water flow. What happens in conduction is atoms of the conductive material successively losing and catching electrons in microscopic scale, resulting in a global trend of moving ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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How does a flute produce its sound? if an object for instance does not create successive compressions and rarefactions in air but only creates continuous compressions would we be able to hear these compressions as sound ?
Yes , because we hear the sound of a moving jet (it creates only compressions in air but the air b... | Recorder, whistles, flutes, etc. all belong to a family of instruments that use the same means of producing sound: a jet of air, a knife edge and a tube. The recorder illustrate this 'jet-edge system' very clearly. Inside the beak of a recorder mouthpiece there is a slot-shaped tube called the windway. Air blown into o... | {
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"source": "stackexchange",
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Density of states in a system of interacting electrons When we are introduced to the density of states in typical band-theory problems we neglect interaction between electrons, and thus we define the density of states of a sigle particle as:
$D(E)=2\int_{1st BZ}\delta (E-\epsilon_\mathbf{k})d\mathbf{k}$
since the possi... | $\mu$ would be a proper generalization of a single partile's engery in many body systems.
$$G=G(N)$$
$$\mu =\frac{\partial G}{\partial N} =G(N+1)-G(N) $$
$$\text{Density of states:} \quad \rho(\mu) := \frac{\partial N}{\partial \mu} $$
| {
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Difference between a wavevector and wavefunction I often see both terms used in textbooks, but I am not sure whether I understand the difference between them. Both describe the state of a system, however, they seem different in some ways. From what I have found, what is important in the wavefunction is its direction. W... | The notion of vectors is not only useful in our common three-dimensional geometry (with the x-y-z basis), but it is also widely used in other more abstract areas. In quantum mechanics, where objects are described as complex wavefunctions, it is useful to express the wavefunction as a superposition of some well-chosen i... | {
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How do I find a stream function given a volumetric flow rate? How do I find a stream function given a volumetric flow rate?
The flow only occurs in one direction, between 2 plates, and I have no knowledge of velocity.
I know that volumetric flow rate = change in stream function between two points but I have no idea ho... | If it is pure shear flow, with the top plate moving with velocity V and the lower plate stationary, the stream function is given by $$\psi=Q\left(\frac{y}{h}\right)^2$$where Q is the total volumetric flow rate, equal to Vh/2, h is the distance between the plates, and y is the distance measured upward from the bottom pl... | {
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Transition probability derivation: How to prove $\lim_{\alpha\rightarrow\infty} \frac{\sin^2\alpha x}{\alpha x^2} ~=~\pi\delta(x)$? How to prove
$$\lim_{\alpha\rightarrow\infty} \frac{\sin^2\alpha x}{\alpha x^2} ~=~\pi\delta(x)~?$$
I have encountered this limit while learning time dependent perturbation and transition... | The short answer is that $\sin^2 (ax) /ax^2 $ becomes increasingly localized at zero. The effective domain shrinks like $1/a$ while its value at zero is $a$. Moreover, $\int_{-\infty}^\infty \sin^2 (ax) /ax^2 = \pi$. The rest is math.
| {
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Why is the interaction energy of a dipole and a magnetic field *negative* when they are parallel? The interaction energy between a magnetic moment, $\mu$, and an applied magnetic field, $B$, is given by
$$\varepsilon=-\mu \cdot B$$
That negative sign is confusing my inuition. If we expand the equation based on the defi... | The formula that you quote is the potential energy when a magnetic dipole of moment $\vec \mu$ finds itself in a uniform magnetic field $\vec B$.
You have noted that it is a dot product.
The zero of potential energy has arbitrarily been chosen to be when the magnet moment and the magnetic field are at right angles to... | {
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How should I be thinking about phonons? I have started learning solid state physics and i am struggling with the concept of phonons.
I understand how the vibrational wave can be quantised (because of quantum oscillators) and thought of as a partial in analogy to a photon. But it is said the phonons in a crystal are th... | You are correct that phonons are quantized sound; sound because they require coherent vibrations. Phonons correspond to the vibrational modes of the unit cells of the crystal. The lowest order mode is when they all move together, which is similar to sound, and are called the acoustic mode; all higher order modes are c... | {
"language": "en",
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Where does the energy go when engine braking? If you're in gear in a car and not accelerating, the car slows down faster than it would from just air resistance and tire deformation. In normal braking, the energy is turned into heat from the brake pad rubbing on something connected to the tire. Where does your car's kin... | I always wondered the same thing, my guesses are all kinetic energy in tires turns into KE in engine and then lost through heat, as you may notice engine reving up when you down shift , the engine isn't getting any energy from fuel it must from the tires, so its the opposite the tires move the engine.
| {
"language": "en",
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Feynman path integral course online There are a lot of books dealing with Feynman path integrals. Are there any online courses introducing Feynman path integrals and their applications?
| If you are looking for a full semester course online, the Urbana campus at the University of Illinois offers a good calc sequence online called Netmath, and they cover working with and applications of path integrals.
Or are you looking for something shorter, or free, or cheaper:
There are some online textbooks/notes p... | {
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Why do we take $h$ as "height from surface to bottom" when calculating liquid pressure? In the following image, pressures of points x, y and z are $P_{x}, P_{y}$ and $P_{z}$ respectively, and they all are equal. My question is, why?
The amount of matter on x is much more than amount of matter on y. Why do they still ha... | A simple though experiment is to fill all the space up to the surface by water and remove the walls. Now you can easily believe the formula $p = \rho g z$ (as the same amount of water rests on the points x, y and z) and the water is static as all forces cancel. If you now insert the (infinitesimally thin, totally rigid... | {
"language": "en",
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In what direction should you throw a 1 kg uniform sphere in order to put it into lower Earth orbit? If you are standing on Earth's surface, in what direction (and at what speed) should you throw a 1 kg uniform sphere of radius 0.1 meters in order to put it into lower Earth orbit? Assume that there is air, but it is not... | If we ignore air resistance for a moment, then all orbits in an inverse square force like gravity are closed. this means that if you throw something hard enough it will complete one orbit then return to its starting point i.e. your hand.
So if you throw the object downwards it obviously hits the Earth, and if you throw... | {
"language": "en",
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Can I catch a particle (such as electron) and say it's left hand
*
*Why is only the left hand electron coupled to weak interaction?
*How can I tell the chirality of an electron?
| The chirality (left handedness/right handedness) of a fermion originates by the way it transforms under the appropriate representation of Lorentz algebra (in general the Poincare algebra). In this way, we can state that the left handed electron and the right handed electron are two fundamentally different entities (as ... | {
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How to prove that gravitational potential energy of a body of mass $m$ at a height $h$ is $mgh$? Many introductory physics books just write that potential energy of a body of mass $m$ at a height $h$ as $U_\text{g}=mgh$. However, they never show how this was derived. I'm interested in knowing this derivation – if poss... | By definition we know that-
Gravitational Potential Energy of a body is the work done against gravity in raising it to a certain height h.
