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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Evaluating an Integral with a Cross Product

Problem: A infinite thin spherical shell with radius $r_0$, centered around the origin, is homogeneously charged with surface-charge-density $\sigma$ and rotates with the constant angular verlocity $\...
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'Electrostatic field of a circuit' What does it mean?

I've recently gotten into electrodynamics, I've studied electrostatics & magnetostatics, so far I know these facts: Electrostatic fields are produced by static charge configurations Magnetic ...
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Dipole moment for dielectric shell with specific boundary conditions (different charge densities)

To find the dipole moment in a dielectric sphere, I normally use the bound charges density which helps me to find the potential differences and then negative gradient of that potential can give me the ...
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1answer
31 views

Dipole moment for dielectric materials vs conducting materials

Let's assume we have a polarized charged sphere which is uniformly polarized (it is one of the Griffiths example). Since it is uniformly polarized, I assumed the material is dielectric. What would be ...
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64 views

What are the differences and advantages of the integral and differential forms of Gauss's law? [duplicate]

What are the differences and advantages of Gauss's Integral and Differential Equations? Thank you so very much!
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What is a permanent magnet and how do I calculate the magentic force exerted by a permanent magnet?

So far my understanding of a permanent magnet is: Since elements like iron have atoms with orbitals that have many electrons with the same spin, these cause the metal to have a magnetic field, which ...
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Why does an inductor oppose the change in current (magnetic field)?

May I get a physical interpretation on this question? What is happening in the inductor when the current is running through it and what is physically happening when the current starts changing?
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1answer
57 views

Why is the 4D $U(1)$ electric 1-form symmetry a global symmetry?

I am reading the paper Generalised Global Symmetries to understand higher-form symmetries. The first example in Section 4 that the authors talk about is the free Maxwell theory in 4d, i.e., pure $U(1)$...
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26 views

Magnetic field due to a dielectric sphere

There is a nice example in Griffiths where he calculated magnetic field of a spherical shell/sphere for uniform surface charge $\sigma$. The argument was, since the surface current is $K= \sigma r \...
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1answer
29 views

Electric field due to changing uniform magnetic field

I think this could be a misunderstanding on my part , still I want to clarify this.Suppose in a region of boundless expanse, there is a uniform magnetic field which changes at a rate $\frac{d\phi}{dt}$...
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1answer
43 views

Nature of magnetic field

If I understand correctly a magnetic field is created by the exchange of photons between charged particles. Is it correct to assume that the magnetic field lines are the trajectories of said photons ...
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1answer
57 views

AC currents for self-inductance of a wire

I've been currently reading some stuff on self-inductance of a wire. There are different regimes where different approximations are used. For the AC currents the notes state that an approximation can ...
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1answer
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How come subatomic particles interact with each other while not being in contact? [closed]

Although electrons stays away from protons an atom remains electrically neutral But two atoms one positively charged and another negatively charged remain so until they comes in contact. Can the ...
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Does magnetic moment change under inversion symmetry?

Since magnetic moment can be view as a small electric current circle. Pictorially, when apply inversion operation, the current direction is reversed, so I think the $\vec{m}\to -\vec{m}$ under ...
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1answer
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Does stacking two magnets together increase the magnetic strength?

Just wondering if adding two (or more) identical magnets together increases their magnetic strength (in Tesla) as I am doing a physics write-up on the Lorentz force (Fleming's left-hand rule) as I ...
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2answers
70 views

Is electric current reference frame dependent?

$$EMF=\frac{dW_{external}}{dq}$$ $$dW_{ext}=\vec{F}_{net}.d\vec{s}$$ $$\vec{F}_{net}=\vec{F}_{inertial}+\vec{F}_{non-inertial}$$ $$dW_{inertial}+dW_{non-inertial}=dW_{external}$$ After reading the ...
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Electric Field in a uniform time-varying Magnetic field

Suppose a homogenous Magnetic Field $\vec{B}$ in vacuum that varies with time, but always points in the z-direction. This induces a curl in the Electric Field $\vec{\nabla} \times \vec{E} = -\frac{\...
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What are estimated magnetic properties of liquid metallic hydrogen?

I understand that liquid metallic hydrogen isn't easy to produce, or keep it stable on Earth, but can be liquid metallic hydrogen magnetised? Does it have magnetic properties at all?
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Accelerating Universe and dark energy or photon absorption or neergy decay?

I’m an RF/microwave engineer with physics background, and had a question regarding dark energy and experimental observations. FYI I don’t qualify as a professional physicist, hence asking a ...
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1answer
36 views

Force on a moving charge in 2 frames

The following simple problem seems to lead to a contradiction when analyzed in different frames. Consider an infinitely long stationary wire with positive line charge density $\lambda_0$ and ...
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2answers
50 views

Regarding the definition of the electrical potential energy

Let us say we have two charge positive $q_{1}$ and $q_{2}$ very far away from each other. Suppose also that $q_{1}$ is stuck in its place. If we want to bring them closer to some distance $b$ apart, ...
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What magnetic interaction makes spin not conserved?

