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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Does a magnetic field sustain a current in a loop?

A particular current produces a constant particular magnetic field. If the fact holds under scrutiny, why doesn't a magnet held within a loop sustain a particular current? Are the electron spins ...
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2answers
1k views

Where does the energy come from when a current heats a wire (resistor)?

I'm trying to figure out an example from a textbook (Demtröder -- Experimentalphysik 2, pg. 198) where the energy transport caused by a current is depicted: Assume you have a wire (with some ...
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1answer
55 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 ...
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1answer
83 views

Is there inductance to a DC circuit?

When a DC circuit is carrying current, large amounts or small, is there induced-emf due to the inductance? Or is it only applied to AC circuits?
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1answer
45 views

In which loop does the induced emf remain constant during the passage out of the magnetic field region?

A rectangular loop and a circular loop are moving out of a uniform magnetic field to a field-free region with a constant velocity V as shown in the figure. Explain in which loop do you expect the ...
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1answer
76 views

What would Maxwell's Equations be if we had magnetic charges and magnetic currents?

Mind you, we still have electric charge and electric currents. But, what would Maxwell's equations look like if we had to take magnetic charges and magnetic currents into consideration? Would there be ...
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1answer
40 views

Evaluate singularity in numerical iterative physical optics integral

My goal is to implement a code to numerically compute the radar cross section (RCS) of an open-ended PEC cavity by the approach of "iterative physical optics" [1]. The idea is to iteratively compute ...
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28 views

E-Field from Magnetic Flux Density

I'm currently trying to go through the book "2008+ Solved Problems in Electromagnetism" by Syed A. Nasar to try to build up my (very) weak ability to solve E&M problems—and having a... really ...
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2answers
106 views

How to measure vector potential such as that of magnetic field?

If we want to measure gravitational potential energy of a body-earth system, we can simply do it by measuring the body's height from ground. How could we can measure vector potential such as that of ...
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4 views

Verification of Fourier transformationn of Io-sinh function [migrated]

I try to match. What could not match $I_o-\sinh$ pair developed by Ben Logan, transform pair also published in The Practical Application of the Fourier Integral Campbell, George A .Sir J.F. Kaiser ...
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3answers
209 views

Conservation of energy in electromagnetism?

How is energy conserved in electromagnetism? Is it hinted only from Lenz's law or from Maxwell? Also, if a solenoid's flow of current produces a magnetic field, and from the magnetic field forces are ...
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1answer
100 views

Why is there a voltage drop across a superconducting inductor?

If there is no electric field inside a perfect conductor, then why is there a potential difference across an (superconducting) inductor?
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147 views

why the inner product between divergence-free current$\vec{J}$, (satisfied $\nabla\cdot\vec{J}=0$) and a gradient field$\nabla \varphi$ is zero?

I read a statement saying that the inner product between divergence-free current and a gradient field is zero. Divergence-free surface current is $\nabla\cdot\vec{J}=0$, and $\vec{J}$ could be ...
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2answers
34 views

Uniformity in a solenoid

I know the magnetic field strength increases as the number of turns in the solenoid increases. However, I've learnt the field inside the solenoid is usually nearly uniform. So, does the number of ...
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1answer
34 views

Transformer leakage reactance

In practical transformers there is always a leakage reactance. If it is possible to construct such a transformer (ideally thinking) having zero leakage reactance will the transformer action work? or ...
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5answers
544 views

Can we say there are 2 EM radiation types?

EM radiation seems to come from two different sources: According to the Maxwell, by acceleration of electrons According to the Bohr, by jumping of electrons between energy levels? Are these two, ...
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1answer
52 views

Magnetic Induction

Consider the figure above in which there is an electric current (constant) in R1 (consider it to be made of a conductor). Obviously there will be a magnetic field induced in R2 (consider it to be ...
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2answers
334 views

Does a current-carrying wire running through the centre of a solenoid experience force?

Imagine looking at a solenoid from above. Current passing is through it in a clockwise direction. The direction of the field lines therefore is towards the bottom of the solenoid. Now pass a straight ...
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3answers
417 views

Moving conducting bar in (changing) magnetic field

In my exam today I've been given this problem, yet even with the results at hand I simply can't warp my head around it; Given the picture below, a bar is placed on two conducting rails with a ...
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1answer
64 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 ...
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52 views

Constructing conserved current given the lagrangian

Consider the following Lagrangian for a massive vector field $A_{\mu}$ in Euclidean space time: $$\mathcal L = \frac{1}{4} F^{\alpha \beta}F_{\alpha \beta} + \frac{1}{2}m^2 A^{\alpha}A_{\alpha}$$ ...
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0answers
58 views

Determining the attraction of a magnetic bob

I have a question concerning the calculation of the attraction to a ferromagnetic material in a coupled B-field, which of $N \geq3$ equally strong cylindrical permanent magnet is produced. ...
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1answer
42 views

Attraction & Lorentz force, at the same time?

If a conductor carrying current is placed inside a magnetic field, we know that there is the Lorentz force pushing the wire. But what about the attraction force between the wire's field and the ...
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3answers
132 views

Electromagnetism: Conductors

Even though the thermal velocity of electron in a conductor is comparatively high, the thermal velocity is not responsible for current flow? Why is this the case?
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3answers
242 views

magnetic scalar potential far above a magnetic film

The situation I am looking at is a magneto-static problem of a finite magnetic film with magnetization $\bf{M}$. I would like to find the the magnetic field far above the plate. My expectation is that ...
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1answer
51 views

Conservation of energy in transformers

How does energy remain conserved in a transformer if emf is increasing, or decreasing? Does the current decreases to accomodate?
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1answer
35 views

Magnetic field above center of current-carrying loop

A current $ I = 2A $ flows in a circular loop coil of radius $R = 10 cm$. Evaluate the magnitude of the magnetic field (in Tesla) at a point $P$ on the axis of the coil, and a distance $z=5cm$ away ...
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77 views

Is it possible to design a large permanent magnet that creates a multi-Tesla field?

