0
votes
1answer
7 views

Equation of motion of a simple pendulum

Consider a simple pendulum of length $l$ moving in $(x,y)$ plane. Assume its point of attachment is being jiggled by external forces with prescribed acceleration $a(t)=(a_x(t),a_y(t))$. Let $\theta(t)$...
1
vote
2answers
10 views

Charge on capacitors in series

'When there are 3 capacitors connected in series, charge across each capacitor is the same.' Okay so say we consider three caps made of plates of the same conducting materials and same dielectric ...
0
votes
0answers
16 views

The Feynman propagator and the $i\epsilon$ prescription

The Feynman propagator is usually represented in the i-epsilon form and texts solve the integral in this form (as opposed to doing the Feynman (time-ordered) contour on the real axis). Restricting ...
1
vote
0answers
13 views

What does it mean to say “the fundamental physics is contained in…”?

This is very common to show up in books. Here's one example from Thomson's Modern Particle Physics. When talking about the general expression for the $a \rightarrow 1+2$ relativistic decay $$\Gamma_{...
1
vote
0answers
18 views

Understanding a Derivation of the Wave Equation for Longitudinal Oscillations

I am working through problem 4.2 in Barton Zwiebach's "A First Course in String Theory." The problem goes as this: Consider a string with uniform mass density $\mu_0$ stretched between $x = 0$ and $...
1
vote
0answers
10 views

why the changing magnetic flux horizontal component does not induce electricity

Textbooks just seem to utter that change in the magnetic flux that is 'piercing' the plane contributes to electricity induction! They don't explain why! Is there any intuitive explanation?
1
vote
1answer
22 views

Does the medium a magnetic field is propagating through affect it's speed?

I'm wondering if a material's magnetic susceptibility or magnetic permeability would have an effect on the speed a magnetic field would propagate through it. I know it won't speed it up but I'm ...
0
votes
0answers
24 views

Doing a physics project with Machine Leaning

for my studies I can choose a question or a problem which I can then try to solve in a project. Since I am interested in machine learning I thought that one could combine this project course with my ...
0
votes
1answer
20 views

Integer number of wavelengths after the wavefunction is normalized in a cubic box

One of the steps when learning quantum mechanics in three dimensions is to normalize $\psi$ in a cubic box of side $a$. In math terms $$\int\limits_0^a\int\limits_0^a\int\limits_0^a \psi^*\psi dxdydz=...
1
vote
0answers
34 views

Susskind's Theoretical Minimum, Error in General Symmetries Section?

On page 136 of Leonard Susskind's The Theoretical Minimum, Suppose the coordinates of an abstract dynamical system are $q_i$. The general idea of an infinitesimal transformation is that it is a ...
1
vote
0answers
20 views

Diagonal lorentz invariant matrix

Is there any general way to show that diagonal Lorentz invariant matrix must be proportional to $g^{\mu \nu}$? I can show it is true, but my proof is not clean enough for such a simple problem.
0
votes
1answer
46 views

Would it be possible to pull Mars and Earth closer each other?

The burden of travelling between Mars and Earth comes from the distance. If the planets were closer, life could spread to Mars more easily, ensuring the survival of the intelligent lifeforms. What ...
0
votes
3answers
27 views

Is a light wave's amplitude stretched, along with the “red shift” stretch - making it brighter?

When light waves are stretched and "red-shifted", is the amplitude of the light wave stretched as well, affecting the intensity/brightness of the light wave?
0
votes
0answers
21 views

Coleman-Mandula theorem and mass gap

I had a couple of naive questions about Coleman-Mandula theorem. One of the assumptions of the theorem is the non-existence of massless particles in the spectrum. Since we do have massless photons ...
0
votes
0answers
27 views

Bose-Einstein condensate effect

Does every boson gas go through the condensate effect? Could there be a boson gas with high energy (photons for example) that would have enough energy to be at different quantum states even at 0 K?
5
votes
1answer
42 views

Is there a delay in the effect of the gravitational or electromagnetic force if a secondary body suddenly appears? [on hold]

If the sun were to suddenly vanish in our solar system, it would take about 8 minutes for the Earth to exit its orbit. This is because the gravitational field of the sun would vanish at a rate of $c$. ...
4
votes
1answer
66 views

Why would speed of light be directional if spacetime is discrete?

