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
1answer
32 views

behavior of dissolved ionic compounds in an electric field

Consider the following setup - An open, insulating box is filled with distilled water, into which is dissolved a significant amount of pure NaCl. Two insulated lumps of conductor, one with a large ...
13
votes
6answers
28k views

Can magnetic fields be redirected and focused at one point?

I know that magnetic fields can be redirected, but... given a situation where you have static magnetic field over a large area, and you want to quickly change the magnetic field strength. Is it ...
0
votes
0answers
26 views

Calculating temperature in a steel rod/bar which is heated by induction?

I'd like to calculate what kind of temperatures I could achieve in a steel (or copper) bar/rod with the help of heat induction from a coil. Firstly, let's not take radiation or convection into ...
0
votes
0answers
23 views

Books on Nikola Tesla [closed]

I'm looking for suggested books on Tesla. Preferably one that includes both biographical information as well as detailed descriptions of his theories/experiments/work.
2
votes
0answers
46 views

Do divergence and curl of Lorentz force have some physical meaning?

Time ago I started thinking about this: if we take the well known Lorentz Force expression, namely $$\mathbf{F} = q\left(\mathbf{E} + \mathbf{v}\times\mathbf{B}\right)$$ and we operate $\nabla\cdot ...
0
votes
1answer
451 views

calculating magnetic field strength for a very small electromagnet

I am trying to calculate the magnetic field (in tesla/gauss) of an electromagnet that is very small and has very few windings. For example 12 windings over 0.003 meters. I know this is not going to ...
1
vote
1answer
36 views

Understanding of dipole moment and its vector property

I have a trouble understanding the electric dipole moment. The electric dipole moment formula is $${\bf p}= \int {\bf r}' \rho({\bf r}')d\tau '$$ I'm interested in the coordinate, the origin of which ...
0
votes
1answer
37 views

How can we verify the direction of electric displacement is radial?

I have two questions about electric displacement $D$. The following is a problem in Griffiths. A long straight wire, carrying uniform line charge $\lambda$, is surrounded by rubber ...
8
votes
8answers
322 views

How do electrons know when a circuit is closed?

I was told that electrons do not begin flowing unless the circuit is closed. The electrons from the battery are not in the ends of wire when it is open, apparently, as there is no reason for them to ...
5
votes
1answer
225 views

$U(1)$ 5-dimensional Kaluza-Klein topological defects

Five-dimensional Kaluza-Klein theory is well-known to predict that the electromagnetic field can be described as a curled additional dimension over four-dimensional spacetime. That is, you only need ...
0
votes
0answers
41 views

A question in electrostatics

The following is from Problems and Solutions on Electromagnetism. The electrostatic potential at a point $P$ due to an idealized dipole layer of moment per unit area $\pmb {\tau}$ on surface ...
1
vote
1answer
34 views

Why is there no electric potential energy when it is directly above an electric dipole?

If you have two opposite charges of equal magnitude, why is the potential energy of another random particle that is directly above or below the middle distance between the two charges have 0 electric ...
2
votes
1answer
76 views

How strong a magnetic field would be needed to deflect cosmic rays?

How strong a magnetic field would be needed to deflect cosmic rays? For example, lets imagine we wanted to protect the occupants of the International Space Station from cosmic rays. I assume we would ...
0
votes
0answers
24 views

What are the differences between an EM wave propagating in a conductor and an electrical impulse traveling down a conductor?

I'm afraid I don't quite understand the fundamental and mathematical differences between a propagating electrical signal in a conductor and an EM wave traveling in a conductor. Situation: I found ...
0
votes
2answers
291 views

How does back-emf oppose the input-current?

A circuit, has current $A$ flowing at a certain $V$. When there is a change in magnetic-flux, based on Faraday's law of induction & Lenz's law, we know that there is change in Potential ...
0
votes
0answers
25 views

On Rayleigh-Jeans black body distribution derivation

When trying to derive the Rayleigh-Jeans distribution function, all authors say that in k-space each solution of the electromagnetic wave functions(of waves inside a cubic black body) represents an ...
4
votes
2answers
7k views

What is the difference between Biot-Savart law and Ampere's law?

