0
votes
2answers
19 views

Gauss law problem

Consider a cube in an variable electric field such that there is no charge enclosed by the cube.Say E = ax^(1/2) (where a is constant) (Base of cube is in the xz plane) and its direction is parallel ...
0
votes
1answer
7 views

Number of electrons in conduction band

As mentioned in a previous question, the number of electrons in conduction band in a semiconductor can be computed as follows: $$N = \int_{E_c}^{+\infty} g_c(E)f(E)dE$$ where $g_c(E)$ is the density ...
0
votes
0answers
6 views

Standing Waves due to 2 waves of different amplitude

If I have 2 waves of different amplitudes, but same frequency and direction, traveling in opposite directions towards each other, let's say transverse waves on a string formed due to partial ...
0
votes
0answers
13 views

Why don't the magnetic dipole moments in a neutron cancel out?

This may be a silly question. I thought magnetic dipoles were dependent on electric charge, so why in a neutron do the dipole moments not just cancel each other out?
1
vote
2answers
37 views

Energy changes due increasing plate separation of unconnected charged capacitor

If a capacitor is charged by a source with constant voltage, $V$, the energy, $W$, stored by a capacitor of capacitance $C$ is given by: $$ W = \frac{1}{2} QV = \frac{1}{2} V^{2}C = \frac{Q^{2}}{2C} $...
1
vote
2answers
935 views

If gravitational waves exist are they technically just another form of light/electromagnetic wave?

I would imagine a gravitational wave would have very similar characteristics to electromagnetic wave, what kind of differences are there?
3
votes
2answers
27 views

Are the partial derivatives of Lagrangian in the varied action functional derivatives?

In particle mechanics Lagrangian $L$ depends upon position, velocity (and may be explicitly on time), whereas in field theory the Lagrangian density ${\cal L}$ similarly (or analogously) depends upon ...
1
vote
2answers
76 views

Why is it harder to open the cap on a bottle than to close it tightly?

In my experience it's harder to turn the tap off a bottle than to put it tightly on. Why?
0
votes
0answers
44 views

How to calculate possible spin of two photon system?

Photon hasn't well defined quantity such as spin. Instead of it, it is characterized by helicity $h$. Let's assume state of two photons in CM frame (with $\mathbf k$ being the momentum of one of ...
0
votes
0answers
8 views

What is magnetic levitation? Can it be done with electromagnets?

I have done a bit of a research on magnetic levitation and maglev trains but I fail to understand the phenomena. I also saw some videos on YouTube , they were about superconductors and magnets. I ...
5
votes
2answers
52 views

Why is the efficiency of human cells less than efficiency of an Otto engine?

I always was thinking (I don’t know why!) that the efficiency of human (and animals and plants) cells should be equal or near to the efficiency of a Carnot engine or at least should be the highest ...
0
votes
1answer
177 views

What happens when two charged capacitors are joined?

Two capacitors are charged and then they are connected with a wire. Then what will become constant after they are connected? Their potential or their charge? Exact question: A capacitor $2C$ is ...
1
vote
2answers
29 views

Understanding Zeeman Splitting

I'm reading a standard modern physics history book ("Inward Bound" by A. Pais), and I realized I don't really understand Zeeman splitting well. In the section I'm reading, there's a short discussion ...
3
votes
1answer
52 views

At what direction does a ray reflect at a 90 degrees corner?

Say we pointed a laser that is projected into a $90^{\circ}$corner of a mirror made by perpedicular walls. I know that you cant use tangent since a derivative of boundary of mirror would not exist ...
1
vote
2answers
352 views

Difference between directional and omnidirectional transmission or reception?

It is suggested that the next generation wireless technology will be highly directional since this increases the signal gain (MIMO) and decreases interference. How will the receiver still be able to ...
17
votes
7answers
4k views

How can a battery charge up another battery to a higher percentage?

Say I have my phone on 5% and a large battery pack on 35% and I charge the phone. By the end the phone is on 100% and the pack is on 12%. How can the battery pack charge the phone up to a higher ...
0
votes
0answers
14 views

Maximum Angular Momentum for an orbit in GR

[The reference for this question is the book Gravitation by Misner,Thorne, & Wheeler.] The trajectories of massive particles around a spherically symmetric body is governed by the effective ...
1
vote
2answers
22 views

Harmonic motion equation - non-null right hand side

Considering the following motion equation : \begin{equation} \ddot x + \frac{a^2 b^2}{c^2} x = -V \frac{a b}{c^2} \end{equation} where $a$, $b$, $c$ and $V$ are all constant. One can identify the ...
0
votes
0answers
11 views

What are the effects of an electromagnetic wave on a moving charge?

