0
votes
0answers
3 views

Is the stability matrix of a linearised RG flow always diagonalisable?

This is a follow up on "Why are the eigenvalues of a linearized RG transformation real?". My question is simple: Is there some physical (or mathematical) reason for the stability matrix of ...
0
votes
0answers
4 views

Do cutouts in 3d objects affect their heat expansion

If you heat (assume evenly) a 5 mm sheet of metal containing a cut away hole 20cm diameter, so it expands...what would happen to diameter of hole? Increase, decrease, stay same?
0
votes
0answers
5 views

Free body diagram of mechanisms

The image below shows a mechanism with 2 pin joints. On one of the pin joints, there is a torsional spring that exerts a torque $M_s = \kappa \theta$ (rest state is at $\theta = 0$). A force $F$ is ...
0
votes
0answers
9 views

Casimir effect with super conductors

Do the plates in the Casimir effect need to be metal? What if one uses super conductors? Is there any difference theoretically/experimentally?
1
vote
1answer
18 views

Movement of a capacitor

What will happen if a capacitor is accelerated or rotated very very fast? Will its capacitance decrease? Will one of the plate be able to hold the electrons which is providing the Potential ...
2
votes
1answer
18 views

Power dissipation in circuits:Why is high voltage used in power lines?

I know this question has been asked before, but there is one doubt I still cannot clear. Power dissipation is proportional to $I^2R$. Does this not mean that it is also proportional to $V^2/R$? If ...
0
votes
0answers
19 views

How thin a practical capacitor can be?

According to the formula of a $parallel$ $plate$ capacitor... $C$ $=$ $\dfrac{\epsilon_0 A}{d}$ The thinner the capacitor, more the charge it will be able to store... And hence the graph should ...
4
votes
0answers
20 views

What would we see if we looked at our Solar system from 2,000 light years away with our current technology?

Assuming the tables were turned and we would live in a system like Kepler-422/423/424, some 2,000 ly away. If we'd look at the Solar system with a telescope like Kepler and using techniques like ...
2
votes
0answers
14 views

Weighing with One-Foot Scale

I stand with left foot on a scale and the other on a piece of effectively incompressible material at the same initial height at the scale's top surface. The weight shown is WL. I repeat with right ...
0
votes
1answer
16 views

Unstable equilibrium in a pendulum

Consider a pendulum with a bob and a massless, rigid, hinged rod attached to the bob. The bob is at rest at the bottom most position. Neglecting friction, is it possible to impart such a velocity ...
2
votes
1answer
41 views

Special relativity and missing factors of $c$

I am doing problems in a textbook called 'Introduction to Classical Mechanics' by David Morin. In one of the questions it says the following: In the lab frame, two particles move with speed $v$ ...
3
votes
1answer
51 views

Vector fields and tensors in E&M

I'm confused by a very basic property of electric fields. The electric field is a vector field. Vectors are tensors. Wikipedia has the following statement in the article about the electromagnetic ...
2
votes
0answers
23 views

Open string 4-tachyon amplitude for cylinder/annulus topology in bosonic string theory

One knows the formula for the open string $4$-tachyon amplitude for the disk topology in the bosonic string theory : it is proportional to the $s \leftrightarrow t \leftrightarrow u$ symmetrisation ...
2
votes
0answers
10 views

Information retrieval from a database

Consider a database $\cal D$ containing $N$ entries $A_0, A_1, ... A_{N-1}$, which are some fixed and unknown strings of $k$ bits; you can access this database sending a coherent superposition of ...
2
votes
6answers
70 views

Expectations values of position in quantum mechanics

In quantum mechanics, we can show that $$ \langle r \rangle^{-1} \neq \langle r^{-1} \rangle $$ I can understand this mathematically as the integrals are different but can anyone explain physically - ...
1
vote
1answer
46 views

Confusion with Newton's Second Law : Position & Time Dependence of Force

For a conservative force $\vec{F}(x,y)$ we have $$\vec{F}(x,y) = - \vec{\nabla}\phi(x,y)$$ where $\phi(x,y)$ is a potential function for $\vec{F}(x,y)$. Again, Newton's second law states that $$ ...
-1
votes
0answers
43 views

Physics11 vietnamese

Một tụ điện xoay C0 có các bản cực bằng nhau hình bán nguyệt. Ban đầu gồm 10 bản cố định và 9 bản linh động mắc xen kẻ nhau. Cho biết diện tích mỗi bản lớn nhất là 18,85mm2; khoảng cách giữa hai bản ...
0
votes
1answer
19 views

“Radiative” particle decay?

