# All Questions

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### distributing charges on conducting plates

Three parallel plates1,2 and 3. Plate 2 is given charge q and the plate 2 and 3 are connected by wire. How to find charge distribution on each plate. Ans= plate 1 has q/2 charge on both sides ,the ...
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### Why is the light reflected at the same angle from mirror?

From the school physics I know that the material objects bounce from the plain surface at the same angle, losing some kinetic energy. In the same school I was taught that the light (and waves in ...
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### Why should(n't) the string snap ?

My question is related to a comment in the margin of the popularising site The Official String Theory Web Site. There are two basic types of string theories: those with closed string loops that ...
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### The effects of angular velocity on special and general relativity

Imagine I have a circular disk in a vacuum. I apply a constant force, so a constant torque on the disk. My first question is: does this disk have a angular velocity speed limit? I believe it does, ...
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### Diffusion modeled by Levy process and Wiener process

I'm reading about diffusion and see that both Levy process and Wiener process can be used to model the diffusion of a particle. Why Levy process is more general than Wiener process, especially in term ...
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### neglect of lattice potential for conduction electrons

Why is it true that in nearly free electron compunds, complete neglect of the lattice potential is usually a good approximation as long as one considers crystal momenta remote from the boundaries of ...
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### Where does this commutator relation come from?

What is the origin of this relation: $$[H,a_n^\dagger] = \epsilon_n a_n^\dagger$$ for Hamiltonian H, creation operator $a_n^\dagger$, and eigenvalue $\epsilon_n$. The square brackets denote ...
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### Interpretation of Orbital Magnetic Moment

The wavefunctions for atomic orbitals have always been described to me one of two ways: As a "smeared out" electron standing wave with integer number circumference of de Broglie wavelengths As a ...
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### Boomerang torque and angular momentum

I'm building a boomerang for a project, but I've made it a goal for myself to have a boomerang which can fly at lest 200 feet, so how would I increase the flight path. Angular momentum increases in ...
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### The original thorium reactor?

With lunar thorium being common, and heavier than iron or nickel, does earth's core have the the occasional nuclear reaction?
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### definition of Entropy for reversible and irreversible process

$\int \dfrac{\delta Q}{T}$ can't be used to calcute entropy of an irreversible process. If you happen to know heat supplied and temperature at which it is supplied for just an instant. Can you then at ...
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### Wind resistance on moving body

Suppose that a vehicle is traveling 60 km/h north, and the wind speed is also 60 km/h going the same direction, would there be wind resistance, and to what extent? Also, if I put my hand outside the ...
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### General question about the potential barrier problem: Why does exp.$kx$ diverge when $x>0$ in the case when $E < V(x)$?

For the two images below, the first potential barrier has particles approaching it where $E > V_o$ & the second has a particle that has $E < V_o$, where $E$ is the energy of the particles ...
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### Why are some elemental materials grey?

How does grey occur in elemental materials such as metals? I believe that grey arises from the simultaneous reflection and absorption of all colors of the spectrum (in different atoms of course), as ...
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### Reflections of objects of a particular color

How can any color reflect the color of light shone upon it? I tested this by shining different colors of light on different objects, and instead of becoming black, it reflected a hazier form of that ...
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### The Color White

How can something that is white reflect all colors? It would have to reflect all frequencies or at least blue green and red, correct? How can a two element compound, such as NaCl reflect all of these ...
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### Angular Power Spectrum of CMB

I was wondering, if anyone could guide me through the different peaks of this spectrum (See below)? I've been reading and reading numerous pages about this, but I can't seem to get my head around ...
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### Ultrasonic cascade sonotrode design question

I wish to experiment with a cascade sonotrode =||<[]-[]==[[]]==[[]]==[[]].. <-something like this. -=||<[]- is the transducer. [large]=small=[large] (small and large diameters) Typically, ...
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### Charge density waves: site-centering v.s. bond-centering

Question about charge density wave (CDW): From this Ref. page 13, why bond-centering charge density wave is naturally compatible with the observed coexistence of charge ordering and ...
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### What is the central charge of the q-state potts model, for large q?

The central charge of a model, is, heuristically related to the number of microscopic degrees of freedom. Is there a simple argument for the asymptotic behavior of the central charge for the q-state ...
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This started with wondering about the nature of certain physical quantities under time-reversal - chiefly, that acceleration retains its magnitude and direction at a given time regardless of the ...
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### Wigner's Theorem and discrete Symmetries

According to my skript: A pure state is a ray: $\quad$ $\{λψ\}$, where $ψ ∈ \mathcal H$, $||ψ|| =1$ fixed and $λ ∈ \mathbb C$, $|λ| = 1$. Pure states are uniquely given by 1-dimensional orthogonal ...
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### Why there's any voltage on P-N junction in diodes?

If all electrons here have their counterparts in protons, on both, P and N sides, then why there's any voltage (built-in voltage, on the bottom of the image)? Both sides should be neutral, shouldn't ...
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### What is the relation between Rössler attractor and thin accretion discs (like in the movie Interstellar)?

