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.
4
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2answers
452 views
Does special relativity make magnetic fields irrelevant?
I've heard that special relativity makes the concept of magnetic fields irrelevant, replacing them with relativistic effects between charges moving in different velocity frames. Is this true? If so, ...
4
votes
4answers
351 views
How to interpret the continuity conditions in the PDEs (for example, Maxwell equations) originated in physics?
I am currently working on PDEs in physics, mostly Maxwell equations. I am a mathematics graduate student, and this question has been haunting me for years.
In PDE theory, or more specifically the ...
4
votes
1answer
121 views
Loopless voltage measurement
I think we are all very well familiarized with the classical voltmeter. Classical voltmeter has two conducting wires that bring two potentials into the box. In the box we have well controlled ...
4
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1answer
462 views
Magnetic fields and friction
If I was to pass a metal object through a magnetic field would there be any friction?
4
votes
2answers
454 views
Canonical momentum in different coordinate system
The canonical momentum is defined as
$p_{i} = \frac {\partial L}{\partial \dot{q_{i}}} $,
where $L$ is the Lagrangian.
So actually how does $p_{i}$ transform in one coordinate system $\textbf{q}$ to ...
4
votes
1answer
364 views
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 ...
3
votes
1answer
47 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?
3
votes
3answers
255 views
Why is copper diamagnetic?
Cu has an unpaired electron in 4s, but it is diamagnetic. I thought that it has to be paramagnetic. What am I missing?
3
votes
4answers
295 views
Collision of two photons
Could someone explain me how will be look like collision of two photons? Will they behave like:
Electromagnetic waves, they will interpher with each other and keep they wave nature
Particles and ...
3
votes
0answers
518 views
how does accelerating charge produce Electromagnetic radiation? [closed]
Suppose an electron is accelerating along $x-$axis. How does it produce electromagnetic radiation?
Which of Maxwell's equation comes in play and how do I derive the wave equation from it?
For ...
3
votes
1answer
218 views
Dirac's quantization rule
I first recall the Dirac's quantization rule, derived under the hypothesis that there would exit somewhere a magnetic charge: $\frac{gq}{4\pi} = \frac{n\hbar}{2} $ with $n$ natural.
I am wondering ...
3
votes
3answers
5k views
Why a glass rod when rubbed with silk cloth aquire positive charge and not negative charge?
I have read many times in the topic of induction that a glass rod when rubbed against a silk cloth acquires a positive charge. Why does it acquire positive charge only, why not negative charge?
It ...
3
votes
1answer
770 views
Why is the lid of the cookware kept on induction cooker not hot?
Induction cookware cooks food by inducing an electro magnetic field in the ferro-magnetic cookware. Since iron offers a lot of resistance to the current, the current is converted into heat in the ...
3
votes
2answers
250 views
Gravitational wave energy
Electromagnetic energy can be related to it's frequency via $E=h\nu$. Is there a comparable relationship between gravitational wave energy and frequency?
3
votes
1answer
532 views
How can I model buckyballs sticking together? (Tiny spherical magnets)
Buckyballs are a desktoy that consist of 216 tiny spherical magnets. You can makes all kinds of interesting shapes and structures out of them (do a Google search).
I want to model them in a physics ...
2
votes
1answer
399 views
Magnetic Fields created by Current-Carrying Wires
The drawing shows two perpendicular, long, straight wires, both of which lie in the plane of the paper. The current in each of the wires is I = 5.6 A. What is the force and torque acting on each wire ...
2
votes
2answers
2k views
Dirichlet and Neumann Boundary condition: physical example
Can anybody tell me some practical/physical example where we use Dirichlet and Neumann Boundary condition. Is it possible to use both conditions together at the same region?
If we have a cylindrical ...
2
votes
1answer
136 views
Does the expression of the orbital magnetic dipole moment have $c$?
The orbital magnetic dipole moment of a particle with mass $m$ and charge $q$ can be shown to be related to the orbital angular momentum through the equation
$$\displaystyle ...
2
votes
2answers
339 views
Electron model under Maxwell's theory
I was not able to recall my memories, so:
What is the formula that states the frequency of electrons revolving around nucleus is equal to the frequency of light (or photon) emitted (or radiated)?
(I ...
2
votes
1answer
2k views
How do I relate the direction of a compass needle to the direction of a current?
A compass needle is placed above the wire points with its N-pole toward the east. In what direction is the current flowing?
If a compass is put underneath the wire, in which direction will ...
2
votes
1answer
229 views
Are perpetual magnetic motors a scam? [duplicate]
I read articles and watched videos and more videos where people say they've built perpetual magnetic motors.
I do not see any proof that this can work and also I do not see proof that it is wrong. ...
2
votes
3answers
941 views
Some questions about car radio and cellphone antennas
1-Why the antenna of the radio of cars is located outside the car and not inside?
2-If the answer to 1 is because that cars are like Faraday cages then how come my cell phone can receive signal ...
2
votes
4answers
601 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 ...
2
votes
2answers
416 views
What is the physical process (if any) behind magnetic attraction?
I understand that the electromagnetic force can be described as the exchange of virtual photons. I also understand that it's possible for virtual photons, unlike their real counterparts, to have mass ...
2
votes
2answers
202 views
Must the action for a relativistic charge 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 ...
2
votes
5answers
2k views
Figuring out North and South Magnetic Poles for Earth
I currently make my living as an electrophysicist, so am a bit embarassed at having not thought this one through, before.
Magnetically, the North pole is.... North. We get out our compass, and wait ...
2
votes
1answer
286 views
Software to calculate forces between magnets
I am working on a complex configuration of magnets and every time I make an experiment something unforseen happens. Now I believe I could speed up the development by sitting down and calculating the ...
