The Stack Overflow podcast is back! Listen to an interview with our new CEO.

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.

Filter by
Sorted by
Tagged with
0
votes
0answers
39 views

Deriving the equations of motion of a real vector field [duplicate]

I'm trying to calculate the euler-lagrange equations of the following Lagrangian density. $$\mathcal{L} = -\frac{1}{2}(\partial_\alpha A_\beta)(\partial^\alpha A^\beta)+\frac{1}{2}(\partial_\alpha A^\...
0
votes
1answer
21 views

Drone's sufficient engine RPMs initiated by Magnetic Fields

I've seen many videos where magnets are used to initiate and boost RPMs. Most of them are fake, where people are using compressed air at the backstage. Did anyone of you actually tried to challenge ...
1
vote
0answers
20 views

Retarded Coulomb's law and EM waves; Feynman texts

In this chapter, Feynman writes down the retarded Coulomb's law, $$\begin{equation} \label{Eq:II:21:1} \mathbf E=\frac{q}{4\pi\epsilon_0}\biggl[ \frac{\mathbf e_{r'}}{r'^2}+\frac{r'}{c}\,\frac{d}{dt}\...
0
votes
0answers
9 views

I know that if $σ$ is the conductivity of the wire, then, by Ohm’s law in its general form we have: $\vec{J}=\sigma \vec{f}$

I know that if $σ$ is the conductivity of the wire, then, by Ohm’s law in its general form we have: $\vec{J}=\sigma \vec{f}$ that $\vec{f}=\vec{f}_{em}+\vec{f}_{app}$ where where $\vec{f}_{em}$ is the ...
0
votes
2answers
20 views

Could a disturbance to a magnetic field be detected near the source?

(Forgive my crude MS paint drawing) Imagine we have a large magnetic field coming from a powerful source. At the edge of the field where it is weak (A), we introduce a disturbance using another ...
0
votes
0answers
20 views

Relation between AC Current and electromagnetic wave in transmission lines?

I m so confused we started by learning about electromagnetic waves in the vacuum , then we went to transmission lines and there the professor mentioned the em waves are traveling through them. I ...
0
votes
1answer
16 views

What distance do I use when examining the effect of an electric field on a point outside of a nonconducting sphere with an arbitrary volume charge?

It's easy enough for me to do this for a ring (as I interpret the field lines as "exiting from the centre of the ring") or a point (simply the distance from the point to the other point), but I just ...
0
votes
1answer
40 views

Nature of induced magnetic field lines in cylindrical capacitor

What is nature of the induced magnetic field lines inside a cylindrical capacitor which has a time variant potential gradient across it? For a parallel plate capacitor, its straightforward: the ...
0
votes
3answers
56 views

Conceptually understanding RL circuits

I'm struggling to conceptually understand the current-time profile of an RL circuit. Specifically, what causes the rate of change of current, $\frac{\partial i}{\partial t}$, to start off high when ...
1
vote
1answer
19 views

Collective dipole oscillation and specular reflection

Let's say I shine light upon a surface and observe a specular reflection. If I look with a electron microscope, I see tiny particles much smaller than the wavelength of light (~10 nm) and also closely ...
-1
votes
2answers
55 views

Could you transmit electricity wirelessly?

Since you can wiggle an electron to get a radio wave And if you 'wiggle' it fast enough you get higher and higher frequency right? So couldn't you just broadcast an electromagnetic beam or whatever ...
0
votes
3answers
105 views

Proof for Vector Identity

I am currently studying electrodynamic and came across the following vectoridentity, but I am unable to prove it: $$ \vec{f} \times ( \nabla \times \vec{f} ) -\vec{f}(\nabla\cdot\vec{f}) = \nabla \...
0
votes
1answer
31 views

In a simple resistor circuit, does the current reach V/R instantaneously?

Essentially, if I were to plot current against time for a simple circuit with a DC cell and a resistor in it, would there be a non-flat profile from time $t = 0$. So, between $t = 0$ and $t = t_1$, ...
0
votes
2answers
45 views

Why can't a current carrying loop (curl of the electric field exists) produce a time varying magnetic field?

If a time varying magnetic field can give value to the curl of an electric field, why not the other way round? That is, why can't an enclosed loop with some emf produced (basically a current carrying ...
0
votes
1answer
24 views

I know that $I=\int_AJ(\vec{r},t)d\vec{A}$ now can I say that $I(\vec{r},t)$?

I know that $I=\int_AJ(\vec{r},t)d\vec{A}$ and if $J(\vec{r},t)=\sigma(\vec{r},T) \vec{E}(\vec{r},t)$ we have $I=\int_AJ(\vec{r},t)d\vec{A}=\int_A\sigma(\vec{r},T) \vec{E}(\vec{r},t)d\vec{A}$ now can ...
1
vote
1answer
33 views

Can circularly polarised light be observed deterministically in a fixed direction?

