# All Questions

Question 1: I believe both questions are in the context of Deep Inelastic Scattering. I've been given the expression for $F_{2}^{ep}(x)=(\frac{2}{3})^2[u(x)+\bar{u}(x)] +(\frac{1}{3})^2[d(x) + ... 1answer 91 views ### Description of charged sphere with Heaviside function in cylindrical coordinates I need to describe density of charge of uniformly charged sphere (radius R, total charge Q, position of centre (0,0,0)) with Dirac delta function and Heaviside step function. The hard part is to ... 0answers 57 views ### Help with a couple of materials science questions My first post on this forum is asking for some assistance in answering some questions. I was hoping as well as being given an answer, you could offer a little explanation so that I can understand - ... 2answers 63 views ### How to calculate the resultant movement of a superposition of harmonic oscillators? If$x_{1}(t) = \cos(\omega t - \frac{\pi}{6})$and$x_{2}(t) = \sin(\omega t)$are two simple harmonic oscillators in the same direction and with the same angular frequency$\omega$, how to ... 1answer 187 views ### Can someone explain diamagnetism in plasma? Diamagnetic plasma has an internal resistance to an outside magnetic field. I thought that a good analogy for this would be electrical resistivity. In high school we learned that different ... 1answer 130 views ### Is there a map which shows all the black holes in our galaxy? I am looking for a map which shows all black holes in the Milky Way. See this exoplanet map for what I mean. If it is not existing (yet). Is there at least an online catalogue which summarizes for ... 0answers 44 views ### Model of scattering in wet things You know that clothes and some surfaces become darker when wet (covered in an earlier question: why wet is dark?), because of the disordered reflections or of the refractive index of water or ... 1answer 91 views ### The Electric Quadrupole I've read the following sentence: "Every electric circuit with two pairs of accessible terminals is called a quadrupole." I was wondering why does it happen that the multipole expansion gives us a ... 1answer 156 views ### Legendre transform for non-local Lagrangians, or Hamiltonian of non-local Lagrangian and their properties This is sort of a multi-part question, mostly dealing with how to treat non-local Hamiltonians and how the corresponding properties of Hamiltonians work in a non-local framework. I proposed an example ... 1answer 260 views ### Force between two finite parallel current carrying wires Remark: This is not a homework question...It is pure out of theoretical interest. I asked this the mathematics-community a couple days ago and got no answer, so I figured I'd try here. Most standard ... 1answer 94 views ### Can sound frequency create more ideal conditions for fire? You may have seen the recent story of a device that engineers claim can extinguish flames using sound frequency. An older article loosely explains the theory behind how this works: Sound travels ... 0answers 25 views ### Do any, all, or no adjustments to standard model change the Weinberg angle or it’s relation to masses or coupling constants? Do any, all, or no adjustments to standard model change the Weinberg angle or it’s relation to masses or coupling constants? If any, which (types of) ones? For the ones that do, what are the relations ... 1answer 73 views ### Experimental evidence for Z boson coupling to right handed fermions I do have a question about electro-weak interactions. I know the Z boson is an admixture of two fields, one that couples only to the left-handed part of the fermions (the neutral field introduced to ... 1answer 21 views ### squared position dependent electric field [closed] Here's an asignment problem that got me puzzled. The thing is that I have to solve a second order non linear differential euqation, the course is an urdergraduate mechanics course. A particle of mass ... 1answer 80 views ### Metric in Lagrangian and the minimum total potential energy principle I was wondering why physical systems "like" to go to the minimum of potential energy and I found this question, that tries to justify the minumum total potential energy principle. I was also reading ... 1answer 71 views ### Torque wrench units My torque wrench has these markings on it: (da. Nm.) and on the line below M. KGS. I can tell from comparing the scale to poundf-foot on the other side that the scale units are kilograms-meter ... 1answer 84 views ### Why is the lyman alpha line asymmetric at high redshift? If someone could explain this to me I'd be very grateful, thanks. 0answers 34 views ### Optics - Photodiodes - minimum photon energy that a photodiode will be sensitive to [closed] I'm foraying into the world of photodiodes, and I'm trying to calculate the minimum photon energy required for the photodiode to be sensitive to. Symbolic expressions desired. Minimum detectable ... 2answers 153 views ### What does the “T” stand for in T-duality? First of all, I am not a physicist. I'm a graduate math student and recently I came across the concept of T-duality. Actually I'm studying generalized complex geometry, which according to this paper ... 1answer 81 views ### Entanglement entropy in (1+1)d field theory with dynamical critical exponent$z>1$It was well known that for (1+1)d CFT(z=1) case, we can use the tool of conformal map to derive the formula of entanglement entropy for a finite interval: S ~$c \log L$. L is the length of the ... 2answers 86 views ### How does Ohm's law relate to transferring energy? Potential difference = Current x Resistance where Resistance is a constant. Walking through a circuit I have 1 battery, a wire and 2 components. I start of with 6 volts at the battery and after the ... 2answers 313 views ### Big Bang…or…Everywhere Stretch? Recently I watched a minute-physics video that suggested that a better name for the beginning of time would be "Everywhere stretch" because there wasn't a space-time singularity that formed where the ... 3answers 1k views ### Why do lines in atomic spectra have thickness? (Bohr's Model) Consider the atomic spectrum (absorption) of hydrogen. The Bohr's model postulates that there are only certain fixed orbits allowed in the atom. An atom will only be excited to a higher orbit, if ... 3answers 192 views ### Propagation speed of photons when taking higher-order QFT corrections into account In our group of experimental physicist who have nothing to do with and know very little about quantum field theory, we recently had a question concerning the propagation speed of photons in vacuum: ... 1answer 86 views ### Hadamard Gate application help I need to prove that if you apply Hadamard gates on the input/output before and after a CNOT gate that you get the same answer as flipping the target and control qbits of a CNOT. So for example I'm ... 