# All Questions

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### Moment of Inertia Calculation [closed]

A uniform disc has centre O, radius a and mass 2m. It is free to rotate in a vertical plane about a horizontal axis through O. A particle P of mass m is placed on the highest point of the rough edge ...
52 views

### Dynamical map of open quantum system

Why it is said that the dynamical map of an open quantum system is a semi-group. I think the identity map is always an element. Hence it should be a monoid.
391 views

### How come the length of a wire does not affects on the circuit current?

Today we started learning about the electromagnetic induction. Out teacher gave us the following explanation: Suppose we have a conduction frame inside a magnetic field $\vec{B}$ going towards the ...
138 views

### Questions about the formalism of Quantum Mechanics

I have to do a presentation on this. I'm not expected to do something really detailed, but I'm not understanding the mathematical formalism. I would like to receive general answers to these questions: ...
258 views

### What state does the particle in a box occupy?

My textbook derives the equations for the different energy states $E_n$ of the particle in a box. But my professor in class said this example was a good one because it spoke about the "superposition ...
3k views

### Electric field due to a finite line charge [closed]

I was wondering what would happen if we were to calculate electric field due to a finite line charge. Most books have this for an infinite line charge. In the given figure if I remove the portion of ...
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### Is there serious research on superluminal travel? [closed]

I was wondering if there are any serious attempts, or any ideas for loopholes in the current theories, that would enable us humans to travel with a speed greater than the speed of light?
2k views

### What does 'zero-mean random noise with standard deviation equal to 1' mean?

Can anyone tell me what the meaning is of the phrase: "zero-mean random noise with standard deviation equal to 1"? Also, I want to know why not except zero-mean random noise and standard deviation ...
69 views

### Angular momentum wavefunctions with respect to different axes

I've been learning about quantum angular momentum, and I have a question about the relationship between quantum mechanical angular momentum wavefunctions with respect to different axes. I know that ...
50 views

### How can we see the electrons?

As you can see the blue glows is the electrons flowing inside the cathode ray tube under vacuum condition. My question is how can we see the electrons in the setup? electrons are everywhere but why ...
83 views

### Charles Law inverse for cooling?

Is the inverse of Charles law also true? If I double the volume, the temperature will be halved? In the fire service we ventilate a structure by opening up the ceiling to the attic. If the volume of ...
199 views

### Could time be considered a force

I think I understand the basic concept of spacetime, at least some of it. And when people talk about spacetime they don't call gravity a force, but rather the result of mass interacting with it. And ...
119 views

### Why do inverse square forces not rise asymptotically for $r\to 0$?

An example of an inverse square force is the gravitational force: $$F = \dfrac{G m_1 m_2}{r^2}$$ For large bodies I can understand why the force does not rise asymptotically as you get closer. For ...
1k views

### What determines the forward voltage drop for a diode?

I have always had the idea that the forward voltage drop in a semiconductor diode was related in a simple way to the bandgap energies in the semiconductor. However this is apparently not the case: ...
79 views

### How do I choose the right value of $r$ to find where the electric field is zero?

Sorry for the long question. I'm having a difficult time trying to explain my confusion. I have a positive point charge$\ Q_1 =+q$ at the origin and a negative point charge $\ Q_2 = -2q$ at $\ x=2$ ...
81 views

### Does the path integral measure have dimension?

For example, in the field functional integral: $$\int D\phi \ e^{S[\phi]}$$ Does the $D\phi$ here have dimensions?
117 views

### Is sound a Nambu-Goldstone mode?

The usual sound exists in solids, liquids, and gases, as a long-wavelength excitation with linear dispersion. Can its presence be attributed to the spontaneous breaking of some symmetry? In other ...
64 views

### Since electron clouds of different atoms repel each other, does that mean that touch is the feeling of electromagnetic repulsion? [duplicate]

Also when we rest our hand on an object does that mean we are effectively levitating because of the repulsion of the electron clouds?
166 views

### Physical intuition for quadrupole source

In his Theory of Vortex Sound M. S. Howe defines sources "mathematically" (i.e. dipole is a source that could be described as a vector and than there is proved that it's equivalent to a two point ...
64 views

### General solution to the wave equation proving dependence on $x \pm vt$

I am trying to solve for a general solution to the wave function and demonstrate any solution has the form $f(x,t) = f_L (x+vt) + f_R (x-vt)$ I have used separation of variables f(x,t)=X(x)T(t) to ...
112 views

### Varying wrt metric [closed]

I saw people write $\frac{\partial( F^{ab} F_{ab})}{\partial g^{ef}}$ as $\frac {\partial (g^{ca}g^{db}F_{cd}F_{ab})}{\partial g^{ef}}$ in a way that exposes the dependence on the metric. but ...
56 views

### Units for displacement current density

The displacement current density, $$\mu_0 \epsilon_0 \frac{\partial \textbf{E}}{\partial t}$$ has units of (N/A^2) (C^2/Nm^2) N/Cs = Ns^2/(Cm^2) which is not the same for current density: C/sm^2 ...
68 views

### A single electron (and positron) annihilation, is my face melted?

To help clarify my understanding of scale in regard to microscopic particle energy levels (specifically, an electron), I came up with this thought exercise: What would happen if an electron and a ...
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### Definition of kinetic energy without the second Law of Newton

As I see it, the definition of kinetic energy $$T= {1\over2} m u^2 \text { where u<<c}$$ comes by using the definition of work $$W= {\int F\cdot\ dx }$$ and we use for the meaning of ...
3k views

### How long does it take for water to boil when subjected to intense heat?

I have a rather odd question that may just boil (sorry) down to a fairly simple problem I can't solve. The scenario is this: A person is testing the intensity of a super-focused beam of sunlight in a ...
121 views

### Photon: speed and mass

Is it correct to think that the speed of light does not depend on the speed of light source because photons have no mass, so they have no the kind of inertia that is associated with mass, so they can ...
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### Equation for calculating spring costant

I'd like to design some of my own springs in order to obtain some very specific forces for a project. There are plenty of guides on how to make an arbitrary spring, but none I've read explain how to ...