Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
How do photons evade the unavoidable consequence of angular momentum algebra? I think an inescapable consequence of the angular momentum algebra is that a particle with spin-$j$ must have $(2j+1)$ spin projections in any direction. However, photons seem to evade this conclusion. Why?
| The technical answer is that spin is property associated with representation of the Poincare group P. The representations are are induced from the "little group" i.e. the sub-group of P that leaves the particle's four momentum fixed. For a massive particle we can go to the rest frame in which the four momentum is $p=... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/680862",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Microstates of the canonical ensemble In the micro canonical ensemble the microstates of a system in an arbitrary macrostate, are also eigenstates of the Hamiltonian. Does the same apply to the microstates of the canonical ensemble? Are they eigenstates of the the Hamiltonian? I would expect them not to be, since here ... | At the core of the statistical mechanics using ensembles, there is the possibility of assigning a probability to the set of all possible mechanical states of the system (microstates).
Therefore, the starting point is the identification of such microstates. In principle, any complete set of commuting observables could b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/681132",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
No net generation or recombination of electrons is assumed I am currently studying the textbook Physics of Photonic Devices, second edition, by Shun Lien Chuang. Section 2.1.1 Maxwell's Equations in MKS Units says the following:
The well-known Maxwell's equations in MKS (meter, kilogram, and second) units are written ... | The number/concentration of electrons in a volume may be due to their flow into / out of the volume (electric current), or due to the electrons appearing/disappearing inside of it. In vacumm, the latter possibility can be usually safely ignored (although not in QFT), so we have the continuity equation:
$$\nabla\cdot\ma... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/681233",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
If you were invisible, would you also be cold? If you were invisible, would you also be cold? (Since light passes through you, so should thermal radiation.)
Additionally, I'd like to know if you were wearing invisible clothes, would they keep you warm? In my understanding, the heat radiation from the body would pass th... | Thermoregulation
There are four avenues of heat loss: evaporation, convection, conduction, and radiation. If skin temperature is greater than that of the surrounding air temperature, the body can lose heat by convection and conduction. But, if air temperature of the surroundings is greater than that of the skin, the b... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/681335",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "24",
"answer_count": 9,
"answer_id": 8
} |
Can I conclude that acceleration happens a bit later after force is felt? We define forces like electric force, magnetic force and gravitational force etc, to be caused by field lines such as electric field, magnetic field and gravitation field respectively. Since these fields take time to reach the object on which the... | For the idealization of point particles, the acceleration of a particle at a particular time is determined by the force at that same instant in time.
This is also true for the idealization of rigid bodies, as acceleration of one point means instantaneous acceleration (in general) of all points in the body in order to k... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/681734",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Electric field given flux through a plane Suppose you have a hidden, arbitrary, static charge distribution below the plane $z=0$ and you know the electric flux through the plane at every point on that plane. There is no charge above the plane. Is it possible to determine the electric field at every point above that pla... | I believe this should be possible. Is the region above the plane a charge-free region? If so, then you should be able to write down a general solution to Laplace's equation using separation of variables. Then apply boundary conditions at $z=0$, and $r\rightarrow \infty$, basically at the boundary of the upper hemisp... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/681888",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Finding the resonance frequency for forced damped oscillations I have a problem regarding a forced, damped harmonic oscillator, where I'm trying to find the resonance frequency. I have calculated the frequency for free oscillations as $$\omega_{free}=\sqrt{\frac{\kappa}{I}-\left(\frac{b}{2I}\right)^2},$$ where $b$ is t... | Your equations seem to be correct. There are three types of frequencies to consider:
*
*$\omega_0$ is the frequency of undamped oscillations, i.e. when $b = 0$, aka natural frequency
*$\omega_d$ is the frequency of damped oscillations, i.e. when $0<b<2m\omega_0$
*$\omega_r$ is the frequency at which system gain is ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/682059",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 0
} |
What is opposite to $\mathbf{w}_\parallel$ in a FBD of a box on a ramp? I tried doing research on this but to no avail so my question is this:
If the normal force of an object with mass $m$ on a ramp inclined with angle $0<\theta<90^\circ$ is equal and opposite to the component of gravity pulling the object perpendicu... |
As newtons third law says, there has to be an equal and opposite force of Fw.
Indeed, but your interpreting it wrong. Put it this way,
The force on A due to B is equal and opposite to the force on B due to A. The key point being that the action-reaction pairs are forces on different objects. Not the same object. (the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/682236",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why there is no reaction Deuterium + Deuterium $=\rm {}^{4}He$? Why there is no reaction like
$D+D={}^{4}He$
specified here and in other places like this?
Apparently
$2\times2.0141-4.0026=0.0256$
is positive. What is the problem with this reaction?
| As far as I understand the resulting $^4He$ is highly excited and immediately splits into either $^3He$ and a neutron or $^3H$ and a proton.
Probably this is due to the fact that (in the center of mass frame) the resulting $^4He$ is at rest and thus the excess energy cannot be transferred into kinetic energy and thus h... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/682357",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "12",
"answer_count": 2,
"answer_id": 0
} |
Simple oscillator displacement, speed, and acceleration diagram I'm currently studying the textbook Fundamentals of Acoustics (2000) by Kinsler et al. Chapter 1.2 The Simple Oscillator says the following:
$$\dfrac{d^2 x}{dt^2} + \omega_0^2 x = 0 \tag{1.2.5}$$
This is an important linear differential equation whose gen... | Note that in the diagram below the velocity leads the displacement by $\dfrac \pi 2$ which is the same as the velocity lagging the displacement by $\dfrac{3\pi}{2}$.
So when mentioning phase it is important to state which two quantities are being compared, eg $A$ and $B$, and then whether there is a lead or lag betwee... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/683706",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 0
} |
Fitting of experimental data affected by different kinds of errors It is quite easy to evaluate the best-fit curve for a set of n data points when the dependent variable is affected by a statistical error (namely when you have n triplets $(x_i,y_i,\sigma_{y_i})$. I use $\chi^2 $ minimization (with ROOT software, mainly... | Instead of fitting the function $y=f(\vec x)$ for fixed input parameters $\vec x$ a single time you could change the input parameters $\vec x$ according to your uncertainty model and perform multiple fittings. This yields a distribution of the fit coefficients. The distribution captures the uncertainty of your inputs.
... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/683930",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Do Einstein's two postulates allow for handling acceleration in special relativity or is something else needed? When I was taught special relativity, we started with Einstein's two postulates and worked from there. However we were also taught that a proper resolution of the twin paradox required general relativity - b... | In the twin paradox, which is a veridical paradox, meaning that the conclusion is correct even if on the face of it it seems paradoxical, is a consequence of special relativity even if in the natural interpretation accelerations are required and so one might think GR.
