Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
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Have there been attempts to create heavier antimatter particles? This is a follow-on from previous question:
It seems as though all experiments concerning antimatter have only been conducted with antimatter protons - as they are obtainable through decay.
Have any heavier elements been attempted? eg. has there been anti... | Antihelium has been observed: http://www.theguardian.com/science/2011/apr/24/antihelium-antimatter-brookhaven (sounds like just the nucleus, not a neutral atom).
The AEgIS experiment http://aegis.web.cern.ch/aegis/research.html plans to make neutral antihydrogen atoms and measure their gravitational acceleration.
Also... | {
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Are critical exponents below and above the critical point always same? The scaling relations don't distinguish the the critical exponents below and above the critical value. In the mean field level, I understand these critical exponents are same whatever one approaches the phase transition from the order phase or disor... | Critical exponents are properties of the RG fixed point that drives the phase transition. They are computed by linearising the RG flow equations close to the fixed point. The exponents are the derivatives of the beta functions evaluated at the fixed point. They know nothing of the way you approach the fixed point. In p... | {
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Explain why we use this formula to calculate total resistance Why do we use this formula to find the total resistance? Lets say we have three resistors in a parallel circuit
$$R_t = \frac{1}{\frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3}}$$
Where does it come from?
|
Why do we use this formula to find the total resistance?
Because parallel connected conductances sum just as series connected resistances.
Ohm's law is
$$V_R = RI_R $$
The dual of Ohm's law is
$$I_G = GV_G $$
Since, for parallel connected circuit elements, the voltage across is identical, it follows that, for paralle... | {
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Why is force a localized vector and not a free vector?
A vector which is drawn parallel to a given vector through a specified point unlike free vector in space is called a localised vector. The effect of a force acting on a body depends not only on the magnitude & direction but also on its point of application & line ... | Vectors are considered free (of location) and are the same if they have the same magnitude (length) and orientation. So a vector A in one coordinate system with origin O is the same as a vector A in another coordinate system with origin O* even if O* is moving (translational and/or rotational motion) with respect to O... | {
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Pulling on a weakened rope - where will it tear? Let's say I have a rope of 10m length and it is weakened in 3 spots:
at 2.5m, at 5m and at 7.5m. Weakened means that if enough tension is applied it will tear at these points (all points are equally weakened).
This rope is connected to an immovable wall. There is a perso... | I would say that not in the middle, either one of the other points. This is my argument:
The string when pulled from both extremes (the same in both cases you ask) it will vibrate in its eigen-frequencies, shown in the picture below taken from Wikipedia. This oscillation will add strain to the points of rope.
But as ca... | {
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Benefits of rear spoiler in cars What would be the benefits of rear spoilers in cars, like this one:
| The spoiler spoil the air flow at the rear side. When the car is cruising, from the frame of reference of the car, air flows around it from front to back. If the air flow is smooth and if there is a large velocity at the rear side then there will be pressure drop. Thus, there is a low pressure at the rear side and high... | {
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Why in PN junctions the octet rule is dominating electrostatic repulsion? The depletion region in PN junctions is created by charges from the N part diffusing into the P part, thus completing an octet of covalent bonds in the P part. This shift however leaves positive ions in the N region and negative ions in the P reg... | The electron in the $n$ semiconductor and the hole in the $p$ type semiconductor are delocalised and not bound to any particular atom, so arguments based on completing octets aren't useful.
This diagram shows roughly how the depletion layer forms:
In the $n$ type semiconductor the doping creates donor states in the b... | {
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How does water help extinguish fire? How does water extinguish fire? Heat energy from the fire is transferred to the water, isn't that how it works? How does water deprive oxygen and stop combustion? How is the specific heat of water connected to this? If we use hot water instead cold water, does that make a difference... | To sustain fire, it is true that you need the tri-factor of oxygen,fuel, and heat.
However extinguishing fire through the use of water, is different than one would think.
Indeed, water "sucks" energy in order to change its phase, and thus reduces the heat factor,
but the real crux lies in the water expansion properties... | {
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How to make a rotating linearly polarized (not circular polarization) beam from a single beam? One way to make a linearly polarized beam rotating at frequency $\Delta f\approx10\mbox{MHz}$ is by combining two circularly polarized beams, one left-handed and one right-handed, and where one beam is at a frequency $f$ and ... | A Pockel cell followed by a quarter waveplate will do this. The Pockel cell acts as an electrically controlled waveplate which will give an elliptical state. The waveplate will then convert this to a rotated linear state. Producing a 10MHz driving signal at the several kilovolts required might be a challenge but not im... | {
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What happens to waves when they hit smaller apertures than their wavelenghts? I was wondering this for quite a long time now. Let's say you have a water wave (like ripples, not the ones you see during tsunamis) with wavelength 10 m. Imagine you put a boundary with an opening of 1 m. Will diffraction be observed? Accord... | I cannot speak for how to match with a mechanical resonator but you can match arbitrarily well any EM radiator that is small relative to its wavelength at one (1) frequency. This means that the radiator (loop or dipole, monopole, etc.) will absorb all radiation coming from directions in which it can radiate as a source... | {
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What is the physical meaning of electric potential, potential difference, and voltage? When resembling the electricity flow through a wire to people walking through a street: electrons are people, current is the number of people, resistance is the barriers on the way.
But what is the electric potential in this example?... | In my opinion electrostatic potential at any point in an electric field can be viewed as
Suppose a positive charge makes an electric field
In that Field if any positive or negative test charge is present
Let us assume positive test charge
Then this positive test charge will tend to move away from Source charge due to ... | {
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How is antimatter made? How is antimatter made in laboratory? Can anyone explain, at the particle level, specifically how anti-protons and anti-electrons are made?
| Creating anti-protons is straightforward in principle because any high energy collision produces a shower of protons, antiprotons and various types of pions. The pions decay in a few nanoseconds, so you just have to wait for the pions to decay then separate the antiprotons from the protons.
At Fermilab a 120GeV proton ... | {
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How much entropy is produced in evaporating water due to irreversible evaporation towards equilibrium (humidity=100%)? $dS=dH/T_{boil}$ for the increase in entropy by changing a phase at saturation ($T=373\text K$ for $p=1\text {atm}$ for water). However, water also obviously evaporates below boiling point when equilib... | Specific entropies ($\mathrm{kJ\,kg^{-1}\,K^{-1}}$):
\begin{array}{lrll}
& \mathrm{^\circ C} & \text{liquid} & \text{vapor} \\
\text{Triple point} & 0.01 & 0 & 9.155 \\
\text{Normal boiling point} & 100. & 1.307 & 7.355 \\
\text{Critical point} & 374.15 & 4.430 & 4.430 \\
\end{array}
| {
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Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy? In classical physics we often cast an analogy between translational and rotational systems
Force < > Torque
Energy < > Rotational Energy
Momentum < > Angular Momentum
and considering SI units we have [Force] = N, [Torque] = N-m, [Ene... | Torque and work are different physical quantities,so it makes sense to use different units. Since torque is a vector and work is a scalar, one idea would be to use "$\mathrm{N\times m}$" for torque, instead of "$\mathrm{N \cdot m}$". $\mathrm{N\times m}$ would be consistent with torque, or cross product, while work is... | {
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What make us fall when we move on ice? This is a common phenomenon and most of the people have encountered this while moving on ice:falling down! Why does this occur?
