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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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Are there cases where $\nabla\cdot\iiint\frac{\mathbf{J}(\mathbf{x}')}{\left|\mathbf{x}-\mathbf{x'}\right|}\mathrm{d}V' \neq 0$?

In Jackson's Classical Electrodynamics, Section 5.4 (Vector Potential), the author seems to assume that because $\nabla\cdot\mathbf{J} = 0$, the following holds for the current density (where the ...
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Is there a superposition principle for the motion of interacting charges acted upon by external EM fields?

Consider a system of charges interacting with one another via their EM fields together with external EM fields applied to the system from outside. Suppose we work out the equations of motion ...
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Magnetic field of a ring ( tricky) [on hold]

here in this question the ring is rotated along axis perpendicular to the normal axis . This ring has charge q and rotated with angular velocity w . Then we have to magnetic field on a point on normal ...
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1answer
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Transformation of electromagnetic potential under local U(1) transformation

Let $\mathcal{L}=-(\partial _{\mu} \Phi^*)(\partial ^{\mu} \Phi)$ With $\Phi , \Phi^*$ being complex fields. When looking at local U(1) transformations in class, we saw that $\mathcal{L}$ is not ...
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Why do electrons accumulate on the surface of a wire (along its length) when a voltage source is applied across the wire cross section?

When I was going through how electric fields are transmitted long distances through a wire, I read that there was accumulation of charges along the surface of the conductor to drive the fields to long ...
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Can a magnetically levitating sphere spin in any direction?

I am trying to design a system that levitates a sphere (ball). I have found several solutions, however, it seems to me that the levitating ball only has one degree of freedom, i.e. rotation around the ...
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Energy stored in system or field?

I am having difficulty in understanding whether fields store momentum and energy or particles store them or both fields and particles store them? System of potential energy In the above source it ...
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29 views

What is the frequency distribution of the electromagnetic wave emitted by a charge accelerating along a given trajectory?

Charged particles radiate when accelerated.From a few other answers from Physics Stack exchange like What is the frequency of a photon emitted by an accelerated charged particle? and How ...
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Renaming tensor indices in summation

I am doing some tensor product calculations, which involve terms like this: $F^{\mu\lambda}\partial^\nu A_\lambda$ I am trying to write it as a total differential of some quantity. To achieve this I ...
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How can i calculate the amount of current and voltage consumption for every turn of electromagnetic pulse generator? [on hold]

For example if i use 240 voltage source for electromagnetic pulse coil with 100 turns around the core how could i know the amount of remnant voltage and ampere that come out from the wire at the end ...
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How everyone can benefit from learning physics? [on hold]

What would be benefits of learning physics in a relatively rigorous way (meaning to take a quick look even at the topics that are more advanced, like quantum field theory) for people who don't aspire ...
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Magnetic field like phenomenon associated with Gravitational Field [duplicate]

Like in electrodynamics we have magnetic fields associated with electric fields, is there something similar related with gravitational fields too? I mean both are quite similar fields...(Maybe I'm ...
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Why does a surface charge experience electrostatic pressure from its own field?

In Griffiths he mentions that a surface charge experiences a force in the presence of an electric field, but then he specifies that a force experienced by the surface is one due to its own electric ...
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2answers
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How is there a change in magnetic flux from a moving conductor through a uniform magnetic field?

My textbook states that when a straight conductor with a velocity perpendicular to itself and the magnetic field (see image), an emf will be induced between the two ends of a conductor. This means ...
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Direction of integration and boundary limits in electromagnetism?

I have encountered several problems regarding the choice of direction of integration and the boundary limits, this semester in electromagnetism. Is there some rule, so I don't do it wrong again. In ...
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1answer
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4-gradient of retarded time

To prove explicitely that the Liénard-Wiechert potentials satisfy the Lorenz gauge, one has to find the time derivative and gradient of the retarded time. In this Wikipedia article it's calculated as ...
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Can non-conservative field store energy when work is done?

