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)

2
votes
3answers
461 views

Magnetic B Field of Point Charge Not at Constant Velocity

I'm working on an N-body simulator for charged particles. I know that moving charged particles generate a magnetic field, and another moving charged particle could be effected by this magnetic field. ...
2
votes
2answers
12k 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
689 views

How does a magnet work?

I'm having trouble understanding how a magnet (not the field that is generated as a result but the material itself) work. The particles are aligned in a specific direction to give rise to force but I ...
2
votes
2answers
216 views

Does hitting metals magnetize them?

I was on my workshop lab today and had to file (rub on metal surface with rough surface to smooth-en it) an iron bar. It made iron dust fall of the surface. To mark some points on the bar I then had ...
2
votes
1answer
186 views

Can unbound electrons jump energy levels?

If an unbound electron (or indeed any charged particle) is moving through free space, is there a probability that it can spontaneously change energy by emitting a photon, or does this require the ...
2
votes
2answers
3k views

Why can't electromagnetic radiation from magnets be used for apparent perpetual motion?

If electromagnetic radiation represents a transfer of energy, then does a permanent magnet represent unlimited energy, and if so, why can't magnets be used for perpetual motion? Even if permanent ...
2
votes
2answers
1k views

Does a charging capacitor emit an electromagnetic wave?

Assume you charge a (parallel plate) capacitor. This establishes an electric field (the $\mathbf E$ vector points from one plate to the other) and a circular magnetic field (the $\mathbf B$ vector ...
2
votes
2answers
484 views

Gauss' law giving zero field where field is not zero?

Two plastic sheets with charged densities as shown: I'm trying to find the field at $B$. I obtained the correct answer by adding up the fields created by each charge density. But I realized that ...
2
votes
4answers
1k views

Potential energy of a charged ring

Consider a ring of radius $R$, and charge density $\rho$. What will be the potential energy of the ring in its self field? The best I can do: $$dq = \rho R \cdot \, d \alpha $$ $$E_p = 2 \pi R \cdot ...
2
votes
4answers
3k views

Can heat be transfered via magnetic field in a vacuum?

Say you want to store hot coffee in a container surrounded by a vacuum. To remove all sources of conductive energy loss the container is suspended in the vacuum by a magnetic field and does not have a ...
2
votes
3answers
121 views

Electric field or static electric field around a plugged-in lamp cord (when lamp is not turned on)?

When an electrical cord from, say, a lamp, is plugged into an AC wall socket, I'm aware that an electric field forms around the entire length of the cord and even before the lamp switch is flipped on. ...
2
votes
4answers
206 views

The fainter the star, the further away it is?

If a star appears to be very faint, would it imply that it's really far away or that it's luminosity is minimum?
2
votes
2answers
135 views

Why is the displacement current term needed in the Maxwell's equations?

Why did Maxwell believe that a displacement current term needed to be added to Ampere's circuital law? I have found loads of answers online about the plates acting as capacitors but i don't ...
2
votes
1answer
224 views

Poisson's equation for time dependent charge

Is Poisson's equation valid for a time dependent charge density? I think Poisson's equation is valid just for electrostatic fields. But I saw a paper that's used this equation for time dependent ...
2
votes
3answers
683 views

How does a single charge produce magnetic field?

I have studied in Introduction to electrodynamics (Griffiths) that magnetic field is actually due to effects of relativity unequal Lorentz contraction of the positive charge and negative lines, a ...
2
votes
2answers
1k views

Relationship between surface density and volume density

Often in an E&M problem, I'm having to "chop" an extended object into an infinite sum of smaller extended objects which I know more about to find a potential or electric field or whatever. The ...
2
votes
2answers
330 views

What happens to a photon after it is absorbed by an antenna?

I recently have read about interception of wireless information, however this mentions that people can intercept the information, and then somehow the recipient also gets the information. Regardless ...
2
votes
1answer
225 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
176 views

Why is this not a violation of parity invarance for EM

I read that Wu's experiment illustrates that parity violation is possible for weak processes. In that experiment, when Co-60 undergoes beta decay, the emitted electrons come out opposite to the ...
2
votes
4answers
759 views

What does the work on a current carrying wire in a Magnetic Field?

We consider that the force acting on a current carrying wire placed in a uniform magnetic field perpendicular to the length of the wire is given by $IBl$. If the wire moves by a distance $x$ in a ...
2
votes
2answers
86 views

Copper coils for nMRI

Given the lack of financial efficacy in the medical industry, I was wondering if nuclear magnetic resonance imaging would be possible with a copper/silver coil if the use is short 10 second imaging ...
2
votes
2answers
1k views

Can an electromagnet repel a strong magnet?

When the direction of current is reversed, the polarity of an electromagnet should be reversed. I am trying to understand why this did not happen in an experiment I conducted today. An electromagnet ...
2
votes
1answer
77 views

Proof that 4-potential exists from Gauss-Faraday field equation

This is a problem concerning covariant formulation of electromagnetism. Given $$\partial_{[\alpha} F_{\beta\gamma]}~=~ 0 $$ how does one prove that $F$ can be obtained from a 4-potential $A$ such ...
2
votes
3answers
1k views

Why does charge accumulate at points?

This is a very trivial question, but I can't seem to reason it out, again, as to why charges gather at points and edges.
2
votes
3answers
3k views

1 Tesla electromagnet?

Is it possible to create a powerful electromagnet at home? With use of a ferromagnet it seems so... Using the following formula: $B(Tesla)= k\mu_0nI$. I understand some ferromagnet's like iron could ...
2
votes
1answer
310 views

Why are lab Magnets painted red?

Wy are lab magnets painted red in color? I tried searching everywhere but couldn't get a satisfactory answer.
2
votes
2answers
126 views

Does the Lorentz force change under a boost?

