Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Does the Earth revolve around the Sun? I am aware of this Phys.SE question: Why do we say that the earth moves around the sun? but I don't think this is a duplicate.
In a binary star system, where the masses of the 2 stars are not so different from each other, can we say that each star revolves around the other?
If yes... | In binary systems, each object is so affected by the others gravity that they have significant orbit. The sun has so much inertia that the earth's pull barely moves it, but the earth certainly revolves around the sun. In the reference frame of the Earth however, the Sun does revolve around the Earth.
| {
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"timestamp": "2023-03-29T00:00:00",
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"question_score": "1",
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If the Earth's atmopsphere spins with the earth due to friction, why is there no horizontal spiralling drag? Imagine a bucket of paint with a spinning ball in it. The paint would form a spiral and would not all move in synchronous movement with the ball.
To clairfy - In order for the Earth's atmosphere to appear to us ... | If i interpreted your question correctly and based on my knowledge (i don't know how good it is). This is because there is nothing to apply frictional force on the outer edge of the earth's atmosphere, whereas in case of bucket the water rubs against the boundaries of the bucket which slows down the outer part of the s... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/168882",
"timestamp": "2023-03-29T00:00:00",
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How to calculate the colour a human eye sees when looking at a light spectrum? I have to do a presentation about colourants in Chemistry class (grade 12, advanced) and want to write a program that calculates and visualizes the colours of some simple molecules.
What I need is a formula to calculate the visible colour ... | You will not be able to get the proper color of the narrow bands because they fall outside of the RGB gamut. However you can get their correct hue.
You need color matching functions (usually we use 1931 2° CMFs from the CIE, check online... or here under CMFs category http://www.cvrl.org/)
CMFs multiplied by your spect... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/168978",
"timestamp": "2023-03-29T00:00:00",
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where does electricity come from in piezoelectric? I have some questions here. BaTiO3 is a material which exhibit piezoelectric effect when we give external pressure on it. It contain Ba2+, Ti4+, and O2- ion in its crystal unit. The Ti4+ ion will move up or down, so the crystal will polarize and that it could produce e... | This is due to the molecular arrangement of the atoms in the crystal.
Basically, a piezoelectric crystal contains molecules arranged in, say, tetrahedral fashion.
The dipole moments of each and every atom pairs are well balanced.
When you squeeze or twist the crystal, the equilibrium gets disturbed. That is, one or ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/169049",
"timestamp": "2023-03-29T00:00:00",
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"question_score": "1",
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Is there an infinite amount of wavelengths of light? Is the EM spectrum continuous? The electromagnetic spectrum is a continuum of wavelengths of light, and we have labels for some ranges of these and numerical measurements for many.
Question: Is the EM spectrum continuous such that between two given wavelengths (e.g. ... | Sir Elderberry, Punk_Physicist and the Count Iblis have all given correct answers in principle.
There are two phenomena (really thought experiment, rather than practical, devices) that one needs to heed.
*
*A finite measuring time $T$ can only resolve frequencies to within an uncertainty of the order of $1/T$. This ... | {
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"timestamp": "2023-03-29T00:00:00",
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Approximations of the kind $x \ll y$ I have an expression for a force due to charged particle given as
$$F=\frac{kQq}{2L}\left(\frac{1}{\sqrt{R^2+(H+L)^2}}-\frac{1}{\sqrt{R^2+(H-L)^2}}\right) \tag{1}$$ where $R$, $L$ and $H$ are distance quantities.
Now I want to check what happens when:
*
*$H\gg R,L$
*$R,H\ll L$
... | Let's focus just on the interesting bit of the equation:
$$\frac{1}{\sqrt{R^2+(H+L)^2}} - \frac{1}{\sqrt{R^2+(H-L)^2}} =\\
\frac{1}{\sqrt{R^2+H^2+2HL+L^2}} - \frac{1}{\sqrt{R^2+H^2-2HL+L^2}}$$
Now if $H>>L,R$, we are left just with the terms with $H$:
$$\approx \frac{1}{\sqrt{H^2+2HL}} - \frac{1}{\sqrt{H^2-2HL}}\\
=\fr... | {
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100°C = 100 K =? I'm in first year. Our class is in lesson " Heat and Thermodynamics". While solving a numerical problem of a reversible engine he told us that 100 degree Celsius is equal to 100 kelvin. I inquired but could not get satisfactory answer. Pleas help me understand it.
Here is the numerical, please consider... | A difference in degrees Celsius is equal to one in degrees Kelvin.
So 150 K - 100 K = 50 K = 50 C = 423.15 C - 373.15 C.
The absolute values cannot be compared without taking the 273.15 offset into account.
| {
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Heisenberg uncertainty and probabilistic nature of QM I am trying to understand whether the HUP and the probabilistic nature of QM are orthogonal or not. By that I mean that the HUP fundamentally derives from operators not commuting, which is the important fact here, more than the statistical nature of the LHS in the d... | The $\sigma_A$ on the LHS is defined as
$$\sigma_A = \omega(A^2 - \omega(A)^2I),$$
where $\omega$ is a state (so $\sigma_A$ also depends on $\omega$!) and the way this is defined by von Neumann is by taking a statistically relevant ensemble of very same copies of the very same system in the very same state $\omega$ and... | {
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What do spacelike, timelike and lightlike spacetime interval really mean? Suppose we have two events $(x_1,y_1,z_1,t_1)$ and $(x_2,y_2,z_2,t_2)$. Then we can define
$$\Delta s^2 = -(c\Delta t)^2 + \Delta x^2 + \Delta y^2 + \Delta z^2,$$
which is called the spacetime interval. The first event occurs at the point with co... | Let's put it very simple: They tell you "how far something is apart compared to c"
time-like: if you are fast enough, you can be at (think spatial, like "at the festival") event a and at event b, it is only a "matter of time" until you see the second event
space-like: the two events are too far apart (in space). You ca... | {
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Can net torque $\sum_i\mathbf r_i\times\mathbf F_i$ be expessed as $\mathbf r\times$ (net force) for some $\mathbf r$?
Let $\mathbf F_i$ be forces each of which is applied on $\mathbf r_i$ of a rigid body. Then is there a position vector $\mathbf r$ that satisfies
$$\displaystyle\sum_i\mathbf r_i\times\mathbf F_i =\m... | Yes. The solution is:
$$ \bf{r} = \dfrac{\left( \sum {\bf F}_i\right) \times \left( \sum ({\bf r}_i \times {\bf F}_i) \right)}
{\| \sum {\bf F}_i \|^2} =\dfrac{{\bf F} \times {\bf \tau}}{{\bf F}\cdot{\bf F}}$$
Then you can show that
$$ {\bf r}\times \left(\sum {\bf F}_i \right)= \sum ({\bf r}_i \times {\bf F}_i) = {\b... | {
"language": "en",
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How can you have odd nuclear spin angular momentum but positive parity or vice versa? How does it happen that you can get states like $J^\pi=3^+$ or $J^\pi=2^-$? I think this could be because $\pi=(-1)^l$ so you could have an even state in $l$ but the $J=l+s$ sum could be an odd number?
| Presumably you are talking about mesons.
