1
vote
1answer
72 views

Delta wing flow patterns (straight and yawing)

I was wondering the differences between fluid patterns and their consequences when a delta wing is straight and yawed slightly.
3
votes
2answers
173 views

Can we deviate a bullet from its target? Did anybody try this?

Can we deviate a bullet from its target? Means by applying any strong field at the target? Is there any technique so far like this!
1
vote
1answer
80 views

What are the “other” Hadamard matrices?

The Pauli matrices $$ X = \begin{pmatrix}0&1\\1&0\end{pmatrix}, Y=\begin{pmatrix}0&-i\\i &0\end{pmatrix},\,\text{and}\, Z=\begin{pmatrix}1&0\\0&-1\end{pmatrix} $$ can be used ...
0
votes
0answers
102 views

What does traveling close to speed of light mean?

This has been bugging me for a while. We all know in relativity that if you travel close to speed of light, all sorts of crazy things happen. But what does it mean to travel close to speed of light? ...
0
votes
1answer
209 views

2 spheres both negative or positive [closed]

Two small charged metal spheres A and B are situated in a vacuum. The distance between the centres of the spheres is 12.0 cm, as shown in Fig. 4.1. The charge on each sphere may be assumed to be a ...
4
votes
2answers
101 views

Are the laws of physics bound by the same performance bounds algorithms are bound by?

Consider the problem of finding the convex hull for $n$ points on a plane. No constant time algorithms exist to solve it. Now say you represent the points as poles of equal height that you hammer ...
6
votes
3answers
2k views

Intuition on positive-operator valued measures (POVM)

I'm having a little trouble understanding what positive-operator valued measure (POVM) are- in particular why/how they are non-negative. For instance, if they just represent measurements, what about ...
1
vote
3answers
144 views

Uses of Static Electricity

Is it possible to store static electricity in any type of battery and can they be used in electric appliances(light,bulbs mainly)?
1
vote
3answers
200 views

Is a gapless system always conducting and a gapped system insulating?

In an answer to this question, @user566 mentioned that there is a qualitative difference between gapped and gapless systems; that gapless systems are conducting and gapped system are insulating. Is ...
1
vote
0answers
75 views

Solving for the Nernst Potential in 3D

Hi guys I'm totally stuck on the following question: We know the ionic flux between two chambers due to uneven concentration can be computed by the Nernst-Planck equation $$j = -D_p \left(\nabla C ...
30
votes
1answer
3k views

Are all machines linearly scalable?

For all machines (cars, elevators, computers, etc), when size, power requirements, dimensions are scaled by a constant N, will it work just as is? Will a car with all its parts 10x larger still work ...
3
votes
2answers
341 views

How can water be liquid and gas in the same environment?

Think about it. If there's water in a gaseous form, how can there also be water in a liquid form? The gaseous water would have to have more energy than the liquid, but if it's the same temperature, ...
1
vote
1answer
44 views

Does blowing bubbles at the front of a ship increase efficiencies?

Would this make an air envelope that the hull could smooth past with less friction?
1
vote
0answers
245 views

What is this light pattern caused by LED matrix street light?

At our office, they have installed new downlights in the parking lot (street lights) based on a rectangular matrix of very bright LEDs, which are not diffused. They look something like this one: ...
1
vote
1answer
100 views

What is the purpose of the metal strip??

Context: Two large, flat metal plates are held parallel to each other and separated by a distance $d$. They are connected together at their edge by a metal strip. A thin plastic sheet ...
1
vote
0answers
27 views

Is there a charge across all space? [duplicate]

We're studying electrostatics in class, and the teacher introduced us to Gauss' Law a few days ago as $$\int \vec{E} \cdot \mathrm{d}\vec{A} = \frac{Q}{\epsilon_0}$$ Now suppose that the entire ...
1
vote
1answer
245 views

How to find the Hamiltonian density for electromagnetic field? And, how to solve the stress tensor for electromagnetic field? [closed]

How to find the Hamiltonian density for electromagnetic field? And, how to solve the stress tensor for electromagnetic field?
5
votes
5answers
1k views

How does an object's motion behave if dropped from an aeroplane travelling diagonally upwards?