According to me, it is due to lack of static friction. But why will it be absent here? What is the cause? I have heard that there is a thin layer of wate... | Walking on ice can can be hazardous if you're wearing a nice pair of dress shoes with slick soles. The backward-sloping phase transition between water and ice cannot explain this phenomenon, and it certainly cannot explain why a hockey puck slides so easily on ice. Nor can frictional heating.
That ice is slippery is in... | {
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Is it possible to improve the efficacy of light bulbs with a semipermeable mirror? (The greenhouse bulb) Let us imagine that there exists a material that reflects infrared radiation, but is transparent for visible light. Could we take an incandescent bulb and add this material to the inner surface of the glass hull? Th... | Many light bulbs already do this. See for example this article. I was in the lighting technology business at one point. At that time it was done in some tungsten-halogen incandescents. I don't know current state of things.
| {
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Why does gravity enact force? Gravity would cause two objects in a vacuum to move toward each other. I understand that gravity is a force that exists as a product of energy's original conversion into mass and the continuing change in form of energy and mass. I also understand that the potential energy in the two object... | Generically, if I have a particle which has potential energy $\phi(x,y,z)$, then the force on that particle will be given by ${\vec F} = - {\vec \nabla}\phi$. So, generically, the motion of particles will "try" to minimze the potential energy. In particular, the only points where the particle will not move will be th... | {
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Failure of the Steady State Theory I was reading a journal of astronomy and came to the most famous opponent of Big Bang theory:
The Steady State Theory:
The 20th - century theory was proposed by Hoyle,Gold and Bondie. The theory is based on the Perfect Cosmological Principle which states that universe has no change i... | The following passage has been extracted from the book Parallel worlds:
Finally, in Nature magazine in 1965, Hoyle officially conceded defeat,
citing the microwave background and helium abundance as reasons
for abandoning his steady state theory.
| {
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Why do vapour cones form around jet fighters? Apparently this phenomenon has nothing to do with jets breaking the sound barrier and has something to do with the Prandtl-Glauert singularity as described on Wikipedia. But, the Wikipedia article isn't very detailed and it doesn't explain why the cone arises.
Is there a re... | We have to understand there are two phenomena occurring. One is what creates the vapor. The other is what shapes the vapor into a cone. The vapor is created in this case by the localized pressure dropping below the dew point in the air immediately around the structure moving through it. The cone is formed by the high a... | {
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How fast would someone have to run to travel vertically up a wall? I am currently doing a physics project on the effects of so-called 'super-speed'. I was wondering how fast you would have to run to vertically travel up a wall? That is, to negate the force of gravity. Is it even possible? Help would be appreciated!
| We can take three steps or so against a wall, what we do is not walking or running, we kick the wall about one meter (our leg) before the collision, change the rotation of velocity, thus throwing our body upside and taking advantage of friction.
| {
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Why are ammeters used in series and voltmeters in parallel? As I'm reading a course on electricity, this one says that an ammeter should be branched inline and not on a bridge. Can someone explain to me physically why we branch a amperemeter inline and a voltmeter on a branch?
| The ammeter measures the current flowing through itself. If you want to measure the current flowing through another component, then you must make the current through the ammeter equal to the current through the component. If you wire it in series, that's true. If you wired it in parallel, the current would be unevenly ... | {
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How is Kirchhoff's voltage law understood in the water flow analogy? I met the Kirchhoff circuit laws in the past, but now I'm trying to associate them with a practical representation to be sure to understand them.
Let's start with the Kirchhoff current law: If I say that the electrons are like water going though a pi... | The Kirchoff Voltage law states that the sum of emfs in a circuit is equal to the total potential drop in the circuit.
So for a simple example, where you a 6V cell, for example, and 2 resistors in series.
The 6V cell can be seen as a place where the water is given potential energy - if we imagine a ramp, it would be th... | {
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Poisson brackets of the Kepler Problem For the hamiltonian of a particle of unit mass in a kepler potential:
$$H = \frac{1}{2}\mathbf{p} \cdot \mathbf{p} - \frac{\mu}{r}$$
The angular momentum vector is given by: $\mathbf{L} = \mathbf{r} \times \mathbf{p}$
I know and can show that the poisson brackets of $\mathbf{r} \c... | Let me give a further comment (not exactly an answer)
The quantity $\mathbf{r} \cdot \mathbf{r}$ represents the magnitude of the radius as such it does not change under rotation (poisson commutator with ang. momenutm $L$)
The quantity $\mathbf{p} \cdot \mathbf{p}$ represents the magnitude of (linear) momentum, as such ... | {
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Clausius statement of the 2nd Law I'm slightly messed up with the Clausius statement of the 2nd Law.
I've seen at least two versions, which seem to be conceptually different.
a) It is impossible to transfer heat from a colder body to a hotter body without any other effect.
b) It is impossible to transfer heat from a co... | The statement
without any other effect.
means without external work being acted upon the system. Actually nobody uses the word "effect" and most textbooks use the correct terminology of external work.
Reservoirs are bodies too, therefore 1) directly applies to 2).
| {
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Tesla Coils - Is there a risk that the discharge can create x-rays? I've built a Tesla coil that stands about 3 ft tall and uses a spark gap as the interrupter for the primary circuit. Judging by the size of the streamers it's reaching at least a million volts.
Someone once told me that you have to be careful with Tesl... | I defer to:
https://webhome.phy.duke.edu/~rgb/Class/safety.html
They do list X-ray generation, however the primary risks are ozone (which destroys organic compounds; your body is primarily composed of organic compounds...) with insufficient ventilation and electric shock.
X-rays are typically generated using 20kV to 60... | {
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Influence of applied voltage to an electron of a metal I would like to ask what would happen to the potential well of an electron being trapped in a metal? If I apply a voltage trying to accelerate the electron out of the potential well. Would It make the potential well shallower? Or would it rather shift the fermi lev... | First of all I think we are thinking about the free electrons in the metal rather than the ones which are bound to individual atoms.