First of all, parity is not (-1)^L for a meson, it is (-1)^(L+1). This is because you have to take into account the fermion-antifermion relative parity for the quark-antiquark pair, which is negative.
Secondly, yes, it is L that matters and not J (which includes S). Thus, the lo... | {
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What is the mass of a sphere? A solid sphere of mass M is rotating along an axis.
We can consider it as a collection of large number of point masses, every point mass is moving with respect to center of mass with velocity which depends on its radius from rotating axis.
Then, according to relativity, the mass of every ... | If the sphere is not moving, than is not even rotating on its axis.
I don't really get what you're trying to say. Relativistic mass increases with speed, so if you're considering a moving sphere and a standing one they're not gonna have the same mass, but in the formula of relativistic mass m0 comes in, which is the r... | {
"language": "en",
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Does the direction of the magnetic field inside a solenoid depends on the direction which it is turned? Today trying to explain some magnetic stuff, just came across with a simple (at least seems) question.
My question is brief: does the direction of the magnetic field inside a solenoid depends on the direction which i... | The direction of the magnetic field will correlate to the handedness of the current going through the coil. Basically it doesn't matter how the solenoid is constructed, what matters is the orientation of the current going through it. If you look along the length of the solenoid (through the coil), whether the current i... | {
"language": "en",
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Integral vs differential forms of Maxwell's equations As stated in this post, the integral and differential Maxwell equations should be identical. However, in a text I was reading it states that
The integral forms of Maxwell’s equations describe the behaviour of electromagnetic field quantities in all geometric config... | Dirac delta is the most common distribution in physics.
It is defined by an equation:
$
\int \limits_{-\infty}^\infty \delta(x) f(x) = f(0)
$
One could say that this could be "intuitively: a function, which is $\infty$ at $0$, and $0$ everywhere else, but this makes no sense, since such integral would be $0$ (not $f... | {
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Has anyone tried Michelson-Morley in an accelerated frame? After doing much more digging than I thought I had to do, I found out that the speed of light is NOT invariant in an accelerated reference frame. Has anyone done any experiments to confirm this? In particular a Michelson-Morley experiment in an accelerated re... | The first to do something equivalent to that were Pound and Rebka who first measured the gravitational redshift in 1959. I'm not aware of anyone who actually used a Michelson interferometer in a upright orientation.
| {
"language": "en",
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"source": "stackexchange",
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A cup of water in ZERO gravity What will happen if I try to pour a cup of water in zero gravity, into another empty cup? Will the water come out of the cup? The adhesive force between the water molecules and the interior of the cup should prevent the water from coming out. Is it correct? Or is there something more to t... | Pour? No such thing without gravity.
In NASA TV (see video), I saw the prototype coffee cups. They are shaped with a sharp crease, to allow liquid to ride up the groove. More advanced product would also mix waxy and wettable surfaces to keep it stuck to the inside of the cup but not crawl over the brim, except at the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/170338",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "10",
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Deriving the equations for a moving inertial reference frame I assume $c=1$ in the following derivation:
In order to derive the equations for a moving inertial reference frame, I immediately wrote down the following:
$$ x'=Ax+Bt, \tag{1}$$
$$t'= Dx+Et. \tag{2}$$
In order to solve it I would need 4 independent equations... | You work too hard and the idea of setting $c=1$ may make problems.
I copy here the equations that you obtained and that I found correct. So,
1. The speed of light is the same in each frame implies
$$Act + Bt = c(Dct + Et), \overset{(x=t)}{\implies} Ac + B = Dc^2 + Ec. \tag{i}$$
*Reversing the frames gives indeed
$$... | {
"language": "en",
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"source": "stackexchange",
"question_score": "1",
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Effective Area of Isotropic Antenna: Explanation? I'm reading some introduction to antenna theory and I've often puzzled on the equation:
$$
A_{eff} = \frac{\lambda^2}{4\pi}
$$
which relates the effective area by which an antenna captures radiation to the frequency at which that radiation is.
I have looked at this deri... | Increasing frequency does not change the dv spectral interval of integration, and with an assumption of white noise there is no increase in dissipated power with frequency.
Therefore with resistor power held constant there is a decrease of aperture area with wavelength squared.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/170607",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
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Is there a difference in the energy output of a nuclear fission reaction as opposed to fusion? For example, if I split a Helium atom will I get the same amount of energy as when I fuse Hydrogen into Helium? If there is a difference, what will be the difference (in general not according to Helium/Hydrogen), and why?
| The energy generated during fusion or fission can be seen with this graph:
When a light atom is made into a heavier one by adding nucleons, it will lead to a greater output in energy; but when you reach Iron you can no longer gain energy through fusion. For heavier elements, you begin to lose energy when you fuse them ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Do photons with a frequency of less than 1 Hz exist? A photon with a frequency of less than 1 Hz would have an energy below
$$
E = h\nu < 6.626×10^{−34} \;\rm J
$$
which would be less than the value of Planck's constant. Do photons with such a low energy exist and how could they be detected? Or does Planck's constant ... | You don't need a long antenna to radiate at 1 Hz. You need a long antenna to radiate efficiently at 1 Hz. The efficiency is proportional to the cube of the length of the antenna in wavelengths (look up {electrically} 'small antennas'). A big 1 Hz current in a short wire will radiate very little power, but 1 photon a... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/170828",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Does the line integral definition of Work involve distance or displacement? My textbook reports the following definition of Work:
where ds is the infinitesimal displacement.
I know that an infinitesimal displacement is usually denoted by dr and I also know that the magnitude of dr is given by ds (infinitesimal distanc... | Its just a matter of what you use to call as displacement and as distance .
I have seen the usage of:
*
*dx
*ds
*dr
as the displacement too.
Wikipedia says :
The work done by a constant force of magnitude F on a point that moves a displacement (not distance) s in the direction of the force is the... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/170914",
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Can a virtual image created by a mirror have position in front of the mirror? I dont think so, because then the rays will intersect and the image will be real.
Please let me know if you know a case in which a virtual image is formed in front of the mirror.
| If the image would be formed in front of the mirror it would not be a virtual image anymore, it would mean that your eye would have the impression to see it in front of the mirror.
As soon as the ray is reflected, the eye which interprets light as travelling in straight line necessary sees the object as coming from be... | {
"language": "en",
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Maximum power transfer proof I have the following homework problem.
Consider a power supply with fixed emf $ε$ and internal resistance $r$ causing current in a load resistance $R$. In this problem, $R$ is fixed and $r$ is a variable. The efficiency is defined as the energy delivered to the load divided by the energy de... | Are you sure you mean the internal resistance "r"? The internal resistance typically can not be adjusted, often this question is phrased in terms of the load resistance "R".
