All Questions

0
votes
0answers
2 views

How can beta plus decay be possible?

$\beta +$ decay is where a proton gets turned into a neutron and a positron and a neutrino. However, a neutron is heavier than a proton, so obviously this reaction is endothermic. So then, why does ...
0
votes
0answers
3 views

Electric current dipole moment

Electrical Current Dipole of dipole moment $\mathbf{p_{EC}}$ has unit of $A\cdot m$. My knowledge of dipole moments unfortunately stopped in high school where I learned that dipole moment $\mathbf{p}$ ...
0
votes
1answer
41 views
+100

Probability density during equilibration

An ensemble of states $x$ are initially drawn from a non-equilibrium probability density $P_0(x)$. Over time they will evolve towards the equilibrium distribution $P_\mathrm{eq}(x)$. What I want to ...
0
votes
1answer
28 views

Muzzle Velocity of a Bullet out of a Barrel

We want to know the speed of a bullet, in feet per second, for a given round. We know the weight of the bullet, volume of the casing, amount of gun powder, length of the barrel, and diameter of the ...
0
votes
2answers
24 views

Radio wavelength to penetrate 8 ft of seawater

I am trying to penetrate about 8 ft of seawater with a radio wave. What wavelengths will work? I understand that low wavelengths penetrate seawater the best, but I am looking for a frequency that does ...
0
votes
3answers
38 views

How do I understand Kinetic energy formula? [duplicate]

$$\frac{mv^2}{2}= Kinetic Energy$$ Can you explain me? What is purpose of $v^2$, $mv^2$, I am trying to understand the formula.
0
votes
0answers
35 views

Threshold energy for particle production

For you smart redditors, this is the following problem: Particles are being accelerated (close to the speed of light) and collide on a resting target. Now I have to show that the necessary energy (...
3
votes
1answer
146 views

Quantum description of Raman effect

In the classical description of Raman effect the object of study is the electric polarizability of the system. Since I'm interested in learning the quantum description of the Raman effect and in ...
0
votes
1answer
17 views

Writing a two variable function$ f(x,t)$ in terms of Dirac-Delta $δ(x)$ function and a function $P(t)$?

How to write a two variable function $f(x,t)$ in terms of Dirac-Delta $\delta(x)$ function and a function $P(t)$? For example; I read something in a book. You can find the following picture. But ...
0
votes
1answer
11 views

Fermi level in metals and semicoductors

As we read about Fermi energy, it is the energy of the highest occupied electrons. But if we look at the energy level diagram of semiconductor the Fermi level is situated somewhere between the valence ...
7
votes
1answer
409 views

Why don't all electrons contribute to total orbital angular momentum of an atom?

There are 47 electrons in a Silver atom, but talking about its orbital angular momentum we only take the outermost valence electron which occupies the 5s orbital. Why don't the remaining inner 46 ...
1
vote
1answer
14 views

Boundary condition for Hamilton-Jacobi PDE

For separable solutions to Hamilton-Jacobi PDE (say in 2D), we treat the Hamilton's principal function $S$ as $$S= W(x) + W(y) - E*t$$ and treat the separate parts as constants and find $W(x)$, $W(y)$...
0
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0answers
6 views

Conductance Charged Box

A box with thick and conductive sides with dimensions: Lx, Ly, Lz- while LZ>>Lx,Ly. The long dimension of the box [Lz] can be viewed as infinite so Phi(x,y,z)=Phi(x,y) Given that: Phi(0,y)=-Phi(Lx,...
2
votes
1answer
21 views

Randomly stopped dynamics of $x(t)$: How can I find $\text{var}\{ x(t) \}$?

Consider the simple dynamical equation $$ \dot{x}(t) = u H(t-\tau),$$ where the timescale $\tau$ is an exponentially distributed random variable $\tau \sim \omega \exp\{\omega \tau\}$ and $H(t) = 1-\...
0
votes
0answers
8 views

Radius vs Speed for Flywheel energy storage

If I understand the foemula correctly the equation for kinetic energy of a flywheel is mw^2r^2 whereas the formula for "centrifugal force" is mw^2r. So how come so much focus is on the speed of ...
0
votes
1answer
52 views

Barometer experiment

I was watching a video on YouTube on experiment of barometer. In the experiment there are 2 barometers - one of them is defective, another one is correct. The experiment is given as the air is ...
5
votes
2answers
256 views

Heisenberg uncertainty and Lorentz contraction

Consider a particle in a frame moving with speed $v$ relative to the lab frame. By Lorentz contraction, the width of the wavefunction will be smaller in the lab frame, resulting in smaller $\Delta x$. ...
-1
votes
1answer
83 views

General theory of relativity questions

I do not understand how this theory (the theory of general relativity) would work because gravity is what makes things heavy and weight is what bends things. All of the proof makes it hard to question ...
0
votes
2answers
22 views

Why there are three reactions at fixed beam in F.B.D?

