# All Questions

136,649 questions
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### Understanding angular velocity in spherical polar coordinates

I'm a first year physics student and i've just learnt this equation for angular velocity in spherical polar coordinates: $\omega=\dot{\phi}\mathbf{e_z}+\dot{\theta}\mathbf{e_\phi}$ The diagram i ...
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### $T \bar{T}$ OPE

In page 157 of Di Francesco (Conformal Field Theory) it is said that the holomorphic and antiholomorphic components of the energy-momentum tensor have the trivial OPE $T(z) \bar{T}(\bar{w}) \sim 0$....
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### Signal / electromagnetic wave propagation

The question is rather an everyday question: Think on an electromagnetic wave (e.g. let's say 60 GHz) and I would like to know how to absorb it such that there will be no transmission (through a wall) ...
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### Are there any formulas that calculate the stopping distance of a car on different surfaces that include deceleration?

I already found the formula d = v^2/2ug where 'v' is for velocity, 'u' is for the coefficient of friction between the tires and the road surface, and 'g' is the acceleration of gravity. Are there any ...
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### A problem involving Radiance and Fluence definition

In the book I'm reading the Radiance is defined in this way: $$I=\frac{dP}{d\Sigma'd\Omega}$$ where $d\Sigma'$ is the projection of the surface $d\Sigma$ on the plane perpendicular to the photons flux ...
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### I am confused about moment of inertia

Question There is a homogeneous and nonelastic (solid) rod on the frictionless line with mass M, length L and velocity V. Then line becomes a circle with radius L and our rod starts turning around it. ...
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### What would happen to the water bodies if gravity ceased to exist on Earth?

If water stopped flowing (it would stop flowing downwards from mountains and all) then, would it start rising upwards? If it floats on the space ( I say space because there'd not be any atmosphere) ...
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### Newton's second law in non inertial frames: why am I getting $r_x = 0$?

Consider the following system: and the following frame: A rotating frame $x^{\prime} y^{\prime} z^{\prime}$ whose origin coincides with the inertial frame $x y z$ and the $z^{\prime}$ and $z$ ...
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### Characteristics of the Navier-Stokes equations as a set of PDE's

I am not entirely sure if I should ask this question here or not, but here goes: can anyone suggest any reference (book, article, etc.) about the Navier-Stokes equations from a mathematical point view?...
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I am watching a video about element 173 being the biggest and last element that can ever be assembled. The reason given as the last element that can be created is that the electrons would have to ...
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### Can we exploit non-conservative field forces?

If a body is revolving in a constant non-conservative field and a non-zero work is done on it. Can we repeat the cycle infinite times and get more and more energy there by creating a free energy ...
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### Reason for repulsion of like charges

Like charges repel and unlike attract. Like magnetic poles repel and unlike attract. Since electricity and magnetism are fundamentally linked , is this attraction/repulsion due to the magnetic fields ...
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### Dimensional analysis of Clausius and Clapeyron equation

proof that the following formulas unit are consistent using dimensional analysis.
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### Finding the charge on a capacitor (RC circuit)

I am having trouble finding the charge on the capacitor for $t\rightarrow \infty$ after the switch has been closed at $t=0$. I already know the current $I$. I also have to find out on which sides the ...
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### Thermodynamical conjugate variables

In thermodynamics the potentials are typically only a function of 2 variables, say $$U=U(S,V)$$ with entropy $S$ and volume $V$. I see that conjugate pairs $S,T$ or $p,V$ always have the unit of ...
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### Constraints on the $(4^n-1)$-dimensional Bloch vector

I'm interested in the constraints on the $(4^n-1)$-dimensional generalized Bloch vector (the Bloch vector for $n$ qubits). To the best of my knowledge, these are not analytically characterized for ...
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### Understanding The Electric Globe Valve

To begin, we need to look at the everyday "globe valve," to see how it actually functions. Mainline(constant pressure lines), are coupled with inlet of Valve. Typically flow will stop there when ...
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### How do I derive the relationship between the classic magnetic field and the electromagnetic tensor?

I am trying to derive the relationship between the components of the electromagnetic field tensor and the components of the magnetic field by inverting $F_{ij} = \epsilon_{ijk}\beta_{k}$. The ...
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### Can someone explain, how we get the third formula?

