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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52 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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1answer
90 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.
3
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1answer
47 views
Faraday's law for a current loop being deformed
I'm working through Jackson's Classical Electrodynamics text (3rd version, chapter 5.15) about Faraday's law:
Faraday's law is pretty familiar:
$\int_c E \cdot dl = -\frac{d}{dt}(\int_s B \cdot n ...
3
votes
1answer
60 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
1answer
70 views
How does one determine whether an object will make an EM wave refract in a qualitative way?
for example, i have a vague notion that the actual answer is that the permittivity and permisivity are different in each different material, so all waves refract at every boundary, but we only call it ...
2
votes
1answer
90 views
What is the meaning of a constant magnetic scalar potential?
Let a spherical shell of inner radius $a$ and outer radius $b$ have a uniform magnetization $\mathbf{M}=M\,\hat{\mathbf{z}}$,
$\hskip2in$
I've found that the magnetic scalar potential ...
2
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1answer
51 views
Field Tensor and classical limits
I would be very grateful if someone would kindly explain this generalization of the Lorentz force law to the special relativity domain. Please bear with me.
Classically, the Lorentz force law is ...
0
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59 views
Curie's principle in electromagnetic field theory
I am looking for some explanation and if possible also some references about the applications of Curie's principle in electromagnetic field Theory, precisely in the computation of magnetic (resp. ...
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98 views
Equation of motion of charged particle under effect of magnetic field
A charged particle with specific charge q/m diverges from the origin [to the first quadrant] by angle X with x axis and with velocity v. To bring the particle back to the x axis, a magnetic field B is ...
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109 views
covarient derivative of electromagnetic field tensor
I'm trying to prove the energy momentum tensor in curved spacetime for Electromagnetic field is Divergence-less directly(Without using general lie derivative method which can prove any energy momentum ...
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0answers
35 views
How do capacitance field fluxes change matter?
I asked my teacher at college how a field flux affects matter and he said nobody knows. Surely someone must. How does it change it?
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0answers
74 views
Lorentz force in homopolar motor
There are a lot of videos showing the simple homopolar motors in action.
Let's look at this one: http://www.youtube.com/watch?v=6jyraFLqfqE
It is supposed to be working due to the Lorentz force.
...
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0answers
61 views
cgs Gauss' system of units
I had never seen this system until today, and I'm really confused. I've read the wikipedia article about it but I still don't know how to change between this and the international system. For example, ...
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24 views
Is a Magnetostatic Approximation Appropriate for a Coilgun Application?
I have little experience with magnetodynamics. Any help would be greatly appreciated.
I have an overdamped RLC circuit as the firing mechanism of a coilgun. A small cylinder of iron (a nail or ...
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0answers
141 views
What is the current induced in a straight wire when a magnetic field is turned from on to off?
Consider a straight wire which runs along the y-axis. A B-field exists which is uniform and only in the x-direction:
$$\vec{B}=(B(t),0,0)$$
The B-field at time $t_0$ is 'switched off' in ...
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0answers
45 views
Should a 1D Guassian wave oscillate?
I wrote a few lines that numerically solve Maxwell's equations.
The result is a moving wave that looks like a single pulse.
This looks strange to me because I expect waves to move in oscillator ...
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0answers
44 views
Object accelerated by electromagnet
If I have an electromagnet (powered by a battery) and a metallic object, the
power generated will depend on the voltage of the battery and the resistance of the coil.
If I accelerate the object, will ...
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72 views
Minimum energy magnetic field has zero current?
Following section 9.2 of Bellan's "Fundamentals of Plasma Physics" suppose we have some domain (I assume simply connected) $D$ with a magnetic field $\mathbf{B}$ inside, but no electrostatic field. ...
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53 views
Calculating attracting magnets?
I have two identical bar magnets...
Each bar magnet can lift an object with 10lb of force.
When they attract each other, I believe that each magnet applies a force of 10lb to attract the other.
That ...
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38 views
State equations in magnetic systems
Assume there is an isotropic magnetic material in a homogeneous field $\vec{H}$ inside of a long coil. The work done to the material is given by $\delta A = H dM$, where $M$ is the magnetisation of ...
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41 views
How to calculate how weak does a magnet get when you get an other magnet closer to it?
I heard that when you take two magnets and get them closer together so they reject each other (north pole to north pole or south pole to south pole) they weakens. Does anybody knows how to calculate ...
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0answers
61 views
Currents and fields of a magnetron
I can't make heads or tails of the descriptions of magnetrons (i.e. microwave) I'm reading online. Hyperphysics does the best job of illustrating fields and currents, but the story of its operation ...
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0answers
100 views
Requesting decent analysis on FEMM simulation data
I have been running simulations in FEMM for different magnetic configurations as an attempt to understand better how do they work and why it is impossible to come up with the holy grail of energy: a ...
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0answers
18 views
Segregating a collection of “pinned” but freely rotating bar magnetics into two populations with antiparallel orientations
I have a collection of bar magnets on a frictionless plane, immobilized at their centers, but able to rotate freely. I flick each magnet to randomize its rotational orientation.
Is there a magnetic ...
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0answers
220 views
How to calculate the force between two different magnets?
I'm doing an experiment and I need to calculate the force between two magnets with different stats, different sizes and flux density.
I learned physics in high school so I know all the terms.
In my ...
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113 views
Why are magnetic lines of force invisible?
We can very well feel the magnetic field around a magnet, but we can't see it. Why is that so?
Also, can we cut a portion of the field and use it?
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71 views
How to get electromagnetic wave from transmitted power?
If a question states that a transmitter emits 90W power, is it meant as average power density or Poynting vector magnitude?
