1
vote
0answers
117 views

How does a battery work?

How does the positive terminal of the battery attract electrons? Does the electric field emitted by the positive terminal attract the electrons?
1
vote
1answer
61 views

Electron electric field

As we know the fundamental unit of charge in our universe at the time of electrodynamics was an electron, and in any frame of reference, its radius is a finite number and assuming uniform charge ...
5
votes
3answers
109 views

What happens inside a body when it rotates?

I'm studying rigid body dynamics lately. I came across the definition of torque, and though I've found a lot of explanations as to why there is an r there (the moment), all of them are mathematical ...
42
votes
7answers
7k views

What technology can result from such expensive experiment as undertaken in CERN?

I wonder what technology can be obtained from such very expensive experiments/institutes as e.g. undertaken in CERN? I understand that e.g. the discovery of the Higgs Boson confirms our understanding ...
0
votes
0answers
53 views

Definition of force in a scalar field theory

How do we define the force for a general scalar field theory? In particular what is the scalar force of the below equation of motion: where $\tilde{T}$ is the energy stress tensor of matter and $A$ ...
1
vote
0answers
51 views

Cooling 8 L of frying oil using ice [closed]

I have 8L of frying oil (canola) - if I put it in a metal canister after frying and put that canister in ice water, what is the minimum amount of ice water that I need to bring the oil to a ...
2
votes
0answers
76 views

How to prove the energy of gravity in general relativity is non-local?

Every textbook in general relativity containing the energy of gravity all says that the energy of gravity is non-local and every energy-momemtum density received is pseudo-tensor, but "having not ...
1
vote
1answer
88 views

Higgs Bosons and photons

The higgs boson is an elementary particle so how it can decays into another elementary particle; the photon? And why the photon doesn't interact with the Higgs field?
2
votes
2answers
903 views

Relationship between surface density and volume density

Often in an E&M problem, I'm having to "chop" an extended object into an infinite sum of smaller extended objects which I know more about to find a potential or electric field or whatever. The ...
11
votes
2answers
374 views

Why are non-Abelian gauge theories Lorentz invariant quantum mechanically?

I seem to be missing something regarding why Yang-Mills theories are Lorentz invariant quantum mechanically. Start by considering QED. If we just study the physics of a massless $U(1)$ gauge field ...
0
votes
1answer
57 views

Friedmann equations question

Friedmann equations for critical density is: $$\rho_c = \frac{3H^2}{8\pi G}$$ Is there any other way to write this equation? For example: $$\rho_c = \frac{3}{8\pi GH^2}$$ I saw the above form on ...
1
vote
0answers
44 views

quantum hall effect and hamiltonian [closed]

I need a good book or lecture on quantum hall effect. my supervisor wants me to find the Hamiltonian for the Qunatum hall effect, he want me to find this from this relation if it looks like a 2nd ...
1
vote
1answer
117 views

MIcrocanonical and Canonical - The thermodynamic limit

Considering a two level system with energies $ 0 $ and $ \epsilon$, we write out the single particle partition function with ease to be, also N-particle partition function for non-interacting ...
2
votes
2answers
79 views

Could galactic rotation be similar to an irrotational vortex?

Could black holes' near light speed rotation cause galaxies to move like an irrotational vortices?
1
vote
1answer
55 views

How to prove this equation $W_b= {P_2V_2 - P_1V_1 \over 1-n}$

$W_b = \int$Pdv $PV^{n}$=C $P=CV^{-n}$ W=$\int cv^{-n} $dv =c$\int_{v1}^{v2}V^{-n}$ dv =c[${v^{-n+1} \over 1-n}]_{v1}^{v2}$ How to get rid of the (-n+1)? to prove $W_b= {P_2V_2 - P_1V_1 \over ...
2
votes
1answer
95 views

References on experimental realization of quantum one-dimensional infinite-well model

Can someone point me to the examples, or references, of experimental realization of the quantum one-dimensional infinite-well model? [EDIT] The motivation of this question is the curiosity - though ...
2
votes
1answer
174 views

Is there an alternative to dark matter?

I've been reading about dark matter. I understood that its existence is inferred by the discrepancy between the gravity exerted by a galaxy (total mass) and the movement of the stars that compose it. ...
0
votes
0answers
54 views

Is there a machine readable format of the data tables in Particle Data Group PDFs?

