Questions tagged [electromagnetism]
The classical theory of electric and magnetic fields, both in the static and dynamic case. It also covers general questions about magnets, electric attraction/repulsion, etc. Distinct from electrical-engineering.
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Electric charges on compact four-manifolds
Textbook wisdom in electromagnetism tells you that there is no total electric charge on a compact manifold. For example, consider space-time of the form $\mathbb{R} \times M_3$ where the first factor ...
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Average force between two parallel finite wires with AC
Imagine 2 parallel antennas (wires) of equal length (a) with a distance r between them.
Both have AC currents with identical sine waveforms (equal frequencies and amplitudes). They are also in phase ...
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Special conformal transformations and image charges
Let us consider a grounded conducting sphere with radius $r$ and a point charge $e$ at a distance $R>r$ from the center of the sphere. For simplicity, we can choose the sphere centered at the ...
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Electromagnetic waves accelerating reference frame
A charge is at rest in an inertial reference frame.
Consider this situation:
From the point of view of an observer who is accelerating relative to the charge. Would the observer observe ...
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Is the magnetic field at the centroid of a closed loop connected at two different points to a battery always zero?
Given a closed 2-dimensional conducting loop, the terminals of a battery are connected at any two points on the loop (but not the same point).
As an example, consider a circle of radius $R$; the two ...
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Solving Maxwell equations on curved spacetime
I have difficulties to understand how to solve the Maxwell equations on curved spacetime. I want to solve the equations in the weak regime $g_{\mu\nu}=\eta_{\mu\nu}+h{\mu\nu},~ h_{\mu\nu}\ll 1$ ...
user255856
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Boundary conditions on current carrying wire
I'm trying to simulate Maxwell equations by finite elements method for a current carrying wire. My 3d geometry consists of a cylinder and a box containing it. I will use a mixed formulation and ...
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Does a rotating magnetic monopole have electric and magnetic moment in classical view?
Would a rotating sphere of magnetic monopole charge have electric moment ?
In a duality transformation $E\rightarrow B\cdot c$ etc. how is the magnetic moment translated $m = I\cdot S $ ?
$M_{el} = \...
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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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QED lagrangian: gauge fixing term
I have a question about the structure of the QED lagrangian, in particular the free photon lagrangian which is contained in it. My premise is: I only know how to exploit canonical quantization in ...
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Has the Helmholtz decomposition of the $\mathbf{E}$ field from the Liénard–Wiechert potentials been worked out?
If you look at Maxwell's equations for $\mathbf{E}(\mathbf{x},t)$ they split neatly into two categories. They are:
\begin{align}
\nabla\cdot\mathbf{E}(\mathbf{x},t)&=\frac{\rho(\mathbf{x},t)}{\...
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Do divergence and curl of Lorentz force have some physical meaning?
Time ago I started thinking about this: if we take the well known Lorentz Force expression, namely
$$\mathbf{F} = q\left(\mathbf{E} + \mathbf{v}\times\mathbf{B}\right)$$
and we operate $\nabla\cdot \...
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How to calculate the magnetic dipole moment of a bar magnet?
I want to calculate the force between two magnets. In a previous Phys.SE question, I was told that I needed to use the dipole-dipole interaction, but that equation depends on $m$, the magnetic dipole ...
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What would Maxwell's equations look like in a universe which followed Galilean transformations?
I was wondering how the electromagnetic force would behave in a Gallilean transformation universe. Would the magnetic force be non-existent?
We know that Gallilean transformations are Lorentz ...
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Should the energy-momentum tensor be invariant under gauge transformations?
For example, consider the electromagnetic theory given by
\begin{align}
I=-\frac{1}{4}\int d^4x\, F_{\mu\nu}F^{\mu\nu},
\end{align}
where $F_{\mu\nu}=\partial_\mu A_\nu-\partial_\nu A_\mu$. The action ...
