Questions tagged [electromagnetism]
The classical theory of electric and magnetic fields, both in the static and dynamic case. It also covers general questions about magnets, electric attraction/repulsion, etc. Distinct from electrical-engineering.
19,213
questions
0
votes
0
answers
6
views
Terminal velocity of a bar magnet in a long conducting tube (quantitative)
If a bar magnet is dropped into a copper (conducting) tube, it reaches a terminal velocity. I'm trying to derive this terminal velocity.
We know from Lenz's law that $e=\frac{-d\phi}{dt}$.
I'm facing ...
1
vote
0
answers
12
views
What actually happens when light goes into a waveguide?
I have only dealt with a rudimentary theoretical treatment of how light propagates inside a waveguide, such as those available in Griffiths.
One thing bugging me is that I still do not know how ...
- 659
0
votes
1
answer
14
views
Flux change through a loop
I'm having difficulties understanding this thing. It would be lovable if someone could show me its derivations and prove it:
In a conducting loop, there is a relationship between the total displaced ...
- 873
-1
votes
0
answers
21
views
I am trying to derive an expression for the magnetic field of charged particles travelling at significant fractions of speed of light [closed]
I tried using lorentz transformation , which gives me correct answer for speed of light c but when i try to input any other values nearby c it becomes a constant
B = 10-7 . q/r² ( (v - u)/ 1- uv/c²)
1
vote
1
answer
37
views
Permittivity real and imaginary parts with similar value possible?
Here's the context; I'm studying biological tissues that are supposed to behave like dielectrics. Using the modified cole-cole equation for theoretical predictions:
$$\tilde{\varepsilon}_r (\omega )= \...
- 189
1
vote
2
answers
3k
views
Transformers: How does current in primary coil change?
I was doing a question on transformers and found this really confusing question:
A 100% efficient transformer converts a 240V input voltage to a 12V
output voltage. The output power of the ...
CommunityBot
- 1
2
votes
4
answers
445
views
Derivation of covariant derivative
I'm currently doing Introductory QFT and was confused about the origin of the additional terms in the covariant derivate. My understanding is as follows:
If we begin with the Dirac Lagrangian ...
CommunityBot
- 1
0
votes
2
answers
530
views
Direction of Magnetic field caused by straight current-carrying conductor [closed]
The question: Two crisscrossing insulated wires contain currents as displayed. Find the total mag field at points P and Q, assuming the horiz current moves (a) right and (b) left. QUESTION is for part ...
CommunityBot
- 1
0
votes
1
answer
44
views
Why electric field increase near the cable connected in AC 220V socket even current is zero?
If I put low frequency electric field meter near the connected cable in AC 220V socket, he show 1200 V/m even if device is turn OFF(zero current), if disconnect cable from socket, field drop to 8 V/m.
...
- 36k
0
votes
0
answers
34
views
Assessing Error Step Size in Continuous Charge Distributions
For obtaining the electric field for bodies at the macroscopic scale, the discontinuities in the charge distributions are ignored. A continuous function for the charge distribution is considered.
But ...
- 1
0
votes
3
answers
1k
views
Magnetic field for a cylindrical shell in which current flows
If we have a cylindrical shell along the z-axis, with current density $\vec j= j_0 \vec e_z$ and small radius $R_i$ and big radius $R_a$. I tried to find the magnetic field inside by using Ampere's ...
- 133k
0
votes
0
answers
41
views
Force acting on an infinite wire with current inside a magnetic medium
I've recently stumbled on an electromagnetics problem that I don't quite understand.
There is an infinite wire carrying current I that is placed inside a medium with magnetic permittivity μ1. At a ...
0
votes
0
answers
36
views
Displacement current and magnetic field in capacitor with dielectric
I have a capacitor with round plates of radius $r_{2}$ kept at a distance $d$, with a voltage $V\left(t\right) = V_{0} \cos \left(\omega t\right).$ Inside, there is cylindrical material with a certain ...
- 1,150
1
vote
2
answers
280
views
Coulomb Gauge misunderstanding
If we have $\vec A(\vec r,t)$ and $\phi (\vec r,t)$ and we make the following gauge transformations:
$$\vec A(\vec r,t)'= \vec A(\vec r,t) + \nabla f(\vec r,t)$$
$$\phi(\vec r,t)'=\phi(\vec r,t) - \...
CommunityBot
- 1
3
votes
1
answer
536
views
Opacity/transparency of conductive meshes to charged particles (electrons/ions)
When using a conductive (metal) mesh, effectively a metallic woven fabric, in vacuum applications as a "grid" for charged particle optics, how does one calculate (or at least estimate) the opacity or ...
CommunityBot
- 1
0
votes
1
answer
1k
views
Motion of a magnet inside a solenoid
I have a homework problem where a bar magnet is placed halfway inside a horizontal solenoid. A cut away shows that the magnet's north pole points to the left of the page and the south pole is outside ...
