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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290 views
Do intergalactic magnetic fields imply an Open Universe?
According to a recent paper on the arXiv, they do. How credible is this result? The abstract says:
The detection of magnetic fields at high redshifts, and in empty
intergalactic space, support ...
6
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0answers
99 views
Nuclear Magnetic Resonance (NMR) Conceptual Questions
Let $M$ be the magnetic moment of a system. Below are the Bloch equations, including the relaxation terms.
$$\frac{\partial M_x}{\partial t}=({\bf M} \times \gamma {\bf H_0})_x-\frac{M_x}{T_2} $$
...
5
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0answers
113 views
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, published in PHYSICAL REVIEW A 86, 040101(R) (2012).
You should check it ...
5
votes
0answers
191 views
Neodymium Electromagnet?
I am interface designer that occasionally steps out and does some product/concept design. The last one I'm working on is something that would require me to make a matrix of little electro magnets, and ...
5
votes
0answers
146 views
Can I integrate out the fermion field that is not gapped?
This piece of argument has been repeated again and again by experts, that is
Since the fermions are gapped, then I can integrate it out.
but I have no idea of what will happen if the fermions ...
5
votes
0answers
194 views
1-form formulation of quantized electromagnetism
In a perpetual round of reformulations, I've put quantized electromagnetism into a 1-form notation. I'm looking for references that do anything similar, both to avoid reinventing the wheel and perhaps ...
4
votes
0answers
92 views
Is it correct to sum over either index of the metric the same way?
I don't know if the following is correct, i want to compute the following derivative
$$\frac{\partial }{\partial (\partial_{\mu}A_{\nu})}\left(\partial^{\alpha}A^{\beta}\partial_{\alpha}A_{\beta} ...
4
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0answers
75 views
Is the search for a Simple-group-based Electro-Weak theory over?
Just wondering:
We know that, in its current form of the $SU(2)_L\times U(1)$, the electroweak theroy rides a wave of huge success. However, is it not possible that the correct simple group ...
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41 views
Order of magnetic phase transitions
Is there any phase transition occur in paramagnetism to diamagnetism transitions state. What should be the order and how will I calculate the order?
4
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0answers
106 views
Does a rotating magnetic monopole have electric and magnetic moment in classical view?
Would a rotating sphere of magnetic monopole charge have electric moment ?
In a duality transformation E->B.c etc. how is the magnetic moment translated m = I.S -> ?
Mel = d/dt(-Qmag/c).S ?
A more ...
4
votes
0answers
130 views
Semiclassical QED and long-range interaction
I'm interested in the (very) low energy limit of quantum electrodynamics. I've seen that taking this limit does not yield Maxwell equations, but a quantum corrected non-linear version of them.
If ...
3
votes
0answers
62 views
How can I find the position of an image charge when the boundary is parabolic or hyperbolic?
If the position of some charge Q is known, the boundary condition is u=0 on some parabolic surface, and we know the image charge has its electric volume of Q', then how can I determine the position of ...
3
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0answers
44 views
Effective aperture of isotropic antenna
I have always taken for granted that 'the aperture of a loss-less isotropic antenna is $\dfrac {\lambda^2} {4\pi}$'.
On a whim, I tried to look up how this expression was derived, but so far I have ...
3
votes
0answers
79 views
Materials with different gravitomagnetic permeability?
If you start with general relativity, and assume small perturbations around a nearly flat metric, it is possible to obtain linearized equations of gravity that look a lot like Maxwell's equations, ...
3
votes
0answers
79 views
Theoretically speaking what are the limits for super-capacitors in terms of their storage capability?
According to wikipedia, commercially available super-capacitors in 2010 can store 30Wh/kg, with 85Wh/kg achieved in a lab in 2011.
Have any theoretical calculations been done to show what can be ...
3
votes
0answers
128 views
Question about laws of conservation in electrodynamics
One way of deriving laws of energy, impulse and angular momentum of electromagnetic field conservation is following:
Introduce two values below:
$$
\mathbf P = \frac{c}{4 \pi}[\mathbf E \times ...
3
votes
0answers
95 views
Can you safely draw sparks from the nose of an electrified boy?