The power dissipated in the external load is:
$$
P=\frac{V^2 R}{(r+R)^2}\;,
$$
which you need to maximize. If you maximize with respect to R, you ... | {
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Does spacetime have symmetric curvature around an object? If yes, then why do planets revolve around the Sun in elliptical orbits? Does spacetime have symmetric curvature around an object? If yes, then why do planets revolve around the Sun in elliptical (as opposed to circular) orbits?
| To a first approximation the spacetime curvature around the Sun is indeed spherically symmetric. I say to a first approximation because the masses of the planets (particularly Jupiter) also produce curvature and this breaks the spherical symmetry. However let's ignore this for now because I don't think it's relevant to... | {
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Hamiltonian from a Lagrangian with constraints? Let's say I have the Lagrangian:
$$L=T-V.$$
Along with the constraint that $$f\equiv f(\vec q,t)=0.$$ We can then write:
$$L'=T-V+\lambda f. $$
What is my Hamiltonian now? Is it
$$H'=\dot q_i p_i -L'~?$$
Or something different? I have found at least one example where usin... | Comments to the question (v2):
To go from the Lagrangian to the Hamiltonian formalism, one should perform a (possible singular) Legendre transformation. Traditionally this is done via the Dirac-Bergmann recipe/cookbook, see e.g. Refs. 1-2. Note in particular, that the constraint $f$ may generate a secondary constraint
... | {
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When a pn junction is formed, why is a positive region of charge formed on the n side of the junction? I understand that when electrons diffuse from n-side to p-side, negative charge is developed on the p-side. But the mere absence of electrons on the n-side doesn't make that positively charged. The n-side must be neu... | The pn-junction consists of fixed and mobile charges. The n-side has an abundance of electrons and the p-side had an abundance of holes. These are the mobile charges.
There is an abundance of charge carriers because of doping. Foreign atoms are introduced into the crystal. Some have an additional electron in the outer... | {
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Why invariance is important? The concept of invariance seems to have a great importance. Indeed, the fact that the laws of Electrodynamics are not invariant in every inertial reference frame led to the theory of Special Relativity which in the end makes those laws be invariant.
As I understand invariance, a physical la... | Invariance may be connected with two things:
First, invariance usually means a symmetry of the system under certain transformations. This symmetry and it's accompanied conserved quantities mean that there is a set of observational objects, that do not change. Moving epistemologically on this, we may deduce that the onl... | {
"language": "en",
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"source": "stackexchange",
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Microscopic definition of dynamic pressure of fluids So we have a moving fluid and we know from Bernouli's equation that there is a term called dynamic pressure(not to be confused with the hydrostatic pressure of the fluid).
So,what exactly is it and how can it be explained microscopically?
Note:do not involve relativi... | Wiki/google quote:
Dynamic pressure is the kinetic energy per unit volume of a fluid
particle. Dynamic pressure is in fact one of the terms of Bernoulli's
equation, which can be derived from the conservation of energy for a
fluid in motion.
So Dynamic Pressure is just local impulse/energy of movement that is b... | {
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Most True to Life Physics & Biology Simulation Engine? I'm a programmer. I code in C++, C#, HTML5, and PHP. There are many graphics engines I have at my disposal. The question is: Does there exist a graphics engine that is as true to our reality as possible given our current understanding of physics? For instance, I ca... | The scales and scopes of the models we do have are far larger than cellular level. Further, while there are skeletal models for animal bodies, the models for their motion is very much top-down modelling rather than bottom-up modelling. That is, an actual human's motions will be recorded and interpolated into the model,... | {
"language": "en",
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"source": "stackexchange",
"question_score": "8",
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Usage of singular or plural SI base units when written in both symbol as well as name I have multiple doubts related to the usage of singular or plural SI base units when written in both symbol as well as name.
I have framed this question under two parts, namely, Part (a) and Part (b). Each part has three sentences whi... | cms and kgs are wrong. The SI units are abbreviations which are also used in the plural. You will write 2.6 m/s or 1 m/s, but say "2.6 meters per second" or "1 meter per second" respectively.
Keep in mind the SI units are also used in tons of other languages that do not form the plural by attaching an -s. The units loo... | {
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"source": "stackexchange",
"question_score": "5",
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What's my $\mathrm dM$? Gravitational Potential inside a circle of mass
I'm trying to find the gravitational potential for an arbitrary point within a ring of uniform mass density. The point is constrained to be in the same plane as the ring.
So we start with:
$$\Phi=\int G\frac{\mathrm dM}{r}$$
Let's assume that the... | $\mathrm dM$ is just $\rho \mathrm dV$, where $\rho$ is the density and $\mathrm dV$ is the volume element.
In your case then $\mathrm dM = \delta(r-R)\delta (\theta - \pi/2)\lambda r^2 \, \mathrm dr \, \mathrm d\theta \, \mathrm d\phi = \lambda R \, \mathrm d\phi$
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Why is the Specific Heat of Helium 36 times greater than Xenon? Given that these are single atoms, why should the specific heat difference be so great? And more specifically, why does it take 36 times as much energy to raise the temperature of a given mass of Helium compared to Xenon?
| The key question is "Per mole or per gram?" Because the both values can be found tabulated as "specific heat" in various sources. Perhaps it would be useful to distinguish "molar specific heat" from "specific heat per unit mass".
You seem to be using the intuition for the molar quantity, so if the table is by mass, the... | {
"language": "en",
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Confusing, can a motor on a moving disc stop the disc? I've been troubled trying to solve this problem.
If a motor is fixed on a moving rotating disc and the motor's rotor has another disc attached to it. What would happen if we attached the motor's disc to the the disc its on and powered the motor's to rotate the dis... | Yes you can use the motor to slow and stop the disk, but you have to have (1) Alignment of the motor and its load angular momentum with the disk's angular momentum, and (2) enough motor load (moment of inertia) and motor speed to fully transfer the disk's angular momentum into the motor and load.
The process of momentu... | {
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Could I break the sound barrier using circular motion? (And potentially create a sonic boom?) Ok, Lets say I get out my household vaccum cleaner, the typical RPM for a dyson vaccum cleaner reachers 104K RPM, Or 1.733K RPS. In theory, this disc would be travelling with a time period of 0.00057692307 seconds, If we take ... | I want to add to the other answers that when an object is rotating at a supersonic speed, an observer will be hit by a rapid series of sonic shock waves, as the shock wave is an ever-expanding spiral. This is what makes supersonic propellers so terribly loud.
The images below depict the process. The red circle in the m... | {
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can a really loud sound travel in space? I heard in a discovery news video that there is one particle every cubic cm in space. So, if i were to vibrate a circular body of say mass $10^7$kg at $10^{22}$Khz, would i be able to hear a sound from say 1 metre away in space, assuming i will somehow live? Im asking this becau... | Assuming a loose definition of the word "sound," the answer is yes.
