# All Questions

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### Relation between Canonical and Grand Canonical Ensemble for a two levels system

For a two-level system of classical non interacting distinguishable paricles, labelling energy levels as $+\epsilon, -\epsilon$, in the canonical ensemble I found for the partition function ...
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### EM: invariant property in electromagnetic relativity

I have problems answering the following question: "What is an invariant property in electromagnetic relativity? Which of the following is an invariant quantity for the electromagnetic waves?: ...
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### Expectation value

Which of the following equations is correct? why?
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I have an AdS space written in Poincare' coordinates and in global coordinates. The transformation that brings one set in the other is a conformal transformation?
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### Are alpha and beta rays also generating electromagnetic rays?

To discover radio waves in 1886, Hertz used electric sparks between two iron balls. These sparks caused radio waves which generated a current between other balls. The oscillating charges caused a ...
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### problem in making hologram (3 sided)

why the image become illusion in transparent glass ? (in making hologram).
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### Pauli matrices and $SU(4)$ [on hold]

together with the 2-dim identity matrix, the pauli matrices build a basis of the vector space of the 2x2 hermitian matrices. The set of i*pauli matrices build a basis for the SU(2). Assuming, we ...
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### Can a theoretical object exist as such that its mass overcomes gravitational collapse

A daydream the other day led me to the question if "a universe filled with water (or any other substance for that matter), instead of being mostly void" could exist within the rules of our current ...
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### Electrical and magnetic energy

I have problems answering the following question: What is the ratio of the electrical energy to the magnetic energy stored in a unit volume of the atmosphere just above the Earth's surface, where ...
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### Derivation of Euler-Lagrange equation from principle of least action

When deriving the Euler-Lagrange equation for a field $\phi$ the term $$\int\textrm{d}x^{\mu}~\partial_{\mu}\left( \dfrac{\partial \mathcal{L} }{\partial(\partial_{\mu}\phi)}\right)\delta\phi$$ is ...
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### Tidal heating of body

I am trying to make a plausible, though not necessarily accurate model of tidal heating of a body moving close by a gravitational attractor, not necessarily in orbit and not considering the spin of ...
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### Infrared Camera spectral filtering with an uncooled camera?

I have been doing experiments with thermal cameras and stumbled upon CO2 detection. Having ordered a filter I was full of joy and immediately tried it out - nothing happened. So I talked to the ...
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### EM waves: Circularly polarized light incident at Brewster angle

I am having problems in answering the following question: " Circularly polarized light is incident at the Brewster angle on a dielectric surface. With explanation, describe the polarization of the ...
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### Thermodynamics : Heat transfer : Conduction or convection?

I have an hot object, that is place in a room of 20°C. My goal is to be able to estimate how warm the object will be in T seconds. To be able to do that, I must first understand what is really ...
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### Simple harmonic motion angular frequency

I'm struggling with this physics question. "A simple harmonic wave of wavelength 16 cm and amplitude 2.5 cm is propagating along a string in the negative x-direction at 35 cm/s." a) Find its angular ...
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### What kind of other light sources can disturb the detection of LASER

My project objective is to detect different lasers , for example Laser Range Finder: pulse width 10ns and repetition rate is ...
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### Torsion-free, symmetric connection and non-coordinate basis

The torsion tensor is defined as (Hawking p.34) \mathbf{T}(\mathbf{X},\mathbf{Y}) = \nabla_{\mathbf{X}}\mathbf{Y} - \nabla_{\mathbf{Y}}\mathbf{X} - [\mathbf{X},\mathbf{Y}]. ...
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### Under what conditions does a beam splitter entangle two input photons?

There is a dispute on PhysicsForums related to what are the conditions necessary for two photons to be entangled by a beam splitter. Lots of references given by the forum users but they never arrive ...
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### How to Calculate Engine Torque from Drive Torque @ The Wheels for All wheel drive and Front wheel drive?

Would it be possible for someone to explain how engine torque is calculated from drive torque at the wheels and how this differs between AWD and FWD. Thanks.
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### How would one attach black hole to starship in the Black hole Starship model?

A black hole starship is a theoretical idea for enabling interstellar travel by propelling a starship by creating an artificial black hole and using a parabolic reflector to reflect its Hawking ...
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### Majorara zero mode in Ising chain, not exactly zero subtlety

We know the transverse field Ising model with N sites(open boundary), can be mapped into N free fermions(there are 2N modes if including the negative energy counterparts) With property: ...
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### Intuitive understanding of vector spaces

I want to understand the intuitive meaning of vector spaces (Hilbert, Banach, metric, normed). I have read a lot of texts and understand the mathematical formalism given but how does it correspond to ...
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### How do I find the relation between the accelerations of the ring and the disc (see image)?

What is the relation between the accelerations of the ring and the disc (see image)? Both the ring and the disc have mass $M$. The ring has a radius $R$ and disc has radius $2R$. They are connected ...
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### Hydropower formula:

If I had a 10 kW hydro electric generator (including impeller drive), what formula would I use to calculate how much water per second would need to be pumped into the impeller at 700 kpa to generate ...
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### Scale invariance and stress energy tensor

I have seen in a paper [1] that in a quantum field theory scale invariance takes place provided the stress energy tensor is traceless. How this is true? References: "INFINITE CONFORMAL SYMMETRY IN ...
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### On a AC to DC adapter there are +12v wire with 24A and -12v have 0.5A. How many amp this adapter will provide in total?

