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.
1
vote
1answer
70 views
Ginzburg-Landau model for superconductivity
Could someone kindly elaborate more on the Simple Interpretation section from this Wikipedia Article? I refer to the part on the natures of $\alpha , \beta$. Why can one assume that ...
1
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 ...
1
vote
4answers
62 views
North and south of magnetic field
The current I is flowing upward in the wire in this figure. The direction of the magnetic filed due to the current can be determined by the right hand rule.
Can we determine the north and the south ...
0
votes
0answers
29 views
Energy or Work done to pull an Iron cyclinder into a Solenoid
RadiI have been following the calcuations from these lecture slides here (slide 11). Where the slides attempt to approximate how much a solenoid pulls, by working out the energy required to pull an ...
0
votes
0answers
13 views
How to generate sloped streamslice plot of magnetic field in Matlab [migrated]
I am currently working on my bachelor thesis, where I am calculating trajectory of electrons and ions in Hall Effect Ion Thruster. However, to be sure what I am calculating, I need to know how does ...
0
votes
0answers
67 views
The electric field inside a uniformly polarized cylinder
Given a uniformly polarized cylinder with some polarization $P\vec{z}$ which has radius $R \gg d$ where $d$ is the height and $R$ is the radius. The cylinder's center is on $\vec{z}$ and it's on the ...
0
votes
0answers
54 views
The gauge-invariance of the probability current
It is simple to show that under the gauge transformation $$\begin{cases}\vec A\to\vec A+\nabla\chi\\
\phi\to\phi-\frac{\partial \chi}{\partial t}\\
\psi\to \psi ...
0
votes
1answer
58 views
Magnetic Field and the Speed of Light
Is it just a historical choice that both magnetic field and the Lorentz force equation include the speed of light?
I figure that whoever wrote up the equations (in cgs!) could have put both factors ...
1
vote
1answer
77 views
Uncertainty-principle and the Maxwell formalism of electromagnetic waves
An electromagnetic wave (like a propagating photon) is known to carry it's electric and magnetic field-vectors perpendicular and each depending on the differential change of the other thus "creating" ...
3
votes
1answer
48 views
CGS Units for Magnetism
Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
3
votes
1answer
38 views
inductance value of a single turn thin circular coil
the question may seem simple but I haven't found any fitting formula yet. The problem is the following:
consider a single-turn, circular coil made of reasonably thin wire (diameter of the wire much ...
-4
votes
3answers
177 views
There must be free positive charges, moving oppositely to electrons for the wire with current to stay neutral
All popular expositions (e.g. these ones) of relativistic electromagnetism claim univocally that electrons in motion become more dense due to the speed. They teach that Lorentz contraction of charges ...
2
votes
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$. ...
4
votes
2answers
109 views
Vector potential
I have difficulty understanding the following vector calculus example. Text can be found here. It is the 5th Q&A -- starting with equation (31.1035).It concerns finding the vector potential of a ...
2
votes
2answers
60 views
EM Fields in a Rotating Frame of Reference
I'm struggling on my approach to the problem of figuring out E and B fields in a non-relativistic way for a rotating frame of reference in the x-y plane around the z-axis. I am attempting to do this ...
1
vote
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 ...
0
votes
1answer
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 $V^+$ ...
3
votes
2answers
125 views
Physical explanation for why total internal reflection occurs
I have been trying to understand total internal reflection (and have read several posts on this site already). Mathematically, I feel that I understand how the evanescent wave decays exponentially as ...
0
votes
1answer
56 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 ...
-1
votes
0answers
54 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 ...
0
votes
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 ...
1
vote
3answers
73 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 ...
0
votes
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 ?
0
votes
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 ...
2
votes
1answer
102 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
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 ...
1
vote
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 ...
2
votes
2answers
55 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
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 ...
0
votes
1answer
82 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 ...
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 ...
0
votes
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 ...
0
votes
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 ...
4
votes
2answers
84 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 ...
0
votes
0answers
37 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, ...
5
votes
3answers
175 views
Faraday's law - does the induced current's magnetic field affect the change in flux?
I've had this conceptual problem with Faraday's law and inductance for a while now.
Take the example of a simple current loop with increasing area in a constant field (as in this answer). So ...
-2
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
67 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?
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 ...
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 ...
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 ...
1
vote
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
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
40 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 ...
0
votes
1answer
81 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.
...
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
111 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.
1
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 ...
1
vote
1answer
38 views
Negative emf in AC generator
At a certain instant in AC generator, when the normal of the plane (rectangular coil) makes an angle of 270 degrees with with the magnetic induction B, the value of emf is:
$E = -NAB\omega$
My ...
0
votes
2answers
90 views
Difference between electrostatic and magnetic field [closed]
In this, F (the force set by the field) is perpendicular to the speed of the particle. In which case does F change the absolute value of V? Why or why not?
