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3 views

How does an optical chopper work?

How is an optical chopper eliminating drift? Or in more concrete words: Why is a chopped signal eliminating drift (DC -> AC)?
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1answer
18 views

Creation and annihilation operators for fermions from anticommutator

In a question, I was given that $a^{\dagger}a + a a^{\dagger} =1$ and asked to show what a|n> and $a^{\dagger}$|n> would be, given that H|n>=($a^{\dagger}a$ + 1/2). I am getting the wrong ...
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1answer
19 views

What determines the focal point of a mirror or a lens?

Definition:The focal point of a lens or mirror is the point in space where parallel light rays meet after passing through the lens or bouncing off the mirror. A "perfect" lens or mirror ...
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2answers
37 views

Variation of entropy in a transference of heat

When we put two bodies with different temperatures in contact, there will be a heat transference between them. The variation of the entropy of the system is well known (this is computed in most ...
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1answer
16 views

How to work out tension required for static equilibrium in a trebuchet? [closed]

I have the above system The dotted line at 45degrees of the horizontal is a rope with tension T The total mass of the system is 6035kg with the centre of mass located 0.82m from the pivot. The length ...
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0answers
32 views

Does aerogel “blow up” in a vacuum?

Discussions in comments below How can aerogel be lighter than air? really make an answer to this important. The conclusions there are that aerogel could in some cases be "lighter than air" ...
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1answer
46 views

If two opposite charges are placed next to each other and a field line leaves from one, at what angle does it reach the other one?

Two charges $+q_1$ and $-q_2$ are placed at A and B respectively, A line of force emerges from $q_1$ at angle $\alpha$ with line AB. At what angle will it terminate at $-q_2$? My approach: Since no ...
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0answers
11 views

How to determine the pivot point in a system?

I am just introduced to highschool physics and was wondering about the following thought-experiment. Suppose in the following setup m1 and m2 have the same mass. This system is floating in outer ...
3
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2answers
38 views

How does one divide by a vector when calculating pressure?

Sir $P=F/A$ And since $F$ and $A$ both are vectors but $P$ is scalar. So doesn't it violates that "Division is NOT defined for vectors"?
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0answers
26 views

Question on four velocity condition for timelike observers

I am a bit rusty on GR, I have the condition $u_{\mu}u^{\mu} = -1 $, in some notes I am given that we can obtain: $$-1 = u^{2}_{t} [g_{tt} +2 \Omega g_{t \phi} + \Omega^{2} g_{\phi\phi}] \tag{1} $$ ...
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0answers
38 views

What makes the electron, as an excitation in a field, discrete?

In standard quantum mechanics, the wave function have discrete energy-values due to a potential. However, my very limited understanding of QFT is that electrons are excitation in the Dirac field, and ...
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0answers
21 views

Ricci Tensor component, index lowering doubt

$v^j$$v^k$$[R(e_i,e_j)]e_k$.$e_i$ gives $v^j$$v^k$$R^l$$_k$$_i$$_j$ $e_l$.$e_i$ Then $v^j$$v^k$$R^l$$_k$$_i$$_j$ $g$$_l$$_i$ Finally, $v^j$$v^k$$R$$_i$$_k$$_i$$_j$ Which upon summation on i gives $v^j$...
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1answer
40 views

How does the period of an oscillating mass-spring system comply with the relativistic time dilation as viewed by a moving observer?

Consider a simple mass-spring system. The spring is attached to the ceiling in the lab frame of reference and a block of mass $m'$ is suspended from the spring. The mass starts to oscillate with ...
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0answers
33 views

Connection between Hamiltonian and density matrix

I found this equation in my notes from a seminar on BCS theory $$ H_{m,n} = \frac{dE}{d\rho_{n,m}} $$ where $H_{m,n}$ are elements of the Hamiltonian in matrix representation, $\rho_{n,m}$ are ...
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3answers
47 views

Why does mass-spring system tend or prefer to oscillate with natural frequency? Can you explain it theoretically rather the derived version of it?

Why the mass spring system prefers to oscillate at natural frequency, why does it rather oscillate at other frequencies? Every object in the universe is always vibrating, is it true? if it is the ...
2
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1answer
27 views

Haag's comment on the relation between fields and particles

I am very confused by the statement made in Haag's, Local Quantum Physics: Fields, Particles, Algebras (page 46): ... the idea that to each particle there is a corresponding field and to each field a ...
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2answers
41 views

How to Apply Gauss Law? [duplicate]

Lets say that we have a point charge $q$. Let's draw a Gaussian sphere at a distance $d$ from the charge, as no charge is enclosed inside the sphere the integral $$ \oint_A \vec E \cdot \text d \vec A ...
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0answers
12 views

Why do computational models of electron-electron collisions avoid using cross-sections?

