# All Questions

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### What is wavelength of electric field generated in a wire? [closed]

I read link given below: http://en.wikipedia.org/wiki/Speed_of_electricity Above link says _The speed at which energy or signals travel down a cable is actually the speed of the electromagnetic ...
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### How can I change the divergence angle of a single mode fiber

For my project I use the end of a single mode fiber as a "transmitter". I need to set the divergence to 20 micro radians. Is there an equation how to calculate the divergence and the necessary optics ...
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### “find eigenvalue for some fractional power of operator”

If $A$ is an operator and $Aφ=αφ$ ( $α$ is eigenvalue ) then $A^nφ=α^nφ$ and n is a positive integer. Question is that if $n=1/m$ ($m$ is a positive integer) then does it true that say ...
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### Why can't we see black holes?

Is it all because they are so black? But even our tech is not able to see black holes. We can only tell where black holes are, when they are eating stars. Or it's because they consume light ?
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### What is the effect of torque steering on a motorcycle with a long wheel base?

From experience, it appears motorcycles with a large wheelbase coupled with a long handlebar (say a modified Harley Ape Hangar) shows a noticeable tendency to veer to a side during acceleration vs a ...
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### Can quantum entanglement be simulated on a digital computer to any degree of precision?

First principles modelling of physical phenomena has been very successful in physics. The largest limitation is perhaps the fact that many QM problems are NP hard so we would need really powerful ...
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### Rigorous proof of Ampère's law from the Biot-Savart law

This question focusses on the tridimensional distribution of current, different from the case discussed in another post of mine, whose answer relates to a linear distribution where ...
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### 1kg mass impacting at half light speed - effects?

Such a mass would have kinetic energy approximating a 1 mega-tonne thermonuclear weapon. So, what would such an object do if it hit the Earth? We know how destructive such an energy release can be, ...
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### How does a magnetic field cause objects to move?

I understand electricity and magnetism and the relationship between them. What I never understood, and never actually seen explained in any book or website, is how opposite magnetic fields cause ...
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### What is a Geodetic?

What is a geodetic? First I thought it was a typo (hehehe..). To me, the term "geodetic" is used to describe a curve that belongs to a congruence of geodesics...but, I also think I'm wrong. ...
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### Conversion of energy to matter? [on hold]

Now, this answer has sort of been asked before. But I haven't really found the answer that I'm looking for. Exactly what is the mechanism by which energy can be converted into matter? By ...
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### Is the internal energy of an ideal gas always $\frac{3}{2}nRT$?

Is the internal energy of an ideal gas always $\frac{3}{2}nRT\,?$ I saw in Wikipedia that it is $\alpha nRT\;.$ Is $\alpha$ always equal to $\frac{3}{2}$ for an ideal gas?
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### rare High altitude balloon footage (highest peak before balloon burst 23034 metres) without fishe-eye lens

Why does Earth appear flat so HIGH UP in the sky? https://www.youtube.com/watch?v=jDs2T2PgxNM
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### Why can't we destroy energy?

From a wikipedia article: In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither ...
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### Mode groups in an optical fiber

I know what modes in an optical fiber mean but what are exactly mode groups in an optical fiber? From what I read until now, I have the impression that modes that have close propagation constants ...
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### What does electric field of unpolarized light look like when measured?

When we talk about fermions in mixed state, we say that their state can't be described by a wavefunction and just compute all the probabilities using density matrix. That's OK because the ...
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### Are there materials that are opaque to infra sonic waves?

I have read that infra sonic sound waves because of their large wavelengths pass through most materials largely unabsorbed. However, are there materials that absorb infrasonic waves(could be frequency ...
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### Is Planck’s proof of Kirchhoff’s Law of Thermal Emission false, and if not why not?

In his book ‘The Theory of Heat Radiation’, Max Planck adduced his theoretical proof of Kirchhoff’s Law of Thermal Emission. It was recently shown that Planck’s proof is false: “The Theory of Heat ...
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### Microstates, Distribution of Particles, and the Probability of an Empty Compartment

If I have a closed system composed of $N$ particles and $p$ compartments, the total number of microstates available to that system is $$p^N$$ Now say I want to find the probability that any one of ...
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### Does fluctuation really occur in equilibrium as its microstates are allowed to occur by Fundamental Postulate in equilibrium?

The Fundamental Postulate says: In equilibrium, all accessible microstates are equally likely. Accessible means having same energy.(right?) Let a container is taken full of gas having number of ...
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### Transformation of self-dual and anti-self-dual tensors and irreducibility of representations

I am working out exercise 2.5 of Maggiore's book. Part of the exercise is the following: Verify that self-dual and anti-self-dual tensors are irreducible representations of (real) dimension three of ...
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### Inverse Fourier Tranform using convolution

Yeah so inverse Fourier of f^(w)g^(w) is f(t)*f(t) Let inverse of f^(w) be H(t)e^(-t) Let inverse of g^(w) be H(t)e^(-2t) But then I don't know how to to integrate ...
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### How can a moving point charge “update” its fields without emitting an e-m wave?

