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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3answers
940 views

Ferromagnetism and electrical conductivity

Are ferromagnetic materials good conductors of electricity? If yes, please explain with proper examples.
1
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2answers
218 views

Motor Speed depends on voltage or number of turns

What limit the size of motor speed? Does the speed of motor depend on the voltage applied or magnetic field strenght or frequency or what?
14
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4answers
17k views

Where can I find simulation software for electricity and magnets?

Is there easily-available* software to simulate coils, solenoids, and other magnetic and electromagnetic devices? I'd like to play around with some design ideas, such as Halbach arrays, but physics ...
0
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2answers
123 views

Does the ring (in the picture) experience Lorentz force?

Does the ring in the picture below experience Lorentz force? Or should I say it's just some magnetic force? (Notice that the poles repel eachother.) I am unsure because of the setting with the ...
5
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3answers
107 views

Energy of an accelerated charge?

Thought experiment: Let's take a positively charged particle and place it in a vacuum with no electric field. It just sits there. Now, we instantaneously introduce an electric field. The moment ...
2
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1answer
117 views

Can you create a powerful magnetic field out of confined circulating electrons only?

Would it be possible, regardless of how efficient it was to do something like this, or why you would actually do it, to generate a really powerful magnetic field simply by producing a cycling electric ...
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2answers
341 views

Domains of electromagnets

I know that magnetism is due to the alignment of domains of a magnetic substance. Due to this reason, an iron bar may be magnetized by stroking it with a permanent magnet such that its domains are ...
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0answers
50 views

Books about electomagnetism and quantum mechanics [duplicate]

suggest me some books about electromagnetism. anchor tags:everything about ac,dc circuits,magnetic circuits,semiconductors etc things i need to know through my college years(i am currently doing my ...
6
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1answer
345 views

Could the Faraday's rotating wire experiment be explained without using Lorentz force?

I read Faraday's rotating wire experiment which resulted in the invention of the homopolar motor. He was clever enough to think it that way and make the wire rotate around one of the poles of the ...
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1answer
374 views

Force between two solenoids

How does one calculate the magnetic force between two coaxial solenoids, placed in a way their currents are in the same sense? There is a simple way to treat both as dipoles and then calculate the ...
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0answers
33 views

Calculation of Phase Delay of a Curved Planar Dielectric Waveguide (i.e., bend)

I am working on a very phase sensitive planar dielectric waveguide device and I need to calculate the phase delay induced by a bend of some radius, $r$ and of $\phi$ radians for a give planar ...
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2answers
728 views

Can we increase the magnetic flux for a permanent magnet?

When we pass current to the permanent magnet does its magnetic flux, magnetic property of attraction and repulsion increases? if possible Is there any other way for increasing the magnetic flux?
0
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1answer
483 views

Coercivity of a ferromagnetic material?

I understand that coercivity is the field/force required to demagnetize/magnetize a ferromagnetic material. What if we had two opposite magnetic fields of different strengths values H acting on the ...
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0answers
29 views

Effect of shielding on a miniature Helmholtz coil

So a bit of background... I'm a university graduate student with primarily digital signal processing and embedded experience. However I've managed to get myself involved with a project that requires ...
0
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2answers
223 views

Drift Speed and Current in Two Different Inertial Frames

We have a long, cylindrical wire carrying a constant current I in an inertial frame. At a distance of R from the center of the wire, the magnitude of magnetic field is $μI/2πR$. What is the magnitude ...
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2answers
53 views

Electric field around a moving charge

A moving charge has magnetic field around it. Is it just increased electric field in relativistic frame? If yes, does a moving charge has more electric field around it?
0
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1answer
27 views

Does the sign of imaginary part of complex permittivity have any physical meaning?