Generally to apply a voltage we attach connections to each end of the metal and apply a potential difference. One way of thinking about what happens when we apply a voltage is that the fe... | {
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Why doesn't a star's core cool down when it expands as a red giant? When a star starts to run out of hydrogen to fuse, it begins to collapse due to gravity until the central core temperature rises to $10^8~\text{K}$
Then due the force generated by the fusion of helium, the star expands again and becomes a red giant.
So... | First, a star does not become a red giant when helium fusion begins, instead it becomes a red giant earlier when an inert degenerate core of helium forms and a shell of hydrogen begins fusion. When shell hydrogen fusion begins, the star expands to be a red giant.
The core is degenerate (sustained from collapse by el... | {
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Renormalization in non-relativistic quantum mechanics I read many articles about renormalization in the Internet, but as I currently don't know much of QFT (currently just studying classical field theory and QM), and as all this looks quite interesting, I'd like to still get some bit of understanding and feeling of it ... | http://www.roma1.infn.it/~amelino/appunti1.pdf
Here, page 16: "Aside on perturbative renormalizability".
You can find a quite simple but enlightening example of renormalization applied to a non-relativistic theory where you also have the exact energy spectrum and eigenfunctions to be compared to.
| {
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Can action be unbounded from below? While solving the problem in this question, I found cases where the numerical optimization failed, suspecting unboundedness of the function being minimized. The function approximates the action of the system in question.
I decided that this result could be explained by an unbounded f... | Example: Consider an action functional
$$\tag{1} S[q]~=~\int_{t_i}^{t_f} \! dt ~L, \qquad
L~=~\frac{1}{2}m\dot{q}^2-V(q),$$
with Dirichlet boundary conditions (BC)
$$\tag{2} q(t_i)~=~q_i \qquad \text{and}\qquad q(t_f)~=~q_f,$$
where the potential $V$ has a repulsive pole
$$\tag{3} V(q_0)~=~+\infty$$
at $q=q_0$. Th... | {
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single-mode approximation in spinor F=1 BEC with dipolar interactions How can a single-mode approximation be justified in spinor F=1 BEC with dipole-dipole interactions? Or maybe this kind of approximation will never take place and condensate components are always separated. What do You think?
Good literature on the to... | In the field of multi-component condensates, the single mode approximation (SMA) means that different dipole states are assumed to share the same spatial wave function. Thus, there are no dipolar textures.
SMA is well justified when the inter-component (e.g., spin-dependent or dipole) interactions are much weaker than ... | {
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Why does Earth behave like Natural Bar Magnet? What is the reason for the Earth to behave like a bar magnet and have poles (North and South poles)?
| The Earth's magnetic field is caused by eddy currents in the liquid parts of the planet's interior. We believe the field is not due to a permanent magnet because: (1) Its direction and strength change over time, and (2) the planet's interior is hotter than the Curie temperature of its elements, and so a permanent magn... | {
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Making a plastic scintillator glow? I have a plastic scintillator I had purchased off eBay, which is believed to be BC408
http://www.phys.ufl.edu/courses/phy4803L/group_I/muon/bicron_bc400-416.pdf
Upon shining a small UV LED (10-40mA, maybe ~400nm wavelength) on it, it seems to glow mostly from the refraction of the pl... | You can get original Fiestaware pieces online or at some antique stores. The uranium glaze is a decent low-level x-ray and gamma emitter that is safe to handle.
My orange salt-shaker reads a couple hundred Bequerel on a good Geiger tube held a few centimeters away.
You can also order nano-curies of a number of isotope... | {
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Distribution of gravitational force on a non-rotating oblate spheroid Suppose a person is standing on a non-rotating$^1$ oblate spheroid of uniform density. He first stands on one of the poles, then on the equator. In which case is the gravitational force greater?
In Case 1, the distance between the centre of the pers... | Neglecting rotation, it seems to me that if one takes the case where the diameter at the equator is very much larger than that at the pole, we can consider it to be a disk.
Standing on the edge, all the mass is directly below you and all of it contributes to attracting you to the centre. Standing at the centre of the d... | {
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Have you ever seen in your car a mosquito? it's not capable of going 60 mph. How come it can still keep up with the car as if it weren't trying For instance, a fly is staying at the same spot, then your car suddenly moves forward at a rate of 60 mph. The mosquito IS staying at the same spot but for some reason it moves... | The air inside the car moves transiently as the car speed up (is being pushed by the rear of the car, sits, etc), if this happens slow enough this will occur without much turbulence or wind-like movements. The mosquito is pushed by the air (assuming he is flying), so in his frame of reference, relative to the inside of... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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How to represent a laser pulse in quantum optics Every quantum optics textbook that I've found says something like "a coherent state represents the output of a laser", but a coherent state is a static thing (aside from the oscillating phase of the complex parameter); how do you represent a laser pulse propagating in sp... | A coherent state is a monochromatic sinusoidal field. The electric field pulse is inherently not monochromatic, but instead has a spectrum of frequencies which are superimposed on top of one another so as to all add constructively once every pulse repetition time. Therefore, to represent the field pulse in quantum opti... | {
"language": "en",
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How to know if a vehicle is moving without any external source of information? The situation is the following:
I'm inside a vehicle (plane or a car, it doesn't matter) and I need to know if the vehicle is moving at a constant speed BUT I cannot perceive any external change like visual changes, vibration, etc.
How can I... | Does humidity and similar cues count in the answer or not?
For example if moving continously after a while certain environmental parameters will change. Which cannot be screened in the vehicle.
For example humidity, temperature etc..
Assuming the vehicle can actually screen all those parameters. Then one can use non-... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Bose Enhancement Factor How may one explain the fact that the probability of a boson transferring to a state with an occupation number n is 'enhanced' by a factor of (1+n), compared to the classical case? (In the classical case, the probability is supposed to be independent of the occupation of the final state.)
| A simple way to see this is by considering the fact that the probability to transition from a particular initial state to a particular final state is the same as for the inverse process where one considers the transition from the final to the initial state. This is because the square of the modulus of the matrix elemen... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Analytical solution of Liouville's equation for classic harmonic oscillator I'm interested in the analytical solution of the simple PDE:
$$\frac{\partial f}{\partial t} - m\omega^2x\frac{\partial f}{\partial p}+ \frac{p}{m} \frac{\partial f}{\partial x} ~=~ 0.\tag{1}$$
With: $$f(x,p;t\!=\!0)~=~f_0(x,p) \quad \mbox{arbi... | Hints:
*
*OP's eq. (1) is the equation for a constant of motion $\frac{df}{dt}=\{f,H\}_{PB}+\frac{\partial f}{\partial t}=0$ of a harmonic oscillator $H=\frac{p^2}{2m}+\frac{1}{2}m\omega^2 x^2$.
*Let us assume for simplicity that $m\omega=1$, and leave it to the reader to generalize to arbitrary $m$ and $\omega$.
