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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27 views
Phasor representation of voltage in frequency domain
In a text on application of electromagnetism in transmission line, there introduces a phasor for the voltage (in frequency domain)
$$\tilde{V}(x) = V^+e^{-i\beta x} + V^-e^{i\beta x.}$$
Here $V^+$ ...
7
votes
5answers
2k views
What is the electric field generated by a spinning magnet?
Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical ...
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votes
3answers
291 views
charge density in a wire
if we have a infinitely thin and infinitely long straight wire on the z-Axis with given current I(t), how can i compute the charge density?
I figured out that the current-density is given by ...
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vote
3answers
72 views
Microscopic picture of an inductor
I have a good understanding of how inductors behave in electrical circuits, and a somewhat rough-and-ready understanding of how this behaviour arises from Maxwell's equations. However, what I don't ...
5
votes
3answers
218 views
Why is visible light used in Optical fibers (instead of other EM waves)?
Why aren't other electromagnetic waves used in optical fibres instead of visible light?
Is it because the wavelength of light fits the internal reflection/refractive index of the material used for the ...
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votes
1answer
54 views
Where is the amplitude of electromagnetic waves in the equation of energy of e/m waves?
Does the amplitude of the photon oscillations always stay constant and if it is not - what are the physical differences between the photon with higher amplitude in comparison to the one with the less ...
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1answer
108 views
magnetic scalar potential far above a magnetic film
The situation I am looking at is a magneto-static problem of a finite magnetic film with magnetization $\bf{M}$. I would like to find the the magnetic field far above the plate. My expectation is that ...
-1
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0answers
51 views
Could Space Travel be achieved using a very strong magnetic field to warp space?
An Alcubierre Warp Drive could theoretically achieve faster than light space travel but requires exotic matter or negative energy to compress space in front of the spaceship and expand space behind ...
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0answers
40 views
reflected wave in the transmission line
I am reading a book about the telegraph equations on the transmission line, which is illustrated as follow
where the input impedance is $Z_0$ and the impedance of the load is $Z_L$, we setup the ...
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1answer
46 views
EM Waves Energy Loss
Where does the energy go when two photons interfere destructively at a point on a screen in Young's double slit experiment ?
2
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1answer
101 views
Induced Current, Dipole Falling Under Gravity Towards Circular Loop [closed]
Suppose a magnetic dipole $\mathbf{m} = m \hat{z}$ is falling towards a circular loop of radius $b$ under gravity. Assuming the dipole always stays along the $z$-axis of the loop, determine the ...
2
votes
1answer
49 views
Does a microwave resonantly excite the rotational levels when cooking?
Wikipedia states there is no resonance absorption, but says at the same time that the molecules are oscillating like dipoles, which is kind of the same if you are exciting the rotational levels ?
The ...
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2answers
44 views
Electrical force as a replacement for Gravitational force [closed]
Suppose the force between the Earth and Moon were electrical instead of gravitational, with the Earth having a positive charge and the Moon having a negative one. If the magnitude of each charge ...
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vote
1answer
58 views
Could we really charge metal plates using microwaves?
While skimming through Dielectric heating, I read that they use microwaves to charge the plates. How do they do that?
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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 ...
3
votes
1answer
50 views
Uniqueness and existence of polygonal orbits through a spherical shell
Say we have a spherical wire mesh raised to a negative voltage. Then let's say we release a proton from near the surface, and away from the surface, at some angle and speed. Also, imagine that the ...
2
votes
2answers
54 views
What materials focus EM radiation in the 2.4GHz range
If glass and similar materials refract visible light effectively, what materials would be best for focusing lower frequencies of EM radiation, if any? If not, what other methods exist for focusing ...
0
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1answer
115 views
Eddy current losses in electric steel by harmonics of a magnetic field
I am working on an model of a permanent magnet synchronous machine. Right now I am stuck with calculating the eddy current losses caused by the harmonics of the stator magnetic field in the electrical ...
0
votes
2answers
272 views
Can electromagnetic fields be used to deconstruct and reconstruct atoms?
I was thinking one day and came up with a theory after reading about how scientists were studying anti-matter by using electro magnetic fields to separate matter from the anti-matter they made. It ...
14
votes
5answers
967 views
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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1answer
291 views
Is the induced electric field due to time varying magnetic flux always circular?
According to Faraday's law, changing magnetic flux induces an electric field. Is that electric field always circular?
5
votes
2answers
169 views
Effects of a very large magnetic field on the human body
Ever since reading about the NHMFL I have always wondered about this and asked several people without getting a good satisfactory answer. My question is, considering the simplest case let's say a ...
0
votes
0answers
26 views
Cause a change in the magnetic field detectable by a smartphone Magnetometer
I hope someone can shed some light or point me in the right direction.
I have a smartphone that has a Magnetometer built into it. I'm exploring some ideas
of how I might be able to use it. I want to ...
8
votes
1answer
154 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 ...
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1answer
81 views
Importance of Ampere's Law
Consider a hollow cylinder carrying a current $I$ and a wire outside the cylinder carrying a current $I'$.
Let's say the cylinder is symmetrical with even current distribution etc.. so the ...
4
votes
2answers
82 views
What does it mean for electric current to be a scalar?
After reading the reasons for considering electric current as a scalar quantity, I was confused at a point which read "The magnitude and direction of current does not depend upon the direction of the ...
