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.
4
votes
1answer
123 views
Does a lightning rod prevent lightning strikes?
I was always under the impression that a Lightning rod worked to protect a building by being the tallest part of the building and having a low path of resistance to the ground, thus being a good ...
1
vote
3answers
62 views
Magnetic B Field of Point Charge Not at Constant Velocity
I'm working on an N-body simulator for charged particles. I know that moving charged particles generate a magnetic field, and another moving charged particle could be effected by this magnetic field.
...
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 ...
3
votes
1answer
39 views
Magnetic field due to a conducting sphere in unpolarized light
From Jackson, problem 10.3:
A solid uniform sphere of radius $R$ and conductivity $\sigma$ acts as a scatterer of a plane-wave beam of unpolarized radiation of frequency $\omega$, with $\omega R ...
4
votes
2answers
80 views
Tensor fields on space-time
I'm reading landau electrodynamics . For the investigation of the dynamics of a charged particle in an electromagnetic field , the author introduces a four vector potential field on spacetime . The ...
2
votes
1answer
1k views
When does voltage drop occur?
Why or when does it occur in a circuit? What does it imply when you speak of a voltage drop across a resistor? (Obviously, it probably means that the current's voltage before the resistor is higher ...
1
vote
0answers
28 views
How sharp is the direction of magnetic field of a cone magnet?
I heard recently that cone magnets can be used to focus and direct the field sharply at one particular point in a particular direction. I jus want to kno how accurate and sharp this field is? And if ...
2
votes
3answers
106 views
continuity of the electric potential due to a surface charge
The Electric potential due to a charge distribution on a surface is :
$\Phi \left ( x \right )=\int \frac{\sigma \left ( {x^{}}' \right )dx{}'}{\left \| x-x{}' \right \|}da$ I want to show that it's ...
0
votes
0answers
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 ...
0
votes
1answer
95 views
Force of electromagnet on piece of iron
I can find equations to give the force of an electromagnet on a piece of iron when the iron touches the electromagnet.
But what about when the iron is some distance from the electromagnet? ...
1
vote
1answer
60 views
Electromagnetic field to cool a substance?
I saw somewhere that an electromagnetic field would cause a substance to let off thermal energy, ultimately resulting in the substance to cool really quickly.
If this is possible, does the strength ...
2
votes
1answer
124 views
Transformators (primary and secondary windings)
Consider the following problem:
Robby wants to put a light in the shed so he puts a cable between his house and the shed. In the cable there are 2 wires with a combined resistance of $0,92 \space ...
0
votes
3answers
104 views
Violation of Newton's 3rd law
I'm just expressing my guess.
Let two particles A and B experiences forces $F_1$ and -$F_2$ between them and let guess also there are two observer, one is stationary and other is moving with ...
1
vote
2answers
78 views
Eddy currents: Explanation of the force slowing down a moving metal plate
Suppose a solid metal plate moves trough a homogenous magnetic field $\vec{B}$ as shown in the following image. You will see that the plate will slow down (as long as not the entire plate is in the ...
0
votes
1answer
529 views
Coil design and magnet orientation in a unique generator
First, I am not an electrical engineer - I am a hobbyist building a prototype linear-stlye generator based on an idea I had which, for various reasons, cannot be built with magnets that pass ...
0
votes
0answers
66 views
Magnetic Field on a particle between two Helmholtz Coils [closed]
Given a Helmholtz coil of radius $R$ and with a current $I$, and assuming the coil is a circle centered at the origin in the $xy$-plane, what is the magnetic field on a particle at the point $(a, b, ...
2
votes
2answers
53 views
what sort of set up would i need to create an electro magnetic chair with no legs
I am a furniture design student therefore please keep it simple. a system strong enough to hold the avergae male of say 90 kg is this possible even....
3
votes
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 ...
4
votes
4answers
185 views
How does moving charges produce magnetic field?
I'm tutoring High school students. I've always taught them that:
A charged particle moving without acceleration produces electric as well as magnetic field?
It produces electric field because ...
0
votes
1answer
23 views
Doubt on the analysis of $RC$ circuit
I'm studying $RC$ circuits and I'm in doubt on how to deal with one kind of situation. Well, first when we have simply a circuit with just one resistor and one capacitor in series, or a circuit that ...
2
votes
1answer
28 views
Magnetic Force on a Loop and number of turns
Suppose we have a rectangular current-carrying loop with current $i$, then we know that the magnetic force on each side can be found as:
$$F=iL\times B$$
Where $L$ is the vector in the direction of ...
2
votes
2answers
103 views
How & Why does accelerating charges radiate electromagnetic radiation?
Lets consider it case by case:
Case 1: Charge particle is at rest. It has electric field around it. No problem. That is its property.
Case 2: Charge particle started moving (its accelerating). We ...
-4
votes
0answers
17 views
Electric and Magnetic Field [closed]
Accelerated charges directed into regions between parallel plates with E and B fields – how are the charges deflected?
3
votes
1answer
97 views
Is electromagnetism a dead research field?
Are there any phenomena, we consider as electromagnetic and have no explanation for ?
2
votes
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
votes
0answers
47 views
electrostatic repulsion [closed]
Consider a homogeneous ring of radius $R$ made of a thin wire of cross section $A$ with $R^2≫A$. A charge $Q_1=10\mu C$, uniformly distributed over the ring, broke it apart due to electrostatic ...
