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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Why does the vacuum even have permeability and permittivity?

The vacuum is empty, yet it seems to have 2 properties: it's absolute permeability and absolute permittivity, which have specific, finite, non-zero values. Why? Why are the vacuum permeability and ...
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Does special relativity make magnetic fields irrelevant?

I've heard that special relativity makes the concept of magnetic fields irrelevant, replacing them with relativistic effects between charges moving in different velocity frames. Is this true? If so, ...
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Is there just one EM field for the whole universe?

Does our universe contain individual magnetic fields ? For example two different magnets, one here on earth and one on mars. Do both of them have their own magnetic field? Or is there only one single ...
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Does a magnetic field do work on an intrinsic magnetic dipole?

When you release a magnetic dipole in a nonuniform magnetic field, it will accelerate. I understand that for current loops (and other such macroscopic objects) the magnetic moment comes from moving ...
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Nuclear Fusion: Why is spherical magnetic confinement not used instead of tokamaks in nuclear fusion?

In nuclear fusion, the goal is to create and sustain (usually with magnetic fields) a high-temperature and high-pressure environment enough to output more energy than put in. Tokamaks (donut shape) ...
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Why do physicists believe that there exist magnetic monopoles?

One thing I've heard stated many times is that "most" or "many" physicists believe that, despite the fact that they have not been observed, there are such things as magnetic monopoles. However, I've ...
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Why is this vector field curl-free?

The curl in cylindrical coordinates is defined: $$\nabla \times \vec{A}=\left({\frac {1}{\rho }}{\frac {\partial A_{z}}{\partial \varphi }}-{\frac {\partial A_{\varphi }}{\partial z}}\right){\hat {\...
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Do magnets wear out?

Can a magnet ever wear out or lose strength? If you break a magnet it (seemingly) gets weaker, but what about from normal use? Or even very heavy use, like placing 2 magnets facing each other, so ...
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A contradiction between Biot-Savart and Ampère-Maxwell Laws?

I came across a problem that I cannot get my head around. Consider two very small spherical metallic balls given charges $+Q$ and $-Q$. Assume that both can be approximated as point charges. Now, ...
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Can a wire having a $610$-$670$ THz (frequency of blue light) AC frequency supply, generate blue light?

We know that when we give alternating current across a wire then it will generate an electromagnetic wave which propagates outward. But if we have a supply which can generate 610 to 670 terahertz of ...
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How are forces “mediated”?

I hope this is the right word to use. To me, these forces seem kind of fanciful (except for General Relativity and Gravity, which have a geometric interpretation). For example, how do two charged ...
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Apparent Violation of Newton's $3^{\text{rd}}$ Law and the Conservation of Momentum (and Angular Momentum) For a Pair of Charged Particles

Consider a system of the two identical positive point charges situated in free space (isolated from the influence of any other external fields) as shown in the attached diagram. Particle $1$ is at $(a,...
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In electrostatics, why the electric field inside a conductor is zero?

In electromagnetism books, such as Griffiths or the like, when they talk about the properties of conductors in case of electrostatics they say that the electric field inside a conductor is zero. I ...
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Why don't we define potential due to a magnetic field?

We define electric potential and gravitational potential and use them quite often to solve problems and explain stuff. But I have never encountered magnetic potential, neither during my study (I am a ...
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Why does only one side of a neon lamp glow?

When applying DC to a neon lamp, only the negatively-charged electrode glows: The voltages across the lamps are left: DC (left lead positive), middle: DC (right lead positive), and right: AC. But... ...
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What is a “decade” as a unit of measure (ex. a decade of the EM spectrum)?

Reading through papers and online sources about radio galaxies, I kept stumbling across a term--a "decade" of the electromagnetic spectrum. Radio galaxy emission encompasses "11 decades of the EM ...
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Can the Lorentz force expression be derived from Maxwell's equations?

The electromagnetic force on a charge $ e $ is $$ \vec F = e(\vec E + \vec v\times \vec B),$$ the Lorentz force. But, is this a separate assumption added to the full Maxwell's equations? (the ...
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What happens to the magnetic field in this case?

As far as I know, it's possible to create a radially polarised ring magnet, where one pole is on the inside, and the field lines cross the circumference at right angles. So imagine if I made one ...
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How do electrons know which path to take in a circuit?

The current is maximum through those segments of a circuit that offer the least resistance. But how do electrons know beforehand that which path will resist their drift the least?
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Why is UV light visible when reflected off paper?

I was carrying out a photoelectric effect experiment when I realised that the $365$ nm line in the mercury spectrum was surprisingly visible when shone onto a piece of paper. This lies in the UV ...
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Why do we deal only with large scale magnetic fields in astrophysics, and not electric fields?

In astrophysics there is a lot going on about strong, large scale magnetic fields: in stars (prominences), magnetic dynamos, compact accretors collimating jets, etc. There's even a special ...
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Why does the density of electric field lines make sense, if there is a field line through every point?

When we're dealing with problems in electrostatics (especially when we use Gauss' law) we often refer to the density of electric field lines, which is inversely proportional to the radius in the case ...
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Can the center of charge and center of mass of an electron differ in quantum mechanics?

