Electromagnetism

Electromagnetism

  • Hossein Javadi added an answer:
    To understand the relationship between gravity and electromagnetism, we should explain blueshift by using quantum mechanics. What do you think?

    The Pound–Rebka experiment is a well-known experiment to test Albert Einstein's theory of general relativity in 1959. The result confirmed the predictions of general relativity. Proponents of the theory of general relativity offer three different conflicting explanations of these results that are said to be equivalent to each other and therefore are all equally correct. The main problem with this explanation lies in the conceptualization of a physical process by which mass, momentum and energy could be either added to or subtracted from a photon without changing its velocity or angular momentum. Such a mechanism has never been proposed except for a mathematical description of a four-dimensional substance called a “space-time continuum.” This is a non-Doppler explanation of the shifts in which both source, observer and all photons are in the same inertial reference frame and the photons move at exactly c relative to both source and observer.
    Apparent Weight of Photons; Physical Review Letters - PHYS REV LETT , vol. 4, no. 7, pp. 337-341, 1960
    http://www.uam.es/personal_pdi/ciencias/jcuevas/Teaching/Pound-Rebka-PRL1960.pdf

    Hossein Javadi · Islamic Azad University

    Dear Robert
    What I understood the both special and general relativity are explaining what observers can observe. For example, according to special relativity the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source. There is not any reason why the speed of light is constant. See;

    On the constancy of the speed of light: a nature law or a natural accident!?

    http://gsjournal.net/Science-Journals/Essays/View/5482

  • Chetan R. Kathad added an answer:
    Why the permittivity of free space has non zero value while the sucseptibility is zero?

    The electrical permittivity of free space has constant value of the order of 10-12 F/m while the susceptibility has the exact zero value...The basic physical explanation is sought rather than mathematical..

    Chetan R. Kathad · Saurashtra University

    Mircea Pop, The definition suggests that epsilon is a dissipation factor similar to dielectric constant of dielectrics that forms an opposite field to reduce the original one..And naturally we can think putting all the mathematics aside that if any material susceptible to field than it should not permit it 100%, we taught that susceptibility of a vacuum is zero, than it should permit  the field 100% without any dissipation..

  • Robert Shuler added an answer:
    Is it possible to derive the constant & uniform velocity of light & the Lorentz transform without starting from the principle of relativity?
    Originally the Lorentz transform was developed to explain the Michelson-Morley experiment in terms of length contraction due to motion through an ether. Some work was done on how this might produce distortions of electromagnetic forces and interatomic bonds to produce length contraction. Einstein postulated a fully symmetric (i.e. relative) form with no preferred frame of reference, and gave a different derivation based on the principle of relativity, that the laws of physics including the velocity of light should be the same in all inertial frames. It is a pretty large assumption and gives no insight into mechanisms.

    In years of searching I've found only two papers that claim to derive something like the relativistic Lorentz (not the ether one) from more fundamental principles, one by Yilmaz using de Broglie waves which has received no follow up discussion that I can find, and one by Matthew Brown using pseudo-measurement interaction counting which is only on arXiv (and RG in his profile). Are there any others?

    Does it make any difference if relativity can be derived from some mechanism-like postulates? Does it have any implications for understanding things like spooky action at a distance (entanglement)? Or inertia/gravity?
    Robert Shuler · NASA

    In my view, time is controlled by inertia, and whatever interaction determines inertia is more likely to be akin to entanglement than to QFT.  While QFT interactions are limited to lightspeed, it is well known that entanglement interactions have no regard for relativistic limits.  Think of inertia as position entanglement of multiple masses. 

    Over the course of several papers which I have referenced variously in these discussions, I have shown how gravity can be formulated as emergent from inertia, explaining its weakness.  That implies gravity is probably not a QFT type of force (and therefore there are no conventional gravitons).  Direct evidence for this comes from the way the strength of gravity varies in relation to relativist quantities.  Recall the discussions of the sonic world and the characterization of what an effect with Lorentz stiffness greater than c would look like.  It would not be subject to relativistic slowdowns and limits.  Therefore it could maintain a constant acceleration in spite of relativistic time slowdown (which gravity exactly does, and this amount of acceleration leads to the correct orbit for mercury) and it would be able to push things beyond relativistic limits, or horizons, which depending on the views of Stephen Hawking at any given moment, gravity may or may not be able to do (you couldn't actually push sonic objects beyond their limits, but if there were other objects not made of sound, or EM waves, gravity could push them further than sonic or EM forces in whichever case).

