Q stringlengths 18 13.7k | A stringlengths 1 16.1k | meta dict |
|---|---|---|
Why does a wet towel become more hot than a dry one? I remember a long time ago my mum had told me not to use a wet tea towel when taking food out of the oven because you can burn your hands - lo and behold, it came time to make dinner and I did not head my mother's warning from all them years ago and my hand got burne... | Missing from the currently-accepted answer is the importance of steam.
If you use a damp towel to grab a heavy metal dish out of an oven at $\rm 350^\circ F = 180^\circ C$, the heat from the pan will enter the water in the towel much more rapidly than conduction can carry it to your hand. Instead some of the water in ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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How is it possible for other animals to have better night vision than humans, who can detect individual photons? According to the Wikipedia article on night vision,
Many animals have better night vision than humans do, the result of one or more differences in the morphology and anatomy of their eyes. These include hav... | The quote you made from Wikipedia includes one of the major reasons for the superior night-vision in animals: Tapetum lucidum. The tapetum lucidum is a layer just behind the retina which is reflective; as the light enter the eye and hit the retina, it is not fully absorbed by the retina but instead passes through and ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/308735",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "19",
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How does $I = \mathrm{d}q/\mathrm{d}t$ work for a capacitor? When the capacitor is charging in a circuit consisting of a resistor, a capacitor and an alternating sinusoidal generator at $t=0$, the charge across the capacitor is 0 and the current is $I =\mathrm{d}q/\mathrm{d}t$. Does this make the current zero too? Whil... | The value of $f(t)$ at any specific value of $t$ does not automatically necessitate a value for $\frac{df(t)}{dt}$ at that value of $t$. If $q(t)\propto\sin(\omega t)$ then $\frac{dq}{dt}\propto\omega\cos(\omega t)$, which means while $q(0)=0$, you'd have $\frac{dq}{dt}|_{t=0}\propto\omega$, which would be a max for th... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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What happens to temperature as volume increases (charles law) Let us take Boyles law to start.
Assumptions:
*
*Gas is perfect.
*In a massless piston that can be expanded with no friction
*Adiabatic
If we were to decrease the volume of the piston, the pressure inside would go up because the gas molecules would be... | The answer itself is hidden in the second part of your question. P in any gas law refers to the pressure inside the volume of the container [ which is always equal to the external pressure on the container ]
Now, in the second part of your question, the statements
(P held constant)
&
if we were to spontaneously in... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Doppler effect and apparent frequency What is meant by "apparent frequency"?
I mean the answer we get by applying the formula; what does it signify?
If it is the frequency received by the observer, does it mean that the observer receives the same frequency no matter what the distance of the source?
Shouldn't distance o... | Picture yourself at seaside, and consider the frequency at which waves are reaching your feet. This frequency does not depends on where you stand. Distance plays no role.
Now if you move towards the ocean, or towards the beach, the period between two successive waves is altered.
*
*If you run to the beach as fast a... | {
"language": "en",
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When a force is applied on a body, which point does the body rotate around? I understand when a force is applied to a body it causes moments on all points not lying on the force's line of action. So which of these points acts as the pivot point around which the body actually rotates with a specific moment as a result o... | The motion caused by an applied force $F$ can be resolved into 2 components :
*
*A linear acceleration $a=F/m$ of the centre of mass (CM). If the force is an impulse the effect is to change the linear momentum of the CM.
*A rotational acceleration about the CM. The force produces a torque $\tau=r\times F$ about the... | {
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Why viscosity depends on the scale of things? Water feels like honey for bacteria and air is very viscous for small insects.
My question is why viscosity depends on the scale of things?
| To add a more formal perspective to the answers already given, if we non-dimensionalize the fundamental equations for fluid flow, the Navier-Stokes equations, using the characteristic flow velocity $U$ and characteristic length scale $L$, we obtain these equations in a form which only depends on the flow geometry, and ... | {
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Physical meaning of $L_z$ perturbation of rigid rotator The rigid rotor is a classical exercise in introductory quantum mechanics classes. The Hamiltonian is
$$H = \frac{L^2}{2m} \, .$$
Often you are also asked to consider the effect of adding a perturbation such that the new Hamiltonian has the form
$$H = \frac{L^2}{2... | Actually the example of your $H$ is spherical rotor, which is not so interesting. I suppose the example you have could describe a spherical rotor in a magnetic field.
If you're willing to go beyond this, the most common rotors are axially symmetric, with Hamiltonians of the type
$$
H= \alpha L^2+\beta L_z^2\left(\fra... | {
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What constitute the secondary particles beam when a high energy proton beam hits a target material? Basically I want to know what particles emerge along with high energy photons(not sure about it) as the second beam when a specific high energy proton beam is incidented upon a target material such as iridium or gold. Ca... | Here is an event from LHCb
A proton-lead ion collision observed by the LHCb detector .The image above shows a typical high multiplicity proton-lead-ion collision event,
The complexity of the outgoing fragments is large, and it constitutes charged tracks from electrons, muons, protons, nuclear fragments,photons ... in... | {
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Why does a force not do any work if it's perpendicular to the motion? I have a book that says the Moon's orbit is [in this context assumed to be] circular. The Earth does no work on the moon. The gravitational force is perpendicular to the motion. Why is there no work done if support force is perpendicular to the motio... | In case you are interested in a more mathematical approach, this can actually be proven entirely using geometry. To proof this you need to to know a few things about vectors.
If $\vec v=(v_x,v_y,v_z)$ is the velocity vector of the moon then $v_x$, $v_y$ and $v_z$ represent the components of the speed in the x, y and z ... | {
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Speed of block sliding on frictionless ramps Here's the question:
My book says the answer is C. How is it not A?
I know that all the potential energy is transferred to kinetic energy. With algebra, knowing Kinetic energy is (1/2) * m * v^2 and gravitational potential is mgh, I solve for h which results in (v^2)/2g
Ok ... | I got lost in your explanation, so can't help you figuring out where you went wrong. Your attempt to solve it with conservation of energy is certainly the easiest and correct.
Let's analyze the question
*
*smooth ramp: this is meant to mean no friction, no energy is lost
*starts from rest at height $h$: at this poi... | {
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What is the significance of the phase constant in the Simple Harmonic Motion equation? The displacement of a particle performing simple harmonic motion is given by $x = A \sin(\omega t + \phi)$ , where $A$ is the amplitude, $\omega$ is the frequency, $t$ is the time, and $\phi$ is the phase constant. What is the signif... | In the basic SHM equation, you get x=Asin(ωt) where at t=0, the object is at mean position or zero displacement. Now, what is the significance of the angle inside the sine function? It gives you the position of the particle performing SHM. When the angle is π/2, the displacement is maximum i.e A. When it is π, the disp... | {
"language": "en",
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Acceleration of car. One dimensional motion easy problem A car starts from rest and accelerates uniformly over a time of 5.21 seconds for a distance of 110 m. Determine the acceleration of the car.
My attempt at solving the problem:
$$a(x) = \frac{v - u}{t}$$
where
$v =$ final velocity
$u =$ initial velocity
$$$$
I get... | What is your $V_f$ ?
