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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What name would you give to the method of approximating an arbitrary magnet with many smaller dipoles?

Let's say I had an arbitrarily shaped permanent magnet, with total magnetic moment $M_{0}$. Ways to calculate the magnetic field of this magnet include an analytic solution (if one exists), as well ...
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Bandgap Spacing in Photonic Crystals

I am doing some self-study on photonics and have encountered the following question: We know that amorphous electronic crystals such as amorphous silicon have a bandgap. Can amorphous photonic ...
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Question about laws of conservation in electrodynamics

One way of deriving laws of energy, impulse and angular momentum of electromagnetic field conservation is following: Introduce two values below: $$ \mathbf P = \frac{c}{4 \pi}[\mathbf E \times \...
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I lost a factor of two in the electromagnetic field tensor

I apologize for this simple question, but I lost a factor of 2 and can't find it anymore, so now I'm looking on the internet, perhaps one of you has some information about its whereabouts. :-) ...
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How to teach modeling physical systems?

While working as a teaching assistant for introductory experimental physics classes (newtonian mechanics and electromagnetism) for the last couple of years, I faced a certain kind of issue again and ...
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Electromagnetic field and voltage drop in a circuit

I have been thinking for a while about what really causes voltage drop and how to explain it in terms of what the electric and magnetic fields do. So I've been reading a lot of posts here and in other ...
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224 views

Is it possible to eliminate Van der Waals interactions?

I came to know that the friction force actually depends on the surface contact area due to weak interactions (adhesion due to Van der Waals forces) between the atoms of both materials increasing in ...
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1answer
683 views

Distribution of current of a rotating cone

If I have a hollow cone (surface with no bottom cover ) as the one in the picture. The cone has surface charged density $\sigma$. It rotates around the symmetry axis with an angular velocity $\omega$. ...
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1answer
830 views

If we have a current $I$ flowing down a wire, why must the net bound current be zero?

Say we are dealing with a wire that has a current $I$ flowing through it, i.e. $I$ is the free current. Why must it then hold that the net bound current, that is, the bound volume current, $J_b$, and ...
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126 views

Effect of Motion on Capacitors

What will happen if a capacitor is accelerated or rotated very very fast? Will its capacitance decrease? Will one of the plates be able to hold the electrons which is providing the Potential ...
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63 views

Examples of materials with saturation magnetic flux densities over 8T

We regularly hear about magnets breaking the highest flux density records produced by laboratories, obviously the magnetization field intensity is generally not stated, so are there any materials that ...
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509 views

Thickness of electromagnetic waves

Radio wave photons and light photons have a different wavelength. But they also appear to have a much different "thickness" in that light photons "fit" cleanly through small pigeonholes, where the ...
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Force between Point Charge and a Neutral Conducting Sphere

My question is simply an extension of this SE post; I don't understand why $q'+q''=0$ (both charges of hypothetical construct) for the sphere to be neutral. I'm trying to find a parallel between the ...
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What is the meaning of a vector integral over another vector?

Reading Portis's Electromagnetic Fields: Sources and Media I came across this expression for the stored electric energy in a volume in a general medium: $$ U = \int dV \int \mathbf{E}\cdot d\mathbf{D} ...
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Why are solar panels still efficient with cloudy weather?

Recently I did an experiment in college involving solar panels. I noticed that the weather suddenly changed from sunny to cloudy, however the output of the solar panels dropped only very little. My ...
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Trajectory of two charges in free space

I am talking about the "two-body problem" for charges. Apparently, for two identical particles $\mathbf{x,y}$, each with charge $q$ and mass $m$, ignoring the gravitational force, the equations of ...
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What is the radiation field of a black body with temperature $T$ translating at speed $v$?

Suppose I have a spherical black body at temperature $T$ with radius $R$. When stationary in a vacuum at zero Kelvin, it produces an incoherent radiation field according to Planck's law $$ B_\nu = \...
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63 views

An absurd consequence of magnetic field as an axial vector

One of the definitions of magnetic field (in free space): Force on magnetic north pole per unit pole. Magnetic field is an axial vector. So it changes its sign when we use left handed coordinate ...
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What is the critical volume/mass of non-deuterated water that appears blue to our eyes?

A glass of water appears clear, but a swimming pool appears blue. At some point there must be a switch and I am wondering if anyone knows what that point is (or if its so gradual we can't tell). Does ...
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Shape of wavefronts for transverse modes in electromagnetic waveguide

I've been able to confuse myself about the shape (i.e. surface ripple/bend/tilt) of wavefronts for transverse modes in an electromagnetic waveguide, like a rectangular cross section waveguide with ...
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2answers
58 views

Is the Poynting Vector Continuous

Helmholtz equation for the magnetic field is $$∇^2 \mathbf{H} + k^2 \mathbf{H} = \mathbf{0}$$ Assuming time-harmonic waves Ampere's law can be rearranged $$\mathbf{E} = \frac{∇\times\mathbf{H}}{σ+...
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Chemistry without electrons?

