# All Questions

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### Pulling a bar through a magnetic field

This is a question on magnetic flux and induced voltage. I believe that this problem's answer key is incorrect, and I want to determine whether my opinion is sound. I also want to explore one ...
10 views

### The force required to escape an orbit, why is it based off of P.E.?

I'd like to understand the following: If you have a satellite in an orbit that's radius is 3r (r = Earth radius), why is it that to escape this orbit (and Earths gravitational pull) the force ...
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### Does ratio of magnetic length to geometric length of a bar magnet always remain the same?

The magnetic length of a bar magnet is nearly 0.84 times that of the geometric length, which looks quite valid. What would happen if I cut the same bar magnet along the transverse axis? Would the new ...
103 views

### Integral over a product of two Green's functions

Need some help here on a frequently encountered integral in Green's function formalism. Forgive me since I am a junior student. I have an integral/summation as a product of a retarded and advanced ...
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### The meaning of keeping the bare parameters fixed

So, this question concerns two different kinds of renormalization group equations. I would like some clarifications, if possible. The usual RG equations taught in QFT courses, like the ...
11 views

### How to determine the properties of a hypothetical particle

I'm trying to study QFT but have absolutely no clue whatsoever of how to start. I have a theory of quintessence, but I am trying to determine the properties of the spin-0 particle, which I have called ...
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### Partial diagonalization of the Fock matrix

I'm currently writing my dissertation on the application of SCF semi-empirical methods to large systems, in particular proteins, and I'm stuck with a problem: I don't understand why, given the fact ...
249 views

### If we connect a block of p-type semiconductor only to a positive terminal, will it become positively charged?

Connect the positive terminal of a battery to a piece of p-doped semiconductor, say, silicon doped with boron. Will the terminal pull electrons out of the doped silicon, or equivalently, inject holes ...
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### interpretation of fermi dirac statisitcs

i was reading that as temperature increases the energy at which $n(E)=0.5$ shifts to lower energies as these lower energy states become depopulated. Could someone explain that, what it means and why ...
108 views
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### What concepts could I teach to children in a game about quantum-mechanics?

I think that games can also be used to teach things to young people or childs, different than for example a book and could be more "direct" than teaching them all the math background, I would like to ...
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### What formula gives the solunar periods of the Solunar Theory? [on hold]

Challenge Find a general formula that: Given latitude,longitude and date finds (at most) two major solunar periods for that day. Given A table of daily solunar periods for a given month at a given ...
84 views

### Stellar Outflows in Milky Way

There appears to be a lot of evidence that gas flows outward in Galaxies. I've been trying to parse through the available data and am unable to get a clear answer to this question: is there any ...
35 views

### Compute $Z_2$ Invariant of 2D Topological Insulators without Computing the Eigenstates

For 2D Time-Reversal Invariant systems ($T H(\vec{k}) T^{-1} = H(-\vec{k})$), there is a formula by Fu-Kane-Mele in order to determine whether the system belongs to either one of distinct topological ...
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### 2D square lattice, nearest neighbor and next-nearest connected by springs

For my field theory class I am trying to build the Lagrangian for the following system. Consider a 2D square lattice where the nearest and next-nearest neighbor interactions are modeled by springs ...
18 views

### Proving a relation with Four-velocity tensor

I'm trying to show that: $U^a_{\space\space;b}U^bU_a = 0$ (Where U is four-velocity) and I'm stuck on how to go about it. I tried expanding it out into the Christoffel symbols, but that didn't seem ...
60 views

### If you were able to get rid of Hydrogen from a weather balloon as it were rising and expanding would the weather balloon rise further than usual?

I know that the weather balloon will eventually be stopped because of the atmosphere no longer being buoyant but would the balloon be able to go farther than it regularly would?
32 views

### Why are the charge operator $Q$ and the baryon number operator $B$ unbounded?

A friend recommended me to read PCT, Spin and Statistics, and All That written by R. F. Streater and A. S. Wightman. In page 5 to 6, here's what the authors of this book have to say: [...] In ...
51 views

### WKB formula and Langer correction [duplicate]

The general WKB approximation formula states that $$\int_a^b\sqrt{E_k-V(x)} = (k+1)\pi \text{ with } x \in [a,b]$$ for a regular Schrödinger equation (without the $\hbar$ and such). However, in the ...
74 views

### Average surface temperature of Earth [duplicate]

I had a question in my school exam. Will the average surface temperature of the Earth be lower or higher, if there was no atmosphere? Now, the answer expected is "The avg temp will be lower, because ...
26 views

### Allowed system energies from quantized Hamiltonian spectra

To find the allowed energies for a system, I can find the spectrum of the Hamiltonian $\hat{H}_{\psi}$ given a wavefunction $\psi$ representing the state of the system. 3 cases might happen: either ...
24 views

### How do experimental physicists know the decay path of transuranium elements ahead of time?

I have been watching (and enjoying) Dr. Poliakoff's YouTube videos on the synthesis of transuranium elements like Roentgenium and Copernicium, which decay so quickly that they are identified (if I ...
35 views

### Is 3 dimensions in space inferred or tested? [duplicate]

Could we just as easily say space has color because the object we are comparing to space has the some property, what ever the property be? And why would it stick to the 90 degree rule? Why not, say, ...
45 views

### Energy of system in eigenstate of Hamiltonian

I know how to find the spectrum of the Hamiltonian to get the allowed energies for a system. If the spectrum is quantized, I can get definite values for each energy level. But when the system is in ...
230 views

