# All Questions

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### Carroll's derivation of the geodesic equations [duplicate]

In Carroll's derivation of the geodesic equations (page 69, http://preposterousuniverse.com/grnotes/grnotes-three.pdf), he starts with ...
45 views

### Computer simulation/visualization of a ray of light passing near a massive object

I'd like to write a computer program that simulates and visualizes the trajectory of a ray of light as it passes near a massive object (e.g., neutron star). In other words, I'd like to model light ...
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### Dependence of $I_s$ on $T^x$ in a p-n junction

So we have the current through a p-n junction characterized by the equation $$I = I_s * [\exp(qv/nkT)-1]$$ Concerning the forbidden energy game $Eg0$. $I_s$ is proportional to $T^x \exp(-Eg0/nkT)$ - ...
142 views

### In the electric field we have dielectrics.What about the magnetic field?

Do we have insulators in the magnetic field just like we have dielectrics in the electric field? How are they called and how do they work?
136 views

### Modified double-slit experiment - two electron sources instead of two slits

In the famous double-slit experiment, if I inserted two identical sources (i.e electron guns) at the locations of the slits, would I still see an interference pattern on the screen? I would allow ...
475 views

### Using the uncertainty principle to estimate the ground state energy of hydrogen

I have been reading through this estimate of the ground state energy of hydrogen and others like it. In this one it says it is using the uncertainty principal but then proceeded to use the following: ...
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### Standard derivation of Witt algebra

I have been studying Conformal field theory for the past one week from the books by Blumenhagen and Di Francesco etal. If I understand correctly, whenever one talks of 'local (infinitesimal) ...
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### Is gravitational attraction increased by relativistic mass as well as rest mass? [duplicate]

Is gravitational attraction increased by relativistic mass as well as rest mass? If so what would happen if an observer travelled at relativistic speeds? Would it appear to him that the whole ...
617 views

### How does the bulk modulus of air change with rising pressure?

I can't seem to find the answer to what should be a trivial question: I have a rigid air-tight container of fixed volume and I am pumping air inside. The pressure is increasing (very slowly) from ...
50 views

### How to apply voltage source in FEM when solve Maxwell equation?

I need to solve the Maxwell equation of electric field by finite element method. In this function, the right hand side is the current density. However, in my problem, the voltage source with 1 MHz ...
47 views

### coherent length measurement of a monochromatic laser source

i need to learn about how to measure coherent length of a monochromatic laser source.I know that I can use michelson interferometer or spectrometry.For laser which has long coherent length michelson ...
66 views

### What is the meaning when we say something is at the higher potential and the other is at lower and wht is the direction of flow of charge? [duplicate]

Electric potential is defined as the work done in bringing per unit positive charge from infinity to a point in the electric field of some other charge. let me explain what i actually want to ask. ...
93 views

### Is it possible to produce coherent light with a thermal source? [closed]

Coherent light means monochromatic light and alle waves have the same phase difference. This is given for laser, where the resonator is a potential box and the outgoing waves have the same phase ...
50 views

### Do delay differential equations (DDEs) ever describe real-world phenomena? [closed]

I've recently become interested in DDEs, but I don't know much about them. A DDE has the form \begin{align*}\dot{x}(t) = f(t, x(t - \tau)) && \tau > 0\end{align*} My understanding ...
53 views

### Correlation between entangled photon polarisation measurement?

From Malus's law we know that if we measure a the polarisation of light with a filter angle $\theta$ to the direction of polarisation then the intensity goes like: $$I=I_0 \cos^2(\theta/2)$$ Firstly ...
2k views

### How do eagles fly slowly for a long time?

Eagles fly slowly for a long time. Many other species fly faster and move their wings faster. But eagles keep their wings steady, and move only their tail. How do they move slowly in the air, ...
76 views

### Plasma wakefield acceleration for Protons

Laser plasma acceleration is a promising technology that could replace the current method of accelerating particles (which is via electric fields). It is somewhat cheaper also as it makes the ...
32 views

### Can you explain in mathematical term of how the moon makes the earth spin stable?

I heard that our moon keeps the earth's rotation at the same angle. This is, as the Science Channel explained, is essential for regular seasons on earth. I am not interesting in the fact why it is ...
136 views

### Is the dimension “number of particles” a fundamental, or derived dimension (based on mass), or does it depend on the context, or is it dimensionless?