Just want to know the effects with relation to the resulting velocities and positions.
-4
votes
1answer
38 views

use of earth magnetic field

If we use earth magnetic field as a big magnet and take earth opposite charged electromagnet than it is possible that earth opposite charged electromagnet repel from earth surface.
3
votes
3answers
767 views

Why does weak equivalence principle say gravity is equivalent to acceleration?

I am told that the weak equivalent principle, that $m_i=m_g$ (inertial and gravitational masses are equivalent) is equivalent to the statement that in a small system you can't tell whether you are in ...
0
votes
0answers
11 views

Is Schmidt Decomposition well defined for periodic matrix product representation(MPS)?

We are able to perform Schmidt decomposition for open-boundary MPSs with dimension of boundary bond $m_0 = 1$, $|\psi\rangle=\sum w_{a_l}|a_l\rangle_L|a_l\rangle_R$. Because we can make $|a_l\rangle_{...
3
votes
0answers
24 views

Inflaton Decay Rate

After inflation, (p)reheating is supposed as the mechanism which is responsible for restoring hot Big Bang. Always the resulting decay rate of inflatons into bosons and fermions are mentioned without ...
2
votes
2answers
32 views

Sending someone into the future using the sun?

Ok so for starters, i am not nearly as smart as the least-smart person on here, but I have been reading a lot about all sorts of stuff. We now know that the ability to "send" someone to the future is ...
0
votes
1answer
1k views

Why does surface tension of water-ethanol binary mixture decrease with increasing concentration of ethanol?

I was thinking that it must be due to weaker hydrogen bonding in ethanol than in water. But then I learnt that Raman Spectroscopy and viscosity measurements suggest that upto a certain ethanol ...
-3
votes
0answers
22 views

Human Magnetic fields

What exactly causes a person to be extremely sensitive to static electricity? Is it caused from windy days or certain fabrics your body injesting various precious metals, and slowly discharged from ...
0
votes
1answer
19 views

Question on motion of vehicles

What slows us down while we are climbing in a sloped road, and what makes us faster while sliding down? Friction and what else?
-6
votes
0answers
49 views

If you reverse the velocity of all elementary particles, is time moving backwards? [on hold]

Imagine that you reverse the velocity of all elementary particles. Does this mean that time is running backwards, and that it´s the only way to physically realise the past?
0
votes
1answer
105 views

Gauss' Law and enclosed charge

In Halliday and Resnick, I am asked to find the charge on a drum (a cylinder) given the electric field very close to the drum. In the solutions manual, the author appears to enclose the drum with a ...
0
votes
0answers
8 views

How can I get the sensitivity of some eye cone at a specific wavelength?

The following graph shows the sensitivity for eye cones and rods, but I want to know e.g. what is the red cone sensitivity for a wavelength of 500 nm? How can I find an answer for questions like this?...
0
votes
1answer
33 views

Are there any numerical predictions of the Newtonian gravitational constant $G$ from modern theories?

There are no established relationships between the Newtonian gravitational constant $G$ and other fundamental constants, so there is no way to derive its value from other quantities. Its value must be ...
0
votes
0answers
14 views

Is my understanding of creation/annihilation operators' functional dependency correct?

I am trying to gain a little intuition about second quantisation, specifically about creation/annihilation operators. Lets say you quantise the free EM field (in 1d) and end up with the usual: $H=\...
1
vote
0answers
11 views

regarding the zero internal electric field in a conductor (electrostatic case)

Let's say we have a homogenous electric field in the z direction. For computational simplicity let's say we have a thin sheet of metal covering entire the x-y plane. The answer here is straight ...
2
votes
1answer
59 views

Is $A_\mu A^\mu=0$ a good gauge fixing?