Taking a non-conservative field like magnetic field (in presence of current density), when work is done can it store energy? If so how?
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1answer
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Is magnetic potential vector due to the magnetic field or energy stored in magnetic field?

My doubt is whether the magnetic vector potential is due to the potential energy stored in magnetic field or due to the magnetic field. Also I have another doubt. Since in electrostatics potential ...
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How to calculate induced emf of moving rod?

I have the following exercise: A copper rod with length L is moving on two frictionless conducting rails, in a distance a from an very long straight conductor. The rods resistance: R. Ignore any self ...
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3answers
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Which particle mediates the Aharonov-Bohm effect?

BACKGROUND The Aharonov-Bohm (AB) effect induces phase shifts between the two paths that an electron could take around an enclosed magnetic field. In radial coordinates, assume that the magnetic ...
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Polarized Vs Non-Polarized Capacitors

I am wondering what makes a polarized capacitor physically different than a non-polarized capacitor. I am aware they are functionally different. I understand the physics of a non-polarized capacitor, ...
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1answer
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What is $dB/dx$ where $B$ is magnetic field and $x$ is the separation between two magnetic dipoles?

I came across a question regarding the force between two magnetic dipoles $M_1$ and $M_2$ separated by a distance $x$ . Here in this text book I am given the solution is $$ B = \cfrac{\mu_0}{4 \pi}...
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Change in Magnetic Flux: Eddy Currents vs Moving Wire

From what I have read, as a conducting rod moves perpendicular to a uniform magnetic field, this will lead to a change in magnetic flux and therefore an emf will be induced (area being swept as the ...
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Maximum velocity of an electron across a tube with potential [on hold]

Sorry i usually don't ask such fundamental or silly questions but I'm having trouble with this. I'm nearly sure I'm right doing this question based on an x-ray whereby an electron is hypothetically ...
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1answer
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Interface boundary conditions of superconductor

Are the usual interface conditions for electromagnetic fields, i.e. $$\mathbf{n}_{12}\times(\mathbf{E}_1-\mathbf{E}_2) = 0,$$ $$\mathbf{n}_{12}\bullet(\mathbf{D}_1-\mathbf{D}_2) = \sigma_s,$$ $$\...
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1answer
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Why would the peak strength of the magnetic field of an AC-electromagnet be higher than the peak field strength of a DC-electromagnet?

Elsewhere on Stack Exchange several people have said that the average, mean or RMS value of the magnetic field of an AC-electromagnet would be about the same, or slightly lower than, a DC one. But, ...
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Energy in an RL-circuit [on hold]

Given a circuit made of an electrical resistance $R$, a solenoid $L$ and a battery with voltage $U_0$, I have to calculate (i) the total energy up to $t=\frac{L}{R}$ (switch is closed at $t=0$) (ii)...
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Force carrier exchange = attraction. How? [duplicate]

Edit: Mod who closed the question as duplicate: direct me to the answer I get one could explain repulsion between particles using the "2 men on frozen lake throwing a ball between each other" analogy,...
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1answer
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Angle in Biot-Savart formal in conductor loop

I am referring to this problem in Griffiths: My question: where does cos(theta) come from? r and dl are perpendicular to each other so with the formal from Biot-Savart, dlxr (vectors) = dl and ...
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1answer
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Motion of a particle in a uniform magnetic field [on hold]

The questition states: A uniform magnetic field exists given by $\mathbf B=b\ \hat k$ . A particle of mass $m$ and charge $q$ is present in magnetic field has an initial velocity $\mathbf v=v\ (\...
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1answer
44 views

Is photocell current equal zero at threshold frequency?

In photocell is there currant at threshold frequency $f=f_o$? I mean $I=0$ at $f=f_o$?
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1answer
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Magnetic forces and change in the type of kinetic energy

I have been thinking about magnetic forces and how they do no work. When looking at the definition of work, it makes sense as the Magnetic forces only change the direction. My other approach to try ...
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2answers
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Being $E \neq 0$ and $B \neq 0$ in one frame, is it possible to find $E = 0$ or/and $B = 0$ in another?

