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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Deriving the Maxwell Scalar field Equation from the given action principle

I am currently starting to learn general relativity. I have been trying to derive the Maxwell Scalar field equation from the given action $\mathcal{S}[\phi, A, \mathbf{g}]=\int\left(\frac{1}{2}\left(\...
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Does a magnet moving in a uniform electric field experience torque?

Assume a uniform electric field of $E_y$ along $y$ in the lab frame of reference $(x,y,z,t)$. A simple magnet bar is set in motion at $v$ along $x$ in this electric field so that the alignment of the ...
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Variation of phase across the wavefront

I have been going through the theory of diffraction of radio waves when passing through the ionosphere, when I came across the following statement: the amplitude of the wave when it emerges from ...
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Magnetized Steel wire around Soft Iron Core

We all know that a current carrying solenoid of copper is a weak temporary magnet and if we wrap it around a soft iron core it becomes a temporary strong magnet i.e. the iron core magnifies the ...
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Why does the interaction energy for the electric fields of two point charges vanish over any shell smaller than their separation?

A previous question, A triple integral in Spherical coordinates from Jackson's book on Electrodynamics, presents the fairly routine calculation of the interaction energy encoded in the cross-term ...
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Gravitational and Electric Forces in the Movements of Planets [duplicate]

Now that we know about the electric nature of the cosmos, the question arises: "How much are planets affected by electric forces while moving through the universe?" We have been taught that the ...
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1answer
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A triple integral in Spherical coordinates from Jackson's book on Electrodynamics

I have been trying a solution for the following integral from Jackson but i do not seem to go anywhere. Please help. The problem is to compute interaction energy due to 2 charges. Compute following ...
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1answer
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Inductors in Transformers

Why is it the flux change which induces the ac current of primary coil equal to the flux change that primary coil induces on the secondary coil? Is it possible to prove this? slide from video ...
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What does $e$, $i$ and $\vec{p}$ mean in this formula? [closed]

I've found a formula to calculate the $\vec{E}$-field vector of a Hertzian dipole in the far field (don't know if that's the correct term) to draw and simulate the field lines. Link to my source: ...
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Electrostatic force of attraction b/w dipole and charge [closed]

A particle of charge $Q$ is placed on the axis of a neutral cylinder of volume $V$ at a distance $x$ from one end of cylinder. Here $x$ is much larger than linear dimension of cylinder. Find force of ...
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3answers
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How to relate magnetic flux to fluid flux?

I'm trying to grasp magnetic flux as an analogous concept to fluid flux, which is more concrete in my opinion. For example, we can define a vector field $\mathbf{F}(x, y, z)$ which describes the ...
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Can one measure the current in a ferromagnet?

I understand that all magnetic fields are the result of current (charges in motion). I'm now doing research on electromagnetic induction and understand that changing magnetic flux will induce a ...
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Magnetism problem

I encountered a magnetism problem requiring me to find out the minimum velocity (along $\hat{\mathbf{i}}$) given to a charged particle so that it passes through the centre of a circular region of ...
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How are the two definitions of B, as magnetic field and magnetic flux density, same?

My question, is primarily, based on this question, in which the accepted answer asserts that $\vec B \, $ has two definitions, As magnetic field$^*$, using Lorentz force $\vec F=q \vec v \times \vec ...
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1answer
61 views

What's wrong with the theory of an electromagnet? [closed]

Let's imagine we have a uniform magnetic field pointing directly upwards, filling the whole space, so $\mathbf B(\mathbf r)=B\mathbf k, B>0$ for all points in $\mathbb R^3$. This can be seen as a ...
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2answers
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Do photons influence charged particles? [duplicate]

As a neutrally charged particle, a photon shouldn't interact electrically with another charged particle. However, since photons are electromagnetic waves, they should interact with charged particles, ...
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Contact potential between metals

Metals Al and Au are connected in series. An ideal voltmeter, with both of its leads made of Cu, is connected across them. Then how come the voltmeter gives zero reading but why can't we measure ...
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Can a permanent magnet lift objects without touching it? [duplicate]

I’ve been trying to think through this scenario given the fact that magnetism can not do work. If you hold a permanent magnet close overhead a magnetizable object, and if you slowly lower the magnet ...
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Short conductive cylinder inside a coil [closed]

The problem states; Inside a solenoid coil, a non-magnetic, conductive, solid cylinder is placed coaxially. The cylinder is shorter than the coil but has about the same diameter as the coil's turns/...
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Is magnetic flux depending on the orientation of ferromagnetic rotor?

I have attached a picture of Reluctance motor setup. When I turn on the AC source, Magnetic flux lines will build up. My question is, will the magnetic flux will be a function of the angle turned by ...
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The $z$-component of the electric field vector [closed]

It is asked to find the wave equation for the z-component of the electric field vector by using Maxwell’s equations in free space. Here is my work - In free space we have the following relation ...
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Dimension of the constant in Born-Infeld nonlinear electrodynamics

As I know, based on the Lagrangian of Born-Infeld electrodynamics, its constant which shows the strength of electromagnetic field should have the dimension of inverse of length, but in some papers I ...
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Visual understanding of EM fields within a rectangular metal container

I have the following scenario setup inside a rectangular metal container. The container size is 12.03m by 2.39m by 1.84m (lxbxh). Additionally, inside the container I have placed 4 antennas equally ...
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Electromagnetic field tensor (and other tensors) with different sign conventions

In Wikipedia the components of the EM Field Tensor are listed as $$F^{\mu\nu}=\left( \begin{array}{cccc} 0 & -E_x & -E_y & -E_z \\ E_x & 0 & -B_z & B_y \\ E_y & B_z &...
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Electromagnetic wave equation: can we ignore the constant of integration?

