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Questions tagged [electromagnetism]

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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Contraction of spring on passing current

When a current is passed through a spring it contracts and it's potential energy increases . From where does this energy comes from ? Which agent does work? Please help!!
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How could you protect electronics from 100-1000g of acceleration [on hold]

I am currently researching the feasibility of setting up a coilgun that launches Cubesats for a paper.
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Question about Faraday Cage [duplicate]

I am aware that faraday cage is used to protect electronics from damage. However, if I put a signal generator in a faraday cage and turn the signal generator on, can the frequency still be picked up/...
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relativity in an electron beam [duplicate]

If two electrons are travelling side by side with the same velocity with respect to me, they are standing still with respect to each other. For me, they will exert on each other an electrostatic force ...
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Using Maxwell Stress Tensor to get force between two current-carrying wires

I’m trying to find the force per unit length between two parallel wires carrying the same current in the same direction and a distance of 2a apart. I need to use the Maxwell stress tensor and am ...
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5answers
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How does an ideal transformer satisfy law of conservation?

I have read in books and articles that, keeping the power as is transformers increase the voltage and therefore reduce the current as per the formula $$ P = IV $$ and this results to reduction in ...
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1answer
35 views

Electrons in an Inductor

What basically happens to the electrons, does the magnetic field produced by the electron moving in the inductor affect itself. what are the forces involved? Please elucidate the mathematics. The ...
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Will any charge oscillating in space create an EM wave?

Would it be correct to say that any charge oscillating in space (regardless of the spacial amplitude) at a given frequency will emit an EM wave of the same frequency? related: What change in an EM ...
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Electrodynamics of inhomogeneous media

I'm interested in electromagnetic scattering and for that purpose I would like to write out Maxwell's equations for space-dependent permittivity (and permeability). Actually I only need them for ...
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Equivalence between Maxwell's equations and vector Helmholtz equations

When are equivalent the Maxwell's harmonic equations: $$ \nabla\times\left(\nabla\times\mathbf{E}\right)=\mu\epsilon\omega^2\mathbf{E} $$ and the vector Helmholtz equations: $$ \nabla^2\mathbf{E}=\mu\...
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Is an inertial force needed to be added to second Newton's law when transforming from cartesian to cylindrical coordinates?

Let's consider I have a particle moving on the $x,y$ plane. On this particle acts the Lorentz force, however it does not necessary perform a rotation by the axes origin $(x,y)=(0,0)$. I would like to ...
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If is work done by magnetic field is 0 then why does 2 magnets attract when kept together? [duplicate]

If 2 magnets are kept close to each other they attract then shouldn't it be considered as work done by magnetic field?
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Explanation of Lenz's Law phenomena

If we drop a magnet through a copper pipe (without it touching any of the sides), it would fall slower than it would if there were no pipe. Having the pipe otherwise accelerate the magnet would be in ...
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Retrieving the non-relativistic formulas for electric and magnetic fields from relativistic formulas

I was checking the formulas for the electric and magnetic fields components E and B given in this link from Wikipedia: https://en.wikipedia.org/wiki/Classical_electromagnetism_and_special_relativity#...
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2answers
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Momentum density of the EM field - Classical field theory

The Lagrangian density of the EM field is given by $$ \mathcal{L} = \frac{1}{8\pi}\left(E^2-B^2\right) $$ Let $\vec{A}$,$\phi$ be such that $$ \vec{E} = -\frac{1}{c}\frac{\partial\vec{A}}{\partial t} -...
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3answers
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Can a wave's Poynting vector be in the opposite direction compared to its direction of propagation?

Can a wave's Poynting vector be in the opposite direction compared to its direction of propagation, and if so, what physical implications does it have? As I understand, the poynting vector s can be ...
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Faraday Law - Related question

I've been studying Faraday's law. In one example they gave me a circuit and they connected voltemeter with two points $A$ and $B$ where there is Resistor in the right side $\frac{R}{2}$ between the ...
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How primary current and magnetic magnetic field changes if we remove from transformer core

I came up the following question. There is a transformer and we remove 10% from its core. How the magnetic field in core and the current in the primary winding change if there is no load on the ...
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Velocity of Electrons in AC Current

I took a magnetostatic course and got to know that the velocity of electrons remains the same in such a case. But in AC Circuits the magnetic field varies continuously. For e.g., Magnetic Field ...
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How do scientists determine the width a thunderbolt?

As I searched on the internet, I found a website stating that the width of a typical lightening strike is $2.5~$cm wide and $8$ km long. I intuitively understand that scientists can measure the length ...
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1answer
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What drives the charge separation in atoms?

In atoms charges are neatly separated. Instead of pairing which seems natural they all stick together with their peers. What drives this peer behaviour? Why is it stable?
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1answer
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Does it give sense to speak about field distribution of a single photon?

A Point source of light has radial symmetry. If the source gets attenuated so that only a single photon is leaving each hour, can I still argue, that the field of the single photon is radial but the ...
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1answer
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Expression of $\omega$ in fundamental mode of EM cavity resonator

In Physics (Part 2), D. Halliday - R. Resnick, on page 957 it is written that, "We state without proof that the angular frequency of oscillation for the electromagnetic cavity of Fig. 38-8 is, in the ...
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1answer
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Superparticle in Background Gauge Fields

A simple model for a superparticle is $$L=m\int dt\left(\dot{x}^{2}-\frac{i}{2}\psi\dot{\psi}\right)$$ with SUSY algebra $\delta x=-i\epsilon\psi$ and $\delta\psi=-\epsilon\dot{x}$, where $\...
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Does the weight of a magnet increase when an apposite magnet is balanced above then centripetal force is applied by spinning the balanced magne [on hold]

