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.

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2
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4answers
55 views

How do they find the energy of a photon?

Is the actual energy of a photon ever measured? How is it done? I read that a photon is usually identified by diffraction, that means its wavelength is measured, is that right? In this way we ...
8
votes
5answers
2k views

Relativity and Current in Wire

If an observer is stationary relative to a current-carrying wire in which electrons are moving, why does the observer measure the density of moving electrons to be the same as the density of electrons ...
0
votes
1answer
254 views

Proof of Ampère's law from the Biot-Savart law for tridimensional current distributions

Let us assume the validity of the Biot-Savart law for a tridimensional distribution of current:$$\mathbf{B}(\mathbf{x})=\frac{\mu_0}{4\pi}\int_V ...
0
votes
0answers
14 views

Electromagnetic field tensor

How will we derive the component of electromagnetic field tensor by using the electromagnetic field tensor formula; what is the procedure? Can anyone tell me about how I would derive $F_{00}, ...
4
votes
3answers
145 views

Does the force between two point charges change when inertial reference frame changes?

The two point charges are as below: Then the Coulomb force between them is: $$F_1=\frac{q^2}{4\pi\epsilon_0 r^2}$$ When the inertial reference frame moves left at $v$(or the point charges move ...
2
votes
2answers
1k views

Can an insulator have magnetic poles?

Is it possible to make an insulator have two poles like a magnet? If yes, how to make it?
1
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1answer
122 views

Energy conservation of permanent magnets engine

How to explain the fact that magnet can attract an object (apply a work W) without losing a (significant) part of its internal energy? How to apply the energy conservation principle? Please think to ...
0
votes
2answers
29 views

How can space strech a wave?

Because of the expanding of the universe a wave from a far galaxy or star can be streched to more red. But how is space doing that? There must be a kind of 'entanglement' between the wave and space. ...
1
vote
2answers
83 views

How does magnetism work? (Not asking what magnetism does)

To clarify, if I asked "how does electricity work" I would want "electrons flow from a negatively charged area to a positively charged area", not "it powers electronics like phones" as an answer. The ...
0
votes
0answers
9 views

Is the magnetic dipole moment in charged particles the more survivable property than the electric charge?

After asking about Which of the properties of particles are intrinsic properties and why? it was commented "Charge, spin, lepton number, baryon number definitely. Mass maybe." and "is magnetic dipole ...
1
vote
0answers
13 views

Need some help in finding Poynting vector

I am asked to Calculate the Poynting vector of the transmitted wave but I am not sure how. I am able to show the electric field of the transmitted wave as shown above in the picture.
0
votes
1answer
19 views

Zero or non zero electrical potential energy of the system

Say you have this scenario: and you wanted to find the electric potential energy of the +q charge Now if you were to find the potential energy of +Q and +q system and add that to the potential ...
1
vote
1answer
54 views

What does radiofrequency really do in NMR & MRI?

In NMR, the protons’ frequencies are affected by the magnet which makes them precess around its magnetic field so now all protons will have different frequencies according to their environment ...
0
votes
1answer
34 views

Why can only moving metals (aluminium) reflects a magnetic field of a magnet?

A maglev train is riding because the magnets in the train are reflecting their magnetic field on the aluminium rail, so it is lifted up. Also when you rotate a aluminium plate and you hold a magnet ...
-1
votes
0answers
14 views

Why oscillating charges produce magnetic field? [duplicate]

Electron is a charged particle. As far as I know, an electron has an electric field when it is at rest. But when it starts "moving", it creates a magnetic field. How does that happen? How does the ...
2
votes
2answers
153 views

Momentum around an accelerated electron

Assume that an electron is accelerated along the +x-axis. The electron will radiate electromagnetic energy and momentum in every direction. But it seems to me that the EM momentum it radiates in ...
-2
votes
1answer
15 views

calculation od stacked plate capacitor capacitance

how we calculate the capacitance of a stacked plate capacitor ? is it the summation of the capacitance of each capacitor $Ci$ ?
0
votes
2answers
595 views

How does the magnetic field get induced in car's tire?

