# All Questions

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### Was Einstein wrong when he said nothing can go faster than the speed of light?

If the universe is constantly expanding faster than the speed of light, how could Einstein be right?
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### Why do some terms vanish in first-order perturbation theory?

In first order perturbation theory, we usually express the first order perturbation in the eigenket of the perturbed Hamiltonian in the basis of the unperturbed Hamiltonian $H_{0}$: ...
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### Fermi level and conductivity

Can someone in a simple way explain me what the Fermi level is and what does it have to do with conductivity. My teacher said that Cu conducts electric current better than Al because of something in ...
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### Energy and proper time (not relativity)

In a scientific paper, I have found the following sentence: "...given the fact that the system is homogeneous in energy, or equivalently, that it has no proper time scale." I'm not sure about what ...
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### Can a human body supercavitate to survive water impact?

Inspired by this analysis of a human (OK, Captain America) hitting water feet first at terminal velocity, I'm wondering if supercavitation would be possible and whether it would improve your chances ...
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### Field from non-conducting plate?

For a non-conducting sheet, the electric field is given by: $$E = \frac{\sigma}{2\epsilon_0}$$ where $\sigma$ is the surface charge density. This equation holds well for a finite ...
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### Electric field near surface of a conductor?

If the electric field on a positive charge is non-zero, then the charge accelerates in the direction of the field. The field at the surface of a conductor is perpendicular to the surface. Why ...
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### Charge distribution on conductors?

You have seen that the excess charge on an isolated conductor moves entirely to the conductor’s surface. However, unless the conductor is spherical, the charge does not distribute itself uniformly. ...
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### One more time (the dogma of light) [on hold]

Could someone explain why two photons in a straight line to 300000Km/s, keep the 300000Km/s a in relation to the other and it didn't pass Newton mechanics? please Feynman...?
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### How to Determine the voltage polarity of inductor in a circuit?

The Voltage polarity across the inductor is same as that across the capacitor since both are in parallel and the voltage across them must be same. But when Inductor, capacitor and resistor are ...
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### Linear Algebra in Physics [on hold]

I am an AP Physics Student also in linear algebra, and I was wondering if anyone could offer any instances where I could use linear algebra to simplify problems in elementary physics. Essentially, I ...
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### How are symmetries defined mathematically? [duplicate]

I have started working on differential geometry very recently. I am little bit familiar with mathematical concepts such as manifolds, differential forms and associated concepts. As I was speeding ...
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### Calculating the electric potential in cylindrical coordinates from constant E-field

I am having so much trouble with this problem. I feel like I shouldn't be, but I am. A uniform electric field, $\vec{E} = E_0\hat{x}$. What is the potential, expressed using cylindrical ...
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### Thinking a little more (dogma of light) [on hold]

If it were possible to apply Feynman's postulate, about the only straight trajectory, if it were possible, it is possible to apply on this two light pulses in two successive moments, as I say, that ...
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### I don't know how to define inertial frame [duplicate]

I think special relativity don't give a definition of inertial frame. Someone says that G.R. can give a definition, Why?
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### Actions at a distance vs. “proximity actions”

The ancestors could not imagine an action at a distance (in German: "Fernwirkung"). Today physicists don't take serious its opposite anymore (in German: "Nahwirkung"). So my first question is: ...
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### How can we move an object with zero velocity?

Consider there is a box of mass $m$ at rest on the floor. Most books give an example that we need to do a work of $mgh$ to lift the box $h$ upward. If we analyze this work done, the external force ...
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### phase difference between incident plane wave incident on a dipole and radiation fields from dipole

i have an incident plane wave and a dipole, consider that plane wave incident on dipole. at this moment what happen for dipole ? we know that after incident of plane wave on dipole, the radiation have ...
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### Heat production of magnifying glass

Why Magnifying glass(convex lens) produces heat when placed in front of sun?
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### How much power on average does a Wii remote require? [on hold]

Trying to find out how much power is required in a wii remote. Or how much power is required? Or what voltage is needed? any values would be of great help.
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### Need to use lasers,mirrors and prisms for a project.How to make the light visible and presentable at the same time? [on hold]

Alright so I really want to put some art into science for my project.project. I'm thinking of creating a beautiful image,preferably in air.I read about the visible lime-green lasers but i still want ...
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### Why don't we include $\rho g h$ in a hydraulic jack below?