... | {
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Why does the Copenhagen interpretation assert randomness if this cannot be tested? Why does the Copenhagen interpretation of QM assert that random events occur if such a claim cannot ever be proven or disproven?
A related question:
How to tell if QM is really random?
Edit On second thought it could certainly be disprov... | We do experiments and come up with distributions of the variables that can be measured.
There exists a very deterministic theory, Quantum Mechanics, that predicts accurately probability distributions for our measurements and the theory is continuously validated.
Probabilities exist in the classical physics regime and ... | {
"language": "en",
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How does a single charge produce magnetic field? I have studied in Introduction to electrodynamics (Griffiths) that magnetic field is actually due to effects of relativity
unequal Lorentz contraction of the positive charge and negative lines, a current- carrying wire that is electrically neutral in one inertial system... | A single charge cannot produce a static magnetic field. It can produce a time-dependent magnetic field. It is also a relativity effect because both electric and magnetic fields are changed with change of the reference frame.
In a moving reference frame (or when the charge moves) one observes not only electric, but also... | {
"language": "en",
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Semiclassical quantization of bouncing ball Consider an elastically bouncing ball of mass $m$ and energy $E$. This has a triangular potential
$$ V(x)~=~\left\{\begin{array}{ll} mgx & \text{if } x>0, \\
\infty & \text{if } x<0, \end{array}\right. $$
where the $x$-axis points upwards. Let $\hbar = m = g = 1$, so that ... | I think your error is in assuming that $E_{n+1} - E_{n}$ is proportional to $n$. At least, I assume you assumed it; it's the only way I can see that you could go from the statement
$$
E_{n+1} - E_n \propto E_n^{-1/2}
$$
to the statement
$$
E_n \propto n^{-2}.
$$
Really, what the first proportionality above implies i... | {
"language": "en",
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How noisy are photon detectors? I have a single photon detector and $N$ photons per second arrive at the detector. Then something happens and the number of incoming photons per second changes by the factor of $\alpha$. So now $(1+\alpha) N$ photons per second arrive at the detector.
What are currently the best photon d... | As a practical matter we generally use these devices in cases where the photon arrivals are random. That is the mean rate may be known, but the actual arrivals are distributed according to a exponential time law rather than being periodic.
In those case, counting statistics usually dominate the uncertainty (in low back... | {
"language": "en",
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How does air pressure affect the speed of sound? How does air pressure affect the speed of sound in relation to its kinetic theory etc?
I have tried searching this but i have not found a suitable answer as other websites simply related to air pressure waves.
| To first order, the speed of sound is not affected by pressure. Pressure waves can be shown to fulfill the D'Alembert wave equation $(c_S^2\,\nabla^2 - \partial_t^2)\psi=0$ where the wavespeed $c_S$ is given by:
$$c_S = \sqrt{\frac{K}{\rho}}$$
where $K$ is the bulk modulus of the medium in question and $\rho$ its densi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/146193",
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Why would different metals glow red at different temperatures? According to everything I've been taught about incandescence and black-body radiation, and some quick Googling to confirm I'm not crazy, just about everything, regardless of composition, should start glowing red at about the same temperature- 798K, the Drap... | Different metals would to glow at different temperatures because of their different abilities to hold on to electrons. Some metals hold their electrons very weakly and some hold it very tightly. The temperature at which it glows is dependent on the strength of this force. Things glow either because of absorption or emi... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/146256",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "20",
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Have we proven that higher dimensions exist? Have we proven higher dimensions exist?
| I think dimension means the number of independent coordinates required for describing or specifying the position and configuration in space of a dynamical system. As example when we consider the Minkowski's four-dimension world we consider the position coordinate is homogeneous to time coordinate. So an event is repre... | {
"language": "en",
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"source": "stackexchange",
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What's the difference between frequency domain and time domain spectra?
If I have a mechanical oscillator and want to observe the dynamical behavior of the oscillator, is there any additional information to observe it in time domain and frequency domain? Normally, we observe the frequency domain spectra (power spectra... | A signal spectrum can be decomposed in its component frequencies in two ways:
*
*Time domain - For example with wavelets (you will see the different frequencies along the Y axis and the increasing time in the X axis)
*Frequency Domain - For example with the Fast Fourier transformation or multitaper transformation w... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/146671",
"timestamp": "2023-03-29T00:00:00",
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What caused the expansion of the universe to slow down after the inflationary epoch? As everyone knows, when the big bang happened, the universe expanded at an unbelievable rate and this was called the inflationary epoch (or more popularly cosmological inflation) which lasted for about $10^-$$^3$$^4$ seconds. But after... | If you take and measure the accelerated expansion of space occurring between a random pair of galaxies, say around 20 to 30 million light years away from us, then multiply that acceleration by 13.8 billion years, the age of the universe. Now, compare the answer to the distance they are currently apart. The numbers don'... | {
"language": "en",
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"source": "stackexchange",
"question_score": "2",
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Nature of Cooper pairs Some people say it is bound state, some say it is not. Which is more accurate?
Problem is that I read in some books, including Ziman, that Cooper pairs are bound states but my teacher says that it is not true and that Bardeen had to explain it many times even to his peers.Now, i know that it has ... | As Cooper wrote in his paper (1956), one Cooper pair is a bound-state. The point is : it's a bound-state if you consider 2-electrons on top of the Fermi sea with an attractive potential. In the many-body case, better not to think in term of bound state. A Cooper pair is a fictitious particle which has no clear meaning,... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/146821",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Everyone calls Electromagnetic Induced Transparencyan interference phenomenon, but is it also an interference phenomenon in classical systems? Electromagnetically induced transparency is a hot topic in physics. However I'm curious about its mechanics in physics. Physicists think that it's a phenomenon of interference f... | EIT and the mechanical analogy you posted are examples of coupled resonator systems. In the RLC circuit, the system includes two RLC circuits that are coupled. The EIT system consists of a pair of two-level systems that couple to a single shared level. An RLC circuit and a two-level system both exhibit resonance effect... | {
"language": "en",
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Is a photon really massless? If a photon travels at a speed of light and its massless then it must have no energy but this is not the case as we see in photo electric effect. Also help me to know what are photons made of, how are they created?.
| When people claim that a photon is massless, they mean that a photon has zero rest mass. In special relativity, the formula for the energy of a particle with mass $m$ possessing a momentum $p$ is
$$ E = \sqrt{p^2c^2 + m^2c^4}$$
If we set $m = 0$ for a photon, we'll end up with
$$E = pc$$
Here the momentum of a photon... | {
"language": "en",
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Meaning of Einstein's equation $E=mc^2$? Meaning of Einstein's equation $E=mc^2$? How can a $1\,\mathrm g$ mass possess energy equal to $9\times10^{13}\,\mathrm J$? What does it actually mean?
| The rest energy of mass...is really a vibratory energy, caused by gravitons colliding with the protons and nuetrons mass, as the gravitons flow thru them at the speed of light, in a vibratroy fashion. The total energy of gravitons contained in the mass, is transferred to the mass (proton), for example. I can send you... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Laplace equation vs Gauss theorem Two concentric conducting spherical shells of radii $a_1$ and $a_2$ ($a_2>a_1$), are charged to potentials $φ_1$ and $φ_2$, respectively. Determine the electric potential and field in the region between the shells. Also determine the charge on the inner shell..?
