All Questions

0
votes
0answers
26 views

Lorentz-like equation from Geodesic equation

In Hobson's General Relativity, it explains the following (p.173) Consider the limit of a weak gravitational field in a coordinate system in which $g _ { \mu \nu } = \eta _ { \mu \nu } + h _ { \mu \...
2
votes
1answer
34 views

Why do pipe organ tubes need to have different thicknesses?

In a pipe organ, there are many tubes—long and short, thick and narrow. Each tube presumably resonates at a specific frequency based on its length and produces a distinct note. Why do they have to be ...
1
vote
1answer
26 views

Work from string wrapped around cylinder

A cylinder, on a level surface angularly accelerates as a string wrapped around is put under tension. Because this string causes the cylinder to roll without slipping, the tension force cannot to any ...
1
vote
0answers
19 views

Why an insufficient overlap cause vanishing exchange interaction?

Why should the exchange interaction vanish if the atoms do not have sufficient overlap in their overfunctions? For exchange interaction not to vanish, the only requirement seems to be that the ...
1
vote
1answer
57 views

Rotation matrix - levi-civita symbol

I'm trying to solve the following problem: Given a rotation matrix $R_{ij}$, show that $$n_k=\frac{-R_{ij}\epsilon_{ijk}}{\sqrt{(3-tr(R))(1+tr(R))}}$$ and that $$\sin(\phi)=-\frac{\epsilon_{ijk}...
13
votes
5answers
2k views

What does it mean that physics no longer uses mechanical models to describe phenomena?

I've just started reading Sommerfeld's Lecture on Mechanics, with no background in physics (only in math). Can you explain to me what the author means with the bold sentence? Mechanics is the ...
-3
votes
0answers
41 views

How to transform following formulae to make it stright line : $T = k\sqrt{ h } + \tfrac12 T_0$? [on hold]

SI units in the formula: $$ k = s \frac{1}{\sqrt{m}} $$ $T$ is in seconds $h$ is in metres $T_0$ (start value of T) is in seconds How to convert the formulae to make it a stright line by ...
-3
votes
0answers
30 views

Worked Solutions for GR [on hold]

Does anyone knwo where I could find worked solutions to the excercises in Hobson Efstathiou lasenby's General Relativity?
1
vote
0answers
14 views

Symmetry operators of a Bloch Hamiltonian

Consider a lattice with a 3 atom basis, e.g. the Lieb lattice, and some completely arbitrary on-site energies and hopping energies and phases between the different atoms. In momentum space we can ...
2
votes
0answers
39 views

Visualizing EM fields

Personally, I have trouble visualizing a flock of birds without the birds. I have the same trouble trying to visualize an EM field without the photons. More than once, I have been hounded off of the ...
0
votes
1answer
27 views

Why does the voltage across resistor R decrease as we close the circuit?

I understand why there should be an increase in voltage across a capacitor during the charging process of it and why there should be a decrease in current I during that process. What I don't ...
0
votes
0answers
10 views

Dependence on sample thickness and incident intensity in X-ray crystallography?

In X-ray crystallography, a sample (let's say a crystal or a quasicrystal) is illuminated with X-rays. These scatter from it and form an interference pattern, which will be composed of Bragg peaks, ...
0
votes
1answer
13 views

How momentum is conserved when left circularly polarized light hits a mirror?

I read that 2 light planes with same amplitude are set perpendicular to each other will create a circularly polarized light, but I am curious that when it is reflected back and is converted from left ...
0
votes
0answers
8 views

Does GGA have any deviations in qualitative properties of band structure not just the quantitative band gap size compared to GW?

I found out several articles that emphasize the underestimate of the band gap by GGA calculation. And the GW method gets better results to experimental values. But it's hard to get advice for the ...
0
votes
0answers
13 views

Derivation of gradient of the expectation of local energy

Background: In Variational Monte Carlo, given a Hamiltonian $H$ and a wave function $\psi_\alpha$ dependent on some parameter(s) $\alpha$, we have defined a quantity known as the local energy, $$E_L =...
-3
votes
1answer
83 views

Is speed of light constant in every inertial reference frame, or does it just seems constant for observers? [on hold]

Here is an hypothetical experiment. (I already used this experiment in previous question but the question was diferent) : Let's call k the distance travelled by light in 1 second We have two ...
1
vote
1answer
31 views

