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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Why is second harmonic Intensity periodic in coherent length?

Solving for the intensity of second harmonic generation we get that intensity is $sinc^2(\pi/2*L/L_{coherent})$. How is calculated that the intensity is periodic in L_{coherent} (coherent length, L ...
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26 views

Problem with understanding boundary conditions in electromagnetism

In some books on electrodynamics they stress that electric current won't radiate if it is placed on a perfect electrical conductor (PEC), citing image theory: exactly opposite current will appear and ...
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0answers
19 views

Is absorption probality modulated by interferance instanteneous or retarded effect?

Let say the absorption probability at some atom 1 location is modulated by photo ionized electron wave (ionized from 1) that scatters by neighboring atom 2 and returns to the 1. (Around the absorption ...
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0answers
12 views

How much damping is there being exerted on a vertically stable magnet moving across a superconductor surface?

I'm thinking of a flat magnet with opposite poles on the wide ends, though perhaps some other arrangement is more stable. Assuming such a magnet is levitating in a completely stable state and is ...
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1answer
62 views

How to conserve momentum when numerically-integrating the path of a charged particle through a magnetic field

I have the python script attached below, which is intended to track the trajectory of a charged particle in a static, uniform magnetic field. It is very simple to calculate the instantaneous force ...
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53 views

Metal Sphere in a Uniform Electric field

An uncharged metal sphere of radius $R$ is placed in a uniform electric field $\vec{E} = E_0 \, \hat{z}$. The field will push the positive charges to the "northern" surface of the sphere, and the ...
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41 views

Gradient of Greens function

This question is about Jackson's equation (10.75) and (10.77) I don't know the step in between these two equations.I'm not sure what our unit vector $n'$ will be here and how can we take gradient of ...
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1answer
48 views

What is the QED model about half wave plate?

Surely classical electrodynamics (CED) has a good and well-known answer about the change of light polarization in a HWP (or Quarter wave plate). I tried to find how does this look like from point of ...
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0answers
24 views

Can orbital angular momentum of an electron be changed?

In a lab at my university they are working on a project that deals with changing the "net magnetization" of a material by "flipping" the spins on some electrons. Is orbital angular momentum not ...
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2answers
37 views

What would happen to a motor without a split-ring commutator?

I realise that a similar question has already been asked (Why does the coil in this apparatus reverse its direction of oscillation?), however after reading the answers, I am still unclear on certain ...
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0answers
17 views

What is special about the orbitals of iron, nickel and cobalt to make them ferromagentic? [duplicate]

Ferromagnetism depends on the arrangement of orbitals and spin in the valence electrons of an atom (below its Curie point). But what is so special about the orbitals of iron, nickel and cobalt, to ...
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0answers
18 views

With the help of a given charge is it possible to obtain a charge many times greater in magnitude

Consider a spherically conducting sphere. If a charge(say positive) is placed near the sphere, then it will lead to separation of charges inside the sphere.My question is, can we create charges more ...
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1answer
31 views

Magnetic field lines are not perfectly straight between magnetic poles

Why is fringing between magnetic poles inevitable? The diagram given below is the incorrect representation of magnetic field lines between two poles:
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1answer
20 views

Mie theory: Interpretation in terms of intensity

I'm trying to understand Mie theory. For this I'm reading the book "Absorption and Scattering of Light by Small Particles" by Bohren and Huffman. The derivation of the formulas is fine, but I'm stuck ...
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0answers
13 views

Polarity of emf induced in open circuits

In a closed circuit, current induced is in such a direction that it opposes the change in magnetic flux. In case of an open circuit like a metallic rod, which end of it will have the higher potential? ...
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1answer
35 views

Why is diamagnetism more common than paramagnetism or ferromagnetism?

Why are there more diamagnetic substances in nature than there are paramagnetic or ferromagnetic substances?
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28 views

What is the electric field in medium 2 if the medium 1 contains an electron?

