# All Questions

3k views

### Proving Ampere's Circuital Law

How to prove Ampere's Circuital Law in case of any conductor. My text gives the proof of only the special case when the conductor is long and straight. I am trying to prove it, but haven't been ...
17 views

### Dirac Matrix property suitable to finding sets of intersecting branes

So, 11 dimensional supergravity has four oft-studied half-BPS states, the KK1 plane wave, the M2 brane, the M5 brane and the KK6 monopole. To figure out if we can find more solutions in the form of ...
85 views

### What really are perturbation expansions?

I'm unsure if this question belongs here or at Math.SE, but since I've got to it by reading some articles about Physics I'm going to post it here anyway. In this particular article (Theoretical ...
433 views

### Why do flat cords tangle less in my pocket?

We've all experienced this: put a pair of earbuds in your pocket, and when they come out, they're a tangled, knotted mess. However, you can also buy versions with flat, ribbon style cables rather ...
170 views

### Decomposing light into frequency spectrum

Light hits a charge coupled element. The wavelength of the light somehow is translated into a color picture. Where can I learn about methods (algorithms) to decompose light hitting a CCD into ...
94 views

### Measuring image distance of a virtual image

How to a measure image distance of a virtual image without calculation? It is impossible? To measure it, we must have a real image to measure the image distance with a screen and a ruler. I saw it ...
103 views

### Yaw angle calculation for a two-wheeled inverted pendulum

[Sorry, if this is a basic question. I'm a software engineer, learning mechanical engineering concepts by trying to build a self-balancing robot] I'm unable to understand the yaw angle equation: ...
94 views

### Manifolds, unit 2-sphere and stereographic projection

I am always passing through this example while reading about manifolds that I don't quite get. It is when describing the unit 2-sphere $S^2$ as an example of a manifold. They say, initially it may ...
76 views

### Quantum probability of entangled spin-1/2 particles?

Let us say that you have two entangled spin-1/2 particles (entangled in such a away that angular momentum is conserved). Let us say particle 1 moves to the left and particle 2 to the right. We measure ...
206 views

### What is the significance of the Fourier coefficients?

Let us take an example, a white ray (which is composed of bunch of frequency components) is passed through a prism, the ray gets split (decomposed) into its elementary vibgyor colours (i.e.different ...
118 views

### Why is this not considered a perpetual motion machine?

You have to assume some wacky things for this scenario, but nothing that defies physics as far as I can tell. I'm going to describe this with a human being initially, because I think it makes the ...
596 views

### Why does sunset over a body of water cause a path of light stretching towards the horizon?

Have you ever notice the sunset's image in the sea? It's like long light path to the end of the horizon! I've attached a sample of this: How can we explain this? I know that it can happen even in ...
93 views

### Trajectories in Rindler space with zero net time dilation

I've discovered a family of curves in Rindler space that have zero net time dilation. However I struggle to see why this should be so, i.e. what the physical significance of these curves is. My ...
269 views

### Real World application of Topological Quantum Field Theory

What is a "killer-app" for the formalism of topological quantum field theory in "established real world physics"? To be more precise, I'm looking for an actual physical experiment, state of matter or ...
106 views

### What is the electric field inside a hollow sphere?

If you have a conducting hollow sphere with a uniform charge on its surface, then will the electric field at every point inside the shell be 0. The reason the electric field is 0 at the center is ...
20 views

### Is there a way to cause scintillation of air particles using an artificial source?

Is there an artificial source that can cause scintillation of air (nitrogen, oxygen, etc. )? What does it emit and what is the range?
946 views

### Number of significant figures

I am looking for an intuitive answer that will explain me why there are only two significant figures in say the number 1500. Also definition from wikipedia: The significant figures of a number ...
111 views

