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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2answers
595 views
How do mirrors work?
Apparently, light is just a certain wavelength, or "the visible spectrum" of electromagnetic waves. If I recall correctly, my physics teacher explained to me that electromagnetic waves are basically ...
2
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4answers
256 views
Relativistic origin of magnetic field
There is an explanation in the Wikipedia. Unfortunately the article is quite verbose and doesn't clearly explain why both positive and negative charges vary density even if only one is moving.
It is ...
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5answers
4k views
Where do magnets get the energy to repel?
If I separate two magnets whose opposite poles are facing, I am adding energy. If I let go of the magnets, then presumably the energy that I added is used to move the magnets together again.
However, ...
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5answers
781 views
Why aren't there compression waves in electromagnetic fields?
I just started learning about optics, and in the book I'm reading they explain how the electrical field caused by a single charged particle could be described by a series of field lines, and compare ...
5
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3answers
4k views
Do magnets lose their magnetism?
I recently bought some buckyballs, considered to be the world's best selling desk toy. Essentially, they are little, spherical magnets that can form interesting shapes when a bunch of them are used ...
4
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2answers
201 views
How does lightning “know” where to go?
If lightning comes down in, say, a large flat field with a lightning rod sticking out of the middle, the lightning will strike the rod.
How does it "know" the rod is there?
Will it always strike the ...
3
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3answers
694 views
What are electromagnetic fields made of?
I am trying to understand electromagnetic fields so I have two question related to them.
What is a electromagnetic field made of? Is it made of photons / virtual photons?
How about a static electric ...
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1answer
541 views
What is the force between two magnetic dipoles?
If I have two current loops parallel to each other with currents I1 and I2 and radii R1 and R2 a distance z from each other, what is the force between them?
What would change if they were two ...
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2answers
329 views
What is an “inclined magnetic field”?
What is meant by an "inclined magnetic field"? How is it different from the usual magnetic field?
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5answers
511 views
Will an emf be induced across the ends of a stationary metal rod placed in a time-varying magnetic field?
Consider a thin metal rod placed in a magnetic field whose direction is constant but whose magnitude is changing with time, with the length of the rod perpendicular to the direction of the magnetic ...
6
votes
1answer
920 views
Why are some materials diamagnetic, others paramagnetic, and others ferromagnetic?
Why are some materials diamagnetic, others paramagnetic, and others ferromagnetic?
Or, put another way, which of their atomic properties determines which of the three forms of magnetism (if at all) ...
6
votes
2answers
353 views
What's a good reference for the electrodynamics of moving media?
The answer to a previous question suggests that a moving, permanently magnetized material has an effective electric polarization $\vec{v}\times\vec{M}$. This is easy to check in the case of ...
6
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2answers
835 views
Is it safe to use any wireless device during a lightning storm?
I need "educated" reasons whether it is safe to use any wireless device during a lightning storm.
Most people said don't use it but they cannot explain why.
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1answer
296 views
Are the Maxwell's equations enough to derive the law of Coulomb?
Are the 8 Maxwell's equations enough to derive the formula for the electromagnetic field created by a stationary point charge, which is the same as the law of Coulomb?
If I am not mistaken, due to ...
5
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2answers
657 views
Why does it seem like a broken magnet's poles flip?
I just took a rare earth magnet out of an old hard drive. Lacking an appropriate screwdriver, force was used, and the magnet broke into two pieces; one about a quarter of the original size and one ...
3
votes
4answers
230 views
How to make something charged using electricity?
If I had a piece of metal and i wanted it to be negatively charged.
How can I do that?
3
votes
3answers
339 views
formula for transparency of very thin film of metal
Is there formula that gives transparency of very thin film of given metal (tens of nanometers) to the visible light/light of given wavelength ? Which properties of metals are needed for the formula ?
...
3
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2answers
154 views
Does constraint for speed of Electric and magnetic fields violates Conservation of momentum or Newton's third law?
