# All Questions

140,265 questions
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### Particle deflection in a magnetic dipole field

Consider a small magnetic dipole of magnetic moment $\vec{\boldsymbol{\mu}} = (\mu_x, \; \mu_y, \; \mu_z)$ sitting at the origin. The magnetic field all around is \tag{1} \mathbf{B} = ...
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### Maintaining phase relationships and other general principles of interferometry

I have a few confusions with some basic principles of interferometry. Bob Campbell's So you want to do VLBI writes: Phase relationships among all points on the wavefront will be preserved ...
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### How to write down the collision integral for any interaction in the Boltzmann equation?

I'm studying the Boltzmann equation $$\Big[\frac{\partial}{\partial}t+\vec{v}\nabla_{r}+\frac{1}{m}\vec{f}\nabla_{v}\Big]f(v,r,t)=\frac{df}{dt}|_{coll}$$ where the ...
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### What is pound-force needed to create given Emf using values given? How to find I (amps) or W (joules) to solve for other unknown, using given values?

E (Emf in volts) = B x L x V x sin θ B (magnetic field in webers/m2) = 0.7303 webers/m2 L (length in meters) = 21.48 inches = 0.545592 meter At all times the magnetic field strength remains ...
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### Producing a pair of pions

I know that two protons colliding at high energies can produce a pion $$p+p \rightarrow p+p+\pi^0$$ But is it possible for two protons to produce a pion pair? $$p+p \rightarrow p+p+\pi^{-}+\pi^{+}$$...
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Suppose that superstring theory (hence T-duality) is a valid theory of Nature. T-duality is a transformation inverts the radius of a compact spatial circular dimension of radius $R$. That is, it maps $... 0answers 24 views ### Could the end cap of the Pascal B (1) survive its trip through the atmosphere? In 1957, the US had a nuclear test where a shaft was dug 152 meters into the ground. A 100 mm thick, 900 kg steel plug was installed and welded at the top of the shaft. Under it was 2 feet of ... 1answer 27 views ### Light speed in vacuum over space and time [duplicate] Is the light speed the same, constant, in all the universe locations? I mean, is it he same in the surface of the Sun compared with speed in space inter galaxies? Also: is light speed now the same as,... 1answer 24 views ### Program for simplifying expressions of many-body Pauli operators I have a system of$N$spins, with the usual$\sigma_i^{x,y,z}$operators defined on each site (operators at different sites commute). I would like to find a program, perhaps in Mathematica, with the ... 1answer 40 views ###$g^{\mu\nu}g_{\alpha\beta}$=? Good day. I am self-studying general relativity, and I have a quick question on the inverse metric tensor: I am aware that$g^{\mu\nu}g_{\nu\alpha}=\delta^{\mu}_{\alpha}$. I do not know, however, what ... 0answers 21 views ### Under what angle should we kick a ball at the foot of a hill in such a way that it falls back to us? [on hold] Under what angle should we kick a ball situated at the foot of a hill with incline$\varphiso it falls back to the starting point? Find the solution for the first rebound. Under what angle should ... 4answers 902 views ### How many photons are emitted by a lightning strike? As the title suggests, I'm curious to know, approximately how many photons are emitted in a single lightning strike? 0answers 40 views ### How to build a circuit that generates a Quantum Coin Flip I'd like to build a cheap and simple quantum coin flip circuit. I have posted in the EE stack exchange about the specific circuit design, but was wondering if anyone in physics would have enough ... 1answer 43 views ### What is the purpose of this wire loop around a guy-wire? I recently visited this tower on top of a rock. There is a wire loop around the tower, with extensions going down the rock. This picture provides a good view of the tower and wires. The wires that ... 2answers 63 views ### How do we decide the proton wave function? The fully symmetric spin-up proton spin-flavour wave function in the constituent quark model is usually presented as follows: \begin{align} \frac{1}{\sqrt{18}} ~ ( &2 |u\uparrow ~ u\uparrow ~ d\... 2answers 34 views ### Newtonian Mechanics - Pulley block system, finding acceleration The question is to find acceleration of the block m1 in the above system (please look at diagram). So I took tension along String 2 to be T and thus Tension along String 1 would be 2T. Acceleration ... 