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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Self-energy of electron from classical reasoning

If it takes energy to group charge together(self energy) how can it be possible for every single electrons, etc, to have exactly same amount of charge? (think of if we hold some sand in our hand, then ...
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148 views

Textbook on classical E&M in curved spacetime

Can anyone recommend a good reference for classical electrodynamics that goes over electrodynamics in curved spacetime that doesn't assume much knowledge of GR -- that is it builds up the tensor ...
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84 views

Independent components in a 4-vector representing massless fields

In Ryder Page141, it is written "the electromagnetic field, like any massless field, possesses only two independent components, but is covariantly described by a 4-vector $A_{\mu}$". Why are there ...
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100 views

Why is an electric and magnetic field present in all universe?

the Earth has its fields due to the motion of molten metals in the core but how does this whole universe has its fields without any discontinuity? And we can say for sure that fields are present in ...
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1answer
60 views

Collision of light waves & matter

When light or electromagnetic wave hits an obstacle, what happens? Are the reactions times always negligible? By reaction I mean all that happens after the hit like reflection for example.
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1k views

Flux cutting and flux linking?

I am a bit confused about the difference between flux cutting and flux linking when talking about magnetic fields and induced EMF. I was originally under the impression that flux cutting was when ...
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84 views

Is dimensional analysis always sufficient to establish equivalence of quantities?

In dealing with the Biot-Savart law, it was argued that $$ q\frac{d\vec{s}}{dt}\equiv Id\vec{s} $$ using the fact that the units are equal. Does this kind of argument always work? It seems too ...
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146 views

$E,H,\rho,\vec{j}$ intensive? $B,D,\Phi,\vec{A}$ extensive? Why not $ DdE,BdH ,\Phi\delta\rho,\vec{A}\cdot \delta \vec{j}$ as infinitesimal work?

$u$ is the e&m field energy $\frac{\partial u}{\partial t}=H\cdot \frac{\partial B}{\partial t}+E\cdot \frac{\partial D}{\partial t}$ Compared with thermo physics: $PdV,TdS,\mu dN$ ...
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110 views

Why gauge invariance for electromagnetic fields?

What is the physical constraint that gauge invariance is a required condition for electromagnetic fields? What would happen if the electromagnetic fields were not gauge invariant?
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122 views

All possible electromagnetic Lorentz invariants that can be built into the electromagnetic Lagrangian?

Given the electromagnetic Lagrangian density $$ \mathcal{L}~=~-\frac{1}{4}F_{\mu\nu}F^{\mu\nu}~=~\frac{1}{2}(E^2-B^2) $$ is a Lorentz invariant, how many other electromagnetic invariants exists that ...
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77 views

The question about quantization of free EM field

Let's have the free EM field theory with Coulomb gauge: $$ \partial^{2}A_{\mu} = 0, \quad A_{0} = 0, \quad (\nabla \cdot \mathbf A ) = 0. $$ One of the ways of quantizing the field is the following. ...
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83 views

How do wind turbines cause lightning?

I saw this paper about Lightning discharges produced by wind turbines More lightning strikes wind turbines than comparable non-moving structures. What would be the physical cause of this? Does it ...
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460 views

Is the canonical momentum conserved when a particle moves in magnetic field?

Here is a question about the canonical momentum that I had asked some days ago, but I still have one point that I am not understand. Considering a particle moves in a magnetic field with charge $q$ ...
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1answer
100 views

What is the Significance of Magetic Field Energy Density?

If we consider electrostatic field energy density for an electrical field then by analysis we find that it is the representation of the total electrostatic potential energy of the considered ...
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386 views

Maximum potential of a van de graaff generator

I know that we can calculate the maximum potential of a van de graaff generator by ( radius* electric field in which corona discharge begin to form in the surrounding gas( according to wikipedia 30 ...
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2answers
133 views

If you “disobey” the constraints of the Kramers-Kronig relations, what happens? Do you get non-physical results?