We have$$
\text{work}~~=~~\text{force}~\times~\text{displacement}
\,.$$
Here force of the body is the weight acting vertically downwards=$mg$ and displacement is $h.$
So,$$
\text... | {
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Event horizon, and Hawking radiation When the escape velocity from EH is c, then how can Hawking radiation escapes it? Does it travel faster than c? Or, it escape from outside the EH? If so, how a BH can evaporate beyond EH?
| In quantum field theory, particle antiparticle pairs spontaneously pop out of the vacuum all the time, essentially borrowing energy from the vacuum and then subsequently annihilate each other.
However, when an event horizon is present, you can get situations where the pair is generated very close to the horizon. If th... | {
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Could there be a "massive gravity" theory? If we talk about a "quantum theory" of General Relativity, we know that the particle that mediates the gravitational force would be the so called Graviton, a massless particle with spin-$2$.
I wonder if (and if it would work) there could be also a different theory like a "mass... | Observation puts an upper limit to graviton mass of 10$^{-23}$ eV/c$^2$. https://physicsworld.com/a/motions-of-the-planets-put-new-limit-on-graviton-mass/
| {
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Commutation relations in quantum mechanics As we know, simple harmonic oscillator can be solved only by commutation relations between creation and annihilation operators, and the Hamiltonian expression. The spin energy is either solved only using commutative relations between spin operators in axes ($J_i$) and $J^{2}$.... | If your Hamiltonian belongs to a Lie algebra for which you can solve the initial value problem in the corresponding group then you can use geometric quantization to solve the corresponding Schroedinger equation. This is because the solution of the Schroedinger equations is just $\psi(t)=e^{-itH/\hbar}\psi_0$, and $e^{-... | {
"language": "en",
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How can a photon oscillate when it experiences no time? How can light experience change if it is motionless in time?
| The electric field at a point A changes based on the electric current at A and the spatial variation of the magnetic field at A.
The magnetic field at A changes based on the magnetic current at A (which is zero, since there are no magnetic charges) and the spatial variation of the electric field at A.
So that's what ha... | {
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Condition for looping the loop Consider a ball tied to a string and it is imparted a velocity we have studied that condition for looping the loop is that tension at the uppermost point must be zero, but why is this condition imposed please explain?
If tension becomes zero at some point below the uppermost point won't t... | An object that is not acted upon (with tension) will follow a parabolic curve. So instead of a circle, the object will track a parabola for the duration of the time where tension is zero.
It is the tension that forces an object to follow a prescribed path.
| {
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How do snails avoid being cut while crawling over a razor blade? This YouTube video shows a snail crawling over the edge of a razor blade without being harmed.
The author claims that:
To us, slug and snail slime is gooey and gross but the mucus is one of nature's best lubricants and allows a mollusc to glide over sh... | *
*Snails are light
*They move not by sliding but through a wave of "up-forward-down" along their "foot/belly", quite a continuous equivalent of the discrete motion of the caterpillar.
| {
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Connection between Hamiltonian version of the least action principle and probability amplitude in the Schrödinger equation If I'm not mistaken, Schrödinger was influenced to look at wave equations because of de Broglie's assertion about particles having a wavelength. He started with the Hamiltonian equation which is re... |
What I don't get is how he ended up with an equation that is a probability distribution.
He ended up with Schroedinger's equation, but he did not think it describes probability. He thought more along the lines of de Broglie idea, that the electron is some kind of wave, and $\psi$ - solution to the equation - expresse... | {
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Boiling as apparent violation of the second law of thermodynamics One of the statements of the second law is that no agency can be built whose sole effect is to convert some amount of heat entirely to work.
But in case of boiling, the temperature being constant, entire heat supplied is converted into work, namely the w... | You misquoted the 2nd law. Here is what it really says (Moran et al, Introduction to Engineering Thermodynamics):
It is impossible for any system to operate in a thermodynamic cycle and deliver a net amount of energy by work to its surroundings while receiving energy by heat transfer from a single thermal reservoir... | {
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Is green superheated plasma possible? I've noticed that video games like Fallout 4 depict the plasma fired from plasma weapons as green. Is there a way hot plasma can look green in real life? I know some gases emit green light when ionized by a current, but is there any other way?
| I feel kind of weird answering my own question, but after some digging, I found out that it's possible to make plasma jets look green.
http://www.mdpi.com/2079-6412/1/2/117/htm
http://www.neutronsources.org/news/scientific-highlights/good-results-obtained-with-a-novel-technology-to-produce-large-scale-neutron-converter... | {
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Gravitational waves: simulations of signal I am self-learning GR.
I was wondering if there is any open source software to help learn more about the signal processing of gravitational waves. E.g. a software that injects a signal into random noise and outputs data accordingly.
I am interested in trying to do some signal ... | LIGO actually has a really nice step-by-step tutorial on this: https://losc.ligo.org/s/events/GW150914/GW150914_tutorial.html
It is in python, but should be accessible
There is more in LIGO Open Science Center: https://losc.ligo.org/tutorials/
| {
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"source": "stackexchange",
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Do solid objects really behave like fluid matter on extremely long timescales? Freeman Dyson's fascinating paper Time without End: Physics and Biology in an Open Universe contains the following passage:
I next discuss a group of physical processes which occur in ordinary matter at zero temperature as a result of quant... | I think what Dyson's saying is that, while a solid generally does not deform permanently unless enough pressure is applied to break bonds between atoms or molecules, statistical properties of quantum mechanics, analogous to the probability of electron tunneling, will lead to solids changing shape under the influence ... | {
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Could we curve the flight path of a photon? I was wondering about photon's interaction with matter, and why photons dont slow down. They seem to always bounce in a straight line at the same speed (I think), as if some force is charging them forward after the bounce. First, what is this? I've heard of experiments where ... | The lenses in my reading glasses bend the path of photons, as does gravity. In matter photons move at slower speed than $c$. In Bose-Einstein condensates they can even be brought to a halt.