Is there any relationship between the Rössler attractor and thin accretion disks, like the accretion disk(s) in the movie Interstellar?
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### Transistor circuit question [migrated]

I'm trying to solve the transistor question. But , I'm stuck on the I2. Is I2 equal I0 ? The ...
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### In the Penrose interpretation of QM, do electrons have a location in the atom before it is measure/observed?

I know that Bohm's interpretation says that it has a location before it is observed. Is that because it is a realist interpretation of QM? Do all realist interpretation of QM believe that electrons ...
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### Electric force vs. Magnetic force

$E$, electric field $\to$ $H$, magnetic field $D$, electric flux density $\to$ $B$, magnetic flux density $D=\epsilon E$ $\to$ $B= \mu H$ Electric force calculation is based on electric field: ...
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### Is it possible to have both a conduction current and a displacement current at the same time?

According to Ampere's law, is it possible to have both a conduction current and a displacement current at the same time? In the classical derivation of the displacement current using a parallel plate ...
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### Which formula is correct for the velocity of a mathematical pendulum?

Suppose I have a mathematical pendulum swinging in a plane, and the angle between the vertical direction and the pendulum is $\phi(t) = \phi_0 \cos\omega_0t$. The length of the pendulum is $l$, its ...
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I have a back helping/crunch bench that rotates around and is stopped by a series of chains linked. My weight is around 66 kg, other people that might use it weight around 91 kg, designated as $M$. ...
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### Electron doesn't have a location before it is measured?

Does this apply to all theories or just some? What about interpretations that are realist and don't require an observer, in those interpretations does an electron have a location?
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### How fast would an SUV have to go in order to stop my car in its tracks and send me backwards about 55 ft? [on hold]

I was going 20 mph, the roads were damp (no puddles), the SUV was twice the weight of my car (that is, about 2,000 vs. 4,000), impact was at driver's side headlight, my car stalled as soon as we ...
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### Floquet and Bloch's theorems : connection?

It is often stated that Bloch's theorem and Floquet's theorem are equivalent, even the Bloch's theorem is often referred as Floquet-Bloch theorem. However, it seems quite confusing to me since the ...
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### Why do images reflected off the surface of water deform when the water is disturbed?

When looking at the surface of water straight objects look straight in the reflection but when you disturb the water they become shapeless or fluctuate. What causes this?
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### Probabilities of pure states and density operators

According to my skript: A pure state is a ray: $\quad$ $\{λψ\}$, where $ψ ∈ \mathcal H$, $||ψ|| =1$ fixed and $λ ∈ \mathbb C$, $|λ| = 1$. Pure states are uniquely given by 1-dimensional orthogonal ...
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### Superposition of electromagnetic waves and energy localization

Sorry for the long question. I didn't know how to make it shorter. I'm trying to understand how energy is spatially localized in an electromagnetic wave. My premises are: Electromagnetic energy is ...
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### Is there a portable method of preventing all IR radiation from being emmited?

Me and some friends were recently disuccing "How cool it would be to be invisible", from a military viewpoint, I said that Camoflage, when done correctly, works surprisingly well, so invisibility was ...
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### Law of reflection for a moving mirror

An elementary fact that people learn about mirrors is the law of reflection, that the angle of incidence of a light beam striking the mirror (as measured with respect to a normal) equals the angle of ...
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### Efficiency larger than one?

The efficiency of a heat engine is the work we can do divided by the heat we take out of the hot reservoir. This quantity is always $\le 1$. The efficiency of a heat pump is the heat we can release ...
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### Size of Image when we using converging lens [on hold]

An objct 5cm in length is held 25cm away from a converging lens of focal length 10cm .draw the ray diagram nd find the position ,size nd the nature of the image formed
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### Heating of two identical balls

You are given two identical steel balls of radius (say)5 cm. One ball is resting on a table, the other ball is hanging from a string. Both balls are heated (with a blow torch) until their radii have ...
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### Fifth force in string theory

I can't seem to get a clear description of what the "fifth force" in string theory is. What is the fifth force in string theory? What does it do? What mediates it?
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### Are there any pigments which can be altered via the use of electricty?

I was looking at alternative visual output methods for computer, phone and the like and was wondering, are there pigments, Red, Green and Blue preferably, that can have their intensity/opacity ...
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### Entropy of loops in the PV plane

The change in entropy of the Carnot and reversible cycles is said to be 0. Several other loops are supposed to have a non-negative change in entropy. This presents 2 problems which I cannot ...
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### Can a fuzzball collapse into a singularity?

I've been reading Kip Thorne's Black Holes and Time Warps and got curious about extreme states of degenerate matter; a little research reveals that since the book was written quark and preon ...
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### Example of law of conservation of energy

Law of conservation of energy states that "Energy of the universe remains constant. Energy can neither be created nor be destroyed, but it can be transformed from one form to another". So I ...
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### Do electrons have a location before they are observed/measured?

Is this in all interpretations of QM? What about interpretations that are realist (MWI, penrose, ect)?
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### Cosmic Background Radiation and redshift vs. temperature

I get that the CMB has gone from high energy photons, to low microwave photons today. And that is probably due to the redshift from the expanding Universe. But, since CMB is a black body it is ...
What is the imparted angular momentum to a rigid body if the impulse force is offset by a distance $h$ from the center of mass and the imparted momentum from the center of mass is $mv$? For a ...