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.

learn more… | top users | synonyms (5)

0
votes
0answers
28 views

Variation of Electrostatic Potential with Distance in a Uniform Field

I came across this question in a problem book: If a proton is released from rest in a uniform electric field, does the electric potential at those points where the electron moves increase or decrease. ...
4
votes
1answer
215 views

Will we need to update Electrodynamics?

A contradiction to the Electrodynamics by the experiment. The author has said that, accordning to the experiment, photon is no more gauge invariant? Why is that? An important thing is that Although ...
2
votes
1answer
81 views

What is equation to find force of magnetic attraction

I was wondering as a student, when I was playing with magnets I thought like Gravitational force equation of Newton's Gravitation Theory, can we also calculate force of an magnetic attraction on an ...
0
votes
1answer
52 views

Conservation of energy in transformers

How does energy remain conserved in a transformer if emf is increasing, or decreasing? Does the current decreases to accomodate?
2
votes
0answers
65 views

One question about derivation of Maxwell equations

I saw the following way of derivation of Maxwell equations: author starts from Lorentz transformations for the 3-vector of force, then he applies them for the Coulomb law, after that gets the Lorentz ...
1
vote
0answers
29 views

Velocity in Motional EMF

To calulate motional $\epsilon$, we use the following formula: $\epsilon$ = $-vBL$ To calculate the velocity so the motional $\epsilon$ can be calculated, what is the proper formula from what is ...
5
votes
2answers
146 views

Direction of current flow

In one of my homework questions, I was given the following setup. I was asked to determine if the induced current flows from X to Y, or from Y to X. From Lenz's Law I know that the current in the ...
0
votes
0answers
22 views

Torque per unit length on infinite rotating charged cylinder

For homework I have the following question, but I am stuck on finding the torque on the cylinder. An infinite cylinder of radius $R$ carries a uniform surface charge $\sigma$. We start rotating the ...
1
vote
0answers
22 views

Behavior of multiple flat coils in a cylinder

I am trying to wrap my head around how this would work. The setup is quite simple - there are multiple flat coils sealed in insulated disks, and an active coil with alternating current underneath. ...
1
vote
2answers
63 views

Electric Potential of a sphere given electric field

I'm studying for a course in electromagnetism, and I've been given an electric field for which I need to find the associated scalar potential. The field is the field generated by a sphere of radius ...
0
votes
0answers
14 views

Temperature stability of permanent magnets

Hello physicists/engineers, I have a somehow very specific question I hope some expert can answer me: I'm working at university and try to build an experiment which requires comparable high magnetic ...
5
votes
1answer
87 views

Do radio waves travel around the Earth or through it?

Whenever you hear someone illustrating/describing the transmission of radio waves they always make it seem like they'd travel perfectly around the Earth to another distant location. For example, a ...
3
votes
2answers
78 views

Is the magnetization direction of a permanent magnet fixed?

I am now playing with a permanent magnet made of Neodymium. It is impressively strong. A question is, is the magnetization direction of the magnet fixed relative to its crystal structure?
0
votes
1answer
38 views

internal conversion and the electromagnetic force

I have read that the mechanism behind internal conversion, in which a nuclear transition leads to the ejection of an electron in one of the lower atomic orbitals, is related to the fact that the ...
1
vote
0answers
26 views

Physical, intuitive reason for divergence of dielectric constant at electronic percolation transition?

Several papers such as this (warning, PDF) and this (PDF again) talk about how, near the electronic percolation transition for a metallic 2D film, the real part of the dielectric constant diverges ...
0
votes
2answers
50 views

Induction on a circuit with swich

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 ...
1
vote
1answer
45 views

Any simple reason why Helium in the ground state is diamagnetic?

I know the electrons are in the spin singlet state, and the spatial part of the wave function is an S-state. But that is not sufficient for it to be diamagnetic.
-2
votes
1answer
136 views

Poynting's theorem and escaping particles

I've edited my original question into horrible monstrosity (I apologize to all who spent their time on it). Here is rephrased version which hopefully will be clear. Lets have following model ...
3
votes
1answer
77 views

How does determinism manifest out of QFT?