Why we need Lapla... | Consider that the electrostatic potential $\varphi$ isn't directly observable. The potential $\varphi+C$ where $C$ is a constant gives the same electric field, and so the same physics. Because changing $\varphi$ by a constant should give the same physics, you cannot conclude that the charge on the shell is $4\pi\epsilo... | {
"language": "en",
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Structure of white light? White light is a mixture of different wavelengths.
If so what will be the structure of a beam of white light ? Is there a separation between different colours ? what does it actually mean ?
Does a beam of white light shows any spacing between different wavelengths ?
| The general idea is that if you had empty space, with only visible white light moving through it, and you put a line of electrons a few thousand nanometers long, they would start accelerating up and down, maybe in a pattern like this:
where each tick of the horizontal axis is something like hundreds of nanometers and ... | {
"language": "en",
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Is it possible to create a parachute large enough to stop all velocity? This idea came to me while playing Kerbal Space Program. I noticed that the larger my parachute was, the slower my rocket would fall back down to Kerbin. I would like to know if it is possible to create a parachute so large in the real world that i... | It would be possible in theory, but only in a very side-thinking way: if you make a parachute so large it encapsulates the whole Earth, it will in effect act as a balloon and not fall down, due to the internal pressure of the atmosphere.
This wouldn't work in practice for obvious reasons, but maybe in Kerbal you might ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Does the conductivity of a wire in a vacuum decrease over time? Does the conductivity of a wire in a vacuum decrease over time, say over the period of years or decades? In other words: Does current degrade a wire, making it less conductive? If so, by how much, and why does this occur? Does it have something to do with ... | The line itself does not change much over years. What changes and therefore needs maintenance on power transmission lines is insulators, connectors and spacers. Insulators get dirty or simply break, connectors work loose due to thermal expansion and contraction, mechanical stresses and oxidation, and spacers can be d... | {
"language": "en",
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How do radio waves reach receivers without being canceled out by interference?
When I think of waves traveling through a medium, I tend to think of the double slit experiment or waves in a pond. In those cases, waves are canceled out by destructive or constructive interference.
I'm curious as to how the EM wave can be... | What makes you think they don't?
When I was a wee lad, my physics teacher showed a video to me of exactly that. A man was driving down a road listening to a radio station (BBC if memory serves). At regular intervals, the music would get quieter and cut out.
The issue was that the road in question was nearly along the l... | {
"language": "en",
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Will a person experience micro or zero gravity inside an aeroplane in free fall Let us say that a person is inside a stationary aeroplane being held by a stationary helicopter in the sky. Now, the cable which supports the aeroplane breaks. Thus the aeroplane is now not bound by anything.
1) Will the person inside the a... | 1) will feel zero gravity. The confusion could be that some (including the NASA website) call microgravity to free fall apparent gravity, others to case where gravity is very small, such as in an asteroid.
2) it depends, if it is on the seat he will now only feel the force of the cushion trying to gain its original sh... | {
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"url": "https://physics.stackexchange.com/questions/147787",
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Is the world we are living in discretized? I do not know how to use professional words to ask my question, so I will try to use a layman language. Please bear with me for a moment.
A ROUGH GUESS
The world our eyes are seeing every moment is a picture reflected in our eyes. I guess our eyes are like cameras, that are ta... | Fortunately for experiments in physics we have better proxies than the accuracies of our five senses. We have detectors and computers and ....
With these tools a theory of how the universe is made has been developed, from elementary particles with the theory of quantum mechanics building up the observables around us, ... | {
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Why is the energy of particles in accelerators much higher than the energy of the particles they are trying to find? I have been wondering. In the LHC, or other particle accelerators for that matter, they are colliding particles with energies above TeV. The LHC is going to be 14 TeV or something like that the next time... | The proton is not fundamental. It is made up of quarks and gluons. It is these constituents that are colliding in the LHC to produce, in your example, a Higgs boson. The quarks and gluons only carry a fraction of the energy of the proton.
In addition, the colliding gluons or quarks in general do not have the same momen... | {
"language": "en",
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Are there eight or four independent solutions of the Dirac equation? I edited the question as a result of the discussion in the comments. Originally my question was how to interpret the four discarded solutions. Now I'm making a step back and hope that someone can clarify in what sense it is sensible to discard four of... | OP's solutions $u_3$, $u_4$, $v_3$, $v_4$ don't solve the Dirac equation. For example, let's try $u_3$:
$$
\begin{align}
0&=(i \gamma^\mu\partial_\mu -m) \psi, \qquad\text{with}\enspace \psi= u_3 = (0,0,1,0)^T e^{-imt}\\
&=\big[i\big(\gamma^0\frac{\partial}{\partial t}+\vec\gamma\cdot\nabla
\big)-m\big]u_3\\
&=[i\gamm... | {
"language": "en",
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"source": "stackexchange",
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What is "trivial" about the trivial topological superconducting phase? Once more I am stuck on my favorite word: "trivial". I am reading a bunch of stuff about topological superconductors at the moment and people keep talking about having to distinguish between the topologically "trivial" and "nontrivial" phases.
To ha... | "Trivial" is used because the equation reduces to something a bit simpler. Take the equation
$E_{\pm}=\pm \sqrt{\left(\frac{p^2}{2m}-\mu\right)^2+|\Delta|^2p^2}$
Setting $\Delta$ to $0$ gets rid of the last term, and making $\mu<0$ means that the first term will reduce to
$$E_1=\pm \left(\frac{p^2}{2m}-\mu\right)$$
whi... | {
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What is the effect of water 'climbing' over a cup via a wet string? I have noticed (with tea bags, etc) that if you get the whole string wet, and you have one end inside the cup and the other end below the level of the liquid (outside), the water drains down the string outside the cup at a slow pace... Seeming to defy ... | This effect is called Capillary Action. Yes we do in fact observe it in nature in a large scale: How do you think plants are able to "suck up"1 water through its roots and send it to the leaves? One of the major forces responsible for it is capillary action.
