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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Would a magnetic current cause a looping electric field?

An electric current, composed of electrons moving in a wire, creates a magnetic field looping around the wire. Would a current of magnetic monopoles moving through a wire create an electric field ...
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Retarded potentials are wrong? (if we are to consider the domain restriction onto $ t_{r}$)

I believe what I am considering is brand new, as I cannot find any specific use of this anywhere online and would like to know your thoughts The solution to maxwells equations are: $A(r,t)= \frac{\...
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Solving differential equations for the Lorentz force law

Assume that a particle of mass $m$ and charge $q$ is moving with velocity $v$ in a uniformly constant electromagnetic field given by $E = (0, E_y, E_z)$ and $B = (0, 0, E_z)$, where $(x,y,z)$ are the ...
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Does flow of charge depend on electric potential or electric potential energy?

Suppose I have a positively charged object A (100C) and two other charged objects B (10C) & C (5C) in the vicinity of A i.e. in A's electric field. They are both equidistant from A. So, we can ...
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How to calculate retarded potential (for magnetic field calculation) for a group of current carrying loops where the magnitude of current is changing?

For constant current, the magnetic field seems to be easy to calculate because of Biot-Savart Law. For a loop having varying magnitude of current (simplifying an electromagnet), however, it seems that ...
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Does the Internal magnetic field inside of a current carrying conductor experience Lorentz force?

According to the most depictions of a Lorentz force I've found, Outer magnetic field of a wire reinforces the external magnetic field on the other side, and weakens it on the other, so the conductor ...
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Electromagnetic Field Angular Momentum

It's a well-known fact that the Electromagnetic Field has somewhat of a "intrinsic" angular momentum. However, I'm struggling to find a clear mathematical description of this fact. I find a ...
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Deep Ineslastic Scattering for neutron and proton in weak-interaction

I'm studing Deep Inelastic Sacttering theory to study the parton model. I have studied the scattering $$e+q \to e+ q$$ And the structures functions $$F_1 = \frac{1}{2} \sum_q e_q^2 \cdot q(x)$$ and $...
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Dipole moment in cylindrical symmetry

So I am trying to find the dipole moment for an uncharged sphere in an electric potential, and I am stuck in the following step: $\vec p = \int_V \rho_{density}(\vec r')\vec r' dV'$ Since we are ...
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Isn’t this a paradox?

Imagine a plate beeing irradiated from all sides with a specific wavelength. One side of the plate is smoth and reflects most of the incoming radiation, the other side is covered in grooves with the ...
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Maxwell Equations Solution for single charged particle

Is it possible to find an analytic solution to maxwell equation when I have only one charged particle (and without any neglections): With $\rho=q\delta(r-r_a)$, $J=q\dot{\vec{r_a}}\delta(r-r_a)$ and $...
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Do transformers return current to source when magnetic field collapse at primary coil?

When we close the circut containing an inductor and a battery, the current starts flowing, which builds magnetic field around inductor, changing magnetic field produces its own current in opposite ...
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How fast does an electromagnetic field come into existence? [duplicate]

In this video by the YouTube Channel "Veritasium" it is explained, that electrical energy is in fact not travelling through the wires of a circuit (in the form of electron flow), but instead ...
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Can Lorentz force polarize a neutral dielectric atom?

Is it possible to electrically polarize an atom (typically a dielectric) with magnetic field? For example, if dielectric fluid flows near magnetic field, does it shift each atom's positive charge down ...
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Why don’t we say that magnetic flux changing with position induces an emf?

As per Faraday’s law of electromagnetic induction, an emf is induced with the rate of change of magnetic flux. But according to one of Faraday’s experiments, relative motion between two coils, or ...
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Finding the current in the falling circular metal plate at any time $T$. (Magnetic field and Electricity) [closed]

A circular metal plate starts falling from rest, straight vertically in region of uniform magnetic field of intensity $B$. The magnetic field is parallel to the surface of earth and to the plane of ...
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Why does the Stefan-Boltzmann Law contain no information about the distance from the radiating body?

The Stefan–Boltzmann law states that the power radiated per unit area of a black body is equal to: $$ \frac{dP}{dA} = \sigma \cdot T^4 \,\,\,\,\,\,\,\left[\frac{W}{m^{2}}\right] $$ This power per unit ...
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Do $LC$ tank oscillators violate the conservation of linear momentum?

