0
votes
1answer
41 views

Power and frequency of photons and its relationship with biological safety

I understand that x-rays are more dangerous than radio waves because they are of higher energy, since they have higher frequency. However, it’s less dangerous to stand near a radio station with a ...
1
vote
0answers
116 views

Problem in understanding Feynman's explanation of the Dirac-Delta function [duplicate]

This is quoted from Feynman's Lectures' Normalization of the states in $x$: We return now to the discussion of the modifications of our basic equations which are required when we are dealing with ...
0
votes
3answers
343 views

Primitive unit cell of fcc

When I consider the primitive unit cell of a fcc lattice (red in the image below) the lattice points are only partially part of the primitive unit cell. All in all the primitive unit cell contains ...
0
votes
1answer
36 views

How do I calculate the speeds in x, y, and theta of a spinning rod?

Suppose the object looks like the diagram below (it is the horizontal rod). On the vertices, are two velocity vectors $v_r$ and $v_l$. The question is: how is the motion of the rod affected by these ...
0
votes
0answers
137 views

Relative displacement of a point in a deformed object

I have a 3d model (nodes and elements) and it undergoes translation and rotation at different time steps Ti. I want to know the displacement relative to some point in the model which is kept as fixed ...
2
votes
1answer
74 views

Response functions for the quantum harmonic oscillator

I'm going through problems in Quantum Field Theory for the Gifted Amateur, and have been trying to understand a problem on the forced quantum oscillator [$L = \frac{1}{2}\dot{x}(t)^2-\frac{1}{2}m\...
2
votes
1answer
3k views

Degrees of freedom in a diatomic molecule [duplicate]

We know that a monatomic compound can only have 3 degrees of freedom as we can consider it to be a point mass. However now that we consider a diatomic molecule, there are 3 degrees of freedom in ...
1
vote
1answer
36 views

How the number of binary collisions increases with centrality faster than the number of participant in heavy ion collisions?

How the number of binary collisions increases with centrality faster than the number of participant in heavy ion collisions at different particle colliders?
0
votes
1answer
62 views

OPE of parity even theories in CFT.

If I consider an OPE of some operators, which belong to a theory where parity is not violated, will I have a constraint on the kind of operators appearing in the right hand side ? For example, I ...
0
votes
1answer
67 views

Could you explain this flow of calculation?

I am reading this book, Quantum Optics by Walls and Milburn. I am working on Chapter 6 which is about the Stochastic Methods. I don't understand a calculation in this chapter. Let $w(t)$ be the ...
0
votes
1answer
329 views

Induced emf vs Potential difference

Let a square conductor frame length l, resistance R,be pulled out with a constant velocity v from a magnetic field B perpendicular to plane of the frame .Then an emf e=Blv is produced across the frame....
13
votes
2answers
549 views

Must bounded operators have normalisable eigenfunctions and discrete eigenvalues?

When we have bound states, to my knowledge, we have states that are normalisable and a discrete energy spectrum. However, in the case of scattering states that have a continuous energy spectrum, the ...
3
votes
3answers
227 views

Is Bekenstein entropy limit inconsistent with universal continuity?

It is unknown whether the universe is discrete or continuous in its intricate quantum level structure. See for example: Can universal continuity be experimentally falsified? Is the universe finite ...
0
votes
1answer
267 views

Derivation of equation of path difference in double slit

$p$ = Path difference $r$ = Distance travelled by the rays $x$ = Perpendicular distance between interference of the rays to the medium point of the incident rays $a$ = Vertical distance ...
1
vote
1answer
90 views

Can one make a propagating field along a waveguide only have longitudinal/transverse components?