Is there a machine-readable format of the tables of data in PDG documents such as this one? http://pdg.lbl.gov/2011/download/rpp-2010-booklet.pdf Something such as JSON, XML, CSV, HTML, or anything ...
6
votes
4answers
1k views

Pulling apart two interleaved phone books

The TV show Mythbusters had an episode in which they interleaved two phone books and dramatized how hard it was to pull them apart. (A long time ago, a phone book was a book that had an index of phone ...
0
votes
1answer
73 views

How to calculate current flowing down a wire connecting charge to ground

If charge Q is connected to ground through a wire, how would you calculate the current that flows as a function of time? No other information is given. I have not seen this type of problem in the ...
0
votes
0answers
57 views

Effect of an asymmetric weight distribution on a hack squat machine

In a hack squat machine (see figure above), does it matter if I put more washers on one side? May this asymmetry cause an asymmetry of my effort in performing this exercise? For example, if I put ...
6
votes
1answer
1k views

What is a Physically Accurate Explanation for the Kutta Condition?

Countless arguments between highly intelligent people have been waged (on this very site in fact) as to exactly how lift can be explained in an experimentally and mathematically rigorous way. Taking ...
5
votes
5answers
329 views

Forces while squeezing a toothpaste tube

When I squeeze a tube of toothpaste, I am working with 2 squeeze forces toward tube. What causes the toothpaste to go out of the tube? (instead of remaining stationary)
6
votes
1answer
89 views

Why does this condition ensure that the residue of the propagator is 1?

The corrected propagator is given by $$\Delta'(q)=\frac{1}{q^2+m^2-\Pi^*(q^2)-i\epsilon}$$ ($\Pi^*$ is the sum of all irreducible one-particle amplitudes) I get that the residue of the original ...
2
votes
1answer
96 views

Dirac Notation Question Appearing In a Projection

So I have a part of the energy eigenvalue equation that look like this: $$ \delta(\hat{x})|n\rangle $$ Where n is the energy basis of the Hamiltonian I'm considering. To deal with this, I tried ...
0
votes
0answers
32 views

Invisible stars due to finite photons [duplicate]

When we study black body radiation, we often make calculations assuming a continuum of radiation with some amount of flux. In reality, there is a very very large number of photons being emit per unit ...
1
vote
1answer
139 views

Buoyancy / Drag Problem

Buoyancy / Drag Problem Just a little bit of help would be nice. I have a spherical particle of radius $R$ and density $\rho$, surrounded in a fluid of density $\phi$ and viscosity $\eta$. I'm ...
0
votes
1answer
62 views

Getting maximum power/brightness from 5 lightbulbs with different resistances [closed]

Hi I am trying to figure out the best way to arrange 5 lightbulbs to get all of them to have the maximum brightness with a 30V battery. The 5 five lightbulbs have different resistances: 3ohms, 6ohms, ...
4
votes
2answers
368 views

At what speed would you die upon impact with water?

If we hit the water at great speeds, we die. This is because the water has no time to "move out of the way" and acts as a "solid" surface. At what falling speed can we consider the water as a ...
4
votes
1answer
117 views

When is quantum optics “correct”?

What is the regime under which we may consider quantum optics description of light a good approximation of a more correct theory such as QED? By quantum optics I mean describing the electromagnetic ...
2
votes
3answers
217 views

Detecting negative energy products in particle accelerators

Are the detectors in a typical particle accelerator experiment, either in Fermilab, or now in LHC, sensitive to negative energy particles? How would a negative energy particle, (say, a negative ...
1
vote
1answer
1k views

Calculate coefficient of friction from deceleration with and without conservation of energy

How can I solve the following problem? A block that moves at the speed of 120 cm / s keep going for 70 cm before stopping. What's the coefficient of friction between block and table calculated ...
0
votes
2answers
87 views

How to deal with a $0\Omega$ resistor connected in parallel? [closed]

There is a side without resistance, what is the equivalent resistance? Can anyone help me solve this?
-1
votes
1answer
125 views

How fast must Nadia travel so that she is the same biological age as her twin upon returning to Earth? [closed]

Two twins, Nadia and Aidan, decide to have an adventure when they turn 21. Aidan chooses to travel to a distant star 10 light years away at a speed of 0.8c. Nadia decides to travel to a closer ...
4
votes
2answers
571 views

A basic math identity often used in integrals [closed]

I'm just wondering about why $y_i=A_{ij}x_j$ implies $$d^Ny=|\det A|d^Nx.$$ I see that $\det A$ is the product of the eigenvalues of a diagonal matrix but still don't exactly see how. Please help.
2
votes
0answers
73 views

What is the definition of topological when talking about topological phases of matter?