    Thanks for bringing this up.  I forgot to include this very important conclusion (about the Lorentz stiffness of gravity) in my summary.

  • Laurent Damois added an answer:
    How can I generate an azimuthal or axial magnetic field whose strength is proportional to 1/r^2?
    The behavior of surfaces described by the Heisenberg model in the presence of an external magnetic field have like solution skyrmions. However, it is necessary that the magnetic field is proportional to 1/r^2. Does anybody know how this kind of field can be generated?
    Laurent Damois · Institut National des Sciences Appliquées de Lyon

    If you know the equation of the B Field you want, and if the B Field is a static Field, the current density distribution is: j = Curl(B)/µ

  • Marek Wojciech Gutowski added an answer:
    I need to build a setup for generating a dense magnetic field and would like to know if an assembly of solenoids is a good option?
    I am working on the concept of 'force on a current carrying conductor in magnetic field' for which I need to generate approx. 1-D magnetic field. Would an assembly of solenoids be a good option or is there an alternative?
    Marek Wojciech Gutowski · Institute of Physics of the Polish Academy of Sciences

    Suppose your experiment is designed for presentation purposes only, and usage of the expensive equipment should be avoided.  The field generated by a solenoid (perhaps two Helmholtz coils would be preferred, as they generate much more uniform field between them) will be, for all practical reasons, of order 30-50 mT or less.  Inserting a  soft magnetic core inside your solenoid should improve the field 10 times or even more.  Next you should think about your "current carrying conductor".  Perhaps a single copper wire (ribbon?), with, say 1A current, may appear insufficient.  Instead of DC current in your solenoid you can consider a pulsed mode of operation with much higher current (by discharging a capacitor).  Then, however, you should be ready for making such a discharge unidirectional, not oscillating.

  • Ivan Fernandez-Corbaton added an answer:
    Is there any good method that can be used to detect the polarization of the field near focus?
    If the polarization is changed dramatically near the focus, can we image the polarization in an experiment?
    Ivan Fernandez-Corbaton · Karlsruhe Institute of Technology

    Dear Xiaoyang,

    I will give you a theorists answer. This may not be what you are looking for, but I hope you find it useful in some way.

    First of all, there is one thing that you can do if you are content with getting an idea of how the field looks in the focus, even though is only numerical. You can extract the map of polarization in coordinate space (a numerical result is not necessarily an observable quantity). For this to work, you need to have a very good idea of what the beam is at the plane of the input of the lens. You then use the aplanatic lens aproximation to compute the plane wave decomposition of the focus field. You can find a nice description of the procedure in section 3.5 of this book

    http://books.google.de/books?id=FUAAxtxvGI4C&printsec=frontcover&dq=principles+of+nano+optics+novotny&hl=en&sa=X&ei=hfvTU7j3C6KBywO08IGACw&redir_esc=y#v=onepage&q=aplanatic&f=false

    Now for the discussion. I would say that, strictly speaking, there is no way to measure the polarization of a focused field in coordinate space. By "polarization in coordinate space" I mean the spatially varying polarization vector. I assume that this is what you want to measure.

    The reason why I say that there is no way to mesure it is because it is not an observable of the field. Observables of the field are its frequency, its angular momentum, its linear momentum, etc ... . Now, there is an observable related to polarization in momentum space. It is called the helicity of the field. By polarization in momentum space, I mean the polarization of each and every plane wave composing the field. If all of the plane waves composing a field are left hand polarized, the helicity of the field is 1,  and it is -1 if they are all right hand polarized. You can of course have fields with mixed helicity.

    In theory, you can measure the power in each helicity component of a field. In practice, at optical frequencies, this is not trivial since it is hard to have objects that respond to just one of the helicities. At lower frequencies such objects exist, for example helical conductors

    http://link.springer.com/article/10.1007/s11182-009-9249-9#page-1

    I do not know of such objects at optical frequencies, but some nanostructure materials may do the job some day.