Hi can someone explain how we get the third formula ?
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### Would protons in a straight line ever break their alignment?

Let any amount of arbitrarily spaced protons appear along a line segment of length $x$ in an otherwise empty universe at $t+0$ What mechanism, if any, could cause the breaking of said alignment ...
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### How can magnetic fields be used to stop a rotating wheel?

How can a rotating wheel stop using magnetic power? For example, consider a rotating iron wheel: can it be made to stop suddenly with using magnetic power? If so, how?
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### Does the 2nd law of thermodynamics hold inside the event horizon of a black hole?

Suppose I am an observer inside the event horizon of a black hole, and I am part of a system comprising myself, an object and a singularity. Now I throw the object towards the singularity; what ...
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### What is Incoming wave boundary condition (IWBC) in context of nuclear fusion reaction?

For calculation of fusion cross section at sub barrier energies, instead of regular boundary condition we impose Incoming wave boundary condition (IWBC). How does this different from normal boundary ...
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### Information across the event horizon and the implications of Entaglement

What would it imply if Entagled particles remained entangled after one of the elements of the set crossed an event horizon.
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### Combine coherence tomography with second harmonic generation

I was given the task to combine the OCT procedure with second harmonic generation. Problem is I am completely new to this stuff. Could anyone give me a few proposals which I could use as starting ...
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### How is it possible that doubling a force on a fixed mass causes its acceleration to triple?

So the suggested solution is A. But here's what I don't get, when the F is doubled, how is it possible that the acceleration(A) tripled? If the answer is m.a should the question be "a person needs ...
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### QFT Complex scalar field and commutators

The conserved charge is $$Q=i\int\ d^3x(\phi\pi-\phi^\dagger\pi^\dagger)$$ Expressing this in terms of creation annihilation operators gives Q=i\int d^3 x \frac{d^3p d^3k}{(2\pi)^3(2\pi)^3}\frac{i\...
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### Breaking wave behind a paddle steamer

While travelling on an 1890's paddle steamer (PS Marion) I noticed that the wake from the paddles consists of a series of waves from each paddle that progressively reduce in amplitude. Some distance ...
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### What is the set of independent parameters that can be used to describe a quantum particle like photon or electron?

How many independent parameters are there for fully describing a quantum particle like photon or electron?
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### Thought experiment: if an observer can travel at the speed of light [on hold]

What would the observer see if he could travel at the speed of light and shot a photon beam at the same time?
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### Do photons have multiple peaks or only one? [on hold]

Energy of a photon is given by E = h x frequency Thus, if an photon hitting a detector delivers 10 peaks/ second, it will transfer 10 h/s per 1 photon (6.626x10^-33 J) I.e the EM wave delivers 6....
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### Why is $\langle{\dot A}A^* \rangle=0$ for dynamical variable $A$?

In page 179 of Hansen and McDonald's book, Theory of Simple Liquids, 3rd edition, 2006, an identity of correlation functions was deduced. Here $C_{AB}(t)$ is the time correlation function between ...
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### When air is ionised, which component gases are actually ionised?

Air is a mixture of O2, N2 etc. Which gases are ionised at STP, and is there a different level of ionisation for different gases?
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### Which properties does tensorial product have with respect to scalar product?

Are the associative and distributive properties preserved? In Cohen-Tannoudji's Mécanique quantique vol. I, the scalar product in the $\epsilon=\epsilon_1 \otimes\epsilon_2$ space is defined as ...
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### Introducing Spin operator in QM

What was the experimental facts used in the construction of spins in Quantum Mechanics? What forced W.pauli to invent the idea of spins ? Can there be other internal degrees of freedom yet to be ...
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### What is going on in “nonlinear gravity from entanglement in conformal field theories”?

EDIT: I am now convinced that the sign of the logarithmic terms in the equations after 3.29 and 3.30 are wrong (unless I have missed something else). These identites come from looking at ...
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### CFT in momentum space

Is there a way to see the conformal symmetry in momentum space in a CFT? I mean if I can recover the conformal group in some way in momentum space.
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### Help me calculate the photons flux on Earth surface

I want to find the PFD in small intervals of wavelengths based on this dataset (https://www.nrel.gov/grid/solar-resource/spectra-astm-e490.html) see second link in Data Files. I did it and have my ...