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votes
0answers
36 views
Magnetic field of a Ferromagnetic cylinder [closed]
I have been stuck on this problem for hours and have no idea how to go about this problem.
You have an infinitely long cylinder made of linear ferromagnetic material of relative permeability Km, ...
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0answers
31 views
Why is Electromagnetic Spectrum Bounded? [closed]
The Electromagnetic Waves having frequencies between $10^{4}Hz$ and $10^{20}Hz$, is called as an electromagnetic spectrum. Why these limits? What is the reason for spectrum to have boundaries? We can ...
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0answers
36 views
Finding the magnetic dipole moment [closed]
I have been stuck on this question for awhile and it should be really easy.
The relative permeability of cobalt is $u_r = 600$ and the density of atoms is $N_e = 9.02*10^{18} [m^{-3}]$, calculate the ...
2
votes
1answer
106 views
Aharonov-Casher effect for charged particles
All the explanations of the Aharonov-Casher effect seem to imply it only "works" for neutral particles with a magnetic moment. This seems to stem from the duality of the A-C effect with the more known ...
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votes
1answer
70 views
Is it possible for a charged, fast-moving object to slow down and enter geo-stationary orbit?
I've had a wild idea which I can not discuss at length in this forum, but it comes down to the following problem:
A sphere of radius R=~10μm and mass m=~10-16Kgr is travelling towards the earth at v ...
2
votes
2answers
65 views
Attraction and repulsion of Magnetic materials [duplicate]
Why are diamagnetic materials repelled when placed in magnetic field and why are paramagnetic materials attracted when placed in magnetic field?
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1answer
62 views
Magnets and speed of light
I am in no way a physicist but I do have a fascination with physics. My question is if magnets are being explored / studied as a potential source to achieve the speed of light and if that is even ...
0
votes
1answer
20 views
Work due to introducing a dieletric in a capacitor
I have a doubt about a couple of exercises tha asks to find the work done on introducing a dieletric between the plates of a capacitor. Yes, this question is in the general case, how do we procede? I ...
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2answers
288 views
Proving the consistency of Faraday's law of electromagnetic induction
Here is a question which frequently occurs on my school exam paper:
"Prove that Faraday's law of electromagnetic induction is consistent with the law of principle of conservation of energy." What ...
4
votes
2answers
78 views
Forces as One-Forms and Magnetism
Well, some time ago I've asked here if we should consider representing forces by one-forms. Indeed the idea as, we work with a manifold $M$ and we represent a force by some one-form $F \in ...
1
vote
2answers
117 views
Lorenz gauge fixing
Is it always possible to define function $\psi$ satisfying the Lorenz gauge equation
$$
\partial_{\mu}\partial^{\mu} \psi + \partial_{\mu}A^{\mu} = 0?
$$
1
vote
2answers
148 views
Energy stored in space/frequency electric field
I've come across a problem with finding the energy stored in time/frequency electric field. In space/time we have (taking $\epsilon = 1$)
$$ Energy = \frac{1}{2} \int_V |\mathbf{E}(\mathbf{x},t)|^2 ...
12
votes
3answers
561 views
Shine a light into a superconductor
A type-I superconductor can expel almost all magnetic flux (below some critical value $H_c$) from its interior when superconducting. Light as we know is an electromagnetic wave. So what would happen ...
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1answer
79 views
measuring electromagnetic induction
There is a famous law which says that a potential difference is produced across a conductor when it is exposed to a varying MF. But, how do you measure it to prove? It is quite practical.
...
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1answer
43 views
Induced voltage by a rotating ring
Two concentric rings dielectrics and uniformly charged are suspended on the same floor.
The outer ring has a radius R and mass M,
while the other has radius r << R and mass M.
The outer ring ...
0
votes
1answer
39 views
What limits the maximum sustainable surface charge density of a sphere in space?
Suppose I charge a sphere and leave it in vacuum for 10 years. After that time, I want its surface charge density to be in the order of 10^5C/m^2. Would that be possible? Would it depend on the ...
7
votes
5answers
510 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 ...
2
votes
1answer
152 views
Length of wire required for solenoid to produce desired magnetic field
In a question:
To construct a solenoid, you wrap insulated wire uniformly around a plastic tube 12cm in diameter and 50cm in length. You would like a 2.2 A current to produce a 2.6 kG magnetic ...
2
votes
2answers
144 views
Reference request for the Aharonov-Bohm effect
I am looking for a good reference to an online source or book, on the magnetic Aharonov-Bohm effect. I have read the appropriate sections from the book by Griffiths and Ballentine, and still haven't ...
1
vote
1answer
43 views
Understanding drift velocities in currents
I have a doubt about the understanding of drift velocities in a current. My problem is that the textbook speaks very loosely about this. It's like: "well, if we apply a field $E$ then the charges will ...
4
votes
2answers
110 views
Cancel out Earth's Magnetic field
Is there any tools except helmholtz coil to cancel out earth's magnetic field to calibrate magnetometers in practice.
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2answers
264 views
Does a current carrying wire produce electric field outside?
In the modern texts of electromagnetism in the presence of stationary currents the electric field is assumed conservative $\nabla \times E =0 $. Using this we get $E_{||}^{out}=E_{||}^{in}$ which ...
3
votes
1answer
64 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 ...
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1answer
81 views
Energy stored in fields
How to intuitively think of energy stored in a magnetic/electric field?
Kindly answer in a bit simple terms without referring to mathematics.