Suppose we obtain a solution for each of $\mathbf B$, $\mathbf E$ of maxwell equations in the vacuum ($\rho=0$). Clearly, for any constant vector $\mathbf k, \mathbf m$, $\mathbf {B+k}$ and $\mathbf{E+...
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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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1answer
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Point of stokes theorem - conservative field

I am working through electrodynamics at the moment and I have a rather elementary question - which I apologize for. But after some research on google I am still not sure if I have understand "the ...
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4answers
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Sinusoidal currents in AC circuits

Why is every branch current in any arbitrarily complex AC Circuit always a single sinusoidal function, i.e., of the form $I(t)=I_o \sin(\omega t + \phi)$? Clarifications : By a "single sinusoid" I ...
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Ampere's Circuital Law Not Working

$$\int{b.dl}=U_oi_{inside}$$ Now whatever direction we take for the loop, the upper side and the lower side of the square will be opposite in magnitude and for the left side and right side magnetic ...
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Direction of induced current [closed]

In the image, the loop ABCD is moving away from the current carrying straight wire. I am just confused about the direction of induced current in the loop. The book shows that the current is anti-...
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How to show that the electric field at any point in the interior o a hollow spherical shell without Gauss? [closed]

I need help with this question: Consider a hollow spherical shell with uniform surface charge density. By considering the two small patches at the ends of the thin cones in Fig. 1.37, show that ...
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Can a time derivative of the electric/magnetic field jump across a material interface?

So I just wanted to see if anyone could offer some suggestions. So in my mind this seems impossible, in the case of electric field a jump in time derivative of that field would indicated in my mind ...
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Finite Element Method To Model Magnetic Fields

I've had a trawl through but not found anything I could really follow so I'm asking a question, if it is a repeat, apologies. I'm working on modelling a physical problem in the magnetic field from ...
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Gauss law for an arbitrary dielectric with a dielectric constant that varies accross its volume

someone knows what is the form for the Gauss law or Poisson equation for an arbitrary dielectric with a dielectric constant that varies accross its volume?
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Why is average magnetisation zero in a normal (Non-superconducting) state

Considering type-I superconductors (specifically the magnetisation curves), why can one assume that the magnetisation is zero when we consider a normal state (when $H>H_C$)? EDIT: For ...
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How come that the magnetic field is not uniform around a trace over a ground plane

I have conducted an experiment, which for me gave some quite unexpected results. I measured the amplitude of a magnetic field around a conductor and expected to see an amplitude that decreases with ...
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Can we talk about velocity field and acceleration field in 1D?

It's well know that a charged particle radiate when it is accelerated. We can rely on two different formulas. The first one it's from Jefimenkos : $$\mathbf{E}(\mathbf{r}, t) = \frac{1}{4 \pi \...
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Is the vector potential component $A_\phi$ for a dipole necessarily 0 due to symmetry about the $z$-axis?

Consider an electric dipole (+$q(t)$, -$q(t)$) [where say $q(t)=q_0\cos\omega t$] is placed along the $\hat z$ axis. In the spherical polar coordinates, its vector potential $$\vec{A} = A_r \hat r + ...
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Conformal Invariance of Maxwell's Equations

I am currently doing some conformal field theory (in four dimensions) and want to show the invariance of Maxwell's equations under conformal transformations, in particular \begin{align} \partial_\...
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Time Average Spin Angular Momentum

I’m a bit confused about time averages in electromagnetism using complex amplitudes. Specifically about the angular momentum of the fields- the spin part is proportional to $\bf{E} \times \bf{A}$. In ...
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Time varying magnetic field and induced electric field

Well I would like to start with asking few questions for you to provide a detailed solution and whose details has been clearly mentioned in the above image. It would be helpful if you could clearly ...
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1answer
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When are the off-diagonal terms of magnetic susceptibility tensor non-zero?

The susceptibility tensor is defined as $$ \bf{M}_i = \chi_{ij} \bf{H}_j. $$ From looking at a material's geometry and crystalographic properties, is it possible to say whether the off-diagonal ...
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Induced Electric Field due to Infintely spread out, changing magnetic field

If I have a uniform, downward pointing magnetic field which is spread across a very large region and the magnitude of the magnetic field is increasing with time, what is the shape of the induced ...
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Electronic explained by hydraulic

Where I can find a book or website (or any source) which contain many electric compounds (capacitor, transistor, diode … and more) explained by a flow of water (hydraulic analogy) which is more easy ...
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Reflection coefficient phase using Kramers Kronig, formula equivalence

Why are the following two formulas for the phase of the reflection coefficient equivalent (according e.g. to [1])? \begin{align} \Theta(\omega_0) & = - \frac{\omega_0}{\pi}P\int_{0}^{\infty}\frac{...
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Magnetic Field in Perfect Electric Conductor

I know that one of Maxwell's equations states that the curl of the electric field is proportional to the time derivative of the magnetic field. We know that the electric field in a perfect electric ...
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Why does the charge required on each plate of a capacitor to maintain a certain voltage decrease with the distance between the plates?

The $Q$ charge and the voltage $V$ of a capacitor are linked by $$Q=C\times V$$ For a parallel plate capacitor $C\propto \frac{1}{d}$. This implies for a given voltage $V$ between the plates, the ...
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Photons as propagators of an electro-magnetic field

What does it mean when somebody, let's say a random person on the crosswalk waiting for the sign to go green, say that a "photon is a propagator of the Electromagnetic field"? I don't know if it's a ...