Classical electrodynamics is deterministic. QED is indeterministic, or probabilistically random. Yet they agree with each other? What am I missing?
2
votes
1answer
103 views

Poynting's Theorem simplified?

How can I apply Poynting's theorem to any system that has a magnetic field & electric field, to state conservation of energy? How does Poynting's theorem state conservation of energy in EM? I ...
2
votes
1answer
65 views

Value of weak force coupling constant

I'm trying to get my head around the weak force coupling constant $\alpha_w$ but getting confused by different resources. Hyperphysics suggests it is tiny compared with the strong force coupling, ...
2
votes
1answer
69 views

Why is a Electron negative not both?

I was pondering on the question of magnetic dipoles but then I had an thought that electron and other particles are either positive or negative or have no magnetic charge but then I thought of ...
2
votes
1answer
143 views

Does the Bohr van Leeuwen Theorem also apply to ferromagnetism?

I know that the Bohr-van Leeuwen theorem shows that there could be not consistent pure classical explanation of dia- and paramagnetism. Does the same theorem also rule out a consistent classical ...
2
votes
2answers
38 views

Can eddy currents be extremely reduced?

Is it possible to create a certain design of a conductor to reduce eddy currents greatly? So that the magnetic "breaking" can be reduced? Is it even possible for the force generated by the eddy ...
0
votes
3answers
74 views

Does a change in electron's direction affect direction of force?

A current carrying conducting wire is placed in a magnetic field, and hence, accelerates. Assuming the wire is moving in a constant magnetic field, will the electrons change direction due to the ...
0
votes
1answer
87 views

Physical representation of magnetic vector potential

For time varying potential $$ E= - \nabla(v)- \frac{\partial A}{\partial t} $$ now for scalar potential v we can represent the equipotential surface as a perpendicular surface of the direction of ...
2
votes
1answer
66 views

Electric field for two coaxial cylindrical conductors of finite length with Gauss' Law

I'm confused with the Gauss' Law to calculate the electric field for two coaxial cylindrical conductors of finite length. I know that we can use the Gauss' Law to calculate the electric field for two ...
1
vote
3answers
85 views

A potential perpetual motion machine?

If I were to freeze a superconductor, the ones made up of yttrium barium copper oxide, with liquid nitrogen and placed it on one of those strong neodymium magnets, it should float right? Now what ...
2
votes
2answers
58 views

Force on a magnet in a magnetic field

I was wondering what happens when i put a wire in which current flows parallel to a magnet, something like this: where the straight line is the wire, the circle is a cylindrical magnet(with outgoing ...
0
votes
1answer
53 views

Voltage across an inductor contradicts Lenz's law?

Imagine a simple circuit consisting of an alternating current source connected to an inductor. Assume they are connected in the following fashion: AC source - terminal A - Inductor - terminal B - AC ...
0
votes
1answer
58 views

How are contact forces electromagnetic forces?

How are contact forces electromagnetic? What are they: Coulombic forces between polarized molecules in contact? Or they are the magnetic forces due to the motion of subatomic particles? I also would ...
0
votes
1answer
38 views

Potential in the middle of a square [closed]

Find the electric potential in the middle of a square with side length $a$ and charge $Q$. If I put the origin in the middle of the square, for the potential I get: ...
11
votes
8answers
3k views

Is gravity just electromagnetic attraction?

Recently, I was pondering over the thought that is most of the elementary particles have intrinsic magnetism, then can gravity be just a weaker form of electromagnetic attraction? But decided the ...
0
votes
1answer
36 views

Need an analytic model for describing a region with fast transition of magnetic field from a constant value to zero

I would like to describe analytically the electromagnetic fields in a region with the magnetic field changing rapidly from a constant value to zero. It's more or less like the fringe region of the an ...
0
votes
1answer
44 views

Why do electricity generators have to work harder for higher loads?

