# All Questions

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### Connection between fermi velocity and mean (square) velocity of diffusion current

Is there any connection between $v_{F}$ and the $<v^{2}_{diff}>$, lets say for electrons in metals? I have come to a conclusion, that they are the same order of magnitude and their equivalence ...
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### Fermi Level: chemical potential vs. electrochemical potential

In solid-state physics it is understood that the Fermi-Level is the electrochemical potential. The Fermi-Level is defined to: However, in thermodynamics this formula is referred to as the ...
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### Laws first, matter and energy second? [on hold]

What came first, the laws that govern the universe or matter and energy? This question arose from a discussion I was having with my friend.Basically I was in the support that the laws of physics ...
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### Application of surface tension in thermodynamics [on hold]

I'm writing a paper about surface tension, and have problems with finding literature about application of surface tension in measuring some thermodynamics quantities. Can you help me? Thank you
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### Why toilet paper is softer in moist air?

After a hot shower,the air from the bathroom is full of water steam,right? How does the air affect the texture of the toilet paper?I mean,the toilet paper it seems to be softer,but why? Can someone ...
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### Does background noise while strumming a string affect the frequency recorded?

Does the interference of the waves cause instances of destructive interference where there is no amplitude. Technically the wave is still there although its amplitude is 'cancelled' out but those ...
20 views

### What produces this 477 keV spectral line?

Question about a specific line on a gamma spectrum, here. Below is a background gamma spectrum observed by a Ge[li] detector. I've been able to identify all the lines with mostly certainty, apart ...
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### Can we measure everything? [on hold]

Can anything which can't be measured exist in the physical world? Is there anything which is present in this physical world but can't be measured?
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### Measuring Steam Flow with Venturi Effect?

I understand steam is a compressible fluid and that the Bernoulli effect has been generalised for compressible fluids but I'm struggling understand how the Venturi effect can be generalised to allow ...
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### Entropy reversal in magnets

Entropy is an irreversible phenomenon without any energy supplied to reverse it. I was reading about paramagnetic substances and how dipoles align inside them on application of magnetic field. My ...
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### What are some books that introduce me to advanced physics concepts? [duplicate]

I am a student who's about to take the GCE 'A' Levels, and I am very interested in taking Physics as a career. I'd like to read some books on advanced Physics concepts that are specifically beyond my ...
2k views

### Why don't lakes have tides?

There's a tidal effect that we can clearly observe in oceans, which is the effect of gravity from the Sun and the Moon. If gravity affects everything equally, why don't lakes have tides?
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### Is it physics that leads to development of maths or is it maths that leads to the development of physics? [on hold]

Like calculus(which is maths) was discovered from physics. Are there theories in which maths leads to the discovery of new physics theories? So is it physics that lead to development of maths or its ...
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### Crushing pressure (15,750 psi) at the bottom of Marinas Trench [on hold]

If a human would be crushed if he were to scuba dive to the bottom of Marinas Trench then how do fish survive down there without being crushed?
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### How to calculate backreaction in AdS space?

This might be a very straight forward and basic question in GR. I am interested in calculating backreaction due to certain matter field (say, scalar) in AdS space. Should I put the energy-momentum ...
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### Does the Big Bang Theory explain about the formation and destruction of matter?

I read here here that, The Big Bang theory explains how the universe was created. Even though it was unable to explain that how everything in the universe did came to existence from no matter state ...
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### Transition from the second excited state to the ground state in 3d oscillator

The problem: 3d harmonic oscillator is in a second excited state. Suddenly a perturbation is applied which depends only on the length of the position vector $|\vec{r}|$. Can the oscillator fall into ...
36 views

### understanding twin paradox [on hold]

Is it really possible to observe such a phenomena in real life? I am confused. I have studied time dilation in relation to pion decay but am not sure if it will be true for us human being to observe ...
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### Do rotating magnetic fields violate all physical rules? [on hold]

Differential rotation winds toroidal magnetic fields, and is a key for understanding stellar dynamos. At the equator of our Sun the average angular rotational speed is 25.75 days (= 13.980582 ...
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### If black holes almost or barely collide

Consider black holes approaching each other at some relative velocity and varying impact parameters. For impact parameters above some value they will sail past each other. For impact parameters below ...
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### Does a flying bullet get KE from gravity?

A projectile P is shot horizontally at 9.8 m/s from a height of 4.9 m, considering no air friction, after a second it will have v = 9.8 m/s on the x-axis and v = -9.8 on the y-axis. Because of ...
23 views

### Minimum turning circle of an accelerating spacecraft

I think I've worked this out, but my maths/physics knowledge is far from perfect, so I'd love confirmation that it is correct. Basically, from $v =\omega r$ and $a_c = v^2 / r$ (where $\omega$ is ...
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### Magnetic moment of a ferrimagnet of some geometry

How can I find the magnetic moment of a ferrimagnet of given geometry, particularly in terms of its remanent magnetization? That is, say you have a cylindrical magnet of known radius and height, ...
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### Could black holes interact with dark matter at distances far greater than matter?

