# All Questions

2answers
66 views

### comparison of 1D and 3D wave functions

When discussing the Schroedinger equation in spherical coordinates, it is standard practice in QM handbooks to point out that the radial part of the 3-dimensional wave equation bears a strong analogy ...
0answers
6 views

### How would you prove $E = -\vec{\nabla} V$ from the electric potential's line integral?

Basically, we were given an equation: $$V_1 - V_2 = \int_{r_1}^{r_2}\vec{E}\cdot d\vec{r}$$ where $\vec{E}$ is the electric field distribution and $d\vec{r}$ is the displacement vector of the charge. ...
3answers
50 views

### Are communications between computers faster by electrical signals via copper cables or electromagnetic signals?

Assume that there are two computers which are connected with a copper cable, e.g. Ethernet. Also, there is a radio connection between them, e.g. AM radio, in order to exchange data. When we try to ...
2answers
3k views

### Is imagining 10 dimension video by Rob Bryanton has any invalid/wrong information?

I just watched this youtube video by Rob Bryanton and am wondering that is there anything invalid/wrong in term of real physics. I search for Rob Bryanton and it look like he has not-so-good ...
1answer
70 views

### Anderson localization in 1d, 2d and 3d

Why in 1d and 2d systems, all states will be localized for infinitesimal disorder, but in 3d only states with energy lower below mobility edge will be localized?
2answers
36 views

### How to calculate velocities after collision?

I'm currently writing a program for a particle simulator. One of the requirements is that the particles collide in a realistic way. However, I don't know how to calculate the final velocities. For ...
3answers
104 views

### Why are the laws of thermodynamics “supreme among the laws of Nature”?

Eddington wrote The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in ...
1answer
53 views

### Period of Double Slit Experiment

What is the period of the pattern from the double slit experiment? It varies along the pattern right? Namely I'm confused because when considering two point sources (See: Period of Interference ...
1answer
40 views

### Counting of brownian particles: Point Process

Imagine a point process defined by the passage time of purely brownian particles through a given point (in 1D), line (2D) or plane (3D). I'm interested in the variance of the counts (number of ...
1answer
39 views

### Derivation of Lagrangian density for an infinite classical dielectric in interaction with the EM field

I am tasked with reading and reproducing all the steps in J.J. Hopfield's 1958 paper "Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals". Embarrassingly I am stuck ...
2answers
188 views

### The viscous force between the layers of liquid is same, then why there is variation in the velocities of its layers?

I have learned in my textbook that when the liquid flows the bottom layer of the liquid never moves because of friction, but the upper layers move with increasing velocities how it is possible if the ...
2answers
37 views

### Velocity vector for constant acceleration in newtonian mechanics

If the acceleration vector of an object remains constant, is there no change in the direction of the velocity vector? The question applies to any movements in classical physics.
1answer
62 views

### What aspect of quantum mechanics forces probabilities to be (conventionally, at least) central?

I understand how to compute probability distributions and expected values and such from quantum states, but a lot of treatments of QM make it look like this is what the wavefunction is essentially ...
1answer
77 views

### Equations of motion for a pendulum in 3D?

I am trying to solve for the equations of motion to simulate a pendulum. I decided to use the spherical coordinates. The Lagrange equation is: where L = length of the rope ϕ= angle of the ...
2answers
34 views

### Who first verified Coulomb's Law for atoms?

Who first verified Coulomb's Law for atoms? Was it Rutherford's experiment?
1answer
96 views

### Power as a Function of Distance

I am interested in trying to understand the meaning of the integral of power with respect to distance. In the case of force, we have the two formulae: $\int F \, \operatorname{d}\!t = \Delta p$ and ...
0answers
25 views

### The origin of contact noise?

I was trying to measure the noise of a device with metal probes. I was not sure whether I should trust the results because I was told contact noise might contribute to some degree. I am a little ...
3answers
230 views