In classical electromagnetism, a circularly polarised electromagnetic wave is described by an electric field $\mathbf E(\mathbf r, t)=\mathbf E_0 e^{i(\mathbf k\cdot\mathbf x-\omega t)}$, where $\...
0
votes
0answers
13 views

Two conductors with charges $Q_A ,Q_B$ and Conductors connected with a wire(without charge) [closed]

Two conductors with charges $Q_A ,Q_B$ and Conductors connected with a wire(without charge) now what will happen ? My answer I know that $V_B\neq V_A$ So $V_B-V_A=-\int_A^B\vec{E}\cdot d\vec{r}$ ...
3
votes
3answers
79 views

Interpretation of equal absolute values of pressure and tension in the electric or magnetic field along Cartesian axes aligned with the field

For an electric or magnetic field along the $x$ axis, the stress-energy tensor in mixed covariant-contravariant form, in $(t,x,y,z)$ coordinates, is of the form $\operatorname{diag}(1,1,-1,-1)$ (...
0
votes
0answers
25 views

What would happen to the brain if a transcranial magnetic stimulation magnetic field was large enough to encompass the brain

So transcranial magnetic stimulation has an effect on the electron in the brain right. And that means if you have a field of pulsating magnetic field like the one in transcranial magnetic stimulation ...
0
votes
1answer
36 views

Difference of potential between 2 disks of radius $R$ where $d << R$

Well, We have 2 disks of radius R where the distance between these 2 disks : $d << R$. These disks are uniformly charged. I have calculated the electric field near one disk, I have: $$E(z) = \...
0
votes
0answers
3 views

Screened magnetic interactions

I hear all the time people saying magnetic interactions are not screened. I do not understand what it means. Is it related to the nonexistence of magnetic monopole? I assume you can always find some ...
0
votes
1answer
26 views

How to find the mixed tensor, contravariant tensor and tensor trace of $F$

I have a question in particle physics that ask me to find the mixed tensor, contravariant tensor and tensor trace of $F$: Our professor didn't teach us that much about the math of tensor, which makes ...
0
votes
3answers
68 views

Why is flux through a closed surface not infinite? [duplicate]

The Electric field lines coming out of a charge are infinite, So the lines passing through a closed surface should be infinite , right?
-1
votes
0answers
25 views

How to do colour map in MRI?

For this question, I assume if we do a colour map, it is based on the magnitude of the magnetic strength? After that how do we know where each coordinates belong to? Like for example at p= 0 degrees,...
0
votes
1answer
15 views

Is it possible to measure the amplitude of a magnetic field independent of the orientation?

The strength of a magnetic field can be measured as strength in a specific direction. We can calculate the strength of a magnetic field along the field lines from measuring the strength in x,y,z ...
8
votes
4answers
1k views

Can a magnet bend light? [duplicate]

As light is electromagnetic radiation. Then why I don't see any magnet bending light wave? Or why light doesn't diffract whenever it passes by a live wire?
1
vote
0answers
24 views

Metric (in)dependence of the electromagnetic field strength

In GR, the vector potential is defined as $A^\mu$ which is a contravariant vector. Then lowering the indices requires the metric $A_\mu =g_{\mu\nu} A^\nu$, using this vector one defines the field ...
2
votes
0answers
29 views

Pull force of neodymium magnets - is there a way to be more realistic?

I have a mirror attached to a 360 degrees swivel/tilt base. The mirror with the base weights 1 lbs. I glued a N52 round magnet on the base, and another magnet on the wall. Each magnet measures: 1.25 ...
-1
votes
1answer
99 views

What was the real need of the operators of divergence and curl?

As I'm advancing my study in Electromagnetism I'm getting introduced to more mathematical operators which are exclusively used in Electromagnetism and Fluid Dynamics only. Let me try to explain ...
1
vote
0answers
27 views

How to express the wavefunction of a harmonic oscillator in a perturbing electric field?

So I am looking at the problem of the (charged) harmonic oscillator in a weak electric field - the problem that defines e.g. the polarizibility of the oscillator. Let the fieldless Hamiltonian be: $...
0
votes
1answer
21 views

Magnets falling through solenoid

I'm unsure about something in electromagnetism....When a bar magnet falls through a solenoid, does it travel at a constant velocity due to the opposing force caused due to Lenz's Law? Or does the ...
0
votes
1answer
34 views

On seeing and especially in the dark

Our retina detects the change in the electromagnetic field due to the jiggling of the atoms surrounding us as I understand the seeing process. In the total absence of a light "source", however, why ...
3
votes
2answers
66 views

Simple, nonmathematical argument for partial polarization by reflection

I'm teaching calculus-based electricity and magnetism with a sequence of topics in which students learn the basics of electromagnetic waves before the semester in which they get a more general ...
1
vote
1answer
36 views

How does the Lorenz Gauge condition lead to four wave equations?