| {
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Where did all the photon energy go? From the wikipedia page on dark energy, in reference to the total mass-energy of the universe:
The mass–energy of dark matter and ordinary (baryonic) matter
contribute 26.8% and 4.9%, respectively, and other components such as
neutrinos and photons contribute a very small amoun... | The argument is as follows. The mass density of ordinary or dark matter scales as $\sim a^{-3}(t)$, where $a(t)$ is the scale factor, which depends on time $t$. The reason for this scaling should be intuitive. In contrast, the energy density of photons/radiation goes as $\sim a^{-4}(t)$. This is because the number of p... | {
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Breaking the bottom board... when there's two Watching a board-breaking ceremony at my martial arts school, I have seen something very weird no one can seem to explain: when a person attempts to break two boards stacked on top of each other (separated by pennies at the corner), there is a chance that the bottom board w... | It is really a simple answer: think of the Newton balls. Because the boards are touching, momentum is conserved and moves through the machine (in this case 2 boards stacked). Once it reaches the back of the machine the energy is released into the back board and any remaining energy will have the reverse effect and move... | {
"language": "en",
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How to measure an image's contrast? I'm studying Fourier optics and Interferometry and I intend to determine the contrast of an image using computer software. My teacher of Experimental Physics didn't tell me how to do it, and so, I'm requesting some attention from physicists that have already done this kind of analysi... | You could use Matlab: load the image into an matrix, and then find the maximum and minimum entries of the matrix. Plug these into your equation to find the contrast.
Alternatively, for higher accuracy, you could take the lowest and highest (say) 100 values and take the average of those to give you your Imin and Imax, r... | {
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Why is it said that standing waves do not transfer energy? The author of my physics textbook writes that standing waves, unlike travelling waves, do not transfer energy. He says that this is because a standing wave is composed of two travelling waves carrying energy in opposite directions. Is this explanation sufficien... | Consider for example a transverse standing wave on a string, with wavefunction
\begin{align}
\psi(z, t) = A \sin kz \sin\omega t.
\end{align}
The power transmitted by the wave (in the $+z$ direction) is
\begin{align}
P(z,t) = F(z,t)v(z,t) = 4 \mu v\omega^2A^2 \cos kz \sin kz \cos \omega t \sin \omega t
\end{align}
wh... | {
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"source": "stackexchange",
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How does gravity truly work? I Am only 12 years old and I'm constantly wondering and trying understand how gravity really works. On YouTube everyone always talks about objects wrapping space time around themselves and uses the analogy of a trampoline. I still don't understand gravity because if space were like a tramp... | I personally prefer this visual explanation of Einstein's general theory of relativy from Brian Greene, a professor at Columbia University. https://www.youtube.com/watch?v=0jjFjC30-4A
A follow up video answering why don't planets fall toward the sun.
https://www.youtube.com/watch?v=uRijc-AN-F0
| {
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How can a parallel circuit work? The electrons always takes the easiest way in a circuit, right?
So in a parallel circuit, why does the electrons flow through all parts of the circuit and not just the one with the least resistance?
| Imagine a river flowing towards a fork. The water on the left side of the river does find it easier to go on the left fork and the water on the right does find it easier to go on the right.
Now imagine that the right branch looks like more of a side street than a fork. But if there is a slow down in the forward directi... | {
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The force causing galaxies to accelerate away from each other? The galaxies in the universe appear to be accelerating away from each other according to the Doppler shift that is present in the light we receive from other galaxies.
According to Newtonian mechanics for something to accelerate, a force must be applied to ... | Dark Energy is thought to be responsible for an ever expanding Universe. And we cannot understand this phenomenon based on Newtonian Mechanics as it is not applicable in cosmological scales. You can think of this energy as a negative pressure due to vacuum. But we truly don't know anything else about its nature and cha... | {
"language": "en",
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Rotations in Bloch Sphere about an arbitrary axis I am trying to understand the following statement. "Suppose a single qubit has a state represented by the Bloch vector $\vec{\lambda}$. Then the effect of the rotation $R_{\hat{n}}(\theta)$ on the state is to rotate it by an angle $\theta$ about the $\hat{n}$ axis of th... | You can in fact construct a concrete proof by direct computation:
*
*Take a mixed quantum state represented by a density operator, $\rho = \frac{1}{2}(I + \vec{r} \cdot \vec{\sigma})$, where $I$ is the identity operator on a Hilbert space of two dimensions (representing your quantum state). $\vec{r}$ is the Bloch ve... | {
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Why do some stars have a negative parallax? I am constructing a Hertzsprung-Russell diagram for stars within some radius around Pleiades and have repeatedly come across stars that have negative parallaxes. For example, http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-info=XML&-out.add=.&-source=I/239/tyc_main&recno=161838
... | The parallaxes of very distant stars should be zero (or at least indistinguishable from zero). If the parallaxes have an observational uncertainty (which they do), then half of the parallaxes of very distant stars will be negative. I think this is all that you are finding in the case of absolute Hipparcos parallaxes (t... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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The force felt by a pilot when a package is dropped? Here is a physics question that I wish to prove mathematically.
A 100,000 kg aircraft drops a 1000 kg package of supplies over an arctic research station. What approximate force is felt by the 100 kg pilot at the instant of the release?
Is there a reactive force on t... | The floating mechanism that keeps 100000 kg (including plane, load, and pilot), plane afloat, will at the time of release, keep working now on 99000 kg. That means the 99000 kg will feel a 1000 kg upward force/thrust. This force will be also felt by the pilot due to upward thrust of the plane, but it will be negligible... | {
"language": "en",
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Why does moonlight have a lower color temperature? Moonlight has a color temperature of 4100K, while sunlight has a higher color temperature of more than 5000K.
But objects illuminated by moonlight don't look yellower to the eye. They look bluer. This holds for indoor scenes (like my hall) and for outdoor. I find it co... | I took a photo during the day, at 2PM:
I took photo at night (4AM) of the same scene, lit by the full moon. This is a long exposure photo (30 seconds) with roughly the same exposure:
I adjusted its color balance to match the day photo (temp=5100 and tint=+3 in Lightroom). The moonlit scene has a strong yellowish colo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/244922",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Are electric field lines always conserved? Suppose we have a positive +q charge and a -6q charge at some separation. Then will every field line originate from the +q and end up to -6q or will there be some extra lines coming to -6q from infinity because of higher charge to get 6 times the number of field lines? That is... | Electric field line start on a positive charge and finish on a negative charge or start/finish at infinity of start/finish at a neutral point.
Here is a computer generated diagram of electric field lines plotted for a +1 charge on the left and a -3 charge on the right (-6 was not available).
There seem to be a lot ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "9",
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Why does the temperature of the gas in a container moving with constant velocity not change?
Systematic changes do not affect thermodynamic equilibrium.
What does this mean? And what kind of systematic changes are allowed?
The container with gas is stationary till some time then it's given a constant velocity and the... | Consider two gas molecules in a container, moving in opposite directions with speed $v_{g}$.
Case 1: When the container is at rest.
Both gas molecules rebound with their initial velocities and net KE of the gaseous system remains constant.