General setup Consider the following thought experiment: A capacitor $C$ and an inductor $L$ are mounted with fixed joints on a plate in outer space at say $1$ meter apart. They are connected in the $...
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Measure Position of Electron Directly

What is the most directly way to measure the position of free electron in an expirement? I don't asking about theoretical suggestion but rather on practical expirement which people have done.
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Divergence of current density and charge density

In magnetostatic the continuity equation is: $$\frac {\partial \rho}{\partial t} + \nabla \vec j(\vec r)=0$$. In my script the following is said: Since we are in magnetostatic $\frac {\partial \rho}{\...
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1answer
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Noether's Theorem and Poynting Theorem

For simplicity, let's assume the Lagrangian formulation of Noether's theorem, that is, our equations of motion can be derived from the Euler-Lagrange equations, or, simply, that we can use a ...
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1answer
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Why is this the eigenvalue?

The equation I am looking at (the wave equation for the electric field) has the following form: $\nabla_t^2\mathcal{E} + [(\frac{\omega}{c})^2n^2(x,y) - \beta^2]\mathcal{E} = 0$ where $\nabla_t^2 = \...
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Electromagnetic field around a conductor with current

According to this video https://www.youtube.com/watch?v=bHIhgxav9LY I have a question: If the glowing of a light bulb is the result of energy supplied from the electromagnetic field around a conductor,...
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Examples of current densities

I am trying to imagine how do the continuous current density look like. When we speak about charge density, if it is discrete, one can picture multiple point charges in a region in space. If the ...
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Electric field in the center of hemisphere shell without double/triple integrals

I'm trying to derive the electric field in the centre of a solid hemisphere of radius $ R $ where the charge is distributed uniformly. I have seen different methods involving double/triple integrals ...
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What is net dipole moment of a spherically symmetric charge distribution?

There is some unknown spherically symmetric charge distribution $\rho(r)$ with total charge zero. For this charge distribution how will be monopole and dipole moment? Will they be zero or non zero? ...
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Is there a (semiclassical) electric field operator?

So I come from a chemistry background, where the electronic structure of atoms and molecules is central. For practical purposes, we usually work with a charge density operator $$ \hat{\rho}(r) = q \...
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Further uncertainties in magnetostatic

In magnetostatic, the continuity equation is as follows: $$\frac {\partial\rho(\vec r,t)}{\partial t} + \nabla \vec j(\vec r,t)=0$$ In magnetostatic $\frac {\partial\rho(\vec r,t)}{\partial t}=0$ and ...
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Uncertainties in magnetostatic

Today in our lecture we were introduced to Magnetostatics. While I was able to understand mathematically and physically the majority of the new things that were mentioned, there are a couple of things ...
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1answer
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Equivalence between the fictitious magnetic charges and surface current

In my electromagnetics textbook. There is an example that jf a sphere with radius $R$ is uniformly magnetized by $\textbf{M}=M\hat{\textbf{z}}$, we can get the magnetic field by regarding it either ...
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Electromagnetic fields of a spatially bounded time-dependent current which is eventually steady

So here's the set-up: I have a loop in space and fix a point $p$ outside the loop. Then I turn the current on at $t = 0$ (so the current profile is a step-function). Initially at $t=0$ the E&M ...
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Shouldn’t the net Poynting vector at a resistor or incandescent light bulb be zero?

I’ve read that the Poynting vector points into an incandescent light bulb at all instants of time, whether there is direct/constant or alternating current through the wire. This means the bulb is ...
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What direction does magnetic dipole moment feel force in parallel magnetic field?

Suppose I have a magnetic dipole oriented vertically in the $\hat{z}$ direction so $$ \vec{m} = m\hat{z}$$ And the position vector to it is $\vec{r} = z\hat{z}$. It's entirely on the z-axis. We're ...
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1answer
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Ehrenfest theorem: On which classical circle can we find the electrons in an homogenous magnetic field?

