# All Questions

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### Studying Quantum Electrodynamics?

As an electrical/computer engineer, I already have a relatively thorough understanding of classical electromagnetism. From what I understand though, classical EM is only an approximation to quantum ...
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### What is the electric field between and outside infinite parallel plates?

I know that Gauss's law says $$\oint_S {\vec{E} \cdot d\vec{A} = \frac{q_{enc}}{{\epsilon _0 }}}$$ and that because $\vec{E}$ is always parallel to $d\vec{A}$ in this case, and $\vec{E}$ is a ...
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### Simulating electrical sublimation of aluminium foil

When i apply high voltage (through a capacitor bank) across an aluminium foil, it gets converted into plasma. I want to simulate this process and study the properties of the plasma produced ...
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### Thermal resistance of thermal interface materials?

Thermal conductivity are often used for surfaces between the computer chip and the heat sink to increase heat transfer and they want high thermal conductivity to decrease the thermal resistance. By ...
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### Does all information in the universe come from the observer?

In absence of the observer any system undergoes unitary evolution, that is reversible evolution without entropy change. It is believed that the initial state of the universe had very low entropy, ...
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### Rise of 2 liquids in a pipe [on hold]

The figure is a cross-section of a pipe of radius $r$. In the portion ABCD, a liquid of density $\rho$ is present. On the other part is present a liquid of density $\delta$. ...
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### Quantum vector space - A complex vector space

Why is vector space of states, a complex vector space? and not a real vector space or perhaps a space based on a new field altogether, which we would have to create specifically for quantum mechanics? ...
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### Walter Lewin's lecture on Farday's law

In Walter Lewin's lecture on Faraday's law, he does a demonstration with a solenoid where he shows that current is induced in a loop that surrounds the solenoid as the magnetic flux is changing. He ...
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### Compatibility between solutions of explicit Maxwell equations vs. wave equation?

When trying to solve for the allowed propagation frequencies in a cylindrical waveguide, I approached the problem by solving the wave equation for all three components of $\bar{E}$, and subsequently ...
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### double of temperature

I'm not a physicist, but I was having a discussion with a friend and I could not convince her about the main point. I asked her: "If I am in a place which temperature is 10ºC and she is in a place ...
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### Moment of Inertia for a hollow cylinder

This question may be to rudimentary. I have found 2 formulas for calculating the inertia of a hollow cylinder. Which is correct or are they used for different circumstances? They seem to be used ...
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### Charge within cavity of conductor [on hold]

A spherical cavity of radius 6.50cmcm is at the center of a metal sphere of radius 17.5cmcm . A point charge 4.20μC\mu C rests at the very center of the cavity, whereas the metal conductor carries ...
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### Is there work done when two current carrying wires are attracted?

When a current carrying wire attracts(or repels) another wire, the wire moves under the action of a force. Also, it is obvious that the work done is due to force of "magnetic field" on "moving ...
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### When a star turns into a red giant what stops its core contracting

I recently asked this question When do stars become red giants? and am now wondering when the star's core is contracting after it has fused H to He what force stops the contraction just before it ...
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### The Energy involved in the work done here?

When a wire that has current $I$ flowing within it and its in a magnetic field, the wire experience the Lorentz force, and that force moved the wire over a certain distance $x$(no matter how small), ...
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### Change of field intensity/flux density and energy

The answer for this question in the solutions manual is (D). I tried to solve it using $$W_e = \frac12\int_\nu ED d\nu$$ Which gives me (C). Any idea where I have gone wrong?
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### What determines the frequency output of a Barkhausen-Kurz tube?

I looked at the wikipedia entry and it doesn't give a very detailed or thorough description of the principles of operation of Barkhausen-Kurz oscillator triodes. But I do wonder if it is physically ...
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### Rotation operator for a point in a coordinate system linearly derived from Cartesian coordinates

For some experimental and practical reason, I have created a new coordinate system in the form $$x^\prime_i=T_{ij}x_j$$ where $T_{ij}$ isn't a square matrix. $x_i$ is standard Cartesian coordinates, ...
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### what are wormholes made of and how do they function?

what are wormholes made of and how do they function ? Also, what is the relation between black holes and wormhole ?
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### Hot Big Bang vs. Big Bang

This should hopefully be a quick one. Is there any difference between the Big Bang Theory and the Hot Big Bang Theory? Around Cambridge I hear everyone using "Hot Big Bang Theory", for example ther ...
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### Power wasted and power delivered?

If a power source is supplying a current I and a voltage V, that travels to its destination through resistors of restance R is the power delivered at the destination = $VI - RI^2$, where $(Pi=) VI$ is ...
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### What does it mean for a subatomic particle to “have” energy?

From my limited classical interpretation of the universe, i've always found it convenient to think of energy as what happens when an object isnt in its equilibrium position in the universe, and thus ...
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### Why does a nuclear explosion have directionality?

I had used the bomb icon as a joke in this computer/IT question. When I got a ping back this morning, it just occurred to me that almost all images I've seen of the (in)famous mushroom cloud show a ...
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### combination of cells

There are m*n identical cells of emf E and internal resistance r connected in parallel rows. This combination of cells is connected across an external resistance R. For what arrangement of the ...
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### What is the physical origin of acoustic modes in a duct?

