• Andrew W. Beckwith 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.
    Andrew W. Beckwith

    Hi, all. I do believe this question is a bit incomplete, so let me start by posting a link to a physics arxiv article, below


    Why ? Its a bit more nuanced a situation than the initial querry indicates. 

    First, from the article above, the quote

    Does a uniformly accelerated charge actually radiate? In a constant
    gravitational field should free-falling observers detect any radiation emitted by free-falling charges? Is the equivalence principle valid for such situations?
    If the answer to the first question is affirmative, a free-falling charge will radiate according to an observer at rest, because in a constant gravitational field, any particle should move with uniform acceleration. However, an observer
    falling freely with the charge would observe it at rest and no radiation at all.

    end of quote

    Seems definitive? Well, again the article above, shows that there is more to consider, namely....

    From the article above, quote

    Inertial observers have no doubts about the answer to the first question. They will answer it affirmatively by using special relativity and Maxwell’s equations, as it is done in classical electrodynamics texts (see, for example,Ref. 6 whose conventions we adopt). Nevertheless, comoving observers, that is, accelerated observers with respect to whom the charge is at rest, will not detect any radiation because the radiation field is confined to a spacetime region beyond a horizon that they cannot access.

    end of quote

    For co moving observers, this is NOT an obvious answer, and the initial question needs a bit more specific information. 

    I post this not to say that the question as presented is 'bad physics' but to indicate what a mine field this querry really is. Any full answer will require in dept analysis of several specific cases beyond the initial question tenured for us to review.

    Andrew Beckwith

  • Olivier Barre added an answer:
    How can I calculate force between permanent magnet and electromagnet?

    I need to calculate force between Permanent magnet (NdFeb) and electromagnet for different voltages(5-24). They are 5 mm apart in the device.

    Currently I had measure the Magnetic field  for Permanent magnet in terms of Magnetic Fled Vs Distance  (197 mT at 5 mm) and for electromagnet in terms on Magnetic Field Vs Voltage (134 mT for 12V at contact). 

    Any suggestions will be highly appreciated.

    Thanks in advance.

    Olivier Barre

    Dear M Harshad Kamble

    A few years ago, I have done the calculations of forces induced by magnetic fields (The nature of the sources of fields has no importance for the calculation). The best way is the use of the energy principle. This method uses no simplification.
    But, if you want to use this method, you have to use finite element method.
    If you have not such a software. A free software exists and it is limited to 2D modeling. That will not be an issue if your study can be assimilated to a 2D axisymmetric problem.
    Let me know if you want additional information.

    Best regards

  • Thorsten Beuth added an answer:
    Under which conditions are Mie Scattering Amplitudes imaginary?


    I am using Maetzlers Mie Scattering Script from 2002 to calculate Phase Matrix entries. But under no conditions I am able to produce imaginary scattering amplitudes S1 and S2 which is kind of strange to me because in the phase matrix they produce the side entries. I know that some side entries vanish for spheres - or do they all actually?

    So my question is basically: What are the conditions of imaginary scattering amplitudes.

    Thank you and best regards

    Thorsten Beuth

    Thorsten Beuth

    Dear Denis, thank you for the papers, they are very interesting. Unfortunatly they are not specifically what I was looking for. I found the "error" in Maetzlers script by comparing the formulars directly with his written code. At some point he took the absolute value of the scattering amplitudes which denied plausible results concerning the Mueller scattering matrix for the Stokes vector (plausible at least for my purpose) .

    Never trust a code that you have not written on your own :-)

    -----> SOLUTION <-----

    So just if someone else is looking for it: Mie2_tetascan and Mie_tetascan do not give back scattering amplitudes (as written in the technical manual) for calculating the phase scattering matrix. You have to manipulate the next two lines after "a(:,j)=Mie_S12(m,x,u);" to get the imaginary parts of the solution as well.

  • Marcin Nabiałek added an answer:
    How to calculate the mangetic permeability from hysteresis loop (polarization versus magnetic flux) ?

    I would like to ask about the mangetic permeability, I have measured the hysteresis loop, its polarization (T) versus magnetic flux density (mT). do we need to convert first polarization to magnetization or we can calculate from the formula mu = J/ H / mu° +1 ...

    Marcin Nabiałek

    The first magnetization curve tangent to the curve, so as to under it. Yes designate the initial permeability. The maximum permeability is tangent to the curve but as if over her. Touches on his knee Ewing, at the approach to ferromagnetic saturation.