What is the difference between these laws? Which law is more useful? When to use Ampere's law and when to use Biot-Savart law?
1
vote
1answer
112 views

Spherical magnet inside a solenoid

When passing a bar magnet through a long solenoid why is it that the induced emf when the magnet is in the middle of the solenoid is zero? And if a spherical magnet is put inside the solenoid, will ...
1
vote
0answers
12 views

What is the direction of area vector while calculating flux in AC generators?

I was told that in case of an open surface, the direction of area vector is considered to be in the direction of magnetic field but my doubt is won't the direction of area vector change as armature ...
3
votes
2answers
64 views

How can I use a magnet to lift a paperclip?

This question has been asked already, but no satisfying answer came up. All the answers seem to push the problem further, but do not explain clearly what is going on. I will reformulate it for a ...
4
votes
0answers
50 views

Maxwell Equations don't give unique Electric Field?

Consider the class of electric fields given by $$\mathbf{E}=\begin{cases} \ln (Cr)\hat{z} & 0\leq r < R\\ 0 & r> R \end{cases}$$ where $C$ is a constant and $r$ is the polar-distance ...
0
votes
1answer
420 views

How to calculate flux density in a toroidal CT that has an electrical line pass through the center?

I have seen the Biot–Savart equation, but I don't know how to apply it. Does the conductor length need to be considered finite and equal to the core length, or will the calculation be for a conductor ...
1
vote
2answers
43 views

What is meant by “current” in this context?

This is a quote from Optics by Hecht: A stationary charge has a constant E-field, no B-field, and hence produces no radiation—where would the energy come from if it did? A uniformly moving ...
2
votes
2answers
146 views

Is the photon truly not absorbed in Raman scattering?

In reading about Raman Scattering, I was thinking while reading it "okay, incident photo absorbed by molecule, molecule goes to higher energy vibrational state, molecule re-emits photon with either ...
1
vote
0answers
19 views

What's “the” physical explanation of hysteresis in ferrimagnetic materials?

I am wondering what is "the" or "a" physical explanation of hysteresis in ferrimagnetic materials. From what I've read in wikipedia and Cullity's book, there is a simple physical explanation for ...
0
votes
2answers
166 views

Can photons travel through space independent of time?

I read an article about advanced photons and they stated that photons traveled back in time to hit an electron. Can photons really travel back in time and if so, how?
1
vote
0answers
12 views

Field line Direction and exerted force

Magnetic field lines of a magnetic field have different directions. What information about the force exerted on a charge will give us the direction of field lines?
1
vote
0answers
11 views

Force on a magnetic dipole due to a magnetic field

Imagine a solenoid which has current $i$ and is producing a magnetic field $B$ which equals $$B=\mu N i$$ Now, imagine we put a small cylindrical magnet at the end of the solenoid. Then because at ...
0
votes
0answers
24 views

Series expansion of potential due to a dipole displaced from the origin

I learn that we can expand the electric potential in an infinite series of $\rho$ and $\cos(n \phi)$ when solving the Laplace equation in polar coordinates. The problem I want to consider is the ...
1
vote
0answers
44 views

Where to place a charge to establish net force of 0? [closed]

A charge, Q2 = -15.00x10^-6 C, is 10.00 cm to the right of charge Q1 = 18.00x10^-6 C. Where can a third charge be placed, along the line connecting Q1 and Q2, such that it experiences no net force ...
1
vote
1answer
127 views

How do you derive Fleming's left hand rule?

How do you derive Fleming's left hand rule? What is the theoretical explanation for the directions of the magnetic field, current and the force on the current for being oriented in that way relative ...
23
votes
4answers
1k views

Is the existence of electromagnetic standing waves dependent on the observers reference frame?

