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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3answers
27 views

Ampere's law for a rectangular plate?

If a conductive wire was carrying current $I$, it would create a magnetic field: $$B=\frac{\mu_0I}{2\pi r}$$ However, if it's not a wire, but instead a plate how would the expression change/adjust ...
0
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0answers
36 views

Why does thicker aluminum do better at reflecting wifi

I have performed a experiment based one the ability of aluminium reflecting wifi radio waves. I have found out that the more aluminium foil i put on my reflector, the better result I can get. I did up ...
0
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1answer
28 views

Magnetic flux through the torroid

I am trying to derive the magnetic flux through a toroid of inner radius $\rho_0$ and outer radius $\rho_0 + 2a$, having $N$ turns I started off, by saying $$\phi = \int \vec{B} \cdot \vec{dS}$$ We ...
8
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1answer
143 views

Optical chirality and its possible hierarchy of generalizations

Optical chirality refers to a constant of motion of the electromagnetic field, which measures in some sense how chiral a light field is. Specifically, the pseudoscalar quantity $$ ...
2
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1answer
80 views

What is really sought when we purpose Einstein's postulates in Special Relativity?

Special Relativity can be motivated by looking at Maxwell's Electrodynamics and noticing that there is some kind of inconsistency between it and Newtonian Mechanics. Indeed, as Einstein pointed out on ...
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0answers
25 views

Eddy current damping heat generation

Background According to this source (page 7): https://deepblue.lib.umich.edu/bitstream/handle/2027.42/109373/me450w10project16_report.pdf?sequence=1 the "braking" torque a magnetic field on a ...
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0answers
16 views

Current density to find the magnetic field and Lorentz force?

Problems where there is a copper wire of a large diameter, the solution would tend to use current density $J$ to accurately compute the magnetic field $B$ at a distance. Why so? Why not simply use ...
17
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3answers
4k views

Deriving the speed of the propagation of a change in the Electromagnetic Field from Maxwell's Equations

I've been told that, from Maxwell's equations, one can find that the propagation of change in the Electromagnetic Field travels at a speed $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (the values of which can ...
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1answer
30 views

Ampere's law and Stokes' Theorem for current density

In both Griffiths and Jackson, Ampere's Law (or conversely the curl of the magnetic field) is derived by applying Stokes' Theorem to the surface integral of the current density J. The argument relies ...
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2answers
2k views

Why is Curl of Current Density $\nabla \times \vec{J}$ equal zero?

I am revisiting the derivation for $\nabla \cdot \vec{B} = 0$ in magnetostatics for the field $\vec{B}(\vec{r})$ of a charge $q$ at position $\vec{0}$ with velocity $\vec{v}$. It proceeds like ...
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1answer
28 views

Do electrons in a CRT emit radiation?

According to electrodynamics, accelerating charged particles would emit electromagnetic energy as radiation. Isn't it applicable to the accelerated electrons within a cathode ray tube? Do they give ...
0
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1answer
318 views

Force between two solenoids

How does one calculate the magnetic force between two coaxial solenoids, placed in a way their currents are in the same sense? There is a simple way to treat both as dipoles and then calculate the ...
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1answer
52 views

Force on a charged particle in a field of an EM wave [on hold]

For example if there is a charge q with mass m travelling with a velocity v "in the field" of a plane electromagnetic wave that is travelling in the z direction (in free space). My question is what ...
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0answers
22 views

How does cavity resonance produce EM waves?

My understanding is it acts like a capacitor and inductor in a loop. The capacitor releases stored energy which is absorbed by the inductor through a magnetic field which then returns it to the ...
1
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1answer
39 views

Show the Berry phase is invariant under $U(1)$ unitary transform [on hold]

Recall that $$\gamma_n = \oint A_n(R) \cdot dR = \oint \langle\psi_n(R)|i\nabla_R|\psi_n(R) \rangle \cdot dR.$$ Under the $U(1)$ transform, $$\psi_n \to \psi'_n \equiv e^{i\xi_n(R)}\psi_n,$$ where ...
2
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1answer
49 views

Confusion over plant's “magnetic field” being used to reduce wastage of pesticide

