# All Questions

134,310 questions
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### Decay of (pseudo)scalar particles

I consider the process $\phi\rightarrow\bar{\psi}\psi$, where $\phi$ is (pseudo)scalar and $\bar{\psi}$, $\psi$ are anti-fermion and fermion, respectively. So, how I can distinguish the process with ...
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### Gauge singlet under SM

What does it mean when they say something is a gauge singlet under the Standard Model group? I would like to understand this concept of Singlets and Doublets. Thanks in advance.
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### LC-circuit and the current

A capacitor and a coil are charged in the LC circuit. When the circuit switch is closed, the current starts to flow. In theory the vibration and the current is not deteriorate, but why in reality the ...
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### To isolate a particular mode of vibration in a standing wave on a string

Suppose a string bound between two rigid end-points is vibrating and it is a combination of a number of normal modes of vibration, is it possible to isolate a particular mode of vibration in wave by a ...
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### Vectors (with reference to a pendulum)

Why is the accelaration of a pendulum non-zero at it's lowest point?
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### Super confused over simple unit conversion from centimeters to cubic centimeters

I'm stuck in a really stupid question. It seems like the meaning of "centi" is chancing meaning. Because if: $r = 5 cm = 5 \cdot 10^{-2} m$ so $c = 10^{-2}$ But say i need to take the cubic root ...
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### Is discharge tube dangerous

Are those gas discharge tubes dangerous? I found atom spectra interesting, so I booked a 1.5 hour lab session at a local high school. The physics teacher gave me a few hydrogen tubes, helium tubes, ...
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### Change in the entropy of the cube [on hold]

Suppose that a uniform-density cube of volume l³ and mass m is in the room and is under pressure P and at temperature T . Gravitational acceleration is of magnitude g and its direction is downward. ...
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### Unification of gravity and electromagnetism

Are there any attempts at unifying gravity and electromagnetism at least at classical level since Hermann Weyl's idea of gauge principle (1918)? We now have Standard Model which is very successful and ...
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### help with question regarding maxwell distribution law for velocity of gases [on hold]

Here is the question. I have succesfully completed the first question where I had to find out the equilibrium distribution of molecular translational energies. But i'm not able to get the second ...
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### Kac-Moody algebra from WZW model via Poisson brackets

In 'Non-abelian Bosonization in Two Dimensions', Witten shows that the Poisson brackets of the currents that generate the $G\times G$ symmetry of the WZW model give rise to a Kac-Moody algebra upon ...
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### Why are there these other “harmonics” (violin-specific terminology) in violins?

I've often read explanations of how standing waves form on the string of a violin and their harmonics but there is another phenomenon I've never seen explained: When you play certain notes, musicians ...
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### Why is this version of Fourier deconvolution justiifed?

As I understand it, one typically performs Fourier deconvolution for the data produced by some instrument of a linear system by: Taking the observation of some unit impulse signal $m$, with the ...
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### A circularly charged insulating disk rotates

A circularly charged insulating disk rotates around its axis. Why is there a magnetic field around the rotating disc?
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### The potential energía of a pendulum attached to a vertical spring

I have the next problem about lagrangian mechanics: A particle of mass $m_1$ is suspended of a vertical spring of constant $k$ and natural length $l_0$. A second mass $m_2$ is attached to the first ...
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### Simple Hamiltonian in curved space-time

Consider the theory of a free massless scalar field with a non-trivial background metric: $$\mathcal{L} = -\sqrt{-g} \left( \frac{1}{2}\partial_\mu \phi \partial^\mu\phi\right) .$$ (I prefer the '...
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### Are sounds louder at night, in the winter? [duplicate]

Surface temperatures of -40C and +40C are not unheard of in habitable areas on Earth. That's a variation of over 30% in density (for a given barometric pressure). Does that mean that for the same ...
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### Is it a myth that protons and electrons combine to form neutron?

I found some this- Where are the protons and electrons in a Neutron Star? The rest is thought to be mostly neutrons. The crust is made of mostly iron nuclei bathed in freely-moving electrons. When the ...
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### One Electron Universe - Bumping particles back in time?

I understand that John Wheeler's 'One Electron Universe' proposal is not generally accepted, but it's obvious that in disproving such a proposal, or at least discrediting it, we can gain insights to ...
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### Attaining extrema when a stationarity condition has no solution

I was wondering if someone could shed some light on the following for me: If a stationarity (maximizing or minimizing) condition has no solution inside a particular domain, then how do we reason that ...
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### How can Carnot's theorem and the second law of thermodynamics logically equivalent?

Can anyone please explain how Carnot's theorem which states that "working between two same temperature limits,a reversible engine has maximum efficiency " and the second law of thermodynamics ? By ...
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### Centripetal acceleration of Centre of mass of rolling body [duplicate]

Does the centre of mass of body rolling on a stationary floor experience centripetal acceleration downwards towards the IAOR (instantaneous axis of rotation) which here is the point of contact of body ...
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### Is this derivation of the Doppler effect for sound waves valid?

