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.

2,398 questions with no upvoted or accepted answers
Filter by
Sorted by
Tagged with
15
votes
0answers
442 views

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 ...
11
votes
2answers
825 views

Could the Faraday's rotating wire experiment be explained without using Lorentz force?

I read Faraday's rotating wire experiment which resulted in the invention of the homopolar motor. He was clever enough to think it that way and make the wire rotate around one of the poles of the ...
10
votes
0answers
281 views

Do the Planck voltage and the Planck current have a natural physical interpretation in classical general relativity?

Most Planck units are a product of powers of all three of $\hbar$, $c$, and $G$, so we will not be able to fully understand their physical significance until we have a full theory of quantum gravity. ...
10
votes
0answers
1k views

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 wave forms (equal frequencies and amplitudes) . They are also in-...
9
votes
2answers
1k views

Biot-Savart law from Ampère's with multivariate calculus

Let us assume the validity of Ampère's circuital law $$\oint_{\gamma}\mathbf{B}\cdot d\mathbf{x}=\mu_0 I_{\text{linked}}$$where $\mathbf{B}$ is the magnetic field, $\gamma$ a closed path linking the ...
8
votes
0answers
86 views

Do radio beams self-focus in the interstellar medium?

Summary: In a plasma, electromagnetic waves create a ponderomotive force that pushes electrons and ions out of the way. Thus, in an intense laser beam, electrons tends to move away from the areas ...
8
votes
0answers
218 views

Does a classical charge naturally have spin in a non-flat FLRW universe?

I was recently reading a paper https://arxiv.org/abs/0912.0225 which describes Coulomb's law in a closed universe. I've seen the argument from several sources that the total electric charge in a ...
8
votes
0answers
392 views

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} = \...
7
votes
0answers
147 views

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)}{\...
7
votes
0answers
280 views

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 to be centered at the ...
7
votes
0answers
2k views

What is “spin-orbit torque?”

I am trying really hard to understand the concept of spin-orbit torque. It is a new-ish discovery in the field of spintronics and has many applications for magnetic devices. The information that has ...
7
votes
1answer
608 views

Boundary conditions on current carrying wire

I'm trying to simulate by finite elements method Maxwell equations for a current carrying wire. My 3d geometry consists of a cylinder and a box containing it. I will use a mixed formulation and ...
7
votes
1answer
908 views

Question about superconductivity

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
7
votes
0answers
371 views

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 ...
7
votes
1answer
684 views

Hidden momentum

I'm trying to learn about hidden momentum. After reading what I could find with a google search, I understand that it is equal to the momentum carried by radiation, calculated with the Poynting vector....
6
votes
1answer
485 views

Non-Relativistic Electron Hamiltonian

I have determined a Hamiltonian for an electron using an appropriate Lagrangian of the form $$ L=\frac{1}{2m}(m\overrightarrow{v}+\frac{q}{c}\overrightarrow{A})^2-\frac{q^2}{2mc^2}\overrightarrow{A}\...
6
votes
0answers
161 views

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 ...
6
votes
1answer
1k views

Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
6
votes
1answer
160 views

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 ...
5
votes
0answers
128 views

Photons and Electric Current

I am trying to understand how photons as the force carrier for the electromagnetic force (or field) manifest themselves in a flow of electric charge, i.e. an electric current. Both standard (...
5
votes
0answers
150 views

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 ...
5
votes
1answer
128 views

Is electricity perpetual in a superconductor system?

I wonder if electricity consumption happen according to the Law of conservation of energy and Joule effect, in a regular circuit certain amount of energy is transformed into heat by components ...
5
votes
1answer
190 views

Locality in the coulomb gauge of classical electrodynamics

In the coulomb gauge, the equations that describe the dynamics of $\Phi$ and $\vec{A}$ simplify to: $$ \Delta \Phi = - \frac{\rho}{\epsilon_0} \\ \Delta \vec{A} - \frac{\partial_t^2}{c^2} \vec{A} = - \...
5
votes
0answers
114 views

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 ...
5
votes
1answer
763 views

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 $\vec H$....
5
votes
0answers
534 views

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 \...
5
votes
0answers
140 views

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 ...
5
votes
1answer
541 views

Maxwell Stress Tensor at material boundaries

I am trying to grasp the meaning of the Maxwell Stress tensor $T_i^j$ at material boundaries. Concretely, I am trying to calculate the force between two waveguides. The results are given in an article ...
5
votes
1answer
173 views

Can light (electromagnetic radiation) cause electromagnetic induction in a wire?

