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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2k views

Right hand rule to measure direction of electric field [closed]

The diagram: a positive plate is on the left of the page and a negative plate is on the right. The velocity of the particle is going down the page. So, with a right hand rule, I would point my thumb ...
2
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1answer
97 views

Circuit Loop Law Doubt

In a circuit with a solenoid/inductor and a resistor and a battery . Books say that $\Sigma \Delta V=0$ around a closed loop . That means work done by electrostatic field per unit charge is $0$ ...
3
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0answers
330 views

Finding the terminal velocity of a magnet dropped in a solenoid

We have to find proportionality of the terminal velocity with the factors of the system: Plot: a small dipole(mass $m$) with dipole moment $\mu$ is dropped in a long solenoid (radius $r$, ...
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1answer
328 views

How to add a potential term to the Dirac Equation?

I've read that if you have a Hamiltonian for the Dirac Equation, you can add a potential term to it simply by adjusting the momentum operator so that $p^\mu \rightarrow p^\mu-A^\mu$, where $A^\mu$ is ...
2
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2answers
88 views

Gauss's Law understanding

In the case of a point charge $q$ at the origin, the flux of $\vec{E}$ through a sphere of radius r is, \begin{equation} \oint \vec{E}\cdot d\vec{a} = \int \frac{1}{4 \pi \epsilon_0 ...
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1answer
101 views

Gauss's Law - Electric Field outside a Shell?

I'm somewhat confused as to why the electric field outside a spherical shell is $\frac{Q}{4\pi r^{2}\epsilon_{0}}$ Going through the work: $$ Q = 4 \pi r^{2} \sigma ; \frac{4\pi ...
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1answer
715 views

Superconducting diamagnetic sphere in uniform magnetic field

What is the size of the magnetic dipole moment $\vec m$ of a superconducting diamagnetic sphere $radius=R$ in a uniform magnetic field $\vec B_0$? Since there is no free current, we can solve for ...
3
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1answer
325 views

Electromagnetic black hole?

So I was thinking about something for the past while Consider a large spherical foam-ball with homogeneous density. Where a foam ball is defined as an object that can absorb matter with 0 friction ...
2
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1answer
901 views

Magnetic force on a magnetic dipole

Can someone please help me understand and answer the question of "why are the forces on a magnetic dipole different in a uniform magnetic field and non-uniform magnetic field?" I know in a uniform ...
2
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1answer
117 views

Behavior of the electric field on boundary surfaces

Consider this picture. Integrating over this infinitesimal box gives the following equivalencies: $$\int_{\Delta V} d^3r~{\rm div} \vec{E}(\vec{r}) = \int_{S(\Delta V)} d\vec{f} \cdot ...
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0answers
24 views

Faraday Effect, Satellites, and Electromagnetic Atmosphere [duplicate]

If I launched a Satellite into orbit and dropped a silver wire caged in carbon fiber with aerodynamic wings (for support), would the Faraday effect of the wire traveling through earth's ...
7
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2answers
5k 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 ...
2
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3answers
451 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. ...
5
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2answers
140 views

Layered CMOS structure question

I am trying to understand the workings of a CMOS image sensor. I understand that increasing wavelength results in an increased penetration depth in the silicon often used in CMOS image sensors. What ...
1
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0answers
119 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 ...
3
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1answer
167 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
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2answers
156 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 ...
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2answers
456 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} = ...
0
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3answers
205 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 ...
0
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1answer
78 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
172 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 ...
13
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3answers
832 views

Is classical electromagnetism a dead research field?

Is classical electromagnetism a dead research field? Are there any phenomena within classical electromagnetism that we have no explanation for?
2
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2answers
67 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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2answers
11k 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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5answers
29k views

How do moving charges produce magnetic fields?

I'm tutoring high school students. I've always taught them that: A charged particle moving without acceleration produces an electric as well as a magnetic field. It produces an electric field ...
3
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2answers
69 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....
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1answer
669 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 ...
6
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3answers
294 views

Induced current using a reference system bound with a moving charge

Suppose we have a charge moving at velocity $\mathbf{v}$ in the same plane of a square wire. If I sit in a reference frame where the square wire is still, since the charge is moving with velocity ...
3
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3answers
883 views

Why the magnetic flux is not zero?

If $\vec{\mathbf B}=B\vec{\mathbf a}_z$, compute the magnetic flux passing through a hemisphere of radius $R$ centered at the origin and bounded by the plane $z=0$. Solution The hemisphere and ...
5
votes
3answers
11k views

Can the Earth's magnetic be used to generate electricity?

Since the Earth has a magnetic field, can it, in theory, be run through a conductive metal coil to create electricity?
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4answers
2k 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 ...
4
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3answers
2k 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 ...
5
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2answers
265 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 ...
2
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4answers
248 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 ...
2
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1answer
300 views

How does the speed of a DC motor depend on the size of the coil?

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 ...
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3answers
544 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 ...
10
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5answers
2k 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. ...
0
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2answers
6k views

Derivation of the Biot-Savart Law [duplicate]

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 ...
3
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3answers
263 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 ...
5
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1answer
3k 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 ...
1
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1answer
278 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

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 ...
2
votes
1answer
113 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 ...
0
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0answers
103 views

Is this picture of the electron dipole moment correct?

It's the electron spinning on its axis, and a magnetic moment shoots up. The direction of the North-South is opposite what I thought it should be. Why is the vector arrow pointing from North to ...
3
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2answers
661 views

MRI's and Electromagnetic Radiation

If the waves in an MRI can go through our body, why is it that light with its magnetic fields gets stopped at our skin?
3
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1answer
4k views

Charge inside conductor

I know that the E field inside a conductor is zero. What happens if I put a source of charge inside the conductor? Say the conductor was spherical centered on the origin and there exists a charge at ...
-1
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2answers
4k views

intensity of electromagnetic waves

I have a trouble understanding intensity of electromagnetic waves, I already looked at this article and this question but didn't understand completely $Intensity = \frac{\epsilon_0}{2} |\vec ...
1
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2answers
546 views

Do black holes have charges?

Do black holes have charges? If so, how would they be measured? Also, does electricity behave the same way? Black holes affect photons, which are carriers of EM radiation, so do black holes have any ...
9
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2answers
256 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 ...
4
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1answer
2k views

What are the limits of applicability of Coulomb's Law?

Coulomb's law is formally parallel to Newton's Law of Universal Gravitation, which is known to give way to General Relativity for very large masses. Does Coulomb's Law have any similar limits of ...