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Questions tagged [electromagnetism]

The classical theory of electric and magnetic fields, both in the static and dynamic case. Also covers general questions about magnets, electric attraction/repulsion etc. Distinct from electrical-engineering.

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Dependence of electrostatic force on the surrounding medium

Two unlike point charges held at a fixed distance from each other. The force between them is measured. Then a brass rod is placed exactly in the midpoint of the line joining the 2 charges. Will the ...
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Question about action-reaction pair of two wires carrying current in the same direction

Imagine two wires lined up so that they are parallel, and that they both have a current running through them in the same direction. The magnetic field produced by the current of one of the wires (say,...
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Poincaré invariance of linearly polarised plane wave

I am reading a book that just quotes the Lie group generators and the discrete subgroups that leave a linearly polarised plane wave unchanged. And I have no idea how to derive them. Context The ...
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27 views

Conducting wire covered by a paramagnetic dielectric

How does the conductance of a conducting wire change if it is covered by a paramagnetic material comparing to a diamagnetic one? I am interested in the DC or low frequencies response mainly. For me ...
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Equivalence between Maxwell's equations and vector Helmholtz equations

When are equivalent the Maxwell's harmonic equations: $$ \nabla\times\left(\nabla\times\mathbf{E}\right)=\mu\epsilon\omega^2\mathbf{E} $$ and the vector Helmholtz equations: $$ \nabla^2\mathbf{E}=\mu\...
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Can I replace a multipole expansion by a combination of separate dipoles?

If I want to be able to model a magnetic field flux density $\mathbf{B}$ from a magnetic source located at the origin at a position $\mathbf{r}$, it is my understanding that I can represent $\mathbf{B}...
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Hall effect in superconductors

In many papers on Hall effect in superconductors (in the mixed state), experimental results are plotted as magnetic field vs Hall angle. Why we need to consider the Hall angle in these cases? Can't we ...
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75 views

What does 'Oppose a Change in Current' really mean from Lenz Law?

We all know what Lenz Law is, but I have a bit of trouble conceptualizing the phrase above. Does 'Oppose a Change in Current' means it will take more time for the current to increase to its maximum ...
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82 views

How to generate a very steep magnetic field gradient

I am a student that's very new to magnetism in general and I've been reading/learning for the past few weeks. I'd like to build magnetic tweezers to use in materials with kPa order stiffness - so I ...
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22 views

Relation of dispersion for a plasma

Assuming the electric field : $\vec{E} = E_{0}\,e^{i(kz-\omega t)}\vec{e_{z}}$ and the complex relation by doing $\vec{rot}\,(\vec{rot}\times\vec{E})$ with $\vec{rot}\times \vec{E}= i \vec{k}\...
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Why does the Electric field vector curl when the current is not a steady state one?

The math checks out but I was curious about knowing if we could intuitively comment on curling of the electric field when the charges start clumping/non steady-state current is present. Any hint would ...
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124 views

Proof that true passive linear memristors don't exist

This is a sequel to the following (now irrelevent) question: My attempt to implement linear memristor Assume the "conceptual symmetry" of the fundamental passive circuit elements holds: Then let's ...
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1answer
37 views

Can Tesla coils up frequency to visibility

So I recently built myself a small ish tesla coil to make my childhood dreams come true, and to further my small understanding of the electromagnetic force. Some people like the musical tesla coils ...
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41 views

Complex permittivity of mixture

I have been reading a few papers regarding the estimation of the permittivity $\epsilon_{mix}$ of a mixture of materials of permittivities $\epsilon_1$ and $\epsilon_2$ (I am not a phisicist or a ...
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Electromagnetic Movement

I posted this in the r/Physics page on reddit and someone recommended asking you lovely folk. I'm a prop maker and I need some advice from people in the know please. I have been tasked with creating ...
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What are the conditions for molecular,photon resonance other than same frequency?

I know that the basic condition for molecular,photon resonance is same frequency but what are the other condition for this process like phase relationship,round trip of the molecules and so.Also can ...
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When can we say $x$ and $p$ are “independent variable”, in order to find the Vlasov equation?