$V_f$ is not given in the question so you can't use this equation,
$$V_f =V_i +at$$
But the distance is given which will allow you to use
$$S=ut+1/2 at^2 $$
| {
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"source": "stackexchange",
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What causes change in planet's angular velocity? A satellite moving in an elliptical orbit will increase in angular velocity as it nears a planet. I understand that this is consistent with angular momentum. But what causes the increase in angular velocity if there is no torque acting on the satellite?
| There is no torque (with respect to the focus) which implies the angular momentum is constant, but since the angular momentum is $L=mr^2\omega$ and the radius $r$ changes, the angular velocity is not constant. The mechanism resulting this change can be understood as follows.
In an elliptical orbit (with non zero eccent... | {
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A question on Collison of macroscopic particles
Hello,
In the above question I could solve for average elastic force by taking velocity with respect to wall and finding change in momentum of the ball after that divided change jn momentum by time interval. Answer comes out to be option b.
But as it is written in the qu... |
Kinetic energy before the collision should be equal to kinetic energy after the collision
In an elastic collision, the "total" kinetic energy and "total" momentum of the system will be conserved. You missed that point. The individual kinetic energy of the ball and wall could change. But the total kinetic energy of t... | {
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"url": "https://physics.stackexchange.com/questions/310988",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
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M-theory with its 3-form $H$ and the problem of having no Lagrangian This is a standard question about the M-theoretic construction of the 6d (2,0) theory. This is done, in the simplest case, by an M2 brane hanging between two M5 branes. The theory on the M5 branes is the 6d (2,0) SCFT which has 4 fermions, 5 scalars a... | The problem with a self-dual three-form field strength is that the obvious kinetic term vanishes:
$$\mathcal{L}_\text{kin}=H\wedge *H =H\wedge H\,,$$
but since $H$ is a three-form, $H\wedge H=0$, so $\mathcal{L}_\text{kin}=0$. This can happen in $d=4n+2$, when you have a self-dual $(2n+1)$-form (in a chiral theory). In... | {
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Calculating magnetic field I would like to calculate the magnetic field near a neodymium magnet (N35). With near I mean I have got a rod magnet of about 10 x 4 mm and I am interested in the field from 0 mm under the magnet until 10 mm under the magnet down and also from 0 mm until about 10 mm to the right or left. I w... | For a NeFeB cylinder magnet a couple of equivalent quick approximations is that the field looks like that from a uniform density of monopoles on each face (oppositely signed of course), or is the field from a uniform sheet of current circling the axis on the curved outside of the magnet.
The integrals are easy to set u... | {
"language": "en",
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What is a "Standard value"? Temperature:a measure of the warmth or coldness of an object or substance with reference to some standard value.
I really tried searching lots AND lots for what is "Standard value" is... But I still don't understand what does it mean in that sentence.
Can someone please tell me what do the... | The "standard value" in this case refers to the concept of empirical temperature. The link explains with more detail the difference between empirical and thermodynamic temperatures.
In order to define an empirical temperature you need:
*
*A substance with a thermometric property (such as mercury and its volume expan... | {
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Why doesn't current decrease in series combination? I know that the question is quite stupid but I want to get an insight of this case. consider 3 resistors connected in series with a battery, after the current passes through resistor 1 it loses some of its energy, the kinetic energy of the charge carriers will definit... | The question is worded ambiguously. It could be construed to mean why isn't the current less in each additional resistor than it was in the resistor before it. The answers already here answer that question.
The question could also be asking why isn't the current less when it flows through 3 resistors in series than w... | {
"language": "en",
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Do physicists use particle "energy" to refer to kinetic energy? In 1963, this paper was written about the effects of radiation on solar panels. The paper states that:
When electrons at energies greater than 145 KeV and protons at energies
greater than 98eV bombard a silicon crystal, they can displace an atom from t... | Not only is it common to quote kinetic energy as the energy in contexts other than high-energy physics, it's basically also where the whole convention of writing energies in electron-volts comes from: $1\:\mathrm{keV}$ is the kinetic energy that a singly-charged particle, regardless of mass, picks up if you allow it to... | {
"language": "en",
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Translation of Vectors I am a bit confused about translation of vectors. In the first class in physics itself we are told that we can translate vectors as we like to the desired position to do whatever that we are trying to do. For example, if someone draws two random vectors then to get the sum, we translate them, ma... | Vectors that cannot be translated are the ones that depend on the origin of coordinates. First and foremost in the list is the position vector $\vec r$ followed all the others with definitions that depend on the position vector. These would be torque, $\vec \tau = \vec r \times \vec F$; angular momentum $\vec L = \ve... | {
"language": "en",
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Dirac delta function property in a scattering proof I'm studying the proof for the decoherence of the off diagonal elements of a density matrix through scattering with the environment and I'm stuck at a certain point:
My problem is A1.14 relation. (A1.13 as well to be honest, but I guess that the $(2\pi/L)^3$ is just ... | The answer given by jg255 is rather heuristic, but it gets the correct answer. A better approach is to realize that in scattering theory, when we convert scattering amplitudes, which have an energy-momentum conservation delta function, into probablities by squaring them and interpreting the delta-squared as a delta t... | {
"language": "en",
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External work required to move charge
There are 2 charged which are equal in magnitude and opposite in sign, and they are equidistant from the vertical axis AB. The voltage at both A and B would be 0.
If I introduce a new charge at point A, what would be the external work required to get it to point B?
In a case lik... | As you correctly note, the electric field will be perpendicular to the line AB and will not do any work on the test charge if it moves along that path. But, there must be some constraint to keep it on that path.
Assuming there is some constraint, let's now move the test charge from point A toward point B. To get the c... | {
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Terminology of 1PI diagram One-particle irreducible diagrams are diagrams that cannot be broken into two disconnected diagrams by cutting an internal propagator.
*
*Why are the external lines on an one-particle irreducible diagram truncated/amputated?
*What is the motivation for the word 'one-particle' in the term... | *
*It is a convention that simplifies some formulas. You can always choose to include the propagators on the external lines, but their effect is rather trivial so they don't add relevant information to the diagram.
*"One particle" means "one line". The reason for the word "particle" is that lines represent particles ... | {
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What does "dimension" mean in physics? Still I don't get what's the difference between dimension and quantity. Are they same or they are different?
| The dimension, or physical dimensionality, is a descriptor of a quantity. Thus "the distance between New York and Los Angeles" is a quantity, which has dimensions of length.
A bit more precisely, you can see nouns like length, velocity, time, and mass as describing abstract quantities, i.e. "a velocity $v$" specifies t... | {
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Continuous version of Coulomb's law for infinite charge distributions This might seem naive or have a trivial resolution, but I'm still searching for one and have been unable to find it.