This might be a very simple question, so sorry. I have encountered the expression "radiative particle decay" quite a few times now, and none of the sources ever explain what they mean by radiative: I ...
8
votes
2answers
194 views

If I shout at the sky, will some molecules reach escape velocity?

Will the sound waves, as they move higher and through less dense air, conserve energy to the point where some molecules of the rarified atmosphere attain escape velocity?
3
votes
1answer
45 views

Dirac group representation

I am currently taking a representation theory class (from a physicist), and I am very confused about the Dirac groups' irreducible representations. First of all, all the Dirac matrices in the ...
1
vote
0answers
15 views

Pressure required on a wing to lift a plane and the speed required to create this pressure difference

An aircraft has a single wing with a total area of A =1 2.6 m2 (assume that the thickness of the wing is negligible). When in motion, the air flows under the wing at the velocity that the aircraft ...
2
votes
4answers
203 views

Two soft questions about spin and the particle nature of electrons

How can we define spin as the spin of an electron around it's own axis if an electron is described by a probability cloud of finding an electron in a point in space? How does that probability cloud ...
0
votes
1answer
25 views

What is a good book for quantum mechanics and quantum computation? [duplicate]

I am looking for a book in quantum computers for self-learning.The kind of book that teaches quantum-mechanics + quantum-computation. I have basic understanding in calculus , linear-algebra (like ...
-1
votes
1answer
13 views

Calculating impact force for bulk material [on hold]

I'm trying to calculate the impact force of bulk material falling from a belt conveyor. I have specific data, for example, the belt conveyor capacity (300 t/h), height (2 m), material of density (3 ...
0
votes
1answer
10 views

Refraction through converging lens: finding image distance and image height

So in this situation there is a 1.79 meter tall person standing 30 meters from a 150 mm converging lens and I'm trying to find where the image will form and how tall it will be. My problem is in ...
1
vote
0answers
55 views

Gauge invariance in arXiv:1406.1804

I am reading the paper arXiv:1406.1804. I don't show that the invariance of the connection under the gauge transformation (2.48), I have written an email to the authors about this question. However, ...
6
votes
2answers
457 views

Why does electron move in an elliptical path?

According to Sommerfeld's atomic model, an electron moving around a central positively charged nucleus is influenced by the nuclear charge. As a result of which, the electron moves in an elliptical ...
1
vote
0answers
60 views

An Operator Identity relating to Trace [duplicate]

Suppose that $\hat H$ is an operator (typically a Hamiltonian) and $\beta$ is a positive parameter (typically $\beta=1/k_BT$). Show that $$ \mathbf{Tr}\Big(e^{-\beta\hat H}\Big) \geq ...
1
vote
1answer
28 views

How does the weight of the pendulum's bob cause a torque?

I know that a force will cause a torque if the force has a perpendicular component to the distance from the pivot point. However, in the figure above, I can't seem to visualize how the weight of ...
1
vote
1answer
29 views

Household Electricity

What is actually moving in the wires? Electrons and energy, right? With alternating current, electrons move from atom to atom, practically back and forth in the wires. Mainly, it is energy that is ...
2
votes
1answer
40 views

If stars are ionized, where are the electrons?

As far as I know, universe is electrically neutral so, If stars are ionized, where are the electrons?
2
votes
1answer
19 views

Potential Energy in solids: Why are different equations used for deriving lattice constants and for deriving the properties of phonons?

While deriving the equilibrium lattice constants we use expressions for potential like Lennard-Jones potential which have 6th and 12th order terms or Madelung energy for ionic crystals. While ...
2
votes
3answers
59 views

Can mass be directly measured without measuring its weight?

From Wikipedia, http://en.wikipedia.org/wiki/Mass Inertial mass measures an object's resistance to being accelerated by a force (represented by the relationship F=ma). Active gravitational ...
1
vote
2answers
45 views

Why do we use kilograms instead of newtons to measure weight in everyday life?

What was the reason to use kilograms to measure weight (e.g. body weight, market vegetables etc.) instead of using newtons?
3
votes
1answer
105 views

What is the physical interpretation of the linear coefficient in this ODE for projectile motion?