I have noticed some papers having written complex permittivity as $e' + je'' $ and others as $e' - je''$. The data in literature does not specify the sign. What should I use and does the sign of $e''$ ...
0
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1answer
85 views

Wave operator applied to electromagnetic field tensor

I'm trying to understand an argument in "An introduction to general relativity" by Hughston and Todd (p37). Let $F_{ab}$ be the electromagnetic field tensor, I'm trying to show: $$\Box F_{ab} = -4 \...
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1answer
238 views

How is Magnetic force on a current carrying conductor $Blb$

I was reading an answer about torque acting on a rectangular current carrying loop kept in a uniform magnitude field B. Force acting on each sides is $F_1$, $F_2$, $F_3$, $F_4$. It's written here : ...
0
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1answer
118 views

Can a magnetic or electric field deflect subatomic particles?

I always wondered if the subatomic particles can be deflected by a magnetic or electric field. Basically I am asking the viability of the typical "Force Field" in movies.
2
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1answer
157 views

Confusion in Maxwell's derivation of Ampere's Force Law

I am reading Maxwell's "a treatise on electricity and magnetism, Volume 2, page 155" about "Ampere's Force Law". I have some confusion in the following pages: On the top of the page, Maxwell says:...
6
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1answer
359 views

Confusion in Maxwell's derivation of Ampere's Force Law - Part II [closed]

I am reading Maxwell's "a treatise on electricity and magnetism, Volume 2, page 156" about "Ampere's Force Law". I have some confusion in the following pages: My question is of two parts: 1. ...
5
votes
2answers
118 views

Is there any way to justify or derive the form of the Lorentz force from relativity theory?

Lorentz force is in this form: $$\vec{F}=q[\vec{E}+\vec{u}\times\vec{B}]$$ As we know, it is Lorentz-invariant. Is there any way to justify or derive its form from relativity theory?
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0answers
19 views

Parallel wires in a changing non-uniform magnetic field?

Given this diagram: Where four copper wires are parallel to one another, connected to an exterior circuit(not shown) placed in a non-uniform magnetic field $B$ that is changing over time. Due to $...
5
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2answers
5k 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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0answers
35 views

Amplifying an electromagnet's generated field

Given an electromagnet like this: which is essentially an iron casing with coils inside: Is it right that the centre disc would have one polarity whilst the outer housing another? To boost the ...
1
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2answers
138 views

What is the inconsistency between Maxwell's electrodynamics and newtonian mechanics?

As far as I understand, when a modification of a theory is made it is because some observation required this modifcation. Quantum Mechanics is a nice example of that: observations of microscopic ...
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0answers
8 views

Rotating magnetic field dencity

How to calculate the magnetic field density (Tesla) in a rotating magnetic field produced by 3 sets of coils at 120 degree angles.
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1answer
49 views

Are neutrinos affected by electromagnetic forces?

I know they don't interact with matter nor any electromagnetic forces, but I know they have mass, very little mass but they have. When something has mass it must be attracted by gravity and other ...
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2answers
33 views

Why does electromagnetic induction creates both voltage and current but static electricity only creates voltage and no current?

Why is it that electrons do not move the same way in triboelectric as in magnetic induction when both creates voltage? Triboelectricity can even create tens of thousands of volts like Van De Graf ...
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0answers
39 views

Is the magnetic field caused by the lenght contraction, as seen by us, in the frame of a fast moving charged particle? [duplicate]

If for example an electron moves very fast relative to us, then the length contraction as seen by us, causes the electric field lines to compress themselves in the direction of motion. Is this the ...
1
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0answers
74 views

World's Largest Radio Telesope : How accurately does it need to be figured?

In Guizhou Province, China, the worlds largest radio telescope is almost complete, measuring 500 metres across. I am aware that todays largest optical telescope require very accurate figuring and ...
0
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0answers
48 views

Sum rule and dense limit

Is the finite size of lattice constant important for derivation of sum rules, for example, the sum rule for diagonal conductivity $\sigma_{xx}$: $$ \int \limits_{-\infty}^{\infty} d\omega\sigma_{xx}(\...
6
votes
1answer
414 views

Biot-Savart law from Ampère's with multivariate calculus

Let us assume the validity of Ampère's circuital law $$\oint_{\gamma}\mathbf{B}\cdot d\mathbf{x}=\mu_0 I_{\text{linked}}$$where $\mathbf{B}$ is the magnetic field, $\gamma$ a closed path linking the ...
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1answer
6k views

Why does pushing a magnet inside a solenoid produce current?