Consider a hydraulic jack with massless pistons as follows. The famous equation for this system is $$\frac{F_1}{A_1}=\frac{F_2}{A_2}$$ My question is why isn't the equation as follows?  ...
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### Oscillations and Majorana Neutrino

In neutrino oscillations, neutrinos can convert from one flavor to another. This implies individual lepton number is not conserved. Doesn't it also imply that, if the neutrinos have mass, the mass ...
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### How organic impurities in the air affects laser?

We have a Nd YAG laser in our lab. A technician came to our lab (my professor called him for its maintenance) and while he was working on it, he told me that because of the organic impurities in the ...
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### Dirac, Weyl and Majorana Spinors

To get to the point - what's the defining differences between them? Alas, my current understanding of a spinor is limited. All I know is that they are used to describe fermions (I hope I've phrased ...
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### Interpreting perturbation theory in general relativity

In quantum mechanics we start with a Hamiltonian $H_0$ for which we know the exact eigenstates and energy eigenvalues. We perturb it by a known term $H$, and then attempt to compute (approximately) ...
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### Positron and other particles

Positron is a particle which has the same mass as an electron . But it has positive charge on it . When electron and positron combine, they annihilate each other with the release of energy in the ...
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### How do I find the refractive index? [on hold]

Can someone please explain to me Snell's law? How do I determine the refractive indexes for things such as air to water, water to air, water to diamond..etc - I am not sure what the question wants me ...
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### How does airplane flies [duplicate]

What aerodynamic effects actually contribute to producing the lift on an airplane? I know there's a common belief that lift comes from the Bernoulli effect, where air moving over the wings is at ...
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### Hydrostatics , work Power and Energy [on hold]

If a servant lifts 10 cubic meter of liquid from a tank, which is at a depth of 40m . If the work done by him is 1600J, then find the density of the liquid (g = 10 m/s^2)
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### About the dogma of light [on hold]

Nothing is faster than light, but if she herself were the relativistic frame?, wouldn't it stop light, you wouldn't be a problem in itself and also for the quantum mechanics?
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### Work Power & Energy Question [on hold]

If a person starts from rest with an acceleration of 10 m/s^2 and applies force of 1000N then What will be its work done at the end of 20s . If the object moves in the horizontal surface? Can ...
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### How do you solve Laplace's equation for a parallel plate capactior?

I would like to find the analytic solution to the problem of two plates of opposite electric potential. I have already solved this numerically as shown in the picture below. I'm also wondering what ...
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### Applications of Quantum Walks [on hold]

Can someone explain to me what real world applications could potentially benefit from the study of quantum random walks? I have researched a fair amount on how quantum walks operate and their ...
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### difference between dielectric constant and dielectric function

I'm confused by the definition of dielectric constant. We all know that dielectric is function of wavelength, because materials respond in different ways depending on the energy of the light. But what ...
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### Mathematically impossible for a vortex line to have loose ends?

Could someone show the math behind it? Source : "A vortex is a bunch of air circulating around itself. The axis around which the air is rotating is called a vortex line. It is mathematically ...
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### Why does air circulate on an airfoil — The Kutta Condition

Why does the air circulate on a flowing airfoil, thus giving rise to increased velocity (circulation + relative airspeed) above the wing and hence decreased pressure.
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### Why friction force is force of constraint?

My understanding about constraint force is that it is a force which limits the geometry of particle's motion. For example, situations such as the particle trapped in a track or limited in domain can ...
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### Good ways for learning and cramming formulas? [duplicate]

I got an exam coming up and its numerical based (its pre-university level exam) but really tough. I want to know about various ways and methods to learn formulas. I know how can I derive them but its ...
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### How simplify functional derivatives (in path integrals) with mathematica?

Are there any packages that can simplify functional derivatives in path integrals? For instance the expression (integrate over, $x,y,z,u,v,r,s$): ...
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### Questions about spacetime curvature and gravity

These are some further important queries regarding the question here Why would spacetime curvature cause gravity? Q1. Explain the statement “Everything in spacetime moves at the speed of light”. Is ...