  • Manuel Morales added an answer:
    What should be rated higher: experiment or theory?
    Faraday laws of Electromagnetism were followed by Maxwell Equations, which explained Faraday results and predicted Electromagnetic Waves, beyond FARADAY results.
    Manuel Morales

    My findings show that the current scientific method is based on omitted-variable bias which can only lead to obtaining false-positive data.

    I have invited the public in exposing this fundamental defect in order to help science to self-correct and advance as is necessary when a new discovery supersedes previous knowledge (see links).

    • Source
      [Show abstract] [Hide abstract]
      ABSTRACT: Albert Einstein held the belief that quantum mechanics was an incomplete theory and that there were local hidden variables that would give us a complete sense of reality. As the findings show, he was correct about there being hidden variables. However, he was incorrect as to where to find them. The basketball examples serve to illustrate the findings of the Tempt Destiny experiment and the mechanics involved. The "Flawed Scientific Method" illustrations were designed to go with the public invitation to help science self-correct. In essence, this one page document illustrates for the public the mechanics of the discovery of Einstein's nonlocal hidden variables which in turn revealed how the scientific method is fundamentally flawed and how to fix it.

    + 1 more attachment

  • Johneph Sukham 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?
    Johneph Sukham

    Solid State physics and Photonics

  • Anatolij K. Prykarpatski added an answer:
    Can there be gravity from the electromagnetism?

    In some old and modern articles and books there is discussed an idea that gravity law can result from the fact that atraction force between oppsite signes charges is slightly greater than the repulsion between the same sign charges. - yet a  deep physical question arises - why?  One can find in literature the follwing controversial answer - because in reality there exist only ... attraction forces, and the repulsion forces are the resulting oppositely  directed attraction forces caused  by other far distant surrounding  charges, whose existence is owing to the assumed matter neutrality. 

    The idea of ​​the electrical nature of gravitation expressed M. Faraday, John. Maxwell H. Lorentz, O. Heaviside and others. German physicists W. Weber and F. Zolner in 1882 proposed the concept of gravity, based on the difference between the forces of attraction and repulsion of electrical charges, which make up all of the body (the book of F. Zolner
    «Erkliirung der universellen Gravitation», Leipzig, 1882). However, the physical reasons for the difference they have not specified. Much later, a possible cause of the excess power of attraction over the forces of repulsion expressed  by Sir A.  Eddington in his book "Fundamental Theory" (Cambridge, 1946, pp.103). He was guided by the principle of Mach: "Even in the simplest case, we obviously have to deal with possible action
    just two particles, it is impossible to lose sight of the rest of the universe. " Electric field lines of the two charges of opposite sign extend from one charge to the next, and so this system is closed and is independent from the rest of the universe. The situation is different in the charge of one sign - here the lines of force go into space, ending up somewhere on the other sites. So naturally expected that the objects of the universe have some influence on the
    interaction, reducing its strength."
    The Lorentz article "Reflections on gravity" (Proc. Amst. Acad. 11, 1900, p. 559), and then Adamuti (I.А. Adamuti. Gen. Re1at. and Gravitation. Proc. 1 Ехр. Gravitation Symp.
    Bucharest, 1980, p. 202)  develop the electrodynamic theory of gravity. It  like the mentioned model of Lesage, assumes some penetrating  electromagnetic radiation influence on the body from all sides and partially absorbed by them, when this actioin  is not compensated by pressure on the body of this radiation, which is partially screened by one side of the second body. Since the solid angle subtended by the body 2 by the body 1, is inversely proportional to the square of the distance r, the number of them is blocked by the particle radiation proportional to 11^-1 and as the number of absorbed particles is proportional to the  mass, the gravitational force is proportional to the mass of bodies. Consequently, the theory provides the law of gravitation. Speed ​​of gravitation, of course, is equal to the speed of light. However, the theory gives rise to having a large resistance to movement of bodies with  oncoming flux, which is contrary to the experience: no slowing movement of the Earth and other celestial bodies in orbit  rotation is observed.
    Lyttleton and Bondi suggested a   hypothesis  of electrical nature of gravitation, based on the assumption that the proton charge slightly (by 10^-18) is greater than the charge of the electron. (R. А. Lyttleton, Н. Bondi. Gravity and Electricuty. Proc. Roy. Soc,
    1959, А252, p. 313)  They managed to obtain the  Newton's law, explaining  the observed expansion of the universe, the  Blacket law on the magnetic moment of the heavenly bodies and a number of other facts. However, the hypothesis refutes the  direct measurements of the charge of the electron and the proton,  which had proved  that their difference is less than 10^-21.   Despite this, the hypothesis of different tse charges of elementary particles put forward again and again  (N.E. Zaev. "Electromagnetic mass and gravity nature". Journal of the Russian Physical-Mathematical Society, 1992, N2 1-12, p. 32).  The idea of ​​the electromagnetic nature of gravity has been developing  also by A.D. Sakharov, considering the latest results of the quantum fluctuations of fields. (A.D. Sakharov. "Vacuum quantum fluctuations in the curved spacetime and gravity theory". - Doklady AN SSSR, 1967, v. 177, p. 70).  However, the quantum theory of gravity has not been completed and its evaluation is premature. According to A. Barut, gravity is not connected with the static and dynamic effects of electromagnetic and gravitational interactions caused by the electromagnetic radiation produced by the distortion of the structure of elementary charge in the presence of a massive body (А. О. Barut. Gravity and Electromagnetism. - Proc. 2  Marcel Grossmann Meet. Gen. Relativity, Trieste, 1979. Part А. Amsterdam, 1982, p 163).    However, specific evaluations, allowing to  confirm or reject the hypothesis are  not presented.

