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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Gradient of the potential originated from two similar magnetic vector potentials is not the same

The magnetic vector potential $\textbf{A}$ can be defined up to a gradient of a field. Adding or subtracting such gradient should not change the physics of the problem. The same reasoning is applied ...
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3answers
2k views

How does electromagnetic induction happen?

Electric and magnetic fields are different from each other(i think i am correct).. How does changing magnetic field induce electric current???
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0answers
112 views

Is the uniqueness theorem correct in superconductivity?

There is an uniqueness theorem in electromagnetism. It says that the solution of Maxwell's Equations is determined uniquely by boundary conditions. We can treat superconductivity as a completely ...
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1answer
1k views

Why the electric bulb turns on almost instantly when the switch is closed? [duplicate]

The electron drift speed is estimated to be very low.How could there is current almost the instant a circuit is closed?? By the discussions it is known that The information about beginning of the ...
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0answers
50 views

Absorption from a classical to quantum

Today, I learned that using rubidium atoms at very low temperature in a Magneto-optical trap, one can experimentally show that the Lorentz classical derivation of absorption using dipole is valid. The ...
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1answer
77 views

If an Electrical Field can jump over a point on its stright path of propagation?

Consider point B between points A and C on a stright line in vaccum(or any other environment). If the electrical fild $\vec E$ (or an EM wave) should necessarily pass through B to affect C and appear ...
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2answers
682 views

The relation between electric field and magnetic potential

In every electrodynamics book there is one chapter on special relativity which includes one section about" covariant formulation of electrodynamics" which uses tensor to describe the two fields and ...
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2answers
651 views

Deflection the needle of moving compass by magnetic field

I have a question about electromagnetism. Probably I understand something in a wrong way. So, I know that the drift velocity of electrons in conductors is very small ($\sim$ $0.1$-$1$ mm / s). Also I ...
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2answers
821 views

How does one show using QED that same/opposite electric charges repel/attract each other, respectively?

Why do same charges repel each other and opposite charges attract each other (please explain the phenomenon using real laws of nature (QED) not with the approximation model)?
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1answer
768 views

How does charge flowing between emf terminals reduce voltage difference?

I'm currently learning what electromotive force is and while reading my book's description of an ideal source of emf, I had difficulty understanding what these sentences mean: The nonelectrostatic ...
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0answers
77 views

EMF induced in a dynamo is by photons of permanent magnets?

i know little about the quantum field theory and also that the permanent magnets have there fields because of exchange of virtual photons across or around the ends. so when we take a dynamo and the ...
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2answers
223 views

Why 3 dipole terms in a multipole expansion?

As can be seen on this page http://en.wikipedia.org/wiki/Multipole_expansion when we take a multipole expansion without assuming azimuthal symmetry we end up with $2l+1$ coefficients for the $l^{th}$ ...
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0answers
93 views

why boundary condition in steady electric current?

when we electric field between two conductors in certain direction the current density should pass in its direction why current density direction change at boundary although the direction of electric ...
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0answers
78 views

Any quadrupole approximation? Any example?

In atomic and molecular physics we quite often encounter with electric dipole approximation. The dipole approximation we do when the wave-length of the type of electromagnetic radiation which induces, ...
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1answer
2k views

Magnetic field strength in a solenoid

I'm pretty new to physics. I've been conducting some experiments with electromagnets. My practical results don't match up with the theory. The magnetic field in a solenoid of length $L$ around an ...
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0answers
86 views

Boundary conditions for 2D helical waveguide

I'm interested in looking at standing wave solutions for the wave equation on a 2D annulus, with the twist that the annulus is "streched" in to a helix in 3D, but so that the rings themselves are ...
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2answers
2k views

What does the complex electric field show?

We have a complex electric field. Is there any definition for absolute and imaginary part of a complex electric field? What do they stand for?
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3answers
756 views

The moving capacitor

To what extent can a charged capacitor mounted on a moving platform (e.g. a rotating wheel) be considered an electric current generator? Electric current, after all, is nothing more than the transport ...
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3answers
837 views

Is classical electromagnetism a dead research field?

Is classical electromagnetism a dead research field? Are there any phenomena within classical electromagnetism that we have no explanation for?
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0answers
150 views

Variation of electric field and current flowing

Theoretically, a change in either electric or magnetic field will cause a current to flow , I am already familiar to Faraday's law of electromagnetic induction, so I tried to search about producing a ...
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1answer
1k views

Work done on a moving particle in electric field

This is one of the "fast answer" exercises I've been given to train (should be answerable in around 6-7 minutes). I can only think of a very long-round way to solve this. The question is as following: ...
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2answers
495 views

Relation between the spin of a particle and the polarization of it's wave

Is there any intrinsic relation between the spin of a particle, and the degree of freedom of it's polarization? does it holds for any particle-wave couple? like EM-photon, ...
4
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1answer
350 views

Coulomb interaction as virtual particles exchange?

I've been reading about virtual particle exchanges in physics books and in Physics SA posts, where a particle interpretation of gravity and Coulomb interaction is established. The Feynman Diagram ...
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1answer
287 views

EM force, blocking force carrier photons in a static electric field

I am doing some personal research in this specific area and wanted to ask something related to photons and EM force. are involved. Here is a thought experiment that doesn't add up to observed results, ...
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2answers
902 views

What is the physical process (if any) behind magnetic attraction?

I understand that the electromagnetic force can be described as the exchange of virtual photons. I also understand that it's possible for virtual photons, unlike their real counterparts, to have mass ...
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1answer
465 views

Gauge theory in classical electromagnetism

I understand gauge theory as the theory of continuous transformation group which keeps Lagrangian (or dynamics) invariant. So some integral invariants could be found. In terms of classical ...
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3answers
3k views

Basic question on magnetism regarding north and south pole

I am currently busy with some magnetism and quite shockingly (to me at least) I haven't yet read anything about the difference between the north pole and the south pole of a magnet. Before I started ...
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1answer
338 views

How do I find the electric field above the center of a square plate (rather than circular)?

