1
vote
3answers
282 views

Variation - Electric pressure on a sphere?

I solved the following question(Answer is correct): Find the force with which two hemisperical parts of a uniformly charged hollow sphere repel each other?(charge density: +$\sigma$) Answer: ...
1
vote
2answers
142 views

Quantum computing and ambiguity

I do a bit of hobby programming and I often search the internet for little oddities that are fun to ponder over. I have read a few passages that try to explain quantum computing to the layman like ...
0
votes
1answer
42 views

How to obtain ballistic transport in a device?

I would like to know the ballistic device characteristics and illustrations (if any) to understand how can we overcome the dependence of current on carrier mobility .
2
votes
2answers
9k views

What if an asteroid the size of the moon hits earth?

Would we all perish due to excessive heat? Or would that be limited to the area near the impact while the people on the rest of the earth would die from other phenomena such as mega earthquakes, ...
5
votes
3answers
404 views

Normal modes of the 2D double pendulum

I'm performing an experiment with a 2D double pendulum, and in part of it I want to investigate the normal modes of the double pendulum, where the pendula are not of equal length or of equal mass. My ...
1
vote
0answers
37 views

Why does proton and electron attract each other? [duplicate]

Not that their charges are opposite! "+" & "-" are the signs we named them. By nature Why do they attract each other? And Why do they repel each other?
3
votes
1answer
121 views

How do we know what type of gauge field to add to a theory?

I've been watching Leonard Susskind's particle physics lectures and in one lecture, he discusses a very simple gauge theory. We have a complex scalar field $\phi(x)$ with Lagrangian $$\mathscr{L} = ...
2
votes
4answers
631 views

Why would the apple fall under the tree?

Here is an extremely naive question: Why would the apple fall under the tree? I am puzzled by this, because the conventional answer that the gravity between the apple and the earth pulling apple down ...
0
votes
0answers
56 views

How can I describe an equation for multiple objects over time?

Lets say i have $n$ different objects that effects each other only by the classic gravity force. I have their initial locations, masses and velocity's: $$ x_1(0),\cdots,x_n(0) $$ $$ m_1,\cdots,m_n ...
0
votes
0answers
38 views

Mixed-Alloy materials

Short version: Is it a physical problem (crystal structure/grains/redox/etc.) or just a logistics problem (keeping the solutes from homogenizing, molten/solid/temperature related problems) that keeps ...
2
votes
2answers
77 views

Why do rotations of a multicomponent state function take this form?

I am reading Leslie Ballentine's Quantum Mechanics, section 7.2, which is all about the explicit form of the Angular Momentum operators. I understand how he gets the form for the single component ...
3
votes
1answer
264 views

Light Deflection by a Black Hole

I think I've come across a paradox while studying general relativity. Wikipedia states that the deflection angle of light by a point mass is $4GM/(c^2b)$. ...
3
votes
1answer
134 views

Why do we do partial and not covariant differentiation with $x^{\nu}$?

Why when taking the velocity vector we make $$u^{\nu}=\frac{d}{d\tau}x^{\nu}$$ and not $$u^{\nu}=\frac{\nabla}{d\tau}x^{\nu}$$ where in the last equation I meant the covariant derivative. Why?
1
vote
2answers
83 views

How should semiconductors be understood? [duplicate]

In the chemistry setting, a doped semiconductor is a crystal lattice with holes/extra electrons in it. In the band/quantum mechanical picture, these holes/extra electrons can be seen as the amount ...
4
votes
2answers
268 views

Huygen's principle in curved spacetimes

Does Huygen's principle hold in even dimensional (2m+1,1) curved spacetimes, or are there certain necessary conditions for it to hold? In other words, if I have Cauchy data for a field satisfying the ...
2
votes
2answers
191 views

Confused about the theoretical origin of quadratic air drag [duplicate]

Though the mathematical concepts underlying quadratic air drag are quite straightforward (a single variable differential, just like the linear drag equation), my text book (and online text books) ...
2
votes
4answers
173 views

Turning a car on a perfect surface

Say you have a car with perfect wheels and a perfect surface with so much friction that it never skids. When the car is traveling forward, the wheels INSTANTANEOUSLY turn 90 degrees. Does the car ...
9
votes
1answer
229 views

What are the implications for the AdS/Cft program if AdS is unstable?

