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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Energy in magnetic fields

If I calculate the energy contained in the electric field for an electric dipole p in an electric field E, I get (ignoring the terms independent of orientation): $U = - \vec{p} \cdot \vec{E}$ which ...
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5answers
2k 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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4answers
2k views

History of Electromagnetic Field Tensor

I'm curious to learn how people discovered that electric and magnetic fields could be nicely put into one simple tensor. It's clear that the tensor provides many beautiful simplifications to the ...
5
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4answers
2k views

Recovering energy from a modern, magnetic-levitated flywheel

A modern flywheel rotor is suspended in a vacuum by magnetic bearings. This means that nothing touches the rotor as it spins. When time comes that we need to recover that stored kinetic energy, how do ...
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1answer
2k views

Relativistic charged particles in a constant uniform magnetic field

How can i derive the dynamic of a relativistic charged particle in a uniform magnetic field $B=(0,0,B)$?
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1answer
620 views

Angular momentum and EM wave

Is there any sense in saying that circularly polarized EM waves have angular momentum?
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7answers
4k views

What is the electric field generated by a spinning magnet?

Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical ...
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3answers
2k views

Which metals can cause magnetic interference (passively)?

I am developing an application that uses the magnetometer inside smart-phones to detect orientation w.r.t. the Earth's magnetic field. I have noticed that when the phone is held close to a metal ...
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2answers
425 views

What's a good reference for the electrodynamics of moving media?

The answer to a previous question suggests that a moving, permanently magnetized material has an effective electric polarization $\vec{v}\times\vec{M}$. This is easy to check in the case of ...
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2answers
527 views

Calculate the electric field of a moving infinite magnet, without boosting

Consider a rectangular slab of permanently magnetized material. The slab's dimensions are $L_x$, $L_y$, and $L_z$, and the slab is uniformly magnetized in the $\hat{x}$-direction. The slab is not ...
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1answer
494 views

Electric Potential

A nonuniform linear charge distribution given by $\lambda = bx$, where $b$ is a constant, is located along an $x$ axis from $x = 0$ to $x = L$. What is the electric potential at a point on the $y$ ...
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3answers
987 views

How could this person have discovered the resonant frequency from this string of magnets?

I stumbled onto this page http://mylifeisaverage.com/story/1364811/ and the post states that they were all making strings and shapes with these sets of 216 really small spherical earth ...
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2answers
861 views

How is the direction of a compass affected by a current in a wire?

If I place a compass over a wire(such that the wire is positioned north-south) with charge flowing through it, and it points northeast, how can I determine the direction of the electron current ...
2
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1answer
460 views

Magnetic moment of relativistic rotating ring

Let's consider rotating charged ring. Theoretically mass of this ring has no limit as rotation speed increases. So what about magnetic moment of the ring? Is it limited by the value of speed of ...
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1answer
166 views

Are Bloch equations empirical?

Are Bloch equations (which describe the evolution of macroscopic magnetization) empirical? If so, under what assumptions do they hold? $$\frac {d M_x(t)} {d t} = \gamma (\mathbf{M} (t) \times \mathbf ...
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1answer
350 views

Relationship between magnetic resonance linewidth and spin relaxation

First of all, what is the mathematical relationship between measured linewidth (usually in units of magnetic field) and spin relaxation time? I see papers talk about spin relaxation times in terms of ...
5
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2answers
477 views

meaning of an integral in the continuity equation

This is about continuity equation. What does the last integral mean? $$\frac{\mathrm{d}Q_V}{\mathrm{d}t}=\iiint_V \mathrm{d}^3x \,\frac{\partial\rho}{\partial t}=-\iiint_V\! ...
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2answers
282 views

Find drop-off rate of magnetic interference from a mass of pure iron on a magnetic compass

How can I find the magnetic interference a stationary 35000 kg block of 100% pure iron would have on a magnetic compass and what the drop off rate of the interference would be. So if said 35000 kg ...
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3answers
393 views

What is the density of virtual photons around a unit charge?

It seems that virtual photons also exist in vacuum. So the precise question is: What is the additional virtual photon density due to the electric field of a unit charge? Or: How many virtual photons ...
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0answers
2k views

How can I find the potential difference of a plastic sphere in a capacitor?

A thin spherical shell made of plastic carries a uniformly distributed negative charge of -Q coulombs. Two large thin disks made of glass carry uniformly distributed positive and negative charges S ...
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4answers
2k views

Does magnetic propagation follow the speed of light?

Does magnetic propagation follow the speed of light? E.g. if you had some magnet of incredible strength and attached an iron wire that is one light year long to it, would the other end of the iron ...
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1answer
377 views

Amount of free energy in the Earth-ionosphere waveguide

How much free energy is there in the Earth-ionosphere waveguide resulting from the constant bombardment of lightning strikes all over the Earth, and how do you go about calculating an estimate for it? ...
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2answers
1k views

How can I tell which end of a wire will have a higher potential?

I have the following setup: A <---- wire ----> B $V_b - V_a = \Delta v = \text{a positive value}$ I have two questions: Which end of the wire has a ...
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2answers
384 views

What is an “inclined magnetic field”?

What is meant by an "inclined magnetic field"? How is it different from the usual magnetic field?
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0answers
360 views

Skin effect and currents

Here in this picture you can see I_W which is induced by H. But why I_W is not vice versa? Because of $$rot \, \vec B = \mu_0 \, \left( \varepsilon_0 \frac{\partial E}{\partial t} + \vec j ...
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4answers
442 views

What is the medium that allows magnetic fields *or any forcefield* to exist?