I have a PSU of a pc which is 500w. I want to charge a 12v 100ah battery which need 20 amp of current. Is this psu can provide current that I have wanted? Main question is On a AC to DC adapter ...
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### Question about the seemingly loophole in principle of least action

In Lagrangian formalism, given two point $(x_1,t_1)$ and $(x_2,t_2)$, we ask the question which path $x(t)$ make the action $S=\int_{t_1}^{t_2}L\ dt$ stationary and satisfy the boundary condition ...
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### When light reflects from a medium of lower index of reflection to a medium of higher index of refraction, why does the light undergo a phase shift? [duplicate]

I learned in my physics class that there is a phase shift when light reflects off a low $n$ from a higher $n$, but never got the explanation.
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### Dipole moment of a single point charge

Kindly refer to the Multipole Expansion section (chapter 3) of David Griffith's Introduction to Electrodynamics. Let us first discuss about why multipole expansion is needed. As far as I understand, ...
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### Room with open door and heater : use inside boundary conditions or heat source for PDE?

I want to model the stationary temperature in a room (=square) that has one open door (=side) is otherwise perfectly insulated is heated through a (circular) heater that is placed somewhere in the ...
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### How is 6W equal to 40W in LED bulbs adverts?

Every advert I come across for LED bulbs advertise them as the equivalent of a higher W regular bulbs. This makes no sense to me, if the room requires 40W to lighten it up then it'll always require ...
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### About periodicity of coordinates given a metric

If I am given a metric how do I decide which coordinate is periodic? Eg. can I look at metric in plane polar coordinates and tell that θ direction is periodic. Also How do I calculate the period of ...
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### Difference between stiffness and damping?

I understand stiffness as the extent to which an object (e.g. a mass spring) resists deformation from an applied force, or the rigidity of an object. And I understand damping as the energy ...
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### What are the calculations specifically needed for High Altitude Ballooning?

I want to know the lifting gas needed to launch the vehicle including the positive lift I want to predict the possible retrieval site I want to know the burst altitude AND MAYBE ALL WITHOUT ...
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### First Law of Thermodynamics and Signs

I have a question in my text book about the first Law of Thermodynamics. It gives 3 situations with the systems for each in brackets: 1) Rapidly pumping up a bicycle tyre (air in the pump) 2) ...
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### A question about the Fermi acceleration

In high school physics, we know that a charged particle cannot gain energy from a magnetic field. But, it seems that in the so-called Fermi acceleration, the particle is accelerated by the magnetic ...
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### Ratio of the change of internal energy under the specified condition given below [on hold]

What will be the ratio of the change of internal energy of a diatomic gas U1 & U2 for a unit rise of temperature in constant pressure and volume respectively.
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### What is an overview of secondary school astronomy textbooks for students who have taken some calculus?

I teach mathematics at the secondary level. This year my students have founded an astronomy club. Most have finished two semesters of "college-level" calculus and are familiar with the derivative, ...
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### Velocity matrix and non-local pseudo potentials

It is known that velocity of bloch wave functions are related to band energy derivatives: $$v(k)=\frac{1}{\hbar}\frac{\partial \epsilon}{\partial k}$$ However, in the following paper, it is given ...
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### Does the energy inside a capacitor has a variable dependency of the capacity?

While studying energy inside capacitors, I came up with something: If we can calculate the energy inside it as: $$U=\frac{1}{2}QV=\frac{1}{2}CV^2=\frac{1}{2}\frac{Q^2}{C}$$ Then, why in some cases ...
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### How does cavity resonance produce EM waves?

My understanding is it acts like a capacitor and inductor in a loop. The capacitor releases stored energy which is absorbed by the inductor through a magnetic field which then returns it to the ...
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### While deriving Coulomb's Law from Gauss' Law, the power of 'r' comes out to be exactly 2 while experiment shows that it differs about 10^(-16). Why?

While deriving Coulomb's Law from Gauss' Law, the power of 'r' comes out to be exactly 2 while experiment shows that it differs about 10^(-16). Why is it so? I'm referring to the book 'Physics by ...
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### The existence Water nanoparticles [on hold]

Has anyone investigated the properties of water nanoparticles? If yes what are those? If not, why not?
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### Free fall in non-uniform field

Imagine I'm a space-diver, with mass $m_1$, 500 miles above the Earth's surface at $x_i$. I want to calculate my position, velocity, and acceleration as a function of time, accounting for the Earth's ...
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Show for the null geodesic in 3D flat spacetime using polar coordinates so the line element is $ds^2=-dt^2+dr^2+r^2d\phi^2$. Do light rays move on straight lines? My question is that I only learned ...
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### Why is $np$ always equal to $n_i^2$?? Semiconductors

For you guys who studied semiconductor physics must be familiar with the equation: $$np=n_i^2$$ I can understand why this is true for the intrinsic case (the broken bonds would always provide ...
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### Is meters per second equivalent to seconds per meter?

I know this question is probably ridiculous, but bear with me for a moment. This thought emerged while I was converting between nm and wave numbers ($\rm cm^{-1}$). In order to prove this conversion, ...
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### What does it mean by “infinities” when dealing with quantum mechanics?

I found this PDF online here while browsing Nobel Prize winner contributions, which explains a bit about renormalization (a concept for which Kenneth G. Wilson won the Nobel). However I was somewhat ...
### Show the Berry phase is invariant under $U(1)$ unitary transform
Recall that $$\gamma_n = \oint A_n(R) \cdot dR = \oint \langle\psi_n(R)|i\nabla_R|\psi_n(R) \rangle \cdot dR.$$ Under the $U(1)$ transform, $$\psi_n \to \psi'_n \equiv e^{i\xi_n(R)}\psi_n,$$ where ...