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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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65 views

Why are double negative ions (Hydrogen) unstable?

This is not a duplicate. I have read these questions: Can hydrogen have 3 electrons? where Buzz says: The double hydrogen anion H−− does not exist as a stable species. (It occurs as a resonance, ...
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“Derivation” of continuity equation

The surface integral of j over a surface S, followed by an integral over the time duration t1 to t2, gives the total amount of charge flowing through the surface in that time (t2 − t1): $${\...
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1answer
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What unit system is used in Feynman's Lectures on Physics, Pt. 2

In part II, lecture 18, of Feynman's Lectures on Physics, on Table 18-1 Feynman writes Ampère's law as $$ c^2 \nabla \times \vec{B} = \frac{\vec{j}}{\epsilon_0} + \frac{\partial \vec{E}}{\partial t}. $...
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25 views

Quantisation of gauge theory with minimal coupling

I have a question on the quantization of the gauge theory with minimal coupling term. What I understand is that if one is given an action $$ S=-\int d^4 x \frac{1}{4}F^2 \tag1 $$ Since this action has ...
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1answer
33 views

Symmetry Argument of a Line Charge

I am been trying to make sense of my professor's lecture notes on where he talks about line charges; in general, I am lost when it comes to the symmetry argument in the case that $E_\phi=0$ on an ...
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33 views

Could electric dipoles within hadrons provide an alternate explanation for strong force?

We all know that hadrons such as proton and neutrons are made up of quarks and not all of the quarks have the same charge. This would suggest that they have a positive and negative side. In the case ...
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31 views

Magnetic fields and electric fields relation [duplicate]

Why do we treat magnetic fields to be different from electric fields?.I thought special relativity had solved this thing.
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42 views

Clarification on Taylor expansion in linear quadrupole multipole expansion in cartesian coordinates

In my textbook, given the electric linear quadrupole on the z axis, as in the figure, the author considers the functions $$ \frac{1}{r_i} = \frac{1}{\sqrt{(x-x_i)^2+(y-y_i)^2+(z-z_i)^2}}, $$ with $r = ...
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Is Newton's third law held in special relativity regarding Coulomb's force?

Assume a charged mass $(m,+q)$ is revolved around another charged mass $(M,+q)$ at a distance $r$ so that $M>>m$, yet the gravitational effects are negligible. (Such as an electron orbiting a ...
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1answer
35 views

What is a medium?

I was reading this and in there it states that The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. The existence of the hypothetical ...
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33 views

The rate of change of a relativistic electron energy [duplicate]

I am trying to derive the equation for the rate of change of the energy for a relativistic electron. I know I should start from the following equation: $$\frac{d(\gamma m\vec{v})}{dt}=-e(\vec{E}+\vec{...
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34 views

In certain conditions, does a permanent magnet loses its mass over a period of time? [on hold]

While working on an alternative cosmological paradigm of, Eigen-rotational Clusters of String-matter Universe, it was hypothesized that the fundamental matter is, Eigen-rotational elastic string-...
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How to calculate the refractive index of a matter consisting of chargeless particles such as neutrons?

The traditional method for deriving the index of refraction for, e.g., a gaseous medium consisting of atoms/ions, or any plasma medium, according to this note, depends on the density of electrons ...
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2answers
44 views

What causes a magnetic field to be produced at a point in space, when an electric current is flowing nearby?

We know that a current induces a magnetic field along a circular path around it But what exactly causes the magnetic field to produced in that point of space? Is it that change in the direction or ...
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1answer
36 views

How magnetic monopole modifies faraday's law

I have encountered a statement that upon existence of magnetic monopoles, the equation $$\nabla \times \textbf{E} = -\partial_t \textbf{B}$$ must be modified. I only see that the last equation $$\...
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1answer
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Does potential difference depend on the size of an object

Let's say object A has received a number x of electrons and is now at a 1v higher potential relative to a neutral ground. Now if object B which is 2 times bigger than object A receives the same ...
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19 views

Are there experiments specifically testing for electrostatic charge when light enters a medium?

