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Questions tagged [electromagnetism]

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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4k views

Action principle for the Electromagnetism and Gravity

Here is the formula for the stress energy tensor: $$ T_{\mu\nu} = - {2\over\sqrt{ |\det g| }}{\delta S_{EM}\over \delta g^{\mu\nu}} $$ (This follows from varying the total action $S ...
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What is reason for electronic compass calibration?

Most GPS receivers and smart phones contain an "electronic compass", which I understand is generally a Hall effect magnetometer. These devices generally require "calibration", which involves waving ...
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2answers
556 views

Determining the capacitance of a system with a non-homogenous dielectric

In an experiment we were given non-homogenous dielectric substances described by functions of coordinate. How can capacitance be determined from this?
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2answers
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Two charged black holes in equilibrium

Consider a pair of (possibly rotating) charged black holes with masses m1, m2 and like charges q1, q2. It seems that under certain conditions gravitational attraction should exactly cancel ...
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1answer
80 views

Rock chips showing magnetic property?

If i am placing two rock(stone) chips closer with a wider area of contact and rotating another rock stone around these two chips(having close contact) for some time (around 30 sec) and try to separate ...
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2answers
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Drum head coupled with an RLC circuit

I'm thinking about the possibility of an electroacoustic drum that would be tuned with some kind of RLC circuit. The drum head would have an electromagnet attached to its center, which would be ...
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2answers
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What is the force between two magnetic dipoles?

What is the force between two magnetic dipoles? If I have two current loops parallel to each other with currents $I_1$ and $I_2$ and radii $R_1$ and $R_2$ a distance $z$ from each other, what is the ...
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1answer
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Finding induced current in a loop at an instant [closed]

I'm working on this problem; however, I cannot seem to get anywhere. Given information: The rectangular loop in the figure has 2.1x10^-2 ohm resistance. What is the induced current in the loop at ...
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6answers
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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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2answers
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Why doesn't this perpetual motion machine using magnets work?

See image: If I'm not totally dumb the car would move when the right magnet would not be attached to the car, at same distance of the booth magnets. So why would it not move?
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2answers
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Does a nonzero Poynting vector mean that there is propagation of energy?

I don't know how this "paradox" can be solved. I'm given the following system: A permanent magnet with a magnetic field given by ($\hat{a}$ are unit vectors in the x and y directions) $$\vec{H}=H_0\...
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1answer
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Is there something like the Poynting vector for hydraulic circuits?

The Poynting vector is a representation of the energy flux in electromagnetics, showing the amount and direction of power flow at different points in space. In electric circuits, the energy is not ...
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What is the physical meaning of the terms in the multipole expansion?

I have a few questions on multipole expansions and I have read about the topic in many places but could not find an answer to my questions, so please be patient with me. The electrostatic potential ...
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Canonical momentum in different coordinate system

The canonical momentum is defined as $p_{i} = \frac {\partial L}{\partial \dot{q_{i}}} $, where $L$ is the Lagrangian. So actually how does $p_{i}$ transform in one coordinate system $\textbf{q}$ to ...
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1answer
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Is it possible to check from the EM waves(basically Light rays)?

Is it possible to check from the EM waves(basically Light rays), if there are any extraterrestrial objects interference/passage during its travel from the Sun? I mean, can these EMs from the Sun be ...
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3answers
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Lorentz force in Dirac theory and its classical limit

It is well known that in Dirac theory the time derivative of $P_i=p_i+A_i$ operator (where $p_i=∂/∂_i$, $A_i$ - EM field vector potential) is an analogue of the Lorentz force: $\frac{dP_i}{dt} = e(...
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2answers
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Electromagnetic Radiation

If there was a way to see the electromagnetic waves that is all around us, i.e. see the electric and magnetic fields spreading away from an antenna say, how would it look? I mean if we were to freeze ...
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0answers
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How to set the electric potential on a certain boundary to a certain value experimentally?

In electrostatic boundary value problems, usually the potential is set to some function or a constant on a certain boundary. It is never taught or told in textbooks how such assignment might be ...
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1answer
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Hole in the disc shaped magnet

Let's suppose we have a magnetic disc. One half is magnetic north pole and the other half is magnetic south pole. Now suppose we drill a hole in the disc. Does anybody has an idea how the magnetic ...
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2answers
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Why does it seem like a broken magnet's poles flip?

I just took a rare earth magnet out of an old hard drive. Lacking an appropriate screwdriver, force was used, and the magnet broke into two pieces; one about a quarter of the original size and one ...
2
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2answers
782 views

Gauss' law giving zero field where field is not zero?

Two plastic sheets with charged densities as shown: I'm trying to find the field at $B$. I obtained the correct answer by adding up the fields created by each charge density. But I realized that ...
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4answers
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Potential energy of a charged ring [closed]

Consider a ring of radius $R$, and charge density $\rho$. What will be the potential energy of the ring in its self field? The best I can do: $$dq = \rho R \cdot \, d \alpha $$ $$E_p = 2 \pi R \cdot ...
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1answer
864 views

Poisson's Equation

First up, I needed to compute the potential at some external point due to charge stuffed inside the region between two concentric cylinders, the volume charge density being given. Two methods came to ...
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4answers
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What is / are the primary criticism(s) against Einstein-Cartan-Evans field theory?

What is / are the primary criticism(s) against Einstein-Cartan-Evans (ECE) field theory? On Wikipedia the references provided were: arXiv:physics/0607186, MR2372785 (2008j:83049b), MR2218579 (2007a:...
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1answer
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Numeric method to calculate the charge distribution on a conducting surface?

If I have an arbitrary (closed?) conducting surface and a nearby charge density, is there a simple numeric way of computing the induced charge distribution on the surface?
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What is the minimum wavelength of electromagnetic radiation?