We all know that gravitation force between two small (not heavenly) bodies is negligible. We give a reason that their mass is VERY small. But according to inverse square law, as $r\to 0$, then $F\to ... 2answers 45 views ### Absorption of radiation due to temperature I was wondering if the temperature of an object affects the amount of radiation it absorbs. For example, if I have a box that is hotter, will it absorb more energy as compared to the same cooler box? ... 2answers 228 views ### Mathematical proof of non-negative change of entropy$\Delta S\geq0$I understand that we can prove that for any process that occurs in an isolated and closed system it must hold that $$\Delta S\geq0$$ via Clausius' theorem. My question is, how can I prove this in a ... 1answer 34 views ### Didn't Newton's Corpuscular theory fail to explain reflection? In 1637 Descartes gave the corpuscular model of light and derived Snell's law. This Corpuscular model of light was further developed by Isaac Newton in his famous book entitled OPTICKS and because of ... 1answer 59 views ### Newton's second law for rotation Can the second law of motion for rotation,$\vec{\tau}=I \vec{\alpha}$, be used for any axis? Is there any case that acceleration$\vec{\alpha}$is not in the direction of applied torque ... 1answer 85 views ### **Group structure** in Chern-Simons theory? A non-Abelian Chern-Simons(C-S) has the action $$S=\int L dt=\int \frac{k}{4\pi}Tr[\big( A \wedge d A + (2/3) A \wedge A \wedge A \big)]$$ We know that the common cases,$A=A^a T^a$is the ... 1answer 113 views ### How does current flow in a irregularly shaped heterogeneous resistor? The motivation for my question is understanding how electricity gets through your skin as opposed to running along it, and how the presence of things like water on the skin affect the relative ... 1answer 713 views ### on causality and The Big Bang Theory With the notion of causality, firmly fixed by GR, we derived the concept of a singular point from where space-time begun. Causality alone gives us the possibility to talk about a known past (i.e. ... 1answer 46 views ### Why is current a scalar quantity? Current has both magnitude and direction. As per the definition of vector defined in encyclopedia, current should be a vector quantity. But, we know that current is a scalar quantity. What is the ... 5answers 2k views ### How can I prevent my son building up static on his trampoline? Whenever my three year old son plays on his trampoline, it doesn't take very long for him to start building up a significant amount of static electricity. His hair stands on end (which is quite ... 1answer 108 views ### Age of the universe and age of stars The age of the universe is 13.798±0.037 billion years, yet the age of HD 140283 is 14.46±0.8 billion years, how this can be the case? 1answer 34 views ### Is there another way besides$V = k\int dq/r$to find potential? Suppose that the charge distribution$q(r)$in the radial$r$-direction in a thin hollow disc of inner radius$a$and outer radius$b$is given by the power-law equation: $$q(r) = Q_0 \left[1 ... 1answer 157 views ### Atwood machine problem Sorry for the bad drawing, but I hope that this will help you get a hold of the problem. Consider an Atwood Machine with a total of two blocks, a mass less pulley, ideal string. One block rests on ... 2answers 70 views ### PhD or master in physics [on hold] I am 35 years old and I've just got a bachelor in Science. I will pursue higher education in Physics but I have my doubts if either a PhD or a master is more convenient for me. My areas of interest ... 0answers 12 views ### What do we mean exactly by the positive/negative charges in cathode and anode? In batteries, what exactly do we mean by negative and positive charges? My understanding is that the negative charge of the anode is basically an atom with an extra electron in the last orbital and a ... 2answers 83 views ### Could there have been two “Big Bangs”? A couple of years ago, I remember seeing a documentary on the big bang theory. The theory presented was that to explain the cosmic microwave background radiation, there needed to have been two big ... 0answers 24 views ### current density in 1-d I have a slight problem with the notion of the current density in one dimension. For example the probability current in 1-d given by: J(x) = -\frac{1}{m} Im(-i\psi^*\partial_x \psi) calculation the ... 2answers 37 views ### Canonical equal time commutation relations in QED I understand that to quantize the classical electromagnetic field one needs to impose commutation relations and express the field in terms of creation and annihilation operators. I notice that the ... 0answers 26 views ### explain how does Rectenna work to someone with some college level physics I understand that rectifying antenna (rectenna) is supposed to convert electromagnetic energy to electric current however I do not understand how it's really working. I do get that it's kind of like ... 2answers 77 views ### Why objects of different masses accelerate at the same speed under gravity [on hold] If you push massive object it accelerates slowly compared to object which is of same size but less density/mass. However, all objects do Accelerate at the same rate. ... 2answers 161 views ### Pn junction voltage drop? This image from wikipedia, explains that there occurs a potential drop across a pn semiconductor junction, and an electric field confined to the depletion region. I already know the reason for the ... 0answers 42 views ### Feynman rules for real scalar field interacting with electromagnetic field I was wondering if anyone could help guide me in finding the Feynman rules for a real pseudoscalar field (\phi) interacting with the electromagnetic field (F^{\mu\nu}). The (effective) ... 1answer 38 views ### Can excitons be understood in terms of classical quantum physics? From what I understand, an exciton is an electron-hole pair in a semiconductor that exists in a bound state (through the electrostatic potential). I have seen it stated that this pair behaves in a way ... 0answers 5 views ### Bragg's law - intensity “sensitivity” to lattice spacing or scattering angle I don't understand this sentence (emphasis added): A consideration of Bragg's law (nλ = 2dsinθ), i.e. the relationship between scattering angle (θ) and the interplanar spacing (d) shows that if ... 0answers 30 views ### Stable planetary rotation states In reading an article about theories as to how the slow retrograde rotation of Venus may have come into being, the article Why Venus Spins the Wrong Way (Franzen, 2001) stated that it is bound to ... 0answers 31 views ### Best lagrangian point [on hold] which is the best lagrangian point to send a spacecraft for a voyage to Mars and why? will it be L1 or L2? or any other lagrangian point? please advise. 1answer 39 views ### Work in circular motions Suppose that a satellite circles around a planet that exerts 2000N of gravitational force on the satellite. I understand the fact that since the circular motion and the centripetal force are ... 3answers 109 views ### Virtual Higgs boson? Can particles emit a virtual Higgs boson in a similar manner to the way a virtual photon is emitted? 3answers 146 views ### How does one determine whether an object will make an EM wave refract in a qualitative way? for example, i have a vague notion that the actual answer is that the permittivity and permisivity are different in each different material, so all waves refract at every boundary, but we only call it ... 1answer 252 views ### How to initiate fusion explosion without a fission trigger I'd like to know what theoretical methods for initiating a fusion detonation without a fission trigger exist, and which would be the most practical in the near future. I'd also like to know how small ... 1answer 134 views ### What's the relationship between weakly hypercharged bosons and Higgs bosons? In this video, Leonard Susskind does a good job trying to explain succinctly the Higgs field and exactly how it gives elementary fermions mass, except for one point he seems to skip a few things. The ... 1answer 73 views ### Axial forces on a solenoid windings I understand that the windings in a solenoid experience a Lorentz force \mathbf{f} = \mathbf{J} \times \mathbf{B}, which tend to cause an outward pressure where \mathbf{B} is directed along the ... 1answer 33 views ### Wien's Displacement Law for real bodies It is known that for perfect blackbodies,$$\lambda T= c$$where$\lambda= \text{peak wavelength}T= \text{Absolute temperature}c= \text{Wien's constant}\$ ...

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