I am taking a course in classical electrodynamics and I am facing this problem Prove that $A_\mu A^\mu=0$ is a good gauge fixing or not. If it is, there are residual gauge freedom? I know that, ...
11
votes
15answers
442 views

How to pour water from a bottle as fast as possible

When one pours water out of a bottle, it first flows smoothly but then a pressure 'blockage' develops and the pouring becomes interrupted and turbulent, so that the water comes out in splashes. This ...
1
vote
1answer
143 views

How does the Lennard Jones Potential changes for interaction between particles of different sizes?

I am interested in incorporating a Lennard-Jones potential in a simulation. When the interaction only involves the same type of particle, with same characteristics, we can use reduced units, scaling ...
1
vote
0answers
8 views

Plasma generated with gas and how far does it penetrate the skin

I know Far Infrared can penetrate the body’s tissues 1.5 inches (almost 4 cm) beneath the skin. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3699878/ But how far will a plasma field generated with ...
4
votes
3answers
44 views

Why is there a matter-dark matter asymmetry?

It is said generally that nature is symmetric. For example if light behaves as both a particle and a wave, then matter must also do so, which turns out to be true. But we find that the Universe ...
0
votes
0answers
7 views

Create spinning / modulating plasma effect without spinning the whole appartus

I'm trying to create a variable spinning / modulating plasma effect without turning the whole apparatus as seen in the video. https://www.youtube.com/watch?v=Wis_HEa3X84 Is it possible to just use ...
3
votes
1answer
23 views

Does the Bekenstein entropy bound present problems for unitarity in cosmological models that invovle a collapse or bounce?

If we expect the Bekenstein bound, or something like it, to hold in a collapsing universe, will that not eventually force us to accept some loss of information, or is there something I'm missing?
0
votes
1answer
26 views

Composite materials

I'm reading about brake pad material. The paper said ...according to original equipment manufacturer (OEM) the pads are made by combining five types of materials: binders, abrasive, performance, ...
1
vote
1answer
15 views

An approximation question from “Nuclear Models” by Judah Eisenberg and Walter Greiner

I'm looking at page 311 of the book "Nuclear Theory" by Judah Eisenberg and Walter Greiner. Now for identity (49) which is: $$\omega_{1,2}^2=\frac{\omega_\alpha^2+\omega_\zeta^2-2bc\omega_\alpha\...
6
votes
8answers
2k views

Prove that an electron in a hydrogen atom doesn't emit radiation [duplicate]

According to electrodynamics, accelerating charged particles emit electromagnetic radiation. I'm asking myself if the electron in an hydrogen atom emits such radiation. In How can one describe ...
1
vote
1answer
172 views

Spontaneity / Free Energy of Non-Isothermal Process

I'm trying to determine a lower bound for the work input necessary to make an entropy-reducing process "spontaneous" in the sense that the 2nd law is not violated. For a constant temperature and ...
0
votes
0answers
17 views

How to intuitively understand susceptibility formula?

When calculating various susceptibilities, we get below formula again and again. $$\chi \left( {q,0} \right) \sim \sum\limits_{\bf{k}} {\frac{{{f_{\bf{k}}} - {f_{{\bf{k}} + {\bf{q}}}}}}{{{\varepsilon ...
54
votes
3answers
7k views

Is it possible to “see” atoms?

As per my knowledge, atoms are small beyond our imaginations. But there is an image on Wikipedia that shows silicon atoms observed at the surface of silicon carbide crystals. The image: How can we ...
1
vote
2answers
329 views

Why do charge on an object remains unaffected by the motion of the object?

Unlike mass, the charge on an object is said to be unaffected by the motion of the object. This statement in my text book, is not yet understood by me. I don't know theory of relativity. On ...
0
votes
0answers
64 views

Intuition behind deriving the FRW metric [on hold]

I am studying the FRW metric and am looking at a motivation for it here. The motivation seems to use four spatial dimensions. Why do we need the fourth spatial dimension in this? This doesn't seem ...
-1
votes
1answer
492 views

Dirac Delta Potential and bound/scattered states

Why does the attractive Dirac Delta distribution (function) potential $V = \alpha\delta$(x) (for negative $\alpha$) yield both bound AND scattered states? Is this due to the definition of the Dirac ...
4
votes
2answers
59 views

Beyond Hamiltonian and Lagrangian mechanics

Lagrangian and Hamiltonian formulations are the bedrock of particle and field theories, produce the same equations of motion, and are related through a Legendre transform. Are there more such ...

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