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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1k views

What is the effective spring rate of a magnetic spring

Consider a magnetic spring as seen on this YouTube video, but ignore gravity. If I wanted to calculate the effective spring rate (Force vs. Deflection) curve for the top magnet, how would I go by ...
3
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2answers
80 views

Does a source of photon emission “sense” if the photon is absorbed? [closed]

Can the absorption of a photon be 'felt' by the source that emitted it? At least, if we assume that it emits a steady stream of photons? Is there a back reaction of some kind on the source?
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0answers
16 views

Magnetostatic field inside a wire and current density

Right now I'm studying magnetostatics an I ran across a problem when trying to understand a question in a book witch led me to thing that the steady-state current density inside a wire is not ...
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0answers
7 views

Does charge injection from electrodes increases the total charge in a (semi)conductive channel?

When a (semi)conductor is connected to a voltage bias, charge is injected from the electrodes. When steady state is reached (constant current flow), does the total charge in the (semi)conductor is ...
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1answer
416 views

What are the boundary conditions for EM waves normally incident on the interface between two dielectric media?

An EM wave, amplitude $E_0$, frequency $\omega_0$, is incident upon a material with relative permittivity (dielectric function) $$\varepsilon \left( z \right) = \left\{ \begin{gathered}{\varepsilon ...
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1answer
30 views

Using 2 pair of like pole magnet side by side will increase magnetic field?

I would like to know if using two pairs of magnets with like poles side by side will strengthen the magnetic field? As I understand, placing 2 pair of magnetic with like poles side by side will dry ...
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0answers
30 views

Solving Schrodinger equation in magnetic field with mixed gauge condition [closed]

Consider electrons in magnetic field at low temperature, solve the Schrodinger equation to find out Landau level for the vector potential chosen as follows $$ \vec{A}= ...
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2answers
47 views

How does always the magnetic field lag the elctric field in a conductor?

In conductor, the relation between the phase of the magnetic field $\delta_B$ and that of the electric field $\delta_E$ is given by $$\delta_B-\delta_E=\tan^{-1}(\frac{\beta}{\alpha})$$ where $\alpha$ ...
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4answers
9k views

Is Biot-Savart law obtained empirically or can it be derived?

There's already a question like this here so that my question could be considered duplicate, but I'll try to make my point clear that this is a different question. Is there a way to derive ...
0
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1answer
152 views

Magnetic field in-between two antiparallel wires with the same current

If I have two antiparallel wires with the same current at distance $2a$ apart from each other, how would I go about modeling the magnetic field in between the wires? I know the fields will add but I'm ...
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2answers
22 views

Levitation inside a solenoid

Is it possible to levitate something inside a solenoid? If yes, is it possible in any direction (horizontal, vertical)? Also, is it possible when an object is traveling through? Thanks in advance!
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2answers
142 views

Induced current in open circuit and closed circuit

In a closed circuit, electric current is induced so as to oppose the changing magnetic flux as per conservation of energy. However, in open circuit, emf is induced across its ends. How it is related ...
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2answers
229 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
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1answer
886 views

Forces on a wire moving in a magnetic field?

When a wire is placed inside a magnetic field and current starts to flow in that wire, that wire experiences the Lorentz force = $IL \times B$ and beings to move. Aside from induced motional - ...
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0answers
25 views

Induced Electric field around a current carrying wire [closed]

I was trying to work out the induced electric field near an infinitely long current carrying conductor whose current is increasing at a constant rate of $k\,As^{-1}$. Starting with the calculation, ...
0
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1answer
19 views

Do we use rms values or peak value for calculating power of a transformer?

Do we use rms values or peak value for calculating power of a transformer? I have an A levels exam tomorrow and I wasn't able to find any kind of explanation in the book.
4
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1answer
332 views

Boundary conditions on current carrying wire

I'm trying to simulate by finite elements method Maxwell equations for a current carrying wire. My 3d geometry consists of a cylinder and a box containing it. I will use a mixed formulation and ...
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0answers
39 views

Rotating magnet and magnetic field intensity change?

if magnetic disc is rotating on it's axis super fast and electric field is produced(because of time varying magnetic field),does this means that in the frame of non moving observer magnetic field no ...
0
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1answer
203 views

What is the magnetic flux density “outside” the solenoid when AC current is passing through it?

