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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218 views

Drift Speed and Current in Two Different Inertial Frames

We have a long, cylindrical wire carrying a constant current I in an inertial frame. At a distance of R from the center of the wire, the magnitude of magnetic field is $μI/2πR$. What is the magnitude ...
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0answers
11 views

Why does a 2-sided propagating EM wave become 1-sided if B is made proportional to E?

If you simulate the propagation of an electromagnetic wave in 1D free space (no charges or currents) with initial conditions of $E\neq0$ and $B=0$, and you look at a movie of $E$ vs time, then after ...
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1answer
279 views

Does the Lorentz force law explain Flemings left hand motor rule and the right hand dynamo rule?

the Lorentz force on a charged particle $F=qv \times B$ can explain Flemings left hand rule (motor rule) and the right hand (dynamo rule) In the left hand rule, the direction of the current gives ...
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27 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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2answers
171 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 ...
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2answers
2k views

Why doesn't the magnetic field polarize when polarizing light?

If the magnetic field doesn't polarize does it follow the electric field path of propagation? or does it vanish?
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1answer
376 views

Right hand solenoid example

I am just studying for my physics final exam. I came across this example and am not satisfied with the answer. My answer is that Z is South and the current would flow from Y to X because of the ...
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9answers
13k views
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3answers
211 views

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

In a cavity of size $L$, 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 violate ...
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0answers
13 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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0answers
42 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
56 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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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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1answer
36 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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1answer
21 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
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
19 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
32 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
36 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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2answers
3k views

Landau Levels and Magnetic length

I am looking into Landau Levels and keep coming across a magnetic length defined as follows for a 2D system: $l_B=\sqrt{\frac{\hbar c}{e B}}$ I have seen numerous sources say this is: $l_B\approx ...
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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
119 views

Voltmeters and changing magnetic fields

A changing magnetic field pierces the interior of a circuit containing three identical resistors. Two voltmeters are connected to the same points, as shown. V1 reads 1 mV. What does V2 read? ...
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0answers
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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3answers
239 views

What determines the magnitude of force of an electromagnet?

Let's assume that there is an electromagnet with $B = 1 Tesla$ , what is the magnitude of the force it could generate on other dipoles equal to it's surface area? Can the magnitude of force (in ...
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4answers
2k views

Why should Conservative forces have their curl equal to zero?(intuition)

There are several conditions that must be met in order for a force to be conservative. One of them is that the curl of that force must be equal to zero? What is the physical intuition behind this? If ...
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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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1answer
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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4answers
1k views

Is there an underlying physical reason why the Coriolis force is similar to the magnetic component of the Lorentz force?

I couldn't help but notice that the expression for the magnetic component of the Lorentz force, $$\mathbf F = q\,\mathbf v \times \mathbf B\,,$$ is very similar in its mathematical form to the ...
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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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6answers
1k views

What is the physical definition of causality?

Maxwell's equations give a physical relationship between the electric and magnetic fields $\vec E$, $\vec B$ at the same time, which some interpret as changes in one causes changes in the other etc. I ...
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3answers
515 views

How to find the direction of an eddy current?

Suppose there is a magnetic field going from left to right. Suppose a thin sheet of metal conductor (e.g. a 1m*1m square) is dropped through the magnetic field such that the plane of the conductor is ...
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3answers
32k views

Using A.C or D.C to create an electromagnet

Does it matter if I use A.C or D.C as a source to create an electromagnet? Or Does it depend on the voltage of current? For example: I have a coil which has about 50 turns. If I use same amount of ...
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1answer
120 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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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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1answer
124 views

Does Faraday's law work both ways, and if so why don't we use d.c for a generator?

Faraday's law in the integral form can be stated as $V = -d\Phi/dt$, where the right-hand side represents the rate of change of the magnetic flux and the left the voltage difference. In other words, a ...
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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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2answers
122 views

Why does the electric polarisation (polarisation density) have this direction?

It can be inferred that the amount polarisation $ P $ is dependent upon and proportional to the magnitude of the electric field $ E $ applied: \begin{equation} P \propto E. \end{equation} This ...
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2answers
3k views

Why is bandwidth, range of frequencies, important when sending wave signals, such as in radio?

So in wired/wireless networking and radio, signals are sent in form of wave. Then the concept of bandwidth comes in, which is the difference between highest frequency and lowest frequency in a signal. ...
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0answers
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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1answer
1k views

Electric field from conductive to dielectric media

I am interested in the main difference between transitions from electric fields from Conductive to Conductive/ Dielectric to Dielectric and Dielectric/Conductive media. What are the boundary ...
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0answers
18 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
144 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
117 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
29 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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3answers
175 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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1answer
818 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
29 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 ...