Consider the Boltzmann equation in the case of a homogeneous position distribution. $\frac{\partial f(p, t)}{\partial t} = \mathcal{Q}^{ee}[f](p)$ where $\mathcal{Q}$ represents the collision integral....
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0answers
26 views

Question about the VEGAS-algorithm for numerical integration

Disclaimer: I am not quite sure if this question belongs to Physics SE, if not feel free to move it. Question: I am currently using the VEGAS-algorithm (See e.g. here and here) and i am trying to ...
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0answers
14 views

Hubble mirrors shape

The Hubble mirrors are two hyperbolas. I see on Wiki the formulas for the Ritchey–Chrétien telescope. https://en.wikipedia.org/wiki/Ritchey%E2%80%93Chr%C3%A9tien_telescope My question is how is the ...
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0answers
6 views

Position of the object pin to observe parallax

Pin is kept in front of a concave mirror at a distance x. Observer shifts his gaze towards the left,the inverted image of pin appears to the right. Then f<x<2f Here I understand that the ...
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0answers
36 views

Three body problem - Half Life

I did google this up but found nothing! I can't be the first to ask (the vague question) "What is the half life of a gravitating three-body system?" CLASSICALLY this means: Say I have three ...
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0answers
11 views

What is the reheating temperature of neutrinos during the BBN?

I want to integrate a differential distribution function of sterile neutrinos and the boundaries are the decoupling and the reheating temperature of neutrinos, but I don't know what is an acceptable ...
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1answer
27 views

Contribution to chi square

I want to make a residual plot for a Poisson distributed data. Can someone help me in understanding what is the contribution to chi-square for each data point? I want the proof for the error in the ...
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1answer
23 views

How does a photon released from particle A interacting with particle B, entangle them both? [duplicate]

What does a photon carry from its parent particle that when it interacts with another particle, it entangles both the particles?
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1answer
15 views

Does gas of different ionisation energies affect the total impulse generated by an ion thruster?

Required to do a physics practical experiment for a year project and I decided to do something regarding ion thrusters. My idea was to create a simple DIY ion thruster (can be seen below) and put it ...
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0answers
30 views

Inertial mass and gravitational mass [closed]

I would like to ask about gravitational mass. I know inertial mass is changing by motion (speed) according to $m=\frac{m_o}{(1-v^2/c^2)^{1/2}}$ And also that is inertial mass which sits in $E=mc^2$. ...
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0answers
25 views

What could be introduced to provide a source of propulsion through time? [closed]

This Ape has been suspended in a 4 dimensional anti-higgs field time sphere, where as the only source of gravity interfering with this chimp is the interior materials and the chimp itself. What could ...
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0answers
14 views

Deriving Continuum BdG equation from Tight-Binding model for Graphene-Superconductor interface

Background My question concerns Beenakker's paper on "Specular Andreev Reflection in Graphene": cond-mat/0604594 (arxiv)/Phys.Rev.Lett. 97, 067007 (The same topic is also discussed in Rev. ...
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1answer
44 views

Fourier transform of a Bessel function multiplied by a Gaussian

I am computing the diffraction pattern of some specific grating. I end up with the following integral $$I = \int dx\, J_m(x)e^{-ax^2}e^{-ikx}$$ which is the Fourier transform of the Bessel function of ...
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1answer
23 views

2D Ising model entropy as a function of external field

I am trying to figure out what is the entropy expression as a function of the external field in a 2D Ising model with nearest neighbour interaction. My Hamiltonian is the following: $\mathcal{H}=-\...
1
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1answer
17 views

Electromagnetic wave associated with a single photon

To the question regarding the relation between the energy of a photon (E = hf) and the energy of the associated electromagnetic wave, I read somewhere that the energy of an electromagnetic wave is Nhf,...
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1answer
76 views

Can we solve this without using calculus?

Point A moves uniformly with velocity $v$ so that the vector $v$ is continually "aimed" at point B which in its turn moves rectilinearly and uniformly with velocity $u< v$. At the initial ...
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1answer
47 views

Is the speed of electric field of a charged sphere placed in water same as it were in vacuum?