Consider an infinitely long line charge with uniform line charge density $\lambda$. The electric field at any point in space is easi... | Both methods give the same answer, provided one applies them consistently. Coulomb's law actually reads
$$ \vec{E} = \frac{1}{4\pi\epsilon_0} \int_{-\infty}^\infty dx\,\frac{\lambda}{x^2 +r^2} \times \sin{\theta}\, \hat r,$$
where the factor of $\sin \theta$ is what picks out just the radial component, as required by s... | {
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Experimentally measure velocity/momentum of a particle in quantum mechanics In the context of quantum mechanics one cannot measure the velocity of a particle by measuring its position at two quick instants of time and dividing by the time interval. That is,
$$ v = \frac{x_2 - x_1}{t_2 - t_1} $$ does not hold as just af... | For a cold atom experiment, experimentalists use time-of-flight (TOF) measurement to determine the momentum distribution of atoms in the optical trap. Suppose there are an ensemble of atoms trapped in the optical trap, when the optical trap is switched off, the atoms will "fly around" with their momentum. With detector... | {
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Exchange Integral and Derivative respect to a parameter of a Dirac delta-function I'm trying to solve the 6.2 problem of Jackson's Classical Electrodynamics textbook. At some point, to get the desired solution, I have to exchange a derivative applied to a Dirac delta-function with the integral operator:
$$\int_{\mathbb... | By definition, if $T$ is a distribution, then $$\langle \partial_x T, f \rangle := - \langle T, \partial_x f \rangle\tag{1}$$ for every test function $f=f(x)$ in $C_0^\infty(\mathbb R^n)$ (or also $C^\infty(\mathbb R^n)$ if $T$ has compact support as the delta function). Here the derivative is just a bit more complicat... | {
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Biot Savart - Vortex line segment parallel to a plane I am trying to understand what is shown in this book (Applied hydro and aeromechanics), page 187
https://books.google.com.au/books?id=Ds-bd0zAwIYC
The velocity induced at a given point on a parallel plane by a vortex segment of length $b$, and strength $\Gamma$ is s... | For reference, here is the figure:
We are interested in the influence of the wing-bound vortex of length $b$ and circulation $\Gamma$ on point $A$, particularly the velocity vector $V_1$.
The Biot-Savart law in this case can be written as
\begin{equation}
V_1=\frac{\Gamma}{4\pi}\int\frac{\mathrm{d}\vec{b}\times\vec{R}... | {
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Why don't electromagnetic waves need vacuum to move? I understand that electromagnetic waves are caused by the oscillation of electric and magnetic fields. But how? Are there magnetic and electric fields in air, or in vacuum? How does this oscillation form a wave? Sorry, I am a new at this site, please simply put you a... | If you have an electric field ${\vec{E}}$, say, due to a charge density ${\rho}$ and the charged body is in motion then from Maxwell's equation $$\vec\nabla\times\vec{H}=\vec{J}+{\mu_0}{\partial\vec{D}\over{\partial t}}$$
i.e., the motion of the charges immediately produces a magnetic field $\vec{B}$ which, the magneti... | {
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Why does a simple pendulum or a spring-mass system show simple harmonic motion only for small amplitudes? I've been taught that in a simple pendulum, for small $x$, $\sin x \approx x$. We then derive the formula for the time period of the pendulum. But I still don't understand the Physics behind it. Also, there's no an... |
Why does a simple pendulum or a spring-mass system show simple
harmonic motion (SHM) only for small amplitudes?
Simple harmonic motion (in one dimension) is, by definition, a solution to the generic equation
$$\frac{d^2x}{dt^2}+\omega^2x=0,$$
where $x$ is a generic variable (it can be for instance a displacement or... | {
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Current as the time derivative of the charge I have been told that the current $i$ can be defined as
$ i = \displaystyle\frac{dq}{dt} $,
where $q$ is the charge and $t$ is the time.
I do not understand this definition because, if the charges are moving so that the net charge remains constant in an infinitesimally thin... | We define current as change in charge per time through an area $A$, not in and out of a volume. If the charge is moving through this area you get a current as expected because you have a net flow. If equally much charge passes through from both sides, the current is zero and the net charge is not moving.
| {
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"answer_id": 1
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Can an object falling in vacuum generate electricity by itself? When an object falls through vacuum, gravitational potential energy is converted to kinetic energy. Is there some way to get electrical energy out of the equation by itself (i.e. somehow convert the gravitational potential energy to electrical energy)? I... | A sufficiently large object will experience differential gravity ("tidal force") - this could be converted into a small amount of electrical energy by having two heavy spheres separated by a long rope; as they fall there will be a tension on the rope and you could let that tension do work on a generator / dynamo ("comp... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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About effects of torque off center of mass I am trying to deepen my intuition: If in outer space is a rod of length 2 meters standing still. At the both ends it has some heavy wheels of equal mass and the associated motors. If a motor starts spinning one of the wheels ... will the center of mass of the rod stand still ... | Your intuition is correct.
If no external torque is applied, then the angular momentum of the thing doesn't change. That means, if one or both wheels start to rotate, the entire assembly will start to rotate in some way so that the total angular momentum does not change.
You can sometimes see this if you watch dirtbik... | {
"language": "en",
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Looking for a good casual book on quantum physics I'm looking for something that is going to blow my mind without any scientistic ideas (e.g. something that sounds like science, but doesn't have anything in common with science), written by a professional physicist who spent a lot of time considering "what it all means... | Try "What is Quantum Mechanics" by Transnational College of LEX. It is very unorthodox. Under the supervision of a Nobel Prize winner the book was assembled by a number of different people who had to discover quantum mechanics from the ground up. Amazon or your local library. Be sure it's the second edition.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Method of image charges for a point charge and a non-grounded conducting plane I know how to solve Laplace's equation for a point charge in front of a grounded conducting infinite plane. But I want to know what happens (both physics and math) when the infinite conducting plane isn't grounded, or is connected to a poten... | If the problem you're trying to solve only contains one point charge and the conducting infinite plane at potential V, then there is no physical difference between the plane's potential being 0 (grounded) or +V, because the electric potential may be globally shifted by a constant value everywhere...
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/314982",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
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Electron wave and photon wave packet spreading I am looking for a physical interpretation of different behavior of electron and photon wave packets.
The dispersion relationship for a photon in free-space is linear ($\omega \propto k$), while for an electron (or any other massive particle) it is quadratic ($\omega \prop... | A dispersion relation tells you the form of $\omega (k)$. Since $E = \hbar \omega$ and $P = \hbar k$ you can see it as a relation between the energy and the momentum.