What is the definition of topological when talking about topological phases of matter? Why do people think that the fractional quantum hall effect is topological? I think it means that the ground ...
4
votes
0answers
154 views

Level quantization of 7d $SO(N)$ Chern-Simons action

In 3d, one can write down the $SO(N)$ Chern-Simons action to be $$S(A)=\frac{k}{192\pi}\int_{M}\text{Tr}(A d A +\frac{2}{3}A^3),$$ where $A$ is an $SO(N)$ connection. The level quantization can be ...
1
vote
0answers
47 views

How do we predict electrical properties of a material using scattering data and vice-versa?

I know that the band gap is related to conductivity. What I'm wondering is what it is like for an experimentalist who is trying to figure out what an unknown material, a black box, is doing. The only ...
1
vote
0answers
78 views

Nature favours symmetry?

In my chemistry class today, our professor was giving a lecture on symmetry of organic molecules. He said that " Nature favours symmetry as symmetry reduces the energy of the system". But as far as ...
2
votes
0answers
24 views

Can isotropy (or anisotropy) be expressed in terms of intervals ($s^2$) between pairs of events?

Considering a set $\mathcal S$ of events and given the values of intervals $s^2[~P, Q~] \in \mathbb R$ for all pairs of events $P, Q \in \mathcal S$ (up to a common non-zero scale factor): how can ...
2
votes
1answer
74 views

Why does the raising and lowering operator not affect total angular momentum?

My notes define: $$ L_{\pm} = L_{x} \pm i L_{y} $$ and states: $$ [L_{z},L_{\pm}] = \pm \hbar L_{\pm} $$ I'm fine with this as it's easy to show the result with some ugly algebra. It then says: ...
2
votes
1answer
66 views

Typical form of the beta function of the renormalization group

Why in "typical" cases (according to some non-English text I read), does the $\beta$-function have the form $$ \beta (g) = ag^{2} + bg^{3} + O(g^{4})\ ? $$ I.e., why are there no linear or logarithmic ...
0
votes
0answers
84 views

Does the Shockley Diode Equation hold in a dynamic circuit?

Consider the following simple photodiode circuit: I've created a real-life version of this circuit, and I've detected that $I$ is oscillating: $$I(t)=I(t+\tau)\tag{$\tau\approx ...
5
votes
2answers
497 views

Wormholes, time travel, and time dilation

I believe I understand the "wormhole time machine" now, but still have one question regarding what happens if you enter the stationary mouth. A wormhole is created and (of course) has two mouths, $x$ ...
0
votes
1answer
265 views

Can a ball stay still while laying on a inclined plane?

Suppose there is a ball on an inclined plane. So, there are three forces acting on it: normal force from the plane, gravity (which can be decomposed in the component of gravity perpendicular to the ...
6
votes
4answers
346 views

Complex integration by shifting the contour

In section 12.11 of Jackson's Classical Electrodynamics, he evaluates an integral involved in the Green function solution to the 4-potential wave equation. Here it is: $$\int_{-\infty}^\infty dk_0 ...
0
votes
0answers
56 views

Frequency dependence of refractive index $ n $ considering electron oscillator using Electromagnetic Theory

First and foremost, I am referring to Hecht, Zajac E. Optics 4th edition. I am reading the dependence of refractive index on frequency, and it goes well until Hecht jumped on his equation. First we ...
1
vote
0answers
42 views

Deriving Ampère's Circuital Law from Ampère's Force Law?

Ampère's force $d^2\vec{F_{21}}$ of current element $i_2d\vec{\ell_2}$ on $i_1d\vec{\ell_1}$ is$$d^2\vec{F_{21}}=-\frac{\mu_0}{4\pi}i_1i_2\frac{\hat{r}}{r^2}\left[2(d\vec{\ell_1}\cdot ...
2
votes
0answers
55 views

Does a conducting wire give off measurable radiation?

In the Drude model (semiclassical, but should still apply here I think), the conducting electrons are in a constant electric field, and, in between collisions with the lattice ions (that happen, on ...
11
votes
1answer
453 views

What exactly does the Hamiltonian operator tell us?

I'm confused about how energy and time are linked. On the one hand, the Hamiltonian seems to describe the time evolution of the system because in the time dependent Schrodinger equation, $$ \hat H ...

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