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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Maxwells Equation from Electromagnetic Lagrangian

In Heaviside-Lorentz units the Maxwell's equations are: $$\nabla \cdot \vec{E} = \rho $$ $$ \nabla \times \vec{B} - \frac{\partial \vec{E}}{\partial t} = \vec{J}$$ $$ \nabla \times \vec{E} + \frac{\...
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7answers
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What is Electromotive force (EMF)? How is it related to potential difference?

What is Electromotive force (EMF)? How is it related to the potential difference? Is it created by the potential difference in any conductor? Is it a process? Why is it called force? Does writing EMF ...
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2answers
503 views

Must the action be a Lorentz scalar?

Page 580, Chapter 12 in Jackson's 3rd edition text carries the statement: From the first postulate of special relativity the action integral must be a Lorentz scalar because the equations of ...
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3answers
663 views

Do objects have energy because of their charge?

My gut feeling tells me things should have energy because of their charge, like they have energy because of their mass. Is this possible? Has it been shown? If not then what is missing to make such ...
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3answers
426 views

How Does $\epsilon$ Relate to the Dampened Harmonic Motion of Electrons?

I realize that the permittivity $\epsilon$ of a substance is easily calculated based on diffraction angles, but I am not satisfied with merely measuring it experimentally. I wish to understand its ...
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1answer
4k views

Can microwaves affect WiFi?

I listen to the radio via my iPad with wifi. When I switch the microwave oven on, the radio cuts out. When the microwave oven is finished, the radio comes back on. (This is 100% reproducible!) So - ...
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7answers
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 ...
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2answers
683 views

What is the origin of the factor of $-1/4$ in the Maxwell Lagrangian?

I have seen numerous 'derivations' of the Maxwell Lagrangian, $$\mathcal{L} ~=~ -\frac{1}{4}F_{\mu \nu}F^{\mu \nu},$$ but every one has sneakily inserted a factor of $-1/4$ without explaining why. ...
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Mechanism by which electric and magnetic fields interrelate

I read that force due to electric field on some particle in one reference frame can exhibit itself as force due to magnetic field in some other reference frame and that electric and magnetic fields ...
9
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4answers
866 views

Do neutron stars reflect light?

The setup is very simple: you have a regular ($1.35$ to $2$ solar masses) evolved neutron star, and you shine plane electromagnetic waves on it with given $\lambda$. Very roughly, what shall be the ...
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4answers
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Displacement current - how to think of it

What is a good way to think of the displacement current? Maxwell imagined it as being movements in the aether, small changed of electric field producing magnetic field. I don't even understand that ...
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1answer
280 views

What is the action for an electromagnetic field if including magnetic charge

Recently, I try to write an action of an electromagnetic field with magnetic charge and quantize it. But it seems not as easy as it seems to be. Does anyone know anything or think of anything like ...
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925 views

What are the differences between the differential and integral forms of (e.g. Maxwell's) equations?

I would like to understand what has to be differential and integral form of the same function, for example the famous equations of James Clerk Maxwell: How to know where to apply each way? Excuse ...
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3answers
3k views

Do electric and magnetic lines of force physically exist?

As per my imagination any thing can't impose force on the other by not giving even a touch(i,e action at a distance). So I thought there must be some physical existence of lines of force. Although ...
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1k 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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5answers
48k views

Is aluminium magnetic?

From high school, I remember that Aluminium has 13 electrons and thus has an unpaired electron in the 3p shell. This should make Aluminium magnetic. However, the wiki page of Aluminium says its non-...
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382 views

Field created by varying Gravitational field

Changing Electric Field causes Magnetic filed and changing Magnetic Field causes Electric Field. Is there anything similar in relation to Gravitational Field? What sort of field is created by varying ...
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2answers
1k views

current in wire + special relativity = magnetism

Current in wire + moving charge next to wire creates magnetic force in the stationary reference frame OR electric force in the moving reference frame from special relativity due to change in charge ...
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1answer
766 views

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

Consider two circuits containing a battery, bulb, switch and conducting wires but of length 1 meter in one case and 1000KM in other. When switched on at the same time both the bulbs glow ...
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5answers
2k 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 ...
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1answer
1k views

Two electron beams exert different forces on each other depending on frame of reference?

I am sure there is a simple explanation for my confusion, but I am a little puzzled: We are dealing with two parallel electron cannons that each produces a straight beam of electrons. They are placed ...
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4answers
2k views

How to use Ampere's Law for a semi-infinite wire with current?

Suppose that there is a semi-infinite wire which extends to infinity only in one direction. There are no other circuit elements at the other end(finite end) of the wire and the current does not loop. ...
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602 views

Why do electrons and positrons exhibit opposite helical motion in a magnetic field?

When you throw an electron through a solenoid, it moves helically around the field lines, as per this schoolphysics illustration: © Keith Gibbs 2013 Then if we were to throw a positron through the ...
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4answers
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Method of calculating eddy currents of a conductor, is this correct?

