# All Questions

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### Electrical resistance of a metallic conductor in a gas

A long time ago I learned that the electrical resistance of a metallic conductor, when surrounded by a gas, varies with the pressure of said gas. • What is the name (if any) of the law involved? • ...
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### Kinetic Energy of a Symmetric Dumb-Bell

The problem is as follows: Consider the system consisting of the symmetric dumb-bell (two particles each of mass $m$ connected by a light, inextensible rod of length $l$) moving in the plane. Use as ...
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### Do black holes exist? [duplicate]

Do black holes exist from our point of reference? From our point of reference nothing actually goes inside the event horizon right? So is there anything inside the event horizon from our reference? If ...
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### What is the physical significance of the Dipole Transformation of Maxwell's Equations?

The Question Given Maxwell's equations of the form \begin{align} \bar{\nabla}\times \bar{B} = \dfrac{4\pi}{c} \bar{J} + \partial_0 \bar{E} \\ \bar{\nabla}\times \bar{E} = -\partial_0 \bar{B} \\ \bar{...
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### Energy/Momentum required to deflect earth from its orbit

This question occurred to me when thinking about the firepower people have on earth. How hard is it to change the characteristics of the orbit of the earth when an explosion happens on its surface, ...
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### Use of fission products for electricity generation

Why can't we use fissions products for electricity production ? As far has I know fissions products from current nuclear power plants create enough 'waste' heat to boil water; and temperature ...
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### How do I properly write Newton's second law for a particle with drag?

A heavy particle is projected at speed $U$ at an angle $\alpha$ to the horizontal. The particle is subject to air resistance which is experimentally found to vary proportionally to the square of the ...
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### Is passing on knowledge/information from one body to another and the resultant state is ever studied as any branch of physics?

Does any branch of physics, study the passing off of information/knowledge from one body to another ( body would obviously be living beings here, as they only can do so ). And resultant changes in the ...
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### How do cathode ray tubes (CRT) synthesize colours?

I just read about how LCD displays were based only on black&white contrasts, and only colour filters on each subpixel generated the colour of a pixel. However, how does that happen for a CRT? Is ...
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### Is every ordinary coloured transparent paper an optical filter?

Can any red coloured transparent paper be used as an optical filter that only allows red colours through it?
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### Very specific type of GR paper hunt [duplicate]

My General relativity skills suck. I need a good paper that does not start with equivalence principle and pages of elevator experiments derives principles mathematically, not by physical intuition ...
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### Significance of magnetic translation operator defined in fractional QHE's description