From Purcell's Electricity and Magnetism
A spectacular conclusion of one of the popular exhibitions of the time
was likely to be the electrification of a boy suspended by many silk
threads ...
3
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0answers
272 views
I lost a factor of two in the electromagnetic field tensor
I apologize for this simple question, but I lost a factor of 2 and can't find it anymore, so now I'm looking on the internet, perhaps one of you has some information about its whereabouts. :-)
...
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0answers
23 views
Small charged sphere's motion in earth's magnetosphere?
Suppose that a spherical metal sphere with mass $m=10^{-16}kgr$ radius $R=10μm$ charge $Q=10^{-9}C$ travels with $v=c/3$ and is trapped in the earth's magnetosphere at a distance around $r = 1000km$. ...
2
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0answers
46 views
Electromagnetic inertia due to advanced radiation?
The scalar potential $\phi$ and vector potential $A$ at a distance $r$ from a charge $q$ are given approximately by
$$\phi = \frac{q}{r}$$
$$\mathbf{A} = \frac{q\mathbf v}{r}$$
where the constants ...
2
votes
0answers
102 views
Metal sphere and charged ring
I think this is an interesting question, to which I don't really know the answer to. (Also, not a homework question.)
Say you have an uncharged metal sphere constrained to move in the z-axis. There ...
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0answers
54 views
How can I determine the type of magnet used in a DC motor?
I repair electrical motors in a workshop and sometimes I must repair permanent magnet DC motors and other motors that use permanent magnets. I need to specify type of permanent magnet (Ferrite or ...
2
votes
0answers
86 views
Bandgap Spacing in Photonic Crystals
I am doing some self-study on photonics and have encountered the following question:
We know that amorphous electronic crystals such as amorphous silicon have a bandgap. Can amorphous photonic ...
2
votes
0answers
74 views
The force exerted by a ~10 Tesla magnet on a C13 isotope?
How much force would a ~10 Tesla magnet exert on a weakly magnetic C13 isotope? If I made a molecule of diamond with $N$ C13 atoms, how large would $N$ need to be for me to pull on it with something ...
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0answers
292 views
Maxwell's equations in microscopic and macroscopic forms, and quantization
The macroscopic Maxwell's equations can be put in terms of differential forms as
$$\mathrm{d}\mathrm{F}=0,\quad\delta \mathrm{D}=j\implies \delta j=0,\quad \mathrm{D}=\mathrm{F}+\mathrm{P}.$$
...
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0answers
79 views
How much current would be generated by polarity reversal of Earth's magnetic field?
Continuing from my previous question Is reversal of magnetic polarity in a planet an instantaneous occurence?
A change in magnetic flux is expected to generate an EMF.
In the case where the ...
2
votes
0answers
188 views
Are there any non magnetic materials that attract to each other as if they were magnetic?
Are there any non magnetic materials that attract to each other as if they were magnetic?
This is an argument I am having with a friend.
Thanks,
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votes
0answers
134 views
Energy loss rate for periodic movement
How to prove that if a particle performs cyclical motion then its energy loss rate averaged over the period equals averaged radiation intensity?
The energy loss rate is the quantity of energy that ...
2
votes
0answers
152 views
Solar wind and the Earth's magnetic field
I have again an old question from a comprehensive exam I took a couple of months ago. Lucky for me one could pick 5 out of 8 questions, because on some of the problems I didn't even know how to start. ...
2
votes
0answers
194 views
Levitation rotation speed involving laser acceleration, pyrolytic graphite and a vacuum
The experment would involve a small NIB magnet levitating between or on the diamagnetic material pyrolytic graphite, unlike other forms of levitation this doesn't require power to run such as ...
2
votes
0answers
445 views
How to calculate the electric field and potential inside a dielectric surface placed into a homogenous field
In an experiment we placed a long dielectric cylindrical shell (cylindrical tube)(with inner and outer radii r1 and r2, respectively) in a homogenous field such that its axis was orthogonal to the ...
2
votes
0answers
267 views
Gauge invariance and Feynman path-integrals
Let me look at the Hamiltonian of a charged particle in a plane in a constant magnetic field ($\vec{B}$) pointing upwards - then in usual notation it is,
$$\hat{H} = \frac{1}{2m}\biggl(\hat{p} + ...
