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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4answers
6k views

Derivation of Maxwell's equations from field tensor lagrangian

I've started reading Peskin and Schroeder on my own time, and I'm a bit confused about how to obtain Maxwell's equations from the (source-free) lagrangian density $L = ...
6
votes
3answers
4k views

Do mirrors increase the amount of light in a room?

So if you have a light bulb in a room, and you had a tool to measure the amount of light that's in the room, then let's assume the amount of light only caused by the bulb is "1" If you place a mirror ...
2
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2answers
467 views

Software to calculate forces between magnets [duplicate]

I am working on a complex configuration of magnets and every time I make an experiment something unforseen happens. Now I believe I could speed up the development by sitting down and calculating the ...
3
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1answer
1k views

Impedance formula for an edge coupled differential stripline in an asymmetrical stackup?

I've been searching and have not been able to find a formula for calculating the impedance of differential lines on inner layers where the dielectric above and below it is not symmetrical. I've seen ...
7
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4answers
1k views

How does charge work if photons are neutral?

How can an electron distinguish between another electron and a positron? They use photons as exchange particles and photons are neutral, so how does it know to repel or attract?
23
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7answers
4k views

Is it theoretically possible to shield gravitational fields or waves?

Electromagnetic waves can be shielded by a perfect conductor. What about gravitational fields or waves?
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3answers
2k views

How is electromagnetic wave variation distributed in space?

Imagine an electromagnetic wave (a monochromatic one for example). The electric field amplitude and its variations travel in the propagation direction. So, if there really exists a propagation ...
10
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3answers
1k views

What is the symmetry which is responsible for preservation/conservation of electrical charges?

Another Noether's theorem question, this time about electrical charge. According to Noether's theorem, all conservation laws originate from invariance of a system to shifts in a certain space. For ...
4
votes
1answer
217 views

Ground-based anti-matter detector on the cheap?

Given today's revelation of the detection of terrestrial gammay-ray flashes (TGF) produced by thunderstorms and the associated pair production of an electron and a positron, how feasible is it to set ...
2
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7answers
758 views

“Magnetic mnemonics”

Over and over I'm getting into the same trouble, so I'd like to ask for some help. I need to solve some basic electrodynamics problem, involving magnetic fields, moving charges or currents. But I ...
4
votes
1answer
1k views

Self induction: Why is induced voltage smaller than the applied voltage?

Imagine a circuit with a voltage source, a switch and an inductivity all connected in series. First, the switch is open and there's no current and no magnetic field around. If we close the switch, ...
3
votes
1answer
252 views

Physically induced latency in internet connections

As a trade I am a software architect. It does happen that I have to design sites where the servers are located in different parts of the world. Now an essential parameter to consider is latency: that ...
18
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1answer
2k views

Flux through a Mobius strip

I was sent here from mathoverflow, hoping for a complete answer to this: === A friend of mine asked me what is the flux of the electric field (or any vector field like $$ \vec r=(x,y,z)\mapsto ...
14
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3answers
1k views

Shine a light into a superconductor

A type-I superconductor can expel almost all magnetic flux (below some critical value $H_c$) from its interior when superconducting. Light as we know is an electromagnetic wave. So what would happen ...
4
votes
2answers
209 views

Dynamic ferrofluid sculptures [duplicate]

http://www.youtube.com/watch?v=UJJuq_pcyIQ What exactly is going on in the video example? I understand the phenomena occurs because of magnetism but I am trying to figure out the mechanics behind ...
1
vote
2answers
160 views

EM irradiament and multipoles

Why in the irradiament mulipoles of Lienard-Wiechert's potential we say that electric quadrupole give a contribute of the same order of the magnetic dipole? How can we see it from their equations? And ...
11
votes
2answers
586 views

What's the explanation for the Giant Magnetoresistance effect?