As far as I am aware, a plant does not have a magnetic field of any significant magnitude. What I wonder is, is there any electromagnetic related basis for the crop spraying technique outlined in ...
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1answer
18 views

Energy (voltage) correction on energy level between metallic electrodes with dielectric and accounting for work function difference

My goal is to understand how to correct for the field drop and the work function difference when performing electrical measurements on a certain energy level of a sandwiched system. The situation is ...
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0answers
9 views

Lorentzian nature of reflected microwave power in Electron Spin Resonance

Reflected microwave power has Lorentzian shape when plotted against frequency in ESR. How to mathematically prove this?
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1answer
38 views

Derivation of Biot Savart law for a curve

I'm ok with the following expression for Biot-Savart: $$ {\mathbf B}({\mathbf r}) = \frac{{\mu _0}}{{4\pi }}\int\frac{{{\mathbf J}({\mathbf r'}) \times {(\mathbf{r-r'})}}}{{|\mathbf{r-r'}|^3 }}dV'. $$ ...
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0answers
17 views

Amper`s Law and a flat [closed]

Here the question is written in photo.I think that this E should be integrated then should be written in the equation. I can`t find any easier way. Let me know if you see any.
0
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1answer
30 views

Amper`s Law on a ring [closed]

In this question i am having a bit trouble with understanding? And answer does not match totally. My question is that firstly there is no closed loop why is Amper`s Law holding here and second ...
0
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0answers
32 views

Why do metal filings line up in parallel (non-curved) lines on a knife blade?

My girlfriend works at a knife factory and was grinding a new knife model today made of a very hard, magnetic steel. She noticed that the metal filings were lining up on the knife blade in ...
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2answers
220 views

Is the photon truly not absorbed in Raman scattering?

In reading about Raman Scattering, I was thinking while reading it "okay, incident photo absorbed by molecule, molecule goes to higher energy vibrational state, molecule re-emits photon with either ...
2
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1answer
29 views

What's the difference between electromagnets and induced magnetism?

I am studying over the basic introductory concepts of magnetism, and I cannot comprehend definition of the following words below: Electromagnets- objects that are magnetic only when electric current ...
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1answer
49 views

Coulomb law and photons

When we consider process like $e^- e^- \to e^- e^-$ in QED, we see that from exchanges of one photon (tree-level diagrams) one can obtain Coulomb's law, while loop-diagrams give quantum corrections ...
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7answers
251 views

Is the electric field of a volume charge distribution well defined?

Consider a volume charge distribution $\rho({\bf r'})$. The electric field at ${\bf r}$ is $${\bf E}({\bf r})=\iiint\frac{1}{4\pi\epsilon_0}\frac{\rho({\bf r'})}{R^2}\hat{\bf R}\, \mathrm d^3{\bf ...
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0answers
34 views

What is the difference between length and velocity gauge when it comes to a dipole approximation?

Lets say we have plane wave with $\vec E$ perpendicular to $\vec k$. The dipole term will come from $\vec A\cdot \vec p$. Is the electric field longitudinal in the length gauge for the dipole ...
2
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1answer
170 views

The strength and control over the strength of electromagnets, correlation between the strength and weight of the magnets

I am currently working on a project that aims to use the attraction & repulsion between two electromagnets as an alternative to weights, from a physiotherapy and rehabilitation perspective to help ...
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2answers
24 views

gradient strength units in MRI

This may not be the appropriate forum for this but seemed to be the closest. I am trying to understand some concepts around MRI physics and it is common to use external magnetic fields created using ...
3
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1answer
90 views

How to solve “EM wave equation” for the field of uniformly moving charge?

Is it possible to show that the field of a uniformly moving charge, which is according to Biot-Savart law is given by... $${\bf E}({\bf r},t)=kq\left(\frac{1-v^2/c^2}{(1-v^2 \sin^2 ...
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2answers
49 views

Can a grounded spherical shell have a charge?