The KE of the gaseous system is $$KE = 2. \frac {1}{2}mv_{g}^2 = mv_{g}^2$$
Cas... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What Keeps a Pendulum Moving In a Circular Path?
From the figure, we know that $F_{net} = mg\sin\theta$. Now, this force $\vec{F_{net}}$ is in the direction of the velocity $\vec{v}$ of the bob, both are tangent to the path. Therefore, the net acceleration $\vec{a_{net}}$ has no component perpendicular to the path, th... | The picture is valid, if nothing is moving. Otherwise, there has to be some centripetal force to the origin, since there obviously is acceleration on a curved path. The net force has a tangential component (if we are not in the deepest point) and a radial component (if we are not in the highest). This comes from an inc... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/245223",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Entropy generation during irreversible adiabatic expansion During irreversible adiabatic expansion entropy is generated. It means that the gas expands to as greater volume than that during reversible adiabatic expansion for the same change in pressure. How can that be?
| There is viscous dissipation of mechanical energy within the gas, and this "viscous heat generation" causes the final gas temperature to be higher. So the final volume is greater than for the adiabatic reversible expansion. Even though the final volume is greater, the amount of work done is less (because of the lower ... | {
"language": "en",
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Basic Quantum Entanglement Quantum entanglement means that multiple particles are linked together in a way such that the measurement of one particle's quantum state determines the possible quantum states of the other particles. But my question is that if we randomly pick up any pair of particles from anywhere, will it ... | To answer your questions one by one:
*
*No. All the particles are not entangled. If they were so, we'd have quantum computers by now. Entanglement is a phenomena very sensitive to environment and even the slightest interference from outside can diminish the entanglement between two particles.
*Different type of par... | {
"language": "en",
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"source": "stackexchange",
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Fresnel equations for perpendicular angle of entry In many text books one finds for the fresnel coefficients for light perpendicular to the surface:
$$r = \dfrac{n_1 - n_2}{n_1 + n_2}\quad\text{and}
\quad t = \dfrac{2n_1}{n_1 + n_2}.$$
When I try to establish $t$ from $r$ with the relation that the overall intensity st... | $$r^2 + t^2 \neq 1$$
You have forgotten that the refractive index changes, the speed of light in the media changes and thus the intensity is not jut proportional to the square of the electric field intensity, it is proportional to $n E^2$. Thus
$$n_1 r^2 + n_2 t^2 = n_1$$
$$ t^2 = \left( \frac{n_1}{n_2}\right)(1 - r^2)... | {
"language": "en",
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If two objects have the same drag coefficient, but one is much heavier, would it fall faster? Not sure this is a good example but imagine we have two feathers, both with exactly the same drag coefficient, they have the exact same shape and everything, but the only difference is that one of them is somehow much more hea... | Heavier object will have greater force of gravity; due to this simple fact the object with heavier mass will attain terminal velocity at greater speed than the lighter one. I hope this answers your question
| {
"language": "en",
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How to interpret the units of the dot or cross product of two vectors? Suppose I have two vectors $a=\left(1,2,3\right)$ and $b=\left(4,5,6\right)$, both in meters.
If I take their dot product with the algebraic definition, I get this:
$$a \cdot b = 1\mathrm m \cdot 4\mathrm m + 2\mathrm m \cdot 5\mathrm m + 3\mathrm m... | The length of the cross product of two vectors is the area of the parallelogram spanned by them, so the square-meters are the correct unit as well as geometrically meaningful - it's really an area. The $x$-component is the area of the projection of the parallelogram onto the $y$-$z$-plane, the $y$-component the area of... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "28",
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How is Planck's radiation law related to $E=h\nu$ How did Planck conclude $E=h\nu$ from his radiation law?
| I don't know if Planck did it this way, but one can separate out the assumption that the energy carried by light is quantized into photons from the assumption that the energy of each photon is $E = h \nu$. Whatever the form of the energy relation is, it can only depend on the frequency because (in a universe with spac... | {
"language": "en",
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How does wave-particle duality describe Photoelectric effect? I don't know if electrons work as particles or waves or maybe both in photoelectric effect.
How is Photoelectric Effect actually described by Wave-Particle Duality?
| There are a lot of experimental results that are inconsistent with the assumption that electromagnetic radiation is a wave field.
A. Assuming that radiation is a wave field then it would be absorbed gradualy by the electron and the time needed to trasnfer the necessary for extraction energy to the electon is ~1sec. How... | {
"language": "en",
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What is the potential difference across a disconnected diode? I think potential difference across disconnected diode should be zero but how?
the last graph tells that there is a potential difference between the two ends of a diode. then how can be it zero?
| The charge separation shown in the diagram is true, as far as it goes. Keep in mind that the diagram shows an isolated diode, so the resistance between the ends of the diodes is infinite and no current flows.
The driving mechanism behind the separation of charge is thermal motion of the electrons in the diode. If you w... | {
"language": "en",
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Forces that exert torque on a rigid body in rotation when angular momentum is not parallel to angular velocity I'm confused about the rotation of a rigid body, when the angular momentum $\vec{L}$ is not parallel to the angular velocity $\vec{\omega}$. Consider a barbell with two equal masses that rotates around a verti... | In the example cited in the question, the center of mass of the barbell rotates about the pivot axis. Some external force needs to be applied to the barbell to make that happen. Assuming a constant angular velocity, this force is directed radially inward from the center of mass toward the axis. From the perspective of... | {
"language": "en",
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What dictates the range of EM radiation which can be produced by semi-conductor excitation (like LEDs) Exciting semiconductors such as silicon carbide and gallium nitride can cause electroluminescence. I believe that by altering the chemistry of the semiconductor, you can alter the wavelength of the radiation emitted? ... | Here is a pair of bounds:
At the lower energy range, $kT$ is a limiting factor since thermal effects will swamp attempts to invert the electron/hole population. At room temperature $kT$ is in the infra-red at about 0.025 $eV$.
At the upper energy range you need a conduction band state which is still bound to the cryst... | {
"language": "en",
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Is Newton second law covariant or invariant? Is Newton second law covariant or invariant between two inertial frames, moving with uniform traslational motion with respect to each other?
