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.
12
votes
4answers
5k views
Why do electrons occupy the space around nuclei, and not collide with them?
We all learn in grade school that electrons are negatively-charged particles that inhabit the space around the nucleus of an atom, that protons are positively-charged and are embedded within the ...
40
votes
10answers
3k views
How can I stand on the ground? EM or/and Pauli?
There is this famous example about the order difference between gravitational force and EM force. All the gravitational force of Earth is just countered by the electromagnetic force between the ...
13
votes
4answers
426 views
What is the mechanism behind the slowdown of light/photons in a transparent medium?
So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the ...
23
votes
9answers
3k views
Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the ...
28
votes
13answers
7k views
Home experiments to derive the speed of light?
Are there any experiments I can do to derive the speed of light with only common household tools?
14
votes
5answers
967 views
How are forces “mediated”?
I hope this is the right word to use.
To me, these forces seem kind of fanciful (except for General Relativity and Gravity, which have a geometric interpretation).
For example, how do two charged ...
12
votes
6answers
1k views
How can there be net linear momentum in a static electromagnetic field (not propagating)?
I understand from basic conservation of energy and momentum considerations, it is clear in classical electrodynamics that the fields should be able to have energy and momentum. This leads to the usual ...
13
votes
2answers
401 views
Detection of the Electric Charge of a Black Hole
By the "No Hair Theorem", three quantities "define" a black hole; Mass, Angular Momentum, and Charge. The first is easy enough to determine, look at the radius of the event horizon and you can use the ...
6
votes
1answer
200 views
What happens to an embedded magnetic field when a black hole is formed from rotating charged dust?
Black holes have no-hair so there are uniquely specified by a mass, charge and angular momentum. Imagine a cloud of charged rotating dust. There will be a magnetic field associated with the current ...
10
votes
8answers
2k views
Is it possible to separate the poles of a magnet?
It might seem common sense that when we split a magnet we get 2 magnets with their own N-S poles. But somehow, I find it hard to accept this fact.(Which I now know is stated by Gauss's Law)
I have ...
19
votes
6answers
1k views
Do Maxwell's Equations overdetermine the electric and magnetic fields?
Maxwell's equations specify two vector and two scalar (differential) equations. That implies 8 components in the equations. But between vector fields $\vec{E}=(E_x,E_y,E_z)$ and ...
8
votes
9answers
6k views
What's the core difference between the electric and magnetic forces?
I require only a simple answer. One sentence is enough... (It's for high school physics)
12
votes
7answers
2k views
What equation describes the wavefunction of a single photon?
The Schrödinger equation describes the quantum mechanics of a single massive non-relativistic particle. The Dirac equation governs a single massive relativistic spin-½ particle. The photon is a ...
8
votes
8answers
3k views
What is the difference between electric potential, voltage and electromotive force?
This is a confused part ever since I started learning electricity. What is the difference between voltage and electromotive force (emf)? Both of them have the same SI unit, right? I would appreciate ...
8
votes
3answers
843 views
What is the symmetry which is responsible for preservation of electrical charges?
Another Noether's theorem question, this time about electrical charge.
According to Noether's theorem, all conservation laws originate from invariance of a system to shifts in a certain space. For ...
6
votes
1answer
206 views
When a neutral star with a magnetic field collapses to form a black hole, what happens to the magnetic field?
By the no-hair theorem, black holes are only characterized by mass, charge and angular momentum. If the star is neutral, the black hole will have only mass and angular momentum - and therefore it ...
7
votes
3answers
3k views
Phase shift of 180 degrees on reflection from optically denser medium
Can anyone please provide an intuitive explanation of why phase shift of 180 degrees occurs in the Electric Field of a EM wave,when reflected from an optically denser medium?
I tried searching for it ...
2
votes
3answers
276 views
If electromagnetic fields give charge to particles, do photons carry charge?
As I understand these two statements:
An electromagnetic field gives particles charge
A photon is a quantum of electromagnetic field
It must mean that a photon carries charge. But I guess it isn't ...
1
vote
1answer
427 views
Electrodynamics textbook that emphasizes applications
Which textbook in Electrodynamics which emphasizes practical applications and real life examples would you recommend for undergraduates ?
18
votes
3answers
3k views
Why don't electrons crash into the nuclei they “orbit”?
I'm having trouble understanding the simple "planetary" model of the atom that I'm being taught in my basic chemistry course.
In particular,
I can't see how a negatively charged electron can stay ...
14
votes
4answers
4k views
Why glass is transparent?
Once I asked this question from my teacher and he replied "because it passes light", "and why it passes light" I asked and he said "because it is transparent".
Same question again, Why glass is ...
13
votes
7answers
2k views
Is it theoretically possible to shield gravitational waves?
Electromagnetic waves can be shielded by a perfect conductor. What about gravitational waves?
9
votes
3answers
3k views
Can magnets rotate infinitely?
There are many videos on youtube in which people arranged magnets in circle and rotated one placing in middle of that circle on a shaft, and the magnet (magnet motor) starts madly and continues its ...