I know that there are well defined equations explaining the magnetic flux density in the solenoid. However what about magnetic field outside the solenoid? How is the magnetic flux density related ...
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2answers
48 views

Why is the electric potential zero in this case?

The situation: a charge of $-2q$ is situated a distance $r$ to the left of a charge of $q$. What is the electric potential at a point $p$ a distance $r$ to the right of charge $q$? So basically: -2q ...
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0answers
21 views

Purcell's approach to Larmor's formula, assumption?

In 'Electricity and Magnetism' By E.M.Purcell, a derivation is given of Larmor formula (a version of which can be found here). I will give a brief overview here: A particle is considered at ...
1
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1answer
59 views

what do we mean by speed of light dependent on direction?

I have a statement in textbook saying "When the speed of light is independent of direction,the secondary waves are spherical". Can someone tell when it is dependent on direction and how the secondary ...
4
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0answers
35 views

Special Relativity, refractive index and catching up with a wave

Einstein was partially motivated by the following: With Maxwell's equations, a plane wave is a sinusoidal wave that varies in space in time and moving with speed $c$. These variations are linked by ...
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0answers
16 views

How can I approximate calculate the magnetic force between electromagnets?

I try to find an approximate amount of force exerted between two electro magnets. The below site could've helped me but the problem is that the below just talks about the number of coil turns and so ...
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2answers
55 views

Am I wrong about the right hand grip rule?

The right hand grip rule (also known as right hand screw rule) tells you the direction of a magnetic field due to a current. If you point your thumb in the direction of the current, your fingers will ...
4
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7answers
272 views

Is the electric field of a volume charge distribution well defined?

Consider a volume charge distribution $\rho({\bf r'})$. The electric field at ${\bf r}$ is $${\bf E}({\bf r})=\iiint\frac{1}{4\pi\epsilon_0}\frac{\rho({\bf r'})}{R^2}\hat{\bf R}\, \mathrm d^3{\bf ...
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2answers
2k views

Meaning of boundary condition for steady current density?

Although I understand the derivation of boundary condition in case of steady electric current but I did not understand, that the electric field which is in direction of $J$ current density that is ...
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4answers
3k views

Charged particle between two parallel likely charged plates, is it affected by the plates?

Imagine two parallel conductive plates. Charge up both to have the same amount of positive charge. Then put positive test particle between the two. The Coulomb's law is an inverse square law, so one ...
0
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1answer
33 views

Why don't conductive objects change color when electric dipoles are induced in them?

I was playing around with a comb, my hair, and pieces of paper when it occurred to me that inducing a dipole on the pieces of paper meant that many valence electrons previously on the side closer to ...
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2answers
123 views

Calculating the rate of change of Magnetic Field strength [closed]

I'm a bit stuck on this question (which is homework so hints are more welcome than outright answers). The question is: A very long wire carrying a current I is moving with speed v towards a small ...
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2answers
139 views

How to find the parts across which emf is induced?

Is there a general rule that will allow one to find the parts where the emf is induced? Consider for example the following two cases: Case 1: A rod is made to move normally to the magnetic field at ...
4
votes
1answer
677 views

Why are atomic quadrupole moments calculated using nuclear spin?

It's my understanding that electric quadrupoles interact with the gradient of an electric field, and I understand roughly how this works. I am trying to calculate the interaction between an atomic ...
5
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1answer
863 views

Magnetic Levitation : Confusion with Lenz's Law

Considering the above diagram, I am failing to see why the aluminium ring floats upwards rather than oscillates. I understand Faraday's law, but I am struggling with Lenz's law - as there is an ...
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1answer
249 views

Magnetic field inside of a long rotating cylinder

I have a simple question. I need to find out what is the magnetic field inside a rotating long cylinder with a charged density $\sigma$. The cylinder rotates around its axis with angular speed ...
67
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1answer
2k views

Does the 4/3 problem of classical electromagnetism remain in quantum mechanics?