To my understanding recent progress in the study of the non linear stability of AdS spacetime suggest that $AdS$ might be unstable. If this is true, what are the physical and mathematical ...
1
vote
1answer
175 views

GR exercise: falling particles on earth's surface

I'm having some trouble with Exercise 5.1 in Shapiro's BH,WD&NS book, which goes as follows: Consider two particles of mass $m$ at distance $r$ and $r+h$, such that $h\ll r$, on the same ...
6
votes
0answers
110 views

target category of extended field theory

For a topological field theory to be a true “extension” of an Atiyah-Segal theory, the top two levels of its target (ie its $(n-1)^{\text{st}}$ loop space) must look like $\text{Vect}$. What other ...
3
votes
0answers
62 views

How obtain conserved quantities in integrable models in accordance with Liouville's theorem, via Sklyanin Poisson algebra?

In classical integrable models, in the discrete case we have the Sklyanin algebra, $$\lbrace T_{a}(u),T_{b}(v)\rbrace =[r_{ab}(u,v),T_{a}(u)T_{b}(v)].$$ How to prove that the conserved quantities are ...
0
votes
0answers
33 views

Why do physical bodies in the universe follow the law of physics ( or any rule/pattern ) ? [duplicate]

Why do physical bodies in universe are bound to follow a rule ( eg. laws of physics ) ? Is their any imaginary condition possible, where they stop following any rule. And everything becomes random ? ( ...
2
votes
1answer
88 views

Adding matter to make galaxies bigger

What are the actual factors that play a role in the accretion of matter into galaxies? I read about Accretion Disks but I don't quite understand how they work yet.
7
votes
5answers
2k views

Forces other than the fundamental interactions, e.g. friction

Forgive me for the silly question, but I just don't get it. I just completed an elementary course in mechanics, and I am curious to know what I am about to ask. We have, all year, dealt with many ...
1
vote
2answers
296 views

Diffusion and Drift currents in a PN junction

In a forward-biased PN junction, the potential barrier decreases, allowing more majority carriers from one side to diffuse to the other side where they are minority carriers. After they cross the ...
4
votes
2answers
322 views

Can Minkowski spacetime be redefined as a non-flat riemannian manifold?

Minkowski space time is defined in terms of a flat pseudo-Riemannian manifold. I have wondered if it can be redefined as Riamannian manifold and in the case what type of curvature would there appear. ...
3
votes
1answer
82 views

Normalising a wave function in parts?

If we have the wave function $\psi_{100}(r,\theta,\phi)=R_{10}(r)Y_{00}(\theta,\phi)$ when we are normalising it we do the following: $$1=\int| \psi_{100}(r,\theta,\phi)|^2sin(\theta) r^2drd\theta ...
0
votes
1answer
240 views

Transformer leakage reactance

In practical transformers there is always a leakage reactance. If it is possible to construct such a transformer (ideally thinking) having zero leakage reactance will the transformer action work? or ...
1
vote
0answers
80 views

Constructing conserved current given the lagrangian

Consider the following Lagrangian for a massive vector field $A_{\mu}$ in Euclidean space time: $$\mathcal L = \frac{1}{4} F^{\alpha \beta}F_{\alpha \beta} + \frac{1}{2}m^2 A^{\alpha}A_{\alpha}$$ ...
0
votes
1answer
67 views

Magnetic Induction

Consider the figure above in which there is an electric current (constant) in R1 (consider it to be made of a conductor). Obviously there will be a magnetic field induced in R2 (consider it to be ...
4
votes
3answers
803 views

Definition of a year

Is there an acceptable definition of a year (in number of days)? Google Calculator: https://www.google.com/search?q=seconds+in+1+year returns 3.15569e7 seconds and then ...
1
vote
0answers
172 views

Link between Quantum and Classical Mechanics [duplicate]

In classical mechanics we have momentum as generator of translation by following definition: $$f(x+\delta x)=f(x)+[f(x),p]\delta x+....$$ I was wondering whether using this relation and commutation ...
0
votes
2answers
1k views

Find the max projection angle so that distance from point of projection always increases

We want to launch a projectile from the surface of earth so that its distance from the point of projection is always increasing. What is the maximum angle of projection for which this is possible? ...
1
vote
1answer
71 views

“Dictionary” between QFTs in D and D-1 dimensions?

