# All Questions

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### What is the minimum density required for nuclear fission?

This is a followup to a prior question I asked about gaseous nuclear fission. What is the lowest nuclear fuel density required to initiate fission? This is critical for a gas core reactor with a ...
125 views

### One terminal of an ideal voltage source connected to earth->Massive current will flow?

Assume we have a "somewhat" ideal voltage source like a DC power supply powered by mains. Take just one terminal and form a conductive path between it and earth ground. Assuming that no conductive ...
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### Can I run a pulse tube cooler in reverse to generate pressure oscillations?

Since a pulse tube cooler is basically a Stirling engine without moving parts running "in reverse", does that imply that when I keep the cold end at ambient temperature and apply heat to the hot end, ...
81 views

### Why Is Current $\propto$ Number of Loops?

When an additional loop is added to a generator the magnetic flux through the loops does not change yet the current increases. This seems counter-intuitive and has been described as ...
334 views

### Are bubbles in ice at a lower or higher pressure than atmospheric pressure?

When water phase-transitions into ice it expands. The water usually contains dissolved air. Freezing forces the air out of the solution into bubbles. Are these bubbles at a lower or higher pressure ...
464 views

### How energy curves spacetime?

We know through General Relativity (GR) that matter curves spacetime (ST) like a "ball curves a trampoline" but then how energy curves spacetime? Is it just like matter curvature of ST?
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### Restriction of a Lagrangian

I'm wondering if anyone could help me with the following questions. Let $M$ be the Minkowski spacetime, given $f\in C^{\infty}(M) ; f(m)=x^{0}(m)$, with $\{x^{\mu}\}$ being a global Cartesian ...
516 views

### SMBC ball bouncing problem

This comes from a Saturday Morning Breakfast Cereal (SMBC) comic with a joke answer. The problem states: A 5 kilogram ball is shot directly right at 20 meters per second from a height of 10 ...
111 views

### Dimensional analysis of magnetic energy: dimensions of µ0 and H

When calculating the energy difference between the normal and the superconducting state in a superconductor at zero magnetic field, the result is as follows: Now I'm quite confident of this result, ...
790 views

### String Theory and Standard Model in CERN

I don't know how to say it, but in the TV dominatrices and the popular science books we see the string theory as "the best theory to explain everything", and as "the only game in town"... etc. And ...
447 views

### How does autorefractor work?

Autorefractors are being used by eye opticians for eye diagnosis. I searched internet for articles and wiki page as well but I wasn't satisfied. I am interested to know how do they actually work. How ...
81 views

### Is the conjectured noncommutative heavy scalar “brother” of the already detected Higgs boson is a pseudo scalar?

This is a technical (may be trivial?) question about this sigma scalar field advertised by Chamseddine and Connes to improve the electroweak vacuum stability involved by the weak mass of the already ...
176 views

### How do ice spikes form?

I recently saw this picture posted on Twitter which shows a so-called ice spike rising from an ice cube tray. I have read the Wikipedia page, but it doesn't mean much to me. My instinct was that it ...
67 views

### How to determine spatial color distribution? [closed]

Is there an easy way (i.e. free or open source tool) to determine the spatial color distribution (i.e "intensity") in a picture or in a region of interest of this picture? Let's say I used a laser or ...
89 views

### Can we extract all the physics from the line element only? (i.e.) without any knowledge of coordinate system?

We can't use Euclidean coordinates (Cartesian, spherical or polar) to label the points in curved spacetime except for the local points in local inertial frame. So for a general solution of Einstein's ...
75 views

### Photoelectric effect - shining light on a silver ball

Lets say we have a silver ball hanging on an isolator string. The work function $A_0$ of a silver and radius $r$ of the ball are known. Now we shine light of known $\lambda$ on it from all the ...
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This nice image of Dirac cones (from this article), in a ($E,\vec k$ graph) will be an introduction for several questions, in the realm of topological insulators. 1) Does the Dirac cone appears ...
343 views

### Physical Variables of Circular Motion

To me, the definitions of variables involved in circular motion are rather confusing (perhaps due to the lack of understanding on my part), hence the question. As I understand it, there are two ...
88 views

### Would a super-hot object launch itself into the air?

In this XKCD What If, an indestructible hairdryer outputs 2 GW of power, turning the earth under it into gas: Periodic explosions of gas beneath the box launch it into the air, and it starts fires ...
1k views

### How much force needed to push a car on neutral? [closed]

The car is a 2002 Toyota Corolla SE I have in my garage on a stone surface, I.e., the floor itself. What I want to know is how to determine the force needed to get it in motion at all. The curb ...
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### Book suggestion [duplicate]

I know that book suggestions are very repetitive and boring but i try to be specific : I took a Physics 1/2 course and I am currently prepairing my exam. I know that the best way to learn Physics is ...
280 views

### Photoelectric effect - calculating a current in the photocell in which only $5\%$ of photons manage to cause the effect

I have been trying to solve a problem about photocell and have obtained the wrong result. Can anyone tell me where did I go wrong? So it is easier I will write down all my calculations. We have a ...
58 views

### Multiquark states and molecular QCD

Due to some recent "discoveries" (yet to be completely confirmed), we have hinted the existence of tetraquark particles. Why QCD exotics like multiquark states beyond n=2,3 (mesons, ...
193 views

### Is my textbook teaching an incorrect concept of Work?

I fear my textbook is teaching an incorrect concept of Work. I am very frustrated right now since I was struggling to understand the concept in the way that was explained in the textbook and my ...
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### What is the difference between hierarchy picture and composite Fermion in explaining FQHE

Are they equivalent? I came up this question because they both explain states beyond Loughlin state.
126 views

### How is graphene a 2D substance?

How is graphene a 2D substance? It has length, width and some thickness to it, else it would be invisible. Why is it considered a 2D substance?
543 views

### Can one create mass from energy?

Due to $E =m c^2$, one can convert mass to energy. A classic example would be matter/anti-matter annihilation to produce energy (photons, etc.). Can one do the reverse? So could one do something to ...
1k views

### How to find electric scalar potential of infinite wire with Poisson/Laplace equation?

I though it will be easier then calculating the electric field and then integrating, but I am stuck. lets say we have an infinite wire, charged $\lambda$ per unit of length and its located at the ...
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### What is electron density and its unit in plasma physics?