I have a question about "independent variable" in physics, and especially variable in Lagrangian or Density Function. I read several questions about it in this forum and although I have the feeling I ...
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57 views

Which Fresnel coefficients should be used at normal incidence?

There are two sets of Fresnel coefficients depending on the polarisation of the incident light. At normal incidence these equations converge except there is a phase factor of $\pm1$ For the reflection ...
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52 views

Right-Hand Rule and Reactance

In this paper by Guo et al., it is claimed on p. 4 that for a coil in a magnetic field, the induced current in the coil can follow either a right-hand or a left-hand rule, depending on the reactance. ...
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181 views

Understanding the meaning of (and using) Maxwell Stress Tensor

From reading Griffiths, I understand that the total EM force on a set of charges in volume $\mathcal{V}$ can be found as $$ \textbf{F}=\oint_{\mathcal{S}} \overleftrightarrow{\textbf{T}} \cdot d\...
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103 views

Impossible to decompose EM plane wave in spherical waves? (Normalization mismatch)

I am having a problem with an expansion that should be simple. Let's say I solve Maxwell equations in vacuum, but in spherical coordinates. The solutions of the TM family can be found easily to be $$ ...
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1answer
372 views

How resistance in coils affects the damping of oscillations of a magnet through them

I performed an experiment where I connected a magnet to the end of a spring with the north side on the bottom. The magnet at the tip of the coil was aligned to pass through a column of coils of about ...
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What happens to electrons on accelerated charged plate?

If I charge an aluminum circular plate with 10 000 V and rotate it will the electrons stay in place or rotate with plate? Will it behave the same if instead of one single plate I have a pattern of ...
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What is the quantum mechanical interaction cross section of a photon during scattering of photons by electrons present in metals?

I don't how to calculate interaction cross section for this process.Can I use Thomson cross section for this process because this process is similar to Thomson scattering,if not ,How to calculate ...
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56 views

Distinction between actions of fields vs. waves

When a magnet attracts a nail, it is an action of a field. However, wehen electrons move in a radio antenna, it is an action of a wave. At the quantum level, we often hear that an electromagnetic wave ...
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1answer
50 views

Are microwave-generating devices (gyrotrons) in the ITER system phased to heat plasma?

Provided electricity from grid is converted and supplies 24 gyrotrons, which generate electromagnetic waves, which in turn, will transfer their energy to the electrons of the ITER plasma to heat it up....
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309 views

Outside electric field due to an off-center charge inside a conducting shell

Suppose there is an off-center charge inside a copper conducting shell. We know that we can use the method of images to calculate the charge distribution on the inner surface. We also know that the ...
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94 views

The Meaning of a Complex Energy in Electromagnetism

I am currently trying to understand the physics of a fabry-perot resonator with a complex dielectric function. This is in general a standard problem to consider, but including a complex dielectric ...
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166 views

Why doesn't a charge in gravitational field radiate (or free fall)?

I have read these questions: Why do accelerating electrons emit radiation? Why does an accelerated charged particle emit electromagnetic waves? How and why do accelerating charges radiate ...
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Why are subwavelength structures not resolved by light?

I've been looking at the field of metamaterials for quite a while, now. Most of it is predicated on the assumption (which is empirically demonstrable) that electromagnetic waves, when they reach a ...
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3answers
271 views

If a magnetic monopole travels through a wire loop, which way does the current flow in the circuit?

I am confused about the sign convention for the electric field as described in the wikipedia article "Magnetic_monopole" and I would appreciate some help clarifying the topic. Take a positive (north) ...
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79 views

Does a Chern-Simons term break the $F \rightarrow \star F$ symmetry?

When is the electro-magnetic duality $F \rightarrow \star F$ a symmetry of a theory? I know it holds for free Yang-Mills, but would for instance a Chern-Simons term break it or a coupling to matter? ...
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74 views

What actually is the so called “Electromagnetic field”?

Is it just a mathematical construct or Is it something more than that. i read something about Maxwell trying to explain the EM field by giving a physical mechanism as to how the ether might actually ...
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1answer
96 views

How to shield against electric field but not magnetic field at 20 MHz?