Traditionally for a free electron, we presume the expectation of its location (place of the center of mass) and the center of charge at the same place. Although this seemed to be reasonable for a ...
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Kubo Formula for Quantum Hall Effect

I'm trying to understand the Kubo Formula for the electrical conductivity in the context of the Quantum Hall Effect. My problem is that several papers, for instance the famous TKNN (1982) paper, or ...
29
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What different approximations yield Gravitoelectromagnetism and Weak Field Einstein Equations?

This question is inspired by this answer, which cites Gravitoelectromagnetism (GEM) as a valid approximation to the Einstein Field Equations (EFE). The wonted presentation of gravitational waves is ...
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Can a gym be built to supply electricity to homes? [closed]

A moving magnet induces a current in a conductor, then shouldn't we be able to generate electricity through manual labour? I was thinking about building a gym that used magnets as weights. People ...
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Is a suit that hides a soldier's heat signature fundamentally possible?

I recently played "Crysis", a game where the protagonist wears a suit that allows the player to hide both himself and his heat signature. Then I watched Iron Man 3, where a kid suggests that Tony ...
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Basis for the Generalization of Physics to a Different Number of Dimensions

I am reading this really interesting book by Zwiebach called "A First Course in String Theory". Therein, he generalizes the laws of electrodynamics to the cases where dimensions are not 3+1. It's an ...
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What non-metal is attracted by a magnet?

Are there any non-metal objects that are attracted by magnets?
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The exchange of photons gives rise to the electromagnetic force [duplicate]

Pardon me for my stubborn classical/semiclassical brain. But I bet I am not the only one finding such description confusing. If EM force is caused by the exchange of photons, does that mean only when ...
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Radio antennas that are much shorter than the wavelength

From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall ...
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Recommended books for advanced undergraduate electrodynamics

What books are recommended for an advanced undergraduate course in electrodynamics?
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Please help identify this physics apparatus!

This was my grandfather’s and have no idea what it is only that it is some piece of physics equipment! The main black cylinder doesn’t seem like it wants to rotate but not sure if it should?
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How to read Maxwell's Equations?

I was talking with a colleague about Maxwell's Equation, in particular Faraday's Law of Induction, and I realised that I did not understand the following. Usually, this equation is expressed as: The ...
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Is one way glass possible?

I am not talking about mirrors, just a plain window made of glass like material. Would it be possible to allow light pass only in one direction but not the other?
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Detection of the Electric Charge of a Black Hole

By the "No Hair Theorem", three quantities "define" a black hole; Mass, Angular Momentum, and Charge. The first is easy enough to determine, look at the radius of the event horizon and you can use the ...
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What prevents this magnetic perpetuum mobile from working?

As a child, I imagined this device, which may seem to rotate indefinitely. I have two questions. Is this perpetual motion machine already known? If it is, could you please give some references? What ...
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Why is current a scalar quantity?

Current has both magnitude and direction. As per the definition of vector defined in encyclopedia, current should be a vector quantity. But, we know that current is a scalar quantity. What is the ...
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Is the EmDrive, or “Relativity Drive” possible?

In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some ...
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Is it (practically) possible for a large building to be a Faraday cage?

In my sophomore year of high school, my P.E. teachers kept on complaining about how phones didn't have a network connection in our gym, regardless of model, service provider, etc. A couple of feet ...
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Are room temperature superconductors theoretically possible, and through what mechanism?

At the moment, the highest critical temperature superconductor known to science (or myself, at least) is mercury barium calcium copper oxide. With a $T_{c}$ of roughly 133 K, that's well above the ...
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Why is there no gravitational magnetic field? (Or, is there?)

We can think that the electric field and the gravitational field operate similarly in the sense that the forms of their governing laws (namely, Coulomb's law and Newton's law respectively) are ...
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What is the magnetic field inside hollow ball of magnets?

Setup: we have a large number of thin magnets shaped such that we can place them side by side and eventually form a hollow ball. The ball we construct will have the north poles of all of the magnets ...
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Local explanation of the Aharonov-Bohm effect in terms of force fields

Here is an interesting paper for the Physics SE community: On the role of potentials in the Aharonov-Bohm effect. Lev Vaidman. Phys. Rev. A 86 no. 4, 040101 (R) (2012). arXiv:1110.6169 [quant-ph]. ...
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Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
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Why are the electric force and magnetic force classified as electromagnetism?

I confuse the four kinds of fundamental interactions, so I think the electric force and magnetic force should not be classified as a big class called electromagnetism. Here is my evidence: The Gauss ...
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What kind of force can a rock exert which a motor cannot?

Imagine a dam with two doors. We have two cases: First case: there is a rock heavy enough to stop the doors from opening. Second case: there are are two motors or kind of machines (not sure if it ...
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Maxwell in multiple dimensions: What happens to curl?

I read this answer a while ago, and while thinking about $\nabla$, I realized something. Since the cross product can be written as a determinant, in higher dimensions we require extra vector inputs. ...