If you push a bar magnet inside a solenoid, a current is produced. But why is that? I mean, the wire is being moved along the magnetic field, so taking the cross product: $\vec{F} = I\vec{V}\times\...
2
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1answer
151 views

polarization of a transverse wave travelling in ionosphere with polarization direction perpendicular to earths magnetic field

Assume a transverse electromagnetic wave entering ionosphere such that its Electric field of wave is perpendicular to earths magnetic field. Now, i read that as it will enter plasma, the wave will ...
3
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1answer
692 views

Homopolar motor and Lorentz force

My second grader thought making a homopolar motor for her science experiment would be fun. And, it was. Now I am trying to explain how it works and the Lorentz force. Please help me by giving me a ...
0
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2answers
110 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, ...
4
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1answer
67 views

Schrödinger-Pauli Equation Solutions

The Schrödinger-Pauli equation is the non-relativistic limit of the Dirac equation, and therefore describes spin-1/2 particles in an external electromagnetic field. It is given by: $$\left[\frac{1}{...
0
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1answer
30 views

If $T_{kl}=\epsilon_{kil}m_i$, how to show $m_i=0.5\epsilon_{ilk}T_{lk}$? [closed]

In a book I am reading (about magnetic dipole), it is given that $T_{kl}=\epsilon_{kil}m_i$. Then, it says since $T_{kl}=-T_{lk}$, it can be shown that $m_i=0.5\epsilon_{ilk}T_{lk}$. I understand that ...
1
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0answers
17 views

Variation of spin in the rest frame of an electron in an external magnetic field

In Jackson's Classical Electrodynamics, the equation of motion for the angular momentum $\textbf{s}$ of an electron in an external magnetic field is given by (eq. 11.101) $$\left(\frac{d\textbf{s}}{dt}...
0
votes
1answer
30 views

Calculating the B flux density of a flat spiral coil with N turns [closed]

Consider the following topology for a flat spiral coil (with an air gap in the middle). It's been a very long time since I have used Maxwell's Equations and now I find myself trying to figure out the $...
1
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0answers
22 views

Magnetic field between 4 wires with the same current

if I have 4 long wires with a current I and all of them have the same distance from each other (looked at from the front you'd have a square) shouldn't the magnetic field vanish in the middle of all 4 ...
1
vote
1answer
40 views

Rewriting Maxwell's equation in tensor form [closed]

Suppose $F_{ij}=\epsilon_{ijk}B_k $, how to prove the following: $\partial_iB_i=0$ becomes $\partial_iF_{jk}+\partial_jF_{ki}+\partial_kF_{ij}=0$ $B_iB_i$ becomes $F_{ij}F_{ij}/2$ I can see that ...
2
votes
1answer
34 views

Is it possible that propagation of acustic waves leads to emission of radiation?

Question: Consider large cloud of gas. Assume it is electrically neutral (but as always, matter is composed of smaller things which are actually charged). Is it possible that propagation of sound ...
3
votes
1answer
73 views

Can the Biot-Savart law be derived from QED?

It's important that a new theory of physics contains the equations and results of previous related ones. Maxwell theory and QED both have explanations for electromagnetic phenomena so I'm wondering if ...
0
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1answer
23 views

Induced current in a rectangular loop [closed]

The question is How much heat is generated in the resistor during the loop's entire path into and out of the magnetic field? Give your answer in J. I used Faraday's law and managed to get the ...
7
votes
1answer
159 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') =...
5
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2answers
4k views

How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?

The only difference that I know of between Kitchen Microwaves and Wifi signals is how much power is pumped through them. Why is it that Wifi signals, being 1000 times weaker can travel so much ...