For the second order ODE governing the position of a projectile subject to air resistance $$ m\frac{d^2x}{dt^2} +k\frac{dx}{dt}+mg=0 \quad k>0, \> x(0)=0, \> x'(0)=V>0 $$ a ...
2
votes
0answers
31 views

What is the future of complexity theory in black-hole physics and string theory? [on hold]

I found the recent work by Hayden and Harlow and Susskind very fascinating. I have also heard talks by Scott Aaronson about this emerging connection. In particular this idea of understanding ...
1
vote
1answer
24 views

Classical limit of non-interacting, relativistic quantum gas (Kapusta/Gale p.8)

I want to understand two equations in "Finite temperature field theory" by Kapusta and Gale on page 8. The partition function is $$ \ln Z = V\int \frac{d^3 p}{(2\pi)^3}\;\ln\left(1\pm ...
0
votes
0answers
24 views

How to derive the classical Gibbs entropy

I've seen the classical entropy given as $$S/k = - ∫∫ ρ(p, x)ln [ρ(p, x) h^{M}] dp dx$$ where $M$ is the number of unconstrained degrees of freedom. How do you obtain this result from the classical ...
0
votes
1answer
15 views

Fine Structure Degenerate Perturbation Theory Hydrogen

Note: We are dealing with perturbation on the states $|nlm_lm_s>$ where n is the principle quantum number, l is the angular momentum quantum number, and $m_l$ and $m_s$ are the eigenvalues of $L_z$ ...
-3
votes
0answers
50 views

Tension and acceleration in Atwood's machine [on hold]

A device known as Atwood's machine consists of two masses hanging from the ends of a vertical rope that passes over a pulley. Assume the rope and pulley are massless and there is no friction in the ...
0
votes
0answers
36 views

Proof that the trace ${\rm tr}(\rho^2)=1 $ of the square of a pure state density matrix $\rho$ is always one [on hold]

Can someone provide a proof that the trace ${\rm tr}(\rho^2)=1 $ of the square of a pure state density matrix $\rho$ is always one?
2
votes
1answer
20 views

Why is a particular wavelength favored by transmission through a thin film?

A sheet of glass is coated with a $500\text{ nm}$ thick layer of oil ($n=1.42$). For what visible wavelengths of light do the reflected waves interfere a) constructively? b) destructively? ...
1
vote
1answer
49 views

A few questions on wave packets and uncertainty relations

According to Cohen-Tannoudji the wave-function for a one-dimensional free particle can be written as $$ \psi (x,0)=\frac{1}{\sqrt{2 \pi}} \int g(k) e^{ikx} dk.$$ While $g(k)$ is not specified, there ...
3
votes
1answer
34 views

Would an ideal gas be colder at higher altitude due to gravity?

Since gas molecules are affected by gravity, wouldn't that make gas molecules at higher than average elevation slower (at the top of their ballistic parabola) and thus colder than air molecules ...
0
votes
1answer
38 views

Why does a Ball bounce back if Forces are Equal and Opposite?

When a ball is thrown onto a wall, the ball exerts a force onto the wall. According to Newton's Third Law, the wall will exert an equal and opposite force to the ball. Thus, how would the ball be able ...
0
votes
0answers
14 views

Dicke states, spin squeezing and quantum metrology

Dicke states are by definition simultaneous eigenstates of the $J_z$ and $J^2$ operator. What is the difference between these states and Dicke squeezed (DS) states? I know that these are "entangled" ...
2
votes
1answer
30 views

Beads flying out of flask

There is a popular youtube video of some metal beads being thrown out of a flask, literally hovering out of the container My question is how detailed a physical explanation of this phenomenon we ...
0
votes
0answers
14 views

What determines the energy ratio of beta decay products?

What determines for any particular decay how much energy the electron and neutrino get? Is it just that in the CM frame of the W- the electron and neutrino are back to back, but then back in the lab ...
-4
votes
0answers
58 views

Why most problems encountered in quantum mechanics cannot be solved exactly?

Exact solutions of the Schrödinger equation exist only for a few idealized systems. Why most problems encountered in quantum mechanics cannot be solved exactly? Isn't it a failure for quantum ...
1
vote
0answers
14 views

Wigner function vs. backward kolmogorov

We know that a Wigner function is a quasi probability distribution and a backward Kolmogorov equation for a stochastic differential equation (say quantum Lagenvin equation) gives the probability ...

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