# All Questions

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### Scattering, Perturbation and asymptotic states in LSZ reduction formula

I was following Schwarz's book on quantum field theory. There he defines the asymptotic momentum eigenstates $|i\rangle\equiv |k_1 k_2\rangle$ and $|f\rangle\equiv |k_3 k_4\rangle$ in the S-matrix ...
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### Rabi problem for a spin-1 particle

I'm trying to solve, analitically, the transition probability of a spin-1 particle in a magnetic field $$\vec{B}=B_0\hat{k}+b(\cos{\omega t}\hat{i}+sin{\omega t}\hat{j}).$$ In particular I want to ...
856 views

### Commutators involving functions

I am looking for the commutator: $$[e^{aq},p]$$ My approach is to Taylor expand the function: $$[\sum_n \frac{1}{n!}(aq)^n,p]$$ I know that $[q^n,p]=ni\hbar q^{n-1}$ So how do I account for $n$ ...
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### The relation between spectral function and band structure

I am confused by the wavevector in spectral function A(k,w). How to understand this k for a periodic structure? And how is it related to the k (in first Brillouin Zone) we use in the band structure? ...
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### A derivation in Schwinger's proper time approach

I have a question in derivation of Schwinger's proper time method in chapter 2.1 of http://link.springer.com/book/10.1007%2F3-540-45585-X from Eq.(2.20)-Eq.(2.23) to the classical action expression ...
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### Noether's theorem functionals [duplicate]

I'm working through a derivation of Noether's theorem, and at this line $\delta$$L = \frac{\partial L}{\partial \phi}$$\delta \phi$ + $\Pi^\mu \delta (\partial _ \mu \phi)$ The next step is to use ...
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### Determining the phase delay between H and E fields

I want to determine the phase delay between H and E fields in a medium with losses and not electrically charged. In this medium we also have $\sigma_c\approx \varepsilon \omega$. The enunciation of ...
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### Difference between circular motion and rotational motion

Are rotational motion and circular motion different or the same? If different then when can we say that a body is in circular motion, and when it's in rotational motion? I find several answers where ...
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### Lowering and Raising Kronecker Delta

When an index of the Kronecker-delta tensor $\delta_a^b$ is lowered or raised with the metric tensor $g_{ab}$, i.e. $g_{ab}\delta^b_c$ or $g^{ab}\delta_b^c$, is the result another Kronecker-delta ...
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### Bet about the Precession of Book [duplicate]

Say you spin a book and throw it to someone. If you spin it around it's spin while throwing it perpendicular to the spine, it doesn't precess. You spin it around the length (longer dimension) and it ...
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### How to measure the initial velocity of a potato gun?

I have made a 2m potato gun (It's like a cannon - the noise makes your ears buzz for some second :) ) Its uses alcohol as fuel. an igniter fires. then the potato will be shot out. anyway. as it's ...
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### What is a simple calculation to figure out how many watts needed to maintain a hot piece of tungsten?

The specific heat of tungsten is $.13 (\text{kJ /(kg K)})$. 1 cubic cm of tungsten is .0193 kg And the melting point of stainless steel is $1900 \deg \text{K}$ conservatively (giving it plenty of ...
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### How do they measure the rotational speed of cricket balls?

The 2013 Ashes series (a cricket thing between England and Australia) are underway as of today and it seems they have a new (to me) gimmick. It appears that they are now able to measure almost ...
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### Evolution of a system - Thermodynamic potential

There is something I don't understand about thermodynamic potentials. We know that the necessary condition for a system to evolve is an increase in entropy. For a system kept at constant volume and ...
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### Magnetic and Electric fields

Unified field theory says the Magnetic field and Electric field are proven to be two manifestations of the same thing just viewed from different perspectives. That is the magnetic field is essentially ...
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### Confusion between TWO shear stress formulas

There are two formulas that I have encountered so far for shear stress in my Engineering class. Let S be shear stress, F be force, and A be cross sectional area: S = F/A Note: This formula is ...
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### What happens when you pass a current through a coil made of a ferromagnetic substance?

I know when you pass a current through a length of copper coil or a solenoid, there is the induction of a magnetic flux. But what of the coil made of the ferromagnetic material, the permanent type ...
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### Green light in the satellite image

What could be the possible reason for the green border visible in this satellite image ? (Image source)
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### Shaving down length of a GRIN lens to a desired wavelength

I have a GRIN lens that collimates light from a point source at 670nm. I want to shave the thickness of the lens down to use for 530 nm. I can't find anything useful for equations online. None seem to ...
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### Why wet is dark?

When something gets wet, it usually appears darker. This can be observed with soil, sand, cloth, paper, concrete, bricks ... What is the reason for this? How does water soaking into the material ...
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### Do solids have translational energy?