Consider a cylindrical symmetrical infinite wire with radius $R$. The wire has constant charge density $\rho$ and current density $\vec j$ (pointing to the right). This is measured from the stationary ...
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1answer
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Do $\rho$ and $\vec j$ change in a moving frame?

Consider a cylindrical symmetrical infinite wire with radius $R$. The wire has constant charge density $\rho$ and current density $\vec j$ (pointing to the right). This is measured from the stationary ...
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Special relativity when observer is traveling with the electrons creating current

There was a situation that has completely baffled me. I<-- -----wire----- q Essentially there are 3 things going on. Positive Charge: Below the wire ...
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1answer
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Why thickness of metal thin film will affect reflectance?

The reflection of the interface of two layers is described by Fresnel's equations. For instance, the reflectance of an Al bulk at 1 micron is around 0.9, this is the reflectance of the interface ...
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Strangeness Conservation

I am stuck as to how to prove that strangeness is conserved in electromagnetic and strong interactions, while it is not conserved in weak interactions. I know that this can be shown by studying the ...
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1answer
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Calculate how long it takes a neodymium magnet to fall down a copper tube?

Assuming we know the dimensions of the copper tube and the magnet, what other variables would one need to calculate the amount of time it takes a neodymium magnet to fall down the copper tube and how ...
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2answers
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How do phone loudspeakers work?

From what i understand, loudspeakers require AC signals to cause an electromagnet to oscillate due to changes in current direction, thus force direction. How can this happen with a phone's battery/...
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2answers
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Griffith's Electrodynamics problem 1.9 (rotation through (1,1,1))

I was compiling some solutions to Griffith's E&M, and I can't get my solution to square with the others I have found online. The question asks: 1.9 Find the transformation matrix R that ...
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Total reflection and the direction of magnetic force [on hold]

I have the following two simple questions: Say we have a cylindrical glass filled with water. The height of this glass is 10 centimeters, its radius is 1 centimeter. We but a source of light on the ...
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2answers
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Changing Refractive Index

Is it possible that the refractive index of a material could change?if so,what are those factors and what kind of variation could they bring?
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1answer
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A circuit out of a ring in amper law paradox [closed]

Assuming that there is three line of circuit and our amper ring is surrounded just two circuit . According to amper law the magnitude of B is just related to the circuits in the ring . But here the ...
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Magnetic force between broken magnetic circuit

The above image shows the flux flow through a rectangular core. We know the purpose of the core is to concentrate as much of the flux through it as possible, so we'll assume no or minimal stray field ...
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Frequency of the Primary Circuit in a Spark Gap Tesla Coil

In a simple spark gap tesla coil, how does the oscillating current in the primary circuit interact with the supply voltage/current? If the power supply comes from a transformer hooked up to the ac ...
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1answer
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Conductive sphere surrounded by conductive shell kept at constant potential

I am studying electromagnetism and I am a bit confused about the following problem: We have a charged ($+Q$) sphere (radius $R_0$) surrounded by a conductive shell (radius $R_1$ and $R_2$) which we ...
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2answers
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Trouble with Kirchoffs Law and EMF

So I'm having some trouble using Kirchhoff's second law . For example suppose I have a battery that supplies a constant emf $ \epsilon_0$ and is connected to a resistance R and inductance L. I don't ...
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1answer
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Harmonic dependence of magnetic field and scalar potential

I was reading a book about electrodynamics. the book said we shall now consider the special case when 'the scalar potential' and the field component depend harmonically on the $z$ coordinate (...
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demagnetizing a magnet

This is a theoretical question. Some elements described in the problem may be impossible/impractical, but I am just trying to understand the principles. Suppose I have a powerful electromagnet with ...
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Real (loaded) transformer [closed]

I hope you can help me out with the following exercise: We have a real transformer with a load $Z (R)$. Assume the following parameters: $U_1 = 230 V$, $U_2= 12 V$ $L_1 = 10$ mH, $L_{2}= 3$ mH $k= ...