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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Magnetic field lines are closed lines?

How is it proven that the magnetic lines are closed lines? Because the divergence of $\mathbf{B}$ is zero does not sound convincing, since fields like $\mathbf{F} = a\mathbf i +b\mathbf j+c\mathbf k$ (...
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How should I use the complex permittivity of a material?

Here: $$\epsilon = \epsilon' + j *\epsilon''$$ I understand that the first part ($\epsilon'$) is the relative permittivity of a material, while the second part $\epsilon'' = \frac{\sigma}{\epsilon_0\...
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Is there such a thing as infinitesimal electric field?

I am interested in calculating some response properties, namely, susceptibility and polarizability. In principle, susceptibility should be the functional derivative of the electron density to a ...
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What is the irradiance of an extended source?

For example, what is the irradiance at a certain A = 2m^2 surface from a P = 10W tube lamp with significant radius and length - say, r = 15mm L = 100mm - measured at a small distance - say, d = 2m? ...
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Can the electric field have closed field lines?

In electrostatics, we know that $\vec{\nabla}\times\vec{E} = 0$ and so, the field lines can't form loops. But when we have time-dependant magnetic fields, there's the Faraday-Lenz law which tells us ...
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EM Lorentz force field flux in the air gap of a parallel plates charging capacitor?

For a charging/discharging capacitor, simultaneously to the electric field created by the displacement current during charge/discharge cycle, a magnetic field is also generated in the area inside the ...
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How to calculate the field distribution due to a current radiating in one direction?

Given a surface current distribution, $\textbf{J}(r)$, we can calculate the magnetic potential in the Lorenz gauge using $$ \textbf{A}(\textbf{r})=-μ_{0} ∬g(\textbf{r},\textbf{r}')\textbf{J}(\textbf{r}...
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Hello, I am wondering How I can set Equation which describing the motion of an object attracted to magnet

Umm, I imagined the sphere magnet (it can rolling) which has friction. then How I can do make formula about this? I know $$ E = \int \frac{B^2}{2\mu} \,dV $$ and that is same as $1/2mv^2$ ? and $I = 2/...
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How to calculate the electric field of a polarization density?

By polarization density here I just assume I have a "blob" of free positive and negative charges, and instead of describing the system with a charge density $\rho(\pmb{r})$ I want to use the ...
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Steps in Quantizing Electromagnetic Field for the Gauge Condition $A_0=0$

While reading section 9.3 of QFT An Integrated Approach by Fradkin, it is shown (see equations $(9.49)$ and $(9.54)$ of the book) $$B_{j}(\boldsymbol{x})^{2}=\boldsymbol{p}^{2} A_{j}^{T}(\boldsymbol{p}...
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Does the current inside a capacitor with dielectric becomes zero, if a DC (constant)voltage is applied?

Imagine a capacitor with dielectric with conductivity $\sigma$ and constant voltage source with voltage applied say $V$ is applied to it's ends, now using Ohm's Law we have $ \vec{J} = \sigma \vec{E}$....
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3D FDTD - how do I compute number of variables and required number of iterations?

I am trying to solve a an electromagnetism problem via 3D FDTD. I can't quite get (c) and (d). (note: 106 cuts per wavelength has been taken) Here is my solution: I computed (a) and (b) I believe ...
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Why is current density in a conductor of uniform cross sectional area constant at all points?

According to what I was taught, if current was dispersed “uniformly,” current density would remain constant. So, in a conductor, the 'current density should be the same at all points.' But, given that ...
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Apparent instantaneous EM propagation paradox in two concentric solenoids [duplicate]

Assume there are two solenoids $S_1$ and $S_2$ sharing the same axis, one of radius $R_1$ and the other of radius $R_2 > R_1$ resp. For the sake of simplicity, I will assume that $S_2$ reduces to a ...
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Is the induced electric field in a current changing solenoid actually that what we call magnetic vector potential field?

Induced electric field $E$ in a changing current (i.e. changing magnetic field $B$) solenoid image creditis: https://faculty.uml.edu//Andriy_Danylov/Teaching/documents/L18Ch33InducedEcovered.pdf I was ...
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Can the Lorentz force equation in curved spacetime be derived from the Einstein-Maxwell equations?