Along with having vibrational energy, do both crystalline and amorphous solids also have translational energy? I ask because I've always understood solids to have just vibrational motion/energy. But ...
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### infinitesimal and infinity [on hold]

In differentiation and integration we use infinitesimal to evaluate derivative and integral, talking about infinitesimal first this is the no. that we actually can't say how small it is still we are ...
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### Magnetic Energy Density Equations

I have a plasma with a magnetic field through it and I want to know the magnetic energy within the plasma. I have the simple equation $U=\frac{B^2}{\mu}$ for the energy density. But my question is ...
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### We can move with speed of light [on hold]

We all know de-broglie equation $λ = \frac h {mv}$ but let us take it a little further. Let $λ$ be the wavelength associated to an object with mass $m$, $v$ its velocity because of its energy $E_G$ ...
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### Help trying to make circuit! [on hold]

I tried to make circuit. I have battery of 9V and led light with Vf of 3.30. How can i find the cureent of circuit. Basically two wires are going from battery to led bulb making it light but can i ...
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### Luminance from multiple light sources

I have a LED diode: Given Luminance intensity [I] of $1.85$ cd. I want to have Luminance [B] from the light source of $75 \ {\rm cd/m^2}$. From $B=I/S$, where S is light source plane area, I get ...
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### What do water stains make things appear dark? [duplicate]

If a piece of grey tarmac is damp after the rain, it appears darker. If I spill water on my carpet, the damp patch is darker than the rest of the carpet. Generally speaking, a lot of things appear ...
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### If a body slides down a frictionless inclined plane what will be the net normal force?

If a body(m) slides down a frictionless inclined plane (M), will the net normal force between the ground and the inclined plane be (M+m)g ? I feel it should be less than (M+m)g. This is because one ...
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### Why bremsstrahlung occurs only with the nuclei? Why not with the electrons?

In many books I read that bremsstrahlung effect (for e+) only occur when the electron goes near the atomic nuclei. Why is not possible when cross near an atomic electron? Thanks,
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### Relationship between amplitude and frequency of a wave

What is the relationship between amplitude and frequency of a wave? Some say there isn't any relationship, some say that there is, but from their answers the relationship is still unclear.
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### Nobel Prize challenge: Could neutrinos be used for quantum information processing?

We know that two scientists, Takaaki Kajita and Arthur B. McDonald, working on Neutrinos have just been awarded the 2015 Nobel Prize in Physics. My question as a Nobel Prize Challenge Question is: ...
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### Liquid nitrogen and liquid helium

In cryogenics, liquid nitrogen and liquid helium are often used as coolants. Other than their low boiling point of 77K and 4.2K respectively, what properties make them suitable as coolants? Are there ...
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### Is work done by a pseudo force?

If a body is viewed from the frame of another body which is itself accelerating, will work be done by the pseudo force acting on the first body in the frame of second body ( provided the first body ...
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### Hamiltonians, density of state, BECs

When working with Bose-Einstein condensates trapped in potentials, how can one tell what the density of state of a system of identical bosons given the Hamiltonian, $H$? (I have been told that it is ...
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### Attracting atmospheric positive ions to carbon nano tubes for collection

Can nano magnets built into carbon nano tubes be used to attract moving positive ions from atmosphere to increase the resultant positively charged collecting mass?
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### Can meters measure a distance in higher dimensions?

Consider meters as a unit of measurement. Meters can, of course, be used to express distance across a two-dimensional plane. E.g. The distance from your chair to the door is 2 meters. Meters can also ...
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### Timelike Boundary

I was reading in a paper (see 1st paragraph of introduction section in http://arxiv.org/pdf/1510.00709.pdf) that in AdS space, waves can reach the boundary in finite time and, since said boundary is ...
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### Proving the following cycle is equivalent to Carnot cycle

I want to investigate whether the following cycle I'm about to describe is equivalent to Carnot cycle or not. By equivalent I mean, giving two reservoirs($A$,$B$) with temperature $T_1$ and $T_2$ ...
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### Does magnetic induction produce potential across zero resistance wire?

If magnetic field is increased at a constant rate inside a zero resistance wire loop, then, according to Faraday's law , $$\varepsilon = \frac{dΦ}{dt}$$ Thus, a potential difference is produced. ...
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### Big Bang…or…Everywhere Stretch?

Recently I watched a minute-physics video that suggested that a better name for the beginning of time would be "Everywhere stretch" because there wasn't a space-time singularity that formed where the ...
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### All contour integrals in the world are zero…? [migrated]

Using Stoke's theorem, line integral of a vector gives surface integral of curl of the vector, and after that if we apply gauss' divergence theorem in that, it gives vol integral of div of curl of ...
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### Entangled or Separable-State measurement

This image is from a Nature paper. A question about the caption: how is Victor's data used for sorting Alice and Bob's measurements into subsets? I would really appreciate it if you could walk me ...
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### equilibrium state of a system in statistical mechanics

Why we consider the maximum number of micro states or complexions as equilibrium state of a macro state or a system in statistical physics?
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### Can Ultrasound and or Microwaves actually lock mass in place trapping it in standing wave nodes

Ultrasound Standing waves trap small particles and manipulate their velocities and direction and cause them to defy gravity. Source : https://www.youtube.com/watch?v=odJxJRAxdFU Does current physics ...
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### Question on Electromagnetic Induction

If a magnet is moving through a solenoid, why is there no electromotive force being induced while the magnet is at the centre of the solenoid? (Is it because emf is induced in both directions?) Can a ...
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### Capacitors in parrallel [on hold]

There are two $20 \mu F$ capacitors in parallel that are connected in series to one $50 \mu F$ capacitor. The p.d across the parallel pair is $30 V$, what is the charge on each capacitor? The EMF of ...