I've been reading this review on Giant Magnetoresistance, and something about it is bothering me. The basic effect is that, using a special "stack" of layers (alternating between ferromagnetic and not ...
13
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3answers
3k views

The exchange of photons gives rise to the electromagnetic force

Pardon me for my stubborn classical/semiclassical brain. But I bet I am not the only one finding such description confusing. If EM force is caused by the exchange of photons, does that mean only when ...
3
votes
1answer
825 views

How does an electron microscope work?

I am a physics novice. Google tells me that electron microscopes work much like their optical counterparts -- but the analogy falls apart for me when I think about what I'm "viewing." Obviously, you ...
12
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3answers
18k views

Can magnets rotate infinitely?

There are many videos on youtube in which people arranged magnets in circle and rotated one placing in middle of that circle on a shaft, and the magnet (magnet motor) starts madly and continues its ...
1
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1answer
203 views

Electromagnetic weapons: power?

I essentially have three questions concerning weapons based on EM waves or more generally. Focusing on the weapons using radio-waves and/or micro-waves, what power do these types of weapons need to ...
6
votes
3answers
343 views

Controllable faster-than-light phase velocity

This is not another question about faster-than-light travel or superluminal communication. I totally appreciate the speed limit capped by physical laws (or theories.) Just curious, since there is no ...
8
votes
3answers
3k views

Can I levitate an object without using an electromagnet?

I know that it's possible to make an object levitate using an electromagnet to hold it up. But is it also possible to do this with regular magnets? Is there a special kind of magnet I need in order ...
7
votes
3answers
5k views

Is it safe to use any wireless device during a lightning storm?

I need "educated" reasons whether it is safe to use any wireless device during a lightning storm. Most people said don't use it but they cannot explain why.
11
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1answer
523 views

Formulation of Transformation optics using a Material Manifold

Dear Community, recently, Transformation optics celebrates some sort of scientific revival due to its (possible) applications for cloaking, see e.g. Broadband Invisibility by Non-Euclidean Cloaking ...
7
votes
2answers
436 views

Why is the ground state of the ferromagnetic tetrahedron threefold degenerate?

I'm preparing a presentation on Spin-Ice, but something's been bugging me for a while. On the Wikipedia page for Geometrical Frustration, it says the following about easy spins on a tetrahedron with ...
16
votes
1answer
5k views

Kubo Formula for Quantum Hall Effect

I'm trying to understand the Kubo Formula for the electrical conductivity in the context of the Quantum Hall Effect. My problem is that several papers, for instance the famous TKNN (1982) paper, or ...
7
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2answers
537 views

Navy's new Mach 7 Rail Gun Photo

There have been a few articles about the Navy's new Mach 7 33 Megajoule railgun. As a physics teacher, I have a couple of questions about this, and was hoping for some help. Is the kinetic energy ...
17
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5answers
2k views

Home experiments using wireless LAN or mobile phones about electromagnetism?

Are there any nice experiments using wireless LAN access points or routers or mobile phones to demonstrate physical features of electromagnetic fields, especially em-waves? More precisely I am ...
5
votes
2answers
1k views

What does an atom radiate: a wave packet or a single photon?

What does an atom radiate: a wave packet or a single photon?
2
votes
1answer
2k views

Current from induced emf

If the induced emf in a circuit is negative, and current from this emf is the emf over the resistance, what happens to the negative sign in the induced emf when solving for the current? Surely there's ...
6
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6answers
642 views

Interaction ranges in the Standard Model - Electrodynamics vs QCD

as you might know, the Standard Model of physics can be seen as a $U(1)\times SU(2)\times SU(3)$ gauge theory where each symmetry group accounts for different force fields. The behaviour for the ...
5
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4answers
3k views

Is there a travelling speed of for electric field? If yes, what is it?

Consider a conducting wire of 1M and 1000KM. Now if we connect a battery and a bulb to both these wires. Bulb glows instantaneously its because (my guess:) electric filed travels from positive ...
2
votes
1answer
470 views

What is the pressure between two electric dipole sheets of finite extent?