Can light, as an electromagnetic wave, cause electromagnetic induction in a wire by passing near the wire? Does a moving electromagnetic wave cause a varying magnetic field in the region near the wire?...
5
votes
0answers
86 views

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 ...
5
votes
0answers
179 views

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 $\...
5
votes
0answers
6k views

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 ...
5
votes
0answers
152 views

Is the search for a Simple-group-based Electro-Weak theory over?

Just wondering: We know that, in its current form of the $SU(2)_L\times U(1)$, the electroweak theroy rides a wave of huge success. However, is it not possible that the correct simple group ...
5
votes
1answer
12k views

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. ...
4
votes
1answer
105 views

How electromagnetic energy-momentum looks like for arbitrary 4-velocity vector?

I need to expresse the electromagnetic energy-momentum tensor in a vacuum $$T^\nu_{\ \ \ \mu} = F_{\mu\alpha}F^{\nu\alpha} - \frac14 F_{\alpha\beta}F^{\alpha\beta}\delta^\nu_{\ \ \mu}$$ in terms of ...
4
votes
1answer
69 views

What drives the charge separation in atoms?

In atoms charges are neatly separated. Instead of pairing which seems natural they all stick together with their peers. What drives this peer behaviour? Why is it stable?
4
votes
0answers
94 views

Consider two electrons released from rest at a distance $d$. How much energy is radiated?

Abraham-Lorentz? Two electrons d apart have potential energy. Release them, they will be repelled according to Coulomb's law. I could make an assumption about the associated vector potential, but I ...
4
votes
3answers
101 views

Where does the energy go if the current is induced in a perfect conductor?

Let us rotate a closed circuit made of a copper coil in front of a magnet. This will generate an induced current and all the mechanical energy will be converted into heat through Joule effect. If the ...
4
votes
0answers
118 views

Is there a theorem about “Electromagnetic Miracles”?

I listened to a lecture several years ago in which the speaker claimed that there is a theorem that shows that violation of charge conservation under classical electrodynamics is impossible in the ...
4
votes
0answers
285 views

Magnetic flux ropes - are they stable for topological reasons like fluxon/Abrikosov vortex?

In images of sun's corona there are clear bright lines - nearly 1D stable structures interpreted as magnetic flux ropes, often suggested to have a topological nature e.g. (source): They have some ...
4
votes
0answers
174 views

Is the Lorentz force the divergence of a symmetric tensor?

The conservation of momentum and energy in electromagnetism is usually written as $$ \partial_\nu T^{\mu \nu} + \eta^{\mu \rho} f_\rho = 0 $$ where $T^{\mu\nu}$ is the electromagnetic stress-energy ...
4
votes
1answer
73 views

Are these two definitions for propagation speed be the same?

We know for a fact that for any medium with permittivity and permeability $\epsilon$ and $\mu$, the propagation speed of electromagnetic waves is given by $$v = \frac{1}{\sqrt{\mu\epsilon}}$$ We ...
4
votes
0answers
185 views

What is the refractive index of an electron?

Consider a free electron or electron bunch, would it have a corresponding refractive index? At low or high energies, the effects are obviously much different. I am curious to know (I haven't found) ...
4
votes
0answers
371 views

Don't we know how the magnetic field of the Sun is created?

This is how Wikipedia explains stellar magnetic fields: Stellar magnetic fields, according to solar dynamo theory, are caused within the convective zone of the star. The convective circulation of ...
4
votes
0answers
134 views

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? ...
4
votes
0answers
757 views

Why not use the Weyl gauge?

In E&M in Minkowski space, the Lorenz and Coulomb gauges are typically used since they make things vastly simpler. On a curved background, Maxwell's equations (without sources) can be written as: ...
4
votes
0answers
364 views

Is classical electromagnetism conformally invariant? (and a bit of general covariance)

The contest is a flat $4d$ Minkowsky space. A conformal transformation is a diffeomorphism $\tilde x(x)$ such that the metric transforms as \begin{equation*} \tilde g_{\tilde \mu \tilde \nu} = w^2(x) ...
4
votes
0answers
728 views

Landau quantization: degeneracy of first level

In some books the degeneracy of one Landau level in a two-dimensional gas of free electrons is calculated in the following way: Note: The electron spin is not considered. Number of states of a free ...
4
votes
0answers
153 views

Why isn't cosmic microwave background radiation on Earth drowned out by thermal noise

The Penzias/Wilson experiment, which detected the CMB, doesn't sound feasible given thermal noise. When Penzias and Wilson reduced their data they found a low, steady, mysterious noise that ...
4
votes
0answers
139 views

Can the Hall effect drive a current?

In the Hall Effect current is passed through a wide strip of metal in a perpendicular magnetic field. Since (some of) the negative charges in the metal move but the positive ones stay put, the ...