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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Electromagnetic tensor in a FRW metric

In some papers [like https://arxiv.org/pdf/2204.06883.pdf, eq. (31) ], I see that the Electromagnetic tensor field, for a FRW metric (written in a conformal way) \begin{equation} ds^{2} = a^{2}(\tau) \...
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Are electric force and strong force equal in magnitude?

Should the electric force and the strong force be equal for a nuclei to be stable? Because if perhaps, the strong force is now more than that of the electric force, then shouldn't the nucleus collapse ...
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Resolving Faraday law vs. Stokes law for a long, time-varying current-carrying wire

A long wire carries a sinusoidally-time-varying current. What is the $E$ field around (not within) the wire? Assume quasi-stationary operation. Conflict: According to Faraday, the circulation of the ...
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Derivative of a real harmonic signal

simple question I can't figure out: $s(t) = A\cos(\omega t + \phi) = \mathfrak{R}[A e^{jwt}]$ is the temporal function of a real harmonic signal. I don't get how the derivative is still an imaginary ...
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Can we interpret timelike, rest frame motion as velocity in the time direction? If so, does it cause charge to generate a ''$B$ field'' perp. to time?

For background: I've had courses on SR/GR from Hartle, E&M from Griffiths/Purcell, and know E/B fields are unified/transform with lorentz boost. My question is less about 'does SR account for E/B ...
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Why do the Feynman-Heaviside formulae for $\vec E, \vec B$ fields differ from the Lienard-Wiechert fields?

Why do the expressions for Feynman-Heaviside fields look completely different from the Lienard-Wiechart fields though both of which are fields due to a point charge moving along a specified trajectory?...
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Charge in crossed E and B-fields

This post describes the motion of a charge in crossed electric and magnetic fields as: $$ x = r \cos ωt + \dfrac EB t\\ y = r \sin ωt\\ $$ Can you please explain how these equations are derived. Why ...
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Gauss Law Difference between Griffiths and Jackson [closed]

As per Jackson, Gauss's Law is defined as: $$\nabla \cdot \vec E=4 \pi \rho/\epsilon$$ Now as per Griffiths, the same equation is defined as: $$\nabla \cdot \vec E= \rho/\epsilon$$ So the $\pi$ part ...
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Electric field from an oscillating electric dipole

In the case of two charged spheres connected by a wire and separated by a distance d, when we drive the charges back and forth in the wire (by applying an oscillating electric field), we then have a ...
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How to calculate decay in plasmonic wave amplitude travelling inside a metal-insulator-metal waveguide?

I'm working on a system of plasmonic resonators along a waveguide, to calculate the transmitance of the system I'm using a system of coupled differential equations (CMT), the system consists of 8 ...
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What are the conditions for a glow discharge to occur vs an electric arc to occur in a glass tube?

After the invention of vacuum pumps, people experimented with running electricity in evacuated or partially evacuated glass tubes. In some sources people say this results in a glow discharge and in ...
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General solution of wave equation in homogenous, isotropic, linear, source-free material

Starting with Maxwell's equations we have $$ \nabla \times \vec{E} = - \partial_t \vec{B}$$ $$ \nabla \times \vec{H} = \vec{J} + \partial_t \vec{D}$$ $$ \nabla \cdot \vec{D} = \rho$$ $$ \nabla \cdot \...
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Inverse temporal power-law increase of DC voltage under the application of constant DC current

Is there any material in which the DC voltage follows an inverse temporal power-law increase, under the action of a constant DC current? This implies a situation in which a constant current, I, is the ...
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How to get the Magnetic Force from the Electromagnetic Tensor using Hodge Decomposition?

The following notation is used below: d: exterior derivative $\delta$: codifferential (adjoint of d) $\times$: skew-symmetric operator of a $\mathbb{R}^3$-vector $\nabla\times$: Curl operator in ...
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Where the E-M radiation is coming from?

In the case of two charged spheres connected by a wire and separated by a fixed distance $d$, if the charges are changing periodically, then this system (also referred to as an electric dipole) ...
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Does a time-varying electric field, which is produced by a time-varying magnetic field, itself induce an other magnetic field?