We know that in free space, a propagating electromagnetic wave is always transverse. However, along a dielectric waveguide, the propagating wave can have longitudinal components. The exist of the ...
0
votes
1answer
103 views

Finding the range of frequencies to which a receiver should be set

I'm working on a study guide for a test for my Modern Physics class, and ran across a question I'm having trouble with. A radar transmitter emits a pulse of EM radiation with $\lambda=0.225\,\...
0
votes
0answers
41 views

Is there any such thing as gravity? [duplicate]

So I was seeing this documentary and ended up knowing that there is no such thing as gravity. It said that we are not being pulled by Earth but being pushed by the space time curvature acc. to general ...
0
votes
1answer
24 views

Why the centered rectangualr lattice is not considered a special case of an oblique lattice

From Wikipedia, there are 5 different kinds of lattice in 2 dimension: But I am wondering how the third type (the centered rectangular) is different from the first kind (the oblique lattice). The ...
0
votes
1answer
67 views

lattice vibration

I would like to inquire about acoustic and optical branch in Phonons vibration; My question is: what is the physics description when we say what is make acoustic and optical branch appear ...
1
vote
1answer
212 views

Why is there a change in entropy in a irreversible isothermic process?

Suppose you have a irreversible isothermal process. The net change in entropy from the system to the surroundings should be 0 if the temperature does not change. So why is there an increase in entropy?...
1
vote
2answers
86 views

What are the gradient lines of the magnetic field of a cylindrical magnet?

The problem is identical to that for a finite-length solenoid. I was interested in finding the family of curves for the equipotential lines, rather than the B field itself, which is more readily ...
1
vote
1answer
97 views

How does gas spin the turbine in a jet engine?

I am confused about how the energy from the ignited gas spins a turbine in a jet engine, as pictured below                &...
1
vote
1answer
56 views

Derive an equation related to magnetism [closed]

Solve the equations for $v_x$ and $v_y$ : $$m\frac{d({v_x)}}{dt} = qv_yB \qquad m\frac{d{(v_y)}}{dt} = -qv_xB$$ by differentiating them with respect to time to obtain two equations of the form: $$...
0
votes
1answer
50 views

Adiabatic Invariance of 2 mass system

I am trying to solve the second part of Problem 10 from David Tongs CM notes. Specifically we have The neutron star is in a non-circular orbit with $E < 0$. Give an expression for the adiabatic ...
1
vote
1answer
51 views

Trouble Understanding the Concept of Invariance

I am reading the book by Brian Cox & Jeff Forshaw: "Why does E=MC^2". It is getting to the point of explaining what invariance is. According to Cox & Forshaw, invariance states that the laws ...
0
votes
2answers
61 views

What is a center of gravity?

I have tried to search the definition of it up on Google, but the explanations of center of gravity were very confusing and I had trouble understanding them. Can you explain to me what center of ...
2
votes
3answers
132 views

How to measure thickness of an ultra-thin metal layer?

I have a sample of a metal (aluminum/oxide) layer with a gradual thickness ranging from monolayer to 100nm. This layer is deposited on a transparent substrate, like glass. How can I measure this ...
0
votes
1answer
66 views

Is there a special name for thermodynamic process during which no work is performed?

Let $W$ denote the work done on a system during a thermodynamic process. Is there a commonly-accepted, dedicated term for a process during which $W=0$?
2
votes
1answer
139 views

Noether's Theorem for Hamiltonians and Lagrangians

Looking around I see one version of Noether's Theorem that creates conserved quantities from symmetries that preserve the Lagrangian (e.g. http://math.ucr.edu/home/baez/noether.html), and another ...
2
votes
2answers
69 views

What is used to measure the spin of a particle?

I was wondering what is the specific system or method that is used to measure spin of a particle? e.g. In a lab what would they use to tell what a particles spin is? P.S. I am new to stack exchange ...
0
votes
1answer
71 views

Why is sum of Boltzmann factors equal to $1 + \exp(-\epsilon / k T)$?

I am bit confused about how they write $$ Z = \sum e^{- \epsilon_i / kT} = 1 + e^{-\epsilon/kT}$$ Where did the subscript $i$ go and where does the $1+e^{-\epsilon/kT}$ come from?
3
votes
1answer
143 views

Coupled differential equations: how to write in terms of only one coordinate? [closed]

I have a mass-spring system, which is as follows:                          ...
1
vote
1answer
53 views

clouds reflective property

I am a private pilot and see some beautiful clouds in all forms and patterns up in the sky. I wonder what causes some of the clouds to be so shiny, reflecting such an intense light that it's hard to ...
0
votes
1answer
123 views

Physical meaning of eigenvectors of mass matrix

What is the physical meaning of the eigenvectors of the mass matrix? If I consider a 2-dof system with one mass linked to two orthogonal springs and I write the equations in any orthogonal system of ...
0
votes
1answer
75 views

Coma Cluster X-Ray Mass

Does anyone have a reliable reference for the x-ray mass of the Coma Cluster? I've got a value of $3\times10^{13}\: M_⊙$ from Gursky's 1971 report. Anything more recent would be appreciated.
4
votes
1answer
89 views

Hilbert space, does $|r\rangle$ satisfy $\langle r |r\rangle = 1$?

