# All Questions

235 views

### How does one come up with the Coulomb's law?

My teacher mentioned that field line density = no. of lines / area and the total area of a sphere is $4\pi r^2$ and so an electric force is inversely proportional to $r^2$. Actually, why can the total ...
2k views

### Ball rolling down an inclined plane going in to a loop

I apologize if this question is not up to par. When I was doing exercises in basic mechanics I checked the answers and I can't seem to find what I'm doing wrong. Suppose we have a ball with mass $m$ ...
299 views

### Rigid body dynamics of tossing of a coin

While tossing a coin, it is commonly experienced that you get a head, if you toss it up with the head side up, and a tails if you toss with the tails side up. Is there a mathematical proof of this ...
101 views

### Constraints on open strings absent at the perturbative level

Studying Disappearance of moduli for condensate of open strings and Negative open string norms after BRST cohomology? gave me a huge huge shock! Suppose we have N completely wrapped Dp-branes over a ...
1k views

### What does the general solution of the Schrodinger equation represent for the particle in a box problem?

For the particle in an infinitely deep potential well, I have an intuitive picture of the separable solutions of the Schrodinger equation as being the wavefunctions for the different allowed energy ...
921 views

### General relativity and the microscopic/macroscopic distinction

Here is Wikipedia's diagram of the stress-energy tensor in general relativity: I notice that all of its elements are what would be termed "macroscopic" quantities in thermodynamics. That is, in ...
2k views

### Artificial planetary magnetic field

I wonder how difficult it is to create an artificial planetary magnetic field with generators? What power they would need? The question is inspired by thinking about possible colonization of ...
158 views

### Energy Question

Carl throws 100 g of Bohrium-262 (Half life: 0.10 seconds, undergoes alpha decay) to Sukhdeep with a initial horizontal velocity of 20.0 m/s at an initial height of 2.00 m above the ground. What is ...
217 views

### DIfference in Pitch Caused by Water Temperature?

I have recently been intrigued by the following question: What is the difference between the pitch of the noise of dripping water between hot and cold water? For example, would cold water create a ...
5k views

### How can kinetic energy be proportional to the square of velocity, when velocity is relative?

Let's start with kinetic energy (from los Wikipedias) The kinetic energy of an object is the energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a ...
7k views

There are no book recommendations for Astrophysics here. I will write my own answer, but I am also interested in what are others' views on the question (I will NOT mark my own answer as the best one). ...
265 views

### How does light travel create time travel violating causality?

Saw a question about faster than light travel... I still have the same question though none of the answers offered any resolution for me. It is so summarily assumed by all physicists and commentaries ...
381 views

### Would it be possible to create a hadron collider in space?

Would it be theoretically possible to create a hadron collider in space by orbiting particles around a planet?
125 views

### boundary limit conditions in 3D water surface simulation

As is discused on this post, taking some assumptions, the water surface can be simulated by a discrete aproximation of a grid of heights using this formula Where: HT is the new height grid HT-1 ...
250 views

### A step in the derivation of the magnetic moment of the electron in Zee's QFT book

In chapter III.6 of his Quantum Field Theory in a Nutshell, A. Zee sets out to derive the magnetic moment of an electron in quantum electrodynamics. He starts by replacing in the Dirac equation the ...
406 views

### Can high charges (like $1\times 10^{-3}$ coulomb) be acheived?

How can we charge a metal electrode with large charges ? When i saw a video about measuring a charge using a visiostat on a balloon , the charge was 0.6 nano-coulomb. Is the charge of 1 coulomb ...
812 views

### Why does the electric field dominate in light?

I read a book on the wave property of light where the author mentioned that the electric field, instead of magnetic field, dominates the light property. I don't understand why. In Maxwell's theory, a ...
306 views

### The requirements for superconductivity

Which properties are sufficient evidence for a material to be not superconducting? I am looking for a set of statements like If the material is semiconducting, it is not superconducting Edit: I ...
173 views

### What does Metric Transitivity Mean?

Jaynes In his paper "Information theory and Statistical mechanics" says "Previously, one constructed a theory based on equations of motion, supplemented by additional hypothesis of ergodicity, ...
196 views

### Minimum time required to reach a point [closed]

A police inspecter P is at point $(\acute{x},\acute{y})$ and a thief X is at point $(x, y)$. X has a constant velocity $V_x\hat{i} + V_y\hat{j}$, where $\hat{i}$ and $\hat{j}$ are unit vectors in ...
204 views

### About interchange phase of identical particles in Weinberg's QFT book

In Weinberg's textbook on QFT(google book preview), he discussed the phase acquired after interchanging particle labels in the last paragraph of page 171 and the footnote of page 172. It seems he's ...
749 views

### Is it possible to increase refractive index at lower densities?

Are there any materials for which the refractive index decreases with density? It seems that for most materials we would expect there to be a positive correlation between density and refractive index, ...
4k views

### Phase and Group Velocity of Electromagnetic Waves

Moving charges produce oscillating electric and magnetic fields -we have an electromagnetic wave. In terms of moving charges or at the level of charges, what is phase velocity and group velocity of ...
186 views

### Constructing Supersymmetric Lagrangians

It is a very trivial doubt but somehow I am not able to figure it out. While constructing a supersymmetric lagrangian we always even number of fermionic fields. One reason is of course the product ...