It is fixed so no reactions will be present, then how three reactions at fixed beam in F.B.D? In F.B.D, we draw two reactions and moment reaction at fixed beam: Why there are three reactions at ...
0
votes
0answers
7 views

Physics-y resource request on Killing-Cartan forms

Most books that treat nonabelian gauge theory do not contain detailed discussion on Killing-Cartan forms, they'll usually just say that in $\text{SU}(N)$ Yang-Mills theory, one can choose generators $...
2
votes
2answers
64 views

Quantum corrections in path integral

I am working the following exercise: Calculate the generating functional $$Z[j]=\int \mathcal{D}\Phi \exp\left(\frac{i}{\hbar}S[\Phi,j]\right),\quad S[\Phi,j]=\int d^4x(\mathcal{L}(\Phi)+j\Phi),$$ $...
0
votes
1answer
25 views

Is there differential form notation for Maxwell's equation in curved spacetime?

In special relativity, Maxwell's equations may be written as \begin{align*} dF = 0, \\ \star\, d\star F = J. \end{align*} In four-vector notation, this translates to $\partial_{\mu}F^{\mu\nu} = J^{\nu}...
1
vote
2answers
111 views

$\mu \rightarrow e\gamma$ in the $R_\xi$ gauge: trouble with momenta and Dirac matrices

My first ever question on stackexchange! Sorry if it is clumsy... I am trying to follow the computation of $\mu \rightarrow e \gamma$ in Cheng and Li and am confused about the second and third ligns ...
3
votes
0answers
28 views

Inducing coil current by falling?

Typically, if a rod or coil is moved at a constant velocity through a constant magnetic field, a motional EMF will be induced by Faraday's Law but no current because there is no net emf or no closed ...
0
votes
1answer
42 views

Gibbs-Helmoltz equations

Problem: Show that $$E = T^2 \frac{\delta (S-ET)}{\delta T}$$ Attempt: $$ E = T^2 \frac{\delta (S-ET)}{\delta T} = T^2 \frac{\delta (S)}{\delta T} - \frac{\delta (ET)}{\delta T} =T^2 \frac{\delta (S)...
0
votes
2answers
309 views

For the derivation of the form of a paraboloidal reflector, there is much more in need as just Fermats principle?

I am currently learning about Optics using an MIT OpenCourseWare Course. I have in particular a question about the application of Fermat's principle in Lecture 3, i.e. deriving the shapes of parabolic ...
0
votes
0answers
10 views

Acoustics in heterogeneous medium: Reflection and Tranmission

I am trying to understand the effect of changing impedance on acoustic pressure wave. I have attached the image from the book "Fundamentals of Acoustics"(Kinsler et. al)(r1 and r2 are the impedance of ...
2
votes
2answers
57 views

Why is effective mass a tensor?

So I came across the effective mass concept for solids the other day. It was mentioned that the effective mass is a tensor and may have different values in different directions. However, this is stark ...
0
votes
0answers
8 views

Acoustic Plane Wave

Consider an ideal fluid which we have a spherical object on it and a progressive plane wave is heading the object. I have a simple question and it is: If we assume the amplitude of the incident ...
3
votes
6answers
383 views

Acceleration in Circular motion

Can motion of a particle be circular if radial acceleration is zero but tangential acceleration is not 0?
7
votes
4answers
129 views

Why were pulsars not discovered in the visual spectrum?

In 1967, the first pulsar was discovered at a radio observatory in Cambridge. Radio astronomy started around the 1930s, but astronomy in the visible part of the spectrum had been done for a long time ...
0
votes
1answer
23 views

How to calculate surface current density

So I have the question where you have an infinitely long cylinder, with $\vec{B}(r)=\frac{B_{0} r}{R} \vec{e}_{\varphi}$. Then with $\oint \vec{B} \cdot \overrightarrow{d l}=\mu_{0} I_{e n c l}$, with ...
1
vote
0answers
16 views

Adding new fermions to a two-dimensional space

Suppose we have a two-dimensional space where we can seat electrons. Now, is it so that even before we start adding electrons the single electron energy levels already exist and as soon as we begin ...
0
votes
1answer
19 views

How will a polar molecule affect another molecule nearby, why it feels a relatively large force?

