# All Questions

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### What does the behavior of the correlation function look like in the vicinity of the critical point?

What does the g(r) look like near the critical point?
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### Clear view and good lighting of the x ray departments

What is the benefit of clear view and good lighting of the x-ray diagnostic test department?
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### Scientists observe the laws of the physics but, Where do they come from?

Has anyone ever considered how the laws of physics that we study came into being.
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### Deviation from 2D trajectory

I need to find out how far a ball is from the predicted trajectory in 2 dimensional space and I know the start and end position of the ball in both dimensions. Along with that I know the initial ...
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### What's causing the effect of salt in voltage arcs?

I just came across this video demonstrating that salt increases length of voltage arcs. There is no explanation which leaves me quite confused. Does the salt decompose during the process?
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### What is the difference between induced current and eddy current?

I was reading today about electromagnetic induction and eddy currents. But while reading I was unable to differentiate between induced current and eddy current. What is the difference?
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### Is there a way to obtain an RG flow equation for Quantum spin systems using MERA

We restrict ourselves to ground states of translationally invariant 1d quantum systems. I understand that there is the scale invariant MERA which describes quantum critical points in which the ...
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### Integral over a product of two Green's functions

Need some help here on a frequently encountered integral in Green's function formalism. Forgive me since I am a junior student. I have an integral/summation as a product of a retarded and advanced ...
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### Solve the following linear second order circuit [on hold]

I am not able to solve for the second part of the question (when v(t) = 0)
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### Negative energy levels in the diagram for a hydrogen atom

The higher the number of the shell (n), the higher is the energy level of the electron. However, why was it necessary to have negative values. So for example, when $n=1$, the energy could be $5 eV$ ...
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### The explanation of the mass of the photon [on hold]

Actually, I'm not asking a question, I am answering to people who don't understand why they say that photon has no mass and exists at the same time! I am a popularizer of science, specialized in ...
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### Corrected Lorentz transformations [on hold]

The standard Lorentz transformations are not correct because in their derivation a logical contradiction is involved: namely in the derivation is assumed that for any value of the velocity, the ...
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### How would normal matter behave under conditions found in the core of the sun?

Considering how low the power density is at the sun's core, I seem not to be able to expect what would happen to matter in case it was thrown inside the sun's core. For example, let's assume an ...
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### Who foots the (magnetic) energy bill?

Gravitational attraction and electrostatic attraction/repulsion are intrinsic properties of matter, any particle (electron, proton) for some unknown reason can produce KE at a distance. But magnetic ...
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### Quantum chaos in interacting discrete time quantum walk?

I have situation where I am simulating discrete time quantum walk (DTQW) for various graphs. I have two quantum walkers on the graph and they can interact with each other by the fact that where the ...
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### Can an Ergodic dynamical system approach equilibrium?

An ergodic dynamical system $(\Omega,\phi^t,\mu)$ is such that the time average $\bar{f}$ of every function $f\in L_1(\Omega,\mu)$ equal the space average $\langle f \rangle_\mu$, i.e. the system ...
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### How to find the required inlet velocity/pressure to achieve 0.5 m/s in one part of the underground tunnel? [on hold]

I have this problem here related with tunnel and ventilation. As you can see at the picture below, it is a top view of the tunnel. The tunnel has an inlet and an outlet. The air flows from the inlet ...
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### How to calculate the differential scattering cross section?

I am doing some simulations with the Meep code to study some properties of a metal nanoparticle. The situation is this: an incident electromagnetic wave on a metal nanoparticle. By now I know how to ...
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### Calculate object displacement with a radial/centripetal acceleration [on hold]

What i want to do is calculate the displacement of a object with a certain radial acceleration in a moment of time. The object has a velocity, radial acceleration and a tangential acceleration. For ...
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### Expanding universe and the speed of light [duplicate]

When physicists talk about the expanding universe they often say that the distant galaxies are not really "moving" away but instead the space itself between us and them is expanding. If this is true ...
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### What formula connects the moment of inertia and angular velocity? [duplicate]

I need to determine angular velocity of a disc when a man with given mass and speed whacks on the edge of it. I calculated the total moment of inertia of disc and body, how do I calculate the ...
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### The acceleration vector of a simple pendulum

In this picture the acceleration vector $\vec{a}$ points upward when the pendulum is halfway Click To see animated GIF But accodring to this picture, the force acts tangentially: Which means the ...
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### Hermiticity of the quantum field

The quantum field resultant from the quantization of a real classical field is hermitian, but why the quantum field corresponding to a complex classical field should be non-hermitian?
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### General relativity: gauge fixing

In his lectures professor Hamber said that the metric tensor is not unique, just like the 4 vector potential is not unique for a unique field in electrodynamics. Since the metric tensor is symmetric, ...
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### Why can I calculate the value of an energy quantum of an Einstein solid at half the equipartition value of the heat capacity?