Given the Einstein field equations, $$R_{\mu\nu} - \frac{1}{2}R g_{\mu\nu} = \kappa T_{\mu\nu}$$ that imply in particular that $\nabla_\mu T^{\mu\nu}=0$, one can show, using the explicit form of $T^{\...
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Circular motion of proton in special relativity

Consider a proton in uniform circular motion in a plane perpendicular to a magnetic field. The Lorentz force acts as the required centripetal force. At a low speed, we have $\frac{m\cdot v^2}{r}=B\...
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Could you magnetize and at the same time elactrostatically charge a piece of rusted iron?

Pure iron rust (i.e. iron oxide) can be electrostatically charged and also magnetized. In a piece of iron usually the surface starts rusting leaving the interior untouched. Is it possible to have such ...
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What is the significance of the Dipole moment in electrostatics? No one simply multiplies the distance between charges and the magnitude of charges

What is the significance of the Dipole moment in electrostatics? No one simply multiplies the distance between charges and the magnitude of the charge without a reason
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Can you magnetize iron by putting it in the air gap of a two parallel plates capacitor?

We can magnetize an iron piece (i.e. permanently given enough field strength) by putting it inside a solenoid strong homogeneous field and charging/discharging the solenoid several times. I am asking ...
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Bio Savart Law and volume of sphere?

$\frac {4\pi r^3}{3} $ is the volume of a sphere and it appears in this equation. I understand the cross product is the area of a parallelogram, but am curious how this volume is involved in the ...
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Series expansion of unitary operators in terms of other operators

I am reading lecture notes on local gauge invariance, part of Prof. Ethan Neil's course on Quantum Mechanics at the University of Colorado. There, he writes about introducing a so-called comparator $U(...
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Wave Equation in 2D and 3D

I am new to wave mechanics in 2 and 3 dimensions and would like to know: Do you simply add $\partial \psi \over\partial y$ and/or $\partial \psi \over\partial z$ term without the other parts of the ...
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What is the signal velocity in a very thin enameled magnet wire?

I was wondering if in the case of a very thin enameled (i.e. sub-millimeter thick wire insulation dielectric usually made from polyimide) magnet wire the generated electric field of a signal transient ...
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Superconductor behavior near absolute zero

How does critical current change for superconductors as temperatures approach the millikelvin realm, particularly High-Temperature Superconductors?
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How to reduce magnetic field between two magnetic shield sheets (silicon steel sheets)?

I put 2 parallel silicon steel sheets between magnetic field as below: As silicon steel has much bigger permeability than air,magnetic line will go through 2 silicon steel sheets. What I can do to ...
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How is surface charge induced on the surface of circuit wires when connected to a battery?

Understanding: I watched the newest Veritasium electricity video and he said that the electric field inside the wires of the circuit that causes the electric current to flow is created by both the ...
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Do electric fences such as those typically used to fence cattle generate magnetic fields?

I'm using magnetometers near electric fences and I'm wondering if the electric fence should be detectable and if so, how close do I need to be to the fence for there to be a "measurable" ...
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I have found a serious error in a Wikipedia page? [duplicate]

From WP Signal Velocity article: https://en.wikipedia.org/wiki/Signal_velocity Signal velocity is usually equal to group velocity (the speed of a short "pulse" or of a wave-packet's middle ...
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Why the Tokamak Fusion Reactors need to be so big if electromagnetism works better for small distances?

I'm probably asking something dumb, but for the little I think I know, I assume that the bigger the electromagnetism field, the bigger the energetic requirement. I'm assuming that the square cube law ...
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How do you make light?

We make light making a charge oscillate, or heating a body. Are there differences between the two processes? But, above all, are there other ways in which we can produce em radiation?
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Time taken to travel by light a minimum or maximum?

