All Questions

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Difference of potential between points

I have the following question: What is the potential difference between point A - B? Could somebody please explain me how should I count it? The voltage U=18V. I found out that V1=8V, V2=10V, ...
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Suggested operatonal definition for a tensor

The two tensor definitions I'm (newly) familiar with, by transformation rules, and as a map from a tensor product space to the reals, don't tell me what a tensor does, and to the best of my knowledge ...
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Laplace's demon and spontaneous symmetry breaking

One interpretation of Quantum mechanics is the hidden variable theory. This suggests that if we were to have a complete knowledge of the system at one time then the future states of the system are ...
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How Electromagnetic Force is stronger than Gravitational Force

I had read this in text book..Can some explain this. When we hold a book in our hand, we are balancing the gravitational force on the book due to the huge mass of the earth by the 'normal force' ...
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Laser travelling through a radially graded index of refraction

A laser beam propagates through a region whose refractive index varies as $\mu=\mu_0(\frac{r}{r_0})$. At a distance of R the beam makes an angle of 30° with the normal. Find the minimum seperation ...
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Galactic Rotation of gas clouds

I need to use the formula for the radial velocities of gas clouds due to Galactic rotation together with the assumption of a flat rotation curve (V independent of R) to find the Galacto-centric ...
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How many foci does a converging or diverging lens have?

I thought this was a simple question but can't seem to find a direct answer. In ray diagrams with a converging or diverging lens, we always draw F (the focal length) on both sides of the lens. But ...
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Peskin “An Introduction to quantum field theory”, problem 4.1

I'm trying to solve problem 4.1)c) in Peskin's book "An Introduction to quantum field theory" using this answer sheet. I don't understand why lambda correspond to a line in Feynman diagrams. Any idea? ...
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Calculating the electric field of an infinite flat 2D sheet of charge

I was trying to calculate the electric field of an infinite flat sheet of charge. I considered the sheet to be the plane $z=0$ and the position where the electric field is calculated to be ...
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Does the strength of a beam of light determine how much distance it can travel?

If I point a light torch to a cloudless, crystal clear and an absolutely pristine sky, will the light beam exit the planet and travel forever in the vacuum of space? (assuming it doesn't get perturbed ...
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How can we redirect light from any direction to one direction

Not just focused light. I mean whenever light come from sideway or any angle. What device it called to refract light from any angle to one angle
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Are there concave fresnel lens production

I wonder why most fresnel lens is focused convex lens. Are there any usage and production of concave fresnel lens?
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Can we define the zero potential at an imaginary point?

Consider a force field defined as $$\vec{F}(x) = \left(\frac{A}{x^2}-B\right)\hat{i}\space$$ where $A, B$ are positive constants. We want to get the potential energy function for this field. We can ...
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What are $\mu$ and $\nu$ in $g_{\mu\nu}$ metric?

What are $\mu$ and $\nu$ in $g_{\mu\nu}$ metric? Consider the metric $g_{\mu\nu} = \begin{pmatrix} 1 & 0 &0 \\ 0 & r^2 & 0\\ 0 & 0 & r^2\sin^2\theta \end{pmatrix}$
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Why is Lagrangian defined as kinetic energy minus potential energy? [duplicate]

Why is Lagrangian defined as kinetic energy minus potential energy? $$L = T-U$$ Where $T$ is kinetic energy and $U$ is potential energy.
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Can I state that $\Psi (x_1, \dots , x_n,t)= \sum_{i=1}^n a_i \psi (x_i,t)$ via superposition?

Given that the hamiltonian $\hat H$ of a system is a linear operator and $\dot \psi (x_i,t)$ does not depend on spatial coordinates $x_1, ..., x_n$ with bases $\hat e_1, ... , \hat e_n$ can I state ...
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Quantum mechanical expectation of angular momentum along different axes

This is a question from Concepts of Quantum Mechanics by Mathur & Singh, and I don't know where I should start from: Show that, for a state $|j,m \rangle$, corresponding to a definite value of ...
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Why can't we define a unique wavelength for a short wave train?

Here we encounter a strange thing about waves; a very simple thing . . .namely, we cannot define a unique wavelength for a short wave train. Such a wave train does not have a definite wavelength; ...
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vectors, space and coordinate systems, [on hold]

An object moves from the position $r_1 = (1,3,-5)$ to the position $r_2 = (-1,4,8)$ during the time $8~\text{s}$. Find displacement, distance, average velocity, average speed.
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Would our universe seemed static from outside?

As the time we experience does not exist outside our universe and all the change we experience are measured with respect to the time, how our universe would seem from outside? Will be like an static ...
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Notion of distance in a Conformal Field Theory

I'm confused about the how the notion of distance is used in Conformal Field Theory. Let's take for example the Operator Product Expansion (OPE). In a conformal field theory, due to the scale ...
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Is it possible to tell time within a steady state?

(Inspired from this question (A) which unfortunately the link in the selected answer is now 404 broken (which might have contained my answer), thus cannot read any deeper than that) From the above ...
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How did special relativity change physicists views on the two prominent inverse square laws (ie Newton grav and Coulomb's law)?

On page 107 in Hartle's Gravity -- An introduction to Einstein's General Relativity, he says the following With the success of special relativity it became apparent that the Newtonian theory of ...
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To what fraction of the speed-of-light has any multi-atom molecule (or polyatomic ion) been accelerated?

