# All Questions

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### The boundary between polycrystalline and crystalline

My current understanding of solid crystalline-like materials (please correct me if I'm wrong!) is that it is a continuum in terms of crystallinity, from amorphous (basically no periodicity) to single ...
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### Coordinate system

Quoting from 'Nuclear Physics - Theory and Experiment' by RR Roy, BP Nigam 2005 edition Link to text How did the author arrive at equations (23a, 23b,23c)? Chapter 8 Nuclear model II, 8.7 ...
167 views

### Does the Black's equation work for immortal wires / interconnections (or only for mortal ones)?

I'm trying to understand whether the Black's equation is only true for mortal wire / interconnections or it is also applicable to immortal wires (essentially when its $jL$ product is less than ...
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### Mass and Higgs Field and Inertia [duplicate]

I'm starting to wonder if the description of Mass given to us when we hear physicists talk about Mass created by the Higgs field is the same type of mass we ordinarily think of, like inertial mass for ...
11k views

### Why does the refractive index depend on wavelength? [duplicate]

Why do different wavelength get impeded more or less when in different materials? Moving with the same speed, but a longer physical distance would imply that the fields oscillate less times in the ...
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### Preons and 't Hooft condition

If some fundamental particles, like leptons or quarks are composite (e.g.in preonic models), or the same with gauge bosons or the higgs particles. How could it be possible that preons were more ...
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### What would be the result of the collision of two down quarks?

Even if we can't have single quarks in nature because of the charge colour, what would be the result of the collision of two down quarks at high velocities (0,99% c) at high energies, like the ones of ...
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### Is there a way to tell what frequency an unmarked antenna is designed for?

I have accumulated a large amount of R/C gear over the years. I have several antennas which are not labelled as to their original use. This antenna is either for 5.8ghz, 2.4ghz, or 910mhz. The ...
3k views

### Relativity and Current in Wire

If an observer is stationary relative to a current-carrying wire in which electrons are moving, why does the observer measure the density of moving electrons to be the same as the density of electrons ...
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### Material to read before taking College Physics 1? [duplicate]

I'm transferring to a 4-year university from a 2-year college. I earned a Computer Science A.S., but must now combine Computer Science with Advanced Physics for my B.S. I'm actually very excited for ...
266 views

### vacuum expectation value and Casimir effect

Is it correct to say that the VEV of the SM Higgs is 246 GeV? If so, is the VEV a reflection, or measure, of the Casimir Effect?
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### How do we calculate the Higgs potential? [closed]

What is the Higgs potential and how do we calculate it?
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### scaling of motor power

For car engines, the cylinder volume is often associated with the engine power, which suggests scaling of the power as $L^3$ where L is the linear size. Consider a system consisting of a motor and its ...
658 views

### The connection between classical and quantum spins

I have two questions, which are connected with each other. The first question. In a classical relativistic (SRT) case for one particle can be defined (in a reason of "antisymmetric" nature of ...
1k views

### What does a point in space mean? What we humans think a point in space is in scientific term? (a broader picture)

I am a first year and general student. I Am just curious about the vacuum or space. I just don't get what actually space is. Well, I know our scientists are still working on it but still I just want ...
152 views

### About the stability of the ground state of the bosonic string

In Polchinski's string theory vol 1, p. 23, it is said "It is a complicated question whether the bosonic string has any stable vacuum, and the answer is not known." The book was published on 1998. ...
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### Education : Students devising and solving their own problems [closed]

I apologize in advance for the nature of this question. I have been thinking lately of methods to increase the quality of my self-study. I have been meaning, but never actually got to, in addition ...
81 views

### Measurement of event time by different observers

Follow up question to Doppler redshift in special relativity A source of light pulses moves with speed v directly away from an observer at rest in an inertial frame. How will the time of the emission ...
13k views

### If aerographite is lighter than air, why doesn't it float?

Air is 6 times denser than aerographite but looking at pictures or videos presenting the material, I see it resting on tables rather than raising to the ceiling. Also, since the material is made of ...
1k views

### Educational physics stories with a funny punchline [closed]

As a physicist, when I'm giving lectures or during colloquiums, I usually find it necessary to have some appropriate and related jokes in hand. All of the best teachers have some, and they use them ...
2k views

### RC Parallel Circuits [closed]

I am trying to solve the following exercise: A capacitor 27 and $\mu$F is connected to a battery of 21 V through a resistance of 14 M$\Omega$. 1) What 's the charge on his parallel-plate? 2) ...
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### Topological insulators - Surface states have a phase?

