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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29 views

Biological Signal Extraction Methods

EEG / MEG Back in 1929 Hans Berger recorded the first electrical potential variations in the brain from the scalp. Great advances have been made in this science since discover, with many scientists ...
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201 views

Magnetic Field of a Current Loop using Ampère's Law

A typical exercise while introducing the Biot-Savart Law is to calculate the magnetic field caused by a circular current loop at a point P located in its central axis, as shown in the following ...
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4answers
363 views

Do the integral forms of Maxwell's Equations have limited applicability because of retardation?

In the usual bookwork treatment, it is easy to show that the differential and integral forms of Maxwell's equations are equivalent using Gauss's and Stokes's theorems. I have always thought that ...
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2answers
77 views

What physical forces allows for electromagnetic induction

In electromagnetic induction, what force is actually doing the work? what physical force actually drives the electrons around the circuit? Let's say we have a coil and an increasing magnetic field ...
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2answers
159 views

Does the induced Electric field develop instantly or lags by $\frac rc$?

Consider a loop of radius $r_0=3 \times 10^8$ cm. A thin bar magnet is passed through its center. This implies that the magnetic flux through the loop will change. Now according to Faraday's law a non ...
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1answer
72 views

Apparent Violation of Energy Conservation in an Electromagnetic system

As shown in the figure a beam of EMW pulse is propagating towards a nearly point charge.Here if we calculate the total energy of the system by the equation mentioned in photo - we get that the total ...
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1answer
39 views

What does an induced emf do to a loop of varying current?

I know that the magnetic flux through a loop in space is proportional to the current such that $\Phi=LI$. Therefore a change in magnetic flux will produce an emf, so $\mathscr{E}=-L\dfrac{dI}{dt}$. ...
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1answer
102 views

Does the strength of a beam of light determine how much distance it can travel?

If I point a light torch to a cloudless, crystal clear and an absolutely pristine sky, will the light beam exit the planet and travel forever in the vacuum of space? (assuming it doesn't get perturbed ...
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2answers
312 views

Why magnetic monopole found in spin ice don't modify the Maxwell's Equations?

Magnetic monopole predicted by Dirac nearly a century ago was found in spin ice as quasi-particle(2). My question is Why magnetic monopole found in spin ice don't modify the Maxwell's Equations? (I ...
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1answer
72 views

Au, Ag nanoparticles plasmon peak position?

Is the interaction between metallic nanoparticles (~ 20 nm) and light in the UV-Vis-NIR range governed by Mie theory or by Rayleigh scattering theory? Where are the Au and Ag plasmonic peaks located? ...
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1answer
76 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
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0answers
58 views

How to derive De Haas–van Alphen effect?

I was reading Solid State Physics by Kittel and they manage to derive De Haas–van Alphen effect by invoking the Bohr-Sommerfeld model. This feels unsatisfactory to me. Can someone derive this using ...
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1answer
28 views

How do you show that the normal derivative of the magnetic vector potential is discontinous across a surface current?

This is from Griffith's "Introduction to Electrodynamics" 4th edition, problem 5.33. I need to show that $$ \tag 1 \frac{\partial \textbf{A}_{above}}{\partial n}-\frac{\partial ...
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1answer
62 views

Faraday's electromagnetic induction

Does the emf increase when no of turns of the same wire increased? Consider a wire of length 'l' is coiled with an area of cross section 'A' is subjected to a change in magnetic flux. an emf will be ...
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3answers
134 views

What are correlated magnetic moments?

My book has the following sentence and I don't understand what correlation or lack of correlation means: At high temperature the magnetic moments of adjacent atoms are uncorrelated (to maximize ...
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0answers
61 views

Why do we define the magnetic field around a conducting rod as concentric instead of radial?

Why is it that we define the magnetic field around a conducting rod as concentric instead of being radially outwards? Ok, we know from the Lorentz force law, that the magnetic force is the cross ...
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2answers
514 views

Analogy between fluid dynamics and electromagnetism

I've been reading a set of notes (Chapter 13 of Caltech Ph 136 notes from 2004, by Blandford and Thorne) that draw an analogy between fluid dynamics and electromagnetism, identifying the magnetic ...
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5answers
458 views

Gauss's law for induced electric and magnetic field

Let us consider an accelerating charge, $Q$. As it is accelerating it would radiate energy in the form of EM waves, as per the classical postulates of EM theory. As such there would be induced ...
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1answer
155 views

Derivative of the magnetic field to the vector potential

So the magnetic field is defined with the vector potential A as: $$\mathbf{B}=\nabla\times\mathbf{A}.$$ How would I calculate the derivative: $$\frac{\delta}{\delta\mathbf{A}}|\mathbf{B}|^2$$ I ...
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19 views

Is there an expression for field charge analogous to field mass?

In electrodynamics, it is possible to derive an expression for the field momentum for a given moving charge distribution (e.g. a sphere with uniform velocity) and from that infer an "electromagnetic ...
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194 views

Fourier Transform of ribbon's beam Electric Field

I have a monochromatic ribbon beam with $E(x)e^{i(kz-\omega t)}$ being the electric field's amplitude. I want to show that the lowest order approximation in terms of plane waves is ...
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0answers
19 views

Electrical currents

I have a luminous electric plasma light that looks similar to a lava light only electric currents are inside instead of lava.This light is filled with electric current flowing throughout it. if I ...
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0answers
101 views

Free charge density and current in an ohmic material

I have come across what seems a paradox -or at least an exotic conclusion- regarding current conduction in an ohmic material. It is well known that free charge density can only be zero on an ohmic ...
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5answers
355 views

Field inside a wire?