This question on the behavior of a universe with massless electrons got me wondering: what would atoms look like if electrons were not merely massless, but rather the lepton fields just didn't exist ...
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85 views

How is Electromagnetic segmented-capacitor propulsion system even possible?

The alleged invention - patent US10006446B2 - suggest allowing the creation of lifting force (4) on electrically charged rotating plate when the magnetic flux is turned on and directed towards the ...
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63 views

A steady line current moving at a steady velocity can produce non-transverse electric fields. What about moving line charges?

A steady, straight current that is stationary can exert a Lorentz force on a moving charge that is parallel to that current. Conversely, a stationary charge can experience a Lorentz force that is "...
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1answer
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Confused about the anapole (toroidal) moment

I am a bit confused about the existence of the anapole moment. As far as I understand, in order to fully describe a charge-current distribution, one needs, beside the normal electric and magnetic ...
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Being the Earth modelized as a big magnet (or solenoid), which would be the field intensity of such a model inside the solenoid or magnet axis?

This question has arisen from the Carl Friedrich Gauss’ statement that the Earth could be modeled, concerning his calculations about the earth’s magnetic potential, as a big magnet, himself having ...
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Solving the Euler-Lagrange equation with the Axion Lagrangian

I am trying to show that for a constant axion field $\theta(\textbf{x},t)=const.$ the axion Lagrangian $\mathcal{L}_\theta=-\frac{\kappa\theta}{4\mu_0}F_{\mu\nu}\tilde{F}^{\mu\nu}$ does not lead to a ...
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Charge distribution on wire cutting flux

A straight wire of length $\ell$ moves with a constant velocity, $\vec v,$ through a uniform magnetic field, $\vec B$. The field, the wire and its velocity are at right angles to each other. When a ...
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Degrees of freedom in M-B distributions used for solving Poisson equation

I am having some trouble to understand the Maxwell-Boltzmann probability functions or more precisely how to use it in the Poisson equation. The problem is the following: consider the case of an ion ...
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1answer
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Is friction an emergent phenomenon?

The microscopic origin of the frictional forces is due to contact between irregularities of the surfaces as seen here [1]. Electromagnetic interactions are the fundamental forces responsible for ...
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1answer
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Why is magnetic attraction field lines when seen with ferrocell reverse that with iron filings?

Magnetic attraction seen with ferrocell. The magnetic field pattern is the same as that for repulsion when viewed with iron filings. Why?
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Is the speed of Magnetic field infinite?

My real question is: From Amperes law we know that there is no magnetic field outside of the coaxial cable because the magnetic field generated by the inner wire and outer shell are equal but in ...
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Reason for repulsion of like charges

Like charges repel and unlike attract. Like magnetic poles repel and unlike attract. Since electricity and magnetism are fundamentally linked , is this attraction/repulsion due to the magnetic fields ...
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Visualizing EM fields

Personally, I have trouble visualizing a flock of birds without the birds. I have the same trouble trying to visualize an EM field without the photons. More than once, I have been hounded off of the ...
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Is there a physical circumstance where the flow of a system changes from a nodal source/sink to a saddle?

Phase portraits for sinks can often like eddies in water, so I would assume there is some fluid or aerodynamic system where these curves appear. But, the only difference between a saddle and a nodal ...
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How could Proca equation be written in NP formalism

I am trying to do some calculation of fields in curved background. Is there anyone know the complete form of Proca equation $$\nabla_{\mu}F^{\mu\nu}+m^{2}A^{\nu}=0$$and$$F_{\mu\nu}=\nabla_{\mu}A_{\nu}...
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Theoretically, if I swish a laser back and forth fast enough/at the right frequency on a distant surface, will I get an interference pattern?

Just curious. I’m kinda stumped and am interested in how one would go about visualizing/calculating this.
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Positive frequency definition in general spacetime for general fields

In Quantum Field Theory the positive frequency solutions to the classical field equations are quite important since they are the basis of the definition of particles. In Minkowski spacetime we have a ...
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Would the voltmeters give different readings in a circuit with induced current?

$\def\vE{{\vec{E}}}$ $\def\vD{{\vec{D}}}$ $\def\vB{{\vec{B}}}$ $\def\vJ{{\vec{J}}}$ $\def\vr{{\vec{r}}}$ $\def\vA{{\vec{A}}}$ $\def\vH{{\vec{H}}}$ $\def\ddt{\frac{d}{dt}}$ $\def\rot{\operatorname{rot}}...
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Optimal pump wavelength for frequency doubling in periodically poled media

Background I am currently working on achieving second harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using a 1560 nm pump laser. PPLN makes use of quasi-phase-matching of the ...
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Maxwell stress tensor for electromagnetic wave

Sorry if this is a naive question but I've been struggling in trying to proof this for a week. Consider an electromagnetic wave with wave vector $\vec{k}=k\hat{n}$, the Maxwell stress tensor can be ...
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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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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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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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1answer
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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 ...