When I look at the circle of the Dirac cone around the Dirac point of, let's say, $Bi_2Se_3$, then the electron winds around and it is true that it goes from momentum $-k$ and spin-up to $+k$ and ...
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### Are these data enough to solve this simulataneity problem

observers: S and S'. S stands in midway between two events (say, 2 flashes) located 20 metres apart. He sees two flashes occurring simultaneously. S' notes a 20ns difference in them. S' is moving ...
63 views

### How can this formula be analyzed intuitively?

Given $$\bar{U}_{vib}(T) = R \sum_{\text{n.m.}} \left( \frac{\theta_{vib, n.m.}}{2} \right) + \frac{\theta_{vib, n.m.} e^{-\theta_{vib, n.m.}/T}}{1-e^{-\theta_{vib, n.m.}/T}}$$ where ...
256 views

### How exactly are Calabi-Yau compactifications done?

To compactify 2 open dimensions to a torus, the method of identification written down for this example as $$(x,y) \sim (x+2\pi R,y)$$ $$(x,y) \sim (x, y+2\pi R)$$ can be applied. What are the ...
8k views

### Why do nearsighted people see better with their glasses *rotated*?

If you are nearsighted (like me), you may have noticed that if you tilt your glasses, you can see distant objects more clear than with normally-positioned glasses. If you already see completely clear, ...
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### Energy in nuclear reactions

I have two questions, been trying to get definite and intuitive answers to them for some time so hopefully you can help me: 1) I understand both the strong force and binding energy but what is the ...
174 views

### Dirt energy, possible?

Doesn't $E=mc^2$ mean that mass can be converted to energy? But from what I have studied in high school nuclear physics, it seems that the only "$E=mc^2$" we can get is from binding energy between ...
159 views

### Can anyons emerge from momentum-space other than spatial dimensions?

So far in condensed matter physics, I only know anyons(abelian or nonabelian) can emerge as quasiparticles in 2D real-space. But is there any possibility to construct anyons in momentum-space ? And ...
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### Resisting force depends on velocity? [duplicate]

Why does resisting force depend on velocity? I think there is no relation between resisting force and velocity of object. Please speak about it logically.
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### why is mass of air bubble in material medium considered to be negative?

The mass of air bubble in any medium is considered as negative. Is the air bubble is massless. I m in confusion. can we not neglect the mass of air bubble in material medium. But i have found in many ...
2k views

### Is it possible to create a vacuum ballon using concentric partially pressured spheres?

The old saying goes, nature abhors a vacuum, but that isn't exactly true. The universe is full of vacuum; what nature abhors large energy differentials. So for example, a vacuum chamber has to be ...
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### Where does $p^i/p^+$ come from in the EOM of an open string?

I have a stupid question about Eq. (1.3.22) in Polchinski's string theory volume 1. In the equation of motion for an open string, Eq. (1.3.22), X^i (\tau, \sigma) = x^i + \frac{ p^i}{p^+} \tau + ...
5k views

### What force does bathroom scale measure?

When you stand on a bathroom scale, I know that the force displayed is the normal force. Since it is the normal force, then technically the force displayed would actually be Fg= (mass)(acceleration) ...
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### Apart from Alfvenic turbulence, what is the most important unsolved problems in plasma physics? [closed]

Apart from Alfvénic turbulence, what is the most important unsolved problems in plasma physics? Is there a review article describing some important unsolved problems in plasma physics, I repeat, ...
17k views

### How can an object with zero acceleration move?

My physics text has a problem in which it is said that a person moves a block of wood in such a way so that the block moves at a constant velocity. The block, therefore, is in dynamic equilibrium and ...
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### Poincare recurrence and the multiverse

In this paper Susskind claims that a stable de Sitter universe is problematic (among other things) due to the existence of Poincare recurrence, which happen because of finite entropy. I disagree that ...
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### IIA and IIB Compact on 8D

How can compactifying IIA (non-Chiral) and IIB (Chiral) Superstring on $T^2$ (2-torus) gives rise to ($2$ dual descriptions of) the same $\mathcal N = 2$ supergravity in $8$ dimensions? I don't see ...
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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 ...
155 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, ...
111 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 ...
721 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 ...
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 ...