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
108 views

Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...
3
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2answers
162 views

Looking for lists of student difficulties with electromagnetism, circuits (DC and AC)

I'm trying to help with an Instructor's Guide for an introductory calc-based physics text, and it would really help if I could locate some lists of typical student difficulties with topics like ... ...
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6answers
2k views

Why Does Light Not Become Polarized In A Magnetic and/or Electric Field?

I am familiar with the Faraday Effect, but I remain confused as to why the electric and/or magnetic components of light do not naturally align themselves with a magnetic or electric field (in a ...
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1answer
29 views

Derivation of Jefimenko's Equation in Jackson's EMT book

I have been trying to understand the derivation of Jefimenko's equation in Jackson on p.246-247 which can be seen in the photographs attached. First of all I did not fully comprehend the ...
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0answers
14 views

Do/May pure electromagnetic waves ( Non dual natured ) exist?

The light as well as all matter 'particles' show dual nature. But my question is if according to the Maxwell equations, Electromagnetic waves are valid events, can they actually exist in pure wave ...
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1answer
187 views

Remote EEG detection / reading

I recently noted that Phillips, Accenture, and Emotive grouped together to create a proof of concept that would assist ALS patients to gain greater control of their lives through brain, voice, and eye ...
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0answers
13 views

Why doesn't current induced by changes in flux affect the flux, while current induced by a battery does?

Faraday's law states $V=-\frac{\mathrm{d}F}{\mathrm{d}t}$, where $F$ is flux. Since $V=IR$ for passive circuits, $I = V/R = -\frac{1}{R}\frac{\mathrm{d}F}{\mathrm{d}t}.$ But by the Biot-Savart law, ...
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0answers
5 views

How does the phase of an alternating current change when flowing through a multi-layered material?

I have questions regarding the properties of the alternating electrical current in the following imaginary experiment: There is an AC (sinusoidal wave with frequency $f = 5\ \mathrm{Hz}$). I would ...
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1answer
104 views

Ampère's law and superconductors

I have been reading about superconductors and emerge me a inquietude about the explanation of existence of currents inside the superconductor while their magnetic field (inside too) is zero. Based in ...
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1answer
186 views

Surely force on shell can't be balanced by field momentum?

Imagine a particle with charge $q$ at rest at the origin. It is surrounded by a concentric spherical insulating shell, also at rest, with charge $Q$ and radius $R$. At time $t=0$ I apply a constant ...
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0answers
39 views

Flux Linkage Inside of a Conductor

Can someone explain to me why the flux linkage inside of a conductor is dependent on the cross sectional area of the conductor? My book says that d$\lambda$ = $\frac{x^2}{r^2}$$\phi$. Where $\phi$ ...
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1answer
291 views

Question about superconductivity

A long cylinder of radius $R$ is made from two different material. Its radius $r<r_0$ $(r_0<R)$ part is a material with superconducting transition temperature $T_1$, and its $r_0<r<R$ ...
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1answer
31 views

Aharonov-Bohm experiment: A possible way to understand potentials?

I'm posting this question as a function of an another question I found here: What is potential energy truly? The answer of WetSavannaAnimal aka Rod Vance. As I understand the hole point is to see ...
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2answers
606 views

How do magnets work?

I've read a classbook on the field theory (including EM): it perfectly describes quantitive patterns in EM-theory, but I have no luck understanding how and why it works. I mean, magnetic substances ...
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1answer
260 views

Force on a coil inside of a voice coil

as a homework exercise we were given the following question: Given the following voice coil: With depth = 20 mm Permanent magnets with B = 1.23 T, Ur = 1.1 The coils has 20 turns, I = 1 Ampere. a) ...
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3answers
7k views

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 ...
1
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1answer
75 views

electromagnetic induction and magnetic shielding

In the figure I have a circular conducting wire. Somehow, in the middle circular region I have a magnetic field (this means the magnetic field is shielded in this region and it is possible from what ...
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0answers
25 views

Hydrogen atom Ionization by Magnetic Field [on hold]

The source of a magnetic field (it could be a magnetic dipole) is moving at relativistic speed. This magnetic field encounters a hydrogen atom at rest with respect to the source. Will this encounter ...
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1answer
62 views