Let's say we start with no particles: $\mid0\rangle$. We have $\vec{p}\vert0\rangle = 0$, $H\vert0\rangle = 0$, where we are ignoring $\infty$ vacuum energy. Also, $a(\vec{k})\vert0\rangle = 0$ for ...
1
vote
2answers
244 views

How to calculate thrust from opening a pressure vessel in space?

One popular trope in science fiction is the astronaut who (deliberately or inadvertently) punctures an air tank or something and goes rocketing off. How would we calculate the amount of force that ...
0
votes
1answer
140 views

Einstein Equations [duplicate]

I learned about the Einstein Field Equations and also I know they depend on energy-momentum tensors. The solution of the field equations is a space-time metric. $$G_{\mu \nu}=8\pi GT_{\mu \nu}$$ Now,...
0
votes
1answer
83 views

Observing light speed

Consider an observer B is moving with speed $0.8c$ relative to another observer A in standard-configuration (I think it is called that in english: that the system B is only moving along the $x$-axis ...
1
vote
0answers
48 views

What is the equation for the elementary-excitation spectrum of superfluid heluim over whole momentum space?

I have a simple question. What is the equation for the elementary-excitation spectrum of superfluid helium over whole momentum space, not just near the origin (i.e., phonon excitations $\epsilon=cp$) ...
0
votes
5answers
156 views

Pressure variation

Imagine we have a vertically orientated pressurised liquid filled closed pipe. The pipe is shaped like a spring, such that the distance between the two ends can be adjusted vertically, whilst the ...
0
votes
1answer
162 views

Completeness Relations of Polarization Vectors in QCD

What are the completeness relations of the polarization vectors of (external) particles in QCD amplitude calculation? (I assume the polarization vectors depend on the gauge and even so still have some ...
1
vote
1answer
90 views

Is entropy violated inside black holes and worm holes?

Do the laws of thermodynamics hold true everywhere in universe ? What about black holes and worm holes ?
0
votes
1answer
60 views

Does energy flowing from one place to another in an object cause displacement of that object?

Let's consider two situations: a) A solid metal box (not hollow) where some heat is put on its left side. After some time, the heat will have moved to the right and also have spread out throughout ...
1
vote
2answers
74 views

Hot bullets and shot cowboys [closed]

Please excuse the title of the question, there is physics involved here, but I can't think of a way of writing the title without it being more to the point of the question. A cowboy film is pure ...
0
votes
1answer
53 views

Wave nature of an electron

Quantum mechanics tells us that any quantum particle behaves as particle or waves in different situations. Assume that the electron is behaving like wave in some situation, how can I visualise this ...
0
votes
1answer
63 views

When can I apply Lagrangian mechanics?

I am trying to understand Lagrangian mechanics. I am having trouble capturing all of the nuances in one gulp. I can see the equations, but not necessarily the semantics behind such equations. I ...
1
vote
0answers
69 views

Source of a parabolic wavefront

The Huygens-Fresnel diffraction integral has been formulated for incident spherical waves. A source of a spherical wave is kind of intuitive, since the perpendicular rays of the sphere converge into a ...
0
votes
3answers
131 views

Path followed by centre of mass of a ball when ball breaks up in two parts due to internal stresses

A ball was moving along a parabola and it breaks up into two parts. How can I say that the centre of mass will follow the same parabola ? I know that ( for simplicity I am assuming that the ball is ...
1
vote
0answers
68 views

signal from uniformly moving charge

As a charge moves, its field changes, and this change can only be propagated outward at the speed of light. Thus the field lines will be curves that keep changing apparent source point and direction. ...

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