As you suggest, both Einstein and Born did think of... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/683964",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 6,
"answer_id": 3
} |
Why does mass bend the temporal dimension more than the spatial dimensions of spacetime? From my (limited) understanding of general relativity, most of what we experience as gravity is a result of the distortion of the temporal dimension, and not the spatial dimensions. Therefore, most of the spacetime curvature caused... | There is actually no such thing as curvature in one dimension, so the premise of the question is based on a misunderstanding. When we talk about curvature in general relativity, we mean intrinsic curvature, such as the curvature of a basketball that can be detected by a bug that never leaves the surface of the basketba... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/684074",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "13",
"answer_count": 4,
"answer_id": 3
} |
Is the electron a pointlike particle? And if yes, how is that possible, because the energy then would diverge, wouldn't it? My problem is that I read (besides others in this post Why are electrons and quarks 0-dimensional?) that the electron is a point-like particle. My question is on the one hand whether that is true ... | This is an excellent question. We know from experiment that the electron behaves like a point charge to all accessible scales, that is no deviation from the potential of a point charge has been observed. This indeed leads to a diverging self energy. Main stream physics has no answer to the question how this is possible... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/684326",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 4
} |
At what speed would a wind affect a bullet? Firing a gun loaded with the fastest bullet (.220 Swift 1,422m/s or any bullet that is super fast and excellent aero dynamics) in a close range (2cm) from the tip of an air blower. What would be the speed of the air coming out of the air blower to be able to deflect the bull... | Remember that the bullet does not know what the wind speed is. The bullet only knows to travel in its given medium. So if the wind was blowing from the side at the same 1,422m/s, then the bullet would travel sideways in that medium at the same rate that it is travelling forwards. In this case, in one second it would... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/684458",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 2,
"answer_id": 1
} |
In Srednicki's book, when calculating loop corrections to the propagator, why doesn't he include both diagram topologies at second order? This might be a somewhat basic question, so apologies in advance for that. I've only recently started learning QFT, and so I'd really like to make sure I understand this. In Srednick... | The second diagram in Fig. 9.6 is a tadpole diagram, which is zero due to the renormalization condition
$$\langle \phi(x)\rangle_{J=0}~=~0 \tag{9.2},$$
cf. e.g. my Phys.SE answer here
or the last paragraph on p. 67 in Srednicki.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/684623",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Is there a name for the type of boundary condition where the initial boundary values are known but are not held constant over time? I'm exploring the heat equation to model a particular 1D scenario, and I understood the Dirichlet and Neumann boundary conditions, but neither are sufficient for my scenario. Assuming a ro... | Well, after consulting my professor, it seems it was a Neumann boundary condition with zero flux at both boundaries ($\phi(0,t) = \phi(L,t) = 0$)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/684914",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Does Newtonian mechanics work in polar coordinates? Our teacher suggested that Newtonian Mechanics only applies in cartesian coordinates. Is this true?
He gave this example.
Suppose there a train moving with constant velocity $\vec{v}=v_0\hat{x}$, with initial position vector $\vec{r}=(0, y_0)$, where $v_0,y_0$ are con... | Newton's laws are vector relations, which are independent on the coordinate systems. It is likely that the OP misinterprets the statement made by the professor. E.g., one of the following could be the case:
*
*That addition of components of vectors in curvilinear coordinates (such as polar coordinates) is not as simp... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/684991",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 10,
"answer_id": 8
} |
Parallel Dp-branes and force While studying basic Dp-brane dynamics from A. Giveon & D. Kutasov reference, on page 24 they state the following
Since Dp-branes are BPS saturated objects, parallel branes do not exert forces on each other.
It is not clear to me why this must be true even if the statement seems obvious. ... | If you have two objects with an associated potential energy $V(x)$, where $x$ is the distance between the objects then the force attracting them is $V'(x)$. If the objects are stationary then there is no kinetic energy. Therefore, if you know that the energy $H = 0$ is a constant that does not depend on the distance be... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/685137",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How come the number of wandering electrons is same as the number of the positive ions? My book mentions the following:
Cause of resistance : When an ion of a metal is formed , its atoms lose electrons from its outer orbit . A metal ( or conductor ) has a large number of wandering electrons and an equal number of fixed... | The paragraph should be read as "The total charge of the wandering electrons equals the total charge of positive ions". The actual number of positive atoms may be less, but that is offset by the any that are doubly or triply ionized.
The important idea is that the conductor is usually electrically neutral, with charge... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/685325",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 3,
"answer_id": 2
} |
Atwood machine with cylinder I was trying to solve the following problem:
Basically we have a normal Atwood machine, the pulley is negligible and we have two masses, a square that falls normally and a cylinder that also spins. The rope goes round the cylinder so the rope doesn't do weird stuff. They both have mass m, ... | Atwood machine with cylinder
constraint equations:
$$x_1+x_2+\frac{\pi}{2}\,r=L_R\\
x_2+r\,x_3=0$$
where $~L_R~$ is the rope length
from here you can apply the Euler-Lagrange with
the above holonomic constraint equations . you obtain the accelerations $~\ddot x_1~,\ddot x_1~,\ddot x_3~$ and the generalized constraint ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/685927",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Are normal force and apparent weight the same? Are normal force and apparent weight the same thing? I'll let ya know the context from which I am asking this question:
Is there a normal force on an object submerged in water?
So, from what I gathered from this question, you CAN treat buoyancy as the normal force in fluid... | The term “normal force” refers to the perpendicular component of the force from a surface. If there is a scale between the surface and the object under study, then the reading on the scale will give the normal force and the apparent weight. If the object is hanging below the scale, there may be no normal force (or th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/686050",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Why are there just 3 main units ($L$,$T$,$M$) in physics? Most physics books define physical units in terms of length, time and mass. Some books add temperature. And yes, the SI unit system has 7 base units, but some are clearly redundant.
Why are exactly three basic units sufficient?
Or to make the point even more dir... | This answer is inspired by arXiv: 0711.4276 [physics.class-ph].
The paper I referred to argues that, in fact, there are only two fundamental units: length, and time. Mass is not necessary. The reasons is because everything we measure are actually space and time intervals, and never really make any other direct measurem... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/686164",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How exactly do vortices generate sound and cause pressure fluctuations to produce sound waves? I wish to understand qualitatively, how vortices generate sound by creating longitudinal sound waves? Vortices are often mentioned as the cause of sound production for things like corrugated whirly tubes, edge tones, etc. But... | This is a non-linear process and do not think that there is a simple explanation or theory. The starting reference is James Lighthill's On Sound Generated Aerodynamically. I. General Theory (the link is to a recent paper citing the original)
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/686315",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Will a planet rotate if it is the only being in the universe? As a senior student , I have been wondering whatever the word inertia mean . Is inertia lying in the interaction between all the objects , or is it the nature of a space even without anything put into it ? In our life it seems like the latter , since whereve... | If the "planet" is not an elementary particle, irreducible, but made of matter with chemical bonds, etc, governed by EM or EW force then the planet is made of a large number of objects each with its own frame of reference and the rotation would be "felt" by each element of the planet as stress from neighboring elements... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/686772",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 7,
"answer_id": 1
} |
Lag in Direction of Earth-Sun Gravity vector When the earth is orbiting around the sun, it experiences a force vector pulling it towards the sun, which acts as a centripetal force for its elliptical orbit. However, when the earth moves a bit from a given position, wouldn't it take time for the information that it's in ... | Newtonian theory is not adequate to answer this question.
The answer from General Relativity for the gravitational problem is like the answer to a similar question in electromagnetism, when one charge experiences the fields due to another charge. In either case it is helpful to focus ones thoughts on two events called ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/686925",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
How to measure the intensity of a pen-type laser beam? I need to measure the intensity of a green pen-type laser, rated at $500\,\mathrm{mW}$, before and after it undergoes Bragg diffraction by a synthetic Opal cube. I've basically zero experience with such measurements.