Radiation pressure (Jackson exercise)

Here's an exercise from Jackson: A plane wave is incident normally on a perfectly absorbing flat screen. From the law of conservation of linear momentum show that the pressure exerted from the ...
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1answer
34 views

Effect of a magnetic field on a current carrying wire

When a magnetic field is applied perpendicular to a current carrying wire, a Lorentz force $$\mathrm{\vec{F}=i(\vec{L}\times\vec{B})}$$ acts on the conducting wire. Of course the applied magnetic ...
2
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1answer
72 views

Proof that 4-potential exists from Gauss-Faraday field equation

This is a problem concerning covariant formulation of electromagnetism. Given $$\partial_{[\alpha} F_{\beta\gamma]}~=~ 0 $$ how does one prove that $F$ can be obtained from a 4-potential $A$ such ...
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1answer
40 views

Show that a solenoidal field is always a curl of a vector field

Can someone prove that: $$\nabla \cdot \mathbf{B} = 0 \implies \mathbf{B} = \nabla \times \mathbf{A}~?$$ I know that $$\nabla \cdot (\nabla \times \mathbf{A}) = 0$$ identically. But can one prove ...
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1answer
23 views

An equation for any charge distribution in a constant external electric field. Is it correct?

Is the equation (torque, $\tau = p \times E$) correct for any kind of charge distribution? (E is an external constant electric field.) If so, why? Will somebody prove it to me? I'm only sure of its ...
2
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1answer
45 views

Time changing potential gives rise to “force”?

Imagine a charged particle inside a Faraday cage (i.e. charge on outside, zero electric field inside, but non-zero electric potential on the inside). Suppose the charge distributed on the outside of ...
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0answers
79 views

Was Nikola Tesla right about his ether theory?

There are a lot of articles on the net that mention Nikola Tesla's ether (aether) theories (like this). Was there any truth to this idea of his ? Why was it rejected by the scientific community ? Did ...
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1answer
20 views

Poynting vector from 1st term in Lienard-Wiechert field

I start with 1st (non-radiative) term from Lienard-Wiechert fields: $$ \vec{E} = q (1-v^2) \frac{\vec{R_{t'}} - \vec{v}R_{t'}}{(R_{t'} - \vec{v}\vec{R_{t'}})^3} $$ $$ \vec{H} = - q (1-v^2) ...
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0answers
35 views

From Liénard-Wiechert to Feynman potential expression

When studying the potential of an uniformly moving charge in vacuum, Feynman proposes to apply a Lorentz transformation on the Coulomb potential, which reads in the rest frame $ \phi'(\mathbf r',t') ...
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3answers
141 views

What is supposition of equilibrium? How do Rayleigh, Jean know the electromagnetic wave in equilibrium behave?

In a cavity of dimension L, the wave must give zero amplitude at the wall, means wave equation has zero amplitude. Why? Answer from hyperphysics "since a non-zero value would dissipate energy and ...
37
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4answers
7k views

Why doesn't light, which travels faster than sound, produce a sonic boom?

I know that when an object exceeds the speed of sound ($340$ m/s) a sonic boom is produced. Light which travels at $300,000,000$ m/s, much more than the speed of sound but doesn't produce a sonic ...
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2answers
4k views

Can a camera flash light near high voltage wires create medium to pass electricity?

I found a story of a boy who took a picture with flash light near 40,000 voltage lines. When it flashed, current came to his camera (as a spark may be) and passed through his body making him dead. Is ...
2
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2answers
114 views

Far Field Diffraction of EM waves: what does the zero frequency signify?

