# All Questions

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### Internal Energy of an ideal fermi Gas

The internal energy of an ideal fermi gas at temperature 0K is given by $$U=\frac{3}{5}NE_f$$ What conclusion can we draw from this statement.
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### Conceptual question on Gauss's Law

By my understanding, the electric field in the surface integral expression for Gauss's Law represents the total electric field and any point on a closed Gaussian surface. However, when we employ ...
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### Is it possible to see atoms ? There's a wikipedia picture

As per my knowledge atoms are smaller beyond our imaginations But there is a wikipedia image which shows silicon atoms observed at the surface of silicon carbide crystals. Here's the link to the ...
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### Does a vacuum, suddenly opened, become hotter than its surroundings?

Suppose you have an insulated container that is equipped with a valve to let air in. Initially the container is evacuated. You then quickly open the valve, allowing air to rush in. What is the ...
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### Why don't electrons fall on the ground, off the charged wire?

Please, don't mind if my question is silly, but I was always wondering... Imagine we have a negatively charged metailc wire. Let's assume it's just hanging on a thin rope. What force keeps electrons ...
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### Application of BEC in Helium-4

By using the formula for Transition Temperature, we can obtain the Transition temperature for Bose Einstein Condensation to occur at 3.14 K, however, it shows the properties of superfluidity at 2.18 ...
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### Chemical Potential as a function of Temperature

I have considered an ideal fermi gas. Then, we can obtain an expression for chemical potential as a function of Temperature. I want to understand the physical significance to it or what it really ...
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### Getting the velocity of self-diffusion from the einstein relation

I have been reading about self-diffusion of a particle here: http://www-hsc.usc.edu/~rfarley/Diffusion-SI-2003.pdf and on page 9 it gives Einstein's relation as $(\Delta x)^2 = 2 D t$ and I can ...
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### Do we need Escape Velocity

This may seem silly a lot but I really need some clarification on the necessity of escape velocity for a rocket leaving the Earth's gravity. A stone thrown vertically upwards reaches to a certain ...
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### How to explain the constancy of physical structure

My question will may be seems awkward, naïve and my learning level in physics is not very high. But I was wondering how physical structures from the atom to macroscopic ones, including living beings ...
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### Equations of Motion Lab Experiment [on hold]

I did a lab experiment in physics class but I'm having difficulty approaching these questions. I'm thinking that the graph that I'd sketch is a position- time graph which would be the answer for ...
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### Why capacitor pass AC and block DC current? [duplicate]

We know that in circuit capacitor block the DC current and pass AC current. My question is how a capacitor block DC and pass AC?
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### Where do photons go when they are absorbed?

The answer I usually get (and I'm paraphrasing here) is that they disappear and are instead absorbed as heat energy. But I find it hard to believe that the photon simply "disappears." Common sense ...
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### How to find creation and annihilation operators?

I get confused when trying to find this. Please describe it as simply as possible, but keep in mind I have no budget whatsoever to pay for textbooks, so here goes: How do you find the creation and ...
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### Has the cosmic alignment of Earth and the Solar System been explained (CMB anisotropy)

The Cosmic Microwave Background anisotropy is currently aligning Earth and the Solar System with the largest and earliest structure of the universe. Has this been explained yet? Or are the theories ...
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### Does Big Bang Cosmology imply/require infinite space?

The reason I am asking this question is because if all points in space observe recession of galaxies the same as we do from Earth, the universe would have to be infinite (or a closed sphere in 4D or ...
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### Intuition behind Rocket propulsion equation [on hold]

What's the intuition behind the Tsiolkovsky rocket equation: $$v - v_0 = v_{ex} \ln{\frac{m_o}{m}}.$$
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### Phase Transition at Zero Temperature (Not QPT)

As is well known the Ising model exhibits a phase transition, except the one dimensional case in which the phase transition occurs strictly at $T=0$. Now I have always thought that this makes the case ...
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### Solution of the wave equation

The solution of the wave equation $$\frac{d^2y(x)}{dx^2}+k^2 \left(1+\eta(x)\right) y(x)=0$$ Can be found by using the Green function method as y(x)=c_1 ...
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### Spectral lines and QM

In the various presentations I've seen so far in atomic physics of series such as the Balmer series, the wavelength of each spectral line is definite - but in QM, free particles have no definite ...
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### Potential Energy, Kinetic Energy, velocity, question?