    At this point you may be thinking that all the above must be wrong because you can measure the polarization of a beam with just waveplates, linear polarizers and a CCD camera or powermeter. You can of course do that, but this is not a general field. The difference there is that you are working with a special type of light, collimated light.  For proper operation, the waveplates must be aligned so that the collimated beam hits them perpendicularly. They do not work in the expected same for non perpendicular incidence. This is why, even if you place a small piece of waveplate in the focus, it will not do the job for you. What you want is a filter of polarization whose operation is independent of the direction of incidence on the device. That, is related to helicity (the polarization of all the planewaves impinging on the device).

    If you are interested in the use of helicity in focusing experiments, you can check out chapter 4 of my thesis (available in RG and the arxiv).

  • Eric Lord added an answer:
    What is the significance of the Poynting vector in a static electromagnetic field?
    In an electromagnetic field the flow of energy is given by the Poynting vector. For an electromagnetic wave, this vector is in the direction of propagation and accounts for radiation pressure. However, in a static electromagnetic field the Poynting vector can of course be non-zero. It's not easy to understand how there can be a 'flow' of energy in a static situation. Please discuss.
    Eric Lord · Indian Institute of Science

    Yes, of course there is no "velocity" v such that electromagnetic energy flux is wv. That was the mistake made by Umov, cited in a paper* by an earlier contributor to this thread - the mistake of treating energy density as a scalar density.

    (An analogue even simpler than my charge conservation example, illustrating the fallacy, is the flow of a liquid - it flows even though its density is uniform and constant...)

    I've collected and looked into quite a lot of published papers on the topic of electromagnetic energy and the Poynting vector since I started this discussion. Mostly they pursue particular contrived "thought experiments" involving capacitors etc. and are not very enlightening about general principles.

    I agree, electromagnetic energy is not easy to visualize, even when the complications you mention (stress on matter, energy waste in heat or in electronic transport etc) can be dispensed with by considering only electromagnetism in "free space" - i.e, in regions not containing sources and "matter". Then there are only configurations of the fields E and B in space and time to think about.

    If indeed, as you have convinced me, the Poynting vector describes energy flux in some situations but not all, then there ought to be some mathematical entity that does express this flux correctly for all possible solutions of Maxwell's "free space" equations. But I'm at a loss to know what it might be!

    Anyway, thank you for a stimulating discussion.

    _______________________________________________________

    * Experimental test of the compatibility of the definitions of the electromagnetic energy density and the Poynting vector in The European Physical Journal D 09/2004; 31(1):113-120. DOI: 10.1140/epjd/e2004-00135-x

  • Samvel Ter-Antonyan added an answer:
    What are the main astrophysical radio wave sources?
    I have a project and I need to know the sources of electromagnetic radiation along with the processes by which they are emitted in space. Any help would be appreciated.
    Samvel Ter-Antonyan · Southern University and A&M College

    In addition to G. Bothun's comment, look at the Radio Spectrum of Astronomical Sources from the Fundamentals of Radio Astronomy site: http://astro.u-strasbg.fr/~koppen/10GHz/basics.html

  • Daniel Baldomir added an answer:
    Can electric and magnetic forces be viewed as space curvature with particular limitations like gravity?
    Long back perception on gravity was changed from a kind of force to a phenomenon which actually bends space- time in its influence. Is there any chance that even magnetism and electric forces and in fact all 4 types of forces can also be interpreted as such?
    Daniel Baldomir · University of Santiago de Compostela

    Weyl coined the term "gauge transformation" as to a change in the scale for the spacetime. In fact Weyl used the Riemannian metric multiplied by a function (electromagnetic potentials) which is associated to the change of scale (conformal transformations). And it is true that what we understand nowadays by gauge theories are applied in a very different form, i.e. to inner symmetries and no to SO(3,1) one.
    In 1929 he recognized that his theory was wrong and published a paper in Zeitschrift fur Physik that tried to make a connection with quantum mechanics after the publication of London:

    London, F. Quantum Mechanical Interpretation of Weyl's Theory
    . Zeit. f. Phys. 42 (1927)

    Therefore in spite of being a wrong theory of unification and also unable to be interpreted with quantum fields, his importance in theoretical physics was very important for making the geometry of the gauge theories.