If it is invariant then, indipendently from the frame, $\vec{a}=\vec{a'}$ and $\vec{F}=\vec{F'}$ (of course $m=m'$) and this means th... | Newton's second law is covariant, as it does not change its form if we switch to another frame of reference. As already explained by @AccidentailFourierTransform in his comment, Newton's second law is a vector law. This means the quantities in the law are vectors, which have different values in different frames of refe... | {
"language": "en",
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"source": "stackexchange",
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Observer in the double slit experiment with photons In the double slit experiment with photons, the interacting observer is an instrument, detector…
If you replace the detector with a piece of metal with the same mass as the mass of the detector, the wave will collapse?
| No. The strange behavior of the photons is directly related to the observation of which slit the photon passes through. Once it is no longer an observer, and by observer we mean that we detect the presence of a photon, the results change. The original experiment kept the detector in place and simply did not activate... | {
"language": "en",
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Why must we use mode locked lasers? Quick question:
If I have a laser cavity with a bunch of harmonics under the gain curve:
Why do I not always get a pulsed laser? EG:
I mean since these harmonics exist in the cavity arn't they always in phase because the ends are closed and they create standing waves in phase to ea... | There are many kinds of lasers, and mode locking is a technique that is required for most pulsed lasers. Mode locking is used to control the structure and number of pulses present in the laser cavity; usually it is required that only one active pulse be present in the laser cavity, and it endlessly recycles, releasing ... | {
"language": "en",
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How is focal length defined for a two-lens system, separated by a distance $d$? I have found the formula for the effective focal length $f$ of two thin lenses with focal lengths $f_1$ and $f_2$ separated by distance $d$ to be
$$
\frac 1f=\frac 1{f_1}+\frac 1{f_2}-\frac d{f_1f_2}.
$$
However, I can't seem to find how ... | It is the distance from the image plane to the rear principal plane. You can find the location of this plane by projecting the image ray backwards through the system to where it crosses the projection of the object ray. This is sometimes also referred to as the effective focal length (v) of the system, and is true for ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Understanding role of friction in block on block problems Consider two blocks stacked on top of one another. There is friction between the blocks but there is no friction between the lower block and the table. So the only frictional force that tends to retard the lower block is due to the friction between the blocks ri... | In this totally hypothetical situation, everything you said is correct. It's important to remember, however, that there must be a normal force between the bodies for the friction force to exist. In the most intuitive example, this force can be the reaction of the gravitational force (from Earth) from the upper body ont... | {
"language": "en",
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If all the particles of a Bose-Einstein condensate become entangled with each other,does the system still remain a Bose-Einstein condensate? I know that an entangled system is found in a single entangled state and that when you try to observe the individual state of a particle from an entangled system using a reduced d... | An ensemble of interacting particles will, over time, develop entanglement between widely separated parts*, so this is similar to asking whether an interacting system can still be a BEC.
The short answer is yes, but a subtlety is that various authors define BEC in slightly different ways.
One way of defining BEC, as I ... | {
"language": "en",
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Why doesn't an object, despite having a non-zero potential energy stored in it, fall by itself from the elevation? When an object is at a certain height, it has some energy stored in it as we have done some work on it to get it to that height. So when it already has energy, then why doesn't it fall off from the table t... | Although there are excellent answers, I think a more "simplistic" answer is required to correct your thinking.
If you start with a piece of lead (1 kg) on the floor, grab and lift it 1 meter, it will gain (1 x 9.8 x 1 =) 9.8 J of energy. If you now open your hand (release it), it will fall by "it self" and hit the gro... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
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Why do constants have dimensions? I am just a beginner in dimensional analysis, and I see that $G$, the universal gravitational constant, is independent of everything. Speed, for example, depends on distance and time, but $G$ does not depend upon anything.
Then why is $G$'s dimensions not $M^0 L^0 T ^0$, as it is not ... | Newton's constant, $G$, was introduced in his law of gravitation,
$$
F = G \frac{M m}{r^2}
$$
In other words, the constant may be expressed as
$$
G = F \frac{r^2}{M m}
$$
and the dimension of $G$ may be deduced in the same manner as the dimension of velocity in $v=d/t$. Of course, if Newton's law of gravitation holds, ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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What does the direct sum symbol (i.e. $\oplus$) mean in the context of uncertainties I've noticed the symbol $⊕$ used in a context I'm unfamiliar with. In several papers about the the calculation of the uncertainty of quantities measured with hadron colliders.
For example the uncertainty in transverse momentum given b... | This seems to be addition in quadrature of multiple independent uncertainties in a measurement. In particular, if you have a measurement which depends on two quantities $a$ and $b$ whose uncertainties $\delta a$ and $\delta b$ are completely independent and uncorrelated, then their uncertainties will often be combined ... | {
"language": "en",
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How Far Does Gravity Extend? How far does gravity's influence extend?
I've recently (re-)seen 'The Universe' season 1 where it's said that 'take these two dice, place them perfectly still in empty space 1 inch apart and within an hour they'll touch due to their gravitational attraction'.
I get the explanation and it ma... | Well, in your very ideal system and from the classical point of view, yes the dices (considering they as an electromagnetic neutral system) will be in touch at some time independent of where are they at the beginning.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What's the problem with light traveling at speed higher than $c$? I'm trying to understand how Einstein concluded that time is relative based on thought experiments such as a torch attached to the end of a rocket. Based on answers to questions like this one, this one, and this one, I understand that special relativity ... | No physical signal can travel at a speed that is greater than the speed of light because if it did an effect would precede the cause. This is a consequence of the Lorentz transformations.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/249026",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
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All the planets in our solar system rotate 'Anticlockwise', except Venus. Why is the only planet that rotate' clockwise'? All the planets in our solar system rotate 'Anticlockwise', except Venus. Why is the only planet that rotate 'Clockwise'?
| Without having any experimental data at hand, I guess that most planets formed from a uniformly rotating dust disc, and thus their rotational and orbital momentum have the same sign.
However, upon random tangential impacts, some of them (Venus, Uranus..) could change their original axis of rotation, and most probably ... | {
"language": "en",
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Representation under which Pauli matrices transform In Peskin and Schroeder's Quantum Field Theory, there is an identity of Pauli matrices which is connected to the Fierz identity, (equation 3.77)
$$(\sigma^{\mu})_{\alpha\beta}(\sigma_\mu)_{\gamma\delta}=2\epsilon_{\alpha\gamma}\epsilon_{\beta\delta}.\tag{3.77}$$
The a... | Another way to see this is to use the transformation laws for the $\gamma^\mu$ matrices. Note that we can write
$$\Lambda_{1/2}=\left(\begin{matrix}
\Lambda_{1/2L} & 0 \\
0 & \Lambda_{1/2R}
\end{matrix}\right),$$
where $\Lambda_{1/2}$ is the matrix that performs a Lorentz transformation on a Dirac spinor. It is easy to... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Spin Orbit Coupling Hamiltonians I am really struggling with something fundamental.