    Anatolij K. Prykarpatski

    ...Yet, it is an open question! The physical nature of dark matter and dark energy is not  still understood, even more, its existence is ...under question! I guess, the main breaktrhrough one can expect in real understanding the physical nature of  quantum vacuum, zero point energy etc...


  • Hossein Javadi added an answer:
    Is there a relationship between speed and spontaneous symmetry breaking in particle physics?

    Symmetries are not hard to find. Plates in the china shop look the same when rotated; serving platters are identical to their mirror reflection. But much of the world is messy and asymmetric, and discovering how certain symmetries are violated, or broken, can reveal deeper physics.

    Nambu developed a mathematical mechanism for spontaneous symmetry breaking in particle physics. Spontaneous symmetry breaking occurs in systems that under certain conditions are symmetric, but whose lowest energy state is not. A classic example is a hot chunk of magnetic material, in which the atomic-scale “bar magnets” point in random directions, making its interior symmetric under rotation. But as the material cools, these elements align in a single direction, and the metal becomes magnetized. The rotational symmetry is broken in this lowest energy configuration, hiding the symmetry that still exists in the equations of electromagnetism.

    Something similar happens in a superconductor. At high temperature, the electrons in the material are free to roam around randomly, but below a critical temperature their lowest energy state is one in which they pair up. Nambu modeled this behavior in the context of quantum field theory. He was able to explain the expulsion of magnetic fields by superconductors in a new and elegant way involving the breaking of symmetry in the field equations when electrons pair up.


    Spontaneous symmetry breaking (SSB), which is the main subject of my talk, is a phenomenon where a symmetry in the basic laws of physics appears to be broken. In fact, it is a very familiar one in our daily life, although the name SSB is not. For example, consider an elastic straight rod standing vertically. It has a rotational symmetry; it looks the same from any horizontal direction. But if one applies increasing pressure to squeeze it, it will bend in some direction, and the symmetry is lost. The bending can occur in principle in any direction since all directions are equivalent. But you do not see it unless you repeat the experiment many times. This is SSB. A part of Nobel Lecture by Yoichiro Nambu (2008)


    In a particle accelerator, beams of subatomic particles are boosted to nearly the speed of light and then brought into collision with either a stationary target or another beam of accelerated particles coming head-on.


    So, this question arise that what is the relationship between speed and Spontaneous symmetry breaking?

    Hossein Javadi

    Dear Vassili
    "... the maximal attainable speeds are different for different kinds of particles."
    Exatly, it deppends to structure of particles.
    In pair production, a photon that moves with speed c, converts to electron and positron that move with speed v<c, and other bosons also occur. Before pair
    production, there is a photon only. After pair production, there is an electron, a positron and virtual photon (boson) that carries electromagnetic force. In fact; Spontaneous Symmetry Breaking has occurred at speed v<c.
    Theoretically, after big bang:
    Grand Unification Epoch, from 106^-43 seconds to 10^-36 seconds:
    The force of gravity separates from the other fundamental forces (which remain unified), and the earliest elementary particles (and antiparticles) begin to be created.
    Inflationary Epoch, from 10^-36 seconds to 10^-32 seconds.