Let's consider the experiment you proposed. Suppose you place two large diaphragms facing each other in space. They could be simply large sheets of plastic stretched around a metal rim, like a drum head. And "large" in this context means much larg... | {
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Why temperature of liquid drops after spraying through a nozzle? We have tested in our lab as mentioned in the picture.
We connected hot water at $130^\circ F$ at $40 Psig$ to a nozzle (bottle sprayer). We measured the temperature differences inside tank and after spraying and found that there is a temperature drop of... | When you spray through a nozzle the water gets converted into tiny droplets, thus the surface area increases. Due to the increased surface area, the heat dissipates to the surrounding air molecules the water's temperature decreases.
Also, if the surrounding air is at higher temperature than the water then the water wil... | {
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movement of particles in electric field I am confused about a homework problem. Let's assume we have two electrically charged particles of which we know the charge and mass respectively. Let's say that at first they are fixed at some distance $r_1$ and then released simultaneously. I want to find their velocities at di... | In the first case,momentum is conserved because force is applied to each charge from WITHIN the system.So,the center of mass of the system is constant.In the second case,in order for just one charge to move,it has to be put in an EXTERNAL electric field.So,you can see that momentum within the system which consists only... | {
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Does the electric force on a charged particle in a uniform electric field increase? If I have a proton in a uniform field between two parallel oppositely charged plates and the proton accelerates, the electric force acting on it stays constant seeing it is a uniform field and as a result the acceleration of the particl... | NO the force does not change with distance
as E=F/Q
F=QE
electric field is constant and charge is also constant then force is also constant.
| {
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Can sound frequency create more ideal conditions for fire? You may have seen the recent story of a device that engineers claim can extinguish flames using sound frequency. An older article loosely explains the theory behind how this works:
Sound travels in waves, which are simply variations of pressure in a medium—w... | A reasonable sub woofer at sound power level of 130 dB would produce pressure fluctuations of 60 Pa. Compare this to the ambient pressure of 100'000 Pa and you will see that related temperature fluctuations would be negligible.
It extinguishes fire because it pushes the air back and forth. For the small fire in this vi... | {
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Error calculation in parallel resistances This is the question:
There are two resistors with resistance values $R_1=100\pm3$ ohm and $R_2=200\pm4$ ohm. Find the equivalent resistance of parallel combination.
According to what I've learnt, in any expression of multiplication or division, the percentage errors of each ... | Basically what you are doing for $y=\frac{AB}{C}$ is adding up all the percentage errors, which is wrong.
Take log function on both side, so you get $\log(y)=\log(A)+\log(B)-\log(C)$
so for percentage error becomes: $\frac{dy}{y} = \frac{dA}{A} + \frac{dB}{B} - \frac{dC}{C}$
so if you actually follow your step, you get... | {
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$\nabla^{\mu}\nabla_{\mu}$ in general relativity I am trying to work out $\square=\nabla^{\mu}\nabla_{\mu}$ in the metric
$
ds^{2}=-A(r)dt^{2}+B(r)^{-1}dr^{2}+r^{2}d\Omega^{2}
$$
My work:
when applying $\square$ to a scalar $\phi$, then
$
\square\phi=\nabla^{\mu}\nabla_{\nu}\phi=\nabla^{\mu}\partial_{\mu}\phi=g^{\mu\n... | Mm.... At first I repeated your calculation and got the same answer as yours, then I checked the paper you gave and found it consist with (32 a) and it seems not a typo, so I read it from begining - oh brother it's not 2+1 gravity - -b it's 3+1 gravity and you should treat $d\Omega^2$ more carefully:
$r^2 d\Omega^2=r^2... | {
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Relational Interpretation of Quantum Mechanics and Universal Wave Function Why is there no universal wave function for a relational interpretation of quantum mechanics?
"Quantum mechanics is a theory about the physical description of physical systems relative to other systems, and this is a complete description of the... | There is no universal wave function. The universe has many different mathematical descriptions, each corresponding to what a different observer can interact with around them. Each is incomplete, because no observer can interact with whole universe. Each observer, for example excludes themselves from the world they inte... | {
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Compact Disc Optics - Why use a linear polariser and a quarter wave plate? I just came across this website about the application of a quarter wave plate. Link: Compact Disc Optics.
My question is why does the beam need to be linearly and then circularly polarised before sending to the compact disc? And the returned be... | A quarter wave plate together with a polarizing beam splitter is a standard way of building an optical isolator. In an optical isolator the light traveling in one direction is transmitted but the light travelling in the reverse direction is reflected. This is important when reading compact discs because the reflected... | {
"language": "en",
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Why are we allowed to spontaneously break the Higgs field symmetry? In the lepton Lagrangian there are no mass terms allowed for the gauge bosons, due to gauge symmetry. To fix the problem of existing gauge boson masses, we introduce another field (Higgs) and say that here we
1) have self-interactions
(Why do we have... | Essentially, Higgs self-interactions are allowed because they don't violate any laws or symmetries (i.e. the $\phi^4$ term is gauge invariant, Lorentz invariant etc). Informally, a Lagrangian can (and possibly should) consist of any/all combinations of fields, derivatives of fields etc that respect the symmetries of th... | {
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Do magnetic fields cause ionisation of gases? I am doing my final year engineering project on Magnetic Field Assisted Combustion and was curious to see what people thought about it.
Companies sell rare earth magnetic arrangements to be attached to fuel lines of gas burners and they are said to improve combustion effic... | Reading the original question a posible mechanism has suggested itself to me. As the gas travels along the pipe "streaming currents" can be generated by friction. This is a phenomenon in the literature where by electrostatic charge can build up on the pipe and there is a concentration of charged particles in the bounda... | {
"language": "en",
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At what point does force stop translating an object and start purely rotating it? At what point (or distance) from the axis of rotation, does force applied on a rigid body stop translating and purely rotating the body? Can such a point even exist? Does the body always have to translate?
This question assumes that the b... | The only way to purely rotate a rigid body about its center of mass is to apply a pure torque (no net force). If the net force applied is zero then the center of mass is not accelerating.
However and combination of translation and rotation of the center of mass can be viewed as a pure rotation about the instant center... | {
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Charged particle under a uniform electric field
Suppose a charge particle $q$ starts to move without initial velocity under the influence of a uniform electric field $E$ pointing in the positive $x$ direction. Express its position vector in terms of proper time $\tau$.