I keep coming across two versions of the hamiltonian for spin orbit coupling:
$H_{soc}=\frac{\mu_B}{2c^2}(v \times E) \cdot \sigma $
$\mu_B =$ bohr magnetron
$v =$ velocity
$E = $ Electric Field
$\sigma = $ pauli matrices
and
$H_{soc}... | If $\mathbf{E}$ is a radial function, $\mathbf{E}(\mathbf{r})=\mathbf{E}(r)$, then both equations are true.
| {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Confusion about the shock growth I am studying Hamilton's & Blackstock's Nonlinear Acoustics. One of the essential phenomena associated with a finite-amplitude (unidimensional, planar) sound propagation is building the shock due to the super- and subsonic propagation speeds at different phases. The sound propagation sp... |
I don't fully comprehend why the positive (and therefore quicker) parts appear to be retarding and eventually making a N-like sawtooth wave?
I wrote a more detailed answer at https://physics.stackexchange.com/a/139436/59023, but the basic idea is that the larger amplitude parts of the wave have a higher phase velocit... | {
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"source": "stackexchange",
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Rolling without slipping in absence of friction force I'm confused about a rolling without slipping situation. Suppose to have a disk of radius $R$ on a floor, and exert a horizontal force at a certain distance $r$ from the center of mass.
I would like to see in which situation I obtain that the disk rolls without sli... | Everything you have derived is correct.
The reason for your perceived paradox is, I believe, a confusion between force and power. The same force can produce more power if it is being exerted at a greater velocity. When you exert a force at a radius r from the CM, the point of application of the force will accelerate ... | {
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When does the shock occurred? I'm between this 2 conditions that when does the shock occurred over an airfoil.
*
*When the speed of point of an airfoil reach to sonic velocity of environment.
*When the speed of point of an airfoil reach to sonic velocity of local flow over an airfoil.
Which one is correct?
| A compression shock at subsonic flight speed only occurs when a supersonic pocket of air collapses downstream. Neither of your options is correct, and in that shock the speed drops from mildly supersonic (typically Mach 1.25) to the inverse of that Mach number (that would then be typically Mach 0.8).
Acceleration into ... | {
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How much load does an aqueduct support? Recently, I found out about /r/InfrastructurePorn, and I found a particularly interesting photo of the Gouwe Aqueduct in Gouda, NE:
It seems like the bridge that is supporting the boat wouldn't be able to do it. Is the weight of the actual boat being supported by the aqueduct? ... | The ship is floating and so is supported by the upthrust due to the water and so the weight of water displaced by the ship.
If the ship travels very slowly so that the level of water does not rise but rather flows away then the weight supported by the aqueduct does not change between the ship present and no ship presen... | {
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What is $bfr$ in this expression? I am reading 'Fundamentals of Quantum Mechanics' by Sakir Erokoc and came across this expression in relation to transition probabilities:
$$\vec p=e \langle \psi_b |bfr|\psi_a \rangle$$
Which can be found here (p332). I have tried looking in this book and in other places but I can't de... | It's a LaTeX typo. The author meant to write $\mathbf{r}$, i.e. '\bf{r}' where they've defined \bf as a macro for \mathbf. They forgot the slash.
This indicates you should throw the book in the trash, as it was never properly edited.
| {
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Can angular momentum not be conserved in a straight line motion? Consider a particle moving an a straight line, with constant velocity $v$. The angular momentum (pivot point $O$) can be calculated as $$L=mr v_{\theta}$$Where $v_{\theta}$ is the velocity perpendicular to the vector $r$ at each istant.
Now if I calcula... | If you draw similar triangles, then you'll find that $r_A/r_B = v_y/v_x$, and so the product $r_A v_x$ is equal to $r_B v_y$. Try drawing a line from the tip of your lower $\vec{v}$ vector to the tip of your lower $v_y$ component to see this.
| {
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Does curving only the inner surface of a diving mask prevent blurred images? Diving masks often have lenses built into the glass for divers who do not have perfect vision. The lenses in a diving mask faceplate often only have a curved surfaces on the inside glass. Why is this design desirable?
I know that curving of in... | The answer is purely a matter of engineering and cost, I think. I've bought prescription masks in the past, and they are as you say: basically a stock faceplate with a correcting lens attached to the inside face. This allows the manufacturer to stock a standard (flat) faceplate and pull units from stock to build spec... | {
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Where does the energy of a photon trying to escape a black hole go? I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its wavelength gets longer and longer until its energy is... | Since due gravitational time dilation something takes forever to fall inside a black hole from the perspective of an outside observer, there is nothing in it yet that could come out.
The light which is emitted from outside the horizon does reach an outside observer, but since it hasn't yet fallen in, it technically d... | {
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1D Scattering Phase Shift (Finite Well) - Unphysical? I am calculating the phase shift from a 1-dimensional potential well. This seems extremely simple, but I am just getting so confused by it.
Let there be a potential well of depth $V_0$ and spatial width $2a$. Consider a scattering solution with mass parameter $m$ a... | I figured it out. The problem was that I was simply adding the two phase shifts, ignoring the periodicity in the tangent and arctangent functions. In order to get the proper result/plot, one can resort to the addition formula for tangents
$$\delta=\delta_1+\delta_2=\arctan \left(\frac{\frac{\epsilon}{2}\tan(2k'a)-\tan(... | {
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How can a quantum dot be used as a qubit? Many people say that quantum dot is a potential physical representation of qubit. A qubit should have two distinguishable states which may carry quantum information. What are the two states of a quantum dot which could be regarded as the two states of a qubit?
| Quantum dots themselves are not the qubits. The reason to use them is because a quantum dot defines an "island" that can house one single electron which then stores the quantum information.
*
*There can be spin qubits realized within a quantum dot. Once we measure the charge stability diagram for a quantum dot, we c... | {
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Create a laser from sun light I have read on wikipedia the basic theory behind laser. A photon passes through an excited electron which then produces the exact same photon. Light is bounced off mirrors to create more photons.
Is it possible to make sunlight passes through an optical amplifier to create a amplified sun ... | Sun pumped laser
The two most studied lasing media for solar-pumped lasers have been
iodine,1 with a laser wavelength of 1.31 micrometers, and NdCrYAG,
which lases at 1.06 micrometers wavelength... The largest solar-pumped
laser is currently being operated by a research facility in
Uzbekistan. It is a 1 MW solar input... | {
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What are the accelerations of blocks? I've talked with 2 teachers about this situation:
one teacher said he was completely sure that B have twice the acceleration of A, the other said he was completely sure they have same acceleration. Can you have a better look on it? What do think? Consider it has no friction.
|
$$l=x_A+(x_A-k_1)+k_2+k_3+2\pi R$$
$$k_3=x_B-2R-(x_A-k_1)$$
$$l=x_A+x_B+constant$$
so, we have:
$$0=v_A+v_B$$
and$$a_A=-a_B$$
| {
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Difference between theoretical equations and empirical equations Some equations are theoretical in the sense that they are derived from an underlying theory. Other equations are empirical in the sense that they were selected only because they fit experimental data and without a theoretical justification.