If you "disobey" the constraints of the Kramers-Kronig relations, what happens? Do you get non-physical results? I am simulating reflection and transmission off/through a slab of material. I specify ...
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1answer
111 views

physically modelling the Saltatory nerve impulse transmission?

The nerve impulse transmission is specifically a biophysical process. Under a resting stage, the membrane is already polarised (presence of charge on either side leading to a potential difference ...
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255 views

Transmission Line, TEM wave, transverse Laplacian operator (electromagnetic)

Time-Harmonic Electromagnetic Fields (1961) by R. F. Harrington, ISBN 07-026745-6, p. 63, he discusses TEM mode in a transmission line. The wave travels in the z direction. He defines the transverse ...
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126 views

Curie temperature and magnetization?

If an ferromagnetic object is heated and reaches Tc the magnetization gradually drops as we get closer to Tc or it's a instant drop? Can I assume as I heat the object, the magnetization is weakening ...
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1answer
172 views

is it possible to have magnetic flux density B not in the same direction of magnetic field intensity H?

it is said that direction of magnetic flux density B in the same direction of magnetic field intensity H for isotropic media so what is isotropic media and is it possible to have B not in the same ...
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1answer
297 views

Are magnetic forces medium-dependent?

I am learning the Magnetic Effects of Electric Currents at school, and I was given an equation. I was told it would be elaborated on next year. But since I can't seem to bring myself to wait, here it ...
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3answers
965 views

Why does a dielectric have a frequency dependent resistivity?

This question has come about because of my discussion with Steve B in the link below. Related: Why is glass much more transparent than water? For conductors, I can clearly see how resistivity ...
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1answer
580 views

Two electron beams exert different forces on each other depending on frame of reference?

I am sure there is a simple explanation for my confusion, but I am a little puzzled: We are dealing with two parallel electron cannons that each produces a straight beam of electrons. They are placed ...
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1answer
113 views

Trying to understand EM wave and photon

When electrical fields and magnetic fields couple together, it forms electromagnetic waves. And we can "quantized" it and see each "package" of it as photon. So can electrical fields and magnetic ...
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3answers
221 views

Microscopic picture of an inductor

I have a good understanding of how inductors behave in electrical circuits, and a somewhat rough-and-ready understanding of how this behaviour arises from Maxwell's equations. However, what I don't ...
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55 views

How do I obtain a sense of scale when thinking about webers?

This Wikipedia article provides a wonderful way to fathom the scale of the tesla. I cannot seem to find a similar set of examples about magnetic flux. I know that the milli-, micro- and nanoweber are ...
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115 views

Properties of the Faraday tensor for constant fields

I'm doing a special relativity past exam paper and have got caught up with something that I hope someone can help me with! I have to show that for constant fields, the magnitude of A, the ...
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346 views

Does special relativity unify the two phenomena at the base of Faraday's flux law (was Feynman wrong in this case)?

Consider Faraday's flux law for the EMF generated in a conductor loop: $$ \varepsilon = - \frac{d \phi}{dt},$$ where $\varepsilon$ is the EMF, and $\phi$ is the magnetic flux through the loop. ...
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6k views

How do transformers work?

A transformer is basically a primary inductor connected to a voltage $U_P$ which you want to transform. You also have an iron rod and a secondary inductor. So when there is a current $I_P$ the iron ...
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636 views

Vector Potential and Gauge Invariance in Quantum Mechanics

In classical electromagnetism, we are allowed to use gauge invariance through the argument that the only physical observable fields are the $E$-field and the $B$-field. So in that sense the scalar ...
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711 views

How EM waves are produced by accelerating charged particles?