1answer 92 views ### Quantum mechanics, Fourier transformation Why do we use p=-i\hbar\frac{\partial}{\partial x} in quantum physics? (I know the reason for i\hbar, quantization). Is this right to say we can't measure velocity and position of electrons at the ... 1answer 25 views ### Perturbative violation of the unitarity: what is it? Consider the Fermi theory: \mathcal{L} = \frac{G_{F}}{2\sqrt{2}}\bar{n}\gamma_{\mu}(1-\gamma_{5})p \bar{\nu}\gamma^{\mu}(1-\gamma_{5})e $$The cross section of 2 \to 2 scattering calculated ... 1answer 70 views ### Time travel in 4th dimension [duplicate] Can we travel in past in 4rth dimension because according to physics it is not possible to travel in past in even 4rth dimension we can only go in future.pls explain. 0answers 21 views ### Where does R-parity come from? R-parity is a concept used in particle physics and it is defined as:$$P_{\text{R}}=\left(-1\right)^{3\left(B-L\right)+2s}$$where B is baryon number, L is lepton number, and s is spin. By ... 0answers 18 views ### Where is the frame information in a time derivative of a physical vector in a moving frame using limits? In the equation:$$\left(\!\frac{d \vec v}{dt}\!\right)_{\!1}= \left(\!\frac{d\vec v}{dt}\!\right)_{\!0} + \vec\omega_{01}\wedge\vec v$$How could this be translated to a mathematical definition of ... 0answers 15 views ### Time-independent distributions of charge and fields In the introduction of Jackson "Classical Electrodynamics", the author says that the electrostatics studies phenomena that involve time-independent distributions of charge and fields. For time-... 4answers 116 views ### Do photons interact or not directly? All I am asking about is photon (EM wavepacket) photon (EM wavepacket) interaction. I have read this question: If photon-photon interactions are impossible, how are higher harmonics generated? ... 1answer 57 views ### QM explanation why hot oil explodes when water pours on it [on hold] I have read this question: Why does hot oil explode when pouring water on it? It explains in a classical way clearly, that the boiling point of oil is hotter then water, and water changes to steam ... 1answer 18 views ### How does an EMF detector, or something similar, detect freshwater under the seafloor? In the news recently, Woods Hole and Columbia University said they found a gigantic aquifer of freshwater underneath the seabed off the Northeast coast. They said they used electric field detectors ... 1answer 40 views ### Are there no convection currents in ice-boxes and aerogel? Technologies dealt with Ice-boxes have been used for a long time to prevent ice from melting for a longer time than keeping it in open. Aerogel is still an emerging technology (I came across it on ... 2answers 33 views ### Discrepancy in capacitor Let a capacitor be connected to a battery in a circuit. The battery does work worth CV^2 but the capacitor stores only half that amount... We say the other half went away as heat, but how do we ... 2answers 45 views ### Einstein's Calculation (Condensed Matter Physics) (From The Oxford Solid State Basics by Steven H. Simon [Chapter 2.1]) Reading through a condensed matter textbook and struggling to see how the author progresses with the calculation in the chapter ... 1answer 24 views ### Understanding the rolling constraint for one cylinder rolling inside another cylinder This is the problem to find the equation of motion of 2 cylinders in which 1 cylinder is placed inside another cylinder with larger radius as shown in figure. The condition is that both are rolling. ... 3answers 69 views ### Energy Density is expressed in Liters, Liters of What? When looking at energy density of batteries it is often expressed as Wh/L. For instance, Wikipedia says Lithium Ion batteries have a density of 250–693 W·h/L. Liters is supposed to express the volume ... 1answer 93 views ### Schwarzschild Black Holes Page 190 of “Tensors, Relativity, and Cosmology”: Consider a particle falling radially into a black hole with a radial velocity u^1=dr/ds. As the particle is falling radially, we have u^2=u^3=... 3answers 39 views ### Why a Box moving at Constant Velocity isn't doing Work? This was a question from an A-Level past paper and the question is pretty straightforward. I was in a hurry so I couldn't think much on this problem and chose C as my answer, checked it on the ... 0answers 23 views ### What are the necessary or sufficient conditions for a renormalization group scheme to be “valid”? Suppose I have a super operator G which acts on Hamiltonians to produce a new Hamiltonian that is related somehow. For the purposes of this question, suppose that these Hamiltonians are defined on ... 