Questions tagged [electromagnetism]

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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294 views

Magnetizing a ferromagnetic material.

if i pass a DC current directly through iron or steel or something ferromagnetic, Would that magnetize that material? And would it be magnetized permanently? Another question, does anyone know what ...
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Does action at distance in electromagnetism violate energy conservation?

Consider two charges A and B separated at distance D. charge B is attached on spring and can move towards and away from charge A. Now charge A is brought closer to charge B and then it is taken back ...
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103 views

Does a magnet moving in a uniform electric field experience torque?

Assume a uniform electric field of $E_y$ along $y$ in the lab frame of reference $(x,y,z,t)$. A simple magnet bar is set in motion at $v$ along $x$ in this electric field so that the alignment of the ...
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What is responsible for large ppm shifts in HNMR analysis of paramagnetic compounds?

In H-NMR spectra of paramagnetic metal acetylacetonate compounds M(III)(acac)3, large ppm shifts are observed. This is seen with other organometallic compounds as well. What is the origin of this ...
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22 views

Four-velocity of particle in crossed E and B fields

The equation of motion for a charged particle in an EM field is given by (ignoring factors of $c$ throughout) $$\frac{d U^{\mu}}{d \tau}=\frac{e}{m} F^{\mu \nu} U_{\nu}$$ with $U^{\mu}$ being the 4-...
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1answer
114 views

Can one force the octupole moments of a charge distribution (neutral and with vanishing dipole moment) to vanish using a suitable translation?

In a previous question, I noted that if you have a charge distribution with nonzero charge, then it is possible to choose an origin (at the centre of charge) which makes its dipole moment vanish, and ...
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1answer
43 views

Phase difference

Is there a phase difference between the magnetic field and the current producing this field? I have simulation results for three-phase transformer at different frequencies. The spectrum of the ...
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2answers
63 views

Physically intuitive explanation for Hamiltonian of charged particle in EM field

I've recently been looking at the quantum mechanical description of a charged particle in an EM field and have come across this classical Hamiltonian: $$H = \cfrac{(\mathbf p - q\mathbf A)^2}{2m} + q\...
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3answers
68 views

What was the energy of light thought to be before Einstein

I'm studying the photoelectric effect where it turned out that the kinetic energy of the emitted electron was dependent on the frequency of the u.v. light and not its intensity. It was previously ...
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2answers
1k views

How does a magnet cause magnetic force and its magnitude/direction?

Magnetic fields are created due to electron spin in the magnet. But how exactly does an electron "spinning" create forces around the magnet? Also, the magnetic force on a charge moving in the magnetic ...
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268 views

Faraday's law in Earth's magnetic field

If I throw a conducting coil in air which has N loops wound together and the coils starts rotating or spinning in the mid air, will there be any induced EMF/current in the conducting loops due to ...
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5answers
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Would a rotating magnet emit photons? If so, what causes the torque that gradually slows the rotation?

If a magnet is rotating around an axis perpendicular to the axis north-south axis of the magnet (which I assume to be cylindrically symmetric), in space (so no-gravity/freefall or friction), should it ...
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1answer
31 views

Current induced from a changing magnetic field

This question is motivated by Problem 7.12 in Griffiths Electrodynamics book. I have already solved the question, so there is no issue with homework policy. Relevant Equations: $$\Phi = \int{\vec{B}\...
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2answers
40 views

Showing $\partial_{\mu}\tilde{F}^{\mu\nu}=0$ by the antisymmetric properties

The electromagnetic dual tensor is given by \begin{align} \tilde{F}^{\mu\nu}=\frac{1}{2}\epsilon^{\mu\nu\delta\rho}F_{\delta\rho} \end{align} Here, $\epsilon^{\mu\nu\delta\rho}$ is the Levi-Civita ...
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1answer
14 views

No commutator or rectifier?

Homopolar motors and generators don't need a commutator or rectifier. Are there any other motors or generators that don't need a commutator or rectifier? If so, what are they?
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22 views

What is the phase relationship between current and magnetic field?