I was recently considering how to justify the formula relating to the Doppler effect for sound waves to a group of eleventh grade students who are likely encountering it for the first time. The ...
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### Invariant density of Harmonic oscillator

In general, dynamical systems described by a pure Hamiltonian can have an infinite number of invariant densities. In fact, each initial state determines exactly a closed path in phase-space and the ...
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### Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy?

My question could be summarised in: Why do we interpret the accelerated expansion of the universe as the proof for the existence of dark energy? This boils down to putting the cosmological constant ...
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### Why is this the probability that an incoming wavepacket is absorbed by the black hole?

I'm reading Parker's book "Quantum Field Theory in Curved Spacetime: Quantized Fields and Gravity" and when talking about Hawking radiation, there's a claim that I've not been able to understand. He ...
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### Focusing of a cylindrical mirror in X-ray optics

Focusing cylindrical mirror are sometimes used to squeeze a photon beam in one direction. They can be vertically focusing or horizontally focusing, and such mirrors are typically used in X-Ray optics ...
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### Charge operator applied to matrix multiplets

In the context of SM ($SU(3)_C\otimes SU(2)_L\otimes U(1)_Y$) the charge operator is $Q_{SM} = T_3 + \frac{Y}{2}\mathbb{I}_2$ and gives us the fermions charges. Here $T_3=\frac{1}{2}\sigma_3$ is the ...
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### Minkowski diagram of two frames in rest with respect to each other

I'm starting to study Minkowski diagrams, but I can't figure out how should the axis of a $S'$ system look if it is in rest with respect to the frame $S$. I've seen that in all diagrams the origin ...
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### Which electric field does E represents in the Ionospheric Dynamo Current equation?

In Ionospheric Dynamo Current equation $j=\sigma(E+v\times B)$ where $j$ is the current density, $\sigma$ is the tensor conductivity, $v$ is the velocity of the tidal wind velocity (hence the ...
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### What's the nicest proof that two isolates masses orbit their center of mass? [duplicate]

Although it seems intuitive, I haven't found proof of this fact in my textbooks from a purely mathematical standpoint. I was wondering if anybody knows of a particularly elegant proof of this fact, ...
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### Total potential energy and equilibrium

Suppose a deformable body is under an external load. If there is no inclusion of gravity. When the body reach an equilibrium state, will the total potential energy be equal to zero? The total ...
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### Electron density in DFT (*ρ*(r)) and probability density (wave function squared)

Are the electron density in density functional theory, ρ(r), and probability density, defined as wave function squared, the same quantities?
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### Real and fictitious forces exerted on a mass hanging from a string

I am trying to figure out if I am right about this(picture at the bottom): Let's say we have mass M hanging from a string above the surface of the earth, creating θ degrees between it and the north ...
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### How does a Resistor in the Secondary Side affects the Primary Circuit in a Transformer?

Let's assume a 1:1 transformer which has a 20V Source connected on the primary circuit and has no/negligible resistance, on the secondary circuit a 5 Ohms resistor is connected. Using the 'Impedance ...
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### Quantum equations for interaction between two charged particles

Reading in wikipedia about quantum mechanics, standard model of particles, ... . I'm asking myself about the electromagnetism from a quantum particles point of view. Assuming two "fixed" charged ...
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### Quantum Scalar Field Theory with cubic and quartic interaction

If I have a scalar Lagrangian with and interaction term given by cubic and quartic terms (so a scalar theory + $φ^3+φ^4$ interaction), what are the possible divergent 1PI diagrams at one and two loops?...
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### Factor of 4 discrepancy between integral of Planck's law vs Stefan-Boltzmann law

Consider an absolutely black sphere with unit surface area and temperature $T$. Planck's law states for the power emitted per unit area of the emitter per unit solid angle of emission per unit ...
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### Understanding Adiabatic Processes in a practical sense

We all know the basic understanding of the adiabatic phenomenon: ideal gas expands and cools, compresses and heats up. Now by expand, let take a quick look at a scenario: let assume there is a tank ...
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### Canonical transformations preserve Hamilton's equations. Which transformation preserve the Euler-Lagrange equations?

An important aspect in the Hamiltonian formulation of Classical Mechanics are canonical transformations which provide maps between different sets of canonical coordinates. These canonical coordinates ...
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I just started to learned modern physic for a two weeks, and the i got some problem to understand about the resolution of twin paradox I have attached two pages of picture above that makes me ...
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### How can I use a frequency counter to show the rpm of a spinning disc?

I don't want to unbalance the disc by attaching anything heavy to it. Is there a way of doing this optically?
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### What is the relationship of optical mode and data?

This question is on the plot of mode division multiplexing system where it is said capacity crunch can be solved using mdm technique. Now, we know single mode fiber can transmit upto 1Tbps. Now if i ...
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### Why does wave keep on travelling despite destructive interference?

I've problem here. Imagine you've a line of spheres that are attached to each other with springs. You push one sphere down hard. it drags its neighbors down and which in turn pull their neighbors and ...