• 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. 

  • Stan Zurek 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 ...

    Stan Zurek

    Permeability is NOT the slope between polarisation J and flux density B. Both of these will be very similar at low excitation for soft magnetic materials, so that it is often assumed that B = J.

    Permeability, in the understanding of SI units is the slope of the curve of B = f(H) or J=f(H), because by definition B = mu_r * mu_0 * H, where mu_0 is the magnetic constant and mu_r is the relative magnetic permeability (mu_r = 1 for vacuum, by definition).

    For engineering-like applications you can derive the amplitude permeability from the B-H loops. For each loop you detect Bpeak and Hpeak and use them to define mu_r as:

    mu_r = Bpeak / (Hpeak * mu_0)

    This concept is not very precise at high frequencies, because the B-H loops become round and thus Bpeak does not correspond to the same point on the loop as Hpeak. But the concept is good enough for engineering applications, and it can be certainly used as a figure of merit for most studies. 

    The same applies if J or M is used instead of B.

  • 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. 

  • 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

    Although my invite for research contributions are initially focused towards grade school children, I invite my colleagues here at RG to feel free to participate as well (see link).

  • 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

    Dear Colleagues,

    I invite you kindly to contribute to the UJPA Journal Special  Issue entitled

    "Casimir Effect, the Electron Stability and Dynamical Mass Phenomena: Classical and Quantum Field Theory Description"

    In his famous paper "Introductory Remarks on Quantum Electrodynamics" H.B.G. Casimir, being stimulated by his investigation of the vacuum-.uctuation-driven attractive force between conducting plates (the Casimir force), suggested a semiclassical model of the electron within the spirit of Lorentz.s theory of the electron. Such a model, being out of favor in terms of providing a realistic description of the quantum electron, nonetheless it remains potentially not exhausted and still possesses many unveiled physically important features. As it is well known, Casimir suggested two models, the second of which is based on a dense shell-like distribution of charge which might partially (as in the Casimir Effect, generally), or even wholly, suppresses vacuum fields in the interior of the shell. An important question is still up to date left concerning a quantum .eld theoretical description of the related additive in.nite negative mass within the QED renormalization approach. As is well known, the proper-time is attributed to Minkowski. Although it was namely Poincare, who discovered the proper-time, it was Minkowski who recognized its importance in physical theory and showed that it is the only unique variable associated with the source and available to all observers. This idea was later deeply re-analyzed by R. Feynman and further it was suitably developed within the Lagrangian and Hamiltonian description of the electron dynamics. The successful application of the least action principle within the Feynman proper time paradigm is believed to give rise to the charged particle „dynamical" mass explanation, thereby finally solving the well known Abraham-Lorentz-Dirac 4/3-electron mass problem. The further both the classical and quantuum field theory descriptions of these two Casimir related phenomema concerning the classical stability of electron and its related dynamical mass expression are wanted and form a main trend of the Proposal.


    A wide scope of articles touching the topics above are  extremely welcome!

    Sincerely  regards,


  • 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

  • Stephanie Scheidt 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.

    Stephanie Scheidt

    I have had an similar result in a measurement, when the Gaussian probe was come loose. I recomment to check all components of your device.

  • 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. 

  • Mehmood Ali Noor added an answer:
    Can anyone recommend journals reviewing the effects of electromagnetism on plant germination growth?

    Any legitimate journals reviewing the subject of the positive or negative effects of electromagnetism (and MF in general) on the life cycles of plants?

    Mehmood Ali Noor

    You can also go for Arophysics.

  • Charles Francis 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?
    • Source
      [Show abstract] [Hide abstract]
      ABSTRACT: In this paper we derive the uniformity and limitation of the speed of light and all features of the Lorentz transform (mass, time, time skew and length) starting from assumptions (observations) about the effectiveness of electromagnetic interactions on rapidly moving particles in a single reference frame along a single axis, extending this one step at a time. Using the classical law of conservation of momentum, various effects and constraints are extended to include all directions and all types of distinguishable and measurable energy or momentum interactions. In other words, if the speed of electromagnetic phenomena is uniform and finite in any reference frame then it is in all inertial frames and every type of energy is similarly constrained and Special Relativity emerges. Or conversely if the reader prefers, it can be argued that the assumptions used in this paper about effectiveness of interactions follow from Special Relativity. Possible implications of such a derivation are discussed in regards to measurements of entangled particles and universal interactions such as gravity.
    Charles Francis

    Hugo, I have looked at Logunov's work. I believe he was an experimental physicist, not a theorist. If I do not wish to discuss RTG, it is because, imv, it does not merit discussion. There is enough wrong with it that studying well enough to make serious comment is just not worth the effort.

  • Gyan Chandra Chauthwani added an answer:
    Why there should be a resistance for the propagation of electromagnetic waves at all in free space?

    We know that mu0 and epcilonphysically represents the resistance for the propagation of magnetic and electric fields in free space. My question is from where this resistance comes from? Why there should be a resistance in free space at all?

    Gyan Chandra Chauthwani

    If one side we say perfect vacuum is without material, on the other side quantum vacuum says vacuum density is very high.

    I am putting very strong objection towards these contradictory results and statements, because its a very crucial time for physicists to save their theories otherwise complete structure of physics is going to demolished very soon.

  • Bajece Balkan Journal of Electrical added an answer:
    Has anyone used the Hitachi nanopico absorbed current meter/mapper?

    Hello, I have a Hitachi S3500N tungsten SEM and im interested in mapping  DC current distributions in my samples. My Hitachi affiliate offerd me an equipment that they make called the "NanoPico". it is a current amplifier that has an output that can be connected to the SEM to obtain an image. My question is if any one has used it could you provied your insight or perhaps examples. or if any one knows of another option that would be helpful too. 

    Thank you

    Victor I Hernandez

  • Muhammad Javaid added an answer:
    How can I find relative permitivity/ dielectric constant when we know about permitivity of material?

    permitivity of material is 35. so how can i find its dielectric constant/relative permitivity ?

    Muhammad Javaid

    If you know the refractive index of that material then you can easily find the relative permittivity by the relation  n2 =  ϵr

  • Parviz Parvin 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.
    Parviz Parvin

    Moreover, the refractive index is wavelength dependent and usually reduces smoothly at longer wavelengths according to Sellmeier eq. It leads to varying slope dn/dw in terms of wavelength. This optical property is called Dispersion. Prism is a dispersive element. The optical material is chosen with low absorption at operating spectral range.

  • Vladimir Lvovich Bychkov added an answer:
    Why displacement current is necessary in Ampere's law?

    Maxwell modified the ampere's law by the introduction of displacement current and was presented as 4th of the famous Maxwell equations. But I am always not clear about this term. Can any one help in this regard?

    Vladimir Lvovich Bychkov

    It means that the current is not the divergence and is not connected with moving of charges. That is the point/ There is no indication in the law of full current on any charges, and charge conservation. These concepts appeared later than Maxwell involved his ones.

    The problem of the right hand side exists long and there are books devoted to contradictions appearing after incclusion of the displlacement current. The main one that at using of this equation without the  current tert you (using another equation) quickly get the wave equation where E and the B are equal, with the same direction; in points where E=B=0 there is the problem with energy, and there no understanding of medium where this wave propagates. So either to  involve the concept that the current is rot(A) where A is the vector (or vector potential-rot of some fluid flow)  or to consider that electromagnetism exists in vacuum without any medium, as it postulated in modern physics.

  • Johan Frans Prins 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.

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