I'm trying to confirm that the $\Gamma^1_{01}$ Christoffel symbol of the FRW metric is $\dot{a}/a$. I have the FRW metric: $$ds^2=-dt^2+a(t)^2\left[\frac{dr^2}{1-kr^2}+r^2(d\theta^2+\sin^2\theta\ ... 2answers 363 views ### How is extremely low frequency (ELF) radiation collected by a submarine antenna? The U.S. Navy Project ELF managed to generate extremely low frequency (ELF) radiation at down to \approx 76 Hz (implying a wavelength of \approx 3,945 km!). I was curious, what kind of receiving ... 2answers 95 views ### Crane Balancing, Center of mass I am working on the ICPC 2014 Problem C "Crane Balancing" The initial idea was to calculate the center of mass of the polygon, which I did via this equation: Where the Area A: Now, the ... 1answer 27 views ### What can I say about a graph depicting orbit a particle has gone through? Acceleration VS friction I have an orbit in which a particle is told to have gone through. There is a straight part, and a curved part. I am asked to mark the right statements, which are: a. Without any further data, there ... 0answers 33 views ### Internal energy of a molecule and bonding strength I had a lecture on thermodynamics. I was told that the bond strength, \epsilon, is related to the latent heat of sublimation, L_s. L_s\approx L_f+L_v if the melting and boiling points are close, ... 1answer 51 views ### Degrees of freedom of a point mass sliding on a rigid curved wire without friction I am very new to the subject and am going through Structure and Interpretation of Classical Mechanics. One exercise asks to find the degrees of freedom of a number of systems, one of which is a ... 1answer 83 views ### Uncertainty principle within a neutron star or black hole Take the time-energy uncertainty relation, \Delta$$E$$\Delta$$T$$\ge$$\hbar/2$. My question is based on my confusion about the effect this relation may have within the interior of a highly ... 1answer 120 views ### Physical interpretation of single layer potential in the plane Let$\Omega\subset\mathbb{R}^2$be a bounded domain with smooth boundary$\partial\Omega$. The single layer potential with charge density$f$sitting on$\partial\Omega$is defined by ... 0answers 43 views ### Does fixing a metric component have anything to do with diffeomorphism invariance? It is well known that in general relativity, the metrics$g_{\mu \nu}$and$g_{\mu \nu} + \epsilon L_\xi g_{\mu \nu}$are physically equivalent, where$L_\xi g_{\mu \nu}$is the Lie derivative of the ... 2answers 80 views ### Speed to power conversion Here it is stated that there is a cubic relationship between fan speed and power consumption (Pfan ∝ FanSpeed^3). I have the fan speed (m/s), but I need to convert it into power (W). How can I compute ... 0answers 44 views ### Find the fraction of atoms in specific quantum state in stellar atmosphere [closed] Consider gas consisting of hydrogen atoms at temperature about$T \sim 5 \cdot 10^6 \text{ K} \approx 431 \text{ eV}$and concentration$N \sim 10^{11} \text{ cm}^{-3}$. I need to find the fraction of ... 5answers 2k views ### Crash course in classical thermodynamics I've been working on some projects lately where it would be very handy to know more about thermodynamics than I do, but sadly I never had a chance to take a proper thermodynamics course in college. ... 5answers 9k views ### Gauge symmetry is not a symmetry? I have read before in one of Seiberg's articles something like, that gauge symmetry is not a symmetry but a redundancy in our description, by introducing fake degrees of freedom to facilitate ... 1answer 81 views ### Cigarette butt sound wave What happens when I extinguish a cigarette in a recipient with some water in it ? It goes like pshhh, but what creates the sound wave ? Is it the water evaporating ? 0answers 42 views ### Are the edge states of a topological insulator only zero energy states? For non-trivial topological insulators, are the edge states only zero energy states? Or are boundary states of different energies also available? 0answers 58 views ### How can we see that a 4D N = 2 sigma model will yield a 3D N = 4 sigma model when compactified on a circle? I have a question about sigma models in 3D. If we have$\mathcal{N}=2$field theory on$\mathbb{R}^4$and compactify it on$\mathbb{R}^3 \times S^1_R$(in which$S^1_R$is a circle of radius$R$) we ... 1answer 39 views ### Fourier transform of a set of L fermions operators I have a set of L fermion creation and annihilation operators:$\lbrace{\hat{C}^+_1,...,\hat{C}^+_L\rbrace}$and$\lbrace{\hat{C}^-_1,...,\hat{C}^-_L\rbrace}$. Every$\hat{C}^+_l,\hat{C}^-_l$... 