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0answers
185 views
Modified Coulomb potential
I'm working through Byron and Fuller's "Mathematics of Classical and Quantum Physics" and came across this problem:
If the electric potential of a point charge were
$\phi(r) = ...
2
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0answers
75 views
Magnetic Fields
If running a current through a magnet can reverse the magnetic field, then how strong does the current have to be and how would it best be run through the magnet.
2
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0answers
48 views
Degenerated Anderson Model Simulation
I'm trying to simulate the degenerative Anderson model. So depending on an energy difference first orbital and afterwards spin magnetism occurs. First i try to solve an easier ansatz with a limitation ...
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vote
0answers
15 views
Work on Ferromagnetic Object Due to Solenoid
I've been going through some equations and such trying to determine the work done by a solenoid on a ferromagnetic object. I have the following:
Magnetic field due to solenoid:
$\vec{B} = ...
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0answers
30 views
Induced current using a reference system bound with a moving charge
Let's suppose to have a charge moving at velocity $\textbf{v}$ and a square wire.
If I choose a reference system bound with the square wire, since the charge is moving at velocity $\textbf{v}$, in ...
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vote
0answers
78 views
Easy question about magnetism?
I have to build a simple electric motor by attaching a magnet to a battery, extending the terminals of the battery (with stiff wires so they could act as supports), and placing a coil of wire on top ...
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0answers
27 views
Power transfer in a transformer
Can it be proved using the concept of induced emf that power supplied at the primary coil equals power consumed at the secondary. I tried following. Let primary coil be called 1 and secondary be ...
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vote
0answers
43 views
Solving the equation of relativistic motion
How does one solve the tensor differential equation for the relativistic motion of a partilcle of charge $e$ and mass $m$, with 4-momentum $p^a$ and electromagnetic field tensor $F_{ab}$ of a constant ...
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0answers
27 views
Non reciprocal light propagation
In search for some explanation in why magneto-optical materials (like the one used in the Faraday rotator and, consequently, in the "optical diode") act in such a "strange" way, I saw that this kind ...
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vote
0answers
32 views
Back motor effect
Back motor effect is the counter torque which opposes the rotaional motion of the coil in generator.
The back motor effect depends upon the resistance connected to the generator. More is the ...
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vote
0answers
47 views
Fourier Transform of ribbon's beam Electric Field
I have a monochromatic ribbon beam with $E(x)e^{i(kz-\omega t)}$ being the electric field's amplitude. I want to show that the lowest order approximation in terms of plane waves is
...
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0answers
24 views
Show that the plane of incidence is perpendicular to the surface of reflection
Is it possible to derive from the boundary conditions of the Maxwell equations for E and H, that the plane of incidence for an EM wave is perpendicular to the reflection surface? How? If not, what ...
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vote
0answers
37 views
Can someone help me understand this simple derivation for hall voltage?
I have a question regarding the derivation for hall voltage as shown in this image:
How did they get $\Delta V_H = E_H d$ term? ($d$ is the width of the conductor)
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vote
0answers
134 views
Is there any example where electric and magnetic fields are not perpendicular?
Perpendicular electric and magnetic field creates light or other electromagnetic waves. Is it a necessary property to have a perpendicular fields? If not what would happen when the fields are not ...
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0answers
31 views
Energy in the electric field of a translating charged sphere
The energy stored in the electric field of a uniformly charged non-conducting shell moving with a uniform velocity is more than the energy stored in the electric field of the same shell at rest. What ...
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0answers
111 views
How does a Fresnel rhomb work (half and quarter wave plate)?
How does a Fresnel rhomb work (half and quarter wave plate)?
I am aware of birefringence, which creates a phase shift of $\Delta\phi=\dfrac{2\pi\Delta nL}{\lambda_0}$. But this doesn't explain how a ...
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0answers
46 views
Where Is the mechanical energy flow in Poynting's theorem?
Poynting's theorem is usually interpreted as the statement of the conservation of energy, where the change in mechanical and electromagnetic energy within the volume equals the flow of electromagnetic ...
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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 ...