I understand the phase relationship of the electric and magnetic fields. but what is the phase relationship between current in an inductor and magnetic field produced due to this current?
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2answers
3k views

Back motor effect of loaded generators?

The back motor effect (see Counter-electromotive force) is the counter torque which opposes the rotational motion of the coils in a generator when the generator is under load. The back motor effect ...
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2answers
28 views

Confusion about magnetism in a circuit breaker

Why should the core of a coil of a circuit breaker be strongly magnetized when current passes through it, and immediately demagnetized when the switch is turned off? Please try to explain the ...
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3answers
224 views

Emf induced in a rotating disc

A disc is rotating in x-y plane (with uniform angular velocity) in uniform magnetic field oriented along z direction.There will be emf induced in the disc which can be explained by Lorentz force.But ...
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39 views

Deriving the Maxwell Scalar field Equation from the given action principle

I am currently starting to learn general relativity. I have been trying to derive the Maxwell Scalar field equation from the given action $\mathcal{S}[\phi, A, \mathbf{g}]=\int\left(\frac{1}{2}\left(\...
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Turn density impact on B Flux

I am messing around with electromagnets and I am actually making my own winding them by hand. The equations that are used to calculate magnetic field strength do not really have a geometry term in ...
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1answer
36 views

Acceleration through potential difference [on hold]

Fe^1+ ions are accelerated through a potential difference of V volts. What does this statement even mean???Please explain it to me. And please give me a general formula to work out the velocity ...
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0answers
36 views

Electrostatic force of attraction b/w dipole and charge [on hold]

A particle of charge $Q$ is placed on the axis of a neutral cylinder of volume $V$ at a distance $x$ from one end of cylinder. Here $x$ is much larger than linear dimension of cylinder. Find force of ...
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0answers
42 views

Time varying magnetic field and induced electric field

Well I would like to start with asking few questions for you to provide a detailed solution and whose details has been clearly mentioned in the above image. It would be helpful if you could clearly ...
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1answer
273 views

What is the B-field on the axis of a finite Iron-Core Solenoid, a Given Distance Beyond The Tip?

I am building a solenoid electromagnet with a core of known permeability $\mu$. The core needs to stick out slightly beyond the ends of the coils. Let's assume that the magnet is oriented with its ...
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1answer
27 views

Magnetized Steel wire around Soft Iron Core

We all know that a current carrying solenoid of copper is a weak temporary magnet and if we wrap it around a soft iron core it becomes a temporary strong magnet i.e. the iron core magnifies the ...
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127 views

Hodge dual and QED

I was studying the paper Topological massive gauge theories in three dimensions by Deser, Jackiw and Templeton. In the paper, they use Hodge dual for some reason which I don't understand at all. So I ...
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0answers
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Variation of phase across the wavefront

I have been going through the theory of diffraction of radio waves when passing through the ionosphere, when I came across the following statement: the amplitude of the wave when it emerges from ...
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4answers
95 views

Why do bar magnets exert a force on each other?

I've been trying to self-learn electromagnetism. I think I have a basic understanding of Maxwell's equations and the Lorentz force law. By combining the two, you can for example derive Coulomb's law. ...
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3answers
467 views

Is an EMF more/same/less in an insulator than in a conductor?

Is an EMF (electromotive force) more/same/less in an insulator than in a conductor? For example: A loop of copper and a loop of plastic in a changing magnetic field. In which will the emf be the ...
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1answer
41 views

Why does the interaction energy for the electric fields of two point charges vanish over any shell smaller than their separation?

A previous question, A triple integral in Spherical coordinates from Jackson's book on Electrodynamics, presents the fairly routine calculation of the interaction energy encoded in the cross-term ...
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1answer
85 views

A triple integral in Spherical coordinates from Jackson's book on Electrodynamics

I have been trying a solution for the following integral from Jackson but i do not seem to go anywhere. Please help. The problem is to compute interaction energy due to 2 charges. Compute following ...
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1answer
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Gravitational and Electric Forces in the Movements of Planets [duplicate]

Now that we know about the electric nature of the cosmos, the question arises: "How much are planets affected by electric forces while moving through the universe?" We have been taught that the ...
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1answer
37 views

Point of stokes theorem - conservative field

I am working through electrodynamics at the moment and I have a rather elementary question - which I apologize for. But after some research on google I am still not sure if I have understand "the ...
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4answers
419 views

Charged Particles: How is energy conserved?