Mean position of the negative charges and positive changes are not at the same point (polar molecule) how will that affect another molecule nearby, why it feels a relatively large force? Will the type ...
1
vote
1answer
39 views

Tadpole diagrams in 1-loop massive scalar amplitudes?

Consider a massive scalar diagram such as or The loop momentum enters and exits the tadpole vertex, so that in the first diagram the momentum in the propagator connecting the two vertices is zero ...
2
votes
2answers
124 views

What happens when a particle and anti-particle collides of different masses?

If Hydrogen and an 'anti-Helium' come together will we get a hydrogen and a photon? I was wondering if antiparticles can be used to fuel some nuclear reactions and used to develop particles which ...
7
votes
1answer
160 views

2-pt String Sphere Amplitudes

There is a standard lore in superstring theory that 2-pt sphere amplitudes vanish. This is inline with the basic fact that string amplitudes give amputated Green functions with onshell asymptotic ...
0
votes
1answer
38 views

Oscillating spring, speed close to the equilibrium: How is this answer not 1.5? [on hold]

I have this question with the answer listed as $2.0\,\mathrm{m/s}$. "A $1.25\,\mathrm{kg}$ mass on a spring with a constant of $12.0\,\mathrm{N/m}$ is oscillating back and forth. Its maximum ...
38
votes
4answers
3k views

How exactly is a normal force exerted, at the molecular level?

I've been surfing the web for quite a while, finding the answers I would need, but couldn't find a convincing one. First of all I need to remind you that this a very long/continuous question, so ...
0
votes
1answer
26 views

Capillary tube under atmospheric pressure

I found that capillary tubes in refrigration system are tubes with very small diameter and very long length. pressure drops down suddenly due to very small diameter of the capillary and length. ...
0
votes
1answer
135 views

Showing the Minkowski force in the covariant form

So I would like to show \begin{equation} K^{\mu} = \frac{q}{c}F^{\mu\nu}u_{\nu} = \frac{q}{c}\left[\partial^{\mu}(A^{\nu}u_{\nu}) - \frac{dA^{\mu}}{d \tau}\right] \end{equation} and I went about ...
2
votes
1answer
21k views

Relation of potential energy and total energy in Bohr Model of the hydrogen atom

I am currently in the 11th grade. In the Bohr Model of the hydrogen atom, potential energy is two times that of total energy. It means that magnitude of potential energy is two times that of magnitude ...
1
vote
1answer
63 views

Magnetic field and electromagnetic waves

If a magnetic field is just an electric field caused by Lorentz contracted charged particles ( https://the-educational-blog.quora.com/Why-does-electric-current-result-in-a-magnetic-field) then what is ...
-2
votes
0answers
31 views

My new equation of motion is right or wrong [on hold]

Every equation includes acceleration, but form the three equations I made forth equation of motion.![enter image description here](https://i.stack.imgur.com/IUrcH.
2
votes
2answers
138 views

How much in the past are we viewing the accretion disk of a black hole?

Sine the gravity near a black hole is so high, the time moves slower than the observer(in our case we from the earth).In that case, imagine if are viewing a black hole's accretion disk(the innermost ...
1
vote
1answer
164 views

How physics opens a knife with a flick of the wrist?

A person holds the handle of a folding knife in their hand with the pivot point of the blade between their index finger and thumb. The blade points away from the body and the other fingers grip the ...
0
votes
1answer
26 views

Is friction a conservative force when a ball rolls down an inclined plane(pure rolling)?

My book says friction is non dissipative for a cylinder rolling down an inclined plane,so is it conserved?
0
votes
0answers
18 views

Derivation of Raychaudhuri equation - Trace

In Wald (Wald: General Relativity on page 218, equation 9.2.10) is stated that $$v^c∇_cB_{ab}=−B^c_bB_{ac}+R^d_{cba}v^cv_d $$ and to continue in order for the equation to be derived one needs to take ...
-3
votes
3answers
91 views

Fire in my building [duplicate]

If there is a fire in a building and I am on 10th floor and I can't exit the building.Is it safe to jump out of the window with a chair and when chair is close to the ground I just jump of the chair ...
0
votes
0answers
19 views

Resources on Physical Baryon States as mixings of multiplet states

I've been told, and have seen in papers (search for "mixing") that baryon resonances are not plucked from pure group theoretic multiplets, but are most often found to be mixings of states in different ...

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