I have a question about the Lorentz force on a charge $q$: $$\mathbf{F}=q(\mathbf{E}+\mathbf{v}\times\mathbf{B}).$$ I understand that if one performs a Lorentz boost then the electric field ...
2
votes
3answers
271 views

What is the interpretation of the Chern-Simons electromagnetic spin density?

Hans de Vries (who happens to be a no-longer-active physics.SE user) has an online book (referenced below) in which ch. 6 is a presentation of an object he calls the Chern-Simons current, ...
2
votes
3answers
539 views

Unpolarized light vs. randomly rotating polarized light?

I am confused with physical picture about unpolarized light. Is unpolarized light very fast rotating polarized light? or co-existing state of two orthogonal polarization? (or something else?) If ...
2
votes
2answers
393 views

Potential difference with an inductor

As far as I know, the potential difference between two points is defined as the negative line integral of electric field between those 2 points: $$\Delta V=-\int d \ell\cdot\mathbf E$$ I also know ...
2
votes
2answers
88 views

Gauss's Law understanding

In the case of a point charge $q$ at the origin, the flux of $\vec{E}$ through a sphere of radius r is, \begin{equation} \oint \vec{E}\cdot d\vec{a} = \int \frac{1}{4 \pi \epsilon_0 ...
2
votes
2answers
142 views

What is the history behind the factors of 3 in the classification of electromagnetic radiation?

What is the history behind the factors of 3 in the classification of electromagnetic radiation? See e.g. http://en.wikipedia.org/wiki/Radio_spectrum#By_frequency Is this (just) inherited from the ...
2
votes
2answers
2k views

How to capture electomagnetic radiation/waves?

If I wanted to find out what kind of electomagnetic waves "travel" through my room at which frequency, what kind of equipment would I need? Suppose I want to view frequencies from 0 Hz to 6 GHz.
2
votes
2answers
1k views

A wobbly pan on an induction cooker, is it less effective?

According to the Wikipedia, one of the limitations of the induction cooker is that the bottom surface of the pot should be flat. Accordingly, I commented on a question on Seasoned Advise, but I'd ...
2
votes
2answers
675 views

Why does the induced charge have to have the same magnitude as the inducing charge?

Why is it that the total induced charge on a conducting, grounde,d infinite plane must be of the same magnitude as the inducing charge?
2
votes
2answers
1k views

What shape is needed to contain a blade made of plasma?

Although this may stray into the subject of fiction, this question requires physics expertise. If one were able to create a strong enough magnetic field to contain a blade of plasma, what shape would ...
2
votes
2answers
2k views

How to estimate inductive coupling between distant air coils

I have two air coils (assume they are simple, circular wire loops). They both have diameter d. There is a distance D between their centres. D is much greater than d (more than 10x greater) Both ...
2
votes
1answer
371 views

Cherenkov radiation in nuclear bomb

Would Cherenkov radiation occur at the explosion of a nuclear bomb? Suppose it would not be occluded by smoke or anything else for that matter.
2
votes
2answers
478 views

Software to calculate forces between magnets [duplicate]

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
1answer
8k views

Is there a name for the derivative of current with respect to time, or the second derivative of charge with respect to time?

This measurement comes up a lot in my E&M class, in regards to inductance and inductors. Is there really no conventional term for this? If not, is there some historical reason for this omission? ...
2
votes
2answers
177 views

Solution of simple problems using only Maxwell equations in differential form

Solve simple electrostatic or magnetostatic problems using only Maxwell equations. For example: In every book there is an excercise to find a magnetic field outside a thin wire of radius $a$ with ...
2
votes
1answer
103 views

The energy contribution of a frequency at finite temperature

This is from a paper I'm reading: Since each frequency contributes $\hbar \omega/2$ of energy (or at finite temperature, $\hbar \omega /2 \coth(\hbar\omega/2kT)$), we can find the energies for the ...
2
votes
3answers
63 views

movement of particles in electric field

I am confused about a homework problem. Let's assume we have two electrically charged particles of which we know the charge and mass respectively. Let's say that at first they are fixed at some ...
2
votes
2answers
256 views

Natural Frequency of an object and the phenomenon of resonance!

I have read about the term natural frequency in quite a lot of places. But I haven't found an explanation as to what is vibrating. It was pretty awkward when I couldn't clearly answer my little sister ...
2
votes
1answer
205 views

Is it possible to “focus” a radio wave to target an area much smaller than its wavelength?

Recently I was reading about a technology that uses radio waves to stimulate neurons to fire. The radio waves have the advantage of being able to pass through the skull (hence being non-invasive) but ...
2
votes
3answers
475 views

Rigorously prove that electric field is zero in a perfect conductor

I have ran into a problem while trying to prove that the electric field is zero in a perfect conductor My argument went something like this: We know that: $$\vec J = \sigma \vec E$$ In a ...
2
votes
1answer
139 views

Is the Wikipedia article on the Kramers-Kronig relations correct?

Reading the "Physical interpretation and alternate form" section of the Wikipedia article on the Kramers-Kronig relations, it says: The imaginary part of a response function describes how a system ...
2
votes
1answer
104 views

Lagrangian density for the electromagnetic field

I want to know how the Lagrangian density for the electromagnetic field is written in the following form:
2
votes
1answer
193 views

A James Clerk Maxwell Disproof

One of my favorite physicists to learn about was James Clerk Maxwell, for the fact that he unified the study of E&M in physics and he would often disprove theories that did not work as a ...
2
votes
2answers
171 views

Is it possible to design a large permanent magnet that creates a multi-Tesla field?

Is it ever possible to create a LARGE magnet (in meters wide/long) that could potentially create a powerful magnetic field?