Of course photons go $c$ in a vacuum, and seems there's data on electrons getting to $0.999999999976c$ and protons to $0.99999912c$. For the twins paradox to be plausible, one of the twins must ...
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Uncertainty of energy for harmonic oscillator at ground state and first excited state

How does one calculate the energy uncertainty of the harmonic oscillator in the ground state and first excited state?
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How does the solar cell work?

As of i know that a solar cell consists of a transistor(NP) .First step it creates a barrier between the n type and the p type junction.Then following that the sunlight i.e,(the photons) supply ...
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How can I replace the Euler method by RK4 to determine the free fall time? [on hold]

How can I replace the Euler method by Runge-Kutta 4th order to determine the free fall motion in not constant gravitional magnitude (eg. free fall from 10 000 km above ground)? So far I wrote simple ...
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Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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Is my diagram of spacetime curvature valid (relatively)?

I've been wracking my brain trying to understand what "curved spacetime" really is, and I think replacing one dimension with the time dimension then drawing the world-lines through time was the "aha!" ...
254 views

Why is the scalar product of four-velocity with itself -1?

My GR book Hartle says the scalar product of four-velocity with itself $-1$? Consider the definition of four velocity $\mathbf{u} = \frac{dx^{\alpha}}{d\tau}$. Suppose I take the scalar product of ...
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What justification is necessary for convolutional variational principles to be considered legitimate?

I recently asked a related question and was interested in why/or why we cannot use convolutional variational principles in practice or in theory. Summarizing the points I made in the earlier post: ...
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How does one normalize this wavefunction? [on hold]

Here is the question: So I could write $N = \dfrac{1}{{\sqrt{<Ψ|Ψ>}}}$, right? Considering the parentheses in the exponential term, it looks like a good idea to switch to spherical polar ...
25 views

Using a charged capacitor to charge two others

Here is the homework question in question: The figure (below) displays a 13.1 V battery and three uncharged capacitors of capacitances C1 = 4.08 μF,C2 = 6.19 μF and C3 = 3.30 μF. The switch is thrown ...
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SU(3) irreducible representations with tensor method

I am dealing with the tensor product representation of SU(3) and I have some problems in understanding some decomposition. 1) Let's find the irreducible representation of $3\otimes\bar{3}$ we have ...
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What frequencies will get lost/diminished when you listen to a colloidal speaker without the backside

I was checking the connections of my colloidal speakers, so I had to listen to music without the backside of the speakers. There was clearly a difference in the quality of the music, but I couldn't ...
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Can we prove that zero point energy exists everywhere and not just locally around individual particles or our universe as a whole?

If zero point energy only exists locally around the particles that we are using to observe it and not globally then the zero point energy would have to rise in value when we put two particles close ...
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Bandwidth-Pulse Duration Relationship

Assuming Gaussian pulse, the relationship between pulse duration and bandwidth is given by: $$\Delta \nu \cdot \tau \geq 0.441$$ If the rms bandwidth and rms pulse duration are used, then the ...
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Can observations of cosmic super voids teach us anything about time, dark energy, or dark matter?

Can observations of cosmic super voids teach us anything about... Time - If gravity affects time is there a chance we could gain any further information from studying a region with lower than ...
Consider a gauge field $W_\mu = W_\mu^{a} \tau_a$ where $\tau_a$ are the generators of the Lie algebra and $W_\mu^{a}$ just numbers. Then: $$W^2 = W_\mu W^\mu = W_\mu^a\tau_a W^{\mu b} \tau_b = ... 2answers 46 views Would it be possible to make a space elevator only in the atmosphere? I wonder if with current day technology we could make a "floating platform" which hangs from a satellite by a space elevator. This could allow a "launch process" involving floating a balloon to the ... 0answers 19 views Compute Z_2 Invariant of 2D Topological Insulators without Computing the Eigenstates For 2D Time-Reversal Invariant systems (T H(\vec{k}) T^{-1} = H(-\vec{k}) ), there is a formula by Fu-Kane-Mele in order to determine whether the system belongs to either one of distinct topological ... 3answers 327 views Are length contractions limited by Planck length? While we are getting closer to speed of light our length in the direction of the movement is according to Lorentz transformation getting shorter. But we can not (even theoretically) consider length ... 1answer 17 views Laser cutting - ignition How is that sometimes laser cutting by burning material or "flame cutting" causes ignition of the whole material like here where the whole paper starts to burn and sometimes doesn't allowing precise ... 1answer 37 views How do you accelerate atoms/particles? So I'm in 11th grade, and I'm writing a theory for my Physics professor and I need help. I need to know how you can make atoms/particles travel faster. For example, making sound travel a little bit ... 2answers 51 views Balancing a pencil I came across this equation for balancing a pencil while solving some problems:$$ml\ddot { \theta } =mg\theta $$Where l=the length of the pencil, and \theta is the angle it makes with vertical. ... 1answer 22 views Why does this derivation of the dependence of free energy on pressure not work? It is well known that the Gibbs Free Energy of a gas depends on the pressure via the following formula:$$G_m(p) = G^\circ_m + RT\ln{\frac{p}{p^{\circ}}} Where $G_m$ is the molar gibbs free energy ...
I'm trying to demonstrate the Karman-Howarth expression of isotropic turbulence for the velocity correlation functions: $R_{ij}(r)=A(r)_{ij} + B(r)\delta_{ij}$ since: \begin{align} A(r) r^2 + ...