Consider the Einstein field equations. Suppose, for instance, that the RHS, the stress-energy tensor, is uniquely determined by the electromagnetic field. Now, if we imagine a quantized version of ...
3
votes
2answers
2k views

How to measure Entropy?

While $$\delta Q=T\ dS$$ allows for obtaining the entropy change of an isolated system in equilibrium by measuring the heat exchange with the environment, I was wondering whether there are ways to ...
1
vote
0answers
101 views

Determining the attraction of a magnetic bob

I have a question concerning the calculation of the attraction to a ferromagnetic material in a coupled B-field, which of $N \geq3$ equally strong cylindrical permanent magnet is produced. ...
1
vote
2answers
142 views

Work and Kinetic energy rely on each other which came first?

In all the places I've looked kinetic energy is derived from the definition of work, but I don't understand how the definition of work was developed with out the concept of kinetic energy. My question ...
2
votes
3answers
2k views

Why is Avogadro's law always true?

Why is Avogadro's law always true? How and why do equal volumes of gases at equal pressure and temperature contain equal number of molecules? I know it is a fundamental principle in chemistry but I ...
0
votes
2answers
59 views

Increasing resistance, by heat?

I was wondering the magnitude of effect to a conductor's resistance when heated by a few degrees. Can this be controlled way of changing conductor's resistance, or fixing in a system?
1
vote
0answers
49 views

Books on foundations of QM [duplicate]

I am seeking for books on foundations of Quantum Mechanics with subjects like the EPR experiment, Bell's theorem, the problem of measurement, entanglement, decoherence, nonlocality, interpretations, ...
0
votes
3answers
95 views

Black Hole gravity pull [duplicate]

Please I want to Know that if the radius of the black hole = 0 then how it have its surface and how it absorbs things? $$\begin{align} g &= \frac{Gm}{0^2} \\ &= \infty \end{align}$$ then ...
7
votes
1answer
1k views

Do we breathe air by creating a vacuum?

How do we suck air into our lungs, are we generating a vacuum?
5
votes
5answers
2k views

Electron shells in atoms: What causes them to exist as they do?

I have seen similar posts, but I haven't seen what seems to be a clear and direct answer. Why do only a certain number of electrons occupy each shell? Why are the shells arranged in certain distances ...
0
votes
1answer
144 views

I need to know how many pounds of force a 3 million pound object has moving at 1 mph [closed]

How many pounds of force does a 3 million pound object have moving at 0.1, 0.5, and 1 mph?
1
vote
1answer
820 views

Amplitude decrease during geometrical spreading of a seismic pulse

I'm looking at amplitude decrease of a seismic pulse as a result of geometrical spreading. Starting with the energy contained in a unit area, $I = E / (4 \pi r^2)$, where $E$ is the original energy ...
0
votes
1answer
94 views

Spaghettification while still in orbit

THIS POST does a pretty good job of giving an idea as to what the differences in forces becomes as you approach a black hole. For a small enough black hole, you would reach the dangerous tidal force ...
2
votes
3answers
441 views

What does the $y$-axis represent in the atomic spectra and what is its significance?

The picture is an emission spectrum of Helium. The spectrum has sharp lines (peaks) at certain wave lengths characterizing it as helium. Agreed that it characterizes Helium as atomic spectral line ...
2
votes
1answer
304 views

Can the compactified dimensions of M-Theory/String Theory become uncurled?

Is it possible for the curled dimensions described in superstring theories to become uncurled and open up. I have read that the big bang could have been the uncurling over 3 dimensions through ...
0
votes
0answers
61 views

Metric convention conversions between expressions

I'm sure this has caused many people headaches. First, is there a metric $(-+++)\leftrightarrow(+---)$ convention conversion chart where many common expressions are listed? Thank you in advance for ...
1
vote
2answers
80 views

Kinetic Energy of a Particle [closed]

Consider a particle of mass $m$ in $6$ dimension. Its coordinate w.r.t. origin $\left(0,0,0,0,0,0\right)$ is given as $\left(x,y,z,\dot{x},\dot{y},\dot{z}\right)$. If we denote $r = ...

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