What is the significance of the magnetic translation operator used in describing the Fractional Quantum hall effect? I was following Anthony Leggett's lecture video in which he defines these operators ...
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In Weinberg's Lectures on Quantum Mechanics, he mentions Unfortunately, we cannot simply use first-order perturbation theory, with $T_{nuc}$ taken as the perturbation and the state vectors $\Phi_{... 1answer 72 views ### Max. speed that a space probe can communite with the Earth? Question: Which is the average or maximum speed of a telematically sent message that a space probe can send? Is as simple as radio waves travel at the speed of light in a vacuum? Context: I have been ... 1answer 134 views ### Definition of entropy In my textbook there is written $$\Delta S = \int_R \frac{\delta Q}{T}$$ where the$R$means calculated along a reversible transformation. The variation of entropy only depends upon the ... 2answers 281 views ### Force acting on center of mass Pictured above is a biker (traveling in a curve) whose center of mass has two forces acting on it. These two forces - a horizontal centrifugal force F(centripetal)* and a vertical gravitational force ... 1answer 63 views ### Is it possible to incorporate the Lorenz gauge term into the electromagnetic fields? I noticed that the Lorenz gauge term is represented by partial derivatives acting on the four-potential. Is it possible that the Lorenz gauge term could somehow be a similar object that belongs to the ... 1answer 10k views ### Magnetic field inside and outside cylinder with varying current density [closed] I am reading through Introduction to Electrodynamics by David J. Griffiths and came across the following problem: A steady current$I$flows down a long cylindrical wire of radius$a$. Find the ... 1answer 92 views ### Quantum Excitations In the context of quantum mechanics, is the Schrödinger equation actually describing some sort of an actual wave in some field like light in EM field ? So all particles are actually waves in their ... 3answers 85 views ### Why don't the leaves of an electrometer repel each other in water? A normal electrometer filled with air will repel like it should do for electrostatic demonstration, but what if it is filled with water instead or even oil, what will happen? My guess is that the ... 1answer 144 views ### What is$\langle \sigma_\mu \rangle\langle \sigma_\mu \ranglefor the Pauli Matrices? What is \begin{align} \sum_{\mu=0}^{3} \langle \sigma_{\mu} \rangle^2 = ? \end{align}\sigma_{\mu}are the Pauli matrices. The Bra-Ket notation is used in this question: \begin{align} \langle \... 2answers 342 views ### What is the physical consequence of the Lorenz Gauge Term not equaling zero? What happens to the physics of the electromagnetic field if the Lorenz gauge term does not equal to zero? \begin{align} \partial_{\mu}A^{\mu} \neq 0 \end{align} 1answer 150 views ### All possible electromagnetic Lorentz invariants that can be built into the electromagnetic Lagrangian? Given the electromagnetic Lagrangian density $$\mathcal{L}~=~-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}~=~\frac{1}{2}(E^2-B^2)$$ is a Lorentz invariant, how many other electromagnetic invariants exists that ... 2answers 427 views ### What happens to gravity in the middle of the earth? [duplicate] So I was curious about, what if we make a tunnel from one side of the earth, to the other side of the earth? Gravity is ofcourse always negative, which makes you "fall" and not "float". If we take ... 1answer 36 views ### Can an inhomogeneous magnetic field be built to slow down and catch a neutron? Neutrons have a measureable magnetic dipole momentum from their intrinsic spin. Is it possible to slow down and catch the neutron by imposing a force by an inhomogeneous magnetic field. I think the ... 2answers 371 views ### Did the singularity really existed in the begining of the universe? I read in Hawking's brief history of time the following: The final result was a joint paper by Penrose and myself in 1970, which at last proved that there must have been a big bang ... 0answers 167 views ### Can the two electromagnetic field tensors be combined into a more general tensor? Given the electromagnetic field tensor \begin{align} F_{\mu\nu} = \begin{pmatrix} 0 & -E_{x} & -E_{y} & -E_{z} \\ E_{x} & 0 & B_{z} & -B_{y} \\ E_{y} & -B_{z} & 0 &... 0answers 48 views ### Electron propagator in a strong background B field Near a magnetar with B>B_\rm{QED}, the strong B field will suppress the Compton scattering cross section of photons with a specific polarization (E-mode). Some references I know deal this problem ... 1answer 358 views ### Calculate impulse based on mass, acceleration and time elapsed [closed] I want to move to Sprite Kit framework offered by apple which has physics integrated right into it. The back bone of the physics engine is famous Box2D. Sprite Kit has made it pretty easy by wrapping ... 1answer 60 views ### Thrust to Weight ratio in Space with an off set CoM With regards to this thread, Thrust center in space My question is, if the thrust to weight ratio was increased so that it was much higher than the weighted mass of the sphere (ship), would the ... 