I'm stuck on the following. We know that the path taken by light to travel between 2 points A and B corresponds to the path which minimises time elapsed. However, from relativity we also know light ...
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“Standard kinetic energy term of nonrelativistic quantum mechanics” $[\mathbf p\cdot \mathbf A + \mathbf A \cdot \mathbf p]$

On page 187 of Peskin and Schroeder, a certain term in an expression for the amplitude of an electron scattering from a classical magnetic vector potential is identified with “the contribution of the ...
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Spherical polar coordinates [closed]

Please help me in deriving the details equations of motions of a particle under the influence of EM force, coriolis and centripetal force. This is for B.Sc Introduction to Astrohysics. In Some papers ...
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Show that the energy of the electromagnetic field is given by half the norm of the Fourier transform of the electric field

Following Peskin and Schroeder on page 179-180, I am trying to show that $$\frac 12\int d^3x \left(|\mathbf E(x)|^2+|\mathbf B(x)|^2\right) = \int \frac{d^3 k}{(2\pi)^3}|\mathbf k|^2\sum_{\lambda = 1,...
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Do Maxwell's equations still apply in fluids?

Hi I am trying to build an electrical impedance tomography platform. The problem I'm facing mainly is to come up with an algorithm to calculate the variation of conductivity within the lung tissue ...
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Radio Wavefront and Photons?

I have some questions related to radio waves, photons and propagation , transmission and reception. It’s my understanding a radio wave is a changing electrical and magnetic field that travels through ...
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The momentum delivered to completely absorbing surface by an oscillating electron through electromagnetic waves is not ENERGY/$c$. Am I right?

As far as I understood, electron's rest mass is not equal to zero (unlike photons) and hence the momentum associated with oscillating electron != Energy/$c$. So, The momentum delivered to completely ...
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Is it possible for a thermoelectric generator to not have a heat source or heat sink?

Specifically, the type of thermoelectric generator I am suggesting makes use of the ExB drift effect. When electric and magnetic fields are perpendicular to one another, with infinite mobility of the ...
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What are cubic gallons? [closed]

I was working on my physics homework and thge answer came out to gallons^3, and the textbook said I had the right answer, so I'm trying to figre out what exactly a cubic gallon is? does anyone have a ...
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Why is there no deflecting force between two bar magnets?

If we were to hypothetically place a magnet to the right of the magnet above with the North pole of the right magnet next to the South pole of the left magnet, from real-life experience, I know that ...
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What is the "pincentrum" for magnets in the molecule structure?

So my prof explained ferromagnetism and said that permament magnets have "pincentra" so the material becomes magnetic too because the magnetic domains all point in the same direction. I don'...
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Relativistic Euler-Lagrange equations for a four-vector (or one-form) field

I think the best way to ask my question is by considering the maxwell-Lagrangian, $$\mathcal{L}=-\frac{1}{4}F^{\mu \nu}F_{\mu \nu}=-\frac{1}{2}(\partial^{\mu}A^{\nu}\partial_{\mu}A_{\nu}-\partial^{\mu}...
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Nuclear Propulsion System

In a fictional sci-fi setting , is it plausible to: Have a huge nuclear powered aircraft carrier to be propelled by H2-O2 rocket engine (size adjust) where you generate both from sea water on the fly ...
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Current through an ampere loop [closed]

I am trying to apply Ampere's law to find a magnetic field in coaxial cylinders. The inner cylinder with radius $2a$ and outer cylinder is $4a$. Current flows along positive $z$ for inner cylinder and ...
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Can someone explain why this would not create an infinitely rotating disk (perpetual motion) [closed]

This may be a dumb question and the solution may be super obvious but I can't figure it out as hard as I try. I have drawn a sketch of a device I thought of, now I know that we cant create "Free&...
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Has anyone ever put a magnetic or electrostatic dipole on a rotating shaft, spun it and demonstrated reception of a propagating wave in the far-field?

The question If I create a varying electric field and it will then create a varying magnetic field, so will it also create light? Will I see a light ray? got me thinking. I'm pretty sure that if I ...
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Why do we feel electrical shocks?

So electrical energy from source to device flows in the direction of the Poynting vector. My question is what do we feel exactly when we get an electrical shock? Before I learned about Poynting and ...
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Can someone please explain energy of electrons in Bohr's model?

Energy of and electron is $$E = \text{kinetic energy} + \text{negative of potential energy}. \tag{eq-1}$$ But energy of electron in the $n$th orbital is also $$E = -\frac{13.6}{ n^2} \tag{eq-2}$$ ...
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Different wave speeds when considering direction of electric or magnetic field in birefrigent materials

Let us consider a linear birefringent material which is not isotropic and has three principal axes with refractive indices $n_1$, $n_2$ and $n_3$ for the $x$, $y$ and $z$ directions, respectively (we ...
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