# All Questions

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### Show: Lorentz-invariance of solution of Klein-Gordon equation [closed]

Assume $\psi$ is a solution of the Klein-Gordon equation (KGE). Let $\Lambda$ be a Lorentz transformation. Show: $\phi = \psi(\Lambda^{-1} \cdot )$ is also a solution of the KGE. I try to ...
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### Diagram of forces exerted on objects

Assuming that there is an interaction between 1 and 3 (they attract each other), what are the forces between 1 and 2? I know it is as if the force acts on a different body (1+2), but I want to know ...
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### 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?
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### 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 ...
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### 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 (...
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### 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 ...
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### 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 $A$ ...
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### 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 ...
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### 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 ...
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### 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?
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### Could galactic rotation be similar to an irrotational vortex?

Could black holes' near light speed rotation cause galaxies to move like an irrotational vortices?
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$W_b = \int$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 1-... 1answer 120 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 203 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 83 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 ... 5answers 7k 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 153 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 96 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 60.6 ... 1answer 3k 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 866 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 133 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 101 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 299 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 88 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 949 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 'solid'... 2answers 155 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 305 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 4k 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 458 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?