I'm aware of the Faraday effect that occurs when light enters a medium, which suggests that there could be some interactions between magnetic fields and light: https://en.wikipedia.org/wiki/...
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Forces acting on a charged particle [on hold]

List down the forces that act on a charged particle when, a) it is at rest and b) it starts to move.
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2answers
55 views

Is there a connection between spontaneous symmetry breaking and massless photons?

I haven't studied a lot about these topics to put it that way. But I wonder if there is a connection between spontaneous symmetry breaking and the fact that photons are massless? The spontaneous ...
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Why are induced field lines circular?

Whenever we have a changing magnetic flux through a conductor an induced electric field is generated. It is often said that these induced electric field lines are circular in shape.But why is this so?...
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Better book for electrodynamics: Purcell or Griffiths? [on hold]

What do you think would be a better book to study electrodynamics from: Griffiths or Purcell? In terms of theory and practice.
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1answer
32 views

Does Electrostatic potential energy bend Space-time? [duplicate]

Okay, there are various questions. First, "matter and energy bends space-time" does this mean any form of energy can bend space-time? Does theory of relativity assume that there is no other form of ...
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1answer
30 views

How does an electrical field influence a changing magnetic field?

I would like to know what happens in the following situation. (I am not planning on actually doing this - I just want to understand). Suppose I have a permanent magnet contained in a plastic capsule, ...
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1answer
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Relationship between direction of magnetic force and movement of a ring

Suppose a metallic ring is dropped from above a magnet (N is pointing up and S is pointing down). Why does the "direction" of the magnetic force on the ring stay the same when it enters the ring and ...
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1answer
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Why are Cauchy boundary conditions an over-specification of boundary conditions for solving Poisson’s equation?

I was referred to Physics.SE by the following content published in Jackson’s Classical Electrodynamics: This rather surprising result [the fact that the potential within a charge-free volume is ...
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Ponderomotive Force Effect on Conductors

If I place a conductive rod of metal in an oscillating electric field, such that the ponderomotive approximation applies: $$F = \frac{q^2}{4m\omega^2}\ \text{grad} (E^2),$$ where: $q$ is charge, $m$ ...
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39 views

Conservation of energy when placing two inductors next to each other

Say there is a circuit with a current source and an inductor. There is a current $i(t) = at$ going through the inductor. We now place a new circuit with an inductor and a resistor next to it. The ...
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Why does the scattering cross section equal to the sum over all differential cross section; including the incident angle?

According to Beer Lambert's law, the intensity of light passing through a homogeneous medium diminishes at a rate proportional to the incident intensity; i.e. $$ \frac{dI(s)}{ds} = -I(s)\sigma\, , $$ ...
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Can we get energy from alternig locally the earth's magnetic field? [on hold]

Is it possible to create energy alterning between a state in which one system is under the influence of the earth's magnetic field and one in which it is isolated from it? Would it take more energy to ...
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2answers
40 views

How should the electric field produced by an electron look like if electron is described by a wave function?

In quantum mechanics, the position of an object is not definitely known, but instead is described by a probability density function of where it would be located. Then what should the electric field ...
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What is the energy of Evanescent waves in Total Internal reflection?

In TIR, all energy is reflected back as the reflection coefficient becomes 1. Since all energy is reflected back , how come evanescent waves possess energy?
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Solenoid type Conductor [on hold]

A long hollow cylindrical conductor of length L and small radius carries total current I along its curved surface as shown in figure. The magnetic field inside the cylinder is I tried this question ...
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Magnetic field inside hollow cylinder [on hold]

A hollow thin walled conducting long cylinder of radius $R$ is carrying current I along its length. A thin slit of width $h$ is cut along the length of cylinder. What is the magnetic field just inside ...
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48 views

How do I show that the two definitions of the curl of a vector field equal each other? [migrated]

The curl of a 3D vector field is a 3D vector itself and has two definitions - one in integral form and one in differential form. Definition 1: $$ \operatorname{curl}\vec{F}(x,y,z) \, \cdot \, \hat{n} ...
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Lagrangian density of Electromagnetism

