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 (6)

0
votes
2answers
621 views

Hamiltonian and non conservative force

I have to find the Hamiltonian of a charged particle in a uniform magnetic field; the potential vector is $ \vec {A}= B/2 (-y, x, 0)$. I know that $$H=\sum_i p_i \dot q_i -L$$ where $p_i$ is ...
0
votes
2answers
1k views

Radiated power and energy density for a black-body

I am having an hard time trying to understand why the radiated power per unit area $P$ of a black body is given by $$P=\frac{c}{4} u$$ in terms of the energy density $u$ and the velocity of light. I ...
1
vote
1answer
821 views

Magnetic field of permanent magnet [closed]

Hi I am looking for some code to (computationally) calculate the magnetic field of a rectangular permanent magnet in 3D space. Preferably I could edit it to add in my own dimensions. Does anyone know ...
2
votes
1answer
522 views

Electrodynamics and the Lagrangian density

Could anyone tell me what equations can I obtain from the Lagrangian density $${\cal L}(\phi,\,\,\phi_{,i},\,\,A_i, \dot A_i,\,\,A_{i,j})~=~\frac{1}{2}|\dot A+\nabla\phi|^2-\frac{1}{2}|\nabla \times ...
2
votes
2answers
296 views

locality vs non-locality

Suppose we have a circular loop of wire, and we put a long perfect solenoid inside it which is connected to an AC voltage source so that the magnetic field inside it starts to vary by time, does this ...
2
votes
1answer
99 views

Rosenberg-Coleman effect

I know only that it states some variations of the Sun's magnetic fields. What is the Rosenberg-Coleman effect specifically?
1
vote
1answer
98 views

Question regarding the direction of electric field

Suppose that there are negative charges (e.g. electrons) only. There are more negative charges on left than on right. How would electric field be constructed? (So, What would be the direction?) And ...
1
vote
2answers
80 views

What's so special about wave solutions of EM?

Maxwell's equations allow for wave solutions via oscillations between electric and magnetic field content. Couldn't we generate electric waves also if that solution didn't exists? Imagine there was ...
3
votes
1answer
628 views

Passing through the earth's magnetic field

When a projectile such as a jet plane passed through the earth's magnetic field, does an electric current get generated in the projectile? I don't sense it when I fly commercially. Why not?
0
votes
1answer
4k views

Capacitance per unit area and CMOS case

In a modern 0.35μm CMOS processor, the gate oxide thickness is around 80Å = 8nm. This gives us a capacitance per unit area of $430nF/cm^2$ I am not getting how one gets this capacitance per unit ...
0
votes
2answers
934 views

The direction of electric field in a diode

Why electric field pointing to left? (The middle area is depletion region.) (This is without any external electrical voltage applied.)
5
votes
1answer
617 views

Can an induction coil heat two layers of metal?

Imagine we have an induction coil which is strong enough to heat a sheet of metal. We can put a sheet of ferromagnetic metal close to the coil at distance $h_1$, and it gets heated to temperature ...
2
votes
2answers
859 views

D'Alembert operator and special relativity

We are currently covering special relativity in the theoretical physics lectures where we defined: $$ \mathrm ds^2 := \mathrm dt^2 - \mathrm dx^2 - \mathrm dy^2 - \mathrm dz^2 $$ In Road to Reality, ...
1
vote
2answers
164 views

What is the covariant expression for action of the Lorentz force density on charge-current density?

In a continuous medium the Lorentz force density is known to be written in the form: $f_\alpha = F_{\alpha \beta} J^\beta$, where $F_{\alpha \beta}$ is an electromagnetic field tensor, and $J^\beta$ ...
10
votes
2answers
802 views

Forcing quadrupole moments to vanish for a neutral system

For a system of electric charges $q_i$, at positions $\mathbf{r}_i$, with a nonzero net charge $Q=\sum_i q_i$, one can define a "centre of charge" in the obvious way as $$ ...
0
votes
1answer
120 views

4-velocity and electromagnetic fields

Can anyone see a reason for $$\left(1+{U_\rho U^\rho\over c^2}\right)\left(U_\nu{d^2 U^\nu\over d\tau^2}\right)=0$$? Here $U^\rho$ is the 4-velocity for a particle and $\tau$ the proper time. The ...
4
votes
2answers
2k views

First experiment which was used to prove existence of Electromagnetic waves

When I was in high school I heard from my physics teacher in the last year of high school when he was starting the chapter of electromagnetic waves that the first person who proved existence of ...
1
vote
0answers
147 views

Is there an equation for the magnetic field of a conductor attached to a magnet?