In Feynman's Simulating Physics with Computers, Feynman states that "we might change the idea that space is continuous to the idea that space perhaps is a simple lattice and everything is ...
-2
votes
0answers
32 views

What is the time for which this plane is in danger of being hit by this cannon shell? [on hold]

A fighter plane is flying horizontally at a height of $250~\mathrm{m}$ from ground with a constant velocity of $500~\mathrm{m/s}$. It passes over a cannon which can fire a shell at any time in any ...
0
votes
3answers
43 views

gauss law and electric flux

Can you please explain me what is electric flux. I know it is $\oint\mathbf{E}\cdot\mathrm{d}\mathbf{S}$, but in many place its given flux means to flow. But how does electric field "flow"? Also if we ...
-2
votes
0answers
26 views

Parallel combination of cells [on hold]

In this circuit suppose that $E_1 >E_2$. This means that the positive terminal of $E_1$ will have a greater potential than the positive terminal of $E_2$. Then current should flow from the ...
5
votes
1answer
43 views

Use my example to explain why loop diagram will not occur in classical equation of motion?

We always say that tree levels are classical but loop diagrams are quantum. Let's talk about a concrete example: $$\mathcal{L}=\partial_a \phi\partial^a \phi-\frac{g}{4}\phi^4+\phi J$$ where $J$ is ...
4
votes
4answers
563 views

Is the deformation of spacetime, elastic deformation or plastic deformation?

Is the deformation of spacetime, elastic deformation or plastic deformation?
0
votes
0answers
30 views

Swirl velocity from circulation - Vortex

I have a vector field in cartesian coordinate $(x,y)$ with coordinate origin at vortex core. For calculating the swirl velocity $v_\theta$ I followed below steps, Step 1: Find the circle coordinates ...
0
votes
0answers
18 views

Calculating effective dielectic constant

I have a system of metallic nano-particles supported on glass where I have observed a difference in the in-plane dielectric constants of the metal ($\varepsilon_x$ is different from $\varepsilon_y$ ...
1
vote
1answer
32 views

Why do we get a diffraction pattern with a slit whose width is not comparable to the wavelength of the incident light?

In single slit experiments with sodium light whose wavelength is of order micrometer and slit width of order of hundreds of micron, we still observe the diffraction pattern. How do we explain this?
0
votes
1answer
33 views

Acceleration of a pulley system

The figure above shows a pulley system consisting of 3 masses ($m_1$, $m_2$ and $m_3$), a homogeneous wheel (radius R, mass M) and 2 massless pulleyes which are connected by a massless rope. Mass $m_1$...
3
votes
2answers
42 views

Infer the shape of a solid given the moments of inertia

Let's say we have a solid with uniform mass distribution, but we don't know its shape. However, we know the moment of inertia of the solid with respect to as many axes of your choosing as you want (e....
1
vote
0answers
23 views

Collision term in Boltzmann Equation

In Dodelson's Book, chapter 3, we have the collision term in Boltzmann equation is written as $$\int\frac{d^3p_1}{(2\pi)^32E_1}\int\frac{d^3p_2}{(2\pi)^32E_2}\int\frac{d^3p_3}{(2\pi)^32E_3}\int\frac{...
0
votes
2answers
87 views

Intuition for the identity $\epsilon_{ijk}\epsilon_{ilm}=\delta_{jl}\delta_{km}-\delta_{kl}\delta_{jm}$? [on hold]

One of the popular Kronecker delta and Levi-Cevita identities reads $$\epsilon_{ijk}\epsilon_{ilm}=\delta_{jl}\delta_{km}-\delta_{kl}\delta_{jm}.$$ Now, is there an intuition or mnemonic that you use,...
0
votes
0answers
15 views

Can a single photovoltaic semiconductor junction ever produce a voltage much above it's own band gap?

A simple model for a photovoltaic cell is a current source with a series resistor, and a few more items. The current from the current source is some fairly constant proportionality constant of the ...
2
votes
0answers
11 views

Modeling Optical Properties of Doped Crystals

I am interested in modeling the optical properties of crystals doped with lanthanides to investigate their luminescent properties. What would be the best way to pursue this? EDIT (In response for a ...
1
vote
1answer
50 views

If I spin around why do the stars not dissappear?

Galaxies at the far limit of the visible universe are moving away from us near the speed of light. There may be some beyond that that are moving away from us faster, so the universe appears to have an ...
0
votes
1answer
36 views

If I spin around do the stars move faster than light?