If an observer brutally moves a point charge from position A to position B, the electric field produced by the point charge at a given point P changes. Furthermore, a delay is observed before this ...
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### Definition of Position Vectors

I have a few (possibly very stupid) questions relating to position vectors; more specifically my confusion about them. Following Halliday and Resnick's text, we define vectors by their magnitude and ...
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### Reference frequency

In my experiment, I have to obtain a relation between magnetic field of a solenoid and the applied Vp-p. The setup is as follows. http://www.teachspin.com/instruments/faraday/ Does my choice of ...
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### How to produce an illustration: “Error Ellipsoid” in 3D

I need an illustration of a translucent ellipsoid so that I can also see a vector form the origin to an offset center and a vector from that point to somewhere on the surface of the ellipsoid. ...
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### do Significant Figures apply with numbers without units?

say i need to divide a number with a unit to a number without a unit, e.g.: 16.4 meters / 2 Following the rules on significant figures, will the correct answer be 8.2m (since 2 has no unit), or 8 m ...
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### When travelling in the direction of light, what is my relative velocity?

If suppose I am traveling in the same direction as that of light, the relative velocity I am traveling with is speed of light + my speed in the direction of light. Is this correct? Does this have ...
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### 2 airplanes same size and shap different mass

2 identical balls of different mass dropped from same height reach the ground at the same time due to the acceleration of gravity being constant. If I understand correctly the ball with more mass ...
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### Is it possible to avoid static electricity?

Sometimes I touch a car or a door and I feel the static electricity in my hand and it hurts sometimes. I try not to touch anything because I feel like I am going to feel this pain again. Is there a ...
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### potential inside a cylindrical shell in terms of the surface potential?

Given a potential distribution $V(\phi)$ at the surface of an infinite cylindrical shell, is there an easy way to derive the potential inside the cylinder. No charges or currents anywhere.
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### Understanding the concept of temperature vs. mean energy/heat capacity of a system

I need help understanding a concept in thermodynamics. What is the relationship between temperature and mean energy? What is the relationship between temperature and heat capacity? What I know: ...
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### Is a DIY neutrino detector feasible?

How long does a neutrino detector filled with distilled water need to be to pick up neutrinos from one direction at about the level of a distant star within our galaxy? Would an off the shelf ...
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### Pressure inside a light bulb

It is known that nowadays most light bulbs are filled with gas in order to minimise the evaporation rate of tungsten. Is e.g. argon put into the bulb with reduced pressure, so that the glass doesn't ...
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### Charging with No Current

This may be a basic question, but I am not sure about the exact reason, so I'm asking this I've noticed quite a few times that when you plug an iPad charger to a socket and then switch it on, then ...
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### How is there GPE in objects that are on top of a hill?

What I don't understand about gravitational potential energy is that objects have the greatest GPE on top of a hill and it decreases as they go down the hill. At the top of the hill, they are on the ...
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### Application of Snells law

What i know about snell's law: It is applied when a ray of light meets the interface of some other medium and we can find the fourth quantity if we know any of the three quantities in the following ...
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### A modified Stern-Gerlach apparatus devised by Feynman for a thought experiment

I was revising the chapter Spin One in Feynman's lectures. There he considers a somewhat modified apparatus of the Stern-Gerlach type for convenience in explanation in the later parts of the lecture. ...
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### Light from Absolute 0

Would matter that is cooled closest to instant to a near 0 temperature emit light?
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### Difference: Fermi wave length vs. phase-breaking length?

I am reading a quantum transport book, where they often mention: phase breaking length and Fermi wavelength. I have looked up and found that: Phase breaking length= length over which electron remains ...
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### Why work function is not identical to first ionization energy?

From Wikipedia: The ionization energy (IE) is qualitatively defined as the amount of energy required to remove the most loosely bound electron of an isolated gaseous atom to form a cation. ...
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### how would an iron encapsulated electromagnet work?

I have been searching for this for a while now, but, a normal electromagnet works with(example) an iron cylinder and a copper coil around it, but would it work the same if you were to encapsulate it ...
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### Is this a good PhD Thesis Topic in General Relativity? [on hold]

For many reasons I am very uncertain whether I should agree to the following topic for my PhD thesis. Since I have a hard time to figure out if this is the right topic for me, I would love to hear ...
### Callan-Symanzik equation for the QCD scattering cross section of the $e^{-}e^{+}\to q\bar{q}$ process
In Peskin and Schroeder (Section 17.2) it is stated without derivation that the scattering cross section for the $e^{-}e^{+}\to q\bar{q}$ process obeys the following Callan-Symanzik equation:  ...