If $[S_i,H]\neq0$ or $[S^2,H]\neq0$, we might say spin is not a good quantum number in the system. But is there any more practical or detailed criterion? Or certain families of magnetic interaction ...
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Is there a way to express the curl of the electric field in the integral form?

In order to experimentally measure the curl of the electric field from faraday's law... is there a way to equate the curl of the electric field to some integral that are measurable in a practical ...
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Signs in application of Gauss's law to a sheet of charge

In Purcell's Electricity and Magnetism, page 52, the author said that in the following situation: [Where we have a slab of positive charge with uniform density $\rho$. $\,\bf{E_{1}}\,$ is the $E$-...
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Classical Electromagnetic scattering

What are the best resources (books/lectures/articles) to learn Classical Electromagnetic Scattering (forward and inverse modelling)? I am an Electrical Engineer so I would prefer some resource which ...
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1answer
21 views

Electromagnetic field of a spinning cylinder

Let us consider an infinite cylinder of axis $(Oz)$ and radius $R$ spinning at a constant radial velocity $\omega$. We assume that this cylinder is made of a metal that is assumed to be a conductor (...
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1answer
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Reflection and Transmission at oblique incidence Griffiths

I’m working through Griffiths E&M and on page 408 he gives the boundary conditions for oblique incidence. I’ve spent an hour trying to understand how he goes from the boundary condition $$\...
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Investigating the relationship between a changing B field and the curl of the electric field induced

I am currently working on a 12 page lab report on Faraday's law, essentially investigating, how for different magnetic field strength through a stationary loop of wire with N number of turns, effects ...
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1answer
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Strange electromagnetic phenomena

Well, I have some doubts about two electromagnetic phenomena that may be related so I wanted to ask you if you could explain them to me. The first one happend some days ago: I was about to go to bed,...
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1answer
31 views

Force per unit area on conducting sheet in magnetic field [closed]

Question from I.E. Irodov (3.260) I’m having trouble with the (c) part because the external magnetic field should be towards the right to give such a result,hence the force on unit area ...
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1answer
31 views

What is $v(t)$ in a sliding conducting rail in a magnetic field?

This is problem 7.7c from David J. Griffiths - Introduction to Electrodynamics. A metal bar of mass $m$ slides frictionlessly on two parallel conducting rails a distance $l$ apart. A resistor $R$ ...
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61 views

Does the static gravitational field move with the source instantly?

I have read this question: How fast does gravity propagate? where hawkeye says: So what does that mean? It means that the "speed of gravity" is the speed of light … technically. Changes in ...
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2answers
43 views

Gaussian Beam propogation in a fiber

If we suppose that a Gaussian beam traveles in the fiber with diagonal end as it shown in the image, so I have several questions: 1- Do we get in the end of the fiber Evanescent field (Total ...
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1answer
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Scattered fields in surface plasmon excitation with grating

We need to use prism-coupling (e.g. Kretschmann or Otto configuration) or use grating to excite surface plasmon between metal and dielectric. What I understand is that when we use a grating or ...
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1answer
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Poles of a magnet

Why are the poles of a bar magnet not at the extreme ends of the magnet or why is the effective length not the real real length of a magnet?
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1answer
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Is the field strength in a Helmholtz coil really a function of current?

As I understand it, the field strength at the midpoint of two Helmholtz coils is given by: $$B=\left(\frac{4}{5}\right)^{3/2} \frac{\mu_0 nI}{R}$$ All things being equal, it seems that we can ...
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Solving the Line Integral in Ampère's Law mathematical correctly

Imagen we have a infinite long, cylindrical conductor with radius $\varrho_0$ and $\textbf{j}=\begin{cases}j_0 \textbf{e_z} &r\leq \varrho_0\\ 0 &r>\varrho_0\end{cases}$ We have Ampère's ...
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How does magnetic field know the particle's velocity? [closed]

The force on one charged particle due to a uniform magnetic field is $F=qvB\sin\theta$. According to this formula, the faster the particle's velocity, the greater the force of the magnetic field is ...
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Electromagnetic waves & observer effect [duplicate]

So I’ve been doing some research & realised that they done the double slit experiment with large molecules and the molecule behaved as a wave , so does this mean that all matter is really just a ...
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2answers
77 views

Electric field of a point source in QM

We know that radiation from a point source vanishes at large distances. So when an atom emits a photon, the expectation value of the electric field must vanish at large distances. How we can explain ...
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2answers
32 views

Eddy currents in conductors

Eddy currents will flow in closed loops within a conductor this I understand. When a coil is used for induction heating why doesn’t the coil itself become heated due to the induced eddy currents, ...
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What is the radiation field of a black body with temperature $T$ translating at speed $v$?

Suppose I have a spherical black body at temperature $T$ with radius $R$. When stationary in a vacuum at zero Kelvin, it produces an incoherent radiation