Let's say I have a hollow conductive rod, 10mm diameter (O.D.), and I place a magnet of known strength 50mm up the shaft. What is the microTesla (mT) or Gauss (G) of the magnetic field (or flux ...
4
votes
1answer
91 views

Can I find the frequency generated by tapping a battery on a coin?

According To How Stuff Works , if you tap a 9 volt battery on a coin it will transmit Radio waves. I was wondering, because I don't have a radio with me at the moment, Is there a way I could discover ...
4
votes
3answers
6k views

Basic question on magnetism regarding north and south pole

I am currently busy with some magnetism and quite shockingly (to me at least) I haven't yet read anything about the difference between the north pole and the south pole of a magnet. Before I started ...
1
vote
1answer
938 views

Gravity force strength in 1D, 2D, 3D and higher spatial dimensions

Let's say that we want to measure the gravity force in 1D, 2D, 3D and higher spatial dimensions. Will we get the same force strength in the first 3 dimensions and then it will go up? How about if ...
1
vote
1answer
704 views

Quick introduction to electromagnetism / Maxwell's equations [duplicate]

Possible Duplicate: Electrodynamics textbook that emphasizes applications I am a graduate student in applied mathematics and I am looking for a concise introduction to Maxwell's equations / ...
5
votes
1answer
482 views

Does the shielding around a solenoid actuator affect the force on the plunger?

I'm looking to minimize the weight of some solenoid actuators I want to build, so I'm trying to better understand the effects of the shielding / frame commonly found on solenoid actuators. My ...
10
votes
2answers
6k views

How do superconducting materials float in magnetic field?

The movie Avatar got me interested in the subject, but so far I only found sophisticated articles loaded with unfamiliar words. Is there a simple way to explain how magnetic field affects ...
1
vote
3answers
2k views

charge density in a wire

If we have a infinitely thin and infinitely long straight wire on the $z$-Axis with given current $I(t)$, how can I compute the charge density? I figured out that the current-density is given by ...
7
votes
1answer
2k 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 ...
1
vote
2answers
433 views

How is the polarity of a magnet decided (before its creation)?

This is not how to determine a magnet after it's created, but rather before it is created. How is the polarity of a magnet created (why does one side go this way and the other go the opposite)? Is ...
5
votes
2answers
278 views

Why don't Microwave ovens use more energetic EM radiation?

Because in theory, visible light contains more energy than the microwaves actually used to cook your food. 1) Does it have to do with resonance frequency of the $H_2O$ molecule? 2) How can ...
5
votes
2answers
3k views

What is the difference between a photon and a phonon?

More specifically, how does a wave-particle duality differ from a quasiparticle/collective excitation? What makes a photon a gauge boson and a phonon a Nambu–Goldstone boson?
0
votes
1answer
87 views

Numb3rs TV series - Antenna

Most of you might have watched the TV series 'Numb3rs'. http://en.wikipedia.org/wiki/Numb3rs In one of those episodes they were discussing about how to capture the contents on the screen of a ...
4
votes
1answer
411 views

Linear dependence of magnetic potential on current density

I'm a mathematician learning physics to provide some background for my mathematical work (especially pde's!). I have been reading through Jackson's Classical Electrodynamics (3rd edition), and I was ...
8
votes
2answers
3k views

Can I replace neodymium magnets with similar-size electromagnets?

I am interface designer that occasionally steps out and does some product/concept design. The last one I'm working on is something that would require me to make a matrix of little electromagnets, and ...
1
vote
2answers
170 views

What forces affect point charges?

I am working on a point charge simulator, and I was wondering what forces can affect point charges (assuming that they operate in a closed system, with no externally generated light, and initial ...
6
votes
3answers
225 views

Can the poles of a magnet have varying intensity?

In re-reading Is it possible to separate the poles of a magnet? (amongst others) the question mentioned in the title here just occurred to me. It may not be possible, at our current levels of ...
1
vote
3answers
1k views

Is there is any difference between Electric Flux ($\Psi$) and Total number of Electric Field lines ($E\times$Area)?