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Trouble with Classical Derivation of Cherenkov Radiation Mechanism -- Magnetic Field Intensity
I've been going through a translation of Frank and Tamm's original theory on Cherenkov radiation published by Jelley in 1958, and the bottom line is that I'm stuck on one of the intuitive leaps that ...
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Current geometry and Ampere's law
Under the right circumstances, Ampere's law $\oint \vec H\cdot d\vec \ell=I_{encl}$ can be used to deduce the field $\vec H$ at a point from the current enclosed by the circuit which produces $\vec H$....
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Hamiltonian formulation of the semiclassical Model of electrons
I'm currently reading the book Solid State Physics by Neil W. Ashcroft and N. David Mermin. In Chapter 12 they introduce the "Semiclassical Model of Electron Dynamics". In short: After having solved ...
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1-form formulation of quantized electromagnetism
In a perpetual round of reformulations, I've put quantized electromagnetism into a 1-form notation. I'm looking for references that do anything similar, both to avoid reinventing the wheel and perhaps ...
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Why do diamagnetic substances align perpendicular to the field?
My textbook says that Diamagnetic substances align their longest axis perpendicular to the field in uniform magnetic field while paramagnetic substances align their longest axis parallel to the field.
...
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Why does permanent magnet not exhibit macroscopic quantum effect?
Permanent magnets are a result of quantum mechanics, i.e. quantum spin of electrons inside the magnet aligning.
Quantum spin follows the uncertainty principle. If I measure the spin orientation first ...
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(Qualitative) Quantum explanation of simple electric phenomenon
Recently, I came across the following problem in some textbook,
A current of $1 \,\text{mA}$ flows through a wire made of a piece of copper.
and a piece of iron of identical cross-sections welded end-...
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Electron in rotating frame
Suppose there is a stationary electron in an inertial frame $S$. Then there is only a static Coulomb field relative to $S$. However, according to a rotating frame $S'$, whose angular velocity relative ...
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Relation between the electric and magnetic fields for an arbitrary electromagnetic wave
When solving the wave equation for electromagnetic waves,
$$
\nabla^{2} \mathbf{E}=\mu_{0} \varepsilon_{0} \frac{\partial^{2} \mathbf{E}}{\partial t^{2}}, \quad \nabla^{2} \mathbf{B}=\mu_{0} \...
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What is the radiation field of a black body with temperature $T$ translating at speed $v$?
Suppose I have a spherical black body at temperature $T$ with radius $R$. When stationary in a vacuum at zero Kelvin, it produces an incoherent radiation field according to Planck's law
$$
B_\nu = \...
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Are there yet Optical Magnetic Mirrors (OMMs) which reflect via interaction with the magnetic field?
update 2021: As the question has remained unanswered for five years and the field of optical metamaterials has advanced, I think this question can be revisited.
The most familiar mirror relies on ...
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Experiments or phenomenon to falsify the existence of monopoles?
This is a related, but opposite question to this one
I have heard about a lot of things regarding elementary and GUT magnetic monopoles, as well the quasiparticle monopoles in spin ice
Since there's ...
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Spin-dependence of the directionality of dipole radiation
I am interested in understanding how and whether the transformation properties of a (classical or quantum) field under rotations or boosts relate in a simple way to the directional dependence of the ...
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Is there any antenna with a single null?
If we designate the origin (the reference point from which all displacement vectors are measured) $\vec{0}$, and If we consider a sphere $\mathbb{B}\left(\vec{0},\mathcal{R}\right)$ of radius $\...
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When do Lorentz transformations take straight field lines to straight field lines?
If you look at elementary examples, it seems like a Lorentz transformation takes a field pattern with a lot of straight field lines to another field pattern with a lot of straight lines. Examples:
an ...
user4552
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Does electromagnetic field collapse the wave function of charged particles?
In an electron double slit experiment, let's put two charged plates behind the slits in an attempt to move the pattern up and down on the the screen.
What will happen? Will it just shift the ...
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Black Holes and QED
So, in quantum electrodynamics (at least to my rudimentary knowledge), the electromagnetic force is mediated by photons.
