# 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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### Magnetic field lines are closed lines?

How is it proven that the magnetic lines are closed lines? Because the divergence of $\mathbf{B}$ is zero does not sound convincing, since fields like $\mathbf{F} = a\mathbf i +b\mathbf j+c\mathbf k$ (...
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### Wave Equation in 2D and 3D

I am new to wave mechanics in 2 and 3 dimensions and would like to know: Do you simply add $\partial \psi \over\partial y$ and/or $\partial \psi \over\partial z$ term without the other parts of the ...
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### What is the signal velocity in a very thin enameled magnet wire?

I was wondering if in the case of a very thin enameled (i.e. sub-millimeter thick wire insulation dielectric usually made from polyimide) magnet wire the generated electric field of a signal transient ...
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### Superconductor behavior near absolute zero

How does critical current change for superconductors as temperatures approach the millikelvin realm, particularly High-Temperature Superconductors?
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### How to reduce magnetic field between two magnetic shield sheets (silicon steel sheets)?

I put 2 parallel silicon steel sheets between magnetic field as below: As silicon steel has much bigger permeability than air,magnetic line will go through 2 silicon steel sheets. What I can do to ...
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### How is surface charge induced on the surface of circuit wires when connected to a battery?

Understanding: I watched the newest Veritasium electricity video and he said that the electric field inside the wires of the circuit that causes the electric current to flow is created by both the ...
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### Do electric fences such as those typically used to fence cattle generate magnetic fields?

I'm using magnetometers near electric fences and I'm wondering if the electric fence should be detectable and if so, how close do I need to be to the fence for there to be a "measurable" ...
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### I have found a serious error in a Wikipedia page? [duplicate]

From WP Signal Velocity article: https://en.wikipedia.org/wiki/Signal_velocity Signal velocity is usually equal to group velocity (the speed of a short "pulse" or of a wave-packet's middle ...
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### Why the Tokamak Fusion Reactors need to be so big if electromagnetism works better for small distances?

I'm probably asking something dumb, but for the little I think I know, I assume that the bigger the electromagnetism field, the bigger the energetic requirement. I'm assuming that the square cube law ...
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### How do you make light?

We make light making a charge oscillate, or heating a body. Are there differences between the two processes? But, above all, are there other ways in which we can produce em radiation?
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### Time taken to travel by light a minimum or maximum?

I'm stuck on the following. We know that the path taken by light to travel between 2 points A and B corresponds to the path which minimises time elapsed. However, from relativity we also know light ...
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### “Standard kinetic energy term of nonrelativistic quantum mechanics” $[\mathbf p\cdot \mathbf A + \mathbf A \cdot \mathbf p]$

On page 187 of Peskin and Schroeder, a certain term in an expression for the amplitude of an electron scattering from a classical magnetic vector potential is identified with “the contribution of the ...
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### Spherical polar coordinates [closed]

Please help me in deriving the details equations of motions of a particle under the influence of EM force, coriolis and centripetal force. This is for B.Sc Introduction to Astrohysics. In Some papers ...
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### Nuclear Propulsion System

In a fictional sci-fi setting , is it plausible to: Have a huge nuclear powered aircraft carrier to be propelled by H2-O2 rocket engine (size adjust) where you generate both from sea water on the fly ...
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### Current through an ampere loop [closed]

I am trying to apply Ampere's law to find a magnetic field in coaxial cylinders. The inner cylinder with radius $2a$ and outer cylinder is $4a$. Current flows along positive $z$ for inner cylinder and ...
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### Can someone explain why this would not create an infinitely rotating disk (perpetual motion) [closed]

This may be a dumb question and the solution may be super obvious but I can't figure it out as hard as I try. I have drawn a sketch of a device I thought of, now I know that we cant create "Free&...
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### Has anyone ever put a magnetic or electrostatic dipole on a rotating shaft, spun it and demonstrated reception of a propagating wave in the far-field?

The question If I create a varying electric field and it will then create a varying magnetic field, so will it also create light? Will I see a light ray? got me thinking. I'm pretty sure that if I ...
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### Why do we feel electrical shocks?

So electrical energy from source to device flows in the direction of the Poynting vector. My question is what do we feel exactly when we get an electrical shock? Before I learned about Poynting and ...
Energy of and electron is $$E = \text{kinetic energy} + \text{negative of potential energy}. \tag{eq-1}$$ But energy of electron in the $n$th orbital is also $$E = -\frac{13.6}{ n^2} \tag{eq-2}$$ ...
Let us consider a linear birefringent material which is not isotropic and has three principal axes with refractive indices $n_1$, $n_2$ and $n_3$ for the $x$, $y$ and $z$ directions, respectively (we ...