According to wiki:http://en.wikipedia.org/wiki... | As you know the answer should be a hyper trigonometric function instead of a trigonometric one. Your mistake is with lowering/raising of vector components
$$
p^\alpha = m_0 \left( u^0, u^1 \right) = m_0 \left( \eta^{00}u_0, \eta^{11}u_1 \right) = \pm \left( - u_0, u_1\right)
$$
Where the $\pm$ comes from your metric co... | {
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Why does it seem as if big vehicles "attract" mine when I drive close to them? When I drive a car at high speed and when I am near to another big car (like a van, or transport vehicle) I feel an attraction to or something push me toward the other big car. What's the physics in this case?
| There is a great amount of energy involved here for sure. All moving objects meet resistance and the more velocity the more resistance. For objects moving fast there is aerodynamic designs and properties added so it experiences less turbulent force pressing back on the object. Drafting in race car driving is a useful e... | {
"language": "en",
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How does "contamination" through (radioactive) radiation work? Physically, what does it mean when people or objects are contaminated with radiation? Is it because they actually carrying heavy metal particles?
| When a person or an object is contaminated, there is some radioactive material in it (not necessarily heavy metal), either sticking on the surface or ingested.
In the context of radiation protection, you have a radioactive contamination when an unsealed radioactive source produces an unintended and abnormal level of ... | {
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Limits of Integration Trig, Mag Field Infinite Length Wire I don't understand how the limits of integration should be defined when doing basic integrals of trig functions. It seems like it's an arbitrary decision, I don't understand it.
Here's the set up: For the field near a long straight wire carrying a current $I$, ... | I think I figured out my confusion. With these limits defined, in order for cosine to be correct, I have to redefine cos=-adj/hyp. Then everything else works out fine. Weird. It feels wrong to just redefine cosine, but it's true under these defined limits. Is that right, is that something you have to do sometimes? Rede... | {
"language": "en",
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Will the electrostatic force between two charges change if we place a metal plate between them? If a thin metal plate is placed between two charges $+q$ and $+q$, will this cause a change in the electrostatic force acting on one charge due to another? What is the concept behind this? What will happen if the metal plate... | I think that the force exerted by each charge on each other will be equal to $0$, as the electric field of charges will not pass through the metal plate. But the net force on each charges will remain the same and the force will be exerted by the charge induced on the metal plate by both charges.
| {
"language": "en",
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How many atoms are in a piece of paper? How many atoms are there in a common sheet of paper?
The paper is A4, i.e. $210 \, \mathrm{mm} {\times} 297 \, \mathrm{mm}$ $\left(8.27 \, \mathrm{in} {\times} 11.7 \, \mathrm{in}\right).$
| So, a piece of paper is made of wood, and wood is some organic substance. I don't know what the chemical formula is, but let's say it's mostly carbon. In fact, let's just pretend it's all carbon, since you only want order of magnitude. Wikipedia tells me a piece of A4 paper weighs about five grams, and then I divide by... | {
"language": "en",
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Drying clothes with the sun's heat, without any air Will my wet clothes dry if I hang them under the sun, and if there is no air around the clothes? In other words, do I need both air and heat to dry wet clothes, or is heat alone (in the imagined absence of any air) enough to dry wet clothes? Related question : will w... | Your clothes would dry very quickly in a vacuum, assuming that the temperature is still one that one would find on earth. This is because the water would boil out of your clothes. On earth normally, boiling takes a lot of heat energy. This is because of the air pressure. In your scenario there is no air pressure. so th... | {
"language": "en",
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Potential barrier scattering when particle energy equals to the barrier height What happens if we have $E=V$, where $E$ is the energy of a incoming particle and $V$ is the height of a square potential barrier? This wiki page actually gives a finite transmission probability for this case. But what does the wave function... | I have seen references relating the "square potential barrier" to the case of the space between two metal surfaces, but this is incorrect. The square potential barrier would require an infinite electric field at both ends with no field elsewhere. This model is used frequently but I see no practical application.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Beginners Textbooks in physics Hello I am fifteen and I already know everything that my school has been teaching me so I have been going ahead. I have already been studying mathematics far past where I am at school, but I am very interested in physics. I want to learn everything up to advanced topics such as super-st... | You can read the utterly fantastic Feynman Lectures on Physics which is free for online viewing at the link provided. I would also recommend Feynman's Tips on Physics: Reflections, Advice, Insights, Practice - A Problem-Solving Supplement to the Feynman Lectures on Physics. My University uses Giancoli as well as the te... | {
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Why do snorkelers not need to wear corrective glasses when snorkeling with goggles on? I am myopic ~ -2.75 sph +1cyl. When I went snorkeling they tell you not to wear glasses behind the goggles. Surprisingly, underwater, things remain in focus with goggles on even without prescription lenses, while things outside the w... | the assertion that corrective lenses are not needed underwater is incorrect. diving masks with corrective lenses built-in have been in use for decades. the reason you yourself experience this effect is probably contained in Chris' and Martin's comments above.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/175013",
"timestamp": "2023-03-29T00:00:00",
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Are there any scales other than temperature that have different zero points? For most physical measurements, zero is the same regardless of the units used for the measure:
$0 \mathrm{mi} = 0 \mathrm{km}$
$0 \mathrm{s} = 0 \mathrm{hr}$
but for absolute temperatures, different systems have different zeros:
$0 ^\circ\math... | Gauge pressure.
From Wikipedia:
Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Sun and planets orbit each other Do not the planets and the Sun revolve in orbits around each other and the shape of the orbit depends on where the center of gravity of the system is? The greater the mass of the Sun, the closer the orbit approximates a perfect circle.
| No. The shape of the orbit, i.e. how elliptical it is, does not depend on the relative masses of the two bodies.
All objects in the solar system orbit around the centre of mass of the solar system. For obvious reasons, namely that the Sun contain far and away most of the mass of the solar system, the centre of mass of ... | {
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What is the physical process behind wool shrinking when dried? Wool is a fibrous material, but other fibrous materials do not suffer the same problem.
Let us set the scene; a woollen jumper shrinking when put in the washing machine, then the dryer. This involves wetting and heating the jumper, then allowing it to cool ... | It's kind of like "material memory." Wool is pretty kinky originally (as sheared), and the production processes pull the strands straight. Hot washing allows the material to revert to a tightly-wound config, which reduces the external dimensions, aka "shrinking."
| {
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What does it mean that a magnetic field's flux vanishes through any closed surface? I'm reading the Britannica guide to Electricity and Magnetism, and I came across the following quote:
A fundamental property of a magnetic field is that its flux through
any closed surface vanishes.
Can someone explain this in simpl... |
Can someone explain this in simpler terms?
Typically, the closed surface is a mathematical surface (Gaussian surface) which simply defines an 'inside' and 'outside'.
Since, as far as we know, there are no magnetic charges from which magnetic field lines start or end, any magnetic field line entering must exit through... | {
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Phase space Lagrangian? Reading out of this lecture series we define a phase space Lagrangian $\mathcal L$ to be a function of $4n+1$ variables namely $q,\dot q,p,\dot p,t$. My question is, what space is this function defined on? (I know that the $\dot p$ is there for names sake only).
My stab at an answer is it is a ... | *
*If $Q$ is configuration space, then the Lagrangian is a function $L: TQ\times \mathbb{R}\to \mathbb{R}$.
*Let the cotangent bundle $M:=T^{\ast}Q$ be the corresponding phase space.
