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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51 views

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
40 views

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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151 views

Sinusoidal currents in AC circuits

(I had edited this question a number of times but did not receive a satisfactory answer. So I am re-wording this question yet again. The earlier version of this question can be found here : https://...
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2answers
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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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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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26 views

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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13 views

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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1answer
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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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49 views

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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1answer
118 views

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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2answers
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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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44 views

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
36 views

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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7 views

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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21 views

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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1answer
36 views

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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1answer
43 views

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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38 views

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 ...
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What are some good physics project for college students? [closed]

Any project that involves physical principles, from mechanics, waves, electrodynamics, etc
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29 views

Berry curvature from spherical polar coordinates to Cartesian coordinates

Let us consider the typical example of calculating the Berry phase: a spin in a magnetic field. We start with the Hamiltonian $H=-\textbf{B} \cdot \boldsymbol{\sigma} + B$ where $\textbf{B}$ is the ...
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2answers
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Why does vector potential $\mathbf A$ satisfy $\mathbf E = -\nabla \phi - \frac{\partial \mathbf A}{\partial t}$?

As any magnetic field $\mathbf{B}$ is divergence-free i.e. $$\nabla\cdot \mathbf B = 0,\tag{1}$$ by the solenoidal theorem there exists a vector potential $\mathbf A$ that satisfies $$\mathbf B = \...
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What does it mean that the polarization plane “rotates”?

Following description are quoted from Wikipedia. "Optical rotation or optical activity (sometimes referred to as rotary polarization) is the rotation of the orientation of the plane of polarization ...
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38 views

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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24 views

Oscillating Electric Field and Dielectric Constant

Here's an extract from Purcell's Electricity and Magnetism: Every dielectric constant in the table is larger than 1. We should expect that if our explanation is correct. The presence of a ...
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1answer
57 views

Conceptual definition of the $H$-field

I am looking for a conceptual definition or statement for H to help ground my understanding as I work through the math. I often see it explained that D and H are just the components of the fields ...
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4answers
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Can a magnet rip protons from a nucleus?

Pretty self explanatory. I’m wondering if the strong nuclear force could be overcome by a strong enough magnet?
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15 views

How to identify polarized surface charge density in this compound coaxial cable

I am finishing a problem which involves a compound dielectric inside a coaxial cable at a constant potential $V_0$, which is shown in the image below. I managed to find the electric displacement, ...
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1answer
48 views

Why does a conducting cylinder spin when held next to a conducting ring around a solenoid?

My uncle showed me this video. I understand that the magnetic field from the solenoid induces a current perpendicular to it in the ring. However, I don't see why this current would result in a torque ...
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5answers
69 views

How induced EMF possible in a stationary conducting loop?

If a conducting loop is placed in an increasing magnetic field, current starts flowing through it due to induced EMF. But similar to a conducting wire, if the loop is considered to have 0 resistance (...
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23 views

What's the difference between source current and conduction current?

Related Going through Pozar's Microwave Engineering, we have a derivation of Poynting's theorem which begins as follows. "If we have an electric source current $\bar{J_S}$ and a conduction current $\...
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18 views

Is the last step of this method for proving conservation of charge correct?

The following is an attempt to prove the law of conservation of charge in a conductor: Let us say we have a surface $S$ enclosing a volume $V$, in a time interval $\Delta t$, an amount of charge $...
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31 views

How to shift light to a longer wavelength / lower frequency energy level so that it becomes a different “color”?

I need to shift light from a certain wavelength/frequency to another, lower, level. For example, green light down to yellow or orange. I want to reduce the energy level of the transmitted light, for ...
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20 views

How can I make a homemade hemholtz coil?

For a project, I need to make a hemholtz coil capable of generating uniform magnetic fields of between 0.0 and 0.4T and which allows the user to control the strength of said field at any given moment. ...
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1answer
32 views

On proving the $E$-field is null inside the layer of a conductor having a cavity

Let us consider a conductor having a cavity (both can have an arbitrary shape) wherein a positive charge $+q\,$ is located: At the electrostatic equilibrium, the inner wall of the conductor will ...
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1answer
45 views

Velocity-dependent potentials and the dissipation function

From this previous question Charge, velocity-dependent potentials and Lagrangian where the citation is shown at the page 22, §1.5 of the book Classical Mechanics of Goldstein, we read that "an ...
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3answers
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Propagation of EM waves

When we say that electromagnetic waves propagate through space and time. What is the quantity that is actually travelling. We say that electric and magnetic fields are perpendicular to each other and ...
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Mitigation of dead zones when applying circular polarized antenna in shipping containers

I would like to know if I used a circular polarized (RHCP) antenna inside a metal container as show in the configuration below will it reduce the dead zones, as compared to a linear polarized antenna? ...
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1answer
42 views

What is the spin angular momentum of an elliptically polarized photon quantitatively?

The answers to a similar question Is a single photon always circularly polarized? suggest that the angular momentum of the elliptically polarized photon can be known depending on the probability ...
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Magnetic Pendulum Experiment

Suppose we have a pendulum which is fixed at some point, P. We release it from some initial angle $\theta$ and it swings over a solenoid for which there exists some magnetic force. Note that there is ...
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1answer
48 views

Force shield for flying crafts - science fiction? [closed]

In some science fiction films, especially "Alien invasion" types there is a recurring motive of humans trying to attack an Alien spacecraft with missiles but the spacecraft is defended, at least ...
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47 views

geodesic equation for electromagnetic field

I am trying to derive geodesic motion for photons from the Lagrangian of electromagnetism coupled to General Relativity. I tried to use the covariant conservation of the Stress energy tensor: $$\...
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1answer
46 views

Do we need to specify additional conditions in order to find the unique potential satisfying Laplace's equation?

Let us say we have a simple boundary value problem (BVP) in spherical coordinates : $$\Delta \phi = 0$$ along with $\phi=1$ at $\,r=1$, and $\,\phi =0\,$ at $\,\infty$. A surface sphere with radius ...
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1answer
102 views

Method of images in cylindrical coordinates for a circular ring of current

I am trying to determine if the following problem has an analytical solution using the method of images. The problem is an infinitely long (in $z$) cylinder of a material with permeability $\mu_2$ of ...
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1answer
53 views

How did they arrive at the expression for magnetic force on a charge $q$?

I understand that the magnetic force exists as a consequence of length contraction between 2 charge carriers in a current carrying wire (wrt the frame of the moving charge); but I don't understand how ...
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Rotational invariance of the conductivity tensor (Classical Hall Effect)

In classical Hall effect, the conductivity tensor is given as $\sigma = \frac{\sigma_{DC}}{1+\omega_B^2 \tau^2} \begin{pmatrix} 1 & -\omega_B \tau \\ \omega_B \tau & 1 \end{pmatrix}$ where ...
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Metal resistivity temperature dependence in free electron model

In the context of the Drude model, I understand why the conductivity of a metal should decrease with increasing temperature (the conductivity scales linearly with scattering time $\tau$, which due to ...