# 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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### 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 $g_{\mu\nu}=\eta_{\mu\nu}+h{\mu\nu},~ h_{\mu\nu}\ll 1$ ...
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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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### 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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### 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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### How come the magnetic field disappears when a neutron star becomes a black hole, while the rotation remains?

The only question I found is this one, but this considers as non-rotating neutron star collapsing: Our final and most comprehensive test is represented by the collapse to a BH of a magnetized ...
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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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### 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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### 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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### 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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