Since we have from special relativity that $$ E^{2} = p^{2}c^{2} + m^{2}c^4$$
it is clear that we have $E = Pc$ for a photon. Also since the total energ... | {
"language": "en",
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Would a gas "weigh" less than a liquid if they have the same mass? Thought experiment: I acquired two boxes of the same dimensions and same weight. One box contains $1\ \mathrm{kg}$ of water at room temperature while the other box has $1\ \mathrm{kg}$ of water, but in steam form, because the temperature of the box is a... | Actually, if you consider relativistic effects, the box with steam will weight more, not less, since it has more energy and hence more mass. But ignoring these effects, both boxes should weight exactly the same: you have the same number of water molecules in each box, and gravity effect on each molecule is exactly the ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Schrödinger equation and non-Hermitian Hamiltonians Is the Schrödinger equation still valid if we use a non-Hermitian Hamiltonian with it? By this I mean does:
$$\hat{H}\psi(t) = i\hbar\frac{\partial}{\partial t}\psi(t)$$
if $\hat{H}$ is not Hermitian?
| Someone else has already mentioned PT-symmetric Quantum Mechanics. To expand on that, and what I believe is the spirit of your question rather than the letter, sure; you can write down the Schrödinger equation using a non-Hermitian Hamiltonian. The interesting question from a physical perspective is whether this descri... | {
"language": "en",
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Conceptually, why is acceleration due to gravity always negative? As the title states, why is acceleration due to gravity always (-). Say you assign "up" as the positive direction. If an projectile is thrown at a 24 degree angle above the horizontal, I get that acceleration due to gravity before the vertex is negative.... | I think the OP is confusing acceleration and direction of motion. Acceleration does not depend on the direction of the motion. According to the Second law of motion, (for constant mass) $\vec F=m\vec a$, acceleration is in the sense of the resultant force acting on the particle. So, it does not matter that the particle... | {
"language": "en",
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Can something you see through a telescope be behind you? I read somewhere that gravity is able to bend light.
Is there a chance that, if the conditions are right, the light from one star could bend so much through space that when it reaches the telescope we use to look at it, it could actually be behind us?
As an anal... | Yes in principle that is possible, however usually you won't recognize the light as a star anymore, because it has to come very close to a large gravitational field of a black hole. What is indeed observed and you can find pictures of that is black holes acting as gravitational lenses so that you see galaxies or stars ... | {
"language": "en",
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Is weight a scalar or a vector? My professor insists that weight is a scalar. I sent him an email explaining why it's a vector, I even sent him a source from NASA clearly labeling weight as a vector. Every other source also identifies weight as a vector.
I said that weight is a force, with mass times the magnitude of g... | Here, I would suggest using NASA as more authoritative than you teacher to eliminate opinion. https://www.grc.nasa.gov/www/k-12/airplane/vectors.html
Directly from their opening summary: Scalars were quantities without direction, including length, speed, volume, area, mass, density, pressure, temperature...
Vectors a... | {
"language": "en",
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What kind of wave motion is described by grass moving in the wind? You know that sort of 'rolling' illusion when wind blows across long grass, like in the "amber waves of grain" line from America the Beautiful
It's not the same motion as dropping a rock in water, which causes an up and down motion.
And if wind blows a... | Each blade of grass is displaced from its equilibrium position according to the local velocity of the wind. This means the waves are actually periodic variations in the windspeed. This is likely due to vortices generated as the wind passes upstream obstacles, such as hills/woodland/buildings.
Here is a more realistic ... | {
"language": "en",
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"source": "stackexchange",
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Why do we not feel weightless at equator but feel in satellite A person living in his house at the equator goes in a circular orbit of radius equal to the radius of the earth. My question is, why does he/she not feel weightless as a satellite passenger does?
If we compare a geostationary satellite with the earth's equa... | Note: This answer contains the complete solution. The answer has been written such that the more you read, the more clues you get. At the end, you'll find the complete answer. Every line is a new hint. Please read each line and try to solve the question yourself. If you fail to solve, read the next line and try again.
... | {
"language": "en",
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Do 2 conductors (1 grounded via resistor) reach equipotential, before surplus electrons drain to earth? Case I: a negative conductor makes contact with a neutral conductor. Negative donates some electrons to neutral, until there is 0 potential difference. Then they both are slightly negative.
This happens quickly. (Ass... | To understand what happens during a transition, you'll need to go back to the basics.
The negative conductor initially has a surplus of electrons. These electrons don't like to stay close to each other and given enough time they will adopt a configuration which minimizes the potential energy as much as possible. In thi... | {
"language": "en",
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Existence of Higgs Mechanism Given a gauge group G and a subgroup H, does it always exist a Higgs Mechanism breaking G down to H? Is it unique, or with an obvious classification?
| The Higgs field starts out transforming in a representation $V$ of $G$. When acquiring a VEV that spontaneously breaks the symmetry down to $H$, the VEV has to be invariant under $H$, so when we decompose the representation $V$ into representations $\oplus_i V_i$ of $H$, at least one of the $V_i$ needs to be the trivia... | {
"language": "en",
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How can I solve the moment of inertia?
The source tells me to use the formula for a ring, but it is not possible, as the portions are nearer to the axis than a normal ring. How can I find the moment of inertia?
| Use the parallel axis theorem for calculating the moment of inertia relative to a point which is not the center of mas (COM). See for example here in Wikipedia.
$I=I_{\rm COM} - m d^2 ,$
where $d$ is the distance between COM and the point of rotation.
| {
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Difference between these two tensors? (help with index notation) What is the difference between the these two tensors?
$$A_{~~i}^{j} \text{ and } A_{i}^{~~j} $$
In my lecturers notes he states that $A^{~~i}_{j}=(A^T)^{i}_{~~j}$. Why is it this and not $A^{~~i}_{j}=(A^T)^{~~i}_{j}$ ? Thank you.
EDIT
Thank you for you... | Well if you are talking about a symmetric tensor then the quantities are equivalent. But if they are not then it is a similar process to taking the transpose of a matrix.
Conventionally if we have a rank-2 Tensor given by $A^i \;_j$ then the leftmost index represents the rows of a matrix and the columns represented by ... | {
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Is this problem from Griffiths' "Classical electromagnetism" correct? I am talking about pbl 2.46 pag 107.
The potential $$\frac{A}{r} \, \exp{(-\lambda r)}$$ is given I have to find the charge density and the electric field.
The book says the solution for the charge density is: $$\left[\delta^3(r)-\dfrac{\lambda^2}{r}... | This is but the screened Poisson equation. Since the functions are radial, only the radial part of the Laplacian survives,
$$
\nabla^2 (e^{-\lambda r}/r) = \frac{1}{r^2}\partial_r ( r^2 \partial_r (e^{-\lambda r}/r))= \frac{1}{r^2}\partial_r \left ( -\lambda r e^{-\lambda r} +e^{-\lambda r} r^2\partial_r \frac{1}{r}\r... | {
"language": "en",
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"source": "stackexchange",
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Why does space have the topology of a three sphere? Suppose that $U(x)$ is an element of the gauge group say $SU(2)$ and suppose $U(x)=1$ as $|\vec{x}|\to\infty$. Then, why does space have the topology of $S^3$?
This is done in Srednicki page 571. Note that I'm not asking how to prove that $SU(2)\cong S^3$. What I'm a... | The question can be formulated more generally as:
Why is it that when we consider functions over $\mathbb{R}^n$ such that the limit as $|\vec{x}|\to\infty$ is the same in any direction then we can identify their domain with $S^n$?
Note that although the functions that we consider might be $\mathbb{R}^3\to SU(2)$, the... | {
"language": "en",
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What determines the direction of a path on a line integral (vector case)? Line integrals are very important to use in Physics. For example, we calculate work by: W=∫<F,dr>. But I just got confused about something. What determines the direction of motion? The integral limits, or the vector dr?