Here, have a quote from the article mentioned above:
Wicking... | {
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Triple integral $\iiint_{\mathbb{R}^3} d^{3}q ~\delta^{3}(\vec{q})\frac{(\vec{p}\cdot\vec{q})^2}{q^{2}} $ involving Dirac Delta function
I am trying find $$\iiint_{\mathbb{R}^3} d^{3}q ~\delta^{3}(\vec{q})\frac{(\vec{p}\cdot\vec{q})^2}{q^{2}},$$ where $\vec{p}$ is some fixed vector.
The answer should be $\frac{p^2}{... | Hints:
*
*In mathematics, a distribution is usually only defined wrt. smooth testfunctions. However the function ${\bf q}\mapsto({\bf q}\cdot{\bf p})^2/q^2$ is not continuous at the origin ${\bf q}={\bf 0}$. Nevertheless, we can e.g. try to evaluate the triple integral using the following representation of the 3D Di... | {
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If direction of torque is upwards(or downwards), why does the body rotate perpendicular to the direction? We know that torque is given by $$\vec{\tau} = \vec{r} \times \vec{F}.$$ Its direction is given by the right-hand rule which says that torque acts perpendicular to the plane where force applied and position vector ... |
Now, torque τ⃗ is given by the above relation. But in which
direction does it act?
Torque, like angular momentum is a pseudovector and not a vector. It is a conventional way of showing the direction (anti/clock -wise) ot the rotation. It is devised in such a way that you can apply the right-hand rule.
| {
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Why aren't there spherical galaxies? According to the Wikipedia page on Galaxy Types, there are four main kinds of galaxies:
*
*Spirals - as the name implies, these look like huge spinning spirals with curved "arms" branching out
*Ellipticals - look like a big disk of stars and other matter
*Lenticulars - those th... | It is due to the combined effect of rotation and "dissipation". A rotating cloud of gas consists of particles which interact strongly with each other (colliding physically) on relatively short timescales can radiate away some of their energy and momentum by emitting photons. For both of these reasons, a dense cloud of ... | {
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"url": "https://physics.stackexchange.com/questions/148418",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Fluid dynamics - immiscible liquids Question:
A U-tube of uniform cross-section is partially filled with a liquid I. Another liquid-II, which does not mix with liquid I is poured into one side. It is found that the liquid levels of the two sides of the tube are the same while the level of liquid-I has risen by 2cm. If ... | The specific gravities would be the same if the levels of the two side were the same after liquid-II was added.
When I try this, my logic seems to be flawed too. I don't get 1.12.
The level on side II has not changed. The level on side I has risen 2 cm. So 2 cm of liquid-II were added.
Consider the horizontal plane ... | {
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"url": "https://physics.stackexchange.com/questions/148587",
"timestamp": "2023-03-29T00:00:00",
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All geodesics are inextendable? I think the title is true, because geodesics has a tangent vector with a constant length parametrized by an affine parameter.
Probably, it is easier to think about timelike or spacelike geodesics. In this case, its affine parameter measures the length of the curve. It is difficult to im... | The property you are referring to is called geodesic completeness. It is an important concept in the study of singularities in general relativity. There are somewhat trivial examples of geodesic incompleteness where you are just "missing" part of the spacetime, i.e. you could have Minkowski space with a point removed... | {
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Why do the high frequency waves have the most number of modes? While reading the Wikipedia page of Ultraviolet Catastrophe, I came across how Rayleigh and Jeans applied the equipartition theorem. They told that each mode must have same energy. Now as the number of modes are greatest in small wavelengths or large freque... | The "modes" in this case refer to the standing waves that can exist in a cavity. A very nice diagram / explanation is given at http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/rayj.html
To summarize: if you consider a cavity of dimension $L$, the modes that fit inside the cavity have wave numbers $n_1$, $n_2$, $n_3$ ... | {
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Why are unilateral Laplace transforms suitable for causal systems and bilateral aren't? https://en.wikipedia.org/wiki/Two-sided_Laplace_transform#Causality
The above section says that bilateral transforms will not necessarily make sense for causal systems. In the course of advanced engineering mathematics our instructo... | Usually, what you are taking the Laplace transform of, is the system response, $h(t)$, e.g. system's response to a delta input, $\delta(t)$. If system's delta response is nonzero at $t<0$ it would mean the system had been anticipating your delta input at $t=0$ and already started responding, which is non-causal. Theref... | {
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Water behaviour under theoretical near-infinite pressure conditions I've asked a similar question here but the answer given shows the behaviour of water under general conditions.
I'd like to know what the behaviour of water is like as pressures increase towards infinity without being able to escape it's confinement.. i... | Yes, the question is theoretical and so the response. Under enough pressure water will become a solid, regardless of temperature. That is, as far as it is still water. If pressure is high enough, the atoms will collapse and form neutron-degenerate matter (theorized to exist in the cores of neutron stars). I am not sure... | {
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Does the lagrangian contain all the information about the representations of the fields in QFT? Given the Lagrangian density of a theory, are the representations on which the various fields transform uniquely determined?
For example, given the Lagrangian for a real scalar field
$$ \mathscr{L} = \frac{1}{2} \partial_\mu... | Field $\psi_{a_{1}...a_{n}\dot{b}_{1}...\dot{b}_{m}}$ with a given spin and mass (i.e. field which transforms under irrep of the Poincare group) must satisfy some determined conditions called irreducibility conditions:
$$
\tag 1 \hat{W}^{2}\psi_{a_{1}...a_{n}\dot{b}_{1}...\dot{b}_{m}} = -m^{2}\frac{n + m}{2}\left(\frac... | {
"language": "en",
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How can we show that pressure is exerted sideways too? We can show that pressure upward and downward in a fluid is caused by weight of fluid column or volume. A simple derivation of this:
$$\text{Pressure}=\frac{\text{Force}}{\text{Area}}=\frac{\text{Weight of the fluid column}}{\text{Area}}$$
$$~~~~~=\frac{\text{mg}}{... | It is not true that the pression of a fluid is due to gravity. Do you think fluids inside a bottle in the space station have no preassure? The right answer is the one you have quoted. The first link only refers to buoyancy, that does require gravity.
| {
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Best way to heat something in aluminum foil? Let's say we have a wet piece of paper, wrapped in aluminum foil, that we need to heat up in the fastest and most energy efficient way possible (no flamethrower).
What would that be?
Details regarding the methods would be highly appreciated.
| The microwave answer given above is good, especially if you have only one paper wrapped in foil because it would transfer a large fraction of the energy produced to the sample. If you have many of these (for example as a step on an assembly line) then immerse it in a hot medium. This would provide really efficient tr... | {
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Why does the light reflecting off of ocean water sometimes appear 'smoother'? Looking out the window at some water in the Harbour - I noticed that some parts of the water appear 'smoother' than others.