  • Maxim Belkin added an answer:
    How can I go about visualizing first hydration layer oxygen and hydrogen average density?
    I have an MD trajectory file in TIP3P water. I want to use the trajectory to create a 3D visualization of the density map (or average polarization density whichever is easier) of the Oxygen and Hydrogen of the closest water molecules to the molecular surface. How do I go about it? I saw something in chimera tutorial but got confused. Something like the figure 2 in this article http://pubs.acs.org/doi/abs/10.1021/jp709958f is what I need.
    Maxim Belkin · University of Illinois, Urbana-Champaign

    Abhishek,

    Figure 2 in that paper is density plot. You can easily do this type of analysis in VMD. To get density map, use the VolMap plugin. In "Selection" specify "water and oxygen" (or "water and hydrogen"), volmap type - "density", weights - "mass", check the box to average results for the entire trajectory, and output results to a file. You may also increase a bit the default values in the "atom size" field to get slightly smoother-looking results and in the "resolution" field to speed up calculations (at least for the initial test run). The result will be a 3D map, that you can visualize nicely in VMD.

    If you want to get g(r), use the GofRGUI plugin. Don't use GPU-accelerated version for now (or, at least, make sure to compare the results to the CPU version).

    In VMD, you can access both of these plugins under Extensions->Analysis menu. (The latter is called "Radial Pair Distribution Function g(r)" there).

    http://www.ks.uiuc.edu/Research/vmd/plugins/volmapgui/

    http://www.ks.uiuc.edu/Research/vmd/plugins/gofrgui/

  • Eugene A. Tikhonov added an answer:
    How can I measure the ratio of the pulse component and the continuous wave component in a pulse train?
    Generally, the pulse train is displayed as that in the attachment. Here, the background continuous wave is defined as the continuous wave (CW) component and the pulse above the background is defined as the pulse component. The question is, how can I measure the proportion of the CW component in the pulse train? We have transformed the optical signal into the electoral signal by a photodetector, and it is observed with a oscilloscope, but it is difficult to differentiate the CW component.
    Eugene A. Tikhonov · National Academy of Sciences of Ukraine

    Dear Yudong!

    I think that  Your  problem follows from pico,-femtosecond laser emission and analisys of mode locking quality. That is rather old problem and it has  found varios solutions. As for the your narrow question I can say  that measurement difficalties connected with limited dynamical responce of typical oscilloscope with linear amplifier because You need  about three order of magnitude to have good measurement. So  for your measurement You can use  oscilloscope with logarithmic scale amplifier.

    I wish You any success on the way.

  • Tolga Yarman added an answer:
    Does a uniformly accelerated charge radiate?
    In classical electromagnetism, any accelerated charge should radiate. The back reaction on the charge due to radiation is given by radiation reaction which depends on time derivative of acceleration. However for uniformly accelerated charge there is radiation but no radiation reaction. I would appreciate any comment on this apparent paradox.
    Tolga Yarman · Okan Universitesi
    I would like to attract the attention of this distinguished audience to our following work:

    http://link.springer.com/article/10.1140%2Fepjp%2Fi2013-13134-9#page-1

    Cordially...


    Prof. T. Yarman
  • Musdalilah Ahmad Salim added an answer:
    How can I measure the permittivity value and permeability?
    I have powder sample and thin film sample
    Musdalilah Ahmad Salim · Universiti Tenaga Nasional (UNITEN)
    Thank you for all responses. Mr Ravi Kumar, is it possible for dielectric probe kit and resonator used for both type of samples (powder and thin film)
  • Tsvetanka Babeva added an answer:
    What is the reason behind the shift in output transmission spectra due to infiltration of analytes in photonic crystal waveguide sensors?
    In photonic crystal waveguide based sensors, with the infiltration of analytes of different refractive indices there is a shift in the wavelength. What is the main reason behind wavelength shift due to change in refractive index with respect to photonic crystal. Could Raman scattering be related to this phenomenon?
    Tsvetanka Babeva · Bulgarian Academy of Sciences
    May be the interaction appears but the change of RI contrast is very weak to be detected, i.e there is no measurable shift of the spectrum. In other words the sensitivity of your sensors is not high enough to detect these objects.
  • R. G. Rastogi added an answer:
    Does E10.7 take geomagnetic sources into consideration?
    Tobiska (1993, 2000) developed and validated a solar EUV flux model named E10.7 and argued that the long-term variation is identical to the age-long F10.7 while it performed better on a short-term scale. How true is this?
    R. G. Rastogi · Physical Research Laboratory
    one should not mix F 10.7 flux with E 10.7 index.F10.7 is a instrumental measured index of solar EUV radiation. It has been very widely used.It has an annual variation due to the changing distance between sun and earth, but I do not think it has any geomagnetic efffect. E10.7 index is derived from an ionospheric data from mid-latitudes and would have geomagnatic effects incorporated in it