The first port of call appeared to me an instrum... | If you are sure about the shape of the beam (e.g. Gaussian) it's enough to get a power meter and measure the total power. However, we usually used a so called beam profiler. We build it ourself using a ccd chip and matlab, but "every" optical lab has one, nowadays. So, if you don't have one, why not ask a colleague.
An... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/687056",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
What mechanism will force mechanical watch to tick slower when go fast, due to relativistic effects? To make mechanical watch tick slower, watch tick rate must be changed, oscialtion of balance wheel must be SOMEHOW changed, how would speed change oscialtion of balance wheel, due to relativistic effects?
I dont unders... | Your video shows - very nicely - the balance assembly swinging to and fro due to the balance spring. It has a mass (actually a moment of inertia - look at those weights round the rim) that determines how fast this happens.
Suppose I set it up in a laboratory swinging to and fro once every second.
You observe this while... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/687214",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 3
} |
Does anything in an incandescent bulb actually reach its color temperature (say 2700 K)? This question is inspired by a question about oven lightbulbs over on the DIY stack. It spawned a lengthy comment discussion about whether an incandescent lightbulb with a color temperature of 2500 K actually has a filament at a te... | Other answers are good, but it should be noted that the word "incandescent" actually means that the thing is glowing because (or mostly because) of its temperature. The color temperature of incandescent light bulbs (including halogen bulbs) is by definition not cheating: the filament must actually be that temperature.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/687335",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "56",
"answer_count": 4,
"answer_id": 1
} |
Isn't AdS/CFT an end to String theory as a fundamental theory? I start with the Large $N$ QCD paper by 't Hooft.
When 't Hooft published his paper on Large $N$ QCD it was clear why the string theory of hadrons due to Gabriele Veneziano could make sense. But at the same time, it was an end to strings as fundamental enti... | If string theory were really a UV completion of gravity then it would make a prediction for quantum gravity S-matrix elements when Newton's constant is large. But this cannot be done with the perturbative formulation based on the genus expansion. Rather, the main way people have of computing these large $G$ observables... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/687448",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 1,
"answer_id": 0
} |
Does power consumption vary over time as a projectile travels down the barrel of a rail gun? My assumptions (possibly incorrect) for a rail gun are that are that the mass of the projectile is constant, the current through the armature is constant, and the magnetic field strength along the barrel is constant. Therefore,... | There is a back-emf caused by the motion of the projectile through the magnetic field (Faraday's Law of induction). The back-emf is seen as a voltage across the driving terminals. The voltage is proportional to the projectile velocity (ignoring ohmic and contact losses).
So, although the current may be relatively c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/687550",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How did Ernest Sternglass’ phenomenologically incorrect model of the neutral pion predict its mass and lifetime so accurately? In 1961, Ernest Sternglass published a paper where, using what seems to be to be a combination of relativistic kinematics and Bohr’s old quantisation procedure, he looked at the energy levels o... | I don't know about lifetime, but as for mass (and the following is just speculation), one of the (very rare) decay mode of the neutral pion is to gamma and positronium (P.A. Zyla et al. (Particle Data Group), Prog. Theor. Exp. Phys. 2020, 083C01 (2020), p.38). As the decay is very rare, the relevant phase space volume ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/687820",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "6",
"answer_count": 3,
"answer_id": 1
} |
Why do we need the concept of Gravitational and Electric Potential? I understand that we need potential energy for the concept of energy conservation. However, why would we come up with a definition like 'energy required per unit mass/charge to bring the mass/charge from point A to B. The part says 'per unit mass/charg... | It's handy conceptually because it allows you to think about the cause of PE separately from the effect (ie the PE itself), which in turn makes it easier to model physical rules in a way that's more generally applicable.
For example, an object gains PE if you raise it above the Earth's surface. If you imagine a cliff 1... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/687983",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 1
} |
Force of photons from the Sun hitting a football field = weight of 1 dime? I read, I think, some time ago that the "weight" of photons from the Sun hitting an area the size of a football field at noon on a sunny day would be about the "weight" of a dime?
Would appreciate it someone could flesh that out, verify if corre... | Photons are massless so their weight is 0. However, photons do have momentum so they can exert force. This force is due to their momentum and would occur even in the absence of gravity, so it is not a weight.
The solar irradiance during peak hours is approximately $1000 \mathrm{ \ W \ m^{-2}}$ and the size of a footbal... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/688085",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "33",
"answer_count": 3,
"answer_id": 0
} |
Does a blueshift mean that time goes faster? This is a follow-up question to this answer.
The assumption in this answer is that time dilation always causes a small redshift when an observer looks at an object moving at a significant fraction of the speed of light when not taking into account the shifts caused by the di... |
So, if time going slower always causes a redshift, does that mean that if we see a blueshift it means that time appears to move faster?
Yes.
The machine that produces the wave-crests that appear to follow each other extra rapidly, appears to work extra rapidly.
For example if wave-crests appear to follow each other a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/688163",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Dirac-delta-distribution charge densitity Are the charge distributions $$\rho(\vec{r})=\frac{Q}{2\pi R^2}\delta(r-R)\delta(\vartheta-\pi/2)$$ and $$\rho(\vec{r})=\frac{Q}{2\pi r^2\sin(\vartheta)}\delta(r-R)\delta(\vartheta-\pi/2)$$ of a charged circle the same? I would say yes because integrating over them gives the sa... | Two distributions are defined as being equal if, when integrated with respect to an arbitrary test function, they always yield the same result. In other words, $D_1(r,\theta)$ and $D_2 (r, \theta)$ are equal if for all test functions $f(r, \theta)$, we have
$$
\int D_1(r,\theta) f(r, \theta) = \int D_2(r,\theta) f(r, ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/688479",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Would light bend the other way, if I use antimatter instead? Imagine the following setup: an antimatter straw, an antimatter glass filled with antimatter water and we have antimatter atmosphere just in case. My question is: does Snell's law still apply here as though they are regular matter, if I were to observe the st... | I would say no.
If everything is anti-*, also the refractive index will be. Thus, being both negative the resulting bending of light will be the same.
As an example take an electric field and throw an electron through it, the e- will be deflected in one direction.
Now, if you take the anti- of everything the E-filed wi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/688618",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Electric field of an electron in motion in a wire How do I correctly model the electric field of an electron in motion in a wire? I could treat the electron as a point charge moving through the wire. If I use the Liénard Wichert equations, they will predict radiation if the wire turns, since the electron is being accel... | If it's an Ohmic material $\vec{J}=\sigma \vec{E}$. Current density is proportional to electric field.
$J=I/A, E=V/l$ Where $I$ is current $A$ is cross sectional area, $V$ is voltage between endpoints of the wire, and $l$ is length of the wire.
From here we get $\sigma =\frac{Il}{AV}$
Dimensional analysis tells us th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/688744",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Why is the size of image increasing as observer moves away from lens? I was using a convex lens and placed the object at principal axis at a distance from optical center lesser than focal length (between $F_1$ and optical center). Then I started observing the size of the image from other side of lens.
At first I had pl... | It's really not easy to judge the absolute angular size of an object (see Moon illusion). The image you see may get larger relative to the lens frame, but a bit smaller in angular size due to perspective.