I can't seem to understand where the energy comes from when two particles accelerate. I tried searching through the forums and it seems like the answer is always potential energy was the source of ...
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1answer
25 views

Inductors in Transformers

Why is it the flux change which induces the ac current of primary coil equal to the flux change that primary coil induces on the secondary coil? Is it possible to prove this? slide from video ...
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1answer
74 views

What does $e$, $i$ and $\vec{p}$ mean in this formula? [on hold]

I've found a formula to calculate the $\vec{E}$-field vector of a Hertzian dipole in the far field (don't know if that's the correct term) to draw and simulate the field lines. Link to my source: ...
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1answer
112 views

Can we talk about velocity field and acceleration field in 1D?

It's well know that a charged particle radiate when it is accelerated. We can rely on two different formulas. The first one it's from Jefimenkos : $$\mathbf{E}(\mathbf{r}, t) = \frac{1}{4 \pi \...
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3answers
78 views

Sinusoidal currents in AC circuits

EDIT : For this question...."AC Circuits" means "AC Circuits with a sinusoidal voltage source or a current source". How can we always assume that (in steady state) the current flowing in an AC ...
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2answers
252 views

Faraday's law. And Faraday's law of induction

Can anyone properly tell me the difference between the two, and which one historically came first. A majority of sources directly connect "emf" to flux but the 'flux rule' isn't absolute. It isn't ...
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1answer
61 views

What's wrong with the theory of an electromagnet? [on hold]

Let's imagine we have a uniform magnetic field pointing directly upwards, filling the whole space, so $\mathbf B(\mathbf r)=B\mathbf k, B>0$ for all points in $\mathbb R^3$. This can be seen as a ...
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3answers
31 views

How to relate magnetic flux to fluid flux?

I'm trying to grasp magnetic flux as an analogous concept to fluid flux, which is more concrete in my opinion. For example, we can define a vector field $\mathbf{F}(x, y, z)$ which describes the ...
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2answers
72 views

Is the vector potential component $A_\phi$ for a dipole necessarily 0 due to symmetry about the $z$-axis?

Consider an electric dipole (+$q(t)$, -$q(t)$) [where say $q(t)=q_0\cos\omega t$] is placed along the $\hat z$ axis. In the spherical polar coordinates, its vector potential $$\vec{A} = A_r \hat r + ...
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Magnetism problem

I encountered a magnetism problem requiring me to find out the minimum velocity (along $\hat{\mathbf{i}}$) given to a charged particle so that it passes through the centre of a circular region of ...
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1answer
314 views

Direction of electric field between a pair of parallel plates having a positive charge in the space between them

Two parallel plates are kept in such a way that one is earthed and the other one is held at a potential $V_1$. They have a positive charge density $\rho$ in the volume between the inner faces of the ...
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1answer
32 views

Can one measure the current in a ferromagnet?

I understand that all magnetic fields are the result of current (charges in motion). I'm now doing research on electromagnetic induction and understand that changing magnetic flux will induce a ...
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2answers
35 views

Do photons influence charged particles? [duplicate]

As a neutrally charged particle, a photon shouldn't interact electrically with another charged particle. However, since photons are electromagnetic waves, they should interact with charged particles, ...
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1answer
42 views

Why is average magnetisation zero in a normal (Non-superconducting) state

Considering type-I superconductors (specifically the magnetisation curves), why can one assume that the magnetisation is zero when we consider a normal state (when $H>H_C$)? EDIT: For ...
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How are the two definitions of B, as magnetic field and magnetic flux density, same?

My question, is primarily, based on this question, in which the accepted answer asserts that $\vec B \, $ has two definitions, As magnetic field$^*$, using Lorentz force $\vec F=q \vec v \times \vec ...