1answer 40 views ### How does the photon differentiate between events? As far as I know for a photon that's moving at the speed of light (obviously) time comes to a halt and space contracts to a point, making all travels instant from its perspective. Now this is the part ... 2answers 167 views ### Position space wave function of an electron In Wikipedia I find the wave function of a free particle to be $$\Psi(\vec{r},t) = A\,e^{i(\vec{k}\vec{r}-\omega t)}$$ This is is a plane wave moving in the direction of$\vec{k}$with speed (phase ... 1answer 84 views ### Virtual particles and the scaling effect on valence quarks Inside a proton there are 3 valance quarks. In addition, there is constant creation and annihilation of gluon, quarks and anti-quarks. The number of virtual particles we observe depends on how ... 1answer 52 views ### Why is a substrate needed in growing GaN? Is the substrate a layer on which GaN crystal grows on? Is there a way to grow free-standing GaN? Would the resultant crystal be full of defects? 2answers 149 views ### Einstein's mass-energy equivalence [closed] Einstein's wonder equation$E=m c^2,if we take energy is proportional to the mass, we can put single constant to make that equivalent equation. We all know that ... 0answers 32 views ### is there any molecular transition which emits a photon in certain direction i know molecules having magnetic moment would be aligned in certain direction but do they emit photon in any certain direction when excited? are there any molecules which would emit photon in tthe ... 1answer 182 views ### Is dark matter moving faster than light? [closed] My way of thinking is as follows: Consider a balloon. Let this be blown into a nearly elliptical sphere. Now, the temperature inside the balloon is increased by some means; this leads to the ... 2answers 1k views ### In an electron-positron annihilation, in what direction are the photons released? I read that, in an electron-positron annihilation, at least 2 photons are produced, because of the law of conservation of momentum. my question is: in what direction are those photons released? and ... 0answers 26 views ### Interpretation of Mass Continuity Equation in MHD [duplicate] I'm writing up my final-year dissertation and I'm required to give, as part of the introduction, an analysis of all the equations (and their terms) of which I use. Embarrassingly, whilst of course ... 1answer 108 views ### Electric Dipole force exerted by a charged wire on a dipole Why is force exerted by a charged wire on a dipole given asF = P×(dE(r))/(dr)$where P is the dipole moment? Please explain in simple words and avoid using too much technical derivations.I am a ... 1answer 33 views ### Conceptualising the switch from discrete molecules to the continuum (fluid dynamics) I understand that$\rho(x,t)$represents the density of the small volume surrounding the point$x$. However what have I done with the discrete molecules in the process of averaging? 2answers 444 views ### Before Einstein came up with General Relativity, were there serious reasons to doubt Newton's theory? Before Einstein came up with General Relativity, was there any serious reason to doubt Newton's theory (and its various developments)? I only know about the discrepancy in Mercury's orbit, which may ... 1answer 99 views ### What is the limit for the size of a galaxy? The largest known galaxy is just 100x more massive than Milky Way. I wonder if there is some physical mechanism (some equilibrium) which limits the size of galaxies or if it is just because of limited ... 1answer 83 views ### Normal force, work and conservativity I have searched very much on line, both in this site and elsewhere, but found no proof of whether the normal force is conservative or is not, in general. Clearly, if the force is orthogonal to the ... 2answers 531 views ### Why are the proper time and the proper length not defined in the same frame of reference? I've just read this interesting Wikipedia article about time dilation and length contraction in special relativity. . Derivation of time dilation Applying the above postulates, consider the ... 2answers 59 views ### Relativity asymmetry? If you have two bodies that move towards each other at a constant velocity, each body will observe the other body experience time faster. This does not make sense to me because if you had a person ... 1answer 157 views ### Moment of inertia of cylinder [closed] How to calculate moment of inertia of a cylinder about an axis passing through its geometrical center and inclined at an angle (\theta) with vertical. 2answers 65 views ### Expanding universe and speed of light [duplicate] Galaxies are moving away from us proportional to the distance between us and them , but nothing can travel faster than light, so even the farmost galaxies should be travelling away from us along with ... 0answers 81 views ### How is a particle observed in the double slit experiment? I'm having some trouble understanding what is causing so much confusion what this experiment is concerned. As I understand it it is thought that the wave interferes with itself when it's observed, ... 5answers 6k views ### If the Earth is a good conductor of electricity, why don't people get electrocuted every time they touch the Earth? Since the Earth is a good conductor of electricity, is it safe to assume that any charge that flows down to the Earth must be redistributed into the Earth in and along all directions? Does this also ... 0answers 63 views ### Gears in contact? I was doing a practice exam paper question that was along the following lines: A gear,$A$,and moment of inertia$I_A$is spinning about its axis at angular velocity$\omega$. Another gear$B$... 2answers 54 views ### Conservation of Sea water Vs Conservation of Matter I have been listening to people, since my childhood, saying that amount of water in Sea always remains constant even if we draw out much from it, it never changes. They used to emphasize by saying ... 1answer 156 views ### Do Sidebands mean the frequency of an AM radio wave is not constant? I'm studying for A-level now..... I have read some other posts explaining how the sidebands generated after the carrier wave is modulated. So, if there is sideband frequency, the frequency isn't ... 1answer 173 views ### Finding the acceleration of a mass/pulley system without knowing the direction of the displacement I have the following question: I noticed that if I solve for the acceleration assuming that the acceleration is occuring to the left, I get the equation:$\displaystyle a = ...
$$D^0 \to K^+ +\pi^- + \pi^0 + \pi^0$$ cannot occur by first order weak interactions. Could anybody help me confirm this? What I currently have is that the initial charm goes to anti-strange and the ...