Lets deal with wave propagation in a cylindrical duct. We ask the question: "what is the general form of a pressure wave which can propagate through the duct?" In answering this, we assume that the ...
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### Ising model. What is large fluctuations of magnetization?

My background is in mathematics. I have studied the Ising model. The main model of statistical mechanics. Yesterday, I was reading the preliminary notes of the book Statistical Mechanics by Sacha ...
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### Working of a phototransistor

Can anyone explain how photogenerated current in the base gets amplified in a phototransistor. If you use band energy diagram, then I would understand more quickly and clearly.
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### A meaningful distinction between determinism and causality

Causality is generally accepted to be a fundamental physical principle. But quantum mechanics is acausal (e.g. there is no 'why' as to the result of a measurement of the position of a particle in an ...
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### Derivation of 3-D Potential Flows?

Specifically, spherically symmetric source/sink flow in spherical coordinates, then finding the potential of it. (I understand that the potential is just the integral of the flow function) Also, I'm ...
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### Ground velocity of satellite

I was wondering whether there is a simple formula in order to calculate the ground velocity (the velocity with which the satellite is moving along the planet's surface) on basis of the orbital ...
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### 1. The problem statement, all variables and given/known data [on hold]

The problem statement, all variables and given/known data Consider a thin ring of radius a and mass M. A mass m is placed in the plane of the ring (not in the center!). Determine the gravitational ...
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### Quantum entanglement on cosmological scales

This may be a foolish question given my limited understanding of QM but here it is. As I understand quantum entanglement basically means that two particles evolve as a single "unit", i.e., are ...
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### Is assuming spacetime to have |2^N| points overkill?

The physical continuum is commonly assumed to have a mathematical continuum of points, that is, with a cardinality equinumerous with the Power Set of the set of natural numbers. However, since [ZFC + ...
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### D'Alembertian and Laplacian Green's Fucntions

There is a way to obtain the Green's Function for the Laplacian as a limit of the Green's function of the D'Alembertian? For the Laplacian ($-\nabla^2$) we have $$G_1(\vec X) = \frac{1}{4\pi X}$$ ...
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### Formula for molar specific heat capacity in polytropic process

I found this formula for a polytropic process, defined by $PV^n = {\rm constant}$, in a book: $$C = \frac R{\gamma-1} + \frac R{1-n}$$ where $C$ is molar specific heat and $\gamma$ is adiabatic ...
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### Photoelectric effect. current and frequency relationship [on hold]

Does increasing the frequency of photons increase the photo-current? (keeping the light intensity constant) Is this graph correct?
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### Finding the vacuum which breaks a symmetry

I will start with an example. Consider a symmetry breaking pattern like $SU(4)\rightarrow Sp(4)$. We know that in $SU(4)$ there is the Standard Model (SM) symmetry $SU(2)_L\times U(1)_Y$ but depending ...
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### Electric Field inside a regular polygon with corner charges

If we have equal charges located at the corners of a regular polygon, then the electric field at its center is zero. Are there other points inside a polygon where the field vanishes? The simplest ...
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### Constant of motion

An exercise from Goldstein (9.31-3rd Ed) asks to show that for a one-dimensional harmonic oscillator $u(q,p,t)$ is a constant of motion where $$u(q,p,t)=\ln(p+im\omega q)-i\omega t$$ and ...
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### Centrifugal force?

Everyone calls it a pseudo force and that it is not really present, even there are so many machines listed to advantage of centrifugal force ( centrifuge, washing washing drier),and also it has the ...
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### Chemical potential of photons

Why do photons have zero chemical potential and what is its the physical significance? From what I know the chemical potential could be interpreted as the energy per unit particle that is put into a ...
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### Heat flow in a hollow cylinder

Consider a hollow cylinder of different outer radius and inner radius and two different temperatures are maintained at the outer and inner surfaces such that inner temperature is higher. Because of ...
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### Representations and transformations under an $SU(n)$ Lie groups?

I think my problem is that I misunderstand what "transforms under" really means. Let's take $SU(3)$, for the $\mathbf{3}$ with Dynkin indices $(1,0)$, a state transforms like : $ψ→gψ$. For the ...
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### Will it start rolling?

Suppose you have a wheel standing stationary on a rough horizontal surface. Now you apply a horizontal force at the top of the wheel. Now, will the wheel experience any force or will it just start ...
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### Beta Decay Energy

Consider $\beta ^-$ decay. \begin{align*} ^{198} Au \rightarrow ^{198} Hg + e^- + \bar{v_e} \end{align*} The decay energy is given by the difference in mass between multiplied by the speed of light. ...
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### The equation of the location of L1

On http://en.wikipedia.org/wiki/Lagrangian_point#L1 it says that the location of L1 can be determined as $\frac{M_1}{(R-r)^2}=\frac{M_2}{r^2}+\left(\frac{M_1}{M_1+M_2}R-r\right)\frac{M_1+M_2}{R^3}$, ...
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### Collision between 2d circle and flat surface

First of all I want to preface this post by saying that I am absolutely terrible at maths, my level of geometry equals being able to discern a circle from a rectangle but that's about it, as for ...
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### Modern Optics (Fowles) Transmittance as a function of wavelength [on hold]

I need to get the coefficient of finesse to equal ((n^2-1)/(2n))^2 from 4R/(1-R)^2 knowing that the incident beam is normal so that R=((n-1)/(n+1))^2. If this isn't possible I'm not understanding the ...