# All Questions

3 views

### Meaning of contant “c”

I have a puzzlement over Einstein's expression e = mc^2. The constant expressed as "c" is the measured speed at which Light crosses a vacuuum. BUT it is also used to denote the maximum possible ...
3 views

### Fermi surface reconstruction and fermi pockets

Certain quantum phase transitions are characterized by the emergence of some ordering wavevector $K$ : antiferromagnetism, charge or spin density waves, among others. In the case of Néel ...
10 views

### Is there a charge difference between ground and our head?

I commented on a question on Skeptics.SE that it would be an interesting question whether there is a charge difference between ground and our head. The question is not very relevant, but here it is ...
22 views

### Motion of an electron in a magnetic and electric field

I have this problem where i should find the direction and magnitude of the electric and the magnetic force on the electron. E = 1000 N/C B = 2,5 T v = 500 m/s ...
5 views

### PSD image detection

A laser is pointed on a surface, such that it is reflected onto a PSD (position sensing photodiode). Before light reaches the PSD, it goes through a plano-convex lens to get focused. Now, if the ...
23 views

### Intuitive explanation for lower pressure above airfoil

From Bernoulli's principle we know for an inkompressible fluid (constant density $\rho$) in a gravitational potential $\psi=gz$, that we can state the equation along a streamline from point 1 to point ...
24 views

### Is it almost impossible to separate two point charges stuck together?

F=kqQ/r^2, if r-->0 then F--> infinite.
33 views

### Question about a Attractors in Non-linear Systems

I've recently been reading up on non-linear dynamics and came across the concept of attractors. I'd like to ask if the concept of attractors can be used for pedestrian egress from a room? Since ...
16 views

### Can gravitational waves interfere polarize or show any other properties of stndard waves

Is it possible for gravitational waves to be able to produce phenomenon such as interference and polarization etc. which are observed in standard waves. Also is it possible for gravitational waves to ...
16 views

### The quantum state measurement in the eigenbasis of unknown hamiltonian

Let's say we have some physical system in the unknown hamiltonian $H$, which can be represented with the sum of Pauli matrices. For one two-level system it can be set as $H \propto \vec{n}\vec\sigma$, ...
41 views

### WHO WAS FIRST TIME OR MOTION? [on hold]

If motion was first and time present the motion. How can to any velocity break the flow of time? If time present motion no event can break to the flow of time. Only the event of when entropy of ...
8 views

### Why are glass beads used in a fractionating column?

Most sources do not go beyond saying that glass beads help increase the surface area for condensation of the liquid of the higher boiling point while using the method of Fractional Distillation to ...
21 views

### Interaction Hamiltonian coupling question

System is composed of two qubits and the bath is one bath qubit. The interaction Hamiltonian is: $$\sigma_1^x\otimes B_1 + \sigma_2^x\otimes B_2$$ where $B_i$ is a 2 by 2 matrix. I try to interpret ...
13 views

### How do inverse of Kirchoff matrix of a polymer possess the information for its mobility?

In Normal Mode Analysis of polymers like proteins, I have seen that mobility (measures like root mean squared fluctuations) can be found from the eigen values and eigen vectors of inverse of Kirchoff ...
25 views

### What does it mean physically to set $\exp(ikr)=1$ in the dipole approximation?

In the dipole approximation one sets the $exp(ikr)=1+ik.r+...=1$. ($|<f|exp(ikr)\epsilon.r|i>|^2$). Which makes $k.r=0$. What does it mean physically? Is the plane transverse wave converted to ...
45 views

### Quantum entanglement definition

How can we define Quantum entanglement (in QFT)? Can we regard it as organised information or just random data exchange? Why in either cases (Mathematically)?
26 views

### Operator formalism in QFT in Euclidean space-time

In QFT there are two very useful general approaches to study quantum fields (on the Minkowski space-time): path integrals and operator formalism. Sometimes they give the same results, sometimes one ...
16 views

### Can drag force and lift force be in the same direction in the following case?

In the attached picture, can I say that there is a lift force in the Y-direction, and a drag force too in the same Y-direction? $F_L$ proportional to V_fx$^2$ ? $F_D$ proportional to V_py$^2$ ? Is ...
39 views

### Speed of light interpretation [on hold]

I was wondering if any theory interprets the speed of light such that light needs to stay and visit some small open set for each tick of time? Then it could be renamed law of the good guest or ...
5 views

### Intensity fluctuations at the output of a single mode fiber coupled to a He Ne laser

I have coupled a Thorlabs HNL050L-EC - HeNe, 632.8 nm, 5 mW, Polarized Laser to a 2 meter long single mode fiber patch chord in order to select out the TEM00 Gaussian mode. While I am certainly able ...
13 views

### BASED ON NEWTON'S 3RD LAW [duplicate]

Q> my question is based on newton's 3rd law . An elephant pushes a rabbit with 250 N force , then the rabbit should also apply 250 N force but the rabbit is only capable to apply 25 N force . WHY ...
11 views