Can a grounded spherical shell lying inside another bigger non-grounded shell have charge on it? If yes then why? Won't the charge on the grounded conductor flow to the ground?
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0answers
25 views

How to solve the following wave equation? [closed]

We have paraxial beam propagating in a uniaxial anisotropic media. When such is the case wave can be divided into two orthogonal parts: Ordinary wave beam Extraordinary wave beam. The ordinary ...
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1answer
21 views

What would be the suggested material for blocking EM waves of a 600V 160kW 5kHz coil?

I am building up a reactor for heating material inside, but I need its EM waves not to interfere with the rest of my near circuitry. Its working conditions are: 600 VAC 300 A 160kW 5kHz What ...
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1answer
57 views

Effect of magnetic field on stationary charge

Consider a long infinite current carrying wire in N-S direction. Consider a charge +q at a distance of r from wire. Clearly, Lorentz force on charge is 0. But, their is a relative motion between ...
2
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4answers
548 views

Dipole in a spherical cavity in an infinite dielectric

In this example the assumption is made that inside a spherical cavity (with an infinite dielectric) with a dipole placed at its centre, the potential takes the following form: $$\phi_{in}= \frac{p ...
0
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0answers
18 views

Magnetic field in the centre of a square loop [closed]

I've a problem with the Biot-Savart's law. In particular i'd like to know if I could get the magnetic field in the centre of a square loop without integrating. Could you help me? Thankyou so much!
3
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1answer
673 views

Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
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0answers
29 views

Electron travel speed in spinning object [closed]

My question is in the hypothetical circumstance that you could spin a disk or something close to the speed of light how would that effect the travel of electrons through the material. Picture a circle ...
0
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2answers
100 views

What is the inconsistency between Maxwell's electrodynamics and newtonian mechanics?

As far as I understand, when a modification of a theory is made it is because some observation required this modifcation. Quantum Mechanics is a nice example of that: observations of microscopic ...
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0answers
33 views

Amplitude of unpolarized EM wave after a polarizer

The amplitude of a linearly polarized wave after a polarizer is $$E_1 = E_0\cos(\theta)$$ and the intensity is $$I_1 = I_0\cos^2(\theta)$$ Now for unpolarized light, the time averaged amplitude is ...
4
votes
1answer
308 views

What do I see if I move quickly past a charge surrounded by iron filings?

This might be a straightforward exercise, in which case I apologize. Suppose I surround a charge by iron filings initially oriented in some fixed direction, and I then move past the charge at an ...
1
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1answer
133 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
20
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2answers
863 views

Do intergalactic magnetic fields imply an Open Universe?

According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result? The abstract says: The detection of magnetic fields at high redshifts, and in empty ...
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0answers
12 views

Derivation of Poynting theorem in matter

In most textbooks I have read they derive the Poynting theorem using the Maxwell's Equation in vacuum and the fact that the force density $f=\pmb{E} \cdot \pmb{J}$. Then they just generalize it ...
2
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2answers
80 views

What does the Poynting flux represent?

I know that the Poynting flux is the cross product between $\vec E$ fields and $\vec B$ fields, but I'm wondering, are the field lines of the Poynting flux considered as electromagnetic energy? Let's ...
0
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2answers
37 views

An experiment related to Faraday's laws

This question is out of an Indian competition. I do not know calculus yet. I have some idea about how magnetic fields and electric fields work together but i don't know how to go about thinking ...
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1answer
159 views

Find magnetic field in a solenoid for different regions?

A steady current I flows along an infinitely long hollow cylindrical conductor of radius R. This cylinder is placed coaxially inside an infinite solenoid of radius 2R. The solenoid has n turns per ...
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1answer
22 views

Are deflection of compass and speed of electron relative to each other?

If one holds an electron in one hand and a compass in the other hand, and the starts running at the speed of light holding those, then will there be any deflection of the compass ?
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0answers
33 views

Calculating magnetic field strength in gap of tapered toroidal electromagnet

Consider a toroidal electromagnet filled with a magnetic material of large permeability $\mu$. The torus contains a small vacuum gap of length $h$. Over most of its length the torus has a circular ...
3
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1answer
170 views

Hidden momentum

I'm trying to learn about hidden momentum. After reading what I could find with a google search, I understand that it is equal to the momentum carried by radiation, calculated with the Poynting ...