If we look at ... | Under the suggestion of L. Levrel, I'll expand upon my comment.
We can look at the constants included within an equation to get an idea of whether it was theoretically derived, or an empirical result. If we have fundamental constants, such as $\hbar, \epsilon_0, e, c,$ etc, then it was probably theoretically derived. A... | {
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Why current in series circuit is the same? I have read in the internet that the charges do not have any other path to go and they must go through the same in a series circuit,hence the current is same.
It was quite convincing but what confused me was:
"A resistor is a passive two-terminal electrical component that impl... | Consider the following analogy to water pipes: Wires are like pipes already filled with water; the water resembles the movable charge, in case of metal electrons. Voltage is a pressure difference between two points, e.g. because one is higher up than the other. The pipes are really broad in comparison to the slow flow ... | {
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Doubt on series springs Here is a description of the motion of two springs in series. The premise is that the force on the two springs is the same.
This is derived from the following reasoning: when I pull the mass with a certain force $F$ at some point I reach the equilibrium position.
So every piece is stationary a... | I think you are asking why this argument remains valid when the mass is oscillating, and does not only apply when it is static.
I think the answer to this is that the forces in the springs depend only on their extensions ($F=kx$), not how quickly the extension is changing ($F=kx+b\dot x+c\ddot x$). So at any given ext... | {
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Honeycomb Bravais Lattice with Basis I just had my second solid state physics lecture and we were talking about bravais lattices. As far as I understand a Bravais lattice is an infinite network of points that looks the same from each point in the network. For example:
would be a Bravais lattice. On the other hand, th... | The vertices of a two-dimensional honeycomb do not form a Bravais lattice. A non-Bravais lattice is often referred to as a lattice with a basis. Specifically to your question, it can be represented as a two-dimensional triangular Bravais lattice with a two-point basis. Similarly, HCP, diamond, CsCl, NaCl structures are... | {
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Gauss law and electrostatic induction. Concentric spheres Imagine you have hollow concentric spheres A and B with radius a and b (b>a), respectively. If A is a conductor and B has a certain density charge, I´ve been taught that B will induce some net charge over A (appart from what it could have yet). But, if, accordin... | I assume that A is neutral to begin with. Then inside of A cannot be any charge by Gauss law. This means that the inside of A must neutral.
Then you can take the volume between B and A. The boundary of that are the metal spheres A and B. On the whole boundary surface, there cannot be any electric field as that surface ... | {
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How can an accelerating inclined plane prevent a block on it from sliding?
If I increase the force $F$, only the normal force $N1$ acting on $m1$ would increase which has no component along the plane, ie. along $m1gsin\theta$, so how would applying this force prevent m1 from sliding?
When I view it from the accelerate... | What causes the block to slide? That would be gravity.
What tries to prevent this slide? That would be friction.
And friction depends on the normal force $f_k=\mu_k n$.
As you rightfully say, the normal force increases with increasing pushing force $F$, which thus causes increased friction which thus prevents the slide... | {
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Diminishing solar temperature and its effects on earth This is a hypothetical question; considering both the earth and the sun as black bodies. If the temperature of the sun decreased N times, what would be the effect on the radiation intensity received on earth?
(i am inclined to say it would decrease N^4 times using... | Background
The specific intensity or brightness, $I_{\nu}$, is defined as:
$$
I_{\nu} = \frac{ dE }{ dA \ dt \ d\Omega \ d\nu } \tag{1}
$$
where $\nu$ is the frequency, $dE$ the differential energy, $dA$ the differential area, $dt$ the differential time, $d\Omega$ the differential solid angle, and $d\nu$ the differenti... | {
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How to calculate acceleration from discrete samples of velocity? I have a device that I made which is able to calculate distance moved and can determine time.
I take samples of time every x distance. Giving me n velocity samples.
I currently show the user 2 values : max velocity and avg velocity.
What I need help with ... | I think you should take the sample velocities and divide them by the respective times after minusing the previous velocities to obtain the accelerations. If the time interval between calculating the discrete sample velocities are too small, then the above-got values may be taken as instantaneous acceleration. Now plot ... | {
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How do I calculate the differential cross section with respect to the transversal momentum? First of all, sorry for my English, my first language is German.
My problem is: I calculated the matrix element of the quark-gluon-Compton process (q+g -> gamma + q). With the kinematics of scattering in the center-of-mass frame... | The process is discussed at the parton level – both in the initial form and the desired form – so the conversion cannot depend on PDFs.
Now, the Mandelstam variable $t$ is equal to
$$ t = (p^\mu_1 - p^\mu_3)^2 = m_1^2+m_3^2 -2 E_1 E_3 +2 \vec p_1\cdot \vec p_3 $$
in the "mostly minus" metric convention. The masses of p... | {
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Decoupling coupled differential equations in dynamically coupled two state system Consider the following dynamically coupled two state hamiltonian, $$H=-B\sigma_z-V(t)\sigma_x.$$Taking the eigenfunctions of $\sigma_z$ ($|+>$ and $|- >$) as basis vectors, we have the wave function to be $$\Phi=c_
1|+>+ c_2|->$$ and we g... | The system can be separated, but not necessarily in nice form. For instance, the time derivative of the first eq. reads
$$
i\hbar {\ddot c}_1 = - B {\dot c}_1 - {\dot V}c_2 - V {\dot c}_2
$$
Now remove $c_2$ using again the first eq.,
$$
c_2 = -\frac{i\hbar}{V} {\dot c}_1 - \frac{B}{V} c_1
$$
and ${\dot c}_2$ using t... | {
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Dipole moment of a single point charge Kindly refer to the Multipole Expansion section (chapter 3) of David Griffith's Introduction to Electrodynamics.
Let us first discuss about why multipole expansion is needed. As far as I understand, if we have a extended charge distribution (not at the origin), then very far away ... | Quite simply, the dipole moment of a charged system depends on the coordinate origin. There is nothing particularly surprising or unphysical about this, and there are plenty of other quantities (such as orbital angular momentum) with that property. The dipole moment of a charged system is not a quantity that is defined... | {
"language": "en",
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Work done on an object by the internal forces How is the work done by the internal forces acting in a rigid body zero?
Actually I read in a book an example for the same.
Let me present that example here.