I can not find (through Google search engine) that what means electron density and what is its unit? and what is there relation between electron density and electrical conductivity?
2k views

### Does a constantly accelerating charged particle emit em radiation or not?

The Abraham-Lorentz force gives the recoil force, $\mathbf{F_{rad}}$, back on a charged particle $q$ when it emits electromagnetic radiation. It is given by: \mathbf{F_{rad}} = \frac{q^2}{6\pi ...
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### Derivation of freely falling frame in Schwarzschild spacetime

Thinking about the equivalence principle, is there a nice, simple way to show that a local, freely falling frame in Schwarzschild spacetime is described by the Minkowski metric ...
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I am trying to calculate lattice parameter of ZnO (wurtzite). I have found out the formula for calculating lattice parameter of this system from a research article. $\frac{1}{^{d^{2}}}=\frac{H ^{2}+ ... 4answers 368 views ### Vectors with more than 3 components I have some confusion over Vectors, Its components and dimensions. Does the number of vector components mean that a vector is in that many dimensions? For e.g.$A$vector with 4 components has 4 ... 3answers 200 views ### How “things” radiate electromagnetic radiation? [closed] How things radiate electromagnetic radiation? I don't ask why they radiate (higher temperature than 0K) but how they radiate this electromagnetic waves? 1answer 428 views ### Bound states and scattering length What is the relationship between bound states and scattering length? What is the relationship between scattering states and scattering length? When we say, potential is 'like' repulsive for ... 1answer 142 views ### Is a tankless water heater more or less efficient than an electric stove at heating water? Consider the following scenario that I actually encounter frequently: I just finished washing a pot in the sink, and now I want to boil some water for cooking noodles. If I fill the pot with hot ... 1answer 148 views ### String theory and the SM spectrum [closed] Long ago, I realized this: (super)string theory can NOT give a well-defined/unique prediction of why the electron (muon, tau) or the neutrino (any flavor) masses have the masses we measure. String ... 0answers 106 views ### Independency of the frame of reference of the strain rate tensor I've got a problem regarding tensors. Premise: we are considering a fluid particle with a velocity$\mathbf{u}$and a position vector$\mathbf{x}$;$S_{ij}$is the strain rate tensor, defined in this ... 3answers 3k views ### Where does the light go when I turn off the light switch? As above... Surely it must go somewhere? My child asked me this question and it was a difficult one to answer. 3answers 199 views ### How much iron ware to make a Faraday cage In a thunderstorm I was thinking the following: suppose I am rowing in a lake during a thunderstorm. How big a Faraday cage do I need to make to protect myself? If lightning strikes the cage, will it ... 3answers 873 views ### Is it really proper to say Ward identity is a consequence of gauge invariance? Many (if not all) of the materials I've read claim Ward identity is a consequence of gauge invariance of the theory, while actually their derivations only make use of current conservation ... 0answers 24 views ### What is space charge polarizibility? There are not available article in net about space charge polarizibility? Anyone have idea about it that with the derivation of space charge polarizibility? 0answers 67 views ### What is a$U(2)$monopole? What is a$U(2)$magnetic monopole? Anyone has any reference on this? How is this related to Yang-Mills gauge field? 3answers 1k views ### What provides the centrifugal force for planets orbiting a star? this is a question I had when I was a kid. I'm a bit ashamed because I think I am missing out on something very obvious since I have the same question despite almost being an engineer now! From ... 3answers 267 views ### Velocity is “Distance/Time” in a particular direction OR simply “Displacement/Time”? Is velocity: Distance/Time in a particular direction, or simply Displacement/Time? Or both 1 and 2 is the same thing? 1answer 78 views ### How is this not a violation of CPT symmetry? Imagine an electron and a positron, initially held stationary some distance apart at time$t=0$. There is an attractive force between them, so they will approach one another. I am told that all the ... 2answers 743 views ### Double Slit experiment with just one photon or electron I just recently watched a video where they were explaining the double slit experiment. I understand that the interference pattern would be observed if you send photons or electrons one after the ... 1answer 511 views ### Experimental signature of topological superconductor I was wondering if someone can provides some clear experimental signatures of a topological superconductors ? I was thinking about that, because for topological insulator, one of the hallmarks is ... 1answer 97 views ### Rotational Hamiltonian for collective model The effect of rotation can be considered by modelling the nucleus as a rotating body, characterised by some moment of inertia. Then how The Hamiltonian describing rotation is? ... 0answers 802 views ### Difference between Eulerian and Lagrangian formulation of Fluid Dynamics Difference between Eulerian and Lagrangian formulation of Fluid Dynamics. I am completely new to fluid mechanics. Until now definition$F = ma\$ was sufficient for me to solve any rigid body problems ...

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