I have got this strange yet trivial problem. Say, there's an electron inside medium 1. Outside the medium, lets say we have medium 2. What is the electric field at any point in medium 2 due to the ...
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1answer
35 views

Surface potential and work function

I am quite confused to the manifestation of an electric field on the surface of a metal due to the work function. Can anyone give me an intuitive answer behind this? For example: So I am studying ...
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1answer
30 views

At what times is the energy in an LC oscillator completely electric or completely magnetic?

I know that the time period of the LC oscillations is given by $T=2\pi\sqrt{LC}$. At what times is the total energy of the circuit completely stored in the capacitor or completely in the inductor?
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20 views

What is the physical significance of the area and shape of hysteresis curve of ferromagnetic materials?

Why do some materials have thinner/broader hysteresis curves than others and what does this difference in shape imply in terms of magnetic properties of the substances? Is there any significance to ...
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1answer
33 views

What is the source of the Curie point?

I'm seriously revisiting my knowledge on magnetism, and the Curie point has been both enlightening and mystifying. I understand what it does ((ferro)magnetism disappears above it), and have a faint ...
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1answer
39 views

Is there any objective basis for the “south to north” directionality of magnetic fields?

While explaining magnets to some students, I just realized a very basic thing I never thought about: We often say that magnetic fields have a "direction", that magnetic field lines "exit" the north ...
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0answers
27 views

Net magnetic field & magnetization on a ferromagnetic material

A ferromagnetic materiel is saturated by an exterior magnetic field like so: Where $B_1$ is a uniform magnetic field. Then after some time $t$ another magnetic field $B_2$ is introduced as well, ...
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36 views

Gravity Theory Question [closed]

Three Questions: Is the Modern Five Dimension Theory of gravity an adaptation of Einstein's relativity theory? If so, can it allow us to accurately predict the movement, speed and direction of ...
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16 views

What metal can be used as electromagnet core to attract a permanent magnet?

ok. Here are the requirements: 1.The core should not be attracted to permanent magnet when idle(like stainless steel), 2.Should act as electromagnet core with better permeability(like iron). Is ...
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1answer
41 views

Why does a magnet loose magnetic strength when heated?

Magnet manufacturing companies, for example Dexter Magnetic Technologies do not hesitate to explain the effects of heat upon magnets - citing how it degenerates the magnetic properties with levels of ...
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1answer
28 views

Magnetic field and magnetic domains

I have a confusion regarding how magnetic field propagate in a medium (ferromagnets or air etc..). It is knows when a ferromagnet is exposed to an external magnetic field, the magnetic domain of the ...
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7answers
251 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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1answer
30 views

The MRI signal: why do we consider the phase in the MRI signal

I am trying to understand the imaging principles behind MRI and I was looking at some lecture slides found here Specifically, I am looking at slide 41 where we look at some of the equations regarding ...
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1answer
26 views

Imaginary component in capacitive reactance

This is a trivial question in first year but, unfortunately, this popped up in an elementary yet compulsory lab experiment. The capacitive reactance is defined as $$X_{c}=\frac{1}{\omega c}$$ The ...
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28 views

Attraction between electromagnet and a permanent magnet - at which point is the force the strongest?

You have an air-core inductor with current applied, so that the magnet in the picture will be attracted to it. At what position will the force between the two be the strongest and at which the ...
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2answers
252 views

Why is infrared radiation associated with heat?

I am little confused with infrared radiation. I understand that when an object is hotter, it radiates electromagnetic waves of a bigger frequency and this waves are also more energetic, that is why ...
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1answer
24 views

C60 Macro Ojects

What would happen if you could make a ball or hollowed geometric shaped say the size of a basketball made out of C60.. due to its superconductivity could it have exotic or anti- effects on gravity and ...
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0answers
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Clarification of why pressure must be constant at a boundary between two fluids/gases?