*The Hamiltonian/phase space Lagrangian is a function $L_H: TM\times \mathbb{R}\to \mathbb{R}$.
| {
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How do waves have momentum? A question on a practice test I'm taking is as follows:
By shaking one end of a stretched string, a single pulse is generated.
The traveling pulse carries: A. mass B. energy C.
momentum
D. energy and momentum E. mass, energy and momentum
How would one describe the momentum of a wave?
| Without going into wave equations, lets just say the segments of a string does not only move along the vertical direction. There are horizontal movements as well, although to a much smaller amplitude. A segment is being pulled towards the source horizontally when departing the equilibrium position and pulled back whe... | {
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Bell inequality with triplet state Is it possible to prove Bell inequality starting from a state formed from triplet states, i.e. $\frac{1}{\sqrt{2}}(|\uparrow>_A|\uparrow>_B+|\downarrow>_A|\downarrow>_B)$?
If not, why?
I do not see why not, but somewhere it is mentioned something about non-rotational invariance. Mo... | The answer is yes and no, but first, let me point out that you cannot "prove Bell's inequality", the whole point is that you violate the inequality in quantum mechanics.
Now, let me come to the yes/no part:
It's "no, you cannot violate Bell's inequality with this state", if you refer to what according to wikipedia is "... | {
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Why do we use capacitors when batteries can very well store charges? Can batteries be used instead of capacitors? I am trying to figure out a basic, superficial and any obvious difference between the two.
| Practically we use capacitors when we require a large amount of charge to be flown within fractions of seconds.. Battery provides a nearly uniform voltage and effective in long use, but when it comes to discharge a large amount of charge in a fraction of second, battery is ineffective.. How ever by a building a capaci... | {
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Are circles stronger than triangles? I've often heard in engineering that, "there is no shape stronger than a triangle." I also recall that arches are also very strong shapes, which can be crudely described as a perpendicularly-symmetrical half-an-ellipse; Which can be simplified to half a circle.
If there were no conv... | Triangular support(or triangle here) is unparalleled in terms of strength they provide to support load because all the hold mass is properly distributed across the support. You may disagree when g=0 :)
| {
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Does it take more energy to open a door when applying force close to the hinge?
Assuming an ordinary hinged door (without any springs), would it take more energy to open it when applying force in the middle of the door (point b), rather than at the end of the door (point a), where the door knob is?
"Opening the door" ... | If you open a door by pushing it near the hinge, you apply greater force than when you push it near the outside edge, which requires lesser force since the width of the door acts as a lever and force multiplier. As the friction of the hinge and the weight of the door are equal in both cases, and assuming displacement ... | {
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Arrow of time and entropy? The arrow of time is usually defined by the direction in which entropy increases. In a closed system, if there's a max entropy that the system can reach, does that mean time stops or at least become undefined at the max entropy state?
See also: For an isolated system, can the entropy decrease... | This is my first answer here so please don't expect a lot. I watched the BBC show yesterday just about this question. It is called Wonders of the Universe, episode 1 - Destiny.
According to the Professor Brian Cox, yes, arrow of time will stop eventually when maximum entropy is reached. It's going to happen in unimagin... | {
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Physical meaning of the separation constants in Laplace's Equation for Electrostatics In Electrostatics, if we consider a region without charges the electrostatic potential $V$ obeys Laplace's Equation $\nabla^2 V = 0$. We can tackle this with separation of variables. In cartesian coordinates we have $V(x,y,z) = X(x)Y(... |
Now, the constants C1,C2,C3 appearing when we separate variables on
Laplace's equation for electrostatic potential has some physical
meaning? If they do, what is it?
The constants are the related to the square of the spatial (angular) frequency or a spatial growth/decay constant.
For an example of spatial frequen... | {
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How do you prove that $L=I-V+1$ in $\lambda\phi^4$ theory? It is known that the number of loops in $\lambda\phi^4$ theory is given by the formula
$$L=I-V+1$$
where $L$ is the number of loops, $I$ the number of internal lines and $V$ the number of vertices. I would like to know the proof of this statement.
| This formula is actually Euler's formula for planar graphs, and holds for all Feynman diagrams regardless of what theory we are in.
The proof proceeds by induction and is easy if we first disregard the case of crossing lines:
*
*Observe that a one-loop graph has two vertices, one loop, and two internal lines, so th... | {
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Calculating wind force and drag force on a falling object I'm trying to numerically integrate the motion of an object (say, a falling vertical cylinder). Here, there's a drag force: the wind "acting" on the body (presumably adding horizontal velocity) and the air itself slowing down the vertical motion.
Is it correct t... | Yes, the force points along the vector of the relative velocity between the object and the air.
Quadratic drag is an interesting phenomenon. You have to calculate the net velocity vector (which includes a horizontal and vertical component) and compute the force along that axis; when you then decompose it into horizonta... | {
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At what energy consumption would we get a 1 degree rise in the Earth's temperature? If energy consumption continues to rise at (say) 4% per year, how long before the heat dissipation seriously impacts climate?
| I suggest to compare human produced heat with the incident heat of the sun which is around 1 kW/m$^2$. The usual comparison is "the sun delivers more heat in an hour than humans use in a day". While such a comparison may not remain accurate forever, a difference in scale of 7000x suggests that even if humans doubled th... | {
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Chiral anomaly in Weyl semimetal In the presence of electromagnetic fields $E$ and $B$, four current is not conserved in a Weyl semimetal i.e. $\partial_{\mu} j^{\mu}\propto E\cdot B \neq 0$. There are some proofs in the literature where this is proved with the machinery of Lagrangian and Action, but I am looking for a... | Your statement itself is not quite right. What is not conserved is the chiral current, namely the current of fermions at one of the Weyl nodes. The physics can be understood essentially in one-dimensional version of the Weyl metal: consider a 1D electron gas. There are two Fermi points, and the low-energy theory is giv... | {
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How to calculate Electric Field near a charged conducting surface without Gauss' law? I have two problems :
*
*In every textbook I find the use of Gauss' law in calculation of Electric Field near a charged conducting surface. Can it be calculated without Gauss' law?
*Suppose while using Gauss' law to calculate fiel... | Usually, applying Gauss's law to a problem with
$$ \int_A \vec{E} \cdot d\vec{A} \propto Q$$
is only suitable, if one knows, that the electric field is perpendicular to the surface $A$ and is constant in magnitude over the whole surface. This leaves:
$$ E \propto \frac{Q}{A}$$
On can conclude such statements if the pro... | {
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What is meant by the term "single particle state" In a lot of quantum mechanics lecture notes I've read the author introduces the notion of a so-called single-particle state when discussing non-interacting (or weakly interacting) particles, but none that I have read so far give an explicit explanation as to what is exa... | In a physics of nuclear structure, by the term single particle state is typically understood an excitation, that can be attributed mostly to one proton or one neutron that jumped to a higher orbit. Contrary to collective excitation or collective state, which is an excited level, that many nucleons participate in.
| {
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What is the maximum net force of surface tension per gram of water? I've always wondered how much force water exerted through surface tension. By maximum I mean the theoretical pulling/attracting power.