Well, when we do the inter... | The parametrization of your path determines the direction of integration/motion. Let's say you have a semi-circular path of radius 1 moving anti-clockwise (i.e. from the positive x-axis to the positive y-axis to the negative x-axis). We can parametrize it as the following:
$r(t) = (\cos(t), \sin(t))$, $t\epsilon [0, ... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Why do ice cubes shrink in the freezer? OK before you all yell "EVAPORATION", I know that's the boilerplate answer, but why and how.
Back in high school we were taught water, and most other elements and compounds, have three states, solid, liquid, and gas. Which state it's in being dependent on temperature and pressure... | Ice at -10 centigrade has a vapor pressure of 259.9 p/Pa. It evaporates.
Water still evaporates at less than boiling.
| {
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If free quarks can't exist, how did the universe form? As I understand, the Big Bang started with a photon gas that then created the other particles. Thus obviously there would be some free quarks in the early Universe unless quarks are always created in pairs for some reason. How does physics resolve this?
| This article in wikipedia clarifies how the universe has evolved as far as our present understanding of particle physics and general relativity goes.
In particular for the strong interactions, the present theory is QCD,which models quarks and their interactions with other particles.
QCD enjoys two peculiar properties:... | {
"language": "en",
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Gaussian integral formula for matrix product I am looking for a way to prove that
$$ \det (M \cdot N) = \det(M)\det(N) \tag{0}$$
Where $M$ and $N$ are matrices with continuous indices, so that $\det$ is a functional determinant. A way to show that $(0)$ is wrong would also be welcomed.
This question is about the follo... | The following comments seem relevant to OP's problem:
*
*For a matrix $A\in{\rm Mat}_{n\times n}(\mathbb{C})$, define the symmetrized matrix $$A_+~:=~ \frac{A+A^T}{2}.\tag{A}$$
*Then the Gaussian integral reads
$$ \int_{\mathbb{R}^n} \! d^n x ~e^{-\frac{1}{2} x^T A x}
~=~ \sqrt{\frac{(2\pi)^n}{\det A_+}}\tag{B}$$
... | {
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The torque created using an oval chain ring I have seen the explanation on "How bicycle gear works?" but this seem to be using a standard round chain ring. I would like to know if there is any difference when an oval chain ring is used. As I can clearly see, one can have an oval chain ring on a fixed/single gear bike. ... | If you consider the chain ring as delivering torque about a point where the chain to the rear sprocket and front chain ring is tangential (the top portion of the chain is always under tension when delivering power) you'll see that torque is not constant. In the pedal cycle, when the bulge of the chain ring is tangent t... | {
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Generalized forces in Richard Fitzpatrick's Newtonian Dynamics In Richard Fitzpatrick's Newtonian Dynamics, page 128, generalized forces are defined as
\begin{equation} \tag{9.6}
Q_i = \sum_{j=1}^{\mathcal{F}} f_j \cdot \frac{\partial x_j}{\partial q_i}.
\end{equation}
Here, $q_i$ are generalized coordinates, $x_j$ a... | The numerator and denominator in partial derivatives don't cancel out simply. Keep that in your mind!
You should recall the chain rule in partial derivatives.
The formal answer to you question may be:
If $U=U(x_1,\cdots,x_f)$, and each $x_f$ is a function of generalized coordinates $(q_1,\cdots,q_g)$, the potential cou... | {
"language": "en",
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The illumination problem in context of a classical gas? Background
I was reading about the illumination problem:
"In 1958, a young Roger Penrose used the properties of the ellipse to
describe a room with curved walls that would always have dark
(unilluminated) regions, regardless of the position of the candle.
P... | Molecules of gas collide with one another whereas photons do so very rarely.
Another way of putting that is that light travels in straight lines (for the Penrose analysis) whereas gas molecules do not.
| {
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"url": "https://physics.stackexchange.com/questions/318451",
"timestamp": "2023-03-29T00:00:00",
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Expressing position due to gravitational acceleration as a 3-Dimensional differential equation I know that the force of gravity is $F = \dfrac{G m_1 m_2}{r^2}$. Now assume in a one-dimensional system there are two masses in the universe, a planet and an object. The object would have an acceleration of $A = \dfrac{G ... | The easiest thing to do is to start from the gravitational potential
$$
\phi=-\frac{Gm_1m_2}{\sqrt{x^2+y^2+z^2}}
$$
and use $\vec F=-\vec\nabla \phi$, or in components:
$$
F_x=-\frac{\partial \phi}{\partial x}=
-\frac{Gm_1m_2\,x}{(x^2+y^2+z^2)^{3/2}} = m_2 A_x = m_2 \frac{d^2 x}{dt^2}
$$
with a similar approach for yo... | {
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Why doesn't saturation current in the photoelectric effect depend on the frequency of light absorbed by the metal emitter? If current $I$ is given by $I = nAev$, where $n$ is the number of electrons per unit volume, $A$ is the area, $e$ is the charge of an electron and $v$ is the velocity of the electron, it must mean ... | I do not think that the speed of the electron is important, and the reason for this is that the electron will face a repulsion of the electrons on the anode surface, so the importance is the number of electrons that will arrive because they will almost stop
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/318668",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How does one measure the curvature $k$ in FLRW metric? How does one measure the curvature parameter $k$ in the FLRW metric? $$ds^2=-c^2dt^2+a^2(t)[\frac{dr^2}{1-kr^2}+r^2d\theta^2+r^2\sin^2\theta d\phi^2]$$ In particular, what is the convenient equation (involving $k$) that is/can be used to measure $k$?
EDIT I'm look... | This is a very hard question to answer in detail as it requires several pages of mathematics to derive the required formulas (there is no easy fit like $F=-kx$ as you suggested)
I will not derive the formula (it can be found in e.g. Dodelson) but after some work you obtain:
$$\Delta(m-M) = 5\log\left\{ \left( \frac{c}... | {
"language": "en",
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How is entropy a state function? Is there only one reversible way to move from one state to another?