If I take two plane EM waves travelling in opposite direction e.g. $E = E_0 \sin(kx-\omega t)$ and $E=E_0 \sin (kx + \omega t)$, they sum to give a standing wave with a time-averaged Poynting vector ...
1
vote
0answers
14 views

(boundary conditions) Interface between two lossless media

I'm wondering why there's usually no free charges nor free currents in the interface between two lossless media? no free current "I guess" is due to the insulating nature of a lossless media but why ...
3
votes
4answers
166 views

Energy density with complex permittivity?

Question What is the correct form of the energy density when we have a complex permittivity (such as in a dielectric)? Additional information A complex permittivity means that we have: $$\vec ...
0
votes
1answer
31 views

Does the electromagnetic force “split” at lower than normal energies?

Since we have working models for forces "combining" or being describable via a single framework at higher energies (such as the "electroweak" force and the aim of GUTs), does electromagnetism split ...
2
votes
4answers
791 views

How to use Ampere's Law for a semi-infinite wire with current?

Suppose that there is a semi-infinite wire which extends to infinity only in one direction. There are no other circuit elements at the other end(finite end) of the wire and the current does not loop. ...
0
votes
2answers
118 views

Magnetic flux density of a solenoidal electromagnet

I am designing an electromagnet which will be used to test magnetic sensors. So I used to formula found at hyperphysics which relates the magnetic field flux density to the number of turns, the ...
1
vote
4answers
374 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
0
votes
0answers
20 views

Force Exerting on a magnetic train

A question is already asked ( How does this “simple” electric train work? ). But I've got a question about it. In order to calculate the force which is being exerted on the system including magnets ...
4
votes
4answers
457 views

What is the experimental evidence that light is an electromagnetic wave?

Do we have any experimental evidence to confirm that light is an electromagnetic wave? Or is it confirmed simply by Maxwell's equations showing a similarity in speed?
1
vote
3answers
1k views

Generating unlimited electricity through magnets

Well i had seen a toy, which i till date don't know was it battery powered or not? and also could it be used to actually create energy (I know that it is not possible but whats the problem?) ...
8
votes
1answer
5k views

How do 2 magnets spin by themselves if positioned precisely?

A few years ago I went to a museum, where there was a board with 2 bar magnets, on a pole each so they could rotate. If you rotated them so the lined up with the same poles (N) facing each other, ...
28
votes
4answers
18k 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 ...
3
votes
1answer
36 views

Analogue of charge for magnetic fields?

Now, I understand how there are no magnetic monopoles, and hence no analogue of magnetic charge in terms of a scalar field, but is there a similar concept that instead uses a vector or spinor field to ...
1
vote
1answer
304 views

The image current due to moving charges and a current carrying thin wire

Suppose we have a plane conductor and positive charged particles of mass m are moving parallel to the plane conductor at a distance d. The trajectory can be assumed to be straight. An image current ...
1
vote
1answer
120 views

Find magnetic field in a solenoid for different regions?

A steady current I flows along an infinitely long hollow cylindrical conductor of radius R. This cylinder is placed coaxially inside an infinite solenoid of radius 2R. The solenoid has n turns per ...
0
votes
3answers
571 views

Understanding magnetic force on charged particle

if we put any charged motionless or static particle in the constant magnetic field, then why does it don't feel a magnetic force? Mechanism by which electric and magnetic fields interrelate I have ...
0
votes
2answers
258 views

Magnet dimensions for 3-phase permanent generator

I'm making a 3-phase permenant generator. According to Faraday's law, the emf produced is negative number of turns times the change in magnetic flux over the change in time. If it is only the CHANGE ...
0
votes
1answer
52 views

Was decoupling actually detripling?

As I previously understood it, before recombination, high energy photons were bound up in interactions with excited electrons, meaning electrons couldn't settle into orbit with protons and other ...