2
votes
3answers
853 views
How is electromagnetic wave variation distributed in space?
Imagine an electromagnetic wave (a monochromatic one for example)
The electric field amplitude, and its variations travel in the propagation direction.
So, if there really exists a propagation ...
2
votes
1answer
628 views
Current from induced emf
If the induced emf in a circuit is negative, and current from this emf is the emf over the resistance, what happens to the negative sign in the induced emf when solving for the current? Surely there's ...
2
votes
1answer
388 views
What is the pressure between two electric dipole sheets of finite extent?
I have recently become curious about modeling the repulsion of everyday objects in contact with one another. By repulsion I mean as you attempt to walk through a wall, the pain in your nose suddenly ...
1
vote
4answers
59 views
North and south of magnetic field
The current I is flowing upward in the wire in this figure. The direction of the magnetic filed due to the current can be determined by the right hand rule.
Can we determine the north and the south ...
1
vote
2answers
358 views
Tensor product notation [closed]
In the image there is a tensor product:
$$F_{\mu\nu}F^{\mu\nu}=2(B^2-\frac{E^2}{c^2})$$
It's about how this operation on the co- and contravariant field strength tensors can give one of the ...
1
vote
1answer
542 views
How is Gauss' Law (integral form) arrived at from Coulomb's Law, and how is the differential form arrived at from that?
On a similar note: when using Gauss' Law, do you even begin with Coulomb's law, or does one take it as given that flux is the surface integral of the Electric field in the direction of the normal to ...
1
vote
1answer
186 views
Would someone who is only interested in string theory benefit from working out the problems in Jackson electrodynamics?
This is a soft question. I'm not sure if it is appropriate for this site.
Would someone who is only interested in string theory benefit from working out the problems in Jackson electrodynamics? I ...
1
vote
2answers
90 views
Origin of field deduced from potential
Related: Tubelights+power lines pictures?
I would've edited this into the above question, but I realized that there' enough to it to qualify as a new one. Plus this seems to be a confusion of ...
1
vote
1answer
295 views
Poisson's Equation
First up, I needed to compute the potential at some external point due to charge stuffed inside the region between two concentric cylinders, the volume charge density being given. Two methods came to ...
1
vote
1answer
341 views
Electrical eddy current visualization or simulation
Eddy currents are induced in a metal plate when it experiences a changing magnetic flux. Is there a realistic visualization or simulation of eddy currents available? The only picture I found, on ...
0
votes
1answer
77 views
measuring electromagnetic induction
There is a famous law which says that a potential difference is produced across a conductor when it is exposed to a varying MF. But, how do you measure it to prove? It is quite practical.
...
0
votes
1answer
88 views
Is it possible to charge a capacitor to any potential? Does it depend on the dielectric placed between them?
Is it possible to charge a capacitor to any potential? Does it depend on the dielectric placed between them? Or, are there other factors affecting it?
0
votes
2answers
267 views
Bend or concentrate magnetic field?
What are the ways to modify the form of magnetic field from the permanent magnet? For example I have a permanent neodymium magnet. Its magnetic field is distributed at large volume around the magnet, ...
0
votes
1answer
60 views
What are the Electric and Magnetic Properties of Aluminum Oxynitride?
Is Transparent Aluminum (Aluminum Oxynitride) a good conductor of electricity? Also, is it attracted by a magnet?
0
votes
1answer
492 views
Coil design and magnet orientation in a unique generator
First, I am not an electrical engineer - I am a hobbyist building a prototype linear-stlye generator based on an idea I had which, for various reasons, cannot be built with magnets that pass ...
0
votes
0answers
101 views
Intensity of unpolarized light
If the electromagnetic field of an unpolarized plane wave is written as
$$\bar{E}(t,\bar{x})=(\bar{E}_{0x}+\bar{E}_{0y}e^{i\delta(t)})e^{i(\bar{k}\bar{x}-\omega t)}$$
...
0
votes
2answers
81 views
Is reversal of magnetic polarity in a planet an instantaneous occurence?
Just what the title states -
Does reversal of magnetic poles in a planet refer to the point in time when reversal is complete?
OR
Does it refer to the entire drawn out process (assuming the ...
0
votes
0answers
57 views
What should my next step be after drawing a diagram? [closed]
I was training with some example exams, and I found out a problem that I don't know how to start with it, not even a main formula...
Two balls with masses of 0.5g each hang on threads 0.8m long ...
0
votes
2answers
584 views
Can quantum communication really replace electromagnetic waves for telecommunication medium in future?
Currently I am planning to get masters degree. So I am thinking about a subject in which I have to get masters degree. Following are my questions to leading physicists..
Which technology is the ...
0
votes
3answers
663 views
Direction of Magnetic force from a current running through a coil of wire
What is the direction is the magnetic force vectors pointing from a coil of wire that has current running through it?
...
0
votes
1answer
553 views
How is the direction of a compass affected by a current in a wire?
If I place a compass over a wire(such that the wire is positioned north-south) with charge flowing through it, and it points northeast, how can I determine the direction of the electron current ...
-1
votes
1answer
721 views
Violation of Newton's 3rd law and momentum conservation
Why and when does newtons 3rd law violate in relativistic mechanics? Check this link http://www.animations.physics.unsw.edu.au/jw/Newton.htm.
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votes
3answers
172 views
There must be free positive charges, moving oppositely to electrons for the wire with current to stay neutral
All popular expositions (e.g. these ones) of relativistic electromagnetism claim univocally that electrons in motion become more dense due to the speed. They teach that Lorentz contraction of charges ...