### Hydrogen atom: potential well and orbit radii

I happened to open up an old solid-state electronics book by Sah, and in it he says: "it is evident that the electron orbit radius is half the well radius at the energy level En" The orbit radius is ...
32 views

### How do i correctly go from a two variable function to a function of difference?

I would like to know how I can go from a two argument function $g(x_1,x_2)$ formally correct to a function of the difference of the parameters $g(x_1-x_2)=g(x)$ this seems to involve integration over ...
305 views

### How did Newton discover the universal law of gravitation?

I am having trouble comprehending how anyone could come up with this formula: $F = \frac{GMm}{d^2}$ Could someone walk me through this?
372 views

### Does the need for renormalization in QFT vanish once you use a more fundamental theory (e.g., string theory)?

It is often explained that renormalization arises in QFT because QFT is a low-energy effective theory that needs to be replaced by a more fundamental theory at higher energies/smaller distances. While ...
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### Does the speed of light vary depending on what it travels through?

The title is pretty much self explanatory. Does the speed of light vary depending on what it is traveling through? The speed of light through vacuum is 299792458 meters per second but does it change ...
15 views

### How to calculate reflection profile of a beam shining onto a moving mirror?

I want to build a projector, where a beam of light is shining on to mirrors, placed at different angles, which are spinning fast enough to draw lines. The question is, how to calculate the needed ...
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### Finding the current and voltage in a circuit with DC sources

I know that in a circuit with DC sources a capacitor(steady state) can be replaced with an open circuit and an inductor(steady state) can be replaced with a short circuit. My understanding is that ...
32 views

### Relation between Scattering matrix and an effective Hamiltonian

Could somebody provide the proof (or reference to some accessible literature) of relation $$S(E) = 1 + 2πiW^{†} (H_M − E − iπW W^{†} )−1 W \tag{2}$$ of arXiv:0806.4889, which relates $S$-matrix to an ...
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### What is the nearest neighbor distance in Titanium; Scandium; Aluminum?

What is the nearest neighbor distance in Titanium ? Is there database ?
330 views

### Torque on a rotational cylinder in viscous fluid

I've been stuck on what I'm pretty sure is a simple part of a larger question. It's a cylinder (radius a) spinning in a viscous fluid. It's rotating at rate $\Omega$ .During this question we get that ...
38 views

### Using differentials to optimize a function

I've read in a paper by Tevian Dray an alternative way to solve optimization problems manipulating "differentials". Here is an example of how it works (next I quote the paper). Consider the ...
29 views

### How can I find a numerical solution for Einstein's field equation?

If I am given accurate measurements for the mass of an object how would I go about finding numerical solutions for Einstein's field equation? Would I start by solving Newton's equation for a large ...
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### Why does the non-linearity of the string action prohibit stretching due to strong excitations?

From 't Hooft's String Theory lecture notes (paraphrased): To understand hadronic particles as excited states of strings, we have to study the dynamical properties of these strings, and then ...
41 views

### Mass, energy, and entropy.

I have a seemingly simple question about the relation between these three that for some reason doesn't make sense to me. If entropy is the disorder of a system, then a low entropy state is one of ...
142 views

### How can a set of components fail to make up a vector?

Many books in Physics insist to define vectors are objects with components with the property that the components transform in a proper way under a change of coordinates. Now, in mathematics, on the ...
225 views

### Why dimensionality of the Electric Charge varies with the spacetime dimensions?

The point is: We can find via dimensional analysis that the electric charge dimensionality varies with the dimension of space-time. $$[\text{charge}] = eV^{(3-D)/2}$$(You can see below the way I did ...
874 views

### Why does a faraday cage protect you from high currents?

In an electrostatic case it is clear that that in a space enclosed with a conductor (without charge in it) the electric field is zero. This is often demonstrated in physics shows like on the ...
21 views

### Meaning the symbol, $W$ and $dW$

What's the difference between $W$ and $dW$? They are both work done and have similar formulae (same dimension). But I don't know the difference between them. $dW$ here ISN'T power.
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### Finding force acting on an object using work energy theorem

An object of mass 5kg passes through the origin at $t=0$. $x$-component of velocity is 4 and $y$-component is -3. At a later time $t=3$ the object is located at $x=9$ and $y=5$. What is the force ...
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### Is there a difference between Weight and Force of Gravity?

Is there a difference between Weight and Force of Gravity or they just mean the same thing? My textbook does not clarifies this point. Thank you in advance for your help!
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### How to calculate the magnitude of the induced emf in self-inductance and mutual inductance cases?

I'm trying to understand transformers and am reading this site: Mutual inductance and basic operation I am a little confused about how to reason about the voltages involved in a transformer. Assume ...
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### Thermodynamics : Work done on gas inside container by the force due to pressure exerted by external atmosphere

Suppose a container is fitted with a massless and frictionless piston lying on a table such that pressure due to gas is greater than pressure due to external atmosphere. Hence piston will move. Now ...
29 views

### Can a(n electro)magnetic field be amplified to overcome the residual strong force between protons-neutrons in a nucleus?

My knowledge of physics is cursory at best, I never studied it in high school but I enjoy reading about it as a layperson, especially the posts here on PSE. I don't understand any of the complex math ...
63 views

### How does a longer wavelength penetrate deeper with Rayleigh waves?

I'm struggling slightly to understand this idea. I've slowly been building up an explanation, so at this stage it might be just some confirmation I'm looking for, but also some guidance if I'm off ...
222 views

### How to tell if a star is in a galaxy?

An astronomer is studying a star that appears to be in a galaxy. How does the astronomer know the star is actually in the galaxy and not just on the same line of sight as the galaxy? I'm guessing ...