In Volume II Chapter 28 of the Feymann Lectures on Physics, Feynman discusses the infamous 4/3 problem of classical electromagnetism. Suppose you have a charged particle of radius $a$ and charge $q$ ...
1
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1answer
206 views

Opacity/transparency of conductive meshes to charged particles (electrons/ions)

When using a conductive (metal) mesh, effectively a metallic woven fabric, in vacuum applications as a "grid" for charged particle optics, how does one calculate (or at least estimate) the opacity or ...
11
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1answer
254 views

How can I find the position of an image charge when the boundary is parabolic or hyperbolic?

If the position of some charge $Q$ is known, the boundary condition is $u=0$ on some parabolic surface, and we know the image charge has its electric volume of $Q'$, then how can I determine the ...
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1answer
13 views

Light Ray component field strength

When looking at a diagram of the electric and magnetic fields oscilating down the time axis of a light ray diagram the field amplitude and frequency of each component is shown equal.Are they infact ...
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1answer
127 views

Do magnets also attract super conductors?

If a superconductor can be repelled by a magnet (or, well, locked in a certain position by the magnet) can a magnet be attracted to a superconductor. What would happen if you take a peace of metal, ...
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2answers
165 views

Two different electrostatic charges in rotation

It's a theoretical problem. I mean I drawn a circle and I consider this circle exist. I would like to understand how works this device composed of a circle and two charges: Two charges fixed on a ...
1
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1answer
51 views

Unpolarized wave, $\Delta\varphi =\Delta \varphi(t)$?

I have seen a unpolarized wave defined in a number of places (e.g. here) as a wave such that: \begin{align} E_x&=E_0 \cos(kz-\omega t) \\ E_y&=E_0 \cos(kz-\omega t+\varphi) \end{align} Where ...
4
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3answers
401 views

Electromagnetism boundary condition

The dielectric slab surrounded by air as shown in diagram has permittivity $\epsilon$ different from air and permeability almost same as air. Electric field everywhere within the slab is given to be ...
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2answers
278 views

What kinds of behavioural anomalies can a zero-field-cooled (ZFC) / field-cooled (FC) split indicate?

If a material shows a spiltting in the ZFC and FC curves, is it necessarily superparamagnetic, or could there be any other reason for the irreversibility? I have heard spin glasses also show ZFC-FC ...
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0answers
24 views

Do EM waves create action in receiver antenna without reaction?

If EM waves such as radio waves, can induce electrons in the receiver antenna to move, and electrons have a small amount of mass, isn't this action without a reaction? Is it possible to use the ...
0
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1answer
113 views

Magnetization of soft iron and steel

I was reading and I saw that the steel becomes permanent magnet whereas iron becomes a temporary magnet after magnetization during magnetic induction. Why does soft iron make temporary magnets and ...
0
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0answers
19 views

Which is the form of Born-Mayer repulsion term?

I have found in many textbooks different and unclear form of the Born-Mayer repulsion term. I write the interatomic potential as $V(r)= -\frac{e^2}{r}+V_{BM}$ and for $V_{BM}$ I expected something ...
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2answers
60 views

Magnetism In Space

I am not an expert when it comes to physics. I was looking at maglev technology the other day and a thought crossed my mind. Would similar systems work for deep space exploration. I understand ...
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1answer
37 views

Differential Operator

I am trying to understand the following expression \begin{eqnarray} e^{-ik.x}D_{\mu}D^{\mu}e^{ik.x} & = & e^{-ik.x}(i\partial_{\mu}+A_{\mu})(i\partial^{\mu}+A^{\mu})e^{ik.x}\\ & = & ...
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1answer
29 views

Eddy currents in a superconductor

Just had a lesson we just had our teacher introduced the concept of eddy currents, and showed us how a magnet moves slowly through a metal tube due to the opposite generated magnetic field. If you ...
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2answers
47 views

Introducing a phase, what changes?

This question is related to: Mach-Zehnder interferometer and the Fresnel-Arago laws Let us say we have unpolarised wave taking the form: $$\psi=\psi_0 e^{i(kx-\omega t)+i\phi(t)}$$ Where $\phi$ ...