In section 13.2.1 of these notes on acoustic waves it says that the pressure must be continuous at the boundary of two fluids/gases in equilibrium. If they were not it says the following - If the ...
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3answers
173 views

Maxwell's equations - underdetermined - uniqueness

Maxwell's equations can be seen as two dynamical equations (the two curl equations), and two constraint equations (the two divergence equations). So we have 6 unknowns ($E_x,E_y,E_z,B_x,B_y,B_z$). ...
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31 views

Why should we use good conductors as electrodes of a capacitor?

I was asked if and why why should the electrodes of a capacitor should be good electrical conductors. It seemed obvious to me that if I want to form a Helmholtz double layer (because that's what ...
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21 views

Plot current-voltage curve

I have been given the following question as part of an investigation for my Applied Maths course: "[...] Then construct the current-voltage curve, i.e. the dependence of $\left< \dot{\theta} ...
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2answers
116 views

Scalar and Vector Potential

I am a physics undergraduate student currently studying electromagnetics. I have previously studied electrostatics and magnetostatics yet the concept of scalar potential, $V$ and the vector potential, ...
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1answer
18 views

Reflection coefficient of surface with sources

While solving the maxwell equation for surface where both surface charge and current density are present to calculate its reflection coefficient, i.e. $$\nabla \times E=-\frac{\partial}{\partial ...
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1answer
37 views

Can the magnetic field of the earth be utilized in a manmade craft? [closed]

What if there was a magnetic ring around the craft on a gimbal that could freely rotate from positive to negative, then could the craft utilize the magnetic field of the earth without flipping the ...
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1answer
35 views

Period of oscillation of magnet levitated over another magnet

The situation is similar to what we used to do as kids, take a vertical wood dowel, with a ring magnet placed at the bottom, and another ring magnet opposing it, floating on top. More precisely, it ...
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16 views

Wigner-Eckart theorem and Van Vleck paramagnetism

Using the Wigner-Eckart theorem, we can express the matrix elements of Langevin's paramagnetic Hamiltonian $L_z + g_S S_z$ using only the quantum numbers of the total angular momentum, $J$ and $m_J$, ...
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21 views

Effective resistance of a plate with varying conductance

I'm trying to find the resistance across a plate made up of different materials. Do my formulas and method make sense - will they yield the correct answer? Reference I'm using: ...
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0answers
24 views

Would a spinning carbon steel (high magnetic permeability) sphere become magnetized?

Would a spinning carbon steel (high magnetic permeability) sphere "falling" through Earth's magnetic field become magnetized? Would a spinning, evenly charged sphere generate a magnetic field?
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0answers
16 views

Relation between relative dielectric constant, complex permittivty and complex conductivity

To complete Maxwell's equations in a natural medium we need a set of constitutive relations: $$ \vec{D} = \epsilon_0 \vec{E} + \vec{P} $$ with $\vec{D}$ the electric displacement, $\vec{E}$ the ...
3
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1answer
39 views

Relation between the electromagnetic wave and quantum wavefunction

I have been thinking about this for a while. I think I misunderstood something about the basics of quantum waves. Let's look at light diffracted in conditions similar to the double slit experiment. ...
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0answers
15 views

compass with current in a wire [duplicate]

How would you calculate the angle of deflection of a compass placed near an electric current? would you use b fields and the earths bfield to work out the arc length of the compass' movement? thanks
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2answers
35 views

Electromagnetism on a compass [closed]

A current $2\ \mathrm A$ flows in a vertical wire. The value of the horizontal component of earths magnetic field in the region is $2*10^{-5}\ \mathrm T$. A small compass is placed $5\ \mathrm{cm}$ ...
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2answers
42 views

Quantization of the Hamiltonian of a particle in a uniform magnetic field

If a particle of mass $m$ and charge $q$ is subject to a uniform magnetic field and if we have a vector potential $\mathbf{A}$ then we know that classically the dynamics of the particle will be ...
4
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1answer
22 views

Characterisation of passivity

I'm a mathematician trying to work on some problems involving metamaterials. I have some trouble to understand the notion of passive media. Informally, this means that (without sources), a medium ...