I have a conductor with volume $V$, passing a magnetic field($B$) with velocity($v$): I'm trying to calculate the Eddy currents to figure out the magnitude of the drag force($F_d$) generated the ...
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5answers
5k views

What voltages are used to “safely” shock someone (as in a carnival game)

I've had this debate with some coworks. What voltage (rough order of magnitute) is used to safely shock people? "Safe" is a vague term, but as an example, there are arcade games where you hold onto ...
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Understanding the relationship between electricity and magnetism

I keep on hearing that magnetism is just another form of electricity and vice versa. If that's the case why can't we use magnets as batteries, and why aren't my batteries magnetic?
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2answers
421 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 A+\...
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2answers
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Does existence of magnetic monopole break covariant form of Maxwell’s equations for potentials?

Absence of magnetic charges is reflected in one of Maxwell's fundamental equations: $$\operatorname{div} \vec B = 0 \text{ (1).}$$ This equation allows us to introducte concept of vector potential: $$...
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1answer
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Does light induce an electric current in a conductor?

I know that electromagnetic waves induce electric currents in conductors and that's the basis for radio, wi-fi etc. I also know that light is also an electromagnetic wave. So, can light induce a ...
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5answers
1k views

Is the canonical momentum conserved when a particle moves in magnetic field?

Here is a question about the canonical momentum that I had asked some days ago, but I still have one point that I am not understand. Considering a particle moves in a magnetic field with charge $q$ ...
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2answers
518 views

What is the physical significance of the Dipole Transformation of Maxwell's Equations?

The Question Given Maxwell's equations of the form \begin{align} \bar{\nabla}\times \bar{B} = \dfrac{4\pi}{c} \bar{J} + \partial_0 \bar{E} \\ \bar{\nabla}\times \bar{E} = -\partial_0 \bar{B} \\ \bar{...
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111 views

Electric Field “at” the surface of a conductor

It has been pointed out to me that the Electric field exactly on the surface of the conductor is conventionally taken to be $E=\frac{\sigma}{2\epsilon_0}$; does this come from taking the midpoint of ...
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4answers
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The relation between Gauss's law and Coulomb law and why is it important that the electric field decrease proportionally to $\frac{1}{r^{2}}$?

My question relates to the third MIT's video lecture about Electricity and Magnetism, specifically from $21:18-22:00$ : http://youtu.be/XaaP1bWFjDA?t=21m18s I have watched the development of Gauss's ...
3
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1answer
222 views

CGS Units for Magnetism

Why does the formula for magnetic field force include the speed of light in the denominator in cgs units? Where does the extra $c$ go in SI units?
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1answer
338 views

How can radiation be a transverse wave? Does light really resemble a rope? How can a 3D field be a medium for non-spatial 1D waves? Need mental model

I understand longitudinal waves. For example, I've got a clear mental modal of air waves: a slice of air becomes overcompressed, then the slice next to it becomes overcompressed and the first slice ...
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1answer
2k views

Understanding the Seebeck effect

Thermoelectricity is, as I understand it, the difference in voltage between the hot and cold ends of two dissimilar materials. If two materials are connected at two different junctions, the hot ...
2
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2answers
567 views

How does the Lorentz force work?

How does the Lorentz force work? In the context of a homopolar motor. I am a year 13 student so please keep your answers simple! I am having trouble understanding the Lorentz force; does the magnetic ...
2
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1answer
526 views

Electrodynamics and the Lagrangian density

Could anyone tell me what equations can I obtain from the Lagrangian density $${\cal L}(\phi,\,\,\phi_{,i},\,\,A_i, \dot A_i,\,\,A_{i,j})~=~\frac{1}{2}|\dot A+\nabla\phi|^2-\frac{1}{2}|\nabla \times ...
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3answers
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How do I calculate the power consumed by a lightbulb?

I'm studying a lightbulb and its variable resistance, given by the expression: $R(T) = Ro[1 + α(T-T_0)]$, where $R_0$ is the resistance of the lamp at $T_0$. In this case, $R$ is not given by Ohm's ...
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1answer
6k views

what is the specific cause of permanent magnetism [duplicate]

Why can't we answer this simple question? Where does the magnetic field of a permanent magnet come from? What is different about a magnetizable atom that allows it? Why is it perpetual? Or is it ...
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4answers
824 views

Coulomb's Law in the presence of a strong gravitational field

I was under the impression that the $1/r^2$ falloff of various forces were because of the way the area of a expanding sphere scales. But that strict $1/r^2$ falloff would only be globally true in a ...
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2answers
718 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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3answers
1k views

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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1answer
514 views

If a magnetic monopole falls into a schwarzchild black hole, what happens to the magnetic field?

By the no-hair theorem, black holes can only have mass, charge and angular momentum. Does "charge" include "magnetic charge" (such as from a magnetic monopole)? Can black holes have magnetic charge ...
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3answers
567 views

Acceleration of electric charges and radiation

According to classical electromagnetic theory, accelerated charges should emit radiation and lose energy. The reason given in my book why atoms don't emit radiation (say, when the atom moves along a ...
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4answers
3k views

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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1answer
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What is the direction of the induced E field from a changing uniform magnetic field?

If a stationary charge is placed in a changing uniform magnetic field to measure the induced E field at some point, what would be the direction of E? I think E must equal zero normal to B to maintain ...
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2answers
1k views

Cancel out Earth's Magnetic field

Is there any tools except helmholtz coil to cancel out earth's magnetic field to calibrate magnetometers in practice.
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3answers
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formula for transparency of very thin film of metal

Is there formula that gives transparency of very thin film of given metal (tens of nanometers) to the visible light/light of given wavelength ? Which properties of metals are needed for the formula ? ...