If we consider two states $A$ and $B$ on an isotherm and we move from $A$ to $B$ by first reversible isochoric process and then reversible isobaric process. Now the path followed should be reversible since both the ... | Entropy is surely a state function which only depends on your start and end states, and the change in entropy between two states is defined by integrating infinitesimal change in entropy along a reversible path. But heat $Q$ is not a state variable, the amount of heat gained or lost is path-dependent. Once we divided $... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "17",
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How a moving car becomes electrically charged? Car has been electrically charged as it travels along the road.how is this possible?
| If the tires aren't conductive enough, the asphalt/concrete surface of the
road acts like fur, and the tires act like amber, and charge separation
occurs at the contact point, then the movement of the wheel does work on
the separated charge. Charge repels, leaking through the axle to the car body,
accumulates on the ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/319361",
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Stings in the loop space of timelike curves In Smith's paper on homotopy groups for Lorentz manifolds, he builds the loop space of all timelike loops in the following fashion :
*
*Consider all piecewise continuous timelike curves which start and end at point $x$. This include timelike curves with $q$ changes in tim... | Note that the author defines the loop space $T_q$ to be the space generated by loops with $q$ corners. You want to show that $ff^{-1}\sim e$ in the timelike sense. But $ff^{-1}$ will have at least $2q$ corners since you get corners from each copy. So you include all of the curves of this form in your definition of $T_q... | {
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Expansion coefficients in the solution of the Dirac equation for a free particle So my question is why do we need to write the coefficients $b$ (that after the second quantization are going to be promoted as the antiparticle creation operators) as complex conjugate? I mean, why not just write $b$, without the complex c... | It is just notation. If you schematically write
$$
\psi\sim A\mathrm e^{-ipx}+B\mathrm e^{+ipx}
$$
then you can check that
$$
\begin{aligned}
{}[H,A]=-\omega A\\
[H,B]=+\omega B
\end{aligned}
$$
so that $A$ behaves like an annihilation operator (it lowers the energy) and $B$ behaves like a creation operator (it increas... | {
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} |
Photoelectric effect:- Reduction of wavelength increases current? I did a question in which, the intensity of the incident radiation on a metal surface was kept constant but the wavelength of the photons has been reduced. The question inquired what will be the effect on the maximum photoelectric current? The initial wa... | I assume in reality everything is much more complicated, but in the simple model it is quite clear:
A (bound) electron interacts with a photon, and can gain (at most) the photon's energy E. If E is large enough, it can overcome the bound and become a free electron. If E gets even larger, then the electron could even ov... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
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Can space exist without matter or energy? Is it possible for the space to exist without matter or energy?
|
Is it possible for the space to exist without matter or energy?
As a possible mathematical model of nature, yes.
Proof: Newtonian gravity , a classical very successful model, i.e. predictive and not only descriptive. General Relativity which posits that matter and energy form space time , reduces to Newtonian gravit... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/320627",
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Continuous vs discrete set of eigenvectors for a single particle Hamiltonian The Hamiltonian operator of a free particle in three dimensional Euclidean space has an infinite set of eigenvectors lableled by the momentum of the particle, $| p \rangle $. Not only is this set infinite, but it is also continuous. But if we ... | Whether a particular Hamiltonian gives rise to discrete or continuous momentum spectra depends on the energy of the particle(assuming the particle is in energy eigen-state). For instance consider an electron in Hydrogen atom potential but with energy $E > 0$. Then the electrons motion is unbounded and it can have conti... | {
"language": "en",
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What information can we extract from the electronic band structure? I have some difficulty in understanding the electronic band structure.I want know that for a 3D crystal,what information can I extract from its complicated band structure,for example the band structure of the SiC(I downloaded this figure from google).A... | The most useful information you can extract from the band structure for an insulator like SiC involves: (1) the value of the bandgap (the energy difference between the highest occupied band--the valence band--and the lowest unoccupied band--the conduction band); (2) the direct or indirect nature of the band gap (direct... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/320941",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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What causes like electric charges to repel and opposite electric charges to attract at the smallest level? When talking about charged particles, the law of charge dictates that two particles with opposite charge will attract each other and two particles with the same charge will repel each other.
However, I have never... | Short answer:
It is a consequence of
*
*Physics is governed by a stationary action principle
*Locality
*Lagrangian is Lorenz Invariance
*Gauge invariance
Long answer(and still skipping lots of math):
From a relativistic point of view:
Starting from the action principal, we try to write down a simple action involv... | {
"language": "en",
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Quadrivectors in relativity This is what I understood about 4-vectors in relativity.
We define the contravariant and covariant vectors like this :
$$
A^\mu=\begin{bmatrix}
A^0 \\
A^1 \\
A^2 \\
A^3
\end{bmatrix}$$
$$
A_\mu=\begin{bmatrix}
A_0 \\
... | The answer is that it doesn't really matter as long as you are consistent !
To see this, let us define $X^\mu = [a\ b\ c\ d]^T$ and $\tilde{X}^\mu = [a\ -b\ -c\ -d]^T$ we will see that these are equivalent up to a change of basis. Which proves that it is indeed irrelevant as long as we are consistent!
Expanding both ve... | {
"language": "en",
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"source": "stackexchange",
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if white things reflect light and mirror reflect light why don't they look the same We learned at school that white object reflects all the light that falls on it. We also learned that a mirror reflects all light as well. However, we cannot see ourselves in a white object while we can see ourselves in a mirror.
What m... | MIRROR
*
*The thing is that, A mirror is a surface from which light get totally reflected.
*It have a polished surface. We generally see mirror effect from metal surface.
*The light which come in strike at angle $\theta$ to the normal and reflect away at $\theta$ from the normal.
WHITE SURFACE
*
*It is a surf... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/321357",
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Kinetic, potential and total orbital energy in General Relativity In Schwarzschild geodesics the total orbital energy $E$ is
$$E = \dot{t} \left( 1 - \frac{r_{\rm s}}{r} \right) m \, c^2$$
with the time dilation factor $\dot{t}$ in dependence of the local velcity $v$
$$\dot{t} = \frac{1}{\sqrt{ \left( 1-\frac{r_{\rm s}... | Your expression for the total energy is
$$E=\frac{mc^2(r-r_s)}{\sqrt{r(r-r_s)(1-v^2/c^2)}}=mc^2\gamma\sqrt{1-\frac{r_s}{r}}$$
If you wish to split this up into kinetic and potential energy, we recall that the kinetic energy in Special relativity is $E_{\text{kin}}=mc^2(\gamma-1)$, and so we have
$$E=mc^2+E_{\text{kin}}... | {
"language": "en",
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Why do all electromagnetic waves travel at the same speed when travelling through vacuum? What does my teacher mean when he says that all electromagnetic waves travel at the same speed when travelling through a vacuum? If you may, please answer as simple as possible.
| Electromagnetic waves include visible light, radio waves, X-rays, and so on. What distinguishes these different bands of light is their frequency (or wavelength). But what they all have in common is that they travel at the same speed in vacuum.
The reason for qualifying 'in vacuum' is because EM waves of different freq... | {
"language": "en",
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Multiple star system, stable orbits? Inspired by worldbuilding SE, I know that there are relatively stable star systems with two or three suns, but any more than that and they start to become very unstable (e.g. trapezium systems), but I'm more interested in the concept of >3 stars, each of similar mass. How could they... | Perhaps this isn't the answer you're looking for, but star clusters (particularly globular clusters) and galaxies would seem to fit the bill. Galaxies are a bit more of a stretch since this is probably (1) much larger than what you have in mind and (2) typical galaxies are predominantly made of things other than stars,... | {
"language": "en",
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Position vectors I was reading Kinematics from my textbook. It started off by defining vectors and its two types:
1. Free vectors
2. Fixed vectors
That's all good. But then it went on to say that all the vectors we'll be dealing with in 'our' study will be free vectors. After that it defined position vectors. Isn't p... | I believe your book is wrong in assuming that the only vectors required in the study of Kinematics, at any level, will be free vectors. The position vector is a very fundamental vector in Kinematics, and clearly, it is a fixed vector. Why? Because changing it's position will change the position of the object itself, an... | {
"language": "en",
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How is the curl of the electric field of a dipole zero? For a static charge, the curl of the electric field is zero. But in the case of a static dipole the electric lines of force curl. How it that possible?
| I think I do not understand the equation. If you try to find the electric field for a static dipole you have two main way: starting from a potential and then you make the approximation of great distance so that the multipole expansion is truncated to the second order in the charges, or you can compute the electric fiel... | {
"language": "en",
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Magnetic rod in Magnetic Field How does thin magnetic rod suspended in uniform magnetic field oscillate? I want to know what kind of oscillations will they be (displacement will be angular or linear) and which phenomenon (what forces are involved) causes these oscillations? And why it has to be thin?