I'm just a beginner so bear with me. Consider two frames at rest wrt to each other separated by distance enough for light to take a minute or so. At a given instant we create two large dipoles by some ...
2
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2answers
831 views
How to debunk 'The Electric Universe'?
My father is a generally intelligent person, however he has latched onto a theory which I believe to be completely incorrect.
He doesn't believe in gravity, but rather subscribes to an 'Electrical ...
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2answers
297 views
Software for simulating 3D Newtonian dynamics of simple geometric objects (with force fields)
I'm looking for something short of a molecular dynamics package, where I can build up simple geometric shapes with flexible linkages/etc and simulate the consequences of electrostatic repulsion ...
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2answers
143 views
Relativistic charge density contraction in a closed loop
I have heard that magnetism is a relativistic effect of electricity. The law says that charge is conserved, only distances are shrunk. So, I cannot add more charge to increase density. When we have an ...
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0answers
74 views
Einstein +Maxwell 's tensor
Why is it true that we can deduce that Einstein's GR equations coupled with Maxwell's EM equations may be written in the form $$R_{ij}=C(F_{ik}F_j^{\,\,k}-{1\over 4}g_{ij}F_{mn}F^{mn})$$
without ...
1
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1answer
186 views
Non-linear dynamics of classical hydrogen atom
I'd like to know if there have been attempts in solving the full problem of the dynamics of a classical hydrogen atom.
Taking into account Newton equations for the electron and the proton and Maxwell ...
1
vote
1answer
800 views
Moving conductors in magnetic fields: is there electric field or not?
this is my first question on PhysicsSE (I'm already an user of MathSE).
I'm a mathematics students trying to understand Faraday's law, that is
$$\varepsilon= -\frac{d \Phi_B}{dt}$$
where ...
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votes
1answer
160 views
Violation of conservation of energy and potential energy between objects
I would like to clarify my question. I have numbered them to be independent questions
For any conservative fields, $\vec{F} = -\nabla U$. Which means the restoring force is opposite to the ...
19
votes
2answers
633 views
Covariant Description of Light Scattering at a fastly rotating Cylinder
Let us consider the following Gedankenexperiment:
A cylinder rotates symmetric around the $z$ axis with angular velocity $\Omega$ and a plane wave with $\mathbf{E}\text{, }\mathbf{B} \propto ...
11
votes
1answer
438 views
Formulation of Transformation optics using a Material Manifold
Dear Community,
recently, Transformation optics celebrates some sort of scientific revival due to its (possible) applications for cloaking, see e.g. Broadband Invisibility by Non-Euclidean Cloaking ...
8
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3answers
1k views
Electromagnetic Field as a Connection in a Vector Bundle
I would like to know more about Ehresmann connections in vector bundles and how they relate to the electromagnetic field and the electron in quantum mechanics.
Background: The Schrödinger equation ...
14
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5answers
894 views
Is there an intuitive explanation for why Lorentz force is perpendicular to a particle's velocity and the magnetic field?
The Lorentz force on a charged particle is perpendicular to the particle's velocity and the magnetic field it's moving through. This is obvious from the equation:
$$ \mathbf{F} = q\mathbf{v} \times ...
35
votes
7answers
1k views
Cyclist's electrical tingling under power lines
It's been happening to me for years. I finally decided to ask users who are better with "practical physics" when I was told that my experience – that I am going to describe momentarily – prove that I ...
6
votes
2answers
312 views
Radiation Resistance
When a charge is accelerated, it radiates and loses kinetic energy. This can be modeled by having another force act on the charge, which is proportional to the derivative of the acceleration. So if ...
13
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3answers
1k views
What is the answer to Feynman's Disc Paradox?
[This question is Certified Higgs Free!]
Richard Feynman in Lectures on Physics Vol. II Sec. 17-4, "A paradox," describes a problem in electromagnetic induction that did not originate with him, but ...