Based on the Maxwell equations we know that A time-varying magnetic field induces an electric field A time-varying electric field induces a magnetic field Suppose that an electric field, which is ...
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Pauli-Lubanski vector for Maxwell's equation

In the book quantum field theory by Itzykson and Zuber, page 53, the authors prove that Dirac's equation has spin 1/2 by showing that if $\psi$ is a solution to Dirac's equation, then compute that $\...
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How exactly does J. J. Thompson's cathode ray experiment show that electrons are particles?

I've have heard in many places that J. J. Thompson used a magnet and capacitor plates to look at the effects of the electric and magnetic fields on the cathode ray. This way, he was able to deduce the ...
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Integral for $E$ field of spherical shell [closed]

Per Griffiths Electrodynamics problem 2.7, finding the $E$-field of a spherical shell of surface charge dist (sigma); and radius $R$. I don't understand my result or how it jibes with Gauss' law. ...
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Electric Potential in Dielectric [closed]

I am currently preparing for an exam. One of the sample questions was: A rectangular region of space extending from z=0 to $\infty$ having a height $a$ and a width $b$ is filled with homgenoues, ...
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Question about the electric dipole radiation [duplicate]

I can understand why an oscillating electric dipole emmits electromagnetic rdiation-charges are constantly accelerating. But how come fixed electric dipoles with varing charges also emmit E-M ...
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For a given $B$ magnetic flux density in a homogeneous magnetic field what is the separation distance between the flux lines?

For a given $B$ magnetic flux density thus number of lines of force passing through per unit of surface area of a homogeneous magnetic field of parallel flux lines inside a vacuum environment, is ...
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How to derive the formula: $F = q\,v\,B\sin\theta\:$? [closed]

How to derive the formula: $$ F = q\,v\,B\sin\theta \tag{01}\label{01} $$ where $\:\theta\:$ is the angle between vectors $\:\boldsymbol\upsilon\:$ and $\:\mathbf B\:$?
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What are the speeds of light inside a retarder?

A retarder introduces a phase shift between the field components traveling along the fast axis and the slow axis. But at what speeds are they actually traveling? Is the speed of light along the fast ...
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Mathematical equivalent of Fundamental nature of charge [closed]

How to mathematically represent the fact that electric charge is a fundamental quantity? i.e. that it cannot be explained in terms of other things, for example, the normal force can be explained as ...
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Can electric charge/electromagnetics effects of stars, particles, etc., be a major factor in motions attributed to Dark Matter/Dark Energy?

Electromagnetic effects of moving charged stars and particle flux on galaxies rotation and space expansion.
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Why is the field of an electric dipole important in imaging? [closed]

Is it perhaps because the atoms and molecules in our scatterer/object become oscillating electric dipoles due to the electromagnetic field(light) that we shine on it? So can we actually model our ...
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Why are Ohm's law and Ampere's law contrasting in Magnetohydrodynamics?

In the Magnetohydrodynamics the electric current is given by the Ampere's law is: $$\nabla \times \mathbf{B}=\mu_0\mathbf{J}$$ where $\mathbf{B}$ is the magnetic field and $\mathbf{J}$ is the electric ...
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Classic Image problem

The classic image problem: Here, to calculate the potential above an infinite grounded conducting plate and a point charge q held at distance d above the plate, the image method is used. Here, I want ...
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Is Faraday's law applicable exclusively to free charge carriers?

Say I had a loop of some material with no free charge carriers. If the loop were kept in a space with a varying magnetic field, would an EMF be generated along the loop? What about a rod of the same ...
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Does a perfect conductor has unlimited supply of charges?

In Griffiths, it has been written that A perfect conductor would be a material containing an unlimited supply of completely free charges. My question is, how is the unlimited supply of charges ...
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Gyromagnetic precession of electron inside a homogeneous electrostatic field?