On the other hand, in classical general relativity, the Kerr Black Hole ...
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Force from solenoid
I'd like to approximate the force from a solenoid, or at the very least find a formula which is proportional to the force so that I can experimentally find the constant for my particular case. ...
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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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Classical limits of Quantum Electrodynamics?
Quantum Electrodynamics is the theory that studies the interactions between matter and radiation (somewhat).
How would one explain for example the movement of an electron in a constant electric field ...
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Is pair production possible in Classical Field Theory?
It is often said that quantum effects only become manifest in loops, and all tree-level calculations are classical. I am trying to figure out to what extent this claim is true. I know the claim arises ...
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Is there a (semiclassical) electric field operator?
So I come from a chemistry background, where the electronic structure of atoms and molecules is central. For practical purposes, we usually work with a charge density operator
$$ \hat{\rho}(r) = q \...
user312272
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Has anyone derived a classical radation reaction term directly from QED?
As far as I know, pretty much the only aspect of classical EM that's still actively controversial within the physics community is the best way to treat the radiation reaction force exerted on an ...
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An equivalence principle for gauge theories
In GR there is the equivalence principal, stating that in an arbitrary gravitational field, no local non-gravitational experiment can distinguish a freely falling, non-rotating system from a uniformly ...
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What is causing the acceleration of charges in an ideal TEM transmission line?
As I understand, in an ideal TEM transmission line we can set up the telegrapher equations and solve to show that the line propagates voltage and current waves.
My confusion arises when we recognize ...
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Maxwell's Equations In Curved Spacetime Derived Via ADM Formalism VS Differential Forms Discrepancy
I' m trying to understand how the electromagnetic potential for diagonal Bianchi IX models when an electromagnetic field is aligned with one of the three axis is computed. The metric in question is
\...
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Electromagnetic Unruh effect?
The Unruh temperature is given by
$$T=\frac{\hbar\ a}{2\pi c k_B}.$$
If we have an electron field with charge $e$ and mass $m_e$ acted on by an electromagnetic field $\vec{E}$ then very naively maybe ...
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Can a bulk crystal have a spontaneous quadrupole moment?
Crystals of polar space groups e.g. ferroelectrics can have a spontaneous electric dipole moment. Is it possible for a crystal to have a spontaneous electric quadrupole moment (and higher order terms ...
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Is there an equation of state for a non-ideal Fermi gas?
For a gas of self-interacting electrons (i.e. a non-ideal Fermi gas), is there is there any sense in trying to define an equation of state? If so, what is the equation of state for a gas of electrons? ...
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Geometric Algebra formulation of EM in nonuniform dielectric media
Normally, there is a straightforward formulation of Maxwell's equations in free space under the GA framework, which reduces to (picking the right units):
$$
\nabla F = \mu_0 J
$$
with some ...
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How can one confirm creation of magnetic graphene?
Recently my high school physics teacher contacted me looking for an expert to ask about this situation. I am paraphrasing his correspondence below:
I just had a a student accidentally create magnetic ...
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MRI and precession
A lot of explanations of the quantum mechanics of MRI discuss the precession of a proton in an external magnetic field, for example here:
http://www.physicscentral.com/explore/action/mri.cfm
Doing ...
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Why does this 'Quantum Pinned' superconductor allow easy repositioning
I'm confused by videos such as this (popular demonstration of 'Quantum Levitation'):
https://www.youtube.com/watch?v=Ws6AAhTw7RA
So my current understanding of superconductors is that when in the ...
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Ohm's Law for Accelerated Conductors
My questions are about the Ohm's law for a moving conductor in a stationary magnetic field. As we know this law is stated based on following relation with respect to the fixed frame of reference ...
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Is Electromagnetic Mass Possible?
If the sinusoidal electric component of a light wave were off-set to one side of the magnetic component and then the smaller "lobe" were to cancel out with much of the larger side, then where would ...
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