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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Potential Energy of a Sphere

A spherical volume of radius $a$ is filled with charge density $\rho$. What is the potential energy $U$ of this sphere (what is the work done in assembling it)? I can't really wrap my mind around ...
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What would the magnetic field strength of metallic hydrogen be?

Say you have a superconductor made of metallic hydrogen. What would the limit be? I know that the new superconducting magnets such as YBCO can reach field strengths of up to 30 Tesla but given the ...
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Minimal coupling Hamiltonian

A charged particle in an em field can be described by the following Hamiltonian: $${\rm H} = \frac {({\rm p} \ + \frac {q}{c}A)^2}{2m} + U(r)$$ But... what does it mean to square the sum of two ...
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How do you create a grounded metal plane in MEEP? [closed]

I'm using MEEP software to simulate antennae, but I can't find anywhere how to create a metal plane that is grounded. How do I code it to do this?
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How do magnetic fields due to random motions of charges cancel out in a current-carrying wire?

In my physics textbook, it states that magnetic fields due to random motions of charges cancel out at every point I know there's a drift velocity due to force imposed by the electric field, but the ...
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Magnetic Field at centre of Hemisphere

MY APPROACH: I know that we have rotate by an angle theta from centre and take an angular element which corresponds to a ring and write the expression for field accordingly. But my main problem is ...
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Can we make stronger electromagnet by using less voltage and current but very thicker wire and more number of turns in a coil?

If we increase the number of turns in a coil, the resistance increases, to overcome this, we can use very thicker wire and keep the voltage low to keep the current constant, and by the formula, this ...
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Will there be current induced in a coil placed in a time varying Magnetic Field?

If a circular coil is placed in a time varying Magnetic field ,emf is induced but will there be induced current?
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Flux and emf induced graphs when magnet is dropped through a coil

I've been told that if you drop a magnet through a coil the induced emf and flux graphs would look like this: I understand that when the bar magnet is in the middle of the coil the emf induced is ...
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Why is the barrier potential and electric field from n side to the p side?

In the above image taken from my class notes you can see that I have been taught that when a semi-conductor is doped with both P-type impurity and N-type impurity, one on each side, the barrier ...
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What exactly causes the normal force and how is it related to friction? [duplicate]

So I've read many answers on the internet and have not yet come to understand the origin of the normal force. What I've to come to understand about the normal force, is that it is the electromagnetic ...
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The history of Ampère's circuital law and Ampère's force law

How I know and read in the Internet that Ampere realized that two current-carrying wires can exert force on each other. Ampere's force law is: $$d^2\vec{F_{21}}=-\frac{\mu_0}{4\pi}i_1i_2\frac{\hat{r}}{...
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If a toroid uses bismuth core then what will be the field in the core be (slightly) greater or (slightly) less than when the core is empty?

Bismuth is a diamagnetic material so according to me the field should be less as compared to when there is no material in the core.
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Electric potential/gravitational potential

potential is defined as the amount work done by an external force to move from something position at infinity to position R per unit charge if the two charges are positive. then the potential ...
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Particle motion in earths radiation belts. Comparison of gradient drift and drift due to convection and corrotaion

I would like to know if the drift due to the gradient of the earth's magnetic field could be balanced by the sum of the drifts due to corrotation and conviction of the earth's magnetosphere. To my ...
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Definition of electromotive force and conductors moving inside a constant magnetic field

This is a followup question to this one. I already found some similar questions in PhysicsSE (as this one and this one) but I didn't found the answer I was looking for. In a conducting loop wire ...
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Kirchhoff’s rules

Can Kirchhoff’s rules be applied to simple series and parallel circuits or are they restricted for use in more complicated circuits that are not combinations of series and parallel?
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How do we Wick rotate the Maxwell $U(1)$ gauge theory's field strength $F$?