(I've realized that I'm horrible at asking the right question but I believe I pinpointed what it's really about:) Apparently the energy quantum can be calculated by estimating the value of ...
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### Forces acting on two balls in a jar [on hold]

Consider the following situation: Two balls are inside a jar with diameter $D$. The balls are uniform in mass and both have the same diameter $d$. Express the forces that acts at the colored dots ...
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### “Delayed choice” quantum imaging experiment - why wouldn't it work?

I was just reading a Nature news article (Entangled photons make a picture from a paradox, 27 August 2014) about a way to form an image of an object out of photons that have not interacted with the ...
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### Followup to: The relation between energy quanta of an Einstein solid and the equipartition value of heat capacity

As a followup question to this question: The relation between energy quanta of an Einstein solid and the equipartition value of heat capacity and this answer ...
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### Expansion of atoms

If an atom is the smallest particle in the universe, does that mean that the spaces in between the atoms in water when being heated (expansion) are vacuum?
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### Angular momentum of a sphere moving vertically downwards [on hold]

If a solid sphere is moving vertically down with a velocity $v$ and spinning with an angular velocity $\omega$, what is the angular momentum of the sphere about an axis through the sphere which is at ...
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### Finding the direction of the magnetic force acting on a conducting wire

I have a problem in finding the direction of the force when a conducting wire is placed in a magnetic field. If I use Fleming's Right Hand rule I get a circular magnetic field, so what will be ...
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### Will porridge/rice absorb water faster without a lid on it? [on hold]

I just wonder if my porridge would be ready any faster if I take the lid off my tupperware container.
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### Reference books for classical mechanics with good number of solved examples [duplicate]

I am looking for some classical mechanics reference book with a good number of solved problems.
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### Angular momentum and gyro effect on twin wheels

Suppose there are two twin wheels fixed on a vertical rod (z-axis) (with a handlebar, like a bike) by an horizontal spinning axle (x-axis) so that the rotation plane is in the y-axis. The wheels are ...
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### Work done against a resistive force

My past year exam paper had a question about work done against the resistive force, where the answer key said it was resistive force * distance. As I understand it, work done is a measure of impact a ...
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### How do charges actually distribute among capacitors? [on hold]

Two capacitors of capacitance $3 \mu F$ and $6\mu F$, are charged to potentials of 2 V and 5 V respectively. These two charged capacitors are connected in series. Find the potential across each of the ...
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### Torque due to a magnetic field

We have two infinite perpendicular wires, each carrying a current "i" and the distance between them is l. If both the wires are free to move, how will they arrange themselves because of the torque ...
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### If a photon has no mass, how can it be attracted by the Sun?

I read that the photon doesn't have mass, but my teacher says that the photon has mass because the sun can attract it (like in the experiments to prove the theory of relativity). I think that there ...
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### Difference between Urca process and $\beta$-decay in neutron stars cooling

I was reading about the Urca process and its importance in the cooling of astrophysical compact objects. Indeed, it is supposed to be one of the major contribution to the cooling of neutron stars ...
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### Ice and liquid water interacting across a boundary

Imagine we have two thermodynamic systems, one a mass of ice and the other an equal mass of liquid water, with both at 273.16K. Each system is isolated, except that they can interact with each other ...
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### Differential Equations [on hold]

$$\nabla^2 B = - \mu_0^2 \epsilon_0^2 \frac{\partial B}{\partial t}$$ $$\nabla^2 E = \mu_0 \epsilon_0 \frac{\partial B}{\partial t}$$ Find as many E and B field solutions as you can from this. ...
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### Can Allan variance be generalized such that the “Oscillator model” is not presumed?

The definition of Allan variance, $\sigma^2[ \tau ]$, which relates to "stability of clocks" is described on the Wikipedia page as being derived in terms of an "Oscillator model": "The oscillator ...
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### What causes the unexpected change in acceleration for flybys of spacecraft?

If the vehicle is not operating on RTG, then thermal recoils of photons shouldn't be considered as in the case of PIONEER. Then what actually accelerate the spacecraft from expected value (expected by ...
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### How do you go from quantum electrodynamics to Maxwell's equations?

I've read and heard that quantum electrodynamics is more fundamental than maxwells equations. How do you go from quantum electrodynamics to Maxwell's equations?
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### When can I use $\wedge$ instead of curl?

It seems in some circles the wedge product is used in preference to curl. I have a basic understanding of Green and Stokes' formula, I wish to use the $\wedge$ notation from now on. Can someone tell ...
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### Continuous spectra of photons

I guess this is a basic quantum mechanics problem, but I'm not entirely sure of my answer. Suppose we have an electron in a hydrogen atom having the state ...
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### How far is the range of an ultrasonic sensor as far as it can transmit? [duplicate]

How far can ultrasonic waves propagate in the air? (if the energy is enough), how far can ultrasonic waves propagate in the ground (concrete floor)? Are there any articles or experiments about ...
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### When longitudinal waves and transversal waves enter air from a soild, what will happen? How do waves transform?

When ultrasonic waves meet the interface (surface) between two media they will reflect and refract. What is the effect of the acoustic impedance on reflection? Is there any list about acoustical ...