    What is the role of is speed in creation of particle and Spontaneous Symmetry Broken?

    • Source
      [Show abstract] [Hide abstract]
      ABSTRACT: For long time seemed the Friedmann equation is able to explain universe, but in recent years, the cosmological constant was of interest to cosmologists. However, these two equations are unable to explain before the Big Bang. Thus this paper, from a new approach, turns out to merge the fundamental principles of quantum physics, relativity and classical mechanics through a new definition of rest state of particles like photon, and attempts to present the reasons and the possibilities of the existence of the superluminal speeds. So according to this new view some complex concepts and unanswered questions is explained in this paper.
  • Antoine J.H. Acke added an answer:
    What's the relationship between gravitoelectromagnetism and electromagnetism ?

    I worked out this formula B=Bg/4π√(εG)

    Where B is the magnetic flux density at the surface of the Earth or a body, Bg is the gravitomagnetic field of the Earth or a body,  ε is the permittvity of free space and G is the gravitatonal constant.

    I realised that when I substitute the Bg of the Earth, B concided with the experimentally measured magnetic flux density of the Earth's surface.

    Could this be the relation ?

    Antoine J.H. Acke

    Gravitoelectromagnetism (Heaviside, Jefimenko, ...) and Electromagnetism (Maxwell) describe the gravitational and the electromagnetic phenomena in an analogue way. In GEM the gravitational field plays the  role that  the electromagnetic field plays in EM: it intermediates in the interactions between whether or not moving masses just as the electromagnetic field intermediates in the interactions between whether or not moving charges.  The gravitational field is generated by masses, the electromagnetic by charges.  Both are described by an analogue  set of equations.

    GEM as well as EM  perfectly can be explained in the frame of classical  physics starting from the hypothesis that every material object manifests itself in space by the emission -  at a rate proportional to its rest mass -  of mass and energy less entities that run away with the speed of light and that carry information about the position, the velocity and the electric status of their emitter.  Because these entities are grains of information, we call them "informatons".

    • Source
      [Show abstract] [Hide abstract]
      ABSTRACT: The theory of informatons unifies gravito-electromagnetism (GEM) with electromagnetism (EM) by the hypothesis that "information" is the substance of GEM- and of EM-fields. The constituent element of that substance is called an "informaton". The theory starts from the idea that any material object manifests itself in space by the emission - at a rate proportional to its rest mass - of informatons: granular mass and energy less entities rushing away with the speed of light and carrying information about the position, the velocity and the electrical status of their emitter. The GEM- and the EM-field at a point are charactarized as the macroscopic manifestations of the presence of a cloud of informatons near that point, and the quantities "field" and "induction" are identified with respectively the density of the flow and the density of the cloud of information at that point. The laws of GEM and Maxwell's laws are mathematically deduced from the kinematics of the informatons. The gravitational and the electromagnetic interactions are explained as the effect of the tendency of an object to accelerate in order to become blind for flows of information generated by other objects; and gravitons and photons are identified as informatons carrying a quantum of energy, what allows us to understand the dual nature of light.
  • Jeffrey Tuhtan added an answer:
    How are the Biot-Savart law and pressure Poisson equations related?

    Formulations for both are found in many books on fluid mechanics and turbulent flows. However, I am looking for a way to understand the two approaches from a common point of view and would like to find a derivation which goes from one to the other, if possible.

    Jeffrey Tuhtan

    Alex, thank you for the reference, amazing work! Looking through the paper, it seems like Eqn. 16 describes the pressure difference as a function of the chordwise vorticity. Does seem like a reasonable place to start looking for connections between the near body pressure and vorticity?

  • Georg R. Pesch added an answer:
    How can I simulate a magnetic field generated by a single solenoid with COMSOL?

    It might sound simple but I'm having trouble with simulation of a solenoid's magnetic field. most of what I know about electromagnetism is from what we had in Physics II which is not much. But being a chemical engineer it is not very likely not to know COMSOL. So here I am asking for help from those who can explain to me how to simulate the magnetic field of single solenoid with COMSOL. Please have in mind my knowledge of the case is limited and any suggestions and resources are more than welcome.

    Georg R. Pesch

    You might want to look at the AC/DC User Manual of Comsol:


    Page 178 and following deal with coils!

