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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What are the physical interpretations of $\nabla \times \textbf{P}$ and $\nabla \cdot \textbf{M}$?

In the text I am reading, it is made clear that it is incorrect to assume that the electric displacement $\textbf{D}$ is identical to the electric field with the exception that it is raised from the ...
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5answers
390 views

Faraday's paradox

I have studied that Faraday's law of induction and motional emf are two different lines of thinking but are essentially same. But then, how can Faraday's paradox be explained by Faraday's law of ...
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3answers
621 views

Moving conducting bar in (changing) magnetic field

In my exam today I've been given this problem, yet even with the results at hand I simply can't warp my head around it; Given the picture below, a bar is placed on two conducting rails with a ...
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1answer
38 views

What is the connection between linear, circular and elliptical polarization versus perpendicular and parallel polarization?

In the EM course, we suddenly went from linear, circular and elliptical polarization to perpendicular and parallel polarization. What is the connection between these concepts and when it is ...
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0answers
33 views

Vector potential in presence of monopole [duplicate]

In this paper http://www.hcs.harvard.edu/~jus/0302/song.pdf when Song was explaining dirac string. He said "In the presence of a magnetic monopole, the vector potential cannot be defined everywhere. ...
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1answer
46 views

I wnt to know HOW to create TM, TE, and TEM waves; and is it being used in industry? [closed]

Transverse Magnetic (TM) and Transverse Electric (TE) modes So I'm wholly ignorant of any knowledge surrounding these modes, so bear with me here.. One component is infinitesimally small in the ...
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1answer
104 views

Dielectric Tensor vs. Conductivity Tensor in (Cold) Plasmas

I'm studying Waves in Cold plasmas right now, but I guess my question is generalizable. It's about the 4th Maxwell Equation in polarizable / conductive media: $\nabla \times H = \frac{1}{c} ...
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1answer
34 views

What's the relationship between velocity factor and refractive index?

Wikipedia on velocity factor explains: For optical signals, the velocity factor is the reciprocal of the refractive index. On refractive index, Wikipedia also has an explanation of refractive ...
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3answers
533 views

Why do we use the Coulomb potential for the hydrogen atom?

When solving the Schrodinger equation for the hydrogen atom, the Coulomb potential $V = \frac{e^2}{4 \pi \epsilon_0 r}$ is used. The Coulomb potential comes from classical electrodynamics, so why ...
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0answers
58 views

Spontaneous de-excitation of an atom

Consider the Hamiltonian for the process of spontaneous de-excitation on an excited atom, (an electron jumped to a higher energy level while a lower energy level is not complete). This Hamiltonian ...
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2answers
77 views

Is Magnetism and Electromagnetism the Same Thing?

I keep hearing everywhere that magnetism and electromagnetism are different but is seems to me that when a current is moving and it creates a "magnetic field", it is just electrons repulsing other ...
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2answers
64 views

Use of Amperian Loop to Calculate the $B$-field inside a co-axial wire

The problem asks to calculate the magnetic field at point $a$ of a co-axial cable with a wire at the center surrounded by a rubber layer, and then another hollowed cylindrical wire followed by a ...
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34 views

Speed of an electromagnetic soliton in free space

What is the speed of an electromagnetic soliton in free space? Is it equal to 'c' ? P.S. My understanding of the Fourier transform says it's not.
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24 views

Charge on a fork in a microwave

I left a fork in the microwave and when the turntable pushed it against the interior wall, there were sparks and black arc welder-like burns on the wall. I think this means that the microwaves induce ...
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1answer
84 views

How does back-emf oppose the input-current?

A circuit, has current $A$ flowing at a certain $V$. When there is a change in magnetic-flux, based on Faraday's law of induction & Lenz's law, we know that there is change in Potential ...
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2answers
758 views

Can water be magnetized?

This may be a stupid question, so feel free to shoot it down. Assuming all atoms have a magnetic moment, I would assume the water molecule too would have a resultant magnetic moment; ergo, it may be ...
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1answer
33 views

Too many antennas around a single energy transmitting antenna?