CommunityBot
- 1
2
votes
1
answer
233
views
Conformal mapping and van der Pauw
I'm trying to understand the van der Pauw methods with my limited mathematical understanding. If any of my steps are wrong, please correct me.
As far as I understood from a textbook (Mahan, Gerald D. ...
CommunityBot
- 1
0
votes
2
answers
46
views
Maxwell stress tensor on a capacitor late given a dielectric
Given a capacitor with large plates (area $A$) separated by a dielectric with relative permittivity $\varepsilon_r$ and thickness $g$, I believe the Maxwell stress tensor states that the force on each ...
- 5,558
2
votes
1
answer
273
views
How to derive magnetic moment for integrated circuit level?
Imagine we have a circuit on the xy-plane, with a random geometry (it just need to be closed). I want to calculate the magnetic moment of this setup:
$$\vec{m} = \frac{1}{2} \int_V d^3 x' \vec{x}' \...
CommunityBot
- 1
1
vote
0
answers
39
views
About the notation for TEM waves
I just came across this article where the term "$\mathrm{TE}_{101}$ microwave mode" is mentioned. Other than the basics of TEM waves which I learned in Griffiths, this is the first time I ...
- 659
0
votes
0
answers
33
views
Seeking Advanced Study Guidance in Electromagnetism [closed]
I hope this post doesn't break any community rules, I've tried to look for such but couldn't find.
I am concluding my first undergraduate course in Electromagnetism next week. I am confident in my ...
- 206k
4
votes
1
answer
810
views
What's the coordinate of the Landau tube?
The Hamiltonian of a three-dimensional electron gas in a static magnetic field $\vec{B}$ is :
$$\hat{H}=\frac{(\hat{\vec{p}}+e\hat{\vec{A}})^2}{2m}$$
If choosing $\vec{B}=B\vec{e_z}$ and using Landau ...
CommunityBot
- 1
0
votes
1
answer
261
views
How is the index notation for the electromagnetic potentials defined?
I don't understand, how the $\vec{A}$ potential is defined in the covariant formulation of the Maxwell-Equations.
I am using the metric $(+,-,-,-)$.
The electric field (SI-Units) is defined as:
$$
\...
CommunityBot
- 1
0
votes
1
answer
553
views
Lenz law - Direction of induced current
I understand the Lenz Law is such that it tells us that the induced current must be in a direction that makes the end of the solenoid facing the magnet behave as a North Pole to repel the magnet’s ...
CommunityBot
- 1
1
vote
1
answer
73
views
How do I find the wavelength of an EM wave derived from an accelerating electron, given its acceleration?
I understand the Larmors formula, but I can't find any material regarding wavelength of a single photon.
- 38.7k
1
vote
2
answers
163
views
How to solve Relativistic Lorentz Force equation if $\gamma$ is not constant?
I am trying to numerically obtain the trajectory of an electron inside a periodic magnetic field $\mathbf{B}$, taking into account that the relativistic factor $\gamma$ is not a constant (the electron ...
- 1
0
votes
1
answer
257
views
Motor Effect Experiment
I recently saw an demonstration (image below) where the motor effect was being shown. They used the fact that forces come in pairs (Newton's third law) and so when current goes through the copper rod, ...
CommunityBot
- 1
0
votes
0
answers
44
views
A question on perturbed flux function
Given a magnetic field $\mathbf{B}=B_{0}\left(\dfrac{xz}{z_{s}^{2}},\dfrac{yz}{z_{s}^{2}}, 1-2\left( \dfrac{x^{2}}{r_{s}^{2}}+\dfrac{y^{2}}{r_{s}^{2}}\right) -\dfrac{z^{2}}{z_{s}^{2}}\right)$ with the ...
0
votes
0
answers
21
views
What is radial and axial component of magnetic induction of a coil? [closed]
I am sorry if my question is stupid, but can someone explain what are the radial and axial component of magnetic induction of a coil. I am reading an article about the magnetic field generated by coil....
- 11
2
votes
1
answer
290
views
Materials to complement "Interacting Electrons and Quantum Magnetism" by Assa Auerbach
Does anyone know of any material, such as lecture notes from a university, that can complement the sometimes brief chapters of the book "Interacting Electrons and Quantum Magnetism" by Assa Auerbach?
...
0
votes
1
answer
259
views
How does changing the angular frequency ω of the generator affect the current flowing in the circuit and the power transferred to the electric motor?
When an electric motor is used with an electric generator, the situation can be viewed as an RLC circuit where the generator is an AC power source. The source produces a sinusoidal AC voltage with an ...
CommunityBot
- 1
2
votes
1
answer
414
views
Eddy current brake with superconductor
There are eddy current brakes used with metal plates to stop trains. It converts kinetic energy of the metal plate to heat via eddy current on it. So what if we have a superconducting metal plate and ...
CommunityBot
- 1
1
vote
5
answers
4k
views
What's the difference between conduction current density and source (impressed) current density in an $RC$ circuit?