I have recently become curious about modeling the repulsion of everyday objects in contact with one another. By repulsion I mean as you attempt to walk through a wall, the pain in your nose suddenly ...
14
votes
2answers
2k 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 ...
1
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2answers
337 views

Creation of the Electromagnetic Spectrum [closed]

After seeing this image: http://mynasadata.larc.nasa.gov/images/EM_Spectrum3-new.jpg And reading this: "The long wavelength limit is the size of the universe itself, while it is thought that the ...
1
vote
2answers
468 views

Magnetism-Related Terminology

A few questions about a magnet and a paperclip: What do you call a material that attracts another material via magnetism? (i.e. the magnet) What do you call the material that is attracted in #1? ...
13
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4answers
802 views

Nonlinear optics as gauge theory

the widely used approach to nonlinear optics is a Taylor expansion of the dielectric displacement field $\mathbf{D} = \epsilon_0\cdot\mathbf{E} + \mathbf{P}$ in a Fourier representation of the ...
2
votes
3answers
975 views

Does the energy of a magnetic field decrease when it moves a conductor carrying a current?

When a charged particle moves in an electric field, the field performs work on the particle. Thus, the energy of the field decreases, turning into kinetic energy of the particle. Does the magnetic ...
2
votes
1answer
8k views

Is there a name for the derivative of current with respect to time, or the second derivative of charge with respect to time?

This measurement comes up a lot in my E&M class, in regards to inductance and inductors. Is there really no conventional term for this? If not, is there some historical reason for this omission? ...
11
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8answers
4k views

Is it possible to separate the poles of a magnet?

It might seem common sense that when we split a magnet we get 2 magnets with their own N-S poles. But somehow, I find it hard to accept this fact.(Which I now know is stated by Gauss's Law) I have ...
2
votes
2answers
153 views

Utility of displacements potentials in geophysics

In the elasticity theory, you can derive a wave equation from the fundamental equation of motion for an elastic linear homogeneous isotropic medium: $\rho \partial^2_t \overline{u} = \mu \nabla^2 ...
19
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2answers
4k views

Why do earphone pieces repel each other when music is on?

I know it has to do with electricity flowing and generating a magnetic field, but I would like a thorough explanation (with perhaps a picture). In particular: What is in the ear piece? Why do they ...
3
votes
2answers
389 views

Producing energy from magnetic flux loops

I've simplified this down by quite a lot but as far as I understand it, magnetic fields on the Sun's surface twist together and when it gets all a bit too much they release energy. My question is why ...
4
votes
2answers
374 views

Are gauge choices in electrodynamics really always possible?

If $B$ is magnetic field and $E$ electric Field, then $$B=\nabla\times A,$$ $$E= -\nabla V+\frac{\partial A}{\partial t}.$$ There is Gauge invariance for the trnasformation $$A'\rightarrow ...
1
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2answers
807 views

How is electrical energy transmitted?

My original question was in an effort to understand the electrical analogy to Markov chains, which is explained in Snell's article. There are some neat parallels that involve taking a Markov chain ...
7
votes
3answers
295 views

Satellite Power

Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. Since an orbiting satellite is passing through the Earth's magnetic field would a voltage be ...
6
votes
2answers
1k views

Decomposition of a vectorial field in free-curl and free-divergence fields

Is it always possible to do that decomposition? I'm asking it because Helmholtz theorem says a field on $\mathbb{R}^3$ that vanishes at infinity ($r\to \infty$) can be decomposed univocally into a ...
7
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2answers
666 views

Modified Maxwell's equations

In 1985, Harmuth wrote that Maxwell's equations are incompatible with causality, and overcame the problem by adding a term for magnetic dipole currents, and as a consequence the problem of infinite ...
6
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1answer
2k views

Wi-Fi in the presence of very strong magnets?

A friend of mine claims to have been able to surf the Internet without fuss on a Wi-Fi connection while performing NMR on samples he was analyzing. I would have thought the strong magnets needed for ...