  • Myroslav I. Kozak 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?
    Myroslav I. Kozak

    Enter linearly polarized light. Register the usual method through the compensator and polarizer. Be sure to let us know that as a result.

  • Marek Wojciech Gutowski added an answer:
    Any advice on conductivity tensor and lattice symmetry?

    In book it says" If a field in x direction induce any current in y direction,by exploiting the symmetry,one can predict  a same current will arise in -y direction,only consistent possibility is zero current so the conductivity tensor is diagonal  in cubic symmetry lattice"

    What hell he is talking? why field in x direction can induce current in y direction?And if so,why will be same current induced in -y direction? why those stuff related to symmetry? I have no clue about this

    Marek Wojciech Gutowski

    Crystals are not isotropic media, like majority of liquids.  Thus the electric current density in them satisfies the relation j=\sigma E, where j is vector of current density, E is electric field vector, and \sigma is generally the 3x3 tensor of conductivity. \sigma in cubic crystals is not only diagonal, its all three diagonal elements are equal to each other.  This need not to be the case in other symmetries, for example in tetragonal crystals \sigma_xx=\sigma_yy but \sigma_zz usually has different value.  In still lower symmetry you may expect more non-zero elements of \sigma. If you don't see why in crystals the vectors j and E don't have to be parallel, think about a bunch of parallel insulated wires, not aligned with power source contacts attached to their ends.

  • V.G. Irisov added an answer:
    How do I linearize the force generated by two opposite electromagnets for the levitation of ferromagnetic object?

    I want to model my levitation system in simulink, for that I need linearization of force for levitation generated by two electromagnets.

    V.G. Irisov

    You may consider non-stationary setup: magnetic field between two ems has a point where the second derivative over the distance is zero. This is the area there the field is the most linear. Changing the strength of em currents you can shift this area up and down. Doing this periodically with high enough frequency you may get quasi-linear mean magnetic field in a larger area. Just an idea, may not work. 

  • Igor Goliney added an answer:
    Is the mechanical analogy of a child's swing to an electric oscillator valid when passing through resonance?

    When introducing the behavior of RLC series circuits around series resonance, it is often compared to a child's swing. In fact both can be made to reach a large amplitude at resonance. However, can the swing's frequency range from lower to higher values than the resonance one? In case it can, how is the passage to higher frequencies expected to occur?

    Furthermore, could the tippe top's characteristic feature of turning upside down when spun fast enough be explained by the same mechanism?


    Igor Goliney

    A swing is an example of a parametric resonance. A child must bend her knees twice per period of oscillations. Read about it deeper.

    A pendulum is better analogy to the electric oscillations.  Small oscillations of the pendulum are isochronic, which means their frequency does not depend on amplitude. For the arbitrary oscillations the frequency does depend on amplitude. There is an analytic solution of for the pendulum which describes anything in it including the mode of rotation. 

    The  top is a problem of  axial solid body rotation in the gravitational field with one point fixed. It has the analytic solution. I don'k know if anybody bothered to consider what happens if that one fixed point is lost and it turns upside down. 

  • Myroslav I. Kozak added an answer:
    Computing the E field resulting from a short dipole above a stratified media?

    Is there any software (preferably MATLAB) for computing the E field resulting from a short dipole above a stratified media? I found something for a line source, but not a dipole.

    Myroslav I. Kozak

    I'm counting at ovo, therefore am never wrong.

  • Gifuni Angelo added an answer:
    Why is absorption cross section a far field phenomenon, as we calculate it over desired volume of a nano-structure ?

    Why absorption cross section is far field phenomenon, as we calculate it over desired volume of a nano-structure ?

    Gifuni Angelo

     Waseem Raja,

    You talk of nanostructure and of far field phenomenon; but, you do not specify the frequencies of interest.

    You can see if the attached file and/or the references that you find there can help you.

    Best Regards

  • Marcin Nabiałek added an answer:
    Can anyone interpret this VSM graph?

    Recently I have analysed my ferromagnetic sample through VSM and I got result like this. Can anyone interpret this VSM plot of M (moment) vs H (Field) so that it may be helpful for me.

    Marcin Nabiałek

    First time I've seen such a curve and I do not know if it is well made. If so, then maybe something is wrong with the VSM.

  • Edward Lowry added an answer:
    What is the true order of calculation of the EM fields of the classical charge?