How do we Wick rotate the Maxwell $U(1)$ gauge theory's field strength, say in 3 space and 1 time dimensions? Suppose we start with a Lorentz signature with coordinates $(x_0, x_1, x_2, x_3)$, then we ...
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Paul Dirac on Dimensionless Physical Constants and $\alpha\sim\frac{1}{137}$

Paul Dirac gave his viewpoints on the dimensionless constant (click to see the youtube video). For example, he mentioned the fine structure constant $$ \alpha\sim\frac{1}{137}. $$ It is not clear that ...
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WHY does a DC solenoid develop magnetic force even though its inductance is zero?

If the inductance of coil fed by dc voltage is zero then how do dc solenoids work?
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Do we have an electrostatic field in a closed circuit with current?

I was wondering why the voltage drop were the same over resistors in parallel and I got the explanation that $\nabla\times E=0$ for the field in the circuit. This means that the field is conservative, ...
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Finding charge density $\rho(r)$ from long wire of charge

Given a long wire of charge, with uniform charge density $\lambda$ Coulombs per m. For a charge density $\rho(r, \theta, z)$ which depends only upon $r$ and varies linearly from the center and is $0$ ...
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Reading wave function graphs

I got a little bit confused at my EM waves lectures when we were studying the topic of wave propagation. Totally because of my lack of understanding of the wave function graphs. In the chapter, we ...
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Why is magnetostatics defined as $\frac{\partial \rho}{\partial t} = 0$?

I don't see why the idea of steady currents (i.e. magnetostatics) implies that charge density $\rho(\vec{r},t)$ has no explicit time dependence. Is it just coming from magnetostatics being defined as $...
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Problem-solving books on electrodynamics

What are the best books on the problem-solving of classical electrodynamics exercises? I looking for books that help me to solve Jackson and Griffiths exercises, i.e. books on the problem-solving of ...
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Maxwell equations and electric field

How can I use the Maxwell equation to prove that electric field is the sum of a conservative parcel of the electric potential gradient and a non-conservative parcel given by the magnetic potential ...
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27 views

Mechanism for induction of electrical charges

Usually we are just told that if we bring in a positive charge plate close to a neutral one, then the neutral one picks up charge due to induction. However, in no sources I've read (yet) have ...
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1answer
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Difference between covariant derivatives in general relativity and electromagnetism

There are many similarities between the gauging of a $U(1)$ symmetry to obtain the physics of electromagnetism and the gauging of diffeomorphisms to obtain the physics of general relativity. In ...
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Does it make sense to define a notion of charge density for a conductor which has current passing through it?

Suppose you have a conductor like a wire with current flowing through it, does it make sense to define a notion of surface charge density for it? So, for example can free charge exist on the surface ...
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40 views

Behaviour of conductor moving in a magnetic field after a long time

In the above figure, we have a rod moving with some speed in $v$ in a magnetic field $B$, it is said that when at equilibrium the rod generates a surface charge density such that the magnetic field ...
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Why do two wires with current flowing in the same direction attract each other, and two wires with current flowing in opposite direction repel? [closed]

I am looking for a different approach that of the relativistic effect of Einstein.
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Determine formula of thermal Johnson noise with an RLC circuit

So I want to prove that the thermal Johnson-noise of a resistor charges up a parallel LC circuit in a way that its temperature is equal to the resistor temperature, $T_{LC} = T_0$. My approach is: ...
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Energy is not transmitted through the wire

The introduction chapter of "Sound System Engineering, 4th ed." has a section titled "Fields", and in it some quotes that state energy is not transmitted through the wire, rather ...
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Magnetic flux switch , parallel magnetic path actuator, motor/generator

Can you help me understand whether this concept works or is it a misconception. The basic idea is simple, you use a permanent magnet/s and electromagnet within the same core, normally both permanent ...
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Electric Field for a conducting infinite plane formula contradiction

We know the electric field of an infinite plane is $\frac{N}{2e_0}$, where N is the charge density, but for conducting objects its $\frac{N}{e_0}$? This is a contradiction. Or because the plane is ...
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Discrepancy between rotated electric and magnetic fields depending if they're covariant or contravariant

I recently ran into the following issue. I transformed the electric and magnetic fields with the following rotation $$R^{\bar{\mu}}_{\ \nu} \rightarrow \begin{pmatrix} 1 & 0 & 0 & 0\\ 0 &...
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What is the equation for calculating magnetic field for two coils when the currents in the coils have opposite orientation?