    r.g.rastogi.
  • Ahmed Alchalaby added an answer:
    What courses should be taught in a masters program for "Metamaterials"?
    The research center for Nanophotonics and Metamaterials, Saint-Petersburg University ITMO, launches new master program in the field of metamaterials. The course list is available on our website. What have we forgotten or what do you think should be excluded from the course list?
    Ahmed Alchalaby · Dijla University College
    congratulation, the course of nanophotonics by Vladimir M. Shalaev my be helpful
  • Stefano Quattrini added an answer:
    Does light experience a red-shift (blue-shift) when it passes through a static electric field?
    Light experiences a redshift (blueshift) if it passes through a strong static gravitational field, as demonstrated by Einstein. Owing to gravity-electrostatic analogy, why does light not have the same effect if it passes through a static electric field?
    Stefano Quattrini · Sapienza University of Rome
    Proper time variation is the measure of the variation of the local space time curvature. The variation of proper times between the source and observer is at the base of the gravitational Redshift. It is also affirmed by Lev Okun in Gravitation, photons, clocks...
  • Andrew D. Greentree added an answer:
    Why does resolution depend on wavelength?
    We all know the resolution depends on wavelength. However, when a student sees this concept for the first time it does seem paradoxical. After all, the direction in which the wavelength is measured is orthogonal to the transverse dimension of the object we are trying to resolve!

    Any ideas what is the simplest and best mental picture to give a student to help resolve the mental conflict caused by this orthogonality?
    Andrew D. Greentree · RMIT University
    Both @Raymond's and @Baptiste's answers are good and get to the point - especially in the canonical regime of diffraction limited optics. Personally, I think that @Raymond's is the best insight for a conceptual student, because it gets across the need to the spatial information perpendicular to the optical axis, and hence the fact something more than plane waves are required (note that if we are considering fluorescence imaging, then the spatial symmetry breaking is provided by the source).

    Alternatively, Baptiste's point about the Airy function is the mathematical description, and works both in emission and reflection: I know many students who will ONLY accept an answer such as this, grumbling about plane waves as not being rigorous enough ;).

    There are a few other points of view that you may wish to consider, all of which are entirely complementary.

    From a quantum mechanical point of view, you can also motivate this from the cross section of interaction between an individual photon and the object of interest, and that the minimum that this spatial extent can be is given by the wavelength (it can always be larger, based on the coherence length, of course).

    And you can motivate this from an information theoretical basis, which then reduces to @Baptiste's response, but which can then be used as a motivation to explore how the use of additional information or physical responses in the form of nonlinear optical effects can be used to get diffraction unlimited imaging.