This answer gives a great explanation of why the field inside a wire connected to a battery must be equal at all points: Why doesn't the electric field inside a wire in a circuit fall off with ...
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4answers
137 views

Faraday's Law - recursive?

So we know that the EMF is induced by change of flux. The thing that was always confusing me is the following: we start changing the magnetic field which in turn induces electric field which makes ...
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0answers
240 views

Ed Leedskalnin's “Perpetual Motion Holder” (PMH)

Edward Leedskalnin (Coral Castle) describes a perpetual motion holder in his writings. I've built one and pretty much know how it works (I think). It is NOT a perpetual motion device by any means, ...
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1answer
86 views

Understanding the relationship between electric energy and force

I'm trying to do the following problem: One of the three types of radioactive decay is "β decay", during which protons decay into neutrons or vice­versa, emitting either electrons (β­) or positrons ...
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22 views

electron trajectory in Bfield with energy kick at midpoint

So I wanted to get some second opinions on this situation I have thinking about.Lets say an electron enters, normally, a section of bfield that is perpendicular to its motion such that the electron ...
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2answers
129 views

Why can't I connection the anode of a battery to the cathode of an other battery and get a current flowing? [duplicate]

If I touch the anode of a battery or connect the anode the the cathode of an other battery, none (or rather: a very small current) flows between those two. If I instead connect the connect the anode ...
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1answer
74 views

I need help with divergence and gradient? [closed]

$$A_z = \mu{\frac{e^{-jBr}}{4\pi r}}∫I(z')e^{jBz'\cos\theta}dz'$$ Midway into my question, I want to compute: $$-j\left( \frac{\nabla(\nabla\cdot A) }{w\mu\varepsilon} \right).$$ Symbols like $ w, ...
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1answer
241 views

Does magnetic monopole violate $U(1)$ gauge symmetry?

Does a magnetic monopole violate $U(1)$ gauge symmetry? In what sense and why? Insofar as I know, there are at least two types of magnetic monopoles. One is the Dirac monopole while the other is the ...
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2answers
146 views

Why does freshwater attenuate signals less than seawater?

One of the most interesting problem in underwater communication is how deep the device (i.e. submarine) could go to still be able to communicate with on-shore data centers. The attenuation in a ...
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1answer
60 views

potential inside a cylindrical shell in terms of the surface potential?

Given a potential distribution $V(\phi)$ at the surface of an infinite cylindrical shell, is there an easy way to derive the potential inside the cylinder. No charges or currents anywhere.
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Why no longitudinal electromagnetic waves?

According to wikipedia and other sources, there are no longitudinal electromagnetic waves in free space. I'm wondering why not. Consider an oscillating charged particle as a source of EM waves. Say ...
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3answers
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How does a magnetic field cause objects to move?

I understand electricity and magnetism and the relationship between them. What I never understood, and never actually seen explained in any book or website, is how opposite magnetic fields cause ...
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0answers
32 views

can we reuse the motion of a dc motor to power an ac generator

My question is ,when we make a simple dc motor, cant we connect another circuit to the rotating coil and make another circuit for an ac generator
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1answer
5k views

Violation of Newton's 3rd law and momentum conservation [closed]

Why and when does Newton's 3rd law violate in relativistic mechanics? Check this link.
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1answer
56 views

Force acting on a polar molecule

What kind of force does act on a polar molecule? I mean, there must be a force that is acting on the molecule to keep the negative and positive centre away ( like in H2O). Why the positive and ...
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1answer
67 views

Electromagnetism and static electricity

What is the difference between electromagnetism and static electricity? Also electromagnetic waves are mediated by photons , what mediates static electricity?
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3answers
520 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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0answers
39 views

Earth's magnetic field and the equator

Suppose earth is like a bar magnet with the poles acting as N and S. If a charged particle from outer space heads towards the equator, why does it get travel around the field instead of the magnetic ...
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2answers
237 views

Why does light travel as waves? [duplicate]

Why does light travel as waves instead of say just a straight line? What are the forces that make a light photon travel in a wavelike pattern?
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2answers
187 views

Predicting Polarity of capacitor in the given diagram

I have to determine the polarity of the Capacitor in the given diagram. I Approached the problem as follows: When Magnet 1 moves with its North pole towards the coil, emf is induced in the coil as ...
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200 views

ratio between conduction current and displacement current

First, recall that Maxwell displacement current for a plane wave is $$ \vec j_D = \epsilon \partial_t \vec E = \epsilon \partial_t (\vec E_o cos(\vec k \cdot \vec r - \omega t)) = \epsilon \omega ...
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0answers
51 views

Time-dependent electric and magnetic field between two point charges

I would like to get expressions for the time-dependent electric and magnetic fields in the region around two moving point particles. I can't see how this is possible. Clearly you can't use Gauss' Law ...
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0answers
45 views

What are some open problems in classical electrodynamics? [duplicate]

I am about to finish reading 'Introduction to Electrodynamics' by David Griffiths. Throughout the textbook, Griffiths makes frequent references to current literature (mostly articles from American ...
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1answer
183 views

Current distribution on a dipole antenna

It is known that the current distribution on a thin dipole antenna is always sinusoidal: http://www.antenna-theory.com/antennas/dipole.php How can I prove this?
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2answers
122 views

How is the electromagnetic Lagrangian derived?

I've been studying from the book called "path integral formulation" by Feynman and Hibbs. In chapter 4, problem 4.2, they refer to the electromagnetic Lagrangian as: $$ L=\frac{1}{2} m \dot{x}^2+ ...
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2answers
490 views

Why doesn't my mobile phone pick up the electro-magnetic wave emitted by my water heater?

I admit my knowledge of physics is very little, but I did study topics on EM and stuff in school. I was reading about computer networks and I read about antennas and how they are just wires or strips ...