1answer 71 views ### Proving independence of the lagrangian on position of a free particle using the euler-lagrange equation I asked a similar question some time back but am trying to work this from another angle. In deriving the lagrangian of a free particle, we use the homogeneity of space to conclude that the lagrangian ... 3answers 778 views ### What is the logic behind the Fahrenheit scale? The Fahrenheit scale is defined by fixed points on the scale. What interests me is the apparent arbitrary chosen numbers in these fix-points. First wikipedia wites from 32 to 212. -and later in the ... 2answers 159 views ### Torque required to rotate panel on one axis I am working on a project that requires solar panel to rotate around its axis, but I am struck with deciding the specs of the Servo Motor that is capable of doing this. Though, the panel hangs on its ... 4answers 985 views ### How does the introduction of living things into a closed system affect the rate of change of entropy? Does the introduction of living things into a closed system increase or decrease the overall rate of change of entropy of a system? 4answers 534 views ### Aren't all collisions elastic at some level? The more I think about it, inelastic collisions produce heat and sound which imply motion at some scale, right? Are inelastic collisions macroscopic events that ignore motion at microscopic levels? ... 2answers 2k views ### How is parity relevant to determining angular momentum? Question: Particle A, whose spin$\mathbf{J}$is less than 2, decays into two identical spin-1/2 particles of type B. What are the allowed values of the orbital angular momentum$\mathbf{L}$, ... 3answers 99 views ### Does Time change over temperature? [duplicate] I am not a physicist, I am just an engineer. But I dared to ask whether the temperature changes the perception of time. Let's consider a particle that "stops" at absolute zero. I was thinking as a ... 2answers 358 views ### Renormalizing QED with on-shell fermions When renormalizing QED, we calculate the 1 loop correction to the fermion-fermion-photon vertex using the diagram,$\hskip2in$When doing the calculation we typically let the photon go off-shell ... 0answers 37 views ### Am I very wrong with my grey body calculations? I'm trying to calculate the maximum heat that an object on the moon can reach with the energy provided from Sunlight. I've got a total power output of the sun per square meter at the distance of the ... 3answers 88 views ### A charge$Q$is placed at a distance$a/2$above the center of a horizontal square surface of edge$a$[closed] A charge$Q$is placed at$a$distance$a/2$above the center of a horizontal square surface of edge$a$, while a second charge$Q$is placed below one of the vertices of square. Find the flux of ... 0answers 41 views ### Phase added on reflection at a beam splitter? If we have light of a particular phase that is indecent on a beam splitter, I assume the transmitted beam undergo's no phase change. But I thought that the reflected beam would undergo a phase change ... 0answers 21 views ### Why does a circle of light form around a wire when a laser is pointed at it? [duplicate] Referencing this video: https://www.youtube.com/watch?v=9Q_4SNbpBfo Why does a circle of light form? I've seen previous Phys.SE posts on this topic relating the answer to reflection, but I have ... 2answers 56 views ### It's All About Earthquake! [closed] All the following informations i've got are from a book.(i want to check that whether the informations are correct or not,that book isnt a science book though) There is a limit to which earth can ... 0answers 89 views ### What is renormalization? [closed] What is renormalization? I would want a rough description before I go and work on it properly (I did a course on QFT and on SM (which was 3rd course in the series) but skipped the 2nd course which ... 2answers 188 views ### What makes quantum decoherence different from dissipation? From my understanding quantum decoherence and dissipation are completely different ways of modelling information loss to the environment. Dissipation can be modeled using the Caldeira-Leggett model ... 0answers 41 views ### Supersymmetry invariants On page 158 of Fields, the supersymmetry algebra is represented in terms of the action on supercoordinates as $$\delta \theta^\alpha = \epsilon^\alpha$$$\$\delta\bar{\theta}^{\dot{\alpha}} = ...

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