1answer 103 views ### What is dimension? What is the size of dimension? Recently I heard a TED talk by Brian Greene where he was speaking about String Theory working on (10+1) dimensions. Plus he said that we live in only in (3 +1) dimensions. So where are others? ... 1answer 16 views ### Torque and angular acceleration in elliptical orbit I am stuck in a supposedly simple aspect. Consider the Sun-Earth system. The torque is zero and angular momentum is conserved. L = I\omega is constant, but since I changes, \omega should change ... 0answers 18 views ### Problem on kinematics [on hold] Two cars start in a race with velocities u1 and u2 and travel in a straight line with accelerations a1 and a2 . If both reach the finish line at same time,range of race?? 1answer 26 views ### What happens if you connect a wire to the anode of a battery, but not the cathode? My understanding is that the anode of a battery is positive while the cathode is negative. If you connect a wire between the cathode and the anode, then the electrons in the cathode want to flow to ... 0answers 12 views ### Non-diagonal elements in density matrix: interference effect between the statistical mixture In Cohen-Tannoudji, at page 303, vol 1, it is given that Quote: \begin{array}{l}{\text { A calculation analogous to the preceding one gives the following expression }} \\ {\text { for the non-... 2answers 45 views ### Question on the 1/N expansion My question is from Coleman's Aspect of Symmetry, on the large N approximation. We will consider the O(N) version of the \phi^4 theory. Its Lagrangian density is given by:$$ \mathcal{L}=\... 1answer 38 views ### Which puck has the greatest kinetic energy at the finish line? The question is: "Two pucks one with mass 3kg and the other with mass 1kg are pushed along a frictionless table with the same horizontal force from rest from the same starting point to the same ... 0answers 32 views ### Question about Bhabha scattering Feynman Diagram I'm a uni student going into his third year, and thought I might try to read through Griffith's particle physics textbook for the heck of it. I've been trying to become familiar with Feynman diagrams, ... 2answers 73 views ### Flipping a coin with same initial conditions Today, in my physics class my teacher was talking about how we can never predict the outcome of a coin flip. So I thought: Will the outcome of a coin flip be the same if we do not change the initial ... 1answer 29 views ### Having trouble finding fundamental equation fromc_p$,$\alpha$and$\kappa_T$I'm having trouble finding que fundamental equation$u$of a system, since my results don't match those that my professor provides for us. The exercise I need to solve, I think, would be difficult to ... 0answers 10 views ### Why the Chipping Rate of 1.023 MHz for the GPS L1 C/A Signal? Why was the chipping rate for the GPS L1 C/A signal chosen to be 1.023 MHz (1023 chips per symbol, one symbol per ms)? It seems to me like you'd want to get one chip as close as possible to an ... 3answers 49 views ### How to describe the unit vector for a complex wave vector? In optics, we often come across complex wave vectors that describe absorption, dispersion, etc. given as:$\textbf{k} = \textbf{k}_{real} + i\textbf{k}_{imag}$The electric field in phasor notation ... 1answer 35 views ### How to find the equation of motion for a bead of mass$m$threaded on a circular loop with dynamic friction coefficient$\mu_k$? I have been working on a particular physics problem but am not sure if I have analyzed it correctly. Here is the problem: "A bead of mass m is threaded on a metal hoop of radius$R$. There is a ... 1answer 17 views ### Experimental Physics - Defining convolution in terms of equipment resolution So I think this video: https://www.youtube.com/watch?v=N-zd-T17uiE&t=67s By Faculty of Khan does a wonderful job in explaining what convolutions are. We basically consider two pulses$f(\tau)$... 2answers 40 views ### Question on the definition of the potential energy for a two particle system I have some confusion on potential energy of a two particle system. I'm using Section 4.9 from 'Classical Mechanics' by John R. Taylor as reference. Assume two particles are at location$\vec{r_1}\$ ...
Suppose we have an equation $$x^i=y^{ij}z^{i}_{jk}w^k$$ This is supposed to capture the following: $$x^i=\sum _jy^{ij}\sum_kz^{i}_{jk}w^k$$ I don't know how I could turn this into a matrix ...