So I was wondering what the equation is to calculate the magnetic field when the two coils are separated by a distance d, with the currents in the coils in opposite orientation? Any help would be ...
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Manipulating relativistic Newton-Lorentz equations for a particle in fields

I am trying to understand the theory section of this article. In brief, a point particle, of Mass $M$ and charge $+Q$, is injected into a region of uniform magnetic field of strength $B_s$ and a plane-...
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Does the electromagnetic wave have a lateral extension?

An electromagnetic wave is typically shown with this schematic (taken from https://en.wikipedia.org/wiki/Electromagnetic_radiation) We see graphically from the schematic that the amplitude of E and B ...
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How do the evanescent waves contribute/cause a diffraction limit?

In the following article, under the section Physics of the Superlens, it is stated: The light emitted or scattered from an object includes not only propagating waves but also evanescent waves, which ...
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'Isotropy' for Clausius-Mossotti equation

The famous C-M equation $$ \mathbf E_{\text{eff}} = \mathbf E + \frac{\mathbf P}{3\epsilon_0} $$ for an isotropic dielectric made of point dipoles relates the field that a dipole experiences, $\mathbf ...
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How to rewrite the Lagrangian density for the Pauli moment in Dirac theory

I am trying to follow a derivation in a paper (DOI: 10.1119/1.2710486). It defines the Lagrangian density for an ananomalous magnetic moment (Pauli moment) in the Dirac theory as, $$\mathcal{L}_{Pauli}...
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1answer
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Can we say that the curl of $E$ for an electromagnetic wave is zero?

here (page 11) the following statement is written about the curl of the electric field in a TEM wave on a transmission line (⊥ indicates the component on the plane orthogonal to the propagation axis z)...
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Expressing $E$ field and $B$ field using potentials

Why dont we have a term that corresponds to the change in E field in eq1.61b like we have in eq 1.61a?
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Formalizing the Askar'yan Effect

Wikipedia defines the Askar'yan Effect as... [The] Askaryan effect is the phenomenon whereby a particle traveling faster than the phase velocity of light in a dense dielectric... produces a shower of ...
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2answers
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How to compute the field lines of an induced magnetic field inside a capacitor?

Consider a capacitor with a varying voltage applied to it. As the voltage changes over time, the electrical field $\vec{E}$ inside the plates does too. Assumption We assume that the direction of $\...
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Electrostatic and magnetostatic fields

Are electrostatic and magnetostatic fields analogously created whenever there is direct current? If so, does it matter if it’s constant current or constant voltage? In addition, can an electrostatic ...
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Macroscopic field is space-time average of the microscopic fields?

Panofsky and Phillips' Classical Electricity and Magnetism. Chapter 2, Section 3. The authors show, assuming the microscopic field $\mathbf{\mathcal E}$ and the microscopic charge density $\rho_a$ are ...
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Electric and magnetic charge quantization v.s. the radius of the $U(1)$ or $\mathbb{R}$ gauge parameter

We know (say from Griffiths E&M Problem 8.12) that the electric $q_e$ and magnetic charge $q_m$ (with a distance $\vec{z}$ apart) can store the angular momentum in the space: $$ \vec{L}=\int d^3 V ...
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Electric and magnetic charge quantization: other derivations or perspectives?

We know (say from Griffiths E&M Problem 8.12) that the electric $q_e$ and magnetic charge $q_m$ (with a distance $\vec{z}$ apart) can store the angular momentum in the space: $$ \vec{L}=\int d^3 V ...

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