I tried to integrate E due to a line of charge sweeping across the plate, but got bogged down. Any suggestions?
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1answer
200 views

Classically, how can an electron orbiting a proton radiate given its relativistic energy

In classical relativistic Electrodynamics, we are often told that any accelerating point charge inherently radiates (Bremstrallung). (This is the basis for the need for a QM conception of electrons.) ...
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1answer
325 views

Magnetic monopoles in spin ice and Dirac string comparison

In spin ice systems magnetic monopole-like excitations are sources or sinks of $H$, not the $B$ field, why is that? Is it because the strings carries magnetic moment $M$ and not solenoidal $B$ filed ...
4
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1answer
681 views

Galilean invariance of a subset of Maxwell equations

I read in Feynman's proof of Maxwell equations the statement that the subset of Maxwell equations comming from the Bianchi identity: $$ \nabla \cdot {\bf B} = 0, \quad \nabla \times {\bf E} + ...
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1answer
231 views

Magnetic induction in Relativity

As we know magnetic phenomenon is a mere relativistic effect.My question is how to explain the magnetic induction in a relativistic manner?
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1answer
50 views

Is there a heuristic argument for the expression $ \textbf{g} = \frac {\mathbf{S}}{c^2}$?

Electromagnetic momentum density and the Poynting vector are related by the simple expression: $$ \textbf{g} = \frac {\mathbf{S}}{c^2}$$ It can be rigorously derived from Maxwell's equations, but is ...
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1answer
735 views

Conservation of energy and ohms law in a step up transformer

In an ideal step up transformer with a constant resistance attached to the secondary coil, how is energy conserved and ohms law followed at the same time? $$\frac{V_p}{V_s}=\frac{N_p}{N_s}=\frac ...
3
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2answers
798 views

Paramagnet: Negative specific heat?

for a simple paramagnet ($N$ magnetic moments with values $-\mu m_i$ and $m_i = -s, ..., s$) in an external magnetic field $B$, I have computed the Gibbs partition function and thus the Gibbs free ...
3
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3answers
417 views

Magnetic paradox in relativity?

Let 2 electrons A and B be moving parallel with constant velocity $c/10$ in (near) vacuum without a strong gravity field (where $c$ is speed-of-light). A and B create an electromagnetic field that is ...
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1answer
169 views

What is the pressure of a gas required to ionize the gas using an electron gun?

How dense does a gas (Argon in particular ) have to be to in order to ionize it using electron bombardment and weak magnetic fields. Is there a correlation with the density of a gas and the easiness ...
3
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4answers
298 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
628 views

Why moving charges causes Magnetic Field (module and direction)?

Why an constant electric current in a wire produces a magnetic field, that circles that wire? I know that this question was posted before. However, all answers talk about Maxwell equations, axioms, ...
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1answer
221 views

Questions on electric field formed in conductor and the magnetice field it moves through

When a conductor (e.g. a wire) is moving through a uniform magnetic field, it will create a electric field in the wire as the magnetic field exerts a force on the moving conduction electrons in the ...
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3answers
7k views

conservation of energy in Lenz's law

When we bring opposite poles of two magnets together, they attract each other (or vice versa). Now, we can say that the kinetic energy gained by the magnets is due to the attractive force. Similarly, ...
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3answers
108 views

How can macroscopic $\bf D$ be related to microscopic $\bf E$ just by a constant $\epsilon$?

In electromagnetism, the electric displacement field $\bf D$ and magnetic field strength $\bf H$ are macroscopic functions defined in matter, and $\bf E$ and $\bf B$ are microscopic functions. How ...
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3answers
293 views

Dark matter is electrically neutral

I would like to know how come if dark matter was electrically charged it would reflect light. What are the equations or the logic behind it?
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0answers
342 views

Current Carrying Wire Loop near Current Carrying Long Wire

A rectangular wire loop of (width $a$ and height $b$) carrying clockwise current $I_1$ is a distance $d$ below a horizontal infinitely long wire carrying a current $I_2$ to the right. What is the ...
5
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2answers
1k views

Magnetic field lines

When I searched the net about magnetic field lines, Wikipedia told something about contour lines and that magnetic materials placed along a magnetic field has some specific loci, which i did not ...
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2answers
157 views

How do waves meet at a single point?

In principle two objects can never meet,because of electromagnetic repulsions for example if I touch something, I am not actually touching it considering the fact that there is a small region left due ...
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2answers
2k views

Theories that Relate Gravity, Electricity, and Magnetism [duplicate]

There are some people who (without having a stated theory that I know of) insist that Gravity, Electricity, and Magnetism are related. Some point to symmetry in Maxwell's Equations as a potential ...
2
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1answer
189 views

The Doppler effect in a medium like air (sound) versus the electromagnetic Doppler effect

When you have a listener and a source, and when one of the two move relative to the other, the frequency perceived by the listener will be different. Example: If the listener travels toward the ...
3
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2answers
511 views

Electromagnetic Momentum

My book says : The fact that electromagnetic radiation of energy carried momentum was known from classical theory and from the experiments of Nichols and Hull in 1903. This relation is also consistent ...
2
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1answer
82 views

Can a local magnetic field when changed introduce an electric field till infinity?

Here is a question from my school textbook: Here is the solution which they expect : Change in magnetic flux is $\pi a^2 B $, and since $\oint\vec{E}.\vec{dl} = -\frac{d\phi}{dt}$, the total ...