If I have two magnets facing opposite of each and one is hoovering over the other. I then spend the top magnet will it become weightless and no longer have a force on the bottom magnet. At what speed ...
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1answer
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Sliding rod in magnetic field with two loops

The circuit is concatenated with a constant flux $B$ and the rod slides with constant velocity $v$. Find the current through the rod. Before I always solved the problem without the resistor $R_2$ by ...
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Two parallel wires [on hold]

We've got a to parallel wires. We know that on the wire 1 is potential $v_{1}$ beetween its ends, conductivity $σ$, current $I_{1}$, $S$ is cross-section area and $d$ is distance beetween wires, $L$ ...
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On the integration for the electromagnetic density

Upon integrating the electromagnetic hamiltonian density \begin{equation} \mathcal{H}=\left|\textbf{E}\right|^{2}+\left|\textbf{B}\right|^{2}, \end{equation} on a cube of lengths L and volume V we ...
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Lagrangian Mechanics - Charged particle in magnetic field

A particle of mass $m$ and charge $Q$ moves in the equatorial plane ($θ=π/2$) of a magnetic dipole, where the vector potential of the dipole is given by $$\mathbf{A} = \dfrac{\mu \sin \theta }{4 \...
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Looking for summary of electromagnetic fields

Looking for summary of electromagnetic fields, maybe even books and videos or maybe something else that your recommend. that includes maxwell's law integral and differential, maxwell boundary ...
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Specifying a loop of current in MEEP

I'm trying to run a simulation in MEEP where the current source is either a loop, or magnetic dipole (since I'm not particularly interested in results within a few radii of the loop). I want the loop ...
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1answer
28 views

Why do we ignore the negative sign of self induction while considering inductors in an AC circuit?

I have been taught that the back EMF in a coil as per Lenz's law is $$ e =-L \frac{di}{dt} $$ Then why is that during circuit analysis of AC circuits, we consider the e across inductors to be $$ ...
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Can a magnetic field's orientation be transported?

The question might be formulated completely incorrectly, but the essence is : Is it possible to have a magnetic fields orientation be transported across a material, so that you can measure the fields ...
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Magnetic field around solenoid and toroid

Solenoid is proving a little bit confusing While getting through solenoid I found that the field outside it is extremely small and is negligible. Also the field at ends is half of that of center. ...
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5answers
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How can an electrically neutral planetary core be geodynamo?

The standard theory for the earth's magnetic field is that the planet's core is a geodynamo. I know moving electric charges can induce a magnetic field, but I also know that opposite charges produce ...
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Induction heating through a slightly magnetic material

Would be be possible to inductively heat a rod with with ~100 relative permeability enclosed in a tube with ~2 permeability? The gap between the rod and the interior surface of the tube is uniform ...
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Creating light with combining $E$ and $H$ fields

We know that light is an electromagnetic wave. So if we take electric field $E$ and magnetic field $H$, combine them perpendicular to each other, and make them oscillate harmoniously, would it create ...
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How was the value of vacuum permittivity originally found?

The vacuum permittivity appears originally in Maxwell's equations, used to describe electric fields. The permeability of vacuum was defined using Ampere's force law (itself derived from Biot-Savart ...
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1answer
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What is the low level magmatic frequency to be used to get signal out from a metal faraday cage?

I need to track the inventory kept in a sealed metal container at the Gate level before delivery. I am not sure if RFID tracking can help since the signal tends to bounce on and will not reach the ...
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How to find Solenoid's length [closed]

How to find Solenoid's length, given that : Length of the wire is 44m, 2A passing through Wonded around an iron rod with length 10cm, raduis 7cm
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1answer
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Can emf be induced by reducing the radius of a loop?

Faraday's law tells us, that the induced emf in a conducting loop, is equal to the change in magnetic flux per second through the area encircled by the loop. If we decrease the radius of the loop, ...
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4answers
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How does Superposition principle follow from Maxwell's equation's linearity?

It is said that whole of electromagnetism can be completely described by the Maxwell's equations. The thing that intrigues me is that how does superposition principle follow? First, I take an ...
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1answer
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Vector Poisson Equation for a symmetrical geometry

I was recently doing some simulations involving the vector Poisson equation and symmetrical geometries with respect to one plane. My question is basically if the computations involved can be reduced ...
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0answers
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Rotated dielectric slab in a parallel plate capacitor

Between two infinite plates charged with a surface density of $\pm \sigma$ there is a dielectric slab (dielectric constant $\epsilon_r$, the dielectric is linear, homogeneous and isotropic) of ...
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1answer
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Distorsion in Photoluminescence

I am studying photoluminescence, but I do not understand why during measurement it turns out that the signal is distorted due to appearance of a long-wavelength replica of the fundamental laser ...
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1answer
49 views

Can I replace a multipole expansion by a combination of separate dipoles?

If I want to be able to model a magnetic field flux density $\mathbf{B}$ from a magnetic source located at the origin at a position $\mathbf{r}$, it is my understanding that I can represent $\mathbf{B}...
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1answer
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Can I use an electroplating rectifier as power source for electromagnet? [closed]

Can I use a AC to DC electroplating rectifier to create an electromagnet with an output square wave pulse of 10hz? How do I determine the wire lenght, diametre and resistance to match the rectifier ...
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Do two magnets attract each other stronger than one magnet and a metal plate?

In the car, I use a magnetic mount to hold the mobile phone, but it keeps falling when the road is not so smooth. I use a magnet mount to the dashboard and a small metal plate attached to the phone ...
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1answer
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Deducing the direction of the magnetic vector potential from its curl

We have an infinite cylinder of radius $a$ around the $z$ axis. The current density inside the cylinder is uniform: $\vec J=J_0\hat z$. We wish to find the magnetic vector potential $\vec A$ (though I ...