If you have a system of independently radiating electrons/point-charges, the far field distribution of the EM waves can be approximated by the fraunhoffer diffraction integral, or simply by the ...
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0answers
12 views

Electrodynamic near fields around black holes

For the standard Schwarzschild black hole, the temperature of Hawking radiation is simply related to the Schwarzschild radius as $kT = \hbar c/(4\pi r_S)$, meaning that the typical wavelength of ...
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0answers
27 views

Does the Lorentz force law explain Flemings left hand motor rule and the right hand dynamo rule?

the Lorentz force on a charged particle $F=qv \times B$ can explain Flemings left hand rule (motor rule) and the right hand (dynamo rule) In the left hand rule, the direction of the current gives ...
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3answers
90 views

Induction on a circuit with switch

I have the following circuit: It is subject to a steady, time-invariant magnetic field which points out of the paper. At $t = 0$, the switch closes. I thought that the magnetic flux would decrease ...
4
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1answer
222 views

Boundary conditions on current carrying wire

I'm trying to simulate by finite elements method Maxwell equations for a current carrying wire. My 3d geometry consists of a cylinder and a box containing it. I will use a mixed formulation and ...
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1answer
2k views

Proving Ampere's Circuital Law

How to prove Ampere's Circuital Law in case of any conductor. My text gives the proof of only the special case when the conductor is long and straight. I am trying to prove it, but haven't been ...
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0answers
20 views

How do you find current in this electromagnetic induction problem? [on hold]

I'm a bit lost on this. I'm given bfield strength, a velocity, and a length, and also resistance. However, I have no idea how to use any of those variables to find a current.
1
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1answer
39 views

Motional emf with rod

We know that $\mathcal{E} = -N \frac{\mathrm{d}\phi}{\mathrm{d}t}$. When we have a rod such as the one on the left moving through a constant magnetic field, how is it the case that the flux is ...
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1answer
33 views

Concerning railguns and magnetic fields

I'm part of a group working on a Physics II project based on electromagnetism, and my group decided to create a proof-of-concept railgun, shown here: ...
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1answer
28 views

Physical meaning of the separation constants in Laplace's Equation for Electrostatics

In Electrostatics, if we consider a region without charges the electrostatic potential $V$ obeys Laplace's Equation $\nabla^2 V = 0$. We can tackle this with separation of variables. In cartesian ...
0
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1answer
14 views

What are the general rules for determine on which two ends of a bar magnetic are its poles located?

I am wondering. Maybe somebody be nice enough to put a explanation that doesn't involve too much math. I did notice that for bar magnets, their poles are not always on the two ends separated by the ...
3
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1answer
82 views

Physical distribution of salt anions and cations during electrophoresis

If I have a volume of $L$ liters of salt water at a concentration of $\approx N$ mM NaCl and I pour it into an electrophoretic apparatus (like this one: ). Once we turn the apparatus on, and set the ...
0
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1answer
140 views

Magnetic fields in transformers and back emf?

i am having some trouble with understanding the magnetic fields inside transformers and how they relate to back emf. My first query is: is the magnetic field produced inside the soft iron core ...
2
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1answer
53 views

How does an infrared thermometer actually calculate temperature?

I am slightly confused about infrared radiation and the equations related to it. $P = A \epsilon \sigma T^4$ (1) and $B_{\lambda}(\lambda,T) = \frac{2hc^2}{\lambda^5} \frac{1}{e^{\frac{hc}{\lambda ...
3
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3answers
332 views

How do electrons actually move in a circuit?

Last year, we were taught about electricity, about how electrons move in a closed circuit. But as our teacher had not taught us about electric fields yet, she gave us a simplified model of motion of ...
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0answers
11 views

Is there a magnetic force affecting point charges outside of a toroidal coil with sinusoidal current?

To clarify my question a bit more, if we have a toroidal coil with sinusoidal current flowing through wires around the core of the toroid, is there a magnetic force that affects charges outside of the ...
4
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2answers
229 views

Maxwell's equations invariant under all linear transformations?

Maxwell's equations in tensor notation read: \begin{align} \partial_\mu F^{\mu\nu} &= J^\nu \\ \partial_{[\lambda}F_{\mu\nu]} &= 0 \end{align} Consider doing a general coordinate ...
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5answers
3k views

Is the EmDrive, or “Relativity Drive” possible?

In 2006, New Scientist magazine published an article titled Relativity drive: The end of wings and wheels1 [1] about the EmDrive [Wikipedia] which stirred up a fair degree of controversy and some ...
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0answers
46 views

How do I obtain the metric tensor of a constant electric or magnetic field? [on hold]

Consider the empty space with a constant magnetic or electric field. From the Faraday or the Maxwell tensor that describes this situation, how do I obtain its metric tensor? I could try the Einstein ...