I know that electromagnetic radiation slows down in materials eg. water and due to that it refracts at a boundary between materials of different densities. But what about electric field of a charged ...
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0answers
19 views

Effective potential at two loops for supergraviton scattering in M(atrix) theory

I am trying to calculate the effective potential of two D$0$ branes scattering in Matrix theory and verify the coefficients in this paper: K. Becker and M. Becker, "A two-loop test of M(atrix) ...
2
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1answer
19 views

Why is electromagnetic induction a quasistatic approximation?

From Griffiths, Faraday's law is given by: $$ \oint_C \mathbf{E}_{induced} \cdot d\mathbf{l} = - \iint_S \frac{\partial \mathbf{B}}{\partial t} \cdot d\mathbf{a} = - \frac{d \Phi}{dt} $$ On page 323, ...
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0answers
16 views

Second quantization and bosons in Tomonaga's “The story of spin”

TL; DR: According to a book by Tomonaga in a chapter introducing second quantization, the formula $$e^{\pm i \Theta/\hbar} \,\psi(N) = \psi(N\pm 1)$$ is supposed to prove that the second quantization ...
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0answers
47 views

Static or dynamic?

Long time ago, I found a question in the book Introduction to Classical Mechanics-David Morin. The question was Sliding sideways on a plane: A block is placed on a plane inclined at an angle $\theta$....
2
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1answer
16 views

What are the dimensions of the inflation slow-roll parameters?

The inflation slow-roll parameters are: $\epsilon = \frac{M_{pl}^2}{2}(\frac{V'}{V})^2$ $\eta=M_{pl}^2 \frac{V''}{V}$ What are the dimensions of $\epsilon$ and $\eta$? What about $V$ and its ...
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2answers
75 views

What does negative time mean or what does it show?

In this question when I solve for time I get $t=-1$ & $t=5$. What does it actually mean at $t=-1$?
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3answers
115 views

The work done by some weirdly behaving forces in some wierd trajectories

Sometimes we meet some peculiar situations, like some forces act on a body placed in a vehicle at rest. The forces appear as if they never do any work on the body. But, when the vehicle accelerates, ...
1
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1answer
18 views

Ocean waves breaking slowly at the shore during periods of turbulence

During storms, I have often watched ocean waves breaking at the shore as if they were in slow motion. I've wondered if the slow curling of the waves during these times was somehow due to their ...
0
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1answer
53 views

Understanding constraint for not losing contact

What is the exact constraint for two bodies remain in contact? Consider the case of a rod constrained to move downward on an inclined wedge. It is known that the velocity of the wedge and the velocity ...
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0answers
18 views

Assumptions in a Moving Fluid

I saw this paper recently "On Integrals of the Hydrodynamic Equations That Correspond to Vortex Motions" by Hermann Von Helmholtz. It can be found here. In this research paper he discovered ...
3
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2answers
46 views

What happens to the moving ball that collides with wall and the wall disappears at the instant all kinetic energy of ball becomes potential energy?

Consider a wall and a ball that is traveling towards the wall with uniform velocity. Assuming no friction of any kind in this situation. When ball collides with the ball, the ball gets squished as its ...
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4answers
76 views

Why are black holes so special?

When reading about black holes, I have also read about things relating to black holes like holographic principles, parallel universes, black hole connecting multiple universes, etc. But black holes ...
5
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1answer
29 views

On-site symmetries can be gauged, but is a gaugeable symmetry necessarily on-site?

I've always liked lattice QFT because it's mathematically unambiguous and non-perturbative, but it does have two drawbacks: (1) the lattice is artificial, and (2) some features are messy. One of those ...
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5answers
696 views

Which particles does the Higgs Field give mass to?

I have found contradictory information about this. Does the Higgs field give mass only to the $W^+$, $W^-$, and $Z^0$ bosons or does it give mass to other particles as well?
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1answer
21 views

Optimizing engines to produce a certain torque and net force

Say we have $n$ engines sitting on a rigid body. Each engine has position $R_i$ and points in a certain direction and generates a force in that direction, $F_i$. The magnitude of that force ($k_i$) ...
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0answers
21 views

Pressure loss in concentric reducer with low Reynolds number and high viscosity fluid

This is my first question in stack Exchange,I'm making the overall calculus por a piston dossifier of viscous fluid like Honey and Carob Syrup, and I am currently struggling with the minor losses of ...

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