0
votes
0answers
2 views

Do III-V based photovoltaics “glow” (photo-luminesce) when illuminated but not loaded?

In this nice answer I learned that when the silicon photovoltaics cells on the International Space Station are exposed to the sun, they tend to be cooler when electrically loaded than when no current ...
0
votes
0answers
4 views

Lightning, Electrical breakdown, and Current

Recently a discussion came up about this gif: http://i.imgur.com/r9Q8M4G.gifv This lead to a few questions about electrical breakdown of insulators. The disagreement we're having is whether or not a ...
0
votes
1answer
13 views

Can I use an electromagnet as a shark repellent?

I scuba dive, and also spearfish. Doing these sports in the ocean presents obvious challenges since Sharks exist in the ocean. I know about a product called shark shield which uses magnetism to ...
0
votes
0answers
9 views

Are black holes causing expansion?

If the universe is sufficiently large enough, there should be a black hole in every direction of the sky. So, could there be a game of tug-of-war going on between all of the black holes in the ...
0
votes
0answers
7 views

Are elements of statistical ensemble fixed or dynamic?

I will use the simplest example I can think of to explain what I am trying to understand. Consider a system with ~10^23 particles in an equilibrium with fixed values of pressure, volume and ...
0
votes
0answers
4 views

Heat transfer direction in fins

I'm unsure of what exactly is changing the heat transfer direction in the triangular fin. $$q_{x} = -kA(x)\frac{dT(x)}{dx} (1)$$ $$q_{x+dx} = -kA(x)\frac{dT(x)}{dx} - k\frac{d}{dx}[A(x)\frac{dT(x)}{...
3
votes
0answers
8 views

What Is the Roche Limit Factor for a Tension-Free Solid Rubble Pile?

I have reached a confusion when searching for derivations of the Roche Limit, which differ in value and explanation. All equations are given of the form: $ P = r_{M} \sqrt[3]{f\frac{\rho_{M}}{\rho_{...
-3
votes
1answer
39 views

Why $\epsilon_0 = 1/(4\pi k)$ instead of $\epsilon_0 = 4\pi k$?

In my physics book, they consider a positive point charge $q$ and a spherical surface of radius $r$ centered on this positive charge. Then the flux through this surface is given by $$\Phi = \iint \vec{...
2
votes
0answers
10 views

Why is the molecular partition function only broken down into translational, rotational, vibrational, and electronic states, ignoring configurations?

Per Atkins and others, energy is the sum of contributions from the translational, rotational, vibrational, and electronic modes of motion. Since the formula for the partition function is $Q=\sum_i e^{\...
1
vote
0answers
9 views

Non-singlet and singlet flavor combination

What is non-singlet and singlet combination of quark flavors and why are they called in such a way? Does it transform in certain way under global flavor group transformation?
0
votes
1answer
26 views

I cannot verify if given transformation is canonical (CT)

I'm going on a list of exercises and there's 6 days that I can't figure to this. Prove that given transformation is canonical: $\Large Q=\frac{p}{mw}\sin(\frac{mwq}{P})$; $\Large P=p\cos(\frac{...
0
votes
0answers
32 views

What is wrong with my formulas for a mathematical model of a double pendulum?

I tried mathematics but I've had no response in a month. I wanted to create a computer simulation on Matlab, using a model for a pendulum from this study (A double pendulum model of tennis strokes. ...
1
vote
0answers
33 views

String Theory Vs. Holographic Theory of the Universe. Compatibility

String Theory claims strings to be one dimensional objects vs. dimensionless point particles of the Standard Model. String Theory also needs 10 or possibly more spatial dimensions to operate in. ...
3
votes
2answers
36 views

Meaning of “Metrically defined” matrix

I got stuck on understanding stuff from Methods of molecular quantum mechanics written by R. McWeeny. The content is: It must be stressed that the representation property embodied in $\mathcal{A} \...
2
votes
0answers
14 views

Homotopy Theory for Topological Insulators

I'm trying to understand topological insulators in terms of homotopy invariants. I understand that in 2 spatial dimensions, we have Chern insulators since $$\pi_2(S^2) = \mathbb{Z}$$ One subtlety that ...
3
votes
1answer
27 views

LHC: Clumps of protons or continuous stream?

I am curious about the Large Hadron Collider and whether the proton (anti-) beams are clumps of protons (grouped together) or are they roughly equally distributed around the circumference of the ring. ...
5
votes
0answers
25 views

Why can we approximate dynamics of massive particles by masless and vice versa?