Maybe this is a silly question, but why does, say, a gasoline-powered AC generator have to use more gas depending on the load? Let's say I have a 120VAC generator and either a 1A or a 10A load, and ...
8
votes
1answer
265 views

Is it possible to kill a human with a powerful magnet?

I'm asking in terms of physics. Can powerful magnetic induction rearrange spins of my body in such way I will die? How? Or maybe it can rip all iron from me, which would make my blood cells useless? ...
2
votes
1answer
28 views

Taylor expansion of translated fields

First of all, I would like to say that I am somewhat new to four-vector notation. I have a function of a four-vector that I want to expand. $$ A_\mu (\mathbf{x} + \mathbf{x}_0) = A_\mu (\mathbf{x}) ...
1
vote
1answer
34 views

Can magnets destroy notification light on phone? [closed]

I'm posting this question here because it's about magnets and electronics. Please tell me if it's better suited to another site! I've been messing around with some reasonably strong magnets around my ...
2
votes
3answers
322 views

Curl of an electromagnetic wave?

I can't understand the concept of the curl of an electromagnetic wave. $$ \nabla \times E = -\frac{\partial \textbf{B}}{\partial t} $$ All of the examples I find show a current through a conductor, ...
8
votes
2answers
124 views

Counting the number of propagating degrees of freedom in Lorenz Gauge Electrodynamics

How do I definitively show that there are only two propagating degrees of freedom in the Lorenz Gauge $\partial_\mu A^\mu=0$ in classical electrodynamics. I need an clear argument that involves the ...
6
votes
1answer
315 views

Recovering all of Maxwell's equations from the variational principle

Whether you can get the first couple of Maxwell equations from a variational principle? In the second volume of the Landau theoretical physics said that it is impossible.
1
vote
1answer
46 views

How is magnetism ''conducted'' through a non-magnetic metal?

I have a ball of metal about an inch in diameter and a concave disc of another metal (which is magnetic) around the ring of the disc (about $12 {\rm mm}$ in diameter). I don't know which metals they ...
0
votes
1answer
42 views

Electromagnetic waves and group velocity

I have three questions about electromagnetic waves and was wondering whether anybody here could comment on these things: Wikipedia says that there are no longitudinal EM waves, although TM and TE ...
0
votes
1answer
107 views

How do I create a direct current with a magnet? The magnet is not to be moved in the direction of the wire [closed]

How do I create a direct current with a magnet? The magnet is not to be moved in the direction of the wire. In fact I'm looking for the contrary to the drawing from ...
0
votes
0answers
46 views

Why are magnets attracted to negative side in battery

I have been experimenting with magnets, and built an Homopolar motor however it works but the magnet only attracts the negative side of the AA battery (the flat side of a battery) which I thought was ...
0
votes
2answers
74 views

How is the direction of Magnetic/Electric Lines of Force Known?

It is shown that the direction of magnetic line is from north to the south and that of the electric line is from positive to negative. How do we/scientists know that the imaginary lines of force or ...
0
votes
1answer
60 views

How are the E and B fields inside a coaxial cable related to the current and voltage?

Conceptually, I understand that coaxial cables operate in a TEM mode due to the central conductor, and hence the electric field is directed radially outward from the voltage $V_0$ at the centre of the ...
1
vote
2answers
45 views

Energy of an EM Wave and its temperature and amplitude

I'm trying to understand why classical physics fails to explain black body radiation. I'm confused. According to Boltzmann, energy calculation for em wave is based on temperature. According to ...
2
votes
2answers
62 views

Speed of light in different parts of the universe

Do we have enough evidence to claim that speed of light remains the same at 'c' even at the most distant parts of the visible universe?
2
votes
1answer
111 views

How to transform material permittivity tensor from Cartesian coordinates to another orthogonal coordinate system?

I have a material specified by a permittivity tensor written in Cartesian coordiantes: $$\begin{pmatrix} \epsilon_{xx} & \epsilon_{xy} & \epsilon_{xz}\\ \epsilon_{yx} &\epsilon_{yy} ...