Defining pressure from the stress-energy tensor components

Suppose I have a trial metric which when I plugged into the Einstein Field Equations produced a stress-energy-momentum tensor where the following components are non-zero: $T_{tt}$, $T_{tr}=T_{rt}$, $...
0
votes
0answers
32 views

General Relativity Lorentz-like equation

In the literature, it says that, in the weak-field, $$g_{µν} = η_{µν} +h_{µν},$$ slow-motion limit, the Geodesic equation reduces to the Lorentz-like equation. Can anyone explain this?
1
vote
2answers
31 views

Why do we define Energy density

So we define the electrical energy density $(\bar U)$ between a parallel plate capacitor as $$ (\bar U) = \frac{1}{2} \epsilon_0 E^2$$ where $E$ is the electric field between the plates usually given ...
0
votes
0answers
25 views

confusion regarding rolling without slipping

I'm trying to solve the following problem: Where the rolling uniform disk of mass M and radius has initial velocity of $v_0$ and angular velocity $\omega_0$ (it is rolling without slipping, but the ...
0
votes
3answers
58 views

Force due to motion around Sun [on hold]

Think of one person on earth named A . Ignoring Earth for a second, I can imagine him moving around the Sun due to which he should always feel some acceleration towards Sun (Centripetal Force) say F1. ...
0
votes
3answers
49 views

Bending moment in a cantilever beam

If I have a cantilever beam of length L fixed at the left end to a wall and I hang a weight W from it's right free end then why should the bending moment at a point x units to right of the wall be W(L-...
0
votes
0answers
17 views

When can quantum states be distinguished with certainty?

Through a few examples, I'd like to learn under what circumstances can different quantum states be distinguished from each other. So for example, the standard quantum teleportation scheme starts out ...
0
votes
0answers
23 views

Is it there any physics-basis for Tegmark's hypothesis? [on hold]

Tegmark's mathematical universe hypothesis is very interesting (https://en.wikipedia.org/wiki/Mathematical_universe_hypothesis) but it has virtually no support among physicists because it's too ...
0
votes
0answers
28 views

What kind of wave is light? [duplicate]

When people say about wave-like nature of a photon, what do they mean: Pressure-like wavefront - it moves as a sphere (or a bulge on a surface) and then if we measure pressure at one single point (...
0
votes
0answers
14 views

Composite objects, mass, and internal energy in relativity

Why do we define the free Hamiltonian of composite particles as $$H_0 = \sqrt{M(t)^2c^4 + P^2c^2}$$ with $M(t)\neq\sum_im_i$ instead $$H_0 = \sqrt{M^2c^4 + P^2c^2} + U(t)$$ with $M=\sum_im_i$ and $...
1
vote
0answers
30 views

Reference Frame conceptual confusion

I am getting confused as to why a ball still feels gravity when inside a moving car. The point of a reference frame is to reinterpret all the forces acting on a particle in one frame only. Hence all ...
-2
votes
0answers
26 views

Can I ever solve apparent frequency problems without memorizing the various formulae? [on hold]

I have always managed to solve question relating to apparent frequency by luck due to the fact that I have no deep understanding. So can anyone derive just one basic formulae and equally explain how ...
-1
votes
0answers
14 views

Eigen value of a unknown system [on hold]

Why we are using operator for finding the eigen value of a unknown system
1
vote
1answer
45 views

Decomposition of the maximally entangled states

We know that the set of symmetric bipartite pure states is spanned by $S=\{|\phi\rangle^{\otimes 2},|\phi\rangle \in \mathbb{C}^d\}$. I want to know if the maximally entangled state $|\psi\rangle = \...
1
vote
0answers
27 views

General covariance and a running cosmological constant

A running cosmological constant $\Lambda(t)$ can always be included in the perfect fluid source tensor. By the transformation properties of tensors Einstein's field equation is still independent of ...
0
votes
0answers
24 views

Why we get relativistic momentum without length contraction?