8
votes
1answer
193 views
Effect of introducing magnetic charge on use of vector potential
It is well known that Maxwell equations can be made symmetric w.r.t. $E$ and $B$ by introducing non-zero magnetic charge density/flux.
In this case we have $div B = \rho_m$, where $\rho_m$ is a ...
7
votes
2answers
1k 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 ...
6
votes
3answers
1k views
coulomb force in SI and cgs
Coulomb force in SI is
$ F = \frac{Q1*Q2}{4\pi\varepsilon R^{2}} $
while in CGS
$ F = \frac{Q1*Q2}{R^{2}} $
why is it? I mean doesn't it any make difference in dimension? since $ \varepsilon $ ...
5
votes
3answers
241 views
Negative Mass and gravitation
Since Newtonian gravity is analogous to electrostatics shouldn't there be something called negative mass? Also, a moving charge generates electric field, but why doesn't a moving mass generate some ...
4
votes
2answers
551 views
Noticing that Newtonian gravity and electrostatics are equivalent, is there also a relationship between the general relativity and electrodynamics?
In classical mechanics, we had Newton's law of gravity $F \propto \frac{Mm}{r^2}$. Because of this, all laws of classical electrostatics applied to classical gravity if we assumed that all charges ...
7
votes
5answers
838 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 ...
7
votes
5answers
2k views
What is the electric field generated by a spinning magnet?
Consider a cylinder of permanently magnetized material, with uniform magnetization pointing along the cylindrical symmetry axis (the $z$-direction). The magnet is rotating about its cylindrical ...
4
votes
1answer
282 views
Noether theorem and classical proof of electric charge conservation
How to prove conservation of electric charge using Noether's theorem according to classical (non-quantum) mechanics?
I know the proof based on using Klein–Gordon field, but that derivation use ...
4
votes
4answers
2k views
Understanding the diagrams of electromagnetic waves
I'm having trouble understanding the diagrams of elctromagnetic waves.
I have no problem with any concept in classical mechanics, and I think this can be answered without any relativity (which I ...
3
votes
1answer
322 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 ...
2
votes
4answers
2k views
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?
1
vote
2answers
200 views
Point charge potential (sign problem)
I'm a bit embarrassed, but I'm not able to compute the electric potential at point $P$ (at a distance $R$ from the origin) generated by a positive unitary point charge in the origin with the right ...
16
votes
4answers
2k views
Does a magnetic field do work on an intrinsic magnetic dipole?
When you release a magnetic dipole in a nonuniform magnetic field, it will accelerate.
I understand that for current loops (and other such macroscopic objects) the magnetic moment comes from moving ...
8
votes
2answers
902 views
Popular depictions of electromagnetic wave: is there an error?
Here are some depictions of electromagnetic wave, similar to the depictions in other places:
Isn't there an error? It is logical to presume that the electric field should have maximum when ...
4
votes
1answer
399 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 ...
3
votes
1answer
154 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 ...
3
votes
2answers
436 views
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:
...
3
votes
3answers
526 views
How do electromagnetic waves carry quantised energy?
If an electron oscillates about a mean position, it will create a time varying electric filed which in turn will create a time varying magnetic field and so on to create an electromagnetic wave. How ...
8
votes
3answers
1k views
Can superconducting magnets fly (or repel the earth's core)?
If a superconducting magnet and appropriate power supply had just enough $I\cdot s$ (current $\cdot$ length) so that when it was perpendicular to the earth's magnetic field, the force of the ...
7
votes
2answers
437 views
The equivalent electric field of a magnetic field
I know that Lorentz force for a charge $q$, with velocity $\vec{v}$ in magnetic field $\vec{B}$ is given by
$$\vec{F} =q \vec{v} \times \vec{B}$$
but there will exist a frame of reference where ...
7
votes
5answers
1k views
Why do physicists believe that there exist magnetic monopoles?
One thing I've heard stated many times is that "most" or "many" physicists believe that, despite the fact that they have not been observed, there are such things as magnetic monopoles.
However, I've ...
4
votes
2answers
827 views
Make a semi transparent mirror with copper
The question:
How would you make a semi transparent mirror (50% reflection, 50% transmission) with glass with a layer of copper. For light $\lambda$ = 500nm Try to be as realistic as possible
What ...
3
votes
2answers
273 views
Is there any idea why the electric charges of electron and muon are equal?
Is there any idea explaining why the electric charges of electron and muon are equal?
Edit:
The total charge of a particle is proportional to the integral of its own electric field flow through the ...
10
votes
3answers
1k views
Is it possible to mathematically derive the formula for resistance?
Resistance is given by $\rho L/A$, where $\rho$ is the material constant, $L$ is the length, and $A$ is the area.
Is there any way that this can be derived mathematically, or is the only way ...
8
votes
3answers
260 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 ...
7
votes
2answers
405 views
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 ...
7
votes
1answer
1k views
Can microwaves affect wifi - and can they harm us?
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 - ...