### How do exercise gyroscopes work?

I understand they are gyroscopes, and that they resist perpendicular rotation due to angular momentum. But why does rotating it certain ways cause it to accelerate? Also is it possible to use ocean ...
18 views

### If a battery is open on one end but connected on the other, does it raise the potential on the connected end?

If a battery has one end open and other end connected to a circuit will the battery increase the potential at the point of connection?
24 views

### Deriving Ideal Gas law from Hamiltonian Mechanics

I just don't understand the explanation in wikipedia. Is there a nice & elegant way of arriving at the Ideal Gas Law from Hamilton's Equations?
14 views

### How to draw a quantum circuit like this? [on hold]

I want to draw the quantum circuit in which the unitary operations $U$ are performed on three pairs of systems, $(|i\rangle,|j\rangle)$, $(|i\rangle,|k\rangle)$ and $(|i\rangle,|l\rangle)$. However, ...
3 views

### How could I calculate the Heat Index with low humidity?

The formula shown in the Heat Index(HI) requires that the relative humidity should be equal to or greater than 40%, so how to calculate the heat index when the humidity is less than 40%? Or is there ...
10 views

### What is Landau-deGenes Expansion?

I am an undergraduate student in an introduction to condensed matter physics course and I am struggling to understand the process of Landau expansion of order parameter S as it relates to liquid ...
13 views

### Theoritical Electrical Energy of battery

i m a student and looking for some answer related to my research, i m recently working on Ni-fe battery ie. nickel-iron battery. I would like to know what formula or law is applicable to calculate ...
12 views

### Green's function method for ring of charge

If we have a volumetric charge distribution $\rho$ we can find the electrostatic potential by solving Poison's equation $$\nabla^2\phi = -\dfrac{\rho}{\epsilon_0}.$$ If we use the method of Green's ...
320 views

### How can we know the state of a quantum system?

One of the postulates of QM states that given a system in a state $|\psi\rangle$ and given an observable $A$ whose eigenstates are $|\phi_i\rangle$, then the state of the system can be expressed as a ...
20 views

### Was the smaller, warmer Universe of the past conducive to life?

If the Universe began hot and dense, and is now generally cold and sparse, was there a period of the time in the distant past when the temperature of the "Universe" (or a random point in the Universe) ...
18 views

### How does an observer at the center of a black hole collapse explains the singularity?

Suppose an observer at the center of a cloud of mass. Imagine that the cloud collapses with the observer at the center and forming a crust with an almost empty core and forms a black hole. So, from ...
12 views

### Apply torque in 3d [duplicate]

Suppose I have an object with some inertia tensor I and angular momentum l. Suppose I apply a torque t. How does the object's orientation change over time?
16 views

### How does the rolling resistance of sticky tires scale with speed?

If I have a wheel that has a soft tire that sticks to the ground, will the rolling resistance (not the traction of the wheel, I mean how much force it takes to keep it rolling at the same speed) scale ...
47 views

### Can there exist an observer able to observe a collapse of a star into a black hole?

We know that an observer at infinity cannot see a star forming into a black hole as the matter will take progressively longer and longer time to compress (from this observer's point of view). Is ...
35 views

### Breakdown of Snell

I have reason to believe that if we are using an accurately formed thin prism, then very near the working apex, where there are few atomic layers in a latticed prism, there will be inaccuracies in the ...
33 views

### Numerical problem related to Newton's Law of Gravitation [on hold]

Two iron spheres whose volumes are $38810 cm^3$ and $310464 cm^3$ respectively. If they are placed touching each other, calculate the force of gravitation between them. (Density of iron=$7860 kg/m^3$ ...
25 views

### Lab experiment: hysteresis disappears, reappears and then disappears again. How?

In my college laboratory we were assigned an experiment to measure the hysteresis curves of a certain ferromagnetic material at different temperatures. Sounds simple, right? Well, the block of Monel ...
27 views

### Speed of the Coulomb Field Measured To Be “Infinite”

The paper "Measuring Propagation Speed of Coulomb Fields" by R. de Sangro et al points out that: ...the Lienard-Weichert retarded potential leads to the same formula as the one obtained assuming ...
12 views

### What is the difference between Bosonic and Fermionic symmetry protected topological phases (SPT)

I am reading the paper Braiding statistics approach to Symmetry Protected Topological Phases'' by Levin and Gu. In this paper two spin models considered describe spin-1/2 particles in (1+2) ...
16 views

### Does the electrons in Stern-Gerlach aplliance change their energies and how?

When an electron goes thru a Stern-Gerlach appliance it changes its spin projection. Then there must be a change of energy content of the electron due to this event. But it is also known that magnetic ...
I cannot understand how capacitors work. Electric field is supposedly uniform between the parallel plates of a capacitor. But how? Since $E=\frac{kq}{d^2}$, shouldn't the electric field approach ...