Imagine an aeroplane travelling with velocity $v$ at some angle $\alpha$ from East to North. A box is dropped from the aeroplane. What would the projectile of the box be? Would it be a parabola with ...
0
votes
3answers
907 views

What could be added to tap water to reduce air bubble formation on a submerged object?

Submerge your hand in a bucket of water. Some air bubbles appear immediately and others add to the surface of the hand over time. Is there anything that could be added to the water to reduce the ...
0
votes
1answer
111 views

Two uniformly charged spheres are superposed with slight displacement. What's the surface density?

*Note: This is from the Second Volume of Feynman's Lectures on Physics : Mainly Electromagnetism and Matter And this is the excerpt from the book: If the relative displacement of the two spheres ...
1
vote
0answers
60 views

Example of materials with 21 independant coefficients in linear elasticity?

By definition of linear elasticity, the strain et stress tensors are related: \begin{equation} \boldsymbol{\sigma}=\mathbf{C}:\boldsymbol{\varepsilon} \end{equation} and because of minor and major ...
11
votes
3answers
1k views

Changing Rotation Direction in Mid Air

Not sure whether this is an appropriate question for this site, but could anyone explain the physics behind how this skier is able to change his direction of rotation mid-air? https://www.youtube.com/...
2
votes
1answer
50 views

Hydro regime of strongly coupled field theory, low viscosity

I am trying to get an intuition for the following argument: In heavy ion collisions the central collision area can be described by almost ideal hydrodynamics at very early times after the impact. This ...
0
votes
2answers
183 views

Kinetic energy of an electron in synchrotron

The question states as follows: An electron is accelerated in a LINAC (LINear Accelerator) so that it's kinetic energy is 1.00 GeV. Then it is transferred to a synchrotron where it's energy (didn't ...
0
votes
1answer
531 views

What is the net force acting on a climbing man by the rope?

I am confused with determining the net force on the climbing man below by the rope. My analysis is as follows. Let the tension be $T$. As both hands hold the rope then there are $T+T=2T$ upward acting ...
5
votes
1answer
151 views

Two expressions for topological instanton number

I have begun to study instantons and I have the following difficulty: $\newcommand{tr}{\operatorname{Tr}}$ I am considering theory with $SU(2)$ gauge group: $S=\frac{1}{2g^{2}}\int \tr F_{\mu\nu}^{...
1
vote
1answer
78 views

Show that $\mathbf{A}$ is a valid vector potential [closed]

Is $$\mathbf{A} = -\frac{1}{2}\mathbf{r \times B}$$ a valid vector potential in the Coulomb gauge? Here's my work so far. Using the identity for the curl of a cross product I get $$-2\mathbf{\...
0
votes
0answers
26 views

Magnetic field of a living human [duplicate]

Is it true that a human body is like a bar magnet? Say our head is north pole while the legs are south pole, or vice versa? If so, can we confirm it by experiment say with the 360 Pro compass etc?
3
votes
4answers
230 views

Apparent Contradiction with Faraday's Law

Say we have a non-uniform magnetic field that is static in time. Specifically, let's make it: $\overrightarrow{B}(x,y,z) = x \hat{z} $ Now say we have a metal loop in the xy plane which has a non-...
1
vote
1answer
124 views

if light is electromagnetic then can light produce electricity or attract metals

what I mean to say is that if light is electromagnetic in nature then shouldn't it show electric or magnetic properties on matters? Like if light falls on a metal it should produce current due to ...
6
votes
2answers
621 views

Violation of the second law of thermodynamics? [closed]

Here is a machine which seems to violate the second law of thermodynamics: $A$ and $B$ are point black bodies of the same temperature (initially). everything is rotationally symmetric around the ...
2
votes
0answers
93 views

Hysteresis in liquid–solid-phase transitions such as Agar

I'm wondering how it is possible for a substance to have a significantly different melting point than its freezing point. What physical interaction "locks" a substance such as Agar into the phase that ...
2
votes
1answer
308 views

What are the 'types' of parametric down conversion?

I'm looking at photon entanglement, and everywhere in the literature there's a reference to 'type-II' parametric down conversion as a source of entangled photon pairs. I know what parametric down ...
4
votes
1answer
204 views

Why does S-matrix unitarity imply the cross section $\sigma$ $\propto$ $\frac {1}{s}$?

