# All Questions

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How would you go about showing that in the quantum canonical ensemble (that is, in the density matrix and operator formulation), the energy fluctuations, namely $\langle H^2\rangle - \langle H\rangle^... 1answer 587 views ### Neutral shell(with a charge inside) in an electric field A positive point charge$Q$is kept eccentrically inside a neutral conducting shell. An external uniform field E is applied. Then: a) Force on Q due to E is zero b) Net force on Q is zero ... 0answers 62 views ### What is the Levi--Civita connection of a Wick rotated metric? A Wick rotation is a transformation that allows to change from a Lorentzian manifold to a Riemaniann manifold. In the cases when this is possible, is the Levi-Civita connection of the Riemaniann ... 2answers 243 views ### Is a Perfect/Lossless Mirror possible? In traditional mirrors, some of the input light is absorbed by atoms in the mirrors surface and are 'lost' as heat, degrading the quality of the reflected image. Could this loss be compensated by an ... 4answers 1k views ### Why doesn't more light bounce off of things in the manner of sound? If I'm sitting in the den with my door slightly cracked, I can hear my wife washing dishes in the kitchen down the hall. But why can't I also 'see' images of her washing dishes if, say, I looked up on ... 1answer 1k views ### Feynman graphs of Compton scattering Compton scattering is usually described two Feynman graphs (in the second-order perturbative expansion of scattering matrix) that can be described in the following way: annihilation of a photon-... 2answers 10k views ### What is the difference between Biot-Savart law and Ampere's law? What is the difference between these laws? Which law is more useful? When to use Ampere's law and when to use Biot-Savart law? 1answer 828 views ### Ampère's circuital law in differential form I'm having trouble understanding how certain conclusions are made in the explanation of Ampère's circuital law. Here's part of what's in my book: Consider a magnetic field with induction$\vec B$. ... 1answer 368 views ### Casimir forces and its associated Feynman propagator This is a continuation to my previous question, in which I began an attempt solve the Casimir Force problem using path integrals. As one of the answers there suggest I solve the Feynman propagator ... 1answer 69 views ### String vibration In many textbooks an illustration of a vibrating string at a fundamental mode shown (and wikipedia) shown like this http://en.wikipedia.org/wiki/File:Standing_waves_on_a_string.gif). However if you ... 3answers 780 views ### Why didn't we replace our SI units with a better system? [closed] Intro It seems to me that the SI units we use today are nothing but the result of a historical 'coincidence'. I recently began researching about natural (absolute) systems of units, which are ... 1answer 388 views ### Definition of metre We know that 1 meter is the distance travelled by light in vacuum within a time interval of 1/299,792,458 second. My question is why we didn't take a simpler number like 1/300,000.000 or why not just ... 2answers 326 views ### Having trouble understanding the concept of current and Ohms law Circuit in series:$10\,\text{V}$power supply, 2 resistors connected. Each resistor is$10\,\Omega$, total of$20\,\Omega$resistance. If I put an ammeter at the end of the circuit, will the current ... 0answers 234 views ### About the proof of the second Bianchi Identity The second Bianchi Identity is $$\nabla_{[a}R_{bc]de}=0$$ As far as I know, the proof (say, Walfram Mathword) start by stating the representation of Riemann tensor in local inertial coordinates $$... 2answers 176 views ### Understanding emergent phenomena in the block universe. (Reworded question) Each person exists as an unchanging 4D worldtube in the block universe. At each slice of the worldtube there is a present, past and future. However, there is a black box* which appears to exist in ... 0answers 413 views ### Leonard Susskind's videos. Which order? [closed] As a mathematics student with almost no modern physics background (just an introduction to relativity when I was in secondary school) I find Leonard Susskind's lectures videos (freely available in ... 1answer 1k views ### Why does a water drop on a hot plate at 150°C evaporate faster than on a plate at 200°C? I recently read that: A drop of water landing on a hot plate at 150°C (300°F) evaporates in a few seconds. A drop of water landing on a hot plate at 200°C (400°F) survives a whole minute. How ... 2answers 57 views ### Significant error conversion So here is my question: Say we have measured something to be 15,67 mm and the significant error is \pm 0,01mm. then we convert the measurement to meter to be 0,01567m would the significant error ... 1answer 59 views ### Energy in nuclear decays After a nuclear decay is it a necessity that the total energy of the products is more than the energy of the original particle before decaying? (NB: by 'energy' I don't intend to include mass-energy ... 1answer 110 views ### Ohms law and multiple resistors In my physics class, we did an experiment in which we supported Ohm's law. We used a simple circuit that includes one resistor. The relationship of I = VR, and variants thereof, became apparent. We ... 2answers 826 views ### Balloon, lighter than air and vacuum? As I understand, in a balloon/air balloon, a gas burner is used to heat air or using some lighter atom like helium. Since helium or hot air is lighter and less dense than the cool air around the ... 2answers 277 views ### Confused about the concept of time and time dilation [duplicate] I am having a hard time understanding what is time. If scientists define time as a multiple of caesium frequency, then time itself is dependent on motion, so what if I have a number of particles that ... 1answer 1k views ### What are hot electrons? What are they? How are they created? And what do they have to do with plasmons? I searched the web, but I would like more reliable and straightforward sources. 1answer 2k views ### A ball is thrown up, and a second ball is dropped- what is the maximum initial velocity of the first ball so that [closed] This question only exists in one dimension, the y axis. A ball is thrown upwards with an initial velocity V_o from a roof with height h. One second later, a second ball is dropped from the same ... 1answer 208 views ### Help w/ speed of sound experiment report question Experiment was done by using an oscilloscope and a piezoelectric transducer to generate ultrasonic sound waves. We had to move the transmitter whilst receiver remains constant on a angled 1 meter ... 