My question is: Why does the light reflecting off of ocean water sometimes appear 'smoother'?
| Two potential reasons among others: the wind is not uniform, if it blows larger on one sector it will move the water surface stronger. Second, different smal surface waves that come from different directions can interfere either negatively or positively, depending on their relative phases, over some small region, which... | {
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Gibbs free energy intuition What is Gibbs free energy? As my book explains:
Gibbs energy is the energy of a system available for work.
So, what does it want to tell? Why is it free? Energy means ability to do work. What is so special about this energy? Can anyone simply explain?
I just want a math-free intuition.
| First, you have system with some energy, named $U$ by physicists. You think you have all the information you need to characterize the system but then some guy comes near and says:
"Whoa, that's bad, the volume of your system can change."
You say:
"No problem, we just add here $pV$. Our new energy is $H=U+pV$."
"But he... | {
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Must Matter Particles Have A Hard Edge? It's my understanding that electrons are particles, and it's also my understanding that their location while orbiting an atom cannot be determined precisely and must be determined by statistics and probability, almost like electrons can be in multiple places at the same time. Tha... | This is one of the key results of quantum field theory: particles are not single points, they are disturbances in quantum fields that are spread out over space. Typically the disturbance is not spread out very much, otherwise it looks more like what we know as a wave than a particle. The technical term for what you're ... | {
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How did Rutherford's gold foil disprove the plum pudding model? What stops one of the two following scenarios from happening, consistent with the plum pudding model?
*
*The $\alpha$ particle, attracted by the electrons on the outer shell of the pudding, orbits nearly parabolically around the atom, causing the near-1... | Based on thomson's model, all of the alpha particles should go through or reflect back but in Rutherford's experiment, it was more of in between, some reflecting back and some going through, disproving the theory.
| {
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Cooling a satellite Satellites are isolated systems, the only way for it to transfer body heat to outer space is thermal radiation. There are solar panels, so there is continuous energy flow to inner system. No airflow to transfer the accumulated heat outer space easily. What kind of cooling system are being used in sa... | The satellite itself can do with radiative cooling but some instruments on board, e.g., IR sensors, require temperatures as low as than 4 K for which Helium dewars are used. Bolometers require even lower temperatures (in the mK range).
A good summary is available here.
| {
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Can the physical properties of the EM field be described directly from the 4-gauge potential? I'm trying to make an argument that classically, the EM field is considered a more 'real' physical quantity than the potentials, and am tempted to say that the fact that the field carries energy & momentum is evidence of this.... | You can find the Hamiltonian of the potentials if you know the Lagrangian, or you can calculate the energy in reference to the work done on charges in an electric field, which the electric potential sort of comes about secondarily, because it is related to the work. The calculation is done here. As for the equivalence ... | {
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Time slowed by gravity If time moves more slowly on Earth (due to our proximity to a gravitational body) than for someone orbiting Earth in a spaceship, yet the opposite occurs in the frequently cited "twin paradox" of the earth-bound twin vs. the twin in the fast-moving spacecraft, then my question would be this--is t... | You are correct, time dilation will occur for someone staying on the earth surface relatice to somebody in space. And yes, everything that can be measured by a clock, excepth the speed of light, will be perceived as slower, including molecular motions.
| {
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Introducing angular momentum for the first time to a class What is the best way to introduce the notion of angular momentum to a class without making it appear an unnecessary and artificial construction?
| Just two cents: I assume you already introduced Newton's laws, you can say that is something like "When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force"
yu can explain that from the other two Newtons laws it can... | {
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A particular coordinate transformation of a metric tensor So, this was a problem set question for my GR class due yesterday, and I can't for the life of me solve it, it seems I am missing something very trivial. Either the given answer is wrong, or I am.
The given line element is,
$$ds^2 = \left(1 - \frac{r^2}{\alpha^2... | There appears to be a typo in the coordinate transformation for $r$: it should be
$$
r = \rho\,\exp(\tau/\alpha).
$$
With this in mind, your reasoning is correct; if you work out the partial derivatives, you'll find that $g_{\tau\tau} = 1$, and you can work out $g_{\rho\rho}$ in a similar way.
P.S. also note that $t = ... | {
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What is the difference between electromagnet and solenoid? What is the difference between electromagnet and solenoid? Both these terms seem as the same thing to me. The only difference that I can find seems to be that an electromagnet contains a soft iron core. I'm sure there must be some other difference between the t... | A solenoid is an electromagnet but an electromagnet needn't necessarily be a solenoid.
A wire carrying electric current is an electromagnet. In fact, the very first electromagnet was a horseshoe shaped. So a suitable definition of electromagnet would be, anything which produces a magnetic field around itself when a cu... | {
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How are band gap energy, dielectric constant (permittivity) and resistance related to each other? The following three properties are related to current flow:
*
*Band gap energy
*Dielectric constant
*Resistance
I would expect them all to have the same trend (i.e. higher band gap energy would cause higher dielectr... | Dielectric constant is based on relative capacitance to vacuum. And it is usually calculated by using static electric plates. You put two parallel plates in vacuum and measure the capacitance. Then you do the same, but put the medium you want to measure instead. Divide the new capacitance of the new medium with the cap... | {
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What limits the doping concentration in a semiconductor? Si and Ge can be blended in any ratio, $\mathrm{Si}_x\mathrm{Ge}_{1-x},\ 0\le x\le 1$. So do
InxGa1-x.
So what exactly causes doping impurities inside Si/Ge/etc. to saturate at $\sim 10^{-19}\ \mathrm{cm^{-3}}$?
| In the chemistry of liquids and solids, some combinations are "miscible", meaning they mix in any ratio (like water + alcohol or silicon + germanium). Other combinations are not (like water + oil or phosphorus + silicon), in which case there is a certain "solubility", and if you try to put in more than the "solubility ... | {
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Why is Graphene So Strong? There has been a lot of news about Graphene since its discovery in 2004. And as we are all told it is a revolutionary material which is very strong, conductive and transparent.
But what is it about the structure of Graphene which makes it so strong?
| Graphene's strength mainly comes from the strong covalent bonds of the carbon atoms.
Graphite is made of layers of graphene but it is weaker because the layers making up graphite are bonded to each other through London forces hence why the layers can slide past each other and the material is soft. These weak inter-laye... | {
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What is the origin of CMB fluctuations? I have read somewhere that CMB (cosmic microwave background radiation) fluctuations in temperature are linked to mass distribution fluctuations in the early universe (at ~350000 years after Big Bang, which is of course when the cosmic radiation was emitted), and that is used to e... |
The CMB (cosmic microwave background) is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. It shows tiny temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all future structure: the stars and g... | {
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Induced EMF of a rectangular loop should be zero? Considering the shape of a rectangular loop in a changing magnetic field:
The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. Each wire in this loop induces $\epsilon$ opposes the other, and they sh... | You can only calculate the EMF induced from a close loop, instead of "a conductor" as you illustrate in your example A and B.