My last question was wrongly duplicated. We've already known that "p=γmv" . v is particle's velocity depends on our rest frame and mass too. But shouldn't we consider particle's momentum by its ...
0
votes
0answers
11 views

Charge conjugation opearation in odd dimension

In my problem, I use the following set of $\gamma$-matrices (in (2+1) spacetime): $$\gamma^0=\sigma^1;\quad \gamma^{1}=i\sigma^2;\quad \gamma^{2}=i\sigma^{3},$$ where $\sigma^{(i)}$ are usual Pauli ...
1
vote
0answers
29 views

Ordered arrangement of clouds

I happened to take a photograph of a region of clouds ,and it seems to have a rather ordered arrangement ,if one looks towards the left of the image. Would there be a physical explanation for this or ...
1
vote
0answers
15 views

Thermal expansion of a hole in a plate with a temperature gradient

I have a rectangular metal plate with a hole in it (with diameter of 300 mm). The plate has a temperature gradient going from one of its short sides to the other, I can measure the temperature ...
0
votes
0answers
17 views

Stale file handle error [migrated]

I am doing particle physics. I am running code on monte carlo samples. I am getting the "Stale file handle" error only when I run on one specific sample. The rest of them is OK. A screen shot of ...
1
vote
1answer
20 views

Do we consider an object with charge to have negligible mass when we define the concept of electric potential?

In the photo I have made a fixed charge +Q . In the book I read the electric potential at infinity is 0 and to bring charge from infinity to point we have to do work against the repulsive force . And ...
1
vote
1answer
33 views

A fictitious force in an orbiting frame of reference that is not rotating

Say we have an inertial frame of reference where there's a motionless star at the center and a planet is in circular orbit around it. We further assume that the planet is not rotating in the inertial ...
1
vote
1answer
34 views

How much current can be produced by Van de Graaff generators or Pelletrons?

The Van de Graaff generator is a source of high voltage at low current based on the transfer of static electricity, and its higher current successor the Pelletron could deliver more current by ...
0
votes
0answers
18 views

Why, precisely, do three elements in a row (iron, cobalt, nickel) show ferromagnetism, but not the elements below them on the table?

The same is true for Neodymium, Promethium and Samarium, right? But like the first three, they are consecutive, not above and below each other. Somehow only elements in those two ferromagnetic 'hot-...
0
votes
0answers
17 views

2 equal charged particles are fastened to the two ends of a spring of spring constant 100 N/m and natural length 10 cm. . find the extension

(assume extension to be small) I have the answer and I know how to solve this .we're taking equilibrium of each particle at a time and equating kx= kqq/r² but I don't understand how the x is the full ...
-6
votes
1answer
40 views

If we can think of free will as an illusion, why can't we do the same with gravity? [on hold]

I think the problem with gravity has to do with our limited understanding of what energy really is. There is no reason that a graviton would have to exist. Maybe in the current mathematical framework, ...
0
votes
0answers
19 views

Hawking radiation of massive scalar field

There are some calculations about Hawking radiation (expectation value of particle number operator, 2-point function, stress-energy tensor, and so on.), which are often done under the condition that ...
0
votes
0answers
19 views

Velocity of a spool of string being pulled

The question given is In working this, I assumed that it was in pure rolling motion and as such friction was negligible. Working through, I found $ ΔKE = \frac{1}{2}Iω^2 = \frac{1}{2}I\frac{v^2}{R^2}...
0
votes
0answers
8 views

Symmetry propertis of the 3-J symbol from the symmetry properties of CG coefficient

I use the relation between 3-J symbol and the CG coefficient. But using the symmetry propertis of CG coeficient, I am not able to arrive at the symmetry properties of 3-J symbol normally used. Even ...
0
votes
1answer
53 views

A problem in newtonian physics

Hello! I have solved problem 89 using analytic way, i.e, the length of the string will always be constant, so repeated substitutions and differentiating will make way(i got answer as "c"). But is ...
-1
votes
0answers
24 views

How do I find the equivalent capacitance between A and B [on hold]

How do I find the equivalent capacitance between A and B
0
votes
0answers
26 views

How does the collision between two atoms work?

Considering the quantum mechanical model for an atom, what exactly happens when two atoms (say, two Ca2+ ions in a Brownian motion) collide with each other? As I know, this collision is not like a ...
2
votes
0answers
15 views

Is gradual cooling and reheating better than abrupt cooling then re-heating for material durability?

So, I heated some bread on a pan (medium flame). Then, I turned the flame off knowing full well that I would put another bread on it within 10-15 seconds and heat it on medium flame again. I reasoned ...
3
votes
0answers
31 views

Why can't we take a current carrying bar to determine the magnetic field in Biot savart law?

Our teacher told us that to determine the magnetic field by Biot savart law, we should always consider a wire and not a bar. I don't get the reason for the same. Is it because then we have to take in ...

15 30 50 per page