How the electro-magnetic waves are produced by the accelerating charged particle? Graphical explanations are most welcomed. Is the explanation given by the below mentioned article correct regarding ...
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1answer
506 views

A point in multiple magnetic fields

Given two parallel wires carrying a current (e.g. 2 and 3 Amperes) and the distance between them, 5 cm, how do I determine the magnitude of the magnetic field at a point M mid-distance between the ...
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860 views

Tensor product notation [closed]

In the image there is a tensor product: $$F_{\mu\nu}F^{\mu\nu}=2(B^2-\frac{E^2}{c^2})$$ It's about how this operation on the co- and contravariant field strength tensors can give one of the ...
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498 views

Demonstrate magnets adhere to conservation of energy pursuant to the laws of thermodynamics

I am looking for a way to demonstrate that magnets adhere to the laws of thermodynamics, in particular the requirement that energy in a closed system be conserved. To adhere to the requirement that ...
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1answer
234 views

Does the expression of the orbital magnetic dipole moment have $c$?

The orbital magnetic dipole moment of a particle with mass $m$ and charge $q$ can be shown to be related to the orbital angular momentum through the equation $$\displaystyle ...
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1answer
160 views

Does hitting metals magnetize them?

I was on my workshop lab today and had to file (rub on metal surface with rough surface to smooth-en it) an iron bar. It made iron dust fall of the surface. To mark some points on the bar I then had ...
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1answer
128 views

Crushing a magnetic field

What would happen if you crushed a magnetic field to an ever decreasing size? Thanks. EDIT: How small could the field possibly go? Is there a limit on how small it could get? Is there a maximum ...
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2k views

Is the electric field zero inside an ideal conductor carrying a current?

By an ideal conductor, I mean one with zero resistance. Inside an ideal conductor with no current, the electric field is zero, but is the electric field still zero with the ideal conductor carrying a ...
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1k views

Shielding magnetic field without mu-metal

I need to shield my device from magnetic interference, including earth magnetic field (if you move device around, it might be enough to cause slight currents i guess) and magnetic field caused by ...
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2answers
1k views

How are magnetic fields transmitted?

A common analogy for gravity is the ball-on-a-rubber-sheet model. In this model, mass distorts spacetime and creates a 'valley' into which other mass can fall. Is this same principal valid for ...
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1answer
295 views

Complex Potentials, Potentials and Fields

Suppose an electric field $E=-\nabla \psi$ where $\psi=-Q\ln r$ where $Q$ is just some constant and I have found its harmonic conjugate to be $-Q\theta+c$ where $c$ is some constant. What does it say ...
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1answer
9k views

What is the difference between a Rodin coil and a Rodin starship?

I've seen various designs for Rodin coil and a 'Rodin starship'? Are these just regular electromagnets? Or something different? How do they differ from regular electromagnets?
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4answers
628 views

How is this classical “paradox” resolved in electromagnetism?

A magnet and a coil move relative to each other. In the frame of reference of the magnet, there is a magnetic field and consequently a force acting on the charges in the coil according to the Lorentz ...
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446 views

Which symmetry is associated with conservation of flux?

Which symmetry is associated with conservation of flux (e.g., in electromagnetism)? For example, when working with Gauss's law in electromagnetism, net flux through an arbitrary volume element ...
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264 views

Can the field generated by a magnet domain extend to infinity?

As a thought experiment let us assume that we have isolated a magnetic domain. This domain is of finite size and we know its dimensions. Assuming that we can measure an infinitesimal field, will there ...
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1answer
1k views

Understanding the Seebeck effect

Thermoelectricity is, as I understand it, the difference in voltage between the hot and cold ends of two dissimilar materials. If two materials are connected at two different junctions, the hot ...
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1answer
85 views

One point to change electric field

Can there be a charge configuration in space such that at any instant of time I can change the electric field at one and only one point?
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2answers
5k views

What happens if a wind mill rotates in opposite direction?

Suppose if a windmill is made torotate in clockwise direction and due to air flow if it rotates in anti clockwise direction then does it produce electricity?
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413 views

Does magnetised material lose weight?

I've had argument on a forum about this. People think, that: if the same object is magnetised, it has less energy. so by $E = mc^2$ it weighs less than an object that is not magnetised (hence spin of ...
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303 views

Electron gun - electron in cylindrical anode

In an electron gun, the heating filament heats the cathode, releasing electrons by thermionic emission. I've read that "electrons are negatively charged particles and the positively charged ...