13
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4answers
1k views
History of Electromagnetic Field Tensor
I'm curious to learn how people discovered that electric and magnetic fields could be nicely put into one simple tensor.
It's clear that the tensor provides many beautiful simplifications to the ...
7
votes
3answers
1k views
Are Classical Field Theory and Quantum Mechanics of a single particle (nonrelativistic or “classical”) limits of Quantum Field Theory?
Recently I talked about QFT with another physicist and mentioned that the Quantum Field Theory of a fermion is a quantisation of its one-particle quantum mechanical theory. He denied this and ...
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3answers
1k views
Derivation of Maxwell's equations from field tensor lagrangian
I've started reading Peskin and Schroeder on my own time, and I'm a bit confused about how to obtain Maxwell's equations from the (source-free) lagrangian density $L = ...
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2answers
615 views
Deriving the speed of the propagation of a change in the Electromagnetic Field from Maxwell's Equations
I've been told that, from Maxwell's equations, one can find that the propagation of change in the Electromagnetic Field travels at a speed $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (the values of which can ...
9
votes
4answers
854 views
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 ...
9
votes
4answers
550 views
Nonlinear optics as gauge theory
the widely used approach to nonlinear optics is a Taylor expansion of the dielectric displacement field $\mathbf{D} = \epsilon_0\cdot\mathbf{E} + \mathbf{P}$ in a Fourier representation of the ...
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1answer
98 views
Forcing quadrupole moments to vanish for a neutral system
For a system of electric charges $q_i$, at positions $\mathbf{r}_i$, with a nonzero net charge $Q=\sum_i q_i$, one can define a "centre of charge" in the obvious way as
$$
...
6
votes
7answers
3k views
Electromagnetic fields vs electromagnetic radiation
As I understand, light is what is more generally called "electromagnetic radiation", right?
The energy radiated by a star, by an antenna, by a light bulb, by your cell phone, etc.. are all the same ...
5
votes
2answers
318 views
Notation for Sections of Vector Bundles
(Reformulation of part 1 of Electromagnetic Field as a Connection in a Vector Bundle)
I am looking for a good notation for sections of vector bundles that is both invariant and references bundle ...
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2answers
794 views
What does an atom radiate: a wave packet or a single photon?
What does an atom radiate: a wave packet or a single photon?
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4answers
1k views
Visualizing Electromagnetic Waves in 3D Space
I did one module of physics for my GCSE one year ago which taught me about transverse EM waves & the EM spectrum, but since then, I do not understand how a wave would move in 3D space. Can someone ...
11
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3answers
615 views
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?
10
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2answers
620 views
How strong of magnetic field would noticibly attract a person?
There is Iron in blood. Iron is magnetic. Roughly how strong would a magnet have to be to induce a noticeable attraction? It would be nice to know this for several distances. Also, do electromagnets ...
9
votes
2answers
455 views
If you run an electric current through a wire loop, do the accelerated charges radiate?
Does an accelerated charge always radiate?
For example the current electrons in an electric circuit when moving through a turn they are accelerated, do they radiate because of that acceleration?
If ...
8
votes
1answer
155 views
How the inverse square law in electrodynamics is related to photon mass?
I have read somewhere that one of the tests of the inverse square law is to assume nonzero mass for photon and then, by finding a maximum limit for it , determine a maximum possible error in ...
8
votes
3answers
1k views
Is the Abraham-Minkowski controversy resolved?
A paper was published in 2010 claiming to resolve the Abraham-Minkowski controversy. Is this paper viewed as definitive by physicists?
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2answers
2k views
How does reflection work?
In Newton's model of light as being composed of particles, it's easy to imagine reflection as being the rebounding of individual corpuscles off a surface. However, since light can also behave like a ...
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1answer
255 views
Is there a strong force analog to magnetic fields?
In special relativity, magnetism can be re-interpreted as an aspect of how electric charges interact when viewed from different inertial frames.
Color charge is more complex than electric charge, but ...