A man standing a certain height above the ground throws a rock straight up with an initial velocity of $10 \:\mathrm{m/s}$. A few moments later, the rock hits the ground with a kinetic energy of ...
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### What is the minimum velocity a toy car has to travel in order to survive going through a loop of radius 0.2 meters?

I tried height = 2 * radius = 2 * .2 m = .4 m. Potential energy = mgh. Kinetic Energy = $\frac{1}{2}mv^2 = mgh$ Then $\frac{1}{2}v^2 = gh$ $v = \sqrt(2gh) = 2.8 m/s$ The correct answer is 2 ...
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### Why is a hexagon such a stable shape for materials?

A hexagonal lattice is famously the shape of graphene, the source of the 2010 Nobel prize. The shape also shows up in beehives and in the basalt columns of Giant's Causeway in County Antrim. ...
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### Ratio of kinetic energy concerted to internal energy [on hold]

I need help with this question: Consider four identical cars. Two of them collide head-on when both are traveling at 34 m/s. The other two collide head on when both are traveling at 25 m/s. What ...
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### Help! I need to know the category and subcategorie hlp [on hold]

i would like to know the dimensions of aXb in the relation p= b-x^2/at where p is power, x is distance and t is time
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### Estimating divergence of set of vectors

I have a set of points where directions and intensities of a flow are given (in 3D). Is it possible to estimate the divergence of the flow defined by those vectors? I only need a rough estimate and I ...
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### Microscopy: aperture matching and Huygen's principle

I have repeatedly read that the maximum spatial resolution of a microscope occurs when the aperture of the objective and condenser are the same. However, I don't understand why. The resolution is ...
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### Have there been any experiments that have demonstrated a quantum error correcting code?

Has quantum error correction been demonstrated? I know that classical error correction has been demonstrated by [1], they correct a single bit flip error, but has anyone been able to detect and ...
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### Can we change a photon's frequency in mid-air?

Can we have a light source emitting photons in the infrared range and after, lets say, 5 meters, these photons become a photon in the x-ray range? The only way I know we can change a photon's ...
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### Is my answer possible? (Newtonian Third Law Pairs and Forces, Multiple Choice Word Problem)

So I recently took a test and the teacher graded my answer correct, yet my friends saw that the other answer was correct and they protest that it should be the right answer: The question goes: ...
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### How can I use Einstein's field equations?

Every time I try to find the answer to this question I get redirected to different pages that ultimately do not end up answering my question. I have some understanding of Riemannian geometry but have ...
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### Conductor held at constant potential

What does it mean for a conductor to be held at constant electric potential? So if my reference point is infinity, and I integrate inwards over the electric field to any point on the conductor, I will ...
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### If a charge needs potential difference to move

Let's consider a circuit with a ideal source of emf and a resistor. Consider the segment of wire that connects emf with resistor. If we measure potential difference at any point of the wire, this will ...
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### Directly calculate electric potential due to a spherical volume charge

Q: Say we have a uniformly charged sphere with radius $R$, total charge $Q$. Now we need to find out the electric potential inside the sphere, at a distance r from the centre. I understand that we ...
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### Given Force in vector form, how do I find work done? [on hold]

A force of $F=\hat i+2\hat j-3\hat k$ is applied to a particle that moves 10 meters in the direction of $\hat i+\hat j$. How much work is done?
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### What is the universal speed limit relative to? [duplicate]

If all speeds are relative, then what "governing" force is that speed limit relative to? Is there some sort of fixed or absolute grid with locations everything is compared to? Does this also mean ...
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### Voltage induced in coil

Okay, so I have two coils. A ring of radius $R$ is stationary at origin. A coil of $N$ turns and area $A$ carries current $I$ and is moving along the $x$ axis such that $x = x_0 + cos(\omega t)$. ...
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### Are there nonlinear models of quantum mechanics which forbid superluminal signaling?

What would a nonlinear model of quantum mechanics which forbids superluminal signaling look like? Of course, a nonlinear $\psi$-ontic theory with entangled states could have superluminal effects upon ...
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### equilibrium pulley system [duplicate]

the question asks what mass i need for m1 to be in static equilibrium while m2 is given as 60kg. im not sure first of all if that means the two masses have to be equal in length balanced. secondly, ...
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### How do I find the angle given the mass, the acceleration and the force acted on a body?

My Issue I have the mass, the horizontal acceleration and a force that acts on a body. The force forms an angle $\alpha$ of unknown degrees with the x-axis. I would like to know how I can find the ...