In the French wiki article about the Ehrenfest theorem I found these formulas. $${\displaystyle {\frac {\mathrm {d} }{\mathrm {d} t}}\langle {\hat {x}}\rangle ={\frac {1}{m}}\langle {\hat {p}}\rangle }...
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How can a plasma exhibit both quasineutrality and collective motion?

Since, over a Debye Length $\lambda$, very small compared to the characteristic length $L$ of a plasma, a potential due to a source charge is essentially screened, how can plasma particles communicate ...
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How do I find the current density vector in an electromagnet that has a time-varying current?

I know that the current $I$ is a scalar quantity, and to calculate current density $J$ , the cross sectional area $A$ is needed, to give us $J = IA$ . $J$ also has other relations involving ...
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1answer
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How to determine the directional behaviour of the magnetic vector potential?

I was wondering if someone could please help explain the directional behaviour of the magnetic vector potential? I understand that the magnetic vector potential $A$ is perpendicular to $B=∇×A$ but ...
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Maxwell’s first equation in conductor we take E=0. And I have read that’s true for practical time scales. But in 4th equation we use non zero E, why?

\begin{align} \nabla\cdot E&=0\\ \nabla\cdot B&=0\\ \nabla\times E&=-\frac{\partial B}{\partial t}\\ \nabla \times B&=\mu J+\varepsilon\mu\frac{\partial E}{\partial t}=\mu\sigma E+\mu\...
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Time-dependent Magnetic Field "Paradox" with Faraday's Law [duplicate]

I have encountered a "paradox" while studying electromagnetism and I can't seem to understand where I am messing up. Consider a uniform, time-dependent magnetic field B(t) = $t^4 \hat{i}$ in ...
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1answer
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Understanding radiation pressure

I've been told that the radiation pressure of light can be understood(?) by considering a charged particle as a damped driven harmonic oscillator where the forcing term is the electromagnetic field, ...
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2answers
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On what physical principles the measurement of intensity of light is based?

As far as I understand, theoretically intensity of light is defined using the Poynting vector of the corresponding EM field. How is the intensity of light measured in practice? On what physical ...
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What good reason to hang long cable carrying electric current high above the ground? [closed]

Assume no flooding or other natural disasters to damage the cable if it is laid beneath the ground, then what good reason for them to be suspended high above the ground? The cables are insulated and ...
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Is radiation pressure wavelength dependent (does a blue photon move the solar sail more then a red photon)?

I have read this question: Where John Rennie says: Does the wavelength of the light matter? No Now in the same question, another answer, by DavePhd says: Does the wavelength of the light matter? ...
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Is it possible to make a magnetic box that will compensate Earth gravity allowing magnetic objects inside to fly on orbits like in space? [closed]

Is it possible to simulate solar system using magnetic spheres and some kind of box that will compensate the Earth gravity by magnetic force?
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Given a magnet in each known phase of matter, does that magnet contain any lone pairs? [closed]

Given a monopole or dipole magnet in each known phases of matter ( solid, liquid, gas, plasma ), does that magnet contain any lone pairs?
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What is the formula for inductance? [closed]

I am very new to Electromagnetic Induction. I had this question on a test recently. It went like - If both the number of turns and core length of an inductor is doubled keeping other factors constant, ...
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1answer
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What is the electric potential of a particle when it is initially released in a cyclotron?

Lets assume a positively charged particle in a cyclotron. When released it will move to a dee. Hence, does it move to an area of higher electric potential or lower electric potential? In my ...
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2answers
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What are the physical reasons for considering the Maxwell invariant ${\cal G}=E.B$ in the action?

As I know, the Maxwell invariant ${\cal G}=E.B$ is the fundamental invariant such as ${\cal F}=\frac{1}{2}\left( {{{\bf{B}}^2} - {{\bf{E}}^2}} \right)$ that can be used to construct all possible ...
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2answers
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Lorentz force vector

I’m trying to calculate a Lorentz vector in $\mathbb{R}^3$ with those vectors but I keep getting an integer as an answer: There’s a velocity vector of $(3,-2,1)$ , a magnetic field vector of $(1,2,-2)$...
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How is the answer to this question $1/c$ seconds?

This is a question from the YouTube science content channel Veritasium. The below image shows the question. The question is about when will the bulb begin to glow after the switch is turned on. The ...

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