In any case, with a perfect lens you're watching the virtual image at a fixed distance behind the lens, farther fro... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/689407",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
Interference of standing waves inside black body? Does electromagnetic wave inside a cavity (modeling black body ) interfere with each other?
And why in the derivation of Rayleigh law of black body radiation we add energy of different modes (are we supposing constructive interference of the modes inside the cavity)?
| It can be shown that the energy of an electromagnetic field is the sum of energies of its modes. E.g., in a free space, although the waves in different modes do interfere, they all have different frequencies and wave vectors, so that the cross-contributions cancel out (in other words, the EM modes are orthogonal).
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/689792",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Phase difference between two waves in opposite directions Suppose I have two waves travelling along the positive and negative $x$ axis, and are given by : $$y_1=A\sin(kx-\omega t)\,\,\,\,,\,\,\,y_2=A\sin(kx+\omega t)$$
What would be the phase difference between these two waves at a particular point ?
If I define the ph... | Phase difference as a constant, independent on time, can be defined only between two waves with the same wave vector and frequency, which is not the case in the example given in the OP, where the waves propagate in the opposite directions.
More generally, the phase difference is defined between two points in space and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/689913",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
How does potential energy increase with no work? If you're dragging an object up a hill at a constant velocity, work is technically 0 (as acceleration is 0), but potential energy constantly increases. How would you represent this situation mathematically, and how does the potential energy increase despite a lack of wor... | The work-energy theorem should always be your starting point:
$$\boxed{K_1 + W = K_2} \quad \text{or} \quad \boxed{\Delta K = W}$$
You should read this as:
Change in object's kinetic energy equals total work done on the object.
Total work means work done by all forces! If change in kinetic energy is zero, that means ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/690037",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 1
} |
Functional Calculus in QFT Does anybody know some good sources with detailed derivations of the main results we need to compute generating functionals in QFT (and functional calculus used in the subject in general). I find that in mainstream books, such as Peskin and Schroeder Chapter 9, the details are glanced over, a... | While this might sound a bit odd at first, I particularly recommend Nivaldo Lemos' book Analytical Mechanics.
In Chapter 10, it deals with classical field theory, and in order to do so the author chooses to deal with a bit of functional calculus. When doing so,he provides some more detail on the definition of functiona... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/690230",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Conformal Ward identities for local conformal algebra: error in textbook? In Schottenloher's mathematically-oriented CFT textbook, "A Mathematical Introduction to Conformal Field Theory," Proposition 9.8 on page 160 states the conformal Ward identities for 2D CFTs as follows: For all $m \in \mathbb{Z}$, for any primary... | The equation you quote cannot hold for any integer $m$. A function of finitely many variables cannot obey infinitely many independent PDEs!
There are infinitely many local Ward identities but they involve Virasoro descendant fields. See for example Eq. (2.2.15) in my review article: https://arxiv.org/abs/1406.4290
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/690661",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Elastic potential energy and work equations Elastic potential energy is $\frac{1}{2} k x^2$ and work is $F \cdot d$. Why these numbers do not evaluate to the same value in a problem?
The change in potential energy is the work done on a spring - $W = \Delta U$. However, every time I do an example I always get that the w... |
so if potential energy in a spring is 1/2kx^2 and work f*d. Why do
these numbers not come out to the same thing in a problem?
You have to start out with the general definition of work, which is not simply force times displacement, but is
$$W=\int\vec F.d\vec x$$
It only equals $Fx$ if the force it constant and can co... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/690807",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 3,
"answer_id": 0
} |
Relationship between angular and translational velocity on inclined surface I have been researching about rolling motion and I was calculating a way to predict the translational velocity of the object at the bottom of the incline. I know that the kinetic energy of a cylinder undergoing rolling motion is given as
$$E_k ... | You can say that $\omega = v/r$ — but only for the cylinder, not for the water. The cylinder will be rolling without slipping, and so you can only the rotational kinetic energy equation for the cylinder.
The water inside the cylinder will be executing a different sort of motion. The simplest assumption is that the wa... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/691151",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Do counter rotating galaxies have dark matter? Have counter rotating dark matter galaxies been observed?
Counter rotating galaxies, you may already know, are galaxies where some stars or arms rotate in one direction and other stars or arms rotate in an opposite direction, possibly due to the merger of two or more galax... | As you probably know, the presence of dark matter in galaxies can be assumed true due to the analysis of the velocity curves. In 1970, Freeman determined the velocity profiles of galaxies using the 21 cm line and he found that for NGC300 and M33 there should have been much more gravitational mass outside the last brigh... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/691456",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Are the electron and hole densities always equal in an intrinsic semiconductor? In my laser physics book (Laser physics by Hooker and Webb) it is stated that the density of electrons injected into the intrinsic active region of a diode laser is equal to the density of holes. I am not convinced because the electrons do ... | In general, the densities of electrons and holes in the depletion region of a pn junction are not equal, but governed by relation
$$pn=p_B n_B e^{-\varphi_B/V_{th}},$$
where $n_B, p_B$ are bulk carrier densities, $\varphi_B$ is the barrier height and $V_{th}=kB T/q$ (see here). This relation is further modified when th... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/691866",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
How Would a Car Move in Zero-G? Consider a car floating in a microgravity environment. Assuming the engine can still function (i.e. it is surrounded by normal atmosphere; fuel can still be pumped, etc.), in what ways (if any) will the car move when the accelerator is pressed?
There is air moving into the intake and out... |
in what ways (if any) will the car move when the accelerator is
pressed?
For conservation of momentum, the momentum of the exhaust gases rearward out the tail pipe must be equal to and opposite to the momentum of the car forwards. So in theory the car can accelerate in the same manner as a rocket in space, though the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/691962",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 0
} |
Question about the Wave equation I have a question. I was looking for the Wave equation (first Eq. of this wikipedia page).
I saw for the first time a version of this equation during an Acoustic course, where we obtained it for the sound wave combining the Euler equation, the Continuity equation, the general gas equati... | There is no unique answer to this question.
Domain-specific derivations
In electromagnetism the wave equation arises from the Maxwell equations. In elasticity or hydrodynamics it arises from the correspondinge quations for the media. Note that in these latter cases the wave equation is actually an approximation - more ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/692062",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 4,
"answer_id": 1
} |
What is causing this sign difference in the centrifugal term between Lagrangian and Hamiltonian formalism? Consider a central force problem of the form with the Lagrangian
$$
L(r, \theta, \dot{r}, \dot{\theta}) = \frac{1}{2} m \left( \dot{r}^2 + r^2 \dot{\theta}^2 \right) - V(r),
$$
where $r = |\vec{x}|$. Since $\theta... | Just to give a bit of further intuition, what you have done is added a quantity which you expect to be zero “on shell” to the Lagrangian, in order to try to simplify its form. But the Lagrangian is being used in a sort of minimization procedure, we are trying to find a trajectory where nearby trajectories do not change... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/692349",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
On an infinite plane, with gravity the same of that of Earth, how far could light at an arbitrary angle travel until bending to hit the plane? Now, I'm a complete idiot, so bear with me.