0answers 69 views ### Super-String and Anti-Particle I'm afraid this is quite basic question . What I learned from the String theory is that the theory unify the Path-integral, Superposition Principle, and commutation relation . In that context , I ... 1answer 75 views ### Unit determinant for relevant symmetry groups in QFT When treating QFT we want our theory to be invariant under different symmetry groups, for example, the Standard Model is a non-abelian gauge theory with the symmetry group U(1)×SU(2)×SU(3). Moreover,... 1answer 102 views ### moment of inertia of a ring about an axis at 45° to the normal [closed] I wanted to calculate the moment of inertia of a ring about an axis at 45° to its normal outside the plane of the ring . How do i calculate without using integration? I was thinking about using ... 1answer 2k views ### What is the exact cause of flow separation in a viscous fluid? I recently got into a lengthy debate about the exact nature of boundary layer separation. In common parlance, we have a tendency to talk about certain geometries as being too "sharp" for a viscous ... 2answers 407 views ### Intuition Behind Conservation of Angular Momentum I'm having a fairly hard time understanding the intuition behind Noether's derivation of the conservation of angular momentum from the rotational invariance of the Lagrangian, though I do understand ... 1answer 393 views ### Minkowski metric — why does it follow from the constancy of the speed of light [duplicate] In all the sources I’ve been able to find, the Minkowski metric appears ad hoc, or is defined analogously to the euclidean metric. I’d love to see an argument why this metric (time coordinates ... 2answers 434 views ### When can we add a total time derivative of f(q, \dot{q}, t) to a Lagrangian? The other day, I was listening to this lecture on the Lagrangian for a charged particle in an electromagnetic field, and at one point in the video, the lecturer mentions that we can add any total time ... 0answers 57 views ### Can someone explain to me the Rocksar-Kivelson Hamiltonian? The following paper shows the hamiltonian of the 2D quantum dimer gas (page 2) http://www-thphys.physics.ox.ac.uk/people/ClaudioCastelnovo/Talks/050209_MIT.pdf Here are some questions I have. Why ... 2answers 225 views ### First law of thermodynamics applied to atmospheric fluid I was reading an article that talks on a climate prediction model. This article proposes the following equation (energy balance):$$\rho_0 c_v\left( \frac{\partial T}{\partial t} + \sum^{3}_{j=1} ... 3answers 1k views ### Where's the missing helium in the Universe? I'm confused: Big Bang nucleosynthesis is adamant about the 1 neutron to 7 proton ratio which yields 75% hydrogen to 25% helium (with a nominal amount of partially-reacted deuterium and heavier ... 1answer 117 views ### Toroid moments tensor decomposition I am currently working on my bachelor's thesis on the anapole / toroidal moment and it seems that I am stuck with a tensor decomposition problem. I have actually never had a course about tensors, so ... 0answers 45 views ### Subnuclear physics vs wave function This question is more a philosophical question than a physics one. When we appreciate particle physics we study that in order to explain some experimental results we have to introduce a new particle (... 1answer 234 views ### Why are popcorn seeds soft after popping? When a seed of popcorn is heated up in oil, it pops like this: You can take one of these popped pieces and eat it with little to no problem. However, if you get an un-popped seed and sink your ... 1answer 375 views ### Nonzero ground state energy of the quantum harmonic oscillator [duplicate] Since$\frac{1}{2}\hbar \omega$is the zero point energy of the ground state of the harmonic oscillator, then there is no way to extract this energy. Therefore, in what way is this value different ... 0answers 107 views ### Size of an image formed by a concave mirror A short linear object of length$b$lies along the axis of a concave mirror of focal length$f$at a distance$u$from the mirror. What is the size of the image? what i have done so far: since the ... 1answer 336 views ### If nothing can travel faster than speed of light then how the Universe is only 13.7 billion years old? [duplicate] The light would take 93 billion years to reach the edge of universe but nothing can travel faster than the speed of light not even the big bang? 4answers 168 views ### Faraday's Law - recursive? So we know that the EMF is induced by change of flux. The thing that was always confusing me is the following: we start changing the magnetic field which in turn induces electric field which makes ... 1answer 129 views ### Is it possible to find the hydrogen atom's radial wavefunctions? Is there a way to actually find the equation of$R(r)$without looking at a table with these equations already given? I'm given$n$,$\ell$, and$m. 0answers 36 views ### Neutrino mass difference and mixing angles What are the most recent values of the mixing angles associated with the 3 neutrinos flavors and what are their mass differences? Have they been measured in the first place? 1answer 146 views ### Quantum Logic and Quantum Field Theory Quantum Logic is a very interesting and powerful answer to the problem of Quantum Mechanics foundations. Nevertheless this approach is usually developed in a non-relativistic framework. Is it still ... 2answers 255 views ### If we connect a source of electricity in a large water body ,will it be dangerous? [duplicate] I was wondering whether a person will be electrocuted if he is in large pool or a sea which is connected through a source of electricity . As sea water is saline it will conduct electricity so will it ... 0answers 47 views ### How come evaporation is possible? [duplicate] If I have wet clothes and put them on the ground and let the Sun shine on them, they dry as water evaporates. But water should evaporate at 100 degrees Celsius so how come they go dry if it's just ... 0answers 120 views ### Point source approximation I have a 0.05 mm radius sperical source of Photons, and a 10 mm X 10 mm detector aligned to be orthogonal to their distance vector. Distance is D\approx\$ mm. I want to know how good the point ...
I learned that power intensity in EM (electromagnetic) radiation is $$I=\frac12c\varepsilon_0E_0^2$$ This equation implies that the energy in EM radiation is frequency-independent I also learned ...