Magnetic fields are obvious distortions.. of.. something, but what exactly are they distortions of? Massive objects produce curvatures/gradients in space-time resulting in what we observe as ...
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9answers
12k views

What's the core difference between the electric and magnetic forces?

I require only a simple answer. One sentence is enough... (It's for high school physics)
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5answers
10k views

Difference between electric field and electric displacement field

$$\mathbf D = \varepsilon \mathbf E$$ I don't understand the difference between $\mathbf D$ and $\mathbf E$. When I have a plate capacitor different medium inside will change $\mathbf D$, right? ...
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4answers
2k views

Resistance between two points on a conducting surface

Suppose we have a cylindrical resistor, with resistance given by $R=\rho\cdot l/(\pi r^2)$ Let $d$ be the distance between two points in the interior of the resistor and let $r\gg d\gg l$. Ie. it is ...
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2answers
8k views

How is the speed of light calculated?

How is the speed of light calculated? My knowledge of physics is limited to how much I studied till high school. One way that comes to my mind is: if we throw light from one point to another (of known ...
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4answers
285 views

Could ball lightening be a form of plasma?

With regard to the recent arXiv article: J. D. Shelton, Eddy Current Model of Ball Lightening http://arxiv.org/abs/1102.1224 I wonder if this is a reasonable explanation of ball lightening, or if ...
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2answers
908 views

Does van Eck phreaking really work, or is it an urban myth?

Van Eck phreaking, the ability to reconstruct distally the text on a CRT or LCD screen using the leaking em from the target computer, was in the news about five to ten years ago. It is talked about as ...
13
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6answers
2k views

Why do physicists believe that there exist magnetic monopoles?

One thing I've heard stated many times is that "most" or "many" physicists believe that, despite the fact that they have not been observed, there are such things as magnetic monopoles. However, I've ...
4
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1answer
395 views

What is the effect of ice on an antenna?

A local FM radio station transmitting at 89.3 MHz recently announced that it would be running at 50% power due to freezing weather and a forecast of ice accumulation, as "when ice is forecast ... it ...
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6answers
7k views

Electromagnetic fields vs electromagnetic radiation

As I understand, light is what is more generally called "electromagnetic radiation", right? The energy radiated by a star, by an antenna, by a light bulb, by your cell phone, etc.. are all the same ...
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2answers
395 views

Using energy from sun magnetic field

Knowing ...
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2answers
865 views

Studying electrodynamics problems

Suppose an advanced undergraduate student has reached a moderate level of understanding on electrodynamics. Where should he focus on, to sharpen his problem-solving skills? Practicing integrals ...
6
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3answers
419 views

Does bunching reduce synchrotron radiation?

A continuous charge distribution flowing as a constant current in a closed loop doesn't radiate. Is it therefore true that as you increase the number of proton bunches in the LHC, while keeping the ...
7
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3answers
2k views

Are Classical Field Theory and Quantum Mechanics of a single particle (nonrelativistic or “classical”) limits of Quantum Field Theory?

Recently I talked about QFT with another physicist and mentioned that the Quantum Field Theory of a fermion is a quantisation of its one-particle quantum mechanical theory. He denied this and ...
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2answers
712 views

Covariant Description of Light Scattering at a fastly rotating Cylinder

Let us consider the following Gedankenexperiment: A cylinder rotates symmetric around the $z$ axis with angular velocity $\Omega$ and a plane wave with $\mathbf{E}\text{, }\mathbf{B} \propto ...
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3answers
1k views

Popular depictions of electromagnetic wave: is there an error?

Here are some depictions of electromagnetic wave, similar to the depictions in other places: Isn't there an error? It is logical to presume that the electric field should have maximum when ...
4
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4answers
594 views

Radiation from a pair of charged objects orbiting each other

This question on binary black hole solutions, led to me think about the similar question from the perspective of what we know about the Hydrogen atom. Prior to quantum mechanics, it was not ...
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10answers
5k views

Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?

As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the ...
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2answers
2k views

Special designs to reduce the electrical resistance of a wire

The numerical simulation of this nerdy question shows that the resistance decreases with the number of nodes along longest side, and converges to a finite value when the # of nodes approaches ...
2
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1answer
333 views

Cherenkov radiation in nuclear bomb

Would Cherenkov radiation occur at the explosion of a nuclear bomb? Suppose it would not be occluded by smoke or anything else for that matter.
5
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2answers
359 views

What would the electromagnetic field of a massless electron look like

The Standard Model gives non-zero mass to the electron via the coupling to the Higgs field. Issues of renormalizability aside, this is fundamentally unrelated to the fact that the electron couples to ...
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2answers
477 views

Is there an explicit angular momentum in Maxwell equations?

Electromagnetism implies special relativity and then the universal constant "c". And if we set c=1, the coupling constant has units of angular momentum (so in relativistic quantum mechanics we divide ...
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3answers
3k views

Visualizing Electromagnetic Waves in 3D Space [closed]

I did one module of physics for my GCSE one year ago which taught me about transverse EM waves & the EM spectrum, but since then, I do not understand how a wave would move in 3D space. Can someone ...
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1answer
474 views

Electrical eddy current visualization or simulation

Eddy currents are induced in a metal plate when it experiences a changing magnetic flux. Is there a realistic visualization or simulation of eddy currents available? The only picture I found, on ...
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4answers
991 views

When is the force null between parallel conducting wires?

Consider two long wire with negligible resistance closed at one end of the resistance R (say a light bulb), and the other end connected to a battery (DC voltage). Cross-sectional radius of each wire ...