I've recently come across the idea that standing an infinite flat Earth would in theory appear the same as standing inside a hollow e... | Keep in mind that if you shine the light horizontally on this long flat planet, the light will hit the ground at exactly the same instant as if you dropped a stone on the ground, or if you shot a bullet horizontally. They will all hit the ground simultaneously. This is because gravity has nothing to do with the fallin... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/692546",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 1
} |
What Lorentz symmetries do electric and magnetic fields break? When we turn on an external (non-dynamical) electric or magnetic field in (3+1)-dimensional Minkowski space we break rotational invariance because they pick out a special direction in spacetime. Does this also break boost invariance?
What about in (2+1)-dim... | The electromagnetic field is a bivector field. The components called $E_x,E_y,E_z$ are the $tx,ty,tz$ bivector components, and $B_x,B_y,B_z$ are the $yz,zx,xy$ components.
A component of a bivector is unchanged by a rotation in its plane or in any perpendicular plane (perpendicular in the sense that every vector lying ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/692681",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 1
} |
Adding extra gas to a high velocity burner but the temperature in the vessel won't rise So I'm doing a refractory dryout of a vessel and I'm stuck at 350°C. Normally I would add some more gas to the burner and the temperature would go up, but now it just stays at the same level. I've tried to add some more air to the s... | I would guess incomplete combustion. To check this add more gas and check the amount of flame that goes in exchaust, or try to ignite exchaust.
With incomplete combustion it doesnt help to add more air or gas. To solve it you need to improve mixing.
If you are using pressurised gas, it usually does air mixing in a vent... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/692848",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why is the probability of a collision $\frac{dt}{\tau}$ I'm reading about Drude theory in the book Solid State Physics by Ashcroft and Mermin. This book and most other sources I can find simply state that the probability $P$ of an electron "collision" in a time $dt$ is just $$P=\frac{dt}{\tau}$$ where $\tau$ is the rel... | I think I found the answer to my own question.
$P=\frac{1}{\tau}dt$ is a function for computing probability values. Although the function definition has the form of a ratio, it is not a probability value itself, so it doesn't have the form (desired outcomes) / (all outcomes).
$P=\frac{1}{\tau}dt$ is essentially the cum... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/692976",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Can we derive Boyle's law out of nothing? My textbook states Boyle's law without a proof. I saw Feynman's proof of it but found it to be too handwavy and at the same time it uses Boltzmann's equipartition theorem from statistical mechanics which is too difficult for me now.
So to state roughly what Boyle's law is, it s... | Yes, it's almost all very intuitive.
Let's figure out the force $F$ that the gas applies to a (flat) section of its container wall, with area $A$.
You know that $F = ma = \frac{dmv}{dt} = \frac{dp}{dt}$, that is the rate at which momentum is transferred from the wall to molecules of the gas.
This is of course proportio... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/693255",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
"answer_count": 5,
"answer_id": 1
} |
Why is amplitude going to infinity in forced damped oscillator at resonance? I'm trying to find the amplitude of steady state response of the following differential equation:
$$\ddot{x}+2p\dot x + {\omega_0}^2x=\cos(\omega t)$$
A particular solution is
$$x_p=\Re{\dfrac{e^{i\omega t}}{\omega_0^2 - \omega^2 + i2p\omega}... | You algebra is wrong. If $\omega^2-\omega_0^2=-2p^2$ you get $(-2p^2)^2+(2 p\omega )^2= (2p^2)^2+(2 p\omega )^2$= under the square root and, being the sum of two postive numbers, this can never be zero.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/693365",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 0
} |
How Do Pions Mediate The Residual Strong Force? I know that the continuous exchange of gluons between quarks is what holds hadrons together, and that the exchange of pions between nucleons is what creates the strong residual force. However, how exactly does the pion mediate the residual strong force--is it emitted by o... | Here is how I visualize this process. Please tell me if it is incorrect and I will delete:
we imagine two nucleons sitting right next to one another. Note that a pion has two quarks inside it and the nucleons have three each. A quark that just happens to come close to the "surface" of its nucleon at the same time anoth... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/693669",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Electric field of a point very far from uniformly charged rectangle sheet I was wondering what is the Electric field at a point which is very far from a rectangular sheet and it is also above the center of the rectangle. So form a mathematical perspective you get Electric field due to a finite rectangular sheet of char... | For $a \ll r$ and $b \ll r$ the argument of the arctan function (call it $x \equiv ab/4r^2$) is much less than 1. And for $x \ll 1$, we have $\arctan x \approx x$.
Take it from there.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/693777",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 2,
"answer_id": 1
} |
Is there a physically meaningful example of a spacetime scalar potential? From Misner, Thorne and Wheeler, page 115.
0-Form or Scalar, $f$
An example in the context of 3-space and Newtonian physics is temperature $T\left(x,y,z\right),$ and in the context of spacetime, a scalar potential, $\phi\left(t,x,y,z\right).$
I... | There are two different things one can mean by a potential.
The first is in the sense of a gauge field whose derivatives (in some combination) give the field-strength tensor. For example, the electromagnetic potential $A^\mu(t,\vec{x})$ as you mention. One can certainly write down a theory with a scalar gauge field $B(... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/693884",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 3,
"answer_id": 2
} |
Can two waves be considered in phase if the phase angle is a multiple of 2$\pi$? Question is essentially what the title states. Wavefront is defined as the locus of points that are in phase. So I wanted to know if the locus would be the points of only a single circle or multiple circles whose points all have the same d... |
Can two waves be considered in phase if the phase angle is a multiple of 2π ?
Yes. Common cases in modulus arithmetic :
$$ \begin{align}
\phi \mod 360^{\circ} &= 0^{\circ} \to \text{in phase}\\
\phi \mod 360^{\circ} &= 180^{\circ} \to \text{out of phase}\\
\phi \mod 360^{\circ} &= 90^{\circ} \to \text{neither in p... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/694127",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 3
} |
How to make the Moon spiral into Earth? I recently watched a video of what would happen if the Moon spiraled into Earth. But the video is pretty sketchy on the physics of just what would have to happen for that to occur. At first I thought I understood (just slow the Moon down enough), but my rudimentary orbital mechan... | The Earth-moon binary system is emitting gravitational waves, and so is steadily losing energy and the separation between them is decreasing. Eventually, (ignoring the death of the Sun, heat death of the Universe, the possibility of proton decay or the decay of our vacuum state, etc etc), the two bodies will collide.
W... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/694535",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 8,
"answer_id": 4
} |
Why doesn't time contract? I'm tutoring a Year 12 (high school) physics subject which requires me to understand special relativity, in particular, time dilation and length contraction. I have only studied 1 semester of 1st year uni physics, so bear with me for sounding ignorant. I've tried reading parts of Physics for ... | Instead of two observers, it is more clear to think of one observer travelling between two points in the same frame, and the clocks of that frames were previously synchronized. For example, a rocket between Earth and a (future) Mars base.
If the rocket has a really big velocity, the crew will see at arrival, that the t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/694667",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 3
} |
Time constant versus half-life — when to use which? In some systems we use half-life (like in radioactivity) which gives us time until a quantity changes by 50% — while in other instances (like in RC circuits) we use time constants. In both cases the rate of change of a variable over time is proportional to the instan... | Even for radioactive systems the usage can be mixed. Isotopes are reported as half-lives, but individual nucleons or fundamental particles are often reported as lifetimes.