I watched this Nova episode on super massive black holes and learned that the speed at which the outer stars in a galaxy orbit correspond to the size of the black hole at the galaxy's center. They ...
29 views

### How much does a light heat the air around it?

My dad told me to turn off the lights when I'm not using them because it will keep the house cooler. I don't think he is correct that a light can heat up the room. How much of a difference in the ...
19 views

### Compression of non-gaseous substances

I learned about gas laws and their ability to compress. My science teacher told me that solids and liquids are incompressible. But when I learned about nuclear fission in bombs, it talks about ...
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### Why is Cv used in the adiabatic expansion of Carnot Cycle to calculate internal energy

When I took this class years ago, I simply accepted it as fact. However, now that I'm teaching it, the following bugs me....a lot. Why do they use Cv to describe the change of internal energy during ...
29 views

### Can non-color-neutral nucleons exist?

In a proton or neutron, one quark is red, another blue, and the last green, making it color neutral. Is it possible for a nucleon to consist of colors rgg, rbb, rrb, etc? If three quarks of such color ...
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### Why does the sun make me feel warm?

For a while I thought that the reason I felt warmth from the sun was because my skin was being hit by photons, but then I realized that photons also hit me when I take an X-ray, but I don't feel any ...
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### How does the spinglass algorithm work when used for community detection in networks?

I am reading the article "Statistical Mechanics of Community Detection" by Jörg Reichardt and Stefarn Bornholdt, which describes how to use the spinglass model from physics to determine communities in ...
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### Can an atoms energy levels be changed by changing the de Broglie wavelength of electrons?

So an electron can only orbit a nucleus where its wavelength makes a standing wave, leading to discrete energy levels in atoms. But the de Broglie wavelength of an electron = h/mv. So the wavelength ...
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### Is there any situation in which a primordial black hole-sized black hole form AFTER the “Big Bang?”

I have seen that primordial black holes may have formed at the start of the universe due to the density of matter. So, is there any way in which our current Universe could spawn a non-stellar mass ...
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### Before we realized that supermassive black holes were at the center of large galaxies, how did the models explain galaxies?

So, I was watching this show on Netflix about supermassive black holes. I didn't realize that astronomers were surprised to find these black holes at the center of galaxies, because I cannot imagine ...
37 views

### Commutation between Dirac hamiltonian and angular momentum

In reading about angular momentum and spin, I came across a derivation showing that the Dirac Hamiltonian does not commute with orbital angular momentum, and hence L is not conserved. What is it about ...
38 views

### Why is the plaquette operator in the string-net model a projection operator?

In the string-net model, the plaquette operator is defined as $B_P = \sum_{s}a_s B_{P}^{s}$, where $s$ runs over the string types $\{0,1,2,\dots,n\}$. It is claimed on page 19 of ...
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### Laplace transform of partition function a general result or a mathematical result?

In the following derivation I am trying to show that the function $Z_C(\beta)$ is obtained from the function $Z_M(E)$ by Laplace transform. Let, \frac{1}{Z_M}\frac{\partial ...
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### Why the Yang-Mills ansatz is able to describe every interaction we know?

All interactions which seem to rule the microscopic world we have access to are described by the standard model, which is a Yang-Mills theory for the SU(2) x SU(3) x U(1) group. Also, gravity can be ...
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### Do two waves of different frequencies create a resultant wave of lower frequency?

In my results for testing background noise, i found that while strumming a guitar in: a noisy area, the frequency picked up by the mic was 352 Hz while in a quiet area, the frequency picked up by ...
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### Covariant projection method - Meson bound states

I have seen many papers that discuss the production or decay of mesons ( quark bound states ) to make use of the covariant projection method where the product $\upsilon\bar{u}$ of the quark spinors ...
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### Why the shunt resistor would not protect the ammeter when the ammeter connected in parallel?

when you connect the ammeter in parallel it will burn or damage the ammeter. My question is, why the shunt resistor in the ammeter would not protect the ammeter when the ammeter connected in parallel. ...
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### Equations for Big Bang expansion, distance and energy

On the back page of the CPEP poster “The History and Fate of the Universe” a table gives distance scale (size), temperature, and energy (radiation) for seven expansion times (10-43 sec to now). ...
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### Rolling down an arbitrary hill with friction

I had in mind this physical situation of a point mass rolling down a hill of some arbitrary shape ($y=y(x)$, for instance a parabola about $x=0$), with a peculiar friction force whose "expended work" ...
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### what is the real physical sense of moving only in time? is that actual question for today? [on hold]

Best regards, I'm not physicist, but knoweledge that you possess as specialists, is very inspiring for me. My question: It's simple... But possible answer could be interesting. Einstein's theory ...
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### Can someone explain why this QM FTL communication setup is wrong?

So I thought I understood the double-slit experiment and EPR paradox until this setup occured to me. It combines EPR entanglement with "which-way" double slit setups. I know it must be wrong, but I ...
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### What axial forces does the vertical load induce? [on hold]

What axial forces does the vertical load P induce in the members of the system down in the figure? Neglect there weights of the members themselves and assume an ideal hinge at A and a perfectly ...
34 views

### Trivial representation in Clebsch-Gordan decomposition

My professor defined the Clebsch-Gordan series as the direct sum decomposition of the tensor product of two representations of the Lie group SU(2):  D_{j_1} \otimes D_{j_2} = D_{j_1+j_2} \oplus ...
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### Open systems (Conservation of Energy): is a temperature change possible if a tube is thermally isolated?

Full question: consider an perfectly thermally isolated cylinder. Water is flowing through this cylinder from the entry to the exit. Is it possible for the water flowing through the exit to have a ...
27 views

### What is ultimate AC frequency and phenomena related to it?

Just as in title. What is the top AC frequency physically possible to obtain? And are there (and if yes, what?) phenomena occuring only at large frequencies? I'm thinking about a metallic wire. I know ...