The 1972 book by L. Eyges's, The Classical Electromagnetic Field, on p. 184, in $\S$11.7, Integral Forms of The Potential, the statement "We now turn to the problem of finding $\mathbf{A}$ and $\...
0
votes
1answer
29 views

Interpretation of real and imaginary parts of the Poynting vector in a vacuum

It is known that the flux of the Poynting Vector through a certain surface represents the total electromagnetic power flowing through it. Then, its real part is the active power, while its imaginary ...
0
votes
0answers
14 views

Velocity of particles in electrolysis

In an electrolysis (H20) water is split up and the protons start to move towards the anode. So one would be able to calculate the acceleration of the proton from the electric field in that area. ...
4
votes
2answers
1k views

In a double slit experiment, does each and every photon leave a dot on the screen in the bright area?

I have read this question: Why does the photon strike at one or another place on the tape? where PhysicsDave says: All photons passing thru the slits leave a dot on the screen, this is true for ...
0
votes
1answer
31 views

How is the Time Derivative of the the Electric Field Equal to the Current Density in Gaussian Units?

The microscopic form of Ampere's law with the Maxwell addition in Gaussian units states, \begin{equation} \nabla \times \vec{B} = \frac{1}{c} \left ( 4 \pi \vec{J} + \frac{\partial \vec{E}}{\partial ...
1
vote
2answers
63 views

Can a moving current carrying wire produce electric field?

As we know, a current carrying wire can create a magnetic field which is perpendicular to the direction of current (From relativity, we can understand why magnetic fields are produced ) and the net ...
4
votes
3answers
144 views

Stumped on understanding a Feynman lecture about force from wire on magnet

I must (sheepishly) admit that I'm stumped on a beginning page of The Feynman Lectures Volume 2. I have included a picture from the page. [Let me know if I'm breaking copyright, or if I can include ...
0
votes
0answers
17 views

Two solenoids of co-axis in relativity

Lets think about two solenoids sharing their axis. Their radius is almost same, so you can ignore the difference of the area of the cross-section. Two solenoid has same magnitude of current, but ...
2
votes
2answers
87 views

Deriving the speed of light from Maxwell's equations?

Relationship between speed of light and EM force? Can it be said that Maxwell used measurements of the "strength of electric force and strength of magnetic force", to derive the value for the speed ...
1
vote
4answers
100 views

Does the magnetic part of the Lorentz force do work?

The magnetic part of the Lorentz force is $$\vec F_L=q\left(\vec v\times\vec B\right)$$ As this force is always perpendicular to the direction of the movement, we learned that no work is done by it. ...
0
votes
2answers
70 views

What is the strength of gravity compared to electromagnetism?

I would like to know the strength of gravity as compared to electromagnetism. Im not looking for their classical constant values. It can be compared by saying i.e. gravity is a million billion times ...
0
votes
0answers
14 views

Calculate Force on Permanent Magnet from a Coil

I was wondering how I could calculate the force on a permanent neodymium magnet. For this setup, I have a magnet on a scale with a coil directly above it. The magnet and the coil repel each other when ...
0
votes
2answers
70 views

Does a relaxing electron really accelerate?

This is not a duplicate. I am not asking about quantum leaps or quantum jumps or whether the transition is instantaneous (yes I asked that question before here Do electrons really perform ...
0
votes
0answers
15 views

Difference between Motional EMF and Hall EMF

Is there any difference btw motional emf and hall emf ?
0
votes
0answers
11 views

What is the nature of the multipoles used to shape/re-direct an e-beam?

Imagine having an electron beam, just like in an electron microscope. Now imagine that we have some multipoles prepared in the beams path to influence the shape/path of the beam. My question is: ...
2
votes
1answer
30 views

How to find the equation of motion of a particle in an electric and magnetic field?

I'm trying to solve a homework problem. The statement of the problem says: "When the negatively charged plate of a parallel plate capacitor is lit up by light of a certain wavelength, electrons are ...
0
votes
1answer
24 views

Relation derivation based on the definition of Current Density

The current density is defined as: $$\textbf{J} (\textbf{r},t) = \rho(\textbf{r},t) \cdot \textbf{v}(\textbf{r},t)$$ where $\rho (\textbf{r},t)$ and $\textbf{v}(\textbf{r},t)$ is the charge density ...