I'm currently learning for an oral exam in theoretical physics and as a learning aid protocols of older exams exist. In one protocol the question was asked: Why is the scattering cross section $\...
1
vote
1answer
504 views

Relative Minus signs from different Feynman Diagrams

I have a problem understanding the occurrence of a the relative minus signs between contributions, coming from different Feynman diagrams, involving fermions. A simple example is Bhabha scattering $e^+...
0
votes
2answers
735 views

Particle in a Box: Energy Less than the Potential Energy

I am reading quantum mechanics from Shankar's Principles of Quantum Mechanics. On page 157 he defines the box potential $V(x)$ as $$ V(x) = \left\{ \begin{array}{rl} 0 &\mbox{ if $|x|< L/2$} \\...
0
votes
0answers
50 views

Relative time/space - Two clock thought experiment [duplicate]

Please pardon my english. Here is my simple thought experiment. ...
0
votes
1answer
83 views

Hydrogen bond and van der Waals bond

Just now I am studying about hydrogen bond and van der waals bond. I understand and able to differentiate between them but thinking of the similarities, do they have any?
1
vote
1answer
100 views

Determine eigeinvalue and eigenvector of two operators R and L [closed]

Question: Let H be a Hilbert space with countable-infinite orthonormal basis ${|n>}_{n \in N}$. The two operators R and L on H are defined by their action on the basis elements \begin{align} R|n\...
0
votes
1answer
87 views

How can an electron be fired at a target when uncertainty principle says it will spread out around axis of motion?

Consider an electron fired at a target. Taking the axis of motion to be $x$, and position to be $(x,y,z)$ then $\Delta y = \Delta z = 0$ Therefore by the uncertainty principle $\Delta p_y = \...
3
votes
1answer
3k views

Instantaneous axis of rotation and rolling cone motion

Suppose a cone is purely rolling (no slipping) around a fixed axis. I mean, it is revolving around a fixed axis perpendicular to the ground and passing through its vertex and also rotating, so the ...
0
votes
2answers
86 views

Newtonian potential defines size of Universe?

As I understand it the Newtonian potential $\Phi$ at the position of an observer in space is approximately given by: $$\Phi \sim \frac{G M}{R},$$ where $R$ is the radius of a sphere centered at the ...
0
votes
1answer
136 views

Rotation/torque about a point mass on two linked rods

I have two rods with lengths $L_1$ and $L_2$, connected with a joint in the middle and I would like to express the moment(torque) around a point mass (P) at the far end of rod 1. The relation between ...
3
votes
2answers
516 views

How can there be current if all paths have same potential difference

I am learning about circuit analysis now, but there is something that I can't wrap my head around. Imagine this simple circuit: ...
14
votes
7answers
2k views

How can energy be useful when it is 'abstract'?

The topic which haunted me for two years until I gave up on it. But now I am doing engineering and this topic suddenly popped out of my textbook from nowhere. I seriously need to understand this topic ...
-1
votes
2answers
554 views

Why in series combination of circuit has same amount of current flow in every conductor? [duplicate]

Why in series combination of circuit has same amount of current flow in every conductor? Whereas in parallel combination the amount of voltage is same?
3
votes
0answers
124 views

Clarification about Bragg's law explanation

The Wikipedia has this illustration of Bragg's law and then says The two separate waves will arrive at a point with the same phase, and hence undergo constructive interference, if and only if ...
0
votes
0answers
123 views

Standing waves on a compound string

I've been having some difficulties in understanding standing waves. The questions that I have are as follows: 1) Suppose I have a compound string, made up of two strings. What is the correct way to ...
0
votes
0answers
249 views

Would a portable superconducting coil “battery” be impractical due to magnetism?

I'm working on a science fiction story idea, and trying to make sure I get all of the physics right. One of the concepts I'm using is superconducting coils as "batteries," because they can store huge ...
2
votes
2answers
156 views

Why does Fermi level has a probability density of 1/2 while it may lie in the forbidden region?

I dont understand how there is a continuous probability density function in semiconductors, when there are several regions which are restricted by Energy, i.e. forbidden energies. Well i know that in ...

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