I was amazed to see how my car's tire started attracting the nuts and bolts, how in the world did it induce the magnetic field? Never heard of such phenomena!!!
1
vote
0answers
23 views

Electric Field From a point P

A rod of length $L$ and total charge $Q$ creates an electric field at a point $P$, $x$ units away. We need the electric field at this point. I am working on a physics problem for homework and the set ...
5
votes
1answer
305 views

Biot-Savart law from Ampère's with multivariate calculus

Let us assume the validity of Ampère's circuital law $$\oint_{\gamma}\mathbf{B}\cdot d\mathbf{x}=\mu_0 I_{\text{linked}}$$where $\mathbf{B}$ is the magnetic field, $\gamma$ a closed path linking the ...
2
votes
0answers
26 views

Can the permittivity tensor always be diagonalized?

For an anisotropic medium, permittivity is a symmetric tensor, with elements as $\epsilon_{ij}$ How can one be sure that it is diagonalizable, if some elements, say $\epsilon_{xy}$ and ...
0
votes
0answers
9 views

Conformal transformation for a vertical strip at constant potential

Say I've got a vertical strip in the $xz$-plane running from $x=0$ to $x=a$ held at a constant potential $V_0.$ The whole of the $yz$-plane is fixed at zero potential. Now I want to find the potential ...
0
votes
2answers
186 views

Dielectric boundary

I am trying to determine why electric field may be confined to a certain region if there is a large difference in the permitivity for example if electric field flows through water and then reaches a ...
1
vote
1answer
29 views

Open Source Particle Tracing Software

I am fairly new to this forum. I am designing an ionospheric plasma testing environment in a thermal vacuum chamber and to do so I require some form of multi-physics package for simulations .. such as ...
1
vote
0answers
18 views

Relating Poyntings theorem to Lenz and Faraday's law?

In system's similar to a motor, where the armature begins to accelerate simultaneously there is induced $-\epsilon$ to reduce the applied current(hence the applied power $P(t)$ is also reduced), or ...
0
votes
1answer
28 views

Need help to prove the following equation [on hold]

I am able to show the equation at (2) but I am not sure how to show the equation at (3). Any help is greatly appreciated
1
vote
0answers
18 views

What causes that the outer electrons of iron have the same spin and creates a dipool?

Iron has the capability of magnetism. The cause of that is that the spins of electrons in the outer shell are the same so they pushes each other away because of Pauli exclusion principle so they are ...
2
votes
3answers
186 views

What is supposition of equilibrium? How do Rayleigh, Jean know the electromagnetic wave in equilibrium behave?

In a cavity of dimension L, the wave must give zero amplitude at the wall, means wave equation has zero amplitude. Why? Answer from hyperphysics "since a non-zero value would dissipate energy and ...
0
votes
0answers
12 views

Finding the value of E at a point in a 3D field with 2 separate point charges [on hold]

Having some trouble with this question: A charge 30 nC is situated at the point (x, y, z) = (0, 2, 0) and a charge of –30 nC at the point (0, -2, 0) m. Find E at the points: (a) A = (4, 2, 4) m
4
votes
1answer
218 views

Are magnetic hysteresis losses relevant to alternating currents flowing in a wire?

Say we have an AC in a magnetically lossy material, like iron. Because of iron's relatively high permeability, skin effect will be more pronounced than it is in say, copper, so this iron wire isn't so ...
0
votes
2answers
327 views

How come the length of a wire does not affects on the circuit current?

Today we started learning about the electromagnetic induction. Out teacher gave us the following explanation: Suppose we have a conduction frame inside a magnetic field $\vec{B}$ going towards the ...
0
votes
2answers
757 views

How to get the magnetic field strength in space near a solenoid

I am trying to find the magnetic field strength along the axis of a solenoid. If I know the strength of the field at the center of the solenoid, and I know the distance from the center of the ...
3
votes
1answer
103 views

Au, Ag nanoparticles plasmon peak position?