1answer 226 views ### Many worlds interpretation I think many worlds interpretation is inconsistent with the EPR paradox. Quantum mechanics says that particles are really in more places at the same time and the particle is really only probability ... 2answers 2k views ### Why is the direction of a flame is always upward? [duplicate] why a candle's flame direction is always upward? What is the coposition of fire? Don't say it's energy. There is something else. 2answers 2k views ### Why is sound intensity proportional to the square of sound pressure not to sound pressure alone? I am trying to understand the physical principles behind the sound intensity and sound pressure. As far as i know, sound intensity is proportionate to the squared sound pressure. Can someone explain ... 2answers 1k views ### Why can we assume torsion is zero in GR? The first Cartan equation is\mathrm{d}\omega^{a} + \theta^{a}_{b} \wedge \omega^{b} = T^{a}$$where \omega^{a} is an orthonormal basis, T^{a} is the torsion and \theta^{a}_{b} are the ... 5answers 501 views ### Local EPR-experiments with photons in vacuum? The principle of non-locality states "that an object is influenced directly only by its immediate surroundings." (Wikipedia) When two entangled particles are measured in an EPR experiment, we ... 3answers 488 views ### Why does ice melts, waits for 100 degrees and THEN vaporises? Why is not the process of expansion of things continuous? What I am asking is this: Why can't a body be solid, then solid-ish, then solid-like, then liquid-like, then liquid-ish, then liquid, then vapor-like and then vapor? Why is there a rigid temperature ... 1answer 245 views ### Are subatomic partices n-dimensional hyperspheres or another n-dimensional shape? This guy derives the formula for the volume of an n-dimensional hypersphere, which can also be found here on the Wikipedia page, and he shows that it is equal to this equation: V_n = \frac{\pi^\frac{... 6answers 5k views ### When I stretch a rubber band, it breaks. When I hold the broken ends together, why doesn't it join again? The question is simple. When we join the two broken surfaces, what is it that keeps the surfaces from connecting with each other, while earlier they were attached to each other? Also, would the two ... 3answers 2k views ### Derivation of the general Lorentz transformation The standard Lorentz transformation or boost with velocity u is given by$$\left(\begin{matrix} ct \\ x \\ y \\ z \end{matrix}\right) = \left(\begin{matrix} \gamma & \gamma u/c & 0 & 0 \... 0answers 93 views ### stopping, moving of mobile phone when vibrating A mobile phone move aside when it vibrates. How is that happening ? and most importantly is it possible to make any changes to the vibration motor to stop moving when vibrating or any other methods to ... 2answers 125 views ### If a paper disc is cut into a spiral, does its moment of inertia change? It is obvious that there is no change in the mass of it and its radius. But the shape of the object does change. Does it mean its moment of inertia will also change? 1answer 74 views ### Special relativity: how to prove thatg = L^t g L$? We have $$X^\textrm{t}gX = 0 \iff X^\textrm{t}L^\textrm{t}gLX = 0,$$ where$X$is a column vector of length four,$L$is a non-singular$4 \times 4$matrix, 't' denotes matrix transpose, and $$g = \... 0answers 53 views ### Maximum voltage and charge a capacitor can store I'm given a parallel plate capacitor with plate separation of 4mm and plate area 20cm^2. I've calculated the capacitance to be 4.425 x 10^-10 F. I'm asked to also find the maximum voltage and charge ... 3answers 1k views ### The sign of the normal force. The notes state that N=mg. However, if I have a 4\text{kg} box on a table, taking upward as positive$$N=-mg=-4*-9.81$$Is this correct? If it is correct, why do the notes state that$N=mg$. ... 6answers 1k views ### Is there a fourth component to the electric field and magnetic field? The Question If the three vector electric and magnetic fields come from the four component four-potential, then is there a fourth component to the electric and magnetic field? Related Question I ... 2answers 218 views ### Perspective and changes in kinetic energy Say you have two planes flying next to each other at the same speed and one decides to pick up speed by burning a tank of rocket fuel. If someone on the ground wanted to know that plane's new speed ... 0answers 141 views ### What is$\langle G_{\mu\nu}\rangle\langle G_{\mu\nu}\ranglefor the Dirac gamma matrices? Given the following 16 matrix multiplications of the Dirac gamma matrices \begin{align} G_{\mu\nu} = \dfrac{1}{2} \begin{pmatrix} I && \gamma_{0} && i\gamma_{123} && i\gamma_{... 0answers 91 views ### What isT_{\mu\nu}T_{\mu\nu}\$ for the electromagnetic stress-energy tensor?

Given the electromagnetic stress-energy tensor components \begin{align} T_{\mu\nu} = \begin{pmatrix} u_{00} && s_{0 \nu} \\ s_{\mu 0} && \sigma_{\mu\nu} \end{pmatrix}_{\mu\nu} \end{...