Electromagnetism

Electromagnetism

  • 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 · Leibniz-Institut für Angewandte Geophysik

    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 · Advanced Information Microstructures

    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 · N/A

    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 · The Centre for Research and Technology, Hellas

    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 · Indian Institute of Technology Hyderabad

    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 · Jawaharlal Nehru University

    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 · Federal University of Rio de Janeiro

    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 · Landmark University

    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?

      VD=VthF1/2(η)/F0(η) 

    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 · Urmia University

    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 · Institute for Plasma Research

    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 · Tohoku University

    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.
    Zol Bahri Razali · Universiti Malaysia Perlis

    Dear friend, very interesting question.
    However, the easiest way is searching the related document by typing the keywords into google scholar. You will find some related articles.
    If yet to find the articles, do not hesitate to let me know. InsyaALLAH I will help you in detail.
    Good luck. Dr Zol Bahri - Universiti Malaysia Perlis

  • 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 · Magnet Analysis for Government, Industry, and Colleges

    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 · King Saud University

    You can also go for Arophysics.

  • Qian Xu 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.

  • 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?
    Charles Francis · Jesus College, Cambridge

    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 · Nuclear Power Corporation of India ltd.

    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 · University of Gujrat

    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 · Amirkabir University of Technology

    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 · Lomonosov Moscow State University

    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.
  • Jean-Christophe Michel Delagnes added an answer:
    Spontaneous Parametric Down-Conversion is a classical or quantum phenomenon?

    I have seen that both classical and quantum theories are developed for this phenomenon. So please explain me the need for this.

    Jean-Christophe Michel Delagnes · University of Bordeaux

    Quantum description is needed for "small number of photons" phenomena (in the time or in the frequency domain). As mentioned by Alexander it is crucial for instance in entanglement, but also for detailed analysis of parametric amplifier, triple-photon, etc. Otherwise (semi)classical is sufficient (no tremendeous effect on macroscopic quantities).

  • Shian-Loong Bernard Lew 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.
    Shian-Loong Bernard Lew · Taylor's University

    Theory>Experiments. This is my reasoning: theory tells us what is possible and expands our imagination. Experiments tells us what is working now and increases knowledge and understanding. Without theory there will be no experiments to talk about. Even though some would say that experiments (sometimes accidental ones) then to point us to a new theory- I would say that is an exception rather than a norm.  I take inspiration in Einstein's words:

    “Imagination is more important than knowledge. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there ever will be to know and understand.”

    ― Albert Einstein

    Arguing along the lines of Einstein's Gendankens, we realize by definition, it is a thought-experiment. Imaginative thought=>theory>experiments 

  • Korkut Uluaydin added an answer:
    What is the material that can be used to shield EMI within 50 - 60 Hz?

    Either reflections or absorption.

  • Akira Kanda added an answer:
    Does accelerated charge emit photons to all observers?

    We know that a decelerated charge emits photons. So you can visualize an observer in a stationary frame of reference that detects these photons.

    However for another observer in an accelerated frame of reference that is moving, at the same pace as the charge, the charge appears stationary and apparently should not emit photons.

    How do we reconcile these two apparently contradictory positions?

  • Elangovan Sundaram added an answer:
    Why is eddy current path purely resistive in nature?

    In transformer core,etc the eddy current path is assumed to be purely resistive in nature, but there must be some inductance  due to self linkage of the eddy current generated flux with the core itself.The value of this inductance must be very less .Thats why purely resistive  eddy current path is assumed.My question is why this inductance is very less?

    Elangovan Sundaram · ALL INDIA RADIO

    Eddy current is a loss in  the core losses estimated assuming that the operating frequency is constant.So it is for the purpose of calculation ,directly proportional to leakage of flux in turn converted to heat.as the loss is directly proportional to eddy current it is assumed as resistive.

  • Vedvyas Jayprakash Dwivedi added an answer:
    How to give AC current or voltage source excitation in transient (Electromagnetic Analysis) in ANSYS?

    In transient analysis of electromagnets, I need to provide a sinusoidal source. I am able to give the current density at the node but unable to add frequency as input. I am using GUI for my simulation in ANSYS. 

    Vedvyas Jayprakash Dwivedi · C U Shah University Wadhwan City, Surendranagar, Gujarat, India

    Agreed with Feng. If still any doubt, please email me at provc.cushahuniv@gmail.com 

  • Amipara Manilal D added an answer:
    Sensitivity from Biot Savart's law ?

    I need to calculate the sensitivity from the coil using Biot Savart's law how can I calculate current using the position values of cylindrical coil and the rotation matrix the cylindrical coils are  arranged as polygon also we are not considering any electrical ( like resistance or inductance ) parameters. How can I formulate the tripple integral part as a function of current and position 

    Amipara Manilal D · Balaji Institute of Engineering & technology, Junagadh

    According to the Biot-savart law, you need to know the sourcing magnitude current, and its distance vector, which generates field. Cylidrical coil can be taken as uniformly distributed source/sink. At a distance d, amount of field will induce the EMF to sink coil. Produced current magnitude, of course, depend upon loading of sink coil.

    The impedance  part (Losses) included in a Biot-savart equation in the distance form.

    With the above considerations, (Constant source, constant distance) the sensitivity will depend upon the sink coil turns, core  & sink coil losses. This can be improved by reducing distance between source and sink to the possible extent and/or increasing the source current magnitude  to the possible extent

  • Raul Simon added an answer:
    What is the shape of this induced field induced by temporal current variation in a in a coil?

       My question is prompted by Problem P. 4263 (May 2010) of the webpage www.komal.hu, which contains physics problems from Hungary (in English).

    Raul Simon · L A M B

    OK; if that is the case, I consider the question answered. Thanks to you all.

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