Is the interaction between metallic nanoparticles (~ 20 nm) and light in the UV-Vis-NIR range governed by Mie theory or by Rayleigh scattering theory? Where are the Au and Ag plasmonic peaks located? ...
0
votes
0answers
46 views

Kinetic to Electric Energy

Not the best with understanding the formulas involved with the calculations but here is the gist: I want to use Faradays law to convert the kinetic energy from actions such as shaking to produce ...
0
votes
1answer
155 views

Pulling a bar through a magnetic field

This is a question on magnetic flux and induced voltage. I believe that this problem's answer key is incorrect, and I want to determine whether my opinion is sound. I also want to explore one ...
4
votes
2answers
573 views

What happens when you pass a current through a coil made of a ferromagnetic substance?

I know when you pass a current through a length of copper coil or a solenoid, there is the induction of a magnetic flux. But what of the coil made of the ferromagnetic material, the permanent type ...
0
votes
1answer
83 views

What is wavelength of electric field generated in a wire? [closed]

I read link given below: http://en.wikipedia.org/wiki/Speed_of_electricity Above link says _The speed at which energy or signals travel down a cable is actually the speed of the electromagnetic ...
-2
votes
0answers
32 views

Reducing a vector calculus relation [on hold]

Someone can help me? I want to reduce this. $$ (\nabla \times \mathbf{B}) \times \mathbf{B} + (\nabla \times \mathbf{B}) $$ Thanks
-3
votes
0answers
33 views

Free energy in magnetic materials [on hold]

The phenomenological description of magnetoelectrics and multiferroics is based on the series expansion of the free energy is powers of the magnetic and electric fields. WHY IS IT EXPRESSED IN TERMS ...
1
vote
2answers
54 views

What does it mean to be unique in terms of vector potentials?

I was in an electromagnetism lecture, where we were looking at the magnetostatic Maxwell’s equations: $$\begin{align} \nabla\cdot\mathbf{B} &= 0 \\ \nabla\times\mathbf{B} &= \mu_0\mathbf{J} ...
3
votes
1answer
39 views

How does parallel plate lose energy when deflecting charged particles

Say you have a parallel plate setup, each plate is charged with ±Q of charge, and is then disconnected from the power source and is insulated from the environment. There is no way the plates can be ...
11
votes
5answers
423 views

Transformer ratios - 1:2 vs 50:100

I am only the equivalent of a high school student, so please, if possible, don't answer this question with anything too complex or really advanced university level. I am very happy to research new ...
3
votes
0answers
35 views

In what circumstances, can exchange interaction acquire temperature dependence?

Heisenberg exchange interaction (sometimes called as magnetic stiffness?), originating from the Coulomb interaction and the Fermion statistics, is widely used in theories of magnetism. Conventionally, ...
4
votes
4answers
3k views

How does a magnetic field cause objects to move?

I understand electricity and magnetism and the relationship between them. What I never understood, and never actually seen explained in any book or website, is how opposite magnetic fields cause ...
0
votes
0answers
25 views

How can a moving point charge “update” its fields without emitting an e-m wave?

If an observer brutally moves a point charge from position A to position B, the electric field produced by the point charge at a given point P changes. Furthermore, a delay is observed before this ...
2
votes
2answers
124 views

Reference frequency

In my experiment, I have to obtain a relation between magnetic field of a solenoid and the applied Vp-p. The setup is as follows. http://www.teachspin.com/instruments/faraday/ Does my choice of ...
0
votes
1answer
121 views

potential inside a cylindrical shell in terms of the surface potential?

Given a potential distribution $V(\phi)$ at the surface of an infinite cylindrical shell, is there an easy way to derive the potential inside the cylinder. No charges or currents anywhere.
1
vote
0answers
17 views

how would an iron encapsulated electromagnet work?

I have been searching for this for a while now, but, a normal electromagnet works with(example) an iron cylinder and a copper coil around it, but would it work the same if you were to encapsulate it ...
1
vote
1answer
77 views

Light is/produces a electromagnetic wave, but is the opposite also true?

A similar question has been asked, and to some extent the answer is: you need an antenna with $L \approx 200nm$. But it's not entirely clear to me if this is completely hypothetical, as "Marek" also ...