I want to use an electrostatic shield to shield a circuit from an electric field produce by a solenoid. There is component in my circuit that I want to be exposed to the magnetic field of the solenoid,...
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63 views

Vector potential of a partially-known magnetic field

let's consider a three-dimensional space permeated by a known magnetic field $\vec{B}$. Let's consider in this space a topologically spherical surface $\mathcal{S}$ centred in the origin. I put a ...
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1answer
230 views

Do magnets attract magnetic materials by inducing magnetism in them?

Here are 2 concise excerpts from a book I have been reading:- 1.A permanent magnet produces its own magnetic field. An induced magnet is a material that becomes a magnet when it is placed in a ...
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29 views

How to calculate the electrical conductivity of an insulating dielectric?

So I am told that the dispersion relation for an EM wave in a conducting medium is: $$k^2 = \mu_0 \epsilon \omega^2 + i\omega \mu_0 \sigma_n$$ where $k$ is the wavevector, $\omega$ is the angular ...
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Input impedance of a line with complex load

I am currently studying electromagnetism mostly with the help of the book "Microwave Engineering" and when trying to solve problems which main goal is to compute the input impedance of a lossless ...
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Gauge transformations and the angular momentum operator?

In momentum space the complete angular momentum operator $J^{\mu\nu}$ can be written as $$J^{\mu\nu}=p^\mu\partial_p^\nu-p^\nu\partial_p^\mu-iS^{\mu\nu}$$ where $S^{\mu\nu}$ is the spin operator $$...
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890 views

Magnetic field lines vs. Electric field lines

What's the difference between magnetic field and electric field lines? Does $d\vec B$ point in the direction you would experience a force if you were a moving charged particle at that point? I know ...
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Why gravitational waves are not part of thermal phenomena?

Electromagnetic waves are part of thermal phenomena in the form of thermal radiations. But why gravitational waves do not show up as a thermal phenomenon, for example, why gravitational waves do not (...
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How to define a non-uniform mangetic field in Geant4 simulation?

I am getting stuck in creating a gradient magnetic field by Fieldmap method in example of field04 (geant4/extend/field/field04). At the moment, I am doing OK with creating a uniform one by creating a ...
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1answer
66 views

Finding magnetic field of highly symmetric systems by Ampere's Law

Recall how we find the Electric field of spherically symmetric charge distribution, cylindrically symmetric charge distributions and also charge distributions with planar symmetry (infinite flat sheet)...
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Physical implication of $\textbf{E}\rightarrow\textbf{B},~~\textbf{B}\rightarrow -\textbf{E}$ invariance of the Maxwell's equations

An interesting observation to consider about the Maxwell's equation is that in absence of the sources, the equations are symmetric under the interchange $$\textbf{E}\rightarrow\textbf{B},~~\textbf{B}\...
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Is it possible to ensure the diagonalization of the minimal coupling Hamiltonian?

Specifically, we know that the Bogoliubov transformation ensures that a quadratic (bilinear) Hamiltonian of interacting bosons (or fermions) can be diagonalized. Does the Bogoliubov transformation ...
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2answers
167 views

Does accelerating magnet emit radiation?

Just like accelerating charge produces emits radiation, does an accelerating magnet emit electromagnetic radiation?
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What is the difference between source current and current induced due to electric field?

I have seen in some places the following expression for current in an electric conductor (for example Peter Monk's Finite Element Methods for Maxwell's Equations, equation (1.8)):- \begin{equation} \...
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Interaction of charges and EM wave striking the surface of a conductor

Microscopically, what exactly happens between the charges on the surface of a conductor and the $E$ and $H$ components of the obliquely incident EM wave (in a classic viewpoint of field theory)? We ...
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How to think about the transverse force imparted to an electron by an EM wave?

This is a question that came to mind while dusting off the Feynman Lectures. I know he discusses some of this in a subsequent chapter, but I'm trying to understand it based on what has already been ...
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100 views

Coulomb potential in nuclear fission?

In the case of symmetrical fission we must consider the Coulomb potential between the two nuclei : $$ E=k \frac{q^2}{r}= k \frac{(\frac{Ze}{2})^2}{2r_0 (\frac{a}{2})^{1/3}}= k \frac{2 ^ {1/3}}{8} ...