Is it somehow possible to deduce E(z;t) and H(z;t) from it?
Electric ...
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63 views
Would an aluminum cup placed around the back and sides of a disc magnet increase the field strength at the front?
I have read that certain materials, specifically ones with distinct magnetic properties, can influence the way magnetic field behaves around a magnet.
If you were to place a disc magnet inside an ...
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40 views
How fast is an anti-quench?
Is there any evidence it is slower than a quench?
The rapid, energy-rich return of a superconducting electromagnet to the normal state is called a quench, and occurs very rapidly, perhaps almost ...
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0answers
52 views
how is magnetic field always made parallel to the plane of coil in galvanometer?
A galvanometer is constructed like this:
Schematic picture of a galvanometer, from https://en.wikipedia.org/wiki/File:Galvanometer_scheme.svg
It consists of a rectangular coil with a soft iron ...
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232 views
Faraday law, third Maxwell's equation in Mathematica
Three question about this equation:
$
\displaystyle\nabla\times\mathbf{E}=-\frac{\partial \mathbf{B}}{\partial t}
$
1 If I solve this equation with Mathematica, I find the magnetic field ...
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0answers
75 views
Changing the charging properties of an inductive charging coil
I am attempting to modify an existing inductive charging coil (Palm Touchstone) so that it outputs the correct magnetic flux for a different wireless charging standard (Qi). I am doing this as an ...
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41 views
Can ferrofluid be used as a seal in the bearings?
If ferrofluid becomes more dense in the presence of magnetic field, does it allow to use ferrofluid as seal, so that it can be used as oiling and seal at the same time? Maybe somebody knows how ...
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136 views
A crazy problem
Easy
Consider the following figure
Each red point is a particle of a known mass that carries charge Q/2 and connected to a the box by a thread of known length. This problem can be solved easily to ...
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201 views
Proof of equality of the integral and differential form of Maxwell's equation
Just curious, can anyone show how the integral and differential form of Maxwell's equation is equivalent? (While it is conceptually obvious, I am thinking rigorous mathematical proof may be useful in ...
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81 views
Mathematical Formulation of the laws of classical physics
By classical here I mean non-quantum mechanical, and everything that is developed afterwards such as color, etc.
I am wondering if we can look at our Universe mathematically like this:
Let ...
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194 views
Ewings Molecular Theory of Magnetism & Induction Cookers
This question regards Ewings molecular theory of magnetism
I.) Ewing's molecular theory of magnetism describes every magnetic substance as being a collection of dipoles that are initially in a state ...
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217 views
electromagnetics : intrinsic impedance and poynting vector
I know that 1/2 of the real part of the complex poynting vector gives the average power flow per unit area. But what is the significance of its imaginary part ?
What is the significance of the ...
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72 views
How to determine field strength of a dipole embedded in a sphere?
I have a dipole which is embedded in a sphere.
Sections of the sphere may consist of materials of differing magnetic permeability.
I'd like to determine the strength of the magnetic field at ...
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34 views
Can one obtain saturation magnetization from FMR measurements?
Especially for magnetic thin films. Normally this is done by magneto-optical Kerr effect or SQUID measurements. Or is there a way to calculate the saturation magnetization based on other measured ...
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171 views
fringing field capacitance
I am having two doubts about the fringing field capacitance.
1) In most of the formulae, the fringing correction for the capacitance is written per unit length. This is ok if the length is very ...
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0answers
103 views
Intensity of unpolarized light
If the electromagnetic field of an unpolarized plane wave is written as
$$\bar{E}(t,\bar{x})=(\bar{E}_{0x}+\bar{E}_{0y}e^{i\delta(t)})e^{i(\bar{k}\bar{x}-\omega t)}$$
...
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0answers
200 views
How to solve Poisson equation in electrodynamics?
I learned electrodynamics.
According to the vector potential determination,
$$
\mathbf B = [\nabla \times \mathbf A ],
$$
Coulomb gauge,
$$
\nabla \mathbf A = 0,
$$
and one of Maxwell's equations,
$$
...
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0answers
218 views
Modeling a bar magnet's magnetic flux inside a coil
A bar magnet and a coil arranged alongside their common axis is a common setting in teaching. But how does the magnetic flux H(x) (where x is the distance of the bar magnet's center relative to the ...
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190 views
Calculating the number of turns and thickness of an electromagnet
I want to calculate the number of turns of an electromagnet and the thickness of the wire.
But i have tried to search arround in books, and can't really find any thing.
I know my wire is 0,114mm and ...
0
votes
0answers
72 views
How do I write the energy of a constant, uniform 2D charge distribution?
Let's consider a 2D electromagnetic field defined in a square domain $[0,\Lambda]^2$, with periodic boundary conditions, with a constant charge distribution, uniform all over the aforementioned ...
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113 views
What is the electric field part of an EM wave? Radiation field or the induction field?
Look at this image:
I wonder if the electric field is from the induction field from a vibrating electron or the radiation field? If it is from the radiation field, as I suppose, than can someone ...
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44 views
type of Hall sensor conductor
For example, if I have Hall sensor, have can I determine type of Hall conductor material? I want to know is it an electron or is it just a hole.
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19 views
Writing right and left polarized waves in terms of parallel field components
How can I write:
in terms of:
(or vice verse - I can't see how they relate).
I don't see how you can right and left polarized waves in terms of parallel E and B components?
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0answers
17 views
Electron Kinematics for Moving Field Region
I am working out a rough calculation for an experiment I am working on this summer: Suppose an electron is moving in the positive y direction with 300 MeV. There is a circular electric field region of ...
