# 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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### UV absorption mechanism of the wooly hairs of the alpine edelweiss

I've read a paper about this photonic structure with corrugation arranged with gaps of similar wavelength to UV light. but the mechanism of how they interact is unclear to me. I still don't understand ...
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### Physical interpretation of Lorentz and Fano resonance

I'm currently studying about lorentz oscillation and fano resonance (ref: https://doi.org/10.1088%2F0031-8949%2F74%2F2%2F020). According to the lorentz model, also known as driven damped oscillated ...
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### Transformers, AC and DC current

I have a question about transformers and how do they work: I know that the main principles behind a transformer are the mutual induction and Faraday's law; in order to have an emf induced in the ...
1 vote
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### Formal description of an electron absorbing a photon

This question is about how the electromagnetic field interacts with matter. Suppose we have an electromagnetic wave, let us say it is planar but not monochromatic. So the wave is described by an $L^1$ ...
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### How does an electromagnetic wave warp spacetime?

I am trying to understand how an electromagnetic wave would curve spacetime and what effect this curvature in turn would have on the wave. I am imagining an electromagnetic plane wave travelling in ...
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### Induced emf and coil winding direction

Recently, I've learned that induced emf is there to oppose the change in magnetic flux. consider a current increasing over time ($\frac{di}{dt}>0)$, moving left to right as it passes through the ...
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### Does the magnetic field you find depend upon the size of the Amperian loop?

Will the value of magnetic field change if my amperian loops are changed if I am calculating it for the same current? For example, if I take an infinitely long straight conductor, does magnetic field ...
1 vote
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### Do electromagnetic waves really slow down? [duplicate]

I have read somewhere that electromagnetic waves, when refracting only "appear" to slow down. That is when the wave goes thorough a material like glass it vibrates the electrons in the glass ...
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### Direct derivation of retarded fields in electrodynamics

Rather than first selecting a specific gauge, writing a guessed solution for the potentials within that gauge, and then differentiating them to obtain the fields, can we instead directly derive the ...
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### How many times do you need to drop a magnet for it to lose it's power? [closed]

How many times do you need to drop a magnet to the ground for it to lose it's power ? Is there a way this is measured ? Thanks!
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### Changing the magnetic field of earth

I am familiar with the concept of toroid which causes an electromagnetic field. Is it possible, if we make a large enough toroid, to change the magnetic field of earth. If yes, then how will it affect ...
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### Quantised electromagnetic field in quantum optics for an infinite quantiation volume

The electromagnetic field can be quantised by considering the allowed modes in a cube based on the periodic boundary conditions and then quantising those modes. Doing so, we find that the hamiltonian ...
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### Question about electric flux in curved spacetime

I just started physics II (electricity and magnetism) and in learning about Gauss's Law, I came across this definition on Wikipedia: $$\Phi_E=c\oint_{S}F^{\kappa0}\sqrt{-g}\ dS_{\kappa}$$ Where $c$ is ...
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### How to show the ratio (spin angular momentum density)/(energy density) for circularly-polarized gravitational plane wave = $\pm 2/\omega$?

It is well understood that an infinite monochromatic, circularly-polarized electromagnetic plane wave has no angular momentum density. However, a finite monochromatic, circularly-polarized ...
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### How much electromagnetic force is keeping a chair together? [closed]

I’m not a physicist, so excuse my poor wording. In an average chair, roughly how much attractive force from the electromagnetic field is keeping it together? If you want to go further, how much force ...
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### Solve for the shape and curvature of Earth's magnetic field

How can I solve for the shape of Earth's magnetic field (neglecting the effects of the solar wind which distort it). It looks very similar to the field due to a solenoid, but I can only find solutions ...
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### Units of the action of a charged particle in four-potential coupling

I have a question about the definition of the action that Landau defines as: $$S= -\frac{e}{c}\int A^{\mu}dx_{\mu}$$ he says that the $1/c$ factor is introduced by ...
1 vote
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### Why doesn't an EMF source directly affect the internal charges in a wire, but only the surface charges

So for current to flow, an EMF source such as a battery causes surface charges to form a gradient along the wire. These then have local fields which cause current flow by moving internal free charges. ...
1 vote