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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1answer
112 views

Question about complex emissivity and complex permeability

Consider a linear, isotropic material. We can then write: $$\textbf{D}(\textbf{r},\omega) = \epsilon(\textbf{r},\omega)\textbf{E}(\textbf{r},\omega)$$ In phasor notation. The complex emissivity ...
2
votes
1answer
129 views

If Electricity and magnetism are the same viewed from a different reference frame then is electricity medium dependant?

If Electricity and magnetism are the same viewed from a different reference frame (they are the same force as unified by maxwell) then is electricity medium dependant? I came to this question when ...
2
votes
1answer
2k views

Force between two bar magnets

I need to know how to calculate force between two bar magnets. I searched and found an answer on Wikipedia, but I don't really trust Wikipedia, a lot of crazy stuff is posted there. So I searched ...
2
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1answer
270 views

Why are lab Magnets painted red?

Wy are lab magnets painted red in color? I tried searching everywhere but couldn't get a satisfactory answer.
0
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0answers
25 views

Magnetic field distrotion

Does one exist, or do you think it would be possible to design a tracking magnetic field distortion detector to detect fast moving magnetic anomalies in our air space. ie; track a stealth aircraft or ...
7
votes
2answers
246 views

Time dilation only on electromagnetic force?

We've seen by experiment that the speed of light c appears to be constant for each observer (leading to all well-known consequences of relativity). I'm wondering if this appearance of constancy of c ...
0
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0answers
83 views

Energy required to demagnetize a soft iron?

How much energy is required to demagnetize a soft iron ferromagnetic material, that has a very low coercive force And a small hysteresis area? Also, would it be possible to demagnetize that same ...
-1
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2answers
59 views

Electron motion in a wire

For my introductory course to electromagnetism (I'm an undergraduate student, so ELI5), I'm trying to get the right conceptual model of electron movement in a thin wire (with constant but non-zero ...
3
votes
2answers
449 views

Energy density in Magnetic fields?

I don't understand what does magnetic field energy density stand for? The possible work that the magnetic field can do? So we can calculate the amount of "work" it can do just from the energy density? ...
1
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0answers
66 views

Can I transfer electricity through induction?

If I place a solenoid connected to a bulb inside a bigger solenoid which is connected to a power source, will the bulb glow?
7
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1answer
675 views

Can a free particle absorb/emit photons?

As simple as in the title.. I would like to know also some mathematics about it!
3
votes
1answer
5k views

When does voltage drop occur?

Why or when does it occur in a circuit? What does it imply when you speak of a voltage drop across a resistor? (Obviously, it probably means that the current's voltage before the resistor is higher ...
10
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6answers
2k views

Why don't waves erase out each other when looking onto a wall?

If I stand exactly in front of a colorful wall, I imagine the light waves they emit, and they receive should randomly double or erase out each other. So as a result, I imagine I should see a weird ...
2
votes
2answers
82 views

Are there any known, non-spectroscopic effects of EM directly on Light?

Photons have no charge. Light is a form of electromagnetic energy. All spectroscopic effects (to my knowledge) are due to changes in electron state, induced either through an interior or exterior EM ...
1
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0answers
54 views

Benefits of the sign convention for electrons?

Benjamin Franklin considered electrons to be positive, but J.J. Thompson considered them negative. We obviously went with J.J. Thompson's convention. Why? What were the benefits of moving to J.J. ...
4
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5answers
1k views

Relativistic origin of magnetic field

There is an explanation in the Wikipedia. Unfortunately the article is quite verbose and doesn't clearly explain why both positive and negative charges vary density even if only one is moving. It is ...
3
votes
0answers
102 views

Do primary first class constraints change the electric field in the Hamiltonian form of Maxwell's theory?

In my understanding of Dirac's theory of constrained Hamiltonians, the primary (and also the secondary) first class constraints are generators of canonical transformations that do not change the ...
1
vote
1answer
132 views

Show that the plane of incidence is perpendicular to the surface of reflection

Is it possible to derive from the boundary conditions of the Maxwell equations for E and H, that the plane of incidence for an EM wave is perpendicular to the reflection surface? How? If not, what ...
0
votes
1answer
259 views

Very short ($\ell \ll \lambda$) dipole as a receiving antenna

I'm trying to understand how a very short dipole of length $\ell \ll \lambda$ works for receiving radiation. (It is center-fed, and has two thin perfectly conducting arms each of length $\ell/2$ ...
4
votes
4answers
225 views

what are the practical use of LC oscillation?

I get the energy transfer between capacitor and an inductor in a LC circuit but I am not sure what are the practical application of it.? Can someone pls help.?
0
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0answers
89 views

What is the electrical field of a capacitor plate as a charge approaches

The electrical field for a charge density $\varrho(r)$ is $${\bf E}({\bf r})=\frac{1}{4\pi\varepsilon_0}\int \varrho({\bf r}')\frac{{\bf r}-{\bf r}'}{\hspace{.1cm }|{\bf r}-{\bf r}'|^3} \mathrm ...
2
votes
1answer
425 views

Tracelessness of energy-momentum tensor and massless photons

I have read the statement that the tracelessness of the energy-momentum tensor is demanded by the condition of photons being massless. I see how this comes about starting from the canonical ...
2
votes
2answers
3k views

Non-conservative electric fields due to changing magnetic flux?

What I read in several places, tells me that, the fact Coulomb's Law follows inverse-square law and gives a force which is radial, implies that a static electric field must be conservative.(In short, ...
3
votes
2answers
379 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 ...
5
votes
3answers
342 views

Are free electrons in a metal really free

Is it an accurate statement to say that free electrons in a metal experience NO restoring force when they interact with electromagnetic waves? I understand that the electrons exist in a space filled ...
1
vote
0answers
78 views

The curl is not zero?

In Maxwell's equations, the curl of the electric field for a steady state processes (ie. No changing electric or magnetic field) zero. However, if we take a curl along the wire for some distance and ...
1
vote
1answer
64 views

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 ...
0
votes
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???
6
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0answers
108 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 ...
1
vote
1answer
807 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 ...
1
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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 ...
2
votes
1answer
76 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 ...
4
votes
2answers
500 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 ...
3
votes
2answers
574 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 ...
6
votes
2answers
753 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)?
3
votes
1answer
658 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 ...
1
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0answers
76 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 ...
3
votes
2answers
208 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 $l^{th}$ ...
0
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0answers
82 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 ...
1
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0answers
72 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, ...
0
votes
1answer
95 views

Does a battery in a vacuum produce a current?

I've always assumed that the strong resistance of air is the reason there is no flow of electrons between the terminals of a battery until a wire is connect. However, in a vacuum there is no resistive ...
3
votes
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 ...
0
votes
0answers
82 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 ...
1
vote
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?
3
votes
3answers
653 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 ...
12
votes
3answers
664 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?
0
votes
0answers
139 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 ...
0
votes
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: ...
2
votes
2answers
426 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
votes
1answer
317 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 ...