Also, what would happen if you raised or lowered the power? Would it affect life? And how much would you have to rais... | As you can see here http://www.funsci.com/fun3_en/exper2/exper2_05.gif the surface tension depends on the width of the film you are pulling. At the extreme case you would make two surfaces with each water molecule of one half of the water attached to a polar group of the surface. Then contact the surfaces so that each ... | {
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Are there other less famous yet accepted formalisms of Classical Mechanics? I was lately studying about the Lagrange and Hamiltonian Mechanics. This gave me a perspective of looking at classical mechanics different from that of Newton's. I would like to know if there are other accepted formalism of the same which are n... | Gauss's principle of least constraint
Principles of Least Action and of Least Constraint (a review paper by E.Ramm)
If I remember correctly, this principle has been used to derive equations of motion
for Gaussian isokinetic thermostat (i.e., a computational algorithm for maintaining a fixed temperature of the system). ... | {
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Is my proof of the thought experiment that Walter Lewin proposed in lecture 16 valid?
A tennis ball bounces off a wall elastically. The momentum of the wall changes, but the kinetic energy of the wall remains zero. How is that possible?
Walter Lewin Lecture 16 - Ball bouncing on wall?
That proof made sense, but I did... | Assuming a perfectly elastic collision, you are heading in the right direction here. It looks like you're trying to say the thing your ball hits will have 2 times the ball's momentum. You might want to consider $2m_bv_bv'_w=m_w{v'_w}^{2}$ because it looks like something might've gone "wonky" when you added equation (1)... | {
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What compounds or elements only have one phase or two phases? Wood appears to be one. I think gases like helium and hydrogen cannot exist in the solid state under normal pressures, correct? And why do those "phase cheaters"-- those elements/compounds which sublimate directly, skipping a phase, or "procrastinators"-- el... | @HyperLuminal,@Neuneck,@Ernie,@DirkBruere-Few compounds which decompose into simpler substances before reaching their boiling point have only two phases, and the compounds which decompose into simpler substances before reaching their melting point have only one phase
| {
"language": "en",
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How do you define the total rotational energy of an object? This problem arose when I was applying a conservation of energy argument to a mechanics problem, (a spinning coin on a table) and wasn't sure how to define the total rotational energy of the coin. At first I defined it's total rotational energy as about the ax... | Let's do this using angular momentum as a vector. This should clear up the question on using both axes separately or one new one.
The spinning around the y-axis will give an angular momentum in the y-direction: $\vec{L_{y}} = \hat{y} L_{y}$, while the spinning around z-axis gives angular momentum in the z-direction: $\... | {
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Can we find the exponential radioactive decay formula from first principles? Can we find the exponential radioactive decay formula from first principles? It's always presented as an empirical result, rather than one you can get from first principles. I've looked around on the internet, but can't really find any informa... | A simple and direct way to get this exponent and complex eigenvalues is by using Gamow's approach, that was one of first introduced explanations of alpha radioactivity.
It solves Schrodinger equation in WKB approximation, no fancy math or deep knowledge in QM is needed, except being familiar with WKB.
A good source fo... | {
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What is the meaning of " $\Psi$ is not a measurable quantity in itself"? I want to know that why the wavefunction $\Psi$ as a complex quantity (i.e $A+iB$ form) in quantum mechanics and somewhere I have studied that $\Psi$ is not a measurable quantity in itself that's why we multiply it by a it's complex conjugate $\P... | What does measurable mean ?
It means that one can do an experiment and get a value for $a+ib$ , the complex number.
A complex number to be measurable one should be able to measure a value at the same time for $a$ and $b$ and put a point on the complex plane. This means two independent variables, $a$ and $b$ can be mea... | {
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Why do nuclear bomb explosions create an array of visible electric discharges in the form of lightening in the upper atmosphere? I've seen videos on the internet, showing nuclear bomb test explosions, and there appears to be a large amount of visible lightening discharging numerous times over the development of the mus... | The electric charge difference between the earth and the atmosphere grows with altitude, at around 88 DC volts per meter. This electric potential may be shorted out when a thermonuclear explosion releases radiation which ionizes the atmosphere. About 5% of a nuclear explosion's energy is in the form of ionizing radiat... | {
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Is simultaneity testable? I was reading about Andromeda paradox, and I started wondering. How can we know that the situation in the Andromeda paradox is real ? How can you know that simultaneity is real ? How can you say that you are simultaneous with a star fleet coming from Andromeda, if there is no way of knowing it... | It's a paradox about relativity.
How can you know that simultaneity is real?
To keep things simple, we can define simultaneity of two events for a particular observer as light (or information) reaching the observer at the same time. According to this definition, simultaneity is a "real" phenomenon. A layer of complex... | {
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Why can't I harness normal force? Lets say I have my palm flat with a book resting on top of it, and I have my feet on the ground. I extend my arm so that now it's kind of difficult to keep the book up. Why doesn't my hand just produce normal force on the book, cancelling out the force of gravity, and costing me no eff... | I haven't seen this mentioned yet: this normal force is in the context of classical mechanics (high-school or early university physics). That theory only deals with (perfectly) rigid bodies. So you can put a weight on a hypothetically perfectly rigid table and the table does not have to do any work to support it. The t... | {
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Why don't protons just bounce off each other in the LHC? Ok, this might sound like a silly question, but I was wondering, when particles (e.g. protons) are smashed together in the LHC, why do they break up into dozens of other particles, as opposed to just bouncing off of each other elastically?
I'm guessing the full ... | Elastic collisions do happen at the LHC. The TOTEM experiment measures the differential cross section (rate as a function of angle) for proton-proton elastic scattering at the LHC. Here is their latest result. They don't publish an estimate of the elastic cross section, but according to their data it must be at least 2... | {
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The amount of potential energy at the height of h When we lift an object upwards with a constant velocity for a distance of $ h $ the work that we've done is $mgh$ and the work done by the force of gravity is $-mgh$. So the net work on the object is zero and it doesn't gain any energy. how its potential energy will be ... | Gravity doesn't do -mgh work on the object while it is lifted, gravity converts the potential energy gained while lifting the object (mgh) into kinetic energy (1/2 mv^2) after it's dropped.
| {
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What is the uncertainty principle? I looked on Wikpedia for information on the uncertainty principle, but after reading it I still had no idea. I know it has something to do with how many things you can hold at some spot for some amount of time (maybe?). This is inspired by this question.
| In classical physics you are supposed to be able to measure the coordinates and the velocity (really the momentum) of a mass with infinite precision at the same time. If you try this trick in the lab you notice that that's not the case. Either your position or your momentum measurement or both will always show some non... | {
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Where do forces point in an equilibrium system
I have the system above, with three identical balls of weight $W$ and radius $r$. The angle joining the centres is $\theta$, and the coefficient of friction between the balls and the plane is $\mu$ and between the balls $A/C$ and $B/C$ is $\mu '$. The system is in equilib... | Look at a free body diagram.