Is it ever possible to create a LARGE magnet (in meters wide/long) that could potentially create a powerful magnetic field?
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293 views

Why are magnetic field lines perpendicular to the surface of a ferromagnetic material?

It is known that magnetic field lines become nearly perpendicular to the surface of a ferromagnetic material. The quantitative proof which uses the boundary condition requires that magnetization at ...
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44 views

Electromagnetic waves and boundaries

I am reading about dielectric boundaries and electromagnetic plane waves following griffiths ch7. When considering a boundary at z=0 with electric fields perpendicular to the plane of incidence it ...
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1answer
45 views

Torque on wire summarized with magnetic moment

The magnetic moment of a current-carrying wire loop $L$ is $$ \boldsymbol\mu = \frac I2\oint_L\mathbf{r} \times \mathrm{d}\mathbf{r} $$ so the torque it experiences under a uniform magnetic field ...
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1answer
28 views

How you call the constant $\alpha$ within the heat equation in general and in terms of electromagnetism?

The heat equation or diffusion equation does contain a constant $\alpha$. $$\frac{\partial u}{\partial t} - \alpha \nabla^2 u=0$$ How is it called? I'm interested in a general name which can be ...
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2answers
83 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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1answer
46 views

Reflection from multiple thin films: accounting for lost light due to small surface area

I have a problem similar to reflection of multiple thin films. I have light coming in from medium 1 and I want to find the total reflected intensity after being reflected inside 2 layers. However, I ...
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3answers
153 views

Describing a circular current loop as delta functions

It would be really nice to see how Jackson got eqn 5.33 on his example problem for finding the vector potential of a circular current loop $$ ...
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1answer
78 views

Does the weight of an magnet raise when balancing an object?

I was wondering if say I use an magnet which is facing positive and then I expel an another magnet above it which is facing the positive with its positive side and I ensure the magnet does not rotate ...
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2answers
57 views

Magnetic field in materials with non-constant magnetic susceptibility

I'm quite lost what $B$ and $H$ is. It seams to me that most of the texts I read do quite poor job in explaining them properly. They are explained only in cases when magnetic susceptibility is ...
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1answer
58 views

Why Change Of Magnetic Flux Induces Current In circuit?

Why change of magnetic flux with the time through a coil induces current in it? Please tell me that what happen to charge in the coil if the magnet or electromagnet wave around this? Also, why a ...
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1answer
44 views

calculating electrodynamic momentum of a dumbbell (consisting of two point charges) in longitudinal motion

I'm working through a paper on momentum in electrodynamics that requires the integration below and would greatly appreciate any help. I'm pretty sure it evaluates to $2/d$ but I can't quite figure ...
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1answer
73 views

What is the significance of the difference in the eigenvalue equations of Bloch functions for electrons vs photons?

any text on photonic crystals will highlight the almost perfect analogy between electrons in a periodic potential and photons in a periodic dielectric. The analogies are: $$V(\vec r + \vec R) = ...
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1answer
722 views

Magnetic field inside and outside cylinder with varying current density

I am reading through Introduction to Electrodynamics by David J. Griffiths and came across the following problem: A steady current $I$ flows down a long cylindrical wire of radius $a$. Find the ...
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1answer
54 views

Copper coils for nMRI

Given the lack of financial efficacy in the medical industry, I was wondering if nuclear magnetic resonance imaging would be possible with a copper/silver coil if the use is short 10 second imaging ...
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2answers
66 views

Can the magnetic fields of EM radiation be harnessed or measured?

We use the electric component of EM radiation to create the EM radiation and to detect it (antennas and Etc.), but does anyone know of a situation where the magnetic component of EM radiation is used? ...
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1answer
59 views

Magnetic field of a stationary electron

As far as I know, a magnetic field can only be produced by a moving electric charge, or from a particle's spin (this is how a permanent magnet works, all the spins are in the same direction) What is ...
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2answers
50 views

Induction on a circuit with swich

I have the following circuit: It is subject to a steady, time-invariant magnetic field which points out of the paper. At $t = 0$, the switch closes. I thought that the magnetic flux would decrease ...
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2answers
106 views

Does the geomagnetic field rotate?

The Earth rotates about it's own axis. Do the geomagnetic field lines rotate due to this rotation or not?
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4answers
10k 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 ...
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0answers
18 views

Question on envelope-carrier description of traveling wave

I'm doing a research internship in attosecond physics right now, and one of the really important things in the field is the description of a propagating laser pulse as the combination of a slowly (or ...
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3answers
98 views

In scattering, how does a particle 'know' which direction it is being illuminated from?

In scattering experiments, for example light scattering, the scattering strength from different sized particles is depicted as below. What I can't understand is: how does a particle know which ...
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2answers
166 views

What happens from the point when we apply potential difference across an inductor?

I am in a serious doubt about it. Consider a battery of emf E and we connect it to an inductor. Initially the switch is open, now we close the switch. My question is: What mechanism happens just ...