My electromagnetics textbook has a picture of an $RC$ circuit with a current source.
As you can see, there are three current density vectors, $J_i$, $J_c$, and $J_d$. The book labels them as follows:
...
- 206k
0
votes
1
answer
35
views
What is the impressed current density? [duplicate]
I don't have a too solid background for physics and am reading some engineering books of electromagnetics. What is the impressed current density here? I think it is Ohm's law but couldn't find its ...
- 38.7k
0
votes
4
answers
40
views
Comparing Electric Potential Energy of two Charges
I do not know what I am missing here and would appreciate a little help in figuring out the flaw in my logic. I have a row of positive charges and a row of negative charges as shown in the image above....
- 25.2k
0
votes
0
answers
41
views
What are the operators here and how are these formulas derived? [closed]
In (23), are grad and div some kind of scalar operators comparing to $\nabla$ and $\nabla\times$? because tbh I dont know how $\text{curl}(\mu^{-1}\text{curl}\textbf{A})$ turns into $\text{div}\mu^{-1}...
- 206k
0
votes
0
answers
16
views
Effects of Localized Medium Changes on Field Propagation
I've studied various theories related to fields. These theories often include equations describing how the activity of a source is transmitted to other locations. The properties of the medium ...
- 675
2
votes
1
answer
351
views
Velocity gained as a metal object approaches a magnet
I need to find the Kinetic Energy gained as a steel bearing ball approaches a Neodymium magnet. I have the value for the magnetic flux density of the magnet, now I need to find a way to calculate the ...
CommunityBot
- 1
3
votes
3
answers
73
views
Why the $T_1$ recovery & $T_2$ decay are independent?
In MRI, I was thinking that the decrease in transverse magnetization results in the recovery of longitudinal magnetization.
However, recently, I found it was wrong. I saw a diagram
(McRobbie 2014. ...
- 420
2
votes
2
answers
914
views
How do you convert from Townsend Unit $\rm Td$ to $\rm kV/(cm bar)$?
I have found a conversion online which says 1Td = 2.829 V/(cm Torr) and 1Torr = 0.001332 bar and with this;
1Td = 0.3768 V/(cm bar)
However from literature for SF6;
362Td = 86.3 kV/(cm bar) where they ...
- 6,958
0
votes
1
answer
44
views
Most generic form of refractive index tensors
The refractive index of a material is in general a $3x3$ tensor (as in the case of birefringent crystals). From literature, it seems that in the case of transparent crystals, this tensor is in general ...
- 21
0
votes
0
answers
36
views
Calculation of magnetic vector potential A in FEMM [closed]
I don't have too much physics background. In the software FEMM, I simulated a closed loop magnetic circuit, and defined two current boxes (current density is uniform by default) on the left. The ...
- 206k
1
vote
0
answers
33
views
Rotating Tank of Charged Liquid
Consider a long rectangular tank of dimensions a×b, where a≫b, that is rotating at a constant angular velocity and filled with an incompressible charged fluid. How would one find an equation ...
- 11
2
votes
1
answer
638
views
Can a satellite generate electricity by using a planet's magnetic field? [duplicate]
I am entering AP Physics E&M and don't have much knowledge about electricity but always had an idea based on my limited knowledge.
If a satellite had a metal coil inside of it, wouldn't it have an ...
- 2,976
1
vote
2
answers
43
views
Straight wire in moving uniform magnetic field induced emf
My question is a follow-up to this one.
The green dotted line is the arbitrary loop which is constructed in the calculation of emf in the wire of length L. x is a variable which can be changed to ...
- 38.7k
2
votes
1
answer
999
views
Doppler Cone: What exactly is it?
I'm trying to understand Synthetic Aperture Radar (SAR). It appears that it makes use of what's called a "Doppler cone". I understand Doppler frequency shifts of light, but I'm not sure what the cone ...
CommunityBot
- 1
0
votes
1
answer
32
views
How to calculate the magnetic field of a coil? [closed]
I want to find a formula for the magnetic field of a coil in some random point outside of the coil. I understood that I must use the Biot-Savart's law and integrate first over one loop of the coil, ...
- 11
0
votes
0
answers
26
views
Length of helix in one time period [closed]
How to find the length of a helix in one time period?
In the case of a particle suspended at the origin and a magnetic field along the $y$ axis we know that the time period will be equal to $2\pi/aB$ ...
- 3,778
0
votes
3
answers
99
views
What is the net macroscopic electric field at any point inside a dielectric?
This question is based on section 4.2.3 of Introduction to Electrodynamics by David J. Griffiths (Third Edition).
Consider a point $\vec{r}$ inside a dielectric where we are interested in finding the ...
- 11
2
votes
2
answers
196
views
Induced electrical field of infinite straight current carrying wire
I'm kinda confused about the law of induction of a straight wire.
Suppose I have an infinite (infinitely long and thin) wire along the z-axis with current $I=I_0 \cos(\omega t) \, e_z$. The magnetic ...
- 25.2k