As a first approximation, I don't see how a wavelength of less than 2 Planck distances could exist. The question is: Are there any other limits that would come into play before that? For example: ...
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1answer
843 views

Gauge invariance and Feynman path-integrals

Let me look at the Hamiltonian of a charged particle in a plane in a constant magnetic field ($\vec{B}$) pointing upwards - then in usual notation it is, $$\hat{H} = \frac{1}{2m}\biggl(\hat{p} + \...
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5answers
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Work done by the Magnetic Force

The magnetic part of the Lorentz force acts perpendicular to the charge's velocity, and consequently does zero work on it. Can we extrapolate this statement to say that such a nature of the force ...
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2answers
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Pull Force of an electromagnet

How do I calculate the pull force of a cylindrical electromagnet to Iron as a function of distance? Is there any difference betweeen magnets and electromagnets?
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1answer
218 views

What determine whether the dynamical equations are tensor equations or vector equations?

Newton's 2nd law which is central to Newtonian dynamics, is a vector equation $\sum\textbf{F}_{external}=m\textbf{a}$ Same with Maxwell's equations in the covariant form. On the other hand, ...
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Why is the Principle of Superposition true in EM? Does it hold more generally?

In the theory of electromagnetism (EM), why is the principle of superposition true? Can we read it off from Maxwell's equations directly? Does it have any limit of applicability or is it a ...
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What´s the electric field in the entire space generated by a uniform (but not constant) magnetic field?

Let me explain in details. Consider a region in space with no free charges and no free currents, so that the charge density is $\rho=0$ and the volumetric current is $\vec{J}=\vec{0} \text{ }$ in the ...
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3answers
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Can Laplace's equation be solved using Fourier transform instead of Fourier series?

Sorry for the long text, but I am unable to make my question more compact. Any periodic function can be Fourier expanded. Usually, they say in mathematical physics books, if the function is not ...
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2answers
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If electric charges accelerate towards lower potential energies, why do opposite charges attract?

I know my logic must be wrong but I can't figure out why. I know that charges must accelerate towards lower potential energies simply because that's a general rule of nature. However, when you release ...
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2answers
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How is stable levitation possible?

This question is with reference to the video in this blog post: http://www.universetoday.com/90183/quantum-levitation-and-the-superconductor/ My question is the following: how is the disc stable in ...
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1answer
968 views

Induction and Halbach Array

I have recently studied and tried to understand the concept of the Inductrack ( http://en.wikipedia.org/wiki/Inductrack ), a form a magnetic levitation. In the following link they present three ...
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Motivation for Potentials

This is a hypothetical question about "pedagogy". Let's say I am trying to take someone who has just a very small amount of knowledge about Newtonian mechanics and convince them that the Lagrangian ...
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3answers
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Do moving charged particles have both magnetic and electric fields?

Consider a charged particle (electron or proton) at rest. It is surrounded by its own electric field. Now consider an electron moving with certain velocity. Is there still an electric field around it?...
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2answers
287 views

Magnetic Fields

If running a current through a magnet can reverse the magnetic field, then how strong does the current have to be and how would it best be run through the magnet.
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2answers
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How does quantum trapping with diamagnets work?

I just saw this demonstration by someone from a Tel Aviv University lab. What they achieved there is mind blowing. I myself own a levitron that uses the Hall effect to levitate a magnet, the problem ...
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1answer
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How may the magnetic resistance for a magnet moving through a copper pipe be calculated?

When a magnet moves past non-magnetic, electrically conducting material such as copper, the changing magnetic field induces electric eddy currents in the copper, which create their own magnetic field ...
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2answers
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How can a conductor be grounded yet there are induced charges on it?

A classic example for the method of images is the following, quoted from Griffiths's Introduction to Electrodynamics, page 121: Suppose a point charge $q$ is held a distance $d$ above an infinite ...
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Why is the energy density of the electric field produced by two unlike charges positive, even though their potential energy is negative?

Consider two stationary charges, one positive the other negative. Their potential energy is clearly negative. So you would expect that the energy density of the associated electric field would also ...
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1answer
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Would a strong diamagnetic material exhibit inductance?

Do metals with the property of strong diamagnetism also exhibit inductance? Would a fluctuating magnetic field induce a weaker current in a strongly diamagnetic metal as compared against a ...
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3answers
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Why does light in a room not form constructive and destructive interference patterns?

This is something that I have wondered for a long time. How come when I walk around why do I not see random black spots where light has collided destructively and bright spots where it has collided ...
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1answer
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infinite parallel conducting planes

two infinite parallel conducting planes grounded and are separated by a distance d. place a point charge "q" between the two planes, using the "green teoerma reciprocity" how I show that the total ...
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2answers
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How strong of magnetic field would noticibly attract a person?

There is iron in our blood, which is magnetic. Roughly how strong would a magnet have to be to induce a noticeable attraction? It would be nice to know this for several distances. Also, do ...
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1answer
159 views

Quick question concerning the Heisenberg model

I got a small, rather technical question concerning the Heisenberg model. (It is technical indeed.) Consider the Heisenberg Hamiltonian: $H = \sum_{(i,j)} S_{i} S_{j}$ = $- \frac{J}{V} \sum_{q} \...
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Charge on Sphere due to Contact with Capacitor

Question: Consider a parallel plate capacitor which is connected to potential difference $V$. Let there be a small spherical conductor, assume that its radius is much smaller than the distance ...
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2answers
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Magnets arranged in a sphere

If I was to take a bunch of magnets and arrange them in a sphere (And keep them there with glue or plastic or something) so that the north pole faces the outside of the sphere and the south pole faces ...