See for example https://pdg.lbl.gov/2021/web/viewer.html?file=../tables/rpp2021-sum-leptons.pdf where the muon and the tau leptons have their decay ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/694850",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "7",
"answer_count": 5,
"answer_id": 4
} |
How to prove time dilatation from the Lorentz transform? How to prove time dilatation from the Lorentz transform formula:
$$ t' = \gamma\left(t-\frac{Ux}{c^2}\right) $$ (U: the velocity of the referential R' relative to R)
So far I've found this formula :
$$ \Delta t' = \gamma\left(\Delta t-\frac{U\Delta x}{c^2}\right)... | In the formula$$\Delta t'=\gamma\left(\Delta t-\frac{U\Delta x}{c^{2}}\right)$$
we suppose that two events occur in the same place, i.e.$\;\Delta x=0$
we find in the moving reference frame $\mathcal{R}'$ $$\Delta t'=\gamma\Delta t$$
The inverse Lorentz transformation gives $$\Delta t=\gamma\left(\Delta t'+\frac{U\Delta... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/695031",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Stark Effect in Hydrogen Degenerate Perturbation Theory I am going though this example of degenerate perturbation theory.
We are examining the Stark effect in hydrogen for $n=2$. After finding the 4 degenerate cases; $|0, 0⟩, |1,0⟩, |1,1⟩, |1,-1⟩$, we apply the perturbation $\hat{V} = eEr\cos{\theta}$. The matrix repre... | Your matrix contains a (degenerate) subspace spanned by $\vert 1,1\rangle$ and $\vert 1,-1\rangle$ by simple inspection of the original ordering of the basis states.
The similarity transformation $T$ that will bring $\hat V$ to diagonal form $T^{-1}VT$ will only mix $\vert 0,0\rangle$ and $\vert 1,0\rangle$, again by i... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/695137",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
What is the acceleration of a ramp on a table when a body slides on it? I found an Olympiad problem:
Find the acceleration of a ramp on a table when a body slides on it. Assume there is no friction between the body and the ramp, and between the ramp and the table.
I found the final solution to this problem but I do n... | There is no $ma$ in the diagram or in the text. There is the term $Ma$.
You have two bodies in the problem:
*
*the block on the incline, with mass m and acceleration $a_1$ and
*the inclined plane with mass M and acceleration a.
You already have the free body diagrams for each of the two objects.
The inclined plane c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/695249",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Derivative of operator with respect to parameters From Shankar's QM book pg. 56:
For an operator $\theta(\lambda)$ that depends on a parameter $\lambda$ defined by $$\theta(\lambda)=e^{\lambda\Omega}$$
where $\Omega$ is also a constant operator, we can show that $$\frac{d}{d\lambda}\theta(\lambda)= e^{\lambda\Omega}\Om... | For essentially the same reason that it appears in differential equations of functions. The differential equation
$$\frac{\text{d}\theta(t)}{\text{d}\lambda} = \theta(\lambda) \Omega$$
defines a family of operators, given by $$\theta(\lambda) = C e^{\lambda \Omega}.$$ Different choices of the constant operator $C$ lead... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/695526",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why should a clock be "accurate"? Having read that atomic clocks are more accurate than mechanical clocks as they lose a second only in millions of years, I wonder why it is necessary for a reference clock to worry about this, if the definition of the second itself is a function of the number of ticks the clock makes.
... | Time doesn't flow, nor is it perceived, according to the ticking of a clock. If you boil an egg while watching a clock that runs slow, you're going to overcook it, regardless of the fact that the clock says you cooked it for exactly the intended duration. "Boil an egg for 10 minutes" is not a useful instruction if the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/695789",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "27",
"answer_count": 19,
"answer_id": 1
} |
How can a boys mass equal more than a seesaw plank when it is balanced? I had a question that says a boy plays solitary seesaw by placing a long plank over a small rock and sitting at one end of the plank. When the seesaw is balanced the boys mass is likely… the answer is greater than the mass of the seesaw. I don’t un... | Since torque is $\text{weight}\times \text{distance}$ we can write $$W_b\times l=W_p\times L$$ would be the condition when the seesaw is balanced, and $W_b$ is the weight of part of the plank and boy on one side, and $W_p$ is the weight of the plank on the other side, and $l$ and $L$ are the lengths of the plank from t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/695840",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
How can a photon be partially absorbed an electron? I am familiar that the there is a similar question , but elaborating my question " In the photoelectric effect the electron absorbs only a part of the photon 's energy which is needed for liberating the electron and the rest is used as the kinetic energy , why or how ... | The basic reason is because the photoelectric effect happens in metals,and depends on the electron solutions within metalic solids .
In the band theory of solids, a quantum model useful in studying solid state physics, electrons are bound in bands.
In metals, there are electrons in the conduction band, which are consi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/696070",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
In radiotherapy, why do normal tissiue or organ cells not die of radiation? In radiotherapy, why don't normal tissiue cells or organ cells in the way of incoming radiation die, but tumours die instead?
| Living cells which are in the process of actively dividing i.e., replicating DNA strands, peeling them apart and sorting them out, rebuilding them into duplicate genes, and so on are particularly susceptible to any sort of challenge which might create transcription errors- most of which would lead to the death of the c... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/696333",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 6,
"answer_id": 0
} |
Is the momentum in the infinite square well observable or not? I've read in posts such as this and this that the momentum operator is not self-adjoint in the infinite square well because the geometric space is a bounded region of $\mathbb R$, for example $[0,a]$ for a well of width $a$. As such, it leads to weird stuff... | It is not necessary to extend the wavefunctions to the whole real line. As far as I can understand, you are defining an operator $\hat{p}$ in $L^2([0,a], dx)$ with the domain $D(\hat{p}) := \{ \psi\in C^1([0,a])\:|\: \psi(a) = \psi(0)=0\}$ and acting in that way
$$(\hat{p}\psi)(x):= -i\hbar \psi'(x)\:.$$
As you notice,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/696618",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
On "cosmic flux" units I've trouble understanding the following graph, taken from Wikipedia:
It's supposed to show the cosmic ray flux vs particle energy. I've never seen a "flux" written in these units...
Why ${GeV}^{-1}$?
| It is a differential flux. If you wanted to know the total particle rate per unit area, per unit solid angle, per unit time, with units $\rm m^{-2}\,sr^{-1}\,s^{-1}$, you would have to choose an energy interval that you care about and integrate the curve in the figure.
Some authors would write a monstrous differential... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/696760",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Why does the north pole of a magnet always induce a south pole in the near end of a magnetic material? If a magnetic material such as an iron bar is placed near a magnet, the former becomes an induced magnet with the end closest to the north pole of the magnet becoming a south pole and vice-versa for the other end. But... | Electrons are not only electric charges, they are also magnetic dipoles. For the electron see its intrinsic value here). BTW, it does not matter at all for the explanation of the magnetic properties of the magnetic materials whether the observation of the magnetic dipole of the electron is considered as an intrinsic pr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/696872",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Narrow-bandwidth laser and its beam size on uncertainty principle I read that a single frequency laser can have a bandwidth as low as a few kHz, but according to the uncertainty principle, $\Delta x \Delta p = \Delta x \Delta f h/c >=\hbar $, so $\Delta x \sim c/\Delta f$, how come the laser beam can be so narrow spa... | The $\Delta x$ relevant for your calculation is the longitudinal length of the wave. If you have a narrow bandwidth, then you need a lot of wave cycles to define it, and so the wave is long.