We know $\mathcal E = -\frac{d\Phi_B}{dt}$, where $\Phi_B=BA$.
EMF induced in a "conductor" makes no sense because we cannot find the surface $A$ to calculate the magnetic flux $\Phi_B$ through... | {
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Finding the energy lost due to non-conservative forces I stomped across this question and would very much appreciate any form of clarification.
A otter 75kg slides down a hill starting from rest. Hyp = 8.8, height = 6.5, final speed of otter = 9.2 m/s. And it wants me to find how much energy was lost due to non-conser... | As a first estimate you can evaluate energy deficit in conversion from initial potential energy to a final kinetic energy:
$$ \Delta E_{~lost} = mgh_0 - \frac {m{v^{~2}_f}}{2} $$
| {
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Is Parity really violated? (Even though neutrinos are massive) The weak force couples only to left-chiral fields, which is expressed mathematically by a chiral projection operator $P_L = \frac{1-\gamma_5}{2}$ in the corresponding coupling terms in the Lagrangian.
This curious fact of nature is commonly called parity v... | Okay, I think I have an idea why the terminology is used, but I think this argument makes little sense:
The Lagrangian term describing weak interactions is of the form
$$ \bar \Psi \gamma_\mu P_L \Psi W^\mu $$
Under parity transformations $ \Psi \rightarrow \gamma_0 \Psi$ and $ \bar \Psi \rightarrow \bar \Psi \gamm... | {
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Is time travel impossible because it implies total energy in the universe is non-constant over time? I have always argued with my friends regarding Time Travel that it is impossible. My argument has been that it will destroy the theory that all the energy in the universe is constant since when one travels to a differen... | I am not a physicist but all you experts seem to have not understood some basics. Your molecules are not unique - they have always existed as something else. When you go back in time your molecules are already existing as a tree or a bird or grass or Napoleon bleeding Bonaparte, etc. So if you appear in the past what a... | {
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What is the cause for the validity of Newton's third law? What, specifically, causes newton's third law? For instance, if I push on a wall, why is it that I experience a force in the opposite direction?
I seem to vaguely understand that is has something to do with electronic repulsion or molecular compression (maybe... | In John's answer he states:
But we know that momentum is conserved, ...
So we can derive Newton's third law from the conservation of momentum.
But you may ask where does momentum conservation come from?
Noether's Theorem
If we apply Noether's Theorem with the condition that the Lagrangian is invariant over space th... | {
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Do the fields exist without electric charges? I read in an old book on electrodynamics by Pauli that
theoretically there does not exist any need of charges to be there. Fields can even exist without the charges but still independent fields have not been observed still. EMWs are also produced from any accelerated char... | I don't know cases in which the e.m. field can be produced without charges, however I can tell you how to produce an e.m. field without electric charges. Take a neutral particle, meet it with its antiparticle, have them clash, and you'll get gamma rays. The latter are electromagnetic.
Also, gamma emission from excited... | {
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What will happen to a permanent magnet if we keep the same magnetic poles of two magnets close together for a long time? What will happen to permanent magnet's magnetic field or magnetic ability if we keep same magnetic poles of two permanent magnet for long time?
Will any magnetic loss happen over the long period of ... | If we keep two magnets with same poles together, then they'll become weaker overtime. I once deliberately did it to find out what would happen.
This could be explained with Magnetic Domain Theory. In magnets, the magnetic domains are aligned in the same direction, giving them a strong magnetic field. If two magnets ar... | {
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Simple example showing why measurement & interaction are different Does someone know of a clear (pedagogical) example where one can really see(with the math) where interaction and measurement are not synonymous in quantum mechanics?
*
*I know that every measurement involves a certain interaction with the outside wo... | Quantum optics demonstrates the existence of interaction-free
measurements: the detection of objects without light—or
anything else—ever hitting them.
Paul Kwiat, Harald Weinfurter and Anton Zeilinger SciAm November 1996
http://www.arturekert.org/sandvox/quantum-seeing-in-the-dark.pdf
| {
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Solar spectrum units Why is intensity $I$ on a graph of the solar spectrum always showed in units of $[\mathrm{W/m^2/nm}]$ instead of simply $[\mathrm{W/m^2}]$? (The y-axis on the graph.)
It is apparently shown as intensity per wavelength, but why add this extra specification? For me it just complicates matters (it is ... | Intensity has units of watts per area:
$$
\left[I\right]=\rm\frac{W}{ m^2}
$$
where the area is the surface area of the emitting source (in this case, the sun). This tells you the total amount of radiation present (over all wavelengths).
The extra factor of 1/nm in your plot gives the spectral irradiance:
$$
\left[\mat... | {
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Length contraction in cyclic space Consider a flat universe with at least one finite cyclic spatial dimension: travel x meters in one direction, and you will end up back where you started.
For an object that is of small size relative to the scale of the cyclic dimension, relativistic length contraction ought to work ou... | Why do you assume the length needed to "span the cyclic space" would be the same in different frames? With cyclic universes it helps to think of them as equivalent to an infinite universe where matter just repeats cyclically, see the "tiling diagram" with the bee and the spider on this page. From this perspective, it's... | {
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QFT and violation of Heisenberg uncertainty principle In some QFT books is said that a free electron can emit a virtual photon as long as it reabsorbs the photon and returns to its original state within a time:
$$\Delta t<\dfrac{\hbar}{2\Delta E}$$
That inequality DOES VIOLATE the Heisenberg Uncertainty Principle. Why ... | The uncertainty principle is still true in its usual form, but it refers to your knowledge of the state. Suppose your state is just one electron, you can confirm this if you observe the system for a time $\Delta t$ and you don't see additional particles. However, due to the uncertainty principle you can only measure pa... | {
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How are these types of time dilation related? How are these two phenomena related (if at all):
1. Gravitation slowing down time
2. High speed slowing down time
| In relativity there's no objective frame-independent way to compare the rate two clocks at different locations are ticking--different coordinate systems can give different answers (ultimately this is due to the relativity of simultaneity). There is also no frame-independent notion of speed, so you can't say in any obje... | {
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Half-integer eigenvalues of orbital angular momentum Why do we exclude half-integer values of the orbital angular momentum?
It's clear for me that an angular momentum operator can only have integer values or half-integer values. However, it's not clear why the orbital angular momentum only has integer eigenvalues.
Of ... | First of all, notice that from the general theory of angular momentum, the eigevalues $m$ of $J_z$ are integer if and only if $j$ is integer because $$m = -j, -j+1,\ldots, j-1, j\:.$$
At this juncture notice that, passing from Cartesian coordinates to spherical ones, you find $$L_z= -i \partial_\phi\:.$$ From direct in... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/153369",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "25",
"answer_count": 10,
"answer_id": 1
} |
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