I consider "fundamental quantities" to be those that have dimensions that are are like length, mass, time, temperature, and so on. "Derived quantities" have dimensions that can be written in terms of ...
139 views

### Understanding gravitational time dilation / Schwarzschild metric

I've had a look at the answers to these sorts of questions already, but feel like I'm still missing something. Starting with this question, and this one and even this one here. I'm looking at this ...
83 views

### Frames of reference and why are they different

Here is a quote from "The Elegant Universe" by Brian Greene. Imagine two countries that have been at war are sitting down to sign a treaty ending hostilities while traveling aboard a train that is ...
60 views

### D'Alembert Equation and standing wave

As we know the d'Alembert Equation is $$\frac{\partial^2\psi}{\partial x^2}=\frac{1}{c^2}\frac{\partial^2\psi}{\partial t^2}$$ for an undimentional string. Now if we seek standing wave ...
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2k views

### Does holding weights allow you to run faster?

I'm not talking about training with weights so that when you run without them your muscles are used to pushing more weight so with less weight it'll be easier, I'm talking about while actually holding ...
58 views

### Static electricity and insulators

I've read that Stephen Gray in his experiments on electricity, he has found that static electricity can be conducted -transferred- through an insulator thread made of silk. So, how could that happen ...
104 views

### Point with zero electric potential inside triangle [closed]

If we have three equal point charges placed on a two dimensional plane at $(a_x, a_y)$, $(b_x, b_y)$ and $(c_x, c_y)$, what would then be the coordinates of the point at which the electric ...
84 views

### Wave velocity dependence on frequency

Is the velocity of a wave dependent or independent of its frequency? I cannot figure this out on my own. I've asked friends and they do not know.
760 views

### Is there a quick way of finding the kinetic energy on spherical coordinates?

Assume a particle in 3D euclidean space. Its kinetic energy: $$T = \frac{1}{2}m\left(\dot x^2 + \dot y^2 + \dot z^2\right)$$ I need to change to spherical coordinates and find its kinetic energy: ...
195 views

### “Derivation” of Minkowski metric?

Is there a deeper meaning behind the the Minkowski metric? Does it just come from the SR formulae? Or is there some deeper geometrical meaning, maybe in the context of GR?
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### Can I plot a phase diagram (P against T) of a mixture if the mole compositions are known?

Is it possible? Or do I have to actually experiment it in a lab? If it's possible, what data would I need? I'm right now only given the mole percentages and the weight percentages. I do know I can ...
114 views

### Ball on a slope with hollow

The experiment shown in the image suggests that ball B will reach the goal faster than ball A although the balls have identical properties and they start from the same height. The authors even ...
93 views

### Is entanglement a classical phenomena (2)? [closed]

The answer to this question seems to be yes, because you can simulate it with a classical computer and thus by a local classical theory (rule 110 CA) (see this question). However most people ...
129 views

### How to find the maximum area of a slot that can keep water in a trough by surface tension?

Assume I have a trough of water 5mm deep. If I cut a slot (oval) into that trough what is the maximum size/area (length x width) that slot can be without leaking? This question is similar to Max. ...
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### Calculating resistances in parallel

I was playing around with a question involving resistances in parallel. I realized that $n$ equal resistors (resistance $R$) in parallel have combined resistance $R/n$. I proved this using ...
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### Light trajectory

We have observed stars where "we should not" Some people say that gravity can alter light trajectory. Some people say that gravity actually alter the space on which light travels. Which one is ...
408 views

### Question about ohmic conductors

I'm having some trouble understanding Ohm's law. My trouble is with the different ways it is described when referring to ohmic and non-ohmic conductors. If someone can answer this question I think it ...
236 views

### Notation of vectors

It's very common to see $\text{F} = 30 \text{ N}$ when the problem is unidimensional. Yet, force is a vector. Shouldn't I write $|\overrightarrow{F}| = 30 \text{ N}$? Because if I write ...
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### How can I solve this differential eqution with IVP? [duplicate]

$$m\frac{d^2r}{dt^2}=-G\frac{Mm}{r^2}, \qquad r(0)=R ,\qquad \dot{r}=v_0>0$$ describes the free fall of an object of mass $m$. $r(t)$ is the distance of m to the center of the earth which is $R$ ...
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### Resistors in parallel: How do I make this rigorous? [closed]

In my revision guide it says "when two resistors are in parallel, the current has two paths it can flow through so there is less opposition to the current. This is why the combined resistance is less ...
123 views

### Fourier transform of Coulomb potential in 1D

The Fourier transform of the Coulomb potential $V(r)=\frac{k}{r}$ is typically evaluated by computing the Fourier transform of the Yukawa potential given by $V_{Yukawa}=\frac{ke^{-\epsilon r}}{r}$ and ...
53 views

### Entropy Change for a Thermally Isolated System

Clausius' Theorem states that $\int\frac{dQ}{T}\leq0$ for a closed cycle, with equality for a reversible cycle. Suppose we wish to take our system around a closed cycle such that the path from A to B ...