    So to return to your question - just as with the responses, I'd start by giving Raymond's answer, and then quantify with Baptiste's - if you want to extent it with mine, then that's entirely up to you ;).
  • Alexey Kantyshev added an answer:
    Is there any free software for designing FSS (frequency selective surfaces) or EBG structures?
    I need any free software for design of a reflector with an EBG surface.
    Alexey Kantyshev · Informational Sattelite Systems
    Thank you))
  • Oleg Troshkin added an answer:
    Why is the vorticity vector perpendicular to the velocity vector? Is there any physical explanation?
    By vector analysis, one can calculate that vorticity vector should be perpendicular to the velocity vector. But I wonder the physical reason why vorticity vector is perpendicular to the velocity vector? This question can be thought as current - magnetic field vector couples. Why perpendicular?
    Oleg Troshkin · Russian Academy of Sciences
    They may be parallel as in the Arnold-Beltrammy-Childress flow.
  • Raymond Rumpf added an answer:
    What are the factors that influence the angular stability of FSS?
    Frequency selective surface (FSS) is a two dimensional periodic array that has resonant elements. What are the factors that influence the angular stability?
    Raymond Rumpf · University of Texas at El Paso
    Yes, in general the more subwavelength the elements, the better then angular response, usually called the field-of-view. Seek hexagonal arrays and elements that are physically small relative to the operating wavelength. That will be a good start.
  • Michael Shirk added an answer:
    How can we estimate the heating and cooling rate in a femtosecond laser?
    Does anyone know a formula to calculate the heating and cooling rates in a femtosecond laser? How can we measure the temperature during laser processing ?
    Michael Shirk · Raydiance
    "I am a beginner working on the phase transformation of material after laser irradiation."
    If you are looking at phase transformations it is important to understand the material and the timescale of its transformation in relation to the laser. Things are much different if they happen at the surface vs. if they are constrained within a transparent solid. The one thing that is pretty clear is that most phenomena you will be looking at will be driven by the residual heating and shock delivered after the laser pulse-not the initial electron temperature. Shock waves and spallation may be as important (or more important) than the actual temperatures reached in the evolution of your material's phase and microstructure over time.
  • Scott A. Yost added an answer:
    Is the electric field real or only a theory?
    Electric field.
    There are many definitions for it. Let's use this one from Yahoo:

    ·
    An electric field is a property that describes the space that surrounds electrically charged particles or that which is in the presence of a time-varying magnetic field. This electric field exerts a force on other electrically charged objects. The concept of an electric field was introduced by Michael Faraday.
    - Space surrounding a charged particle does not stop close to the particle but extends to universe.
    - That means another charged particle millions km away experience a force immediately; something went faster than light to that second particle. Wow… That is the theory and its seems absurd.
    - Since in this concept, nothing is leaving the charged particle, there is nothing to intercept or nothing to stop travelling.
    - It should not be possible to shield the electric field even with a faraday cage. But the faraday cage works very well.
    - That means the facts are wrong and the theory is right; interesting but absurd again.
    - The electric field has energy to be able to exerts a force on another charged object. Where does that energy comes from. It comes from empty space because before the object was charged, that space was void. That means void can have energy and interact with something. Nothing can interact with something. Let’s be serious and look at these more carefully.
    - The math aspect of the field has been verified many times and seem to be ok. What is probably lacking is the explanation of what is really going on physically
    - One author states that the field is only a concept to help in the math part. It is not a reality. Then what is the reality.

    Is it possible to do an experiment to find out if something is leaving the charged object?
    Yes and it is simple. Place a sensitive electric field detector at 2 meters from a neutral ebonite rod. Zero field.
    Rub the rod with fur and the detector indicates a (-) electric field. If nothing escapes the rod, placing a neutral cardboard between them should make no difference if the field is already everywhere. As soon as the cardboard is in direct line between sensor and charged rod the sensor indicates no field. One cannot stop what is not moving. What is leaving the rod then? Not electrons for sure. This is still open for new explanations.

    Any suggestions?
    Scott A. Yost · The Citadel
    There are a lot of ways to complicate the description of an antenna, but if I just put a straight wire in an electromagnetic field, only the electric field can accelerate the charges along the wire. The response is linear in the component of the electric field along the wire and independent of the magnetic field: a = qE/m for a free charge in the wire, and independent of the magnetic field. If I measure the acceleration and know the mass and charge, I would say that I have just measured the electric field directly, without having learned anything about B. The EM field also interacts with the fixed charges in the wire, but the fact that they are held fixed implies that external forces are acting on the antenna, so that momentum is not conserved and one could not infer anything about the momentum content of the fields from this measurement.
  • A. A. Timopheev added an answer:
    Does a prism affect electromagnetic waves passing through it, other than those light waves visible to the human eye?
    I am a non-physicist.
    A. A. Timopheev · University of Aveiro
    If the prism's geometric dimensions will be much bigger than the ectromagnetic wave length - then there shouldn't be any qualitative difference.
  • Javier Luis López added an answer:
    What is the best workflow for developing a good cfd code?
    I will be more specific here. I am trying to simulate a two dimensional mhd flow through an MPD thruster. I have the euler equations with coupled electric and magnetic field. I prefer using opensource products and I am comfortable with python and fortran. I want to know if a mesh from gmsh or netgen could be directly used for the code. I have written codes only using basic fdm. I haven't even written any complex fvm codes. But I am planning to develop three codes using mhd, DSMC and PIC methods. And I am not an expert in any of these. I am conducting experiments too although I don't need any help in that part. I would really like some expert input on the problem that I have on hand. Expecting some good answers.
    Javier Luis López · Pulsotron SL
    Here it is solved the issue on windows installation.