For example:
In a ... | There will be a magnetic moment induced in the rod. For a small angle displacement from stable equilibrium, the torque $\tau=m\times B=mB*sin(\theta)\approx mB\theta$
This, when equated with the product of the rod's moment of inertia and angular acceleration will give a differential equation that shows that the rod wil... | {
"language": "en",
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Is a mirror-less telescope possible? I was reading about telescopes and the Hubble Telescope for example has a 2.4m mirror which reflects lights to a sensor. Other type of telescopes use lenses to focus light to the imaging sensor.
I was wondering, is it possible to have a telescope without a mirror or lens?
So in Hubb... | Systems other than mirrors can provide telescope function.
There's four other ways to make a telescope.
*
*Like Galileo did, we can use lenses instead of mirrors (this does not work well at large scale).
*One can employ diffraction structures (holograms, or gratings,
or zone plates), but there are significant... | {
"language": "en",
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Equivalent temperature of air to feel as if it was in water I got interested in why, at the same temperature, water feels cooler than air. After a google search, I saw that this question got answered here: Why does water feel cooler than air. I understand the answer given there.; however, is there a formula which I co... | It can't be done to any accuracy really here's why.
You need to have the same heat-flow out of the body into the air (insulator) as into the water (conductor). We need to consider both conductive and convective losses through the medium.
At any given temperature, as air is an insulator, the ratio of conductive to c... | {
"language": "en",
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What determines how much power goes into each diffraction order? Imagine a grating with infinite number of slits, and the spacing D between slits is larger than the wavelength so that there are high order diffractions. In each of the diffraction directions the waves constructively interfere, but what decides the percen... | There are two steps only you need to calculate the intensity distribution behind a multi slit. Firstly you have to calculate the intensity distribution pattern behind a single slit. Secondly you has to calculate the aberration of the pointlike source to all the slits and to the observers screen and by this sumerize th... | {
"language": "en",
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"source": "stackexchange",
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Electric current though liquid - Magnet interaction I f a have a coil with an electric current flowing through it, and I place it inside of a magnetic field, it will move.
Now my question is, what happens if I have a droplet of liquid with an electric current flowing through it, will it also move ? If not, why not ?
| Yes, of course it will move. Here is a familiar science demonstration, the
mercury 'beating heart' BEATING HEART which
shows a crude battery circuit (the mercury and the iron in a
mild acid). When the iron touches the mercury, the battery
puts a current through the contact point, and that current
deforms the mercu... | {
"language": "en",
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Why during annihilation of an electron and positron 2 gamma rays are produced instead of 1? $1 \gamma \rightarrow 1 e^- + 1 e^+$ (pair production)
Then why
$1 e^- + 1 e^+ \rightarrow 2 \gamma$ (annihilation of matter)
instead of
$1 e^- + 1 e^+ -> 1 \gamma$ ?
| The answers given above are incomplete, probably because the OP is misleading. Everyone discussed the case of pair annihilation (or creation, if you wish to consider time-reverse processes) in vacuo, in which case the answers provided are correct.
However, being in vacuo is a useful generalization, but often is also a... | {
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Structure of space Can we consider the structure of space( only space not space-time ) to be that of a vector space? Why can we or why can't.
And why cant we give a vector space structure to space-time?
| A vectors space has a preferred vector, the zero vector. Instead there is no preferred point in classical (flat) physical space. Physical space is better described by a so-called three-dimensional affine space. Metric tools are then represented by a scalar product in the space of translations. This space which descri... | {
"language": "en",
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Is the diffraction pattern of a vertical slit horizontal? I am familiar with the mathematical aspects of single slit diffraction pattern, at the undergraduate level.
Consider the following pictorial representation from the book Optics, by Hecht:
The fact that I find puzzling here is - even though the slit is shown ver... | It is the narrowness in the horizontal dimension which cause horizontal diffraction. The slits are only tall because they are not wide.
Incidentally, the diagram is wrong. It shows light entering the full heights of the slits from top to bottom. If it did that, the fringes would be tall too - like vertical lines.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/324114",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "2",
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Trolley problem
A trolley of mass $300kg$ carrying a sandbag of $25kg$ is moving uniformly with a speed of $27km/h$ on a frictionless track. After a while, sand starts leaking out of a hole on the floor of the trolley at the rate of $0.05kg/s$. What is the speed of the trolley after the entire bag is empty?
I do not... | In some ways this is a "make you think" question because it has a very different answer from a question which has sand added to the trolley.
What you have to realise is that at the instant the sand leaves the trolley its horizontal momentum does not changed so no horizontal force needed to be applied by the trolley on ... | {
"language": "en",
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Is the Charge on proton is $3.2\times10^{-19}$ greater than that of charge on electron? As the charge on electron is $$e^-=-1.6\times10^{-19}C$$ and charge on proton is $$p^+=+1.6\times10^{-19}C$$
Does this mean that the charge on electron is $3.2\times10^{-19}C$ less than that of charge on proton?
| No, The magnitude of charge on both the particles is same but opposite sign.
Actually the negative sign doesn't always mean "less", especially in physics. So, the charge on $e^-$ is not smaller than that of $p^+$.
| {
"language": "en",
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Is there a way to physically resolve elliptically polarized light into circularly polarized components? I was reading about a phenomenon called circular dichroism, in which LCP(left circularly polarized) and RCP(right circularly polarized) light are absorbed to different extents. Hence, when linearly polarized light(LC... | One way to do it is to pass the beam through a quarter-wave plate, then through a traditional beam splitter that separates horizontal from vertical polarization, then through a quarter-wave plate again.
| {
"language": "en",
"url": "https://physics.stackexchange.com/questions/325608",
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"source": "stackexchange",
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Circuit with three capacitors and a switch I have a homework problem where I have a battery connected to a capacitor and a switch that connects the capacitor to 2 others in series/disconnects the circuit from the battery.
The question is:
S is initially closed to the left until c1 is completely charged. Once charged,... | Remember: Capacitors in parallel all have the same voltage across their plates, but can have different charges.