I am puzzled by the following observation. Our descriptions of massless and massive particles are very different. For example, masless particles have only two polarizations, which we call helicities. ...
2
votes
0answers
9 views

Energy-band diagrams of heterojunctions

I'm studying heterojunctions and I'm struggling with drawing their energy-band diagrams. I've been looking for books or something to look up how the diagrams can turn out to be with different types of ...
1
vote
0answers
19 views

Quantum Entanglement Simulation

Suppose there is a digital "quantum entanglement simulator" with a pair generator and two spin detectors, and it Allows to create as many pairs as you want (100 pairs per single click) Allows to ...
1
vote
2answers
64 views

Is this explanation for the Universe's low entropy at its origins plausible?

I've been thinking about this, and it may sound very ludicrous (and it probably is), but I may have come up with a proto-explanation for it. I hope I won't come across as some misinformed perpetual ...
3
votes
2answers
29 views

What frequency is the scratching of finger nails on a blackboard?

This is the frequency/intensity that sets my teeth on edge. Does anybody know what frequency (roughly) it is? I am guessing it is near the top of normal human hearing, 20kHZ, but I'm not sure if ...
0
votes
0answers
19 views

Is there a benefit in a final jump during an explosion?

When in movies a building explodes, the hero usually runs and jumps at the last possible time. Is there any benefit in this final jump? As far as I can see, the speed won't increase once the feet ...
3
votes
1answer
27 views

Does the force on the bottom piece of a hummingbird feeder depend on the height of the water in the reservoir?

Intuitively I would think that the more nectar you put into the reservoir the more weight it will put on the bottom piece (the tray that holds the open pool of nectar that the birds can feed out of). ...
0
votes
1answer
42 views

Why do things move?

I mean, really, why do things even move? Despite the concepts of forces or mass, why does an object move? What is movement? Even in the smallest scales if you think, let's say a plank length, ...
1
vote
0answers
29 views

Rotation of rod [on hold]

A uniform rod of length "L" is free to move and rotate in gravity-free space . when an impulse is given to one end of the rod perpendicular to its length, its center of mass moves with velocity "V". ...
4
votes
1answer
29 views

Confusion about virtual displacements

I am self-studying Goldstein's book "Classical Mechanics", and I need some help understanding the part where Goldstein discusses using Hamilton's principle to solve systems with holonomic constraints (...
0
votes
2answers
35 views

Heated and cooled air effects on combustion

in a turbocharged engine, an intercooler is installed to cool down the compressed air (apparently for better combution) In contrast, in the power plant they use air preheater to heat up the air before ...
1
vote
1answer
25 views

Mechanical, not voltage driven electron motion creating larger net electromagnetic force

Have mercy I'm a physician not a physicist. Electrons' Motion in an electromagnet is on the order of 10$^{-4}$ m/s. That charge can be mechanically rotated in a capacitor with a million times more ...
1
vote
0answers
20 views

Calculating resultant force along vertical axis when using smartphone accelerometer

I'm making an app which is using accelerometer to automatically detect damages on the route while driving. To estimate if damage occurred or not I'm using accelerometer (acceleration along vertical y ...
0
votes
0answers
27 views

How to plot Joukowski airfoil?

We have an asymmetric potential flow past the cylinder (i.e. 2D circle) of radius $R$ of well-known complex velocity $W$: $$ \tilde{W} = v_\infty e^{-i\alpha} + i\frac{\Gamma}{2\pi(\zeta - \mu)} - \...
2
votes
1answer
43 views

How is 'Fog' in summer(sea-elevation) possible?

I am residing in New Delhi (pardon me for any irrelevant data I might include), and I noticed a really interesting thing. It's an average $30^{\circ}\mathrm{C}$ out here, and yet, most/some weather ...
1
vote
0answers
19 views

Poisson brackets among derivatives in the constraints of the Hamiltonian formulation of General Relativity

The issue I have is from the Hamiltonian formulation of General Relativity (GR) and concerns calculation of certain Poisson brackets. In this formulation, we have the 3-dimensional metric $h_{ij}(x)$ ...
-3
votes
4answers
40 views

Is earth continuously changing its orbit size i.e, circumference of the orbit?

One year have 365 days, but in leap year we have 366 days. Does that mean earth is continuously change its orbit size for every four years. like . 365.0 -- 365.1 -- 365.2 -- 365.3 -- 365.4 -- 365.5 -- ...
-3
votes
0answers
43 views

The energy of tides: how does it affect the orbit of the Moon?