    It is known that the Lienard-Wiechert potentials cannot be derived from the wave equation if a radius R of the classical charge is initially assumed to be equal to zero. In original works of both authors, the radius of the charge is assumed to be finite and after calculation of the potentials, R -> 0 (according to Schott, 'the point laws of Lienard and Wiechert).

    But the EM fields are calculated from the potentials under assumption that the charge 'is treated as if concentrated at a point'. In the other words, R = 0.
    So my question is: what is a reason that we must assume R = 0 but not R -> 0 after calculation of the fields?

    I am concerned in it because the procedure with R -> 0 gives the solution of the wave equation corresponding to so called 'longitudinal EM waves (E_{||} ~ 1/distance).

    I add that the existence of such a solution (E_{||} ~ 1/distance) formally doesn't contradict to the Maxwell equation div E = 4\pi\rho because this equation forbids the existence of irrotational component E_{irr} ~ 1/distance. One can see it by solving the Maxwell equations in the gauge which Maxwell itself used, in the Coulomb gauge. The irrotational component isn't identical to the longitudinal component.

    The proof of the absence of E_{||} ~ 1/distance follows from Lienard's expression for the EM field (the book of Schott, Sec. 13).

    Edward Lowry

    The calculation of the EMfield of a classical charged particle can be expressed significantly more simply than appears in traditional presentations.  See the formula at the end of  http://users.rcn.com/eslowry/elmag.htm  . It is more fully explained in  Am J of Physics pg 871, 1963 or email me at  eslowry@alum.mit.edu .  

  • Kimmo Rouvari 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?
    Kimmo Rouvari

    A constant electric field alters the source and the observer in the same way, so they see exactly the same energy packet in the same way.

    That's right, therefore the light source should be used outside the field, only measurement should be done inside the field.

  • Charalampos A. Stergiou added an answer:
    Different ferrites with different permeability spectra show almost the same shielding effectiveness. Permeability plays no role?

    The near-field measurement was performed in the 200kHz-20MHz range and various ferrite plates were tested by placing between two loop probes.

    Charalampos A. Stergiou

    Dear Mr Grabner,

    Thank you for your response. In my tests all ferrites are isotropic. I also want to clarify that this is a measurement in the near field of loop probes.

  • Ganesh Kotnana added an answer:
    How do I draw the photocurrents from a PSD (position sensitive diode)?

    Is there any circuit? I have a DuoLateral PSD to measure magnetostriction?

    Not a cantilever deflection method

    Ganesh Kotnana

    Thank you sir@Orin Laney

  • Vinay Sharma added an answer:
    Does anyone know about variation of consecutive increasing and decreasing trend of coercive field with doping concentration?

    can anybody tell the clarification of variation of consecutive increasing and decreasing trend of coercive field with doping concentration. is it due to variation of grain size?  or oxygen vacancies?

    Vinay Sharma

    Mr Babu please elaborate that what kind of materials you are using and also specify the dopants otherwise .....the main reason of change in coercive field is the oxygen vacancies generated due to doping because grain size doesn't change much but the lattice parameters change due to dopants which may leads to the change in magnetisation................................

  • Jose Hugo Garcia added an answer:
    When do we use conventional hall effect experiments and when do we use QHE experiments?

    is sample's dimension a determinant factor to exert which type of experiment?

    for example if the sample is a bulk material...

    the aim is to find out charge carrier density of the material. is there any other ways for measuring the charge carrier density?

    Jose Hugo Garcia

    Ok, lets answer your question by parts. As you said, the classical Hall effect is a very efficient way to calculate the sign of the charge carriers and the charge carries density in metals.  So, if that is what you want to do you just need a regular magnetic field, a metal bar of reasonable quality and a multimeter and your are done. On the other hand, one of the first practical use of the Quantum Hall effect was to measure the resistivity quantum, which later on was used to defined a new standard for electrical resistance (see art. http://iopscience.iop.org/0034-4885/64/12/201/ )  the quantum Hall effect can also be used to determine the Fine-structure constant with great acuracy. So, as you can see, for practical application, the quantum hall effect can be used for metrology.

  • Stephen O. IKUBANNI 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?
    Stephen O. IKUBANNI

    Okay Prof. I am looking forward.