I am not a physicist. So I am not even sure, if I am phrasing this question properly. Lets say there is a wireless transmitter transmitting energy. Let us say I place $N$ number of antennas around it ...
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1answer
99 views

Asymmetry of induced e.m.f due to a bar magnet falling through a coil

It is well known that as a bar magnet falls freely through a coil, an electromotive force is induced in the coil by Lenz's Law. However, it appears that the peak e.m.f induced (ignoring the ...
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1answer
128 views

Stimulated emission direction

Place a sub-micron clump of crystal violet molecules in front of a multipixel detector. Raise the molecules to an electronically excited state with a beam of 590 nm light, illuminating from the side ...
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1answer
275 views

How is the current flow perpendicular to the wire?

This answer gives a great explanation of how surface charge builds up to force the current to move perpendicular to the wire: http://physics.stackexchange.com/a/102936/41086 However, it fails to ...
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0answers
38 views

How to design a Bitter electromagnet? [closed]

I am designing electromagnets at my home and want to know how can I design electromagnets at home. Bitter electromagnets are really a challenge.
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1answer
264 views

Total Momentum From a Standing Electromagnetic Wave

How does one show the momentum imparted to a perfect conducting resonance cavity (boundary) of any shape by a classical standing electromagnetic wave inside is zero? It should be by conservation of ...
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3answers
636 views

Electromagnetic waves should stop while encountering a conducting shell?

I am a high school student who has just started reading elementary electromagnetism and am a completely beginner in this subject. I have read in books that EM waves are nothing but sinusoidal ...
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1answer
34 views

Ampere's loop inside capacitor

The figure is a view of one plate of a parallel-plate capacitor from within the capacitor. In the question, we are required to rank the 4 paths (a, b, c and d) according to the value of $\oint ...
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3answers
60 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
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5answers
13k views

Can someone please explain magnetic vs electric fields?

I've looked through about 20 different explanations, from the most basic to the most complex, and yet I still dont understand this basic concept. Perhaps someone can help me. I dont understand the ...
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0answers
15 views

Question about calculating the torque of a magnet in an additive field

I did an experiment in the lab today where I placed a magnet in between two coiled magnets, with one coiled magnet having its south side up and north side down and the other coiled magnet with the ...
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1answer
22 views

Does the Magnetic Flux change (at a point) If the wire which causes it was cutted into a half?

If the electric charges move through a wire Does the generated Magnetic flux at any certain point chanage if this wire was cut into a half
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3answers
62 views

How is energy conserved in magnet?

When an object (i.e.magnetic) is attracted towards the magnet, work is done so that the energy is used but from where this energy comes from?
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2answers
45 views

Magnet arcs assemly

Is it possible to join multiple Neodymium magnet arcs (shown blow), which are radially magnetized, to create a cylindrical ring unity that is also radially magnetized? If it is not possible, how to ...
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3answers
165 views

Can the poles of a magnet have varying intensity?

In re-reading Is it possible to separate the poles of a magnet? (amongst others) the question mentioned in the title here just occurred to me. It may not be possible, at our current levels of ...
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0answers
31 views

Electric field due to rotating charged sphere

Consider a spherically symmetric charged object (charge $Q$) rotating about its axis. From Gauss's law we know that all that matters for the electric field $\mathbf{E}$ is the charge $Q$ enclosed ...
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0answers
27 views

Work done by the Lorentz force?

Im trying to calculate the work done on a single wire placed inside a magnetic field. Now for the force I've used the Lorentz force formula to calculate it, for the distance should I used the width of ...
3
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1answer
107 views

Are magnetic hysteresis losses relevant to alternating currents flowing in a wire?

Say we have an AC in a magnetically lossy material, like iron. Because of iron's relatively high permeability, skin effect will be more pronounced than it is in say, copper, so this iron wire isn't so ...
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1answer
101 views

Why are permanent magnets permanent?