2
votes
2answers
46 views
What really is the Magnetic Force on a wire?
I have a doubt regarding the significance of a force on a wire. Well, first of all, I know that if I have a particle and if there are several forces acting over it, then we can compute one total force ...
0
votes
0answers
19 views
What is the “relative permeability” of Neodymium?
I am currently interested in finding a high "relative permeable" material to put into the core of a solenoid. However many sources quote Neodymium's recoil realtive permeability ($\mu_{rec}$) to be ...
0
votes
1answer
71 views
Electric Field in Circular waveguide in TE mode
I know that the magnetic field inside a circular waveguide is
$B_z=B_0\dfrac{\rho}{R}\left(1-\dfrac{\rho}{2R}\right)\cos{\phi}\sin{\frac{\pi z}{d}}$
How can I find the two components of the electric ...
0
votes
1answer
25 views
Doubt in problem regarding resistance a of cylinder
I've been working on some exercises and I'm in doubt if my procedure with this one is correct. We have a hollow cylinder with internal radius $r_a$, external radius $r_b$, resistivity $\rho$ and ...
1
vote
1answer
107 views
Why the magnetic flux is not zero?
According to Maxwell's equations the magnetic flux over a closed surface must be zero, why in this case does not happen?
Thanks for your help
1
vote
1answer
4k views
What is the difference between a Rodin coil and a Rodin starship?
I've seen various designs for Rodin coil and a 'Rodin starship'? Are these just regular electromagnets? Or something different? How do they differ from regular electromagnets?
2
votes
3answers
116 views
Radio antennas that are much shorter than the wavelength
From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall ...
0
votes
0answers
13 views
Potential E alternation
Considering a permanent magnet moving in a trajectory perpendicular to a straight wire of a given lenght, in a way that the magnet expose always the north face towards the wire, no matter the ...
5
votes
3answers
255 views
Negative Mass and gravitation
Since Newtonian gravity is analogous to electrostatics shouldn't there be something called negative mass? Also, a moving charge generates electric field, but why doesn't a moving mass generate some ...
5
votes
4answers
917 views
Can someone please explain magnetic vs electric fields?
I've looked through about 20 different explanations, from the most basic to the most complex, and yet I still dont understand this basic concept. Perhaps someone can help me.
I dont understand the ...
3
votes
1answer
118 views
Can Earth's Magnetic Field Create Electricity?
If the earth has a magnetic field, can it, in theory, be run through a conductive metal coil to create electricity?
6
votes
4answers
3k views
Derivation of self-inductance of a long wire
Currently I am stuck, trying to derive the self-inductance of a long wire. According to literature it should be
$$L=\frac{\mu_r\mu_0l}{8\pi}$$
and in literature its derived by looking at the energy ...
4
votes
2answers
126 views
Can a current carrying loop or wire produces no magnetic field?
A current carrying wire produces magnetic field around it. We can find the direction by Fleming's Right hand rule. We know change in electric filed produces magnetic field and change on magnetic field ...
1
vote
2answers
46 views
Free spin (Curie) Paramagnetism
I'm working through a derivation for Curie paramagnetism and hope someone could help clarify a couple of steps. The way that makes sense to me (although now I have seen the wikipedia derivation below ...
5
votes
2answers
204 views
Why isn't data lost when sent over large distances?
I was thinking about how information is sent, for example through the atmosphere. There are plenty of obstacles, as well diffraction, etc.
Still, no information is lost. How is information sent to ...
1
vote
0answers
95 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 ...
7
votes
3answers
2k views
What is the cause of the normal force? [duplicate]
I've been wondering, what causes the normal force to exist?
In class the teacher never actually explains it, he just says "It has to be there because something has to counter gravity." While I ...
5
votes
3answers
105 views
What causes the permittivity and permeability of vacuum?
When light travels through a material, it gets "slowed down" (at least its net speed decreases). The atoms in the material "disturb" the light in some way which causes it to make stops on its path. ...
3
votes
3answers
178 views
Energy conserved… or not? Confused!
I am confused. Could someone kindly explain what's going on in this question?
A particle of mass $m$ and charge $e$ moves in the $x,y-$ plane. There is a constant magnetic field $B$ that points in ...
8
votes
2answers
187 views
Ferromagnetism with mobile spins
How can electron spins in Iron at room temperature have ferromagnetic order even though they are travelling at very high speeds?
One could argue that spin and motion are completely uncorrelated and ...
1
vote
0answers
30 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 ...
0
votes
2answers
205 views
Derivation of the Biot-Savart Law
Can someone provide a derivation of the Biot-Savart law for electromagnetic induction? To be clear,
$$
d\vec{B}~=~\frac{\mu_0}{4\pi}\frac{I d\vec{\ell}\times \vec{r}}{r^3}.
$$
Is there a simple way ...
1
vote
1answer
28 views
Capacitance of a conducting disk
I'm reading this(PDF) derivation of the capacitance of a thin conducting disk. The surface charge density of such a disk can be shown to be:
$\sigma(r) = \frac{Q}{4\pi a\sqrt{a^2 - r^2}}$ (in ...
2
votes
1answer
111 views
How Special Relativity causes magnetism
So my physics teacher assigned us an article about how special relativity causes magnetism in a wire with a current, even with the low drift velocities of electrons in a current.
It seemed that the ...