There are a few posts here at PSE that explain how the energy of tides comes at the expense of the Moon/Earth angular momentum. There is something that escapes me: how can angular momentum or KE be ...
0
votes
0answers
26 views

RWD Differential steering and front wheels

I am developing a proof of concept autonomous car that will have 4 fixed wheels i.e. non mechanical steering. I will use rear wheels for drive and differential steering. My query is that when I want ...
0
votes
0answers
22 views

Can signal transmission from mobile phones affect battery life of other devices?

Sometimes I hear that you shouldn't keep devices that contain certain batteries close to your mobile phone continuously since signal transmission can affect the lifetime of those batteries. Is this ...
2
votes
2answers
83 views

What is the program of quantum field theory? What is its derivation?

Paraphrasing Griffith's: For some particle of mass m constrained to the x-axis subject to some force $F(x,t)=-∂V/∂x$, the program of classical mechanics is to determine the particle's position at any ...
0
votes
0answers
37 views

Action variables in canonical transformations

Let's suppose we have a Hamiltonian $H(p_k, q_k)$ and we want to transform it via a canonical transformation to one Hamiltonian which doesn't depend on the new coordinates $w_k$, but only in the ...
-2
votes
1answer
30 views

Max weight the lid of a vacuum can hold

I have a box with a lid that has a mass object inside of it. I will be creating a vacuum inside of it with a $4~\mathrm{HP}$ Shop Vac that has a sealed pressure of $59~\mathrm{psi}$ and an airflow of ...
1
vote
0answers
21 views

Optimal axis of rotation of a baseball for lateral movement

Suppose a baseball travels at 90 mph; what is the ideal axis of rotation for lateral movement of the ball? (two-seamer or cutter). So far I think that the following factors play a significant role in ...
0
votes
1answer
47 views

Quantum entanglement of spins

Suppose we have a pair of spins in the state $\left|\Psi\right>=\frac{1}{\sqrt2}(\left|\uparrow\downarrow\right>-\left|\downarrow\uparrow\right>)$. We know that the measurements of the spins ...
5
votes
0answers
25 views

What's the intuition behind the Choi-Jamiolkowski isomorphism?

What is the intuition behind the Choi-Jamiolkowski isomorphism? It says that with every superoperator $\mathbb{E}$ we can associate a state given by a density matrix $$ J(\mathbb{E}) = (\mathbb{E} \...
15
votes
2answers
1k views

What are the odds of finding an electron a long way from an atom? As in metres away?

Pretty self-explanatory, heard that there is a chance that you might find an electron on the other side of the universe, just wanted to know.
13
votes
6answers
2k views

Newton's 3rd law… hitting drywall (which I break) vs hitting a brick (which breaks me)?

According to the Third Newton's law of motion: For every action there is an equal and opposite reaction. So, I understand that if I hit a brick wall with $50\, \mathrm{lbs}$ of force, the ...
1
vote
2answers
28 views

What conditions are necessary for Birkeland currents to form?

If charged particles flow through non-magnetised space will they form Birkeland current? Or can such currents only form along existing magnetic field lines? Part of the reason I ask is this: http://...
0
votes
1answer
18 views

Exact expression for the ground state energy in the 1D Bose-Hubbard model

I heard that Bose-Hubbard model is exactly solvable in 1D. What is then expression for the ground state energy $E_{0}(N, M, ...)$ as a function of total number of particles $N$, number of lattice ...
10
votes
1answer
1k views

If the strong nuclear force is stronger than electrostatic repulsion, why don't nuclei collapse into a point?

Today in class we were discussing the strong nuclear force, and our teacher was explaining about how the strong nuclear force counteracts the repulsion force between protons in a nucleus. When asked ...
1
vote
0answers
23 views

What is the gravitational field inside a hollow spherical cavity embedded in a solid sphere? [duplicate]

Imagine a solid sphere of radius $R$. From the inside of this sphere we remove a smaller sphere of radius $r$. The sphere can be situated anywhere inside the massive sphere. How does the gravitational ...
0
votes
1answer
34 views

Kinematics of Rectilinear Motion [on hold]

A car leaves an outpost and reaches a speed of $60 m/s$ with a constant acceleration of $3 m/s^2$. It continues at this speed for a time then decelerates at $4.2 m/s^2$. How far will the car be after ...
-1
votes
1answer
34 views

what happens to a substance when I bombard it with electrons? [on hold]

What will happen to a gas when I bombard it with an electron beam? Will there be a reaction?

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