    But I heard that openfoam is an "incompressible magnetohydrodynamic equation solver" that works only with non compressed fluids, perhaps it is not the best suite for MHD.
  • Mahboobeh Nosrati asked a question:
    How can I obtain second order permittivity of graphene (trilayer graphene)?
    I want to obtain second order permittivity of trilayer graphene. I have its hamiltonian, energy and wave functions.
  • Daniel Baldomir added an answer:
    Could we save a magnetic monopole inside a hermetic box?
    Dirac magnetic monopole is provided of an string which is infinitely long and which cannot touch matter in the first model of Dirac.
    Daniel Baldomir · University of Santiago de Compostela
    Dear Xavier,

    Sorry because I see that I didn't answer you clearly to your idea of sequester a monopole isolated.
    The answer is not because there must be always a hole or a singularity of the electromagnetic field.
    I add some information more:
    1. Maxwell's equations hasn't duality between electricity and magnetism for the sources of the fields, in fact one of his equations div B= 0 forbids it. It is true that electrodynamics could kept invariant under what are known as duality rotations symmetry. That is to say, its classical physical observables could be invariant under such fictitious rotations as what observed by Oliver Heaviside in the XIX century. This symmetry allows to make classifications of electricity and magnetism within electrodynamics as are made in:
    D.Baldomir and P.Hammond, Geometry of Electromagnetic Systems, Clarendon Press, 1996. With these transformations you can get a divergence of B different of zero but you do not have magnetic monopoles at all. This is only a different form of having the electric and magnetic fields defined into electrodynamics.
    2. In 1931 Dirac showed that it was possible to have Maxwell's equations besides a particle which produced the magnetic field as the electrons or other electric particle in such a form that this could explain how so different particles as the electron and the proton had exactly the same electric charge in spite to be so differente (lepton,hadron). Later more models were developped such as the one of J.Schwinger, Wu and Yang or t'Hooft and Polyakov.

    3. Every model of monopole has a picture of the magnetic monopole and the simplest one is due to Dirac. The magnetic monopole has the minimum magnetic charge g= 137 e/2= 68.5 e, in Schwinger model is the double g= 137 e, etc and the mass is obtained assuming that the classical radius of the electron is the same of the one of the monopole obtaining a huge minimum mass of 2.4 GeV, But in all that I know the mass of the monopole is still an unsolved question.
  • Salman Khaliq added an answer:
    I wanted to know about the short and long end of winding?
    And how it varies with pole no.?
    Salman Khaliq · Hanyang University
    Thanks dear....can you explain a little more that how short pitch it more suitable for less no. of poles and i was concerned about the winding ends. how do we decide that the winding end is/should be long or short?
    Regards
  • Wolfgang Menzel added an answer:
    How can we measure power content of different modes in circular waveguide?
    First three modes...
    Wolfgang Menzel · Universität Ulm
    We scanned the field in front of the circular waveguide and made correlations for the different modes (similar to mode matching). See: Ivan Russo, Winfried Mayer, Stefan Pflueger, Wolfgang Menzel: Correlation-Based Analysis of Mode Converters in Multimode Waveguide. International Journal of Microwave and Wireless Technologies, Vol. 5, Dec. 2013, pp. 721 – 728.
  • Orin Laney added an answer:
    Where can I find the hysteresis curve (and if possible the demagnetizing curve) of Hiperco 50 A and other high magnetic saturation soft materials?
    Soft magnetic materials such as Hiperco 50 A offer outstanding magnetic performance but a lack of information, in particular relative to the B-H curve, and considerably slow down use in the development of an electromagnet. In particular, I am looking for free access bulk magnetic properties of Hiperco 50 A after high temperature treatment for optimized magnetic performance.
    Orin Laney · Atwood Research
    From the manufacturer:
    http://cartech.ides.com/datasheet.aspx?i=101&e=200&c=TechArt

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