  • Sabah Gaznaghi added an answer:
    What is the relation of drift velocity to thermal velocity?

    can we say these two velocities are equal, just in a degeneracy state?


    when : Vth=(2kBT/m*)1/2 Γ(3/2)/Γ(1)

    then : VD=(2kBT/m*)1/2 Γ(3/2)F1/2(η)/Γ(1)F0(η)  is for a 2D material.

    is this equation correct? and why?

    note that

    VD=j/nq : is drift velocity

    F1/2 :  fermi-dirac integral of order 1/2

     Γ : Gamma function

    Sabah Gaznaghi

    Thank you for this great information.

  • Mukul Bhatnagar added an answer:
    Any advice regarding the fermi level in two different metals?

    As we know , when two different metals are connected electrically, the electrons in Metal A with higher fermi level will flow to metal B with lower fermi level. And finally their fermi level become identical.

    Firstly,for my understanding,because the number of  free electrons in metal Awith higher fermi level is larger than it in the metal B with lower fermi level ,then a net electrostatic field is generated and pointed from metal B to A, thus electrons flow from A to B under the E field.Is this correct?

    Secondly,what happen to both metal's fermi level? Finally they are same ,A 's fermi level is pulled down of course,but what about B, is the fermi level of B also be pulled up to meet the fermi level of A in some value ,or just remain unchanged until the fermi level of A lower to the same level as B? If the fermi level of B doesn't change in this process, then where do those electrons come from A go? if those electrons just go into metal B, the number of free electrons in B must increase,then the fermil level must shift up,Is this case?Or the electrons from A don't increase the number of electrons in B ?

    Mukul Bhatnagar

    The value of the fermi level for composite material will be different from that of either A alone or B because when two metals are in contact, they lose their individual characteristics unless there is some part of B or A which remain unchanged.

  • Samik Duttagupta added an answer:
    How can we calibrate an electromagnet which sources pulsed magnetic field?

    I have an electromagnet which is used to source pulsed magnetic fields. For the electromagnet operating with DC source, the calibration can be done with sensitive Hall probe. But for the pulsed case with a pulsed width of ~ 5ms, the Hall probes cannot be used for calibration. In that case, what is the most general way to calibrate the pulsed magnetic field. We can measure the voltage fed to the electromagnet.

    Samik Duttagupta

    Thank you everyone for your answers. It helped a lot. I calibrated the field by two different methods. The calibration by using a detection coil of smaller size than the magnet worked fine and it was possible to calibrate the field. Even it could be checked down to 500 microsec pulse width.

  • Zol Bahri Razali added an answer:
    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.
  • Carl Weggel added an answer:
    Do MAGLEV trains use superconductors to float or electromagnets in the train?

    Whether the magev have superconductors which repel the guide base magnetic field  or the train have electromagnets opposite to guide way to repel it from the guide way?it is obvious that the guide way have electromagnets but i am confused about the train. Which one is correct?if the train have superconductors then how the propulsion system actually works in that train? How it gets the thrust?

    Carl Weggel

    David Cope is correct in his answer; here is additional information:

    The Magnaplane--the first and arguably the best Maglev concept--was invented by Prof. Henry Kolm at MIT and Princeton in the 1970's.  A scale model was built and tested to demonstrate the concept.  The Magnaplane uses superconductors (SC) aboard the train to create an intense magnetic field (~10 teslas).  The train runs in a high-conductivity aluminum tube.  Below 5 to 20 mph, the train travels on rubber tires.  Above 5-20 mph.  the SC magnets aboard the train induce sufficient (repulsive) eddy currents in the aluminum beneath the train to levitate the train ~10 cm above the track.  Propulsion (acceleration and braking) is accomplished as described by David Cope.  The vertical position of the Magnaplane is inherently stable.  One preferred option is to operate the Magnaplane in a fully- or partially-evacuated tube.  (The concept of using an evacuated tube in a subway system was used in the very first mile of the New York City subway system in the ~1880 or ~1890.  See an article in the Smithsonian Magazine.)  In the 1980's, I designed and analyzed lateral, vertical, and longitudinal stabilization systems for the Magnaplane system.

    The German design uses vertical attraction magnets between lower-field, resistive, electromagnets aboard the train, and an iron rail ~0.5 to 1.0 cm located above the electromagnets.  Such a system is inherently unstable, so rapid, powerful feedback circuits aboard the train are required to maintain stability.  Furthermore, the iron "rails" must be meticulously positioned so that the train never contacts the rails. This system is used in a high-speed rail system between Hamburg and Berlin, I believe. 

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