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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1answer
52 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
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2answers
84 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 ...
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0answers
12 views
Work on Ferromagnetic Object Due to Solenoid
I've been going through some equations and such trying to determine the work done by a solenoid on a ferromagnetic object. I have the following:
Magnetic field due to solenoid:
$\vec{B} = ...
2
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1answer
122 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 ...
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3answers
86 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 ...
6
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1answer
98 views
Forcing quadrupole moments to vanish for a neutral system
For a system of electric charges $q_i$, at positions $\mathbf{r}_i$, with a nonzero net charge $Q=\sum_i q_i$, one can define a "centre of charge" in the obvious way as
$$
...
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2answers
64 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 ...
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1answer
498 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 ...
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0answers
36 views
Magnetic Field on a particle between two Helmholtz Coils
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
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2answers
51 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
43 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 ...
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4answers
83 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 ...
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1answer
17 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
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1answer
17 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 ...
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2answers
56 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 ...
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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?
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1answer
92 views
Is electromagnetism a dead research field?
Are there any phenomena, we consider as electromagnetic and have no explanation for ?
2
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1answer
49 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 ...
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0answers
28 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 ...
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2answers
43 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 ...
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0answers
14 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
60 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 ...
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1answer
21 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
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0answers
29 views
Induced current using a reference system bound with a moving charge
Let's suppose to have a charge moving at velocity $\textbf{v}$ and a square wire.
If I choose a reference system bound with the square wire, since the charge is moving at velocity $\textbf{v}$, in ...
1
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1answer
87 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
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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?
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3answers
80 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 ...
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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 ...
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2answers
105 views
What is a “gravitational cell”?
I am not a physicist, and I don't understand the details of electromagnetism. Anyhow, I was looking for how the batteries work in Google. So, I came across this article: "How batteries work: A ...
5
votes
3answers
242 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
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4answers
545 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
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1answer
72 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
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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
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2answers
251 views
Maxwell equation boundary conditions on a conducting sheet
I'm having difficulties solving boundary conditions for an infinitely thin conducting layer in a presence of an alternating field.
I use the Maxwell equations:
$\nabla \cdot \mathbf B = 0$
$\nabla ...
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2answers
88 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 ...
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2answers
38 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
133 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 ...
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1answer
85 views
Magnetostatic energy density — derivation without introducing induchance?
I was looking for a derivation of the expression for the energy density at any point in a static magnetic field. I do know that it is $\dfrac {1}{2 \mu_0}\left|\vec{B}\right|^2$ -- I was just ...
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0answers
78 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
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4answers
2k views
What is the cause of the normal force?
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 ...
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1answer
28 views
2 solenoids: emf and frequency
I'm studying a circuit in AC. I use a function generator and set a waveform.
I have a solenoid and I put a small solenoid inner it.
Could you tell me if there is a relation between frequency and emf ...
5
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3answers
83 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
165 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 ...
7
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2answers
172 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 ...
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0answers
27 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 ...
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2answers
96 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
26 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
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1answer
101 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 ...
2
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1answer
70 views
Trying to understand EM wave and photon
When electrical fields and magnetic fields couple together, it forms electromagnetic waves. And we can "quantized" it and see each "package" of it as photon. So can electrical fields and magnetic ...
2
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1answer
32 views
Falling through the ground [duplicate]
I do not know much about physics but I know that according to Newtons third law of motion when we walk we are pushing the ground down but the ground is pushing us up. What force is making the ground ...
