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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Can you deduce absolute motion through magnetism?

Trying to understand magnetism I stumble into this example: Suppose there are two electrons travelling side by side in deep space (say, between Mars and Jupiter) at 1 cm distance at .99 c. In this ...
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1answer
16 views

Approximating magnetic field at center of current loop by solenoid of 1 turn

It's well known that by application of biot-savart law, the magnetic field at the center of a loop of current is $\frac{\mu_0 I}{2r}$. Now when we consider a solenoid, isn't the resulting magnetic ...
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3answers
4k views

How is bound charge and free charge possible?

I am studying Introduction to Electrodynamics by Griffiths and I came along a concept I cannot seem to understand properly. The concept of free charge AND bound charge. I do not understand how we can ...
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2answers
42 views

Definition of the Lorentz transformations

Until very recently I believed that the Lorentz transformations were defined as "the transformations that carry one inertial reference frame into another". In Wikipedia's page we find something along ...
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1answer
12 views

How to find the force of a magnetic field, resolving the components?

So, in answering this question the book resolves the components of the Force vector in this manner; $\mathrm dF_x= \mathrm dF \sin \theta$ $\mathrm dF_y= \mathrm dF \cos \theta$. And then, ...
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1answer
29 views

How do you calculate the expected force of an electromagnet?

What is the equation to find the expected force of electromagnet? What do the variables mean and how do you find each variable?
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0answers
42 views

How do you find the electric field corresponding to the magnetic field?

If we are given the magnetic field $B$ how can I find the corresponding electric field? The example brought up at the end of my lecture was $B = B_o \cos(ky-\omega t)$ in the $k$-direction. I am ...
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2answers
331 views

Magnet dimensions for 3-phase permanent generator

I'm making a 3-phase permenant generator. According to Faraday's law, the emf produced is negative number of turns times the change in magnetic flux over the change in time. If it is only the CHANGE ...
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1answer
171 views

Cylindrical capacitor in an electric circuit

I've come across a tricky question and would appreciate some hints or explanations as to why the given solution is the way it is. The question reads as follows: A coaxial cable consists of a wire ...
5
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1answer
328 views

Could the Faraday's rotating wire experiment be explained without using Lorentz force?

I read Faraday's rotating wire experiment which resulted in the invention of the homopolar motor. He was clever enough to think it that way and make the wire rotate around one of the poles of the ...
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1answer
27 views

The MRI signal: why do we consider the phase in the MRI signal

I am trying to understand the imaging principles behind MRI and I was looking at some lecture slides found here Specifically, I am looking at slide 41 where we look at some of the equations regarding ...
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0answers
24 views

Lorentz sphere correction

I am studying Lorentz surface correction for magnetic fields. I know that if we consider a uniform distribution of magnetization, the field can be computed as three contribution: $H+H'+H''$ where $H$ ...
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5answers
3k views

How can there be net linear momentum in a static electromagnetic field (not propagating)?

I understand from basic conservation of energy and momentum considerations, it is clear in classical electrodynamics that the fields should be able to have energy and momentum. This leads to the usual ...
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0answers
21 views

What is the answer to the numerical given below? [on hold]

An electron revolves along a circular path of radius 20cm with constant angular velocity 120 rad/s about an axis passing through the centre and perpendicular to the plane of the circle. if the ...
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2answers
228 views

Magnetic flux density of a solenoidal electromagnet

I am designing an electromagnet which will be used to test magnetic sensors. So I used to formula found at hyperphysics which relates the magnetic field flux density to the number of turns, the ...
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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 ...
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0answers
34 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 ...
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1answer
26 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 ...
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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 $$ ...
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1answer
77 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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23 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 ...
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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
29 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
33 views

EM: relative permittivity

After having defined the relative permittivity in lectures, I have problem understanding why it is a complex quantity. Thank you for your help,
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1answer
27 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 ...
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1answer
313 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
51 views

Force on a charged particle in a field of an EM wave

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
20 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 ...
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1answer
35 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 ...
0
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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
35 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 [on hold]

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.
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1answer
30 views

Amper`s Law on a ring [on hold]

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 ...
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0answers
31 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
219 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
26 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 ...
4
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7answers
250 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 ...
0
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0answers
30 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
21 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
89 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
47 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? [on hold]

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 ...
0
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1answer
20 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
56 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 ...