So, when the switch is closed to the left, C1 is has a certain charge and voltage. Then, when the switch is closed to the right, that charge in C1 distributes in such a way so that the voltage across all thr... | {
"language": "en",
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Opposite of particle decay I have read about particle decay, a process in which one particle becomes several other particles. However, I have not been able to find much information about its opposite: several particles combining into one particle. Is such a process possible, and if so, under what conditions? For exampl... | The triple-alpha process is a way $C^{12}$ is formed in stars from three $He^4$ nuclei once (most of) the hydrogen in the core has been burned. It is really two reactions,
$He^4+He^4 \to Be^8, Be^8+He^4 \to C^{12}.$
$ Be^8$ is unstable, but lasts long enough for this to happen if the helium is dense and hot enough.
| {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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How can length be a vector? Length and current both are not vectors. Then how can we assign the vector $l$ to the length of a wire carrying current while calculating for a current carrying conductor in a magnetic field. Also why in Biot—Savart law do we take small length element $dl$ as a vector?
Why is length sometime... | Biot-Savart's law is the cross product of two vectors, the current vector and the vector representing the point whose magnetic field you want to calculate. It is easier to understand using the law for a single charged particle.
$$\vec B = \frac{\mu_0}{4\pi}·q·\frac{\vec v \times \vec r}{{\lvert \vec r \rvert}^2}$$
Make... | {
"language": "en",
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"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
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Current constraints on Dark Matter self-interaction from galactic profiles The self-interaction of dark matter may be small but it cannot be negligible if it is able to dissipate energy to relax into galactic clumps (necessary to explain galaxy rotation curves).
According to some answers in this old question: How Does ... | There is currently no strong evidence that dark matter interacts with ordinary matter via anything besides gravity. There have been proposals for non-gravitational interactions to explain discrepancies with small scale observations. But these discrepancies can also be explained by the effects of ordinary baryonic physi... | {
"language": "en",
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The direction of the induced electric field Recently I got stuck witht the following problem.
Imagine we have uniform a magnetic field which induction points upwards. The fields strength is steadily decreasing. If we put an iron coil perpendicular to the magnetic induction vector, then, obviously, there will be electr... | Given your drawing, equal area loops normal to a uniform, changing field, all we can say is this:
1)The integral around the loops of the electric field dotted into the line element are equal--it says nothing about the direction of the field at any point.
2) (Lenz's law) The direction the current flows is the direction ... | {
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General relativity: How is the 4-velocity and momentum defined in GR? I know that in SR, the 4-velovity $$ u^\mu = (d t/d \tau,d x/d \tau,d y/d \tau,d z/d \tau ) $$ and $$p^\mu = m u^\mu.$$ How do these generalize to GR? I imagine there are new complications, particularly by what we let $p^0$ be. And are these only def... | On a general Lorentzian manifold with a time-positive metric, $d\tau^2 = g_{\mu\nu}dx^{\mu}dx^{\nu}$. In SR $g_{\mu\nu} = \eta_{\mu\nu}$ which gives the familiar relation:
$$
d\tau^2 = dt^2 - dx_idx^i
$$
In GR the Einstein Field Equations tell you what $g_{\mu\nu}$ is so you can compute the altered line element.
Otherw... | {
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Is light deflected by external electric and magnetic field? I recently read about the Maxwell's laws of Electromagnetic Waves and I found that Light is made up of both Electric and magnetic fields.
So now if i pass the light through a capacitor such that the plates are parallel to the light will the light be deflected?... | Looking at the classical electromagnetic wave with E and B fields propagating perpendicularly to each other and the direction of motion. These fields do not carry charge, and it is only charge that is deflected/senses electric and magnetic fields. Classically, there can be superposition of two waves which show interfer... | {
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why is it that only the perpendicular component of a wave can pass through a vertical polarising filter? To me it seems unintuitive that a filter can change the direction of a wave. I also do not understand malus' law, could somebody please show me an intuitive derivation or proof of malus' law.
Is unpolarised light, d... | The component of the field that is parallel to the wire looses its energy in the wire by Ohmic dissipation so that only the perpendicular component remains.
Given this, only the perperdicular component $E\cos\theta$ of $\vec E$ will go through. Since the intensity is proportional to the square of the magnitude of $\ve... | {
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Why mass & energy bend spacetime? I understand how light / matter bend spacetime but I'd like to understand WHY. Is there some kind of interaction?
| As this concept was originated by Einstein,he thought the Newton's law of gravitational is a classical case of two body attraction but for the large scale he generalize the notion of Newton.He thought that space-time is like a trampoline pad and if there is some object(mass) on a trampoline pad it will definitely bend... | {
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Change in shape of Atomic Orbitals We have studied that electron's position around the nucleus is given by probability distribution function, which determines the shape of atomic orbitals. Does the shapes of orbitals change with factors such as electric fields, magnetic fields, high temperatures, intense gravitational ... |
Does the shapes of orbitals change with factors such as electric fields, magnetic fields,
In quantum mechanics, any new extra potentials , as an introduction of an electric or a magnetic field, will lead to new solutions for the orbitals.
Example : the Zeeman effect in hydrogen:
When an external magnetic field is ap... | {
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How to determine the components of a velocity vector undergoing a central force I am busy working on a 3D $n$-body galaxy simulator and I am having some difficulty wrapping my head around determining the initial velocity components of each particle to ensure a circular orbit around a central mass.
I know that the magni... | The velocity in $(x,y)$ plane at the circular orbit can be viewed best in polar coordinates (spherical for $\theta \to \pi/2$ if you have 3D model). Then the initial velocity must have $v_r = 0$ and $\vec v = v_\phi \vec e_\phi$. You can calculate the $v_\phi$ through
\begin{equation}
v = \sqrt{\vec v \cdot \vec v} = \... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/326909",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "1",
"answer_count": 2,
"answer_id": 1
} |
What is the relation between image velocity, object velocity and mirror velocity? Suppositions used:
Velocity of image = VI
Velocity of object = Vo
Velocity of mirror = VM
I Know the fact that VI=-Vo supposing mirror at rest
and VI=2VM supposing object at rest
Now considering both mirror and object in motion, VI=2V... | I have considered an example and proved this formula in the image shown.
*
*The first image has plane mirror and the person at rest.
*Let us now consider the person velocity as $V_o=\frac{Unit Block(UB)}{second}$ towards the mirror and mirror velocity as $V_M=\frac{UB}{2}$ towards the person.
*The second image show... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/326992",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "3",
"answer_count": 5,
"answer_id": 3
} |
Is there any significant difference between a tokamak and a spherical tokamak? The title question is quite self-explanatory. Despite the fact that Spherical tokamaks are more spherical in shape, what else differentiates the ST from the conventional tokamak. I've heard that ST's use reverse field configurations from a w... | To understand the most significant difference, we must first understand how a conventional tokamak generates part of its magnetic field: the poloidal magnetic field is created by driving a toroidal current, i.e. a current along the (toroidal) plasma column. That current is driven by ramping up (or down) a voltage of a ... | {
"language": "en",
"url": "https://physics.stackexchange.com/questions/327083",
"timestamp": "2023-03-29T00:00:00",
"source": "stackexchange",
"question_score": "4",
"answer_count": 1,
"answer_id": 0
} |
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