With red and the contact normal forces, with pink the friction forces, and with gray the gravity forces.
If in the end any of the friction forces come up to being negative, then flip the orientation.
| {
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What is the state of water at exactly 0°C? Theoretically speaking, what is the state of water at bang on 0°C - not any lower or higher?
Any lower would make it a solid whereas any higher would make it a liquid. But what about bang on 0°C?
Thanks in advance
Edit: I understand that other factors are involved, such as pre... | H2O at 0°C is Ice. There is a considerable gap between Ice and Water. After 0°C if you increase the temperature by 0.1°C, that is at 273.1K the equlibrium state occures. This state is called the 'Triple point of water'. This is where water, ice and surprisingly water vapour. After this state if you increase the temp by... | {
"language": "en",
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Is refraction of light a thermodynamic process? Is refraction of light a thermodynamic process?
Can it be explained by conservation of energy?
If so, does temperature has an effect on refraction of light?
| Refraction of Light is not a Thermodynamic process. If you study the QED basis of refraction, you notice that the difference happens in time. The speed of light is constant, and a photon which is refracted, doesn't actually travel any slower, it just travels a longer path, and needs thus more time. If it hit's somewher... | {
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Kinetic energy and Potential of a photon How does the potential and kinetic energy of a photon relate? Do they mean the same thing?
Also how does De broglie wavelength and Potential relate?
| Quote from a webpage a bit over my head :-) greatians.com
Photon has linear momentum. Photon travels in vacuum space at the ultimate speed of light. Photon has the quantized energy of hf as given by eq. WD.1.2.
E = hf … eq.... | {
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Cart speed and wheel rotation Say you have a horse drawn cart. Does the outside of the wheel spin at the same velocity that the cart moves forward?
The reason I ask is because I am working on a problem where a piece of mud detaches from the wheel when it is moving up, and then lands on the wheel again at the same spot... | The rate at which the wheel is spinning is an angular velocity, normally measured in radians/second. The velocity of the cart is a linear velocity - metres/second in SI units. Since the two are different units, they can't really be compared.
However, you can calculate the instantaneous velocity of any given part of t... | {
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Is this symmetry factor in Peskin wrong? I am trying to compute the symmetry factor of a Feynman diagram in $\phi^4$ but i do not get the result Peskin Claims. This is the diagram I am considering
$$\left(\frac{1}{4!}\right)^3\phi(x)\phi(y)\int{}d^4z\,\phi\phi\phi\phi\int{}d^4w\,\phi\phi\phi\phi\int{}d^4v\,\phi\phi\ph... |
What am I doing wrong?
The expansion of $e^x$ is:
$$
e^x=1+x+x^2/2+x^3/3!+\ldots
$$
From expanding the expression:
$$
\left<\phi_x\phi_y\exp{\left(-\frac{\lambda}{4!}\int dz \phi_z^4\right)}\right>\;,
$$
the third order term is:
$$
\left<
\phi_x\phi_y\frac{1}{3!}{\left(\frac{-\lambda}{4!}\right)}^3\int dz \int dw \in... | {
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What's the differences between time in Physics and time in everyday use? OK. This question might sound as not a good question, but the word 'time' is so confusing to me. I mean thermodynamics says time is the product of entropy. Relativity says time is relative. Quantum Mechanics says time doesn't exist, and that we ca... | In layman use, across the world (ignoring time zones), someone who is walking observes the same time as you. In physics, however, that person has his/her own time, relative to that person. In classical thermodynamics, time is the same as layperson's time. It is just described in a different way. Quantum mechanics doesn... | {
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Electric field in a conductor Is it always true that the electric field in a conductor is zero?
What would happen if I put a very big charge inside an ungrounded hollow conducting sphere like this image?
The charges inside the conductor are supposed to rearrange so as to cancel the field created by the big charge.
So ... | If the charge is too large, it will pull electron-positron pairs out of the vacuum outside its surface, and in effect the charged sphere neutralize itself as the positrons fall to it and the electrons (from the pair creations) will fly apart in an explosion electrons.
| {
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Do quarks violate quantization of charge? Does existence of various kinds of quarks like up, down, strange, charm, top, bottom violate the quantisation of charge or just redefine it as up quark have charge +2/3 and have -1/3. Or do things get even complex for unified theories like the proposed string theory?
| The existence of quarks is not seriously in dispute at this point AFAICT. If you want to make something meaningful out of quarks and only quarks having fraction-of-$e$ charges, I think you pretty much have to postulate that electrons are composite.
For instance, the rishon model proposes that all the "fundamental part... | {
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Particle Horizon and CMB If particle horizon is the maximum distance we can see presently in the universe, how come we are able to see CMB? CMB is radiation from surface of last scattering happened at t~380k years.
Suppose universe is expanding at a constant rate ( i.e. no acceleration), will we be able to see CMB aga... | If recombination were been an aisled event, i.e. something that happened in a certain place with some finite spatial extent, then it is possible that light didn't have enough time to reach us from there (this event happened outside the particle horizon).
However recombination was not an aisled event, it happened everyw... | {
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For what values of $\lambda$ is the distribution $(x-i\varepsilon)^\lambda$ positive? I've been reading the famous unpublished paper by Luescher and Mack "The energy momentum tensor of critical quantum field theories in 1+1 dimensions". In the proof of their main theorem, page 7 of the manuscript, they write:
"$$
<0|O_... | I am not sure if you mean $x$ one or two dimensional variable. I don't have the article at hand but I guess we probably mean $D=1$??
The argument goes as follows: you have to use Bochner's theorem, which says that $W(x-y):=W_2(x,y)$ is positive distribution if and only if the Fourier transformation yields is a positive... | {
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How to construct generators and Lie Algebra for Lorentz group? I'm trying to figure out Lorentz group in 2+1. First of all, I'd like to think the special orthgonal group as a combination of rotation and boost in space. Then I construct it as below. First rotation part:
$$
R(\theta)= \begin{pmatrix} cos\theta& -sin\thet... | You should have two boost generators. You have constructed one for boost in the $x$ direction, but there is also one for boost in $y$.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183190",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 1,
"answer_id": 0
} |
What stops the middle point of a power line from falling? Say you have a system that is a uniformly weighted string with slack suspended from two points; i.e. a power line.
There are three forces acting on any given point on this string: string tension going left, string tension going right, and gravity.
Consider the p... | The part at the exact middle of the string has zero mass.
That seems silly, but consider - if you consider a very small section of the string in the middle - say 1 mm - then the pieces of string on either side exert forces with tiny, but nonzero upward components. If you half the length we are considering to 0.5 mm, t... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/183266",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
"answer_count": 1,
"answer_id": 0
} |
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