Would an electron placed inside a homogeneous electrostatic field (i.e. the static field between the parallel plates of a fully charged capacitor) in addition to being accelerated towards the ...
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Magnetic dipole moment due to folded loop [closed]

A steady current i flows in a small square loop of wire of side L in a horizontal plane. The loop is now folded about its middle such that half of it lies in a vertical plane. Let $ μ_1 $ and $ μ_2 $ ...
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1 answer
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Biot-Savart equation not behaving as expected [closed]

I am currently in the process making a large electromagnet with a permeable core that has the following specs: I = 250A N = 300 L = 0.25m relative permability of core = 230000 $B=\frac{\mu_r \mu_0 ...
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Regarding the deflection magnetometer

There is something called tangent law in magnetism which says that if there acts 2 magnetic field - horizontal earth's field (H) and external field (B) - then the resultant magnetic field direction is ...
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1 answer
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Is the skin effect applicable to a solution of ions?

Can the formula $ \sqrt\frac{2}{wµσ}$ be used for solution of salt ions? I saw online that people wrote papers where they use salt water and cell phones to investigate the skin effect, but in other ...
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What is virtual photon concept in classical electrodynamics?

If we observe a charged particle like an electron passing us at some high speed $u$, then as $u \to c$ the field we observe looks like a superposition of plane waves normal to the trajectory of the ...
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Can a x ray see its own reflection ? / Is opisitional force present between two x-ray transmitters? [closed]

If you downstage a particle exselerater to the point before a black Hole is created ,would an x-ray see its self before the big bang,I which creates the black hole?
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Static opened magnetic field line

In every EM textbook, it is written that static magnetic field lines cannot be opened. As an example, one is shown the field lines of a magnet bar where the central field line is obviously straight ...
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How does the electromagnetic field behave in a complex circuit when first energized? Such as in a computer CPU booting up

From what I understand in the case of a simple circuit, the electrons in the circuit wiring are steadily excited, i.e. energized, in a fairly complex way by the external energy input, as the energy ...
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Closed loop integral of Electric Field dotted with dL

I am struggling to wrap my head around Faraday's law: $$\oint\mathbf{E}\cdot d\mathbf{l} =-\frac{d\Phi}{dt} = -\frac{d}{dt}\int_S\mathbf{B}\cdot d\mathbf{S}$$ Integral of an Electric field dotted ...
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Can a divergent vector field in n-dimensions be seen as a divergence-free vector field in n+1 dimensions? [closed]

I raise this question as I demonstrated "magnetic monopoles" to laymen and didn't stop then. When you place axially magnetized supermagnets on a table with the north poles pointing up and ...
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Hysteresis Curve of a Super Conductor

Recently I have been studying magnetostatics. I came across the hysteresis curve for various objects such as transformer, a permanent magnet and other materials. Basically these curves tell how good ...
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1 answer
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"Lorentz gauge" or "Lorenz gauge"?

In electrodynamics there is a gauge condition named after Ludvig Lorenz: $$\partial^\mu A_\mu = 0.$$ In general relativity we also have a gauge condition defined as follow: $$\partial_\mu \gamma^{\mu\...
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Does the force between two magnetic poles ever reach zero?

If we hold two magnetic like-poles together and start to move them away, would the repelling force reach absolute zero at certain point? In that scenario, as a layman, I think that there is something ...
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If magnetic field is a bivector, then what is the pole strength?

I have an intuition that magnetic fields are bivectors. While trying to derive some equations with bivectors, I arrived at this formula $$F=\frac{q_{m_a}q_{m_b}}{r^2}\hat r$$ So we have, $$F=q_mB$$ ...
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How to compute r hat over r squared [closed]

Might be a very simple question to answer but how to compute $\textbf{v} = \frac{\hat{r}}{r^2}$. The answer should be $\nabla \cdot \textbf{v} = \frac{1}{r^2} \frac{\partial}{\partial r}(r^2 \frac{1}{...
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Faraday Rotation and Elliptical Polarization of Light

I have a question about Faraday Rotation. I read that propagation via the ionosphere results in elliptical polarization, so I did some further research and found that light is made up of right and ...
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On a paragraph in Feynman's Lectures on Physics -Volume 1, Chapter 1

One of the paragraphs in the textbook that I don't quite understand is the following: If we were to charge a body, say a comb, electrically, and then place a charged piece of paper at a distance and ...
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2 votes
2 answers
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Symmetry of line integral of $B$ over circle at infinity with finite number of wires cross plane perpendicularly

I am studying problems which look like this: Some number of wires perpendicular to plane, some line segment (here x-axis is taken), find integral along line segment (here x-axis) In answer to the ...
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