In fact I don't even need to spin around, just being on the spinning earth is good enough. Suppose I look out into space at the nearest star, Alpha Centauri, approximately 4 light years away. ...
1
vote
0answers
12 views

Hedin's equations and the ground state energy

Hedin's equations are an iterative scheme to calculate the Green's function $G$, the self-energy $\Sigma$, the vertex $\Gamma$, the polarizability $\chi$, and the screened interaction $W$. However,...
2
votes
1answer
68 views

Why can't we feel acceleration under free fall? [duplicate]

It's a common experience when we are in some vehicle like a car being accelerated we can feel the "invisible push/pull" or acceleration caused due to the change in velocity I assume we can always ...
1
vote
1answer
38 views

Hamilton's principle and virtual work by constraint forces

I have a question about the following pages(pg 47 and 48) from Goldstein's "Classical Mechanics" I do not understand how (2.34) shows that the virtual work done by forces of constraint is zero. ...
1
vote
1answer
34 views

Invariance of Mass in SR: Comparing Taylor and Wheeler's Spacetime Physics with a Comment made in Purcell and Morin's Electricity and Magnetism

Purcell and Morin's Electricity and Magnetism mentions on pp. 241: Mass is not invariant in the same way (in a relativistic theory). We know that the energy of a paritcle is changed by its motion, ...
16
votes
4answers
2k views

Does a “capacitor” for light exist?

If I have a light that is flickering at a frequency low enough to be perceived by the human eye, is there any type of material that exists that will smooth out the appearance of flickering? Similar to ...
-1
votes
0answers
24 views

Fermi's theory of beta decay - density of states?

I am following through these MIT OCW notes on Fermi's theory of beta decay. On page 11 (103 as on page) they wrote the expression: $$\rho(p_e)dp_e = dN_e \frac{dN_{\nu}}{dT_\nu}$$ for the density of ...
0
votes
1answer
24 views

Fin area and heat transfer direction

So, I was studying some fin design in a heat transfer course , and then came the part where the efficiency is to be calculated, then I noticed that when he calculated the surface area , the sides of a ...
-1
votes
1answer
29 views

Electrostatics of hollow conductors

My textbook says: Inside a conductor, electric field is zero. The interior of a conductor can have no excess charge in static situation. Electric field just outside a charged conductor is ...
9
votes
2answers
1k views

How does the LHC explore extra dimensions?

The Large Hadron Collider (LHC) has been smashing particles for a long time and sometimes people say that it has found new dimensions. How is it even possible for a particle accelerator to find new ...
6
votes
1answer
35 views

Bidirectional jerk motion on a stopping vehicle

A stopping vehicle (say a car) has an apparent retardation (which may/may not be constant in magnitude) when force via brakes is applied. I travel by subway trains, and I noticed an odd phenomenon. ...
0
votes
0answers
66 views

Does a “capacitor” for light exist? [duplicate]

If I have a light that is flickering at a frequency low enough to be perceived by the human eye, is there any type of material that exists that will smooth out the appearance of flickering? Similar to ...
-4
votes
0answers
21 views

Motion in one dimension and relative velocity [on hold]

Two bodies $A$ and $B$ are separated by $4~\mathrm{km}$ and $A$ is due west of $B$. If $A$ moves at a speed of $6~\mathrm{km/hr}$ towards the south, what is the relative velocity of $A$ w.r.t $B$?
-4
votes
0answers
14 views

Find the energy of an x-ray photon which can impact a maximum energy of $50~\mathrm{keV}$ to an electron [on hold]

This is an exercise question from the text book Concepts of Modern Physics by Arthur Beiser. Find the energy of an x-ray photon which can impact a maximum energy of $50~\mathrm{keV}$ to an ...
0
votes
0answers
8 views

Why 3D nonlinear waves (e.g Faraday waves) create vortices on the fluid surface that interact and lead to the development of 2D turbulence

When wave became three-dimensional as a result of modulational instability, a floater can be forced to move towards the wave source.
0
votes
0answers
16 views

Thermodynamic system, phonon and electron temperature and dissipation

Consider the following system (rudimental bolometer): A material (imagine a metal) A is initially at $25°\mathrm{C}$ and connected through Van der Waals forces to a material B (metal as well), which ...
0
votes
1answer
34 views

Where does electrical power in PC or other electronic devices goes?

I am wondering where does energy goes for example in PC? As I understand part of it definitely changes to the heat and dissipates, but what about rest of the energy? There is no moving parts except ...
2
votes
1answer
53 views

What is the physics behind these balancing forks?

https://www.youtube.com/watch?v=dTKQAMkGXPI I saw this video and was amazed by seeing this incredible balance. I wanted to know how is this thing balanced and what are other things that can be ...

15 30 50 per page