In this paper (journals.aps.org/prd/abstract/10.1103/PhysRevD.38.478) the author claims that if one uses FRW metric $$ds^2=\sigma^2[-N^2(t)dt^2+a^2(t)d\Omega_3^2]$$ With straightforward manipulation ...
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1answer
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How can I prove Poisson's equation from direct substitution

I’ve got this question where I should take the Laplacian of electric potentials main formula, then it should hold the Poisson’s equation. I’ve somehow gotten close to doing this but I just can’t get ...
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On Magnetic field due to a long straight wire experiment [on hold]

My question is what error can happen in the experiment if distance is ->0?
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Electrostatic energy of any charge distribution sum up to charge Q should be equal over the whole space?

Intuitively I think that even if you have two different charge distribution but their charges are sum up to the same charge Q, their electrostatic energies over the whole space should be same (Maybe ...
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A doubt and different approach regarding “A hypothetical magnetic field existing…”

A popular question from the book HC Verma's Concepts of Physics goes like this:- "A hypothetical magnetic field existing in a region is given by $$ \vec{B}=B_{0} \vec{e}_{r} $$ where er denotes the ...
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Magnetism question from REE (Roorkee Entrance Examination) 1991 [closed]

A short magnet of magnetic moment 6 A.m2 is lying in a horizontal plane with its North Pole pointing 60 degrees east of north.Find the net horizontal magnetic field at a point on the axis of the ...
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49 views

The two definitions of the divergence of a vector field? [migrated]

Now, I am aware that the divergence of a vector field, $\vec{F}$, can be defined in two ways. What I don't understand is why do these equal each other formally? Definition 1: $$\text{div}\vec{F} = \...
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1answer
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Why do metal object in the microwave heat up while the walls stay cool?

I just came accross Why is Microwaved mac & cheese burnt where they touch? and discussed it with a colleague. We understand that metal objects heat up due to the eddy currents induced by the ...
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The definition of quantities in special relativity as upper-index or lower-index

My question is for Minkowski metric $\eta_{\alpha\beta}=\mathrm{diag}(1,-1,-1,-1)$. While defining quantities like the four potential, four momentum or even space-time interval for that matter, why do ...
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1answer
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Why do the sides of the Gaussian pillbox contribute nothing to the electric field? [on hold]

It says that the top and bottom of the pillbox contribute to the $E$ field but the sides don’t. I couldn’t quite understand this, please Help.
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Need Help with Proving Faraday's Law of Electromagnetic Induction with Experiment Results [closed]

I have recently conducted an experiment measuring emf over time and magnetic field strength over time as a bar magnet fell through a coil. The magnetic field was measured by pointing a magnetometer to ...
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1answer
30 views

Magnetic flux through closed surface

I'm having a fundamental problem with Gauss's law for magnetic fields. I understand that what goes in must come out. What I'm having difficulty with is that we are dealing with vector fields measured ...
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1answer
35 views

How does one include the magnetic scalar potential in the electromagnetic four-potential?

The electromagnetic four-potential is typically written as $$A_{\mu}=(\phi,\vec{A}),$$ where $\vec{A}$ is the magnetic vector potential, and $\phi$ is the electric scalar potential. Should I want to ...
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Scalar Potential Boundary Conditions in Magnetic Diffusion

The magnetic diffusion equation, $$ \frac{1}{\mu\sigma}\nabla^2 \vec{H} = \frac{\partial \vec{H}}{\partial t}, $$ in one dimension is written, $$ \frac{1}{\mu \sigma} \frac{\partial^2 H}{\partial x^2} ...
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What are some good model quantum systems in external electric fields?

I want to understand a few things on the behavior of atoms and molecules in (weak, non-ionizing) external electric fields. For this, I am looking for model systems that might have an easily ...
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Cross product in Physics [closed]

The most famous cross product in physics is the one linked to Lorenz's force. In which other models that describe physical interactions is the cross product used? Are there other laws related to the ...