Consider a rigid body having two particles $A$ and $B$. Suppose, the particles move in such a way that the line $AB$... | In a rigid body, the particles remain at their positions irrespective of the body's motion. So Newton's third law is applicable here. Here the two particles exert equal and opposite forces on each other (which you call the internal forces). So the resultant force acting along the line joining the two particles is zero.... | {
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Why does a pot start rotating when coffee is stirred inside? I usually make Turkish coffee as my morning coffee. I have a small somewhat rounded pot with handle on one side. I noticed that when I pour water in and start stirring, pot has a tendency to start rotating in the same direction as I'm stirring.
Why is that?
| It is due to the viscous nature of any liquid. When you stir, the liquid starts spinning and this causes the liquid (the part which is in contact with the pot) to "drag" the pot(due to friction) along with it in the path of its motion.Hope this answers your question.
| {
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Is static friction the only force causing a car to move (without sliding)? A car is moving without sliding means that the friction between wheels and the ground is static friction. This is the force causing an object to move forward, therefore, its direction is the same as the moving direction of the car.
My question i... |
what's the backward force to balance $F_{fs}$ so as to keep the car moving uniformly? Is that the Force produced by engines?
The forwards force comes from the torque produced by the engine of the car and is transferred into the ground via static friction. The retarding force that keeps the car moving at the same spee... | {
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What ds>dQ/T mean? I read the derivation on page 216 over here.
First, it considers an irreversible process between states 1 and 2, followed by a reversible process between states 2 and 1.
From my interpretation, equation 8.31 means that the entropy change of a reversible process is greater than that of an irreversibl... | This is Clausius inequality. The term ds>dQ/T. Holds an impossible reaction which does not obey Clausius statement in 2nd law of thermodynamics. The things in your class thought is about this ds<0. Where you can add c that is entropy generation term. Hope it will help you
| {
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Finding relation between angular acceleration and liner acceleration directed in one direction
In the above given configuration what could be the relation between the angular acceleration of the rod $\alpha$ and the acceleration $a$ of the $2m$ block.
My attempt : What I thought is that the string connected to the ro... | You are almost correct. The linear acceleration of the end of the rod is $\alpha L$ perpendicular to the rod. The component of this acceleration in the direction of the string is $\alpha L\sin A$ where $A$ is the angle which the string makes with the rod (not the angle which the rod makes with the horizontal). So at... | {
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Dynamics of two pulleys I have been trying to do this exercise for 2 days but I'm sure that I'm negleting something...
Here is the sketch of the situation:
$$m_A= 4M \\
m_B= M \\
m_C=2M$$
All the strings cannot be extended and have no mass. The pulleys $c_1$ and $c_2$ have no mass.
The table is rough.
The absolute... | If you take the marked portion of the image then there is a property for pulleys in which forces get magnified.
In your case, the tension on the strings that hold B and C is T
By the property of magnification, The tension U becomes equal to $$U=T+T=2T$$
So the third equation becomes, $$2T-F_a=4Ma$$
Here is another e... | {
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How to derive wave speed/tension relation for the vibrating string? I was studying vibrating strings and in my teacher's notes I found that, generically, if I change the tension on the string by $\Delta T$ then, the speed percentage change can be written as: $\frac{\Delta v}{v}=\frac{1}{2} \frac{\Delta T}{T}$, knowing ... | You can't; it isn't true as you've written it. You can trivially show this by taking some numbers (any numbers at all will do) for $T, \Delta T, v, \mu$ and plugging them in.
What's going on is something else. $\Delta v/v$ can be written as a power series in $\Delta T/T$. What your professor put in the notes is the fir... | {
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Momentum conserving delta-function in the transfer matrix of quantum-field-theoretic scattering theory The $S$-matrix vanishes unless the initial and final states have the same total $4$-momentum, so it is helpful to factor an overall momentum-conserving $\delta$-function:
$$\mathcal{T}=(2\pi)^{4}\delta^{4}(\sum p)\mat... | There is no need of integrating over anything to obtain the 4-momentum conservation. Indeed, if you think in terms of perturbation theory (Feynman diagrams), each vertex conserve momentum, so that the diagram itself automatically conserves momentum.
| {
"language": "en",
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Interacting Hamiltonian commutes with momentum operator? In Peskin's textbook chapter 7 Radiative Corrections: some formal developments (page 212 second paragraph), he describes two-point functions and chooses states to be eigenstates of the full interacting Hamiltonian $H$. Since the momentum operator $P$ commutes wit... | The assumption is that momentum is conserved. So total momentum is a constant of motion. The operator for a constant of motion would commute with the Hamiltonian, because, being a constant of motion, its derivative with respect to time vanishes.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Will the two capacitors be charged in this circuit? Is such a circuit possible to exist with the two capacitors charging?
Because if we considered the outer loop using Kirchhoff's law we get:
ℰ = q1/C1 + q2/C2
But since the two capacitors are initially uncharged, the two terms of the equation increase, that is, q1 and ... | As drawn, the circuit, assuming ideal circuit elements, is problematic for the reason you've deduced (KVL gives a contradiction). One interpretation is that there is infinite large current for an infinitesimal time which instantaneously charges the capacitors to their final steady state voltages.
To gain some insight,... | {
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Difference between radio waves and other electromagnetic waves? Yes radio waves have different frequency and wavelength than others but it seems that radio waves are produced by sinusoidal current while other electromagnetic waves do not need need sinusoidal current to be produced. Is that true ?
| The question appears to be about approaches to generate em radiation: For radio waves making electrons accelerate and de-accelerate in resonant circuits is fine, though by the time you get to microwaves you start to have to look at vacuum tube technology (for example, the klystron and its variants, the magnetron and th... | {
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What is beyond gamma rays and radio waves in the electromagnetic spectrum? The electromagnetic spectrum is commonly referred to as consisting of;
radio-waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays - of increasing frequency from left to right.
But is it possible to get radiation of higher ... | After 6 years you can pretty much answer your question on Wikipedia: Ultra-high-energy gamma rays are relevant in astrophysics and very large radio waves can be used to communicate with submarines.
Ultra-high-energy gamma ray
Extremely low frequency
| {
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Why doesn't orbital body keep going faster and faster? If we consider the change in velocity during an infinitesimal interval of an orbit:
where body B is orbiting body A, we can see that the magnitude of the resultant vector (the green arrow) is greater than the magnitude of the original tangential velocity. Why does... | Any body travelling with increasing velocity increases its kinetic energy $KE$. Since in your system:
$$KE+PE=\text{constant}$$
where $PE$ is potential energy. Therefore increase in $KE$ results into decrease in distance between the objects (so as increase $PE$).
Note: $KE$ is always positive. $PE$ can be positive or n... | {
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "16",
"answer_count": 8,
"answer_id": 0
} |
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