Is there is any difference between Electric Flux ($\Psi$) and Total number of Electric Field lines ($E\times$Area)? $\psi = \Sigma Q$, and $\phi$ = Electric field intensity $\times$ Area where Area ...
0
votes
1answer
145 views

How to count magnetic repulsion

I have two equal flat round magnets. I know amount of force $F$ which attracts iron objects to one of them and geometric characteristics of magnets. I want to fix first of magnet and some additional ...
1
vote
0answers
51 views

Why is low temperature required for optical lattice systems to enter magnetic regime?

"Optical lattice requires a very low temperature scale to enter magnetism regime." Why is that? What I know: optical lattice systems suppress kinetic energy (hopping parameter: $t$ small); ...
1
vote
1answer
184 views

Is the Earth's atmosphere a Faraday cage?

X-ray telescopes are required to be above the atmosphere as the atmosphere blocks EM waves with wavelengths < UV . Does this mean that the Earth's atmosphere can be thought of as a Faraday cage ...
0
votes
1answer
134 views

Will a magnet loose too much strength it cut with a hacksaw?

Can a magnet be cut with a hacksaw, by hand, without losing its essential characteristic of being magnetic? I know i would have to be very careful with heat but I don't know how careful
21
votes
4answers
2k views

Why do Maxwell's equations contain each of a scalar, vector, pseudovector and pseudoscalar equation?

Maxwell's equations, in differential form, are $$\left\{\begin{align} \vec\nabla\cdot\vec{E}&=~\rho/\epsilon_0,\\ \vec\nabla\times\vec B~&=~\mu_0\vec J+\epsilon_0\mu_0\frac{\partial\vec ...
3
votes
4answers
2k views

Magnetic fields and gravitational waves. How far do they reach?

I read that magnetic fields perpendicular to a current shoot out and expand all the way to infinity. Additionally a gravitational wave, no matter how small will also expand to infinity at the velocity ...
7
votes
1answer
77 views

Is it possible to determine astronomically/astrophysically whether other planets have ever encountered polarity reversal?

We know of geomagnetic flip in Earth's history by studying geologic data. Given other planets in the system also possess a magnetic field leads to the assumption that such polarity reversal may not be ...
1
vote
2answers
2k views

separation between slip rings of DC generator

this is a DC generator which converts mechanical energy to Direct Current. The commutators in a DC generator are separated (as you can see in the image). It is explained in our book that it prevents ...
3
votes
2answers
272 views

Reference request for the Aharonov-Bohm effect

I am looking for a good reference to an online source or book, on the magnetic Aharonov-Bohm effect. I have read the appropriate sections from the book by Griffiths and Ballentine, and still haven't ...
1
vote
1answer
2k views

Is the induced electric field due to time varying magnetic flux always circular?

According to Faraday's law, changing magnetic flux induces an electric field. Is that electric field always circular?
4
votes
2answers
1k views

How can I cause a levitating magnet to rotate?

I have a device like this one: http://www.youtube.com/watch?v=kA_hw_lY-OY (All of them work the same) Here's an image: I want to make it spin without touching it with hands. I tried using magnets ...
3
votes
1answer
5k views

Optical constants of noble metals: the Drude model for microwave modelling

I have a question regarding the optical constants of noble metals. According to Johnson and Christy's paper Optical Constants of Noble Metals (Phys. Rev. B 6, 4370–4379 (1972), ...
3
votes
1answer
1k views

How EM waves are produced by accelerating charged particles?

How the electro-magnetic waves are produced by the accelerating charged particle? Graphical explanations are most welcomed. Is the explanation given by the below mentioned article correct regarding ...
1
vote
0answers
60 views

Design and use of the RLC model for signal processing systems and spatial acoustics

Given a frequency response for a signal processing device, set of devices, or physical space it is possible to construct an equivalent (identical frequency response) series RLC circuit. 1) Does this ...
6
votes
3answers
1k views

Why can't a superconductor make a DC motor self sustaining?

Superconducting wire can host a low current magnetic field. I do not know if it supports a corresponding electrical field. Can a superconducting wire that sustains a current accelerate a DC motor? ...