# All Questions

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### What precisely does the 2nd law of thermo state, considering that entropy depends on how we define macrostate?

Boltzmann's definition of entropy is $\sigma = \log \Omega$, where $\Omega$ is the number of microstates consistent with a given macrostate. If I understand correctly, this means that it only makes ...
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### What is angular speed after 2nd collision? speed of block? time? [on hold]

Question with graph: For a) I got that the mass for rod must equal to block for the block to stop immediately after the collision (perfect velocity transfer) so the initial speed for rod after ...
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### Is the moment of a force the same about any point?

I know that when a body is is equilibrium There is zero resultant force in any direction, i.e. the sum of all the components of all the forces in any direction is zero The sum of the moments about ...
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### Show the Lie algebra is the same for $SU(2) \times SU(2)$ and Lorentz group

So I know: $$[\sigma_{I},\sigma{j}] = 2i \epsilon_{ijk} \sigma_{k}$$ So two products of this should give us the Lorentz group: $SO(4) = SU(2) \times SU(2)$ Where $SO(4)$ has 3 Lie algebra which can ...
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### What is clock or phase adjustment in projectors? Why its proper setting eliminates strange behaviour of 1px lines/

I'm doing experiments that require using a big, thin, contrast grid, for example 1 black pixel and then 1 white pixel alternately. I've met with a few DLP projectors and in each of them I had to ...
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### what will happen when you attached your hand to a two 220 volt wire of their phase defference by 120 degree?

What will happen when you attached your hand to a two 220volt wire with their phase difference by 120 degree?
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### Is this picture of the universe valid?

Universe as a whole has zero entropy. Since entropy does not grow, the universe has no time arrow. It does not change over time, at best it only fluctuates. Since following Holographic principle the ...
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### First Order Time Depenent Perturbation theory of particle in magnetic field

So I am dealing with the following hamiltonian, and the following perturbation: $$H=-\mu B_0\sigma_z$$ $$V=\mu B_1(\cos(\omega t)\hat x-\sin(\omega t)\hat y)\cdot{\bf \sigma}$$ I am asked for the ...
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### What is the Electric field of an infinite line charge while on the same axis

Consider a line of charge (w/ charge per unit length lambda(L)) which extends along the x-axis from $x = -\infty$ to $x = 0$. (a) Find the electric field at any point along the positive x-axis (b) ...
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### Spin connection in higher dimension

I have a problem regarding computation of spin connection in the case where One or more dimension is comactified. For example if we take a $D+1$ dimensional bosonic string action and write the D+1 ...
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### Solving relativistic particle decay problem in Landau

In Landau, Lifshitz "The Classical Theory Of Fields" in $\S11$ "Decay of particles" a problem is given: A particle moving with velocity $V$ dissociates "in flight" into two particles. Determine ...
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### What happens to an Electron when its charge is transferred elsewhere

I am vastly uneducated in the areas of physics and chemistry. I am aware that electrons do have a mass, and that they carry a charge. I think that when an electron gains energy it becomes excited and ...
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### Why don't the kinematic equations agree with calculating velocity one second at a time?

A car is moving at a velocity of $10 \, \text{m}/\text{s}$. After point $A$ no acceleration is provided. By simple measurement, the acceleration is found to be $-1 \, \text{m}/\text{s}^2$. Using ...
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### Sum of forces with liquid in rotation

I would like to compute sum of forces on this study (water pass between green/blue walls): A blue container has water inside. A green volume has nothing in it, I consider mass like 0. I know force ...
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### Distance travelled given velocity at various moments of time

I am trying to solve a problem where I need to find the distance travelled at the end of the nth second and my input data contains only velocities at different time instances. Say for instance I ...
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### A graphical proof that the $SU(2)/\mathbb{Z}_2$ vortex is non-orientable

The text, see [1], compares the vortex solutions of a spontaneously broken symmetry $U(1) \rightarrow 1$ and $SU(2)\rightarrow U(1) \rightarrow \mathbb{Z}_2$. The vortices can be classified by ...
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### Why are there interference patterns inside a diffraction envelope?

When double-slit diffraction occurs, there are interference patterns inside, say, the central diffraction maxima (or envelope). I am trying to understand how these interference fringes are created. ...
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### Why maximum energy transfer at natural frequency even if max amplitude occurs below $f_0$

This is a paragraph from my book: "For a damped system, the resonant frequency at which the amplitude is a maximum is lower than the natural frequency.However, maximum transfer of energy, or energy ...
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### Two-Body with external force - energy confusion

Setup Imagine a two-body system of masses under a classical mechanics model. The separation and mass-ratio doesn't matter for this example. Presume they are initially stationary. Now suppose that we ...
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### How do RGB colors work? [duplicate]

They say that all colors can be formed by mixing Red, Green, and Blue appropriately. Is it true? Isn't the Fourier basis infinite dimensional? Or does it turn out to be the case that only three ...
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### why exactly is ice less dense then water?

The answers to this question explain that ice is less dense than water because it has a "crystal structure", but they dont explain what exactly that is and why this happens, also I saw this answer ...
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### Why are neutrino and antineutrino cross sections different?

Particularly in the case of Majorana neutrinos, it seems a little odd that the particle and antiparticle would have differing cross sections. Perhaps the answer is in here, but I've missed it: ...
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### Virtual particles and physical laws

Recently, I was reading about Hawking Radiation in A Brief History of Time. It says that at no point can all the fields be zero and so there's nothing like empty space(quantum fluctuation etc.). Now, ...
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### How do I describe two entangled electrons in the same state except for a different spin

I am trying to formulate the wave function that describes two entangled electons having the same position but opposite spin. According to the Pauli exclusion principle this should be possible. And ...
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### Capacitance of a parallel plate capacitor with joint plates

What is the capacitance of a parallel plate capacitor with plates joint with a thin metal strip diagonally like this |\| I am confused. Is it infinte or 0.
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### Coriolis Force in a Merry go Round

Suppose there is a person standing in a Merry go Round, which is rotating at a constant angular velocity $\vec \omega$. He experiences, of course, a centripetal acceleration $\vec a_{cen}$ and has ...
I'm trying to obtain the Causal Green Function for the D'Alembertian in D=2, I'm finding $$G^{(2)}_C (t; \vec x) = \frac{1}{4\pi} \frac{\Theta(t^2-\vec x^2)}{\sqrt{t^2-\vec x^2}} - \frac{i}{4\pi^2} ... 1answer 17 views ### Work done from force-extension graph The answer is C, but I got B... F at 3cm = 10, F at 6cm = 20 W=F\cdot s (20-10)\times 3=30 I must have made a mistake or stupid assumption then, how am I meant to get 45 N cm? 0answers 20 views ### Gregory-Laflamme Instability Certain solutions to the low energy effective action of string theory, namely black strings and branes, were shown to exhibit an instability via perturbation theory. Specifically, the metric is ... 0answers 17 views ### Arrow of time and virtual reality [on hold] As we know that in classical world we have a clear arrow of time which points towards future..We can see glass falling from table and break but not the opposite in our world and whole universe... If ... 0answers 16 views ### Wannier functions on a ring Let's say I have a single particle hamiltonian in a periodic potential. For example a 1D lattice such that:$$H = -\frac{\partial_x^2}{2m} + V(x)  with $V(x+a) = V(x)$ where $a$ is the lattice ...
A topological space is defined as a non-empty set $X$ together with a given collection of subsets $T$ (topology) of $X$, such that, (i) any union of these subsets is one of the subsets. (ii) any ...