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# 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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380 views

### 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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### What experiments confirmed Poynting's Theorem?

Actually, I have two related questions. The fist is the above question. The second is, what experiments confirmed that $\varepsilon_0\left(\frac{E^2}{2} + c^2 \frac{B^2}{2}\right)$ is the ...
418 views

### What is capacitance, in general?

In circuit analysis software capacitance can be measured between any two nodes of a circuit or of a multiterminal device. In practical terms we take $C_{ij}$, the capacitance between $i$ and $j$ as ...
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### How does $\psi(x) =\exp(\frac{iq}{\hbar}\int^xA(x')\cdot dx')\phi(x)$ remove the gauge field for a free particle?

In what sense does writing $$\psi(x) =\exp(\frac{iq}{\hbar}\int^xA(x')\cdot dx')\phi(x)$$ "formally remove the gauge field" for a free particle in the Hamiltonian H\psi=\frac{1}{2m}(-i\hbar\nabla -...
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### Stability of a dipole magnet undergoing eddy current braking in a cylindrical tube

Consider a cylindrical dipole magnet with strength $B$ undergoing eddy current braking in a conductive cylindrical tube with the axis of the magnet aligned with the axis of the tube. The magnet ...
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### Charge distribution for three connected conductor spheres

In the auxiliary material of the physics textbook of Halliday, first chapter about electrostatics, there is an example that has the following statement and solution: basically there are three ...
22 views

### Poynting vector flux in a tube (geometrical optics)

Let's consider the following tube (defined as the solid described by two arbitrary surfaces dA1 and dA2, and by all the rays which connect their boundaries): I read in a textbook that, because of ...
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### Why is the generalized momentum replaced by the momentum operator but not the ordinary momentum?

I was trying to understand the derivation of the Hamiltonian for a charged particle in an electromagnetic field. https://en.wikipedia.org/wiki/Hamiltonian_mechanics#...
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### Inducing coil current by falling?

Typically, if a rod or coil is moved at a constant velocity through a constant magnetic field, a motional EMF will be induced by Faraday's Law but no current because there is no net emf or no closed ...
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### What would electromagnetic field of concentric ball magnets look like?

What is the scientific consensus for what electromagnetic fields of concentric ball magnets look like? This here is a few assumptions, poles on inside diameter of hollow shell reverse polarity to the ...
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### How is it possible that one face of my electromagnet produces stronger magnetic field than the other?

I work in an experimental research lab. I made an electromagnet by having the machine shop cut through a toroid core and hand-winding wire around it. The objective is to have a region of uniform ...
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### Explain why the magnets move towards eachother in this setup

I have two bar magnets (actually stacks of small magnets, but I believe they should behave the same) attached to a stack of (presumably conductive) coins, and not diametrically opposite. When I ...
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### Why can infinite planes be approximated as Gaussian surfaces?

A little background: I'm an undergraduate studying Electrodynamics, currently in Chapter 8 of Griffiths. A question I came across (8.4 part a for those curious) asks for a calculation of the force ...
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### In Einstein's 1905 paper on electrodynamics, what he meant by energy of electromotive force?

In his 1905 paper, Einstein says that when the magnet is in motion and conductor stationary, changing magnetic field in space develops electric field "of certain definite energy", and this starts ...
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### Electromagnetic field conversion in non-inertial frame

I found these equations for electromagnetic field conversion in inertial frame for non relativistic case. If I have permanent static magnetic field with 3 components $B_x, B_y, B_z$ and permanent ...
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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 ...
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### Why do paramagnetic materials require large fields to achieve magnetic saturation?

I'm performing an experiment on a paramagnetic sample, and the field strengths we are applying range from -7T to 7T, which are impressively large field magnitudes. However, we still are not observing ...
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### Maxwell theory : How to justify the potential gauge fields if there are magnetic monopoles?

Without magnetic monopoles, the Maxwell equations are these (I'm dropping vector notations and all constants, for simplicity): \begin{align} \nabla \cdot E &= \rho_{elec}, \tag{1} \\[12pt] \nabla \...
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### Is there an alternative to SIMION?

Is anyone aware of an alternative, possibly open-source, to the SIMION software? From their website: SIMION is a particle optics simulation software primarily used to calculate electric fields ...
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### Dot convention inductors in series: what is going on

So I'm really confused with mutual inductors and dot convention. If your answer is going to be a link to any website I can assure I read them all and that only left me more confused. So here are my ...
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### How do you measure and distinguish between E and D fields, and B and H fields

I’m pretty familiar with maxwells equations, light waves, fields and materials etc, but I’m not clear on how the the various fields are actually measured. How is this typically done? Can the fields ...
72 views

### Magnet pulling/repelling another magnet beyond the event horizon

I have read this question: If a Kerr-Newman black hole is like a charged, spinning, heavy magnet, what kind of magnet is it like? If you are carrying a magnet, can you tell when you cross the event ...
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### Can the Taylor expansion of Energy/Intensity be explained more in-depth?

In reference to the question, "Why is energy in a wave proportional to amplitude?" Ben Crowell answered with the answer attached. The original post is linked here. I did not respond to the original ...
508 views

### Is the Hamiltonian of a relativistic charged particle in an electromagnetic field only an approximation?

Consider a system of two relativistic charged point particles 1 and 2 which interact through their electric and magnetic fields. The equation of motion for the first particle is then given by the ...
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### Is a dipole fusion reactor feasible?

It is well known that the earth's dipole-like magnetic field is able to confine charged particles to paths that spiral along the field lines. That's why we have auroras and the Van Allen radiation ...
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### Force caused by the magnetic field on the electron spin

I have read there are two ways to express the force caused by a variable magnetic field over a magnetic moment depending on the source of the magnetic moment since it can come from a dipole or a loop ...
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### Non-linearities in Lagrangian of a scalar field coupled to point-like source

I have an exercise where I did not manage to understand the questions. Basically, I have this Lagrangian \mathcal{L}=\frac{1}{2}(\partial \pi)^2-\frac{1}{\Lambda^3}(\partial \pi)^2\...
321 views

### What causes diamagnetism?

What makes a diamagnetic substance repel a magnet? If one explains it on the basis of cancelation of dipole moments in an atom and Lenz's law, shouldn't the diamagnetic substance be attracted to the ...
61 views

### Frequency spectrum of bremsstrahlung

I have tried to find a method to derive the spectrum of the emitted radiation of accelerated relativistic charged particles but I've never found such a method in any books. Does anyone know a method ...
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### How does the “Electromagnetic Position Tracking” work, exactly?

I've been searching the web for the technology of the "Electromagnetic Position Tracking1," and have found the web to be surprisingly ineffectual! I've just came up with some preliminary and basic ...
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### Inductive sensor - How does it work?

I can't understand how the system sketched here physically works: 4 = permanent magnet, with residual magnetization $B_r$ 6 = ferromagnetic core with a coil around it 7 = ferromagnetic toothed ...
446 views

### Poynting theorem on an example?

I understand the basic statement of Poynting theorem of conservation of energy relative to electromagnetic field. However, I fail to apply it to an example. Consider this classical case: ...
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### Four vector potential and discrete parity transformations

I am having trouble understanding the effect of parity transformations on the four-vector gauge field (for example). I am working in three dimensions, but the analysis is probably not that different ...