# All Questions

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?
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 ...
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 ...
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 ...
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 $A$ ...
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 ...
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 ...
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?
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 ...
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 ...
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 ...
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 ...
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 ...
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?
1answer
55 views

$W_b = \int$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 ... 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 ... 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. ... 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 ... 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 ... 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 ... 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 ... 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 ... 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) 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 ... 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 ... 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 ... 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 ... 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, ... 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 ... 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 ... 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 ... 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 ... 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? 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 ... 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. 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 ... 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 ... 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 ... 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 ... 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 ... 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: ... 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 ... 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: $$I(t)=I(t+\tau)\tag{\tau\approx ... 2answers 497 views ### Wormholes, time travel, and time dilation I believe I understand the "wormhole time machine" now, but still have one question regarding what happens if you enter the stationary mouth. A wormhole is created and (of course) has two mouths, x ... 1answer 265 views ### Can a ball stay still while laying on a inclined plane? Suppose there is a ball on an inclined plane. So, there are three forces acting on it: normal force from the plane, gravity (which can be decomposed in the component of gravity perpendicular to the ... 4answers 346 views ### Complex integration by shifting the contour In section 12.11 of Jackson's Classical Electrodynamics, he evaluates an integral involved in the Green function solution to the 4-potential wave equation. Here it is:$$\int_{-\infty}^\infty dk_0 ... 0answers 56 views ### Frequency dependence of refractive index$ n $considering electron oscillator using Electromagnetic Theory First and foremost, I am referring to Hecht, Zajac E. Optics 4th edition. I am reading the dependence of refractive index on frequency, and it goes well until Hecht jumped on his equation. First we ... 0answers 42 views ### Deriving Ampère's Circuital Law from Ampère's Force Law? Ampère's force$d^2\vec{F_{21}}$of current element$i_2d\vec{\ell_2}$on$i_1d\vec{\ell_1}\$ is$$d^2\vec{F_{21}}=-\frac{\mu_0}{4\pi}i_1i_2\frac{\hat{r}}{r^2}\left[2(d\vec{\ell_1}\cdot ... 0answers 55 views ### Does a conducting wire give off measurable radiation? In the Drude model (semiclassical, but should still apply here I think), the conducting electrons are in a constant electric field, and, in between collisions with the lattice ions (that happen, on ... 1answer 453 views ### What exactly does the Hamiltonian operator tell us? I'm confused about how energy and time are linked. On the one hand, the Hamiltonian seems to describe the time evolution of the system because in the time dependent Schrodinger equation,$$ \hat H ...

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