Let me see if I get it right. When an iron bar is attracted by a permanent magnet it becomes a magnet itself because all of its magnetic domains start to point in the same direction. When the iron bar ...
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2answers
75 views

Induction on a circuit with switch

I have the following circuit: It is subject to a steady, time-invariant magnetic field which points out of the paper. At $t = 0$, the switch closes. I thought that the magnetic flux would decrease ...
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2answers
70 views

Deriving Maxwell's Equations from Electromagnetic Tensor

Given $ F_{\mu\nu} = \partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu} $ It is obvious that the diagonals are zero, as $ F_{ii} =\partial_{i}A_{i} - \partial_{i}A_{i} = 0 $ And, ...
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1answer
99 views

How to calculate the force acting on a magnet due to a solenoid

Can I assume both magnet and solenoid as magnetic dipole and use Coulombs law to find the force acting the magnet?
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1answer
147 views

Faraday's law and “infinite induction”

I may have confused after thinking too much about Faraday's law. If an emf is induced in a circuit due to some changing magnetic field, the induced current will be in a direction such that the ...
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0answers
38 views

How to calculate the necessary magnetic induction to magnetize a certain ferromagnetic object and hold it

Well pretty much what I want to do is to build an electromagnetic coil with a nail inside of it for it to hold a steel ball. But I've come into some trouble, because I don't quite get how to calculate ...
0
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1answer
45 views

Why do Magnetic North and South Poles never exist by themselves? [duplicate]

As the title suggest, whilst I was reading I saw written that a 'Magnetic North or South Pole has never been found by itself'. And I was wondering why this was?
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0answers
20 views

The inverse radiation problem

I posted this question on Mathoverflow.net, but it seems that no one show interest. This is the radiation problem. Suppose the current $f(x)\in L^2(\mathbb{R})$ has a compact support in $[-a,a]$. And ...
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3answers
552 views

Coulomb's Law in the presence of a strong gravitational field

I was under the impression that the $1/r^2$ falloff of various forces were because of the way the area of a expanding sphere scales. But that strict $1/r^2$ falloff would only be globally true in a ...
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0answers
24 views

Maxwell Lagrangian at weak fields

In http://arxiv.org/abs/hep-th/9506035 the authors said after writing this equation: $$\frac{1}{4}\eta^{\mu\nu\lambda\rho} F_{\mu\nu}F_{\lambda\rho} = \eta_{\sigma\tau\alpha\beta}\frac{\partial ...
2
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3answers
126 views

Moving coil contradiction of Faraday Induction

In Faraday's Induction Experiment, the e.m.f. induced in the induction coil becomes zero when the relative velocity of the coil and the magnet becomes zero. But one can also argue from a stationary ...
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8answers
15k views

Can magnetic fields be redirected and focused at one point?

I know that magnetic fields can be redirected, but... given a situation where you have static magnetic field over a large area, and you want to quickly change the magnetic field strength. Is it ...
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1answer
24 views

Given $\tilde {E}$, which formula should I use to find $\tilde {H}$

Given $\tilde {E}$, there exist two formulas in my book (Cheng) to compute $\tilde {H}$: Maxwell's formula: $\nabla \times \tilde {E} = -j\omega\mu \tilde {H}$ Plane wave formula: $\tilde {H} = ...
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2answers
400 views

Why are magnetic field lines perpendicular to the surface of a ferromagnetic material?

It is known that magnetic field lines become nearly perpendicular to the surface of a ferromagnetic material. The quantitative proof which uses the boundary condition requires that magnetization at ...
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1answer
201 views

Frequency of rotating coil

Given a coil initially in the x-y plane, rotating at angular frequency $ \omega $ about the x-axis in a magnetic field in the z-direction. This uniform time varying magnetic field is given by $B_z ...
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3answers
117 views

What is supposition of equilibrium? How do Rayleigh, Jean know the electromagnetic wave in equilibrium behave?

In a cavity of dimension L, the wave must give zero amplitude at the wall, means wave equation has zero amplitude. Why? Answer from hyperphysics "since a non-zero value would dissipate energy and ...