The transverse width of a laser beam is limited by diffraction.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/697093",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Are black holes spinning balls of quark-gluon plasma? I had this idea a few days ago that the Higgs event might have been a naked singularity, i.e. the colliding protons (very briefly) fall into a state of infinite density and release two gamma-ray photons as decay products. One thing led to another, and I was led to e... | 1/2 spin particles like quarks are subject to the Pauli exclusion principle. Therefore they cannot contract down to a singularity. However gluons are spin 1 particles and therefore not subject to the Pauli exclusion principle. Thus it could be possible that the singularity in a black hole is made solely of gluons. Phys... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/697216",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Is there a name for this sort of thermo relationship? taking physical chemistry at the moment.
My textbook does not go over the derivation of the relationship below:
$$H_{vap\;/\;sub\;/\;cond}(T')-H_{vap\;/\;sub\;/\;cond}(T)= \int_{T}^{T'}\Delta C_{p,m}\; dT$$
Where $T$ is the enthalpy value at standard conditions, and... | The relation arises from integrating the general partial-derivative expansion
$$dH=\left(\frac{dH}{dT}\right)_PdT+\left(\frac{dH}{dP}\right)_TdP,$$
or—replacing the partial derivatives with the corresponding material properties—
$$dH=C_P\,dT+V(1-αT)dP,$$
with constant-pressure heat capacity $C_P$, temperature $T$, bulk... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/697336",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Is $\Delta$ notation commonly used for the difference in a quantity between two objects? In a standard Atwood machine, the acceleration is
$$a = g\dfrac{m_1 - m_2}{m_1+m_2}.$$
Would writing this as
$$a = g \dfrac{\Delta m}{M}$$
where $M$ is the total mass be an abuse of notation to most physicists?
Alternatively, suppo... | This use is extremely common in thermodynamics, at least. In the Clasius–Clapeyron equation, for instance, $\Delta$ is used to represent simultaneous differences between certain properties of two phases. It would be standard practice to use your $\Delta T$ example to calculate, say, the entropy generation associated wi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/697539",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Why is a relativistic calculation needed unless $pc$ is much smaller than the rest energy of a particle? After introducing the de Broglie wavelength equation, my textbook gives a rather simple example where it asks to find the kinetic energy of a proton whose de Broglie wavelength is 1 fm. In the solution to this probl... | The momentum of a particle of mass $m$ moving at $v$ is $\gamma m v$ where $\gamma$ is given by the following.
$$\gamma = \frac{1}{ \sqrt{ 1 - \frac{v^2}{c^2} } }$$
Let's always take $v$ to be non-negative. If you need a direction then put it in later. Since $v <= c $, $\gamma$ is a real number greater than or equal to... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/697874",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
Uncertainty principle manifesting in $j(j+1)$ vs $j^2$ To motivate my question, please consider a system with total angular momentum $j$. The fact that the largest eigenvalue of $J_z$ is $j$, while $J^2 = J_x^2 + J_y^2 + J_z^2$ has all eigenvalues equal to $j(j+1)$ is often ascribed to the uncertainty principle.
For ex... | The following two operators $A$ and $B$ have the properties that:
*
*$A$ and $B$ do not commute;
*$A^2 + B^2$ commutes with both $A$ and $B$; and
*the largest eigenvalue of $A^2$ equals the largest eigenvalue of $A^2 + B^2$:
$$
A = \begin{bmatrix} \lambda & 0 & 0 \\ 0 & 0 & 1/\sqrt{2} \\ 0 & 1/\sqrt{2} & 0 \end{bm... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/698178",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 1,
"answer_id": 0
} |
Can two passing black holes touch without merging? I've read a report about LIGO saying that the ring-down oscillation showed that it took time for the pair of BHs to fully merge, implying there was a momentary bulge as one circled around and into the other.
I imagine two black holes going past each other, but coming c... | Once something crosses a black hole event horizon, it can never escape. This means that if the event horizons of two black holes touch or overlap, they will merge.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/698304",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
Door Slamming Against Wall There is a door with length $r$ and mass $m$ hinged on a wall with angle $\theta$, and an equally distributed gust of wind with constant velocity $v$ pushing on that door in the direction normal to the wall, as shown in the drawing:
I wish to find a function of $\theta$ by time. This is what... |
Take the sum of the torque about the point A you obtain
$$I_A\,\ddot\theta-\frac 12 F\,r=0\tag 1$$
where
$$I_A=I_{\text{CM}}+m\,\left(\frac r2\right)^2\\
F=v^2\,\cos^2(\theta)\,\rho\,A=v^2\,\cos^2(\theta)\,\rho\,r\,b$$
*
*I is the door Inertia
*b is the door width
*CM center of mass
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/698594",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 1
} |
Constraint rate change problem Two small rings O and O' are put on two vertical stationary rods AB and A'B'
, respectively. One end of the inextensible threads tied at point A'. The thread passes through ring O' and it's other end is tied to ring O. Assuming that ring O' moves downwards at constant velocity $ v_1$, det... | I dont think dy/dt is the velocity $V_2$ as you have taken y from O to a point below A (say it C where you have drawn the line d). Here even AC changes , so only dy/dt won't account for velocity of O . AC+CO=AO.
$$d(AC)/dt + dy/dt =V_2$$ where y=CO.
This is more of a comment rather than an answer.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/698739",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
SAXS vs. X-ray diffraction? Both small-angle X-ray scattering and X-ray diffraction can be used to obtain structure factors, though I imagine the wave vectors accessible to each are different (?). What are the main differences between both, and why are structure factors obtained using the former technique often plotted... | Small-angle X-ray is a device to investigate the structure near the surface (1-100 nano-meters). The x-ray is a high penetrating measurement for the structure of whole crystal structure. Therefore, looking into the surface structure, we make the x-ray a glancing incidence angle (0.1 - 5 degree).
For a glancing angle x-... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/698843",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
In a capacitor, is there energy in the electric field, is there potential energy, or both? The electric field between two capacitor plates is very simple.
$$
\vec{E} = \frac{Q}{\epsilon_0 A} \vec{e}_z
$$
I can get the energy stored in the field by integrating the energy density, $u_e$, over the volume (between the pl... | As Griffiths has said. It is simply a matter of book keeping on whether or not you would like to say that the collection of charges has an associated potential energy to it. Or you would like to say that the E field possesses some energy density. It is the exact same thing, and yes 2 different ways of describing it! Lo... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/699033",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 2,
"answer_id": 0
} |
Necessary and sufficient conditions for operator on $\mathbb C^2$ to be a density matrix Consider a one-qubit system with Hilbert space $\mathscr H\simeq \mathbb C^2$.
Define the hermitian operator
$$\rho := \alpha\, \sigma_0 + \sum\limits_{i=1}^3 \beta_i\, \sigma_i \quad , \tag{1}$$
where $\alpha,\beta_i \in \mathbb ... | A very simple derivation, without using the specific form of the Pauli matrices, can be obtained if one uses that the vector of Pauli matrices transforms as a $\mathrm{SO}(3)$ rotation under the adjoint action of $\mathrm{SU}(2)$ -- i.e, one has that
$$
U (\vec r\cdot \vec\sigma) U^\dagger = (R_U\vec r)\cdot \vec\sigma... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/699132",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "5",
"answer_count": 2,
"answer_id": 1
} |
Will a clock that is isolated and stationary with respect to the CMB report the highest possible value for the age of the universe? We have a very special clock that has existed since the dawn of time. Its purpose is to measure the age of the universe. It is always very far from any massive body or gravitational field... | The fastest clock will be one that is at the center of a large void in the universe. This will be slightly faster than a "standard" clock embedded in an extended region of average mass density which will suffer more gravitational time dilation. (this answer just formalizes the exchange of comments with @PM 2Ring)
Assum... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/699312",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Work of a spaceship in circular motion Say a spaceship is traveling though space in a uniform circular motion.
It's not orbiting any planet, it just flies in circles in an empty space.
The only force working on the spaceship would be the centripetal force caused by the ship's engine.
Thus, the work would be $0$, as the... | Without a force the spaceship would be floating in straight line at constant velocity. The reason why work is zero and the object is accelerated comes from its very definition.
From the second Newton's law:
$$\mathbf F = m\frac{d\mathbf v}{dt}$$
Making a dot product with an infinitesimal displacement:
$$\mathbf {F.dr} ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/699425",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 4,
"answer_id": 1
} |
AdS$_4$ and $\mathbb{H}^4$: What is the difference between them? This figure (source) shows the embedding of 4D hyperbolic space $\mathbb{H}^4$ and 4D de Sitter space dS$_4$ in 5D Minkowski space $\mathbb{M}^5$. $\mathbb{H}^4$ is a hyperboloid of two sheets and dS$_4$ is a hyperboloid of one sheet. However, I also un... | $\text{AdS}_n$ is a sphere of timelike radius in a space of two timelike and $n-1$ spacelike dimensions.* $\text{AdS}_n$ itself has one timelike dimension.
For comparison:
$\mathbb H^n$ is a sphere of timelike radius in a space of one timelike and $n$ spacelike dimensions,** and has zero timelike dimensions itself;
$\t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/699579",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
The kinetic energy of a rotating globe I can find formulas for the kinetic energy of a globe (ball) in motion but not for just rotating. Anyone has the formula to calculate the kinetic energy of a rotating globe?
| The total kinetic energy of a body is sum of translational and rotational kinetic energies
$$K = \frac{1}{2} m v^2 + \frac{1}{2} I \omega^2$$
where $v$ is translational speed of center of mass, $\omega$ is rotational velocity, and $I$ is moment of inertia about axis of rotation. Moment of inertia of a homogeneous solid... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/699787",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Notation for rule of thumb, without breaking dimensional homogeneity? I'd like to know how to write rules of thumb in a concise way, without breaking dimensional homogeneity.
For example, if a runner has an average speed of ~10 km / h, an approximation of the covered distance would be
$\mathrm{distance} \approx \mathrm... | My advice:
*
*Have a symbol for each quantity, including coefficients such as your distance-to-duration ratio; the symbols should represent the quantities, not what they become when nondimensionalized on division by a unit.
*State coefficients' values, where known, in separate equations.
*Trust your reader to remem... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/699915",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 1
} |
Thrusters in space Suppose we propel ourself using thrusters in space. If we have two thrusters pushing against each other like this:
Nothing will happen. We can imagine the "strain" on the vehicle chassis; the forces are not working. All the thruster's fuel is wasted.
Now, if we have two thrusters like that:
The res... |
Consequently, does it mean that ideally we would like to orient the thrusters toward the direction we want to push ?
Assuming a spherical cow (which is a big assumption), your conclusion is correct because there are vector components of the the two thrusters that oppose each other and cancel out.
In practice, you nee... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/700008",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
Data transmission through optical fiber and copper wire I read the following in one reference: A copper wire (twisted pair), the link traditionally associated with low bit rate transmission, is still in use in the modern data centers transmitting data at 20 Gbit/sec. The secret? it does so only over a few meters (the b... | Optical fibers offers best transmission around 1310-1550nm or 850nm of wavelength spectrum depending on type of fiber.
for lower frequencies, the attenuation reaches beyond 10dB per Km.
However there have been increasing trend of transmission of radio waves over fiber through radio over fiber (ROF) in certain conditio... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/700158",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Force between two protons Yesterday my teacher was teaching about the production of photons, he told that photons are produced when the electron move from a higher energy level to a lower energy level then suddenly a idea struck in my mind that if electrons are responsible for photons and photons are responsible for el... | Electrons can make photons, but they aren't the only particles that can make photons. Any particle with electric charge can emit or absorb photons.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/700557",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 5,
"answer_id": 1
} |
Why doesn't the non-degeneracy definition of the metric tensor assure $g(v,v)=0\implies v=0$? We know that a defining property of the metric tensor is that it is non-degenerate, meaning $\forall u,\, g(v,u)=0\implies v=0$. Yet from a textbook I read that $g(v,v)=0$ does not assure $v=0$. Why is this? Can't we simply le... | On Lorentzian manifolds there is an obvious counter example to your claim, namely null-vectors. Let
$$g = \begin{pmatrix}-1 & 0 \\ 0 & 1\end{pmatrix}$$
be the Minkowski metric in 2D. Consider
$$v= \begin{pmatrix}-1 \\ 1 \end{pmatrix}$$
We see that $g(v,v)=0$ although $v \neq 0$. Therefore, your implication is false.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/700726",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 3,
"answer_id": 0
} |
Difference between the wave forms in the water and in the Young double slit experiment We can observe when we cause a slight disturbance at two points on the water surface which is intially totally undisturbed , it will form water waves which would look like as shown in below image:
we can observe that there are const... | Did you know that a water wave in a single slit will NOT make an interference pattern ... but light will! Water and light are similar but also very different. In the DSE there is NO light in the dark areas .... photons will travel to areas that are more "resonant" (path length is a multiple of wavelength), this is qua... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/700854",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 3,
"answer_id": 2
} |
Why do we use different differential notation for heat and work? Just recently started studying Thermodynamics, and I am confused by something we were told, I understand we use the inexact differential notation because work and heat are not state functions, but we are told that the '$df$' notation is only for functions... | "What is the thermodynamic reason for describing them as not being changes in anything?"
Well, what would they be changes in? There isn't some quantity of heat belonging to something of which $\delta Q$ is a change; it's only heat while the energy is flowing. A similar remark applies to work, which is also energy in tr... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/701026",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 4,
"answer_id": 2
} |
Intuitive explanation for coefficient in the Larmor formula So the Larmor formula tells us the total power radiated by an accelerating point charge that doesn't go too fast with respect to the speed of light is $P=\frac{2}{3}\frac{q^2 a^2}{c^3}$ (written in CGS units).
Now my question is: Is there an intuitive explanat... | The 2/3 comes from the average value of $\sin^2\theta$ in the angular integration.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/701159",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Force acting on a negative particle in a magnetic field I have recently learned about magnetic fields and particles. The recent one I have learnt is the right hand rule. The force F acting on a negative particle is always opposite to the force we get from the right hand rule, if I understand the right hand rule correct... | $\vec{V} × \vec{B}$ follows the right hand rule, where:
$\vec{V}$ is pointer finger.
$\vec{B}$ is the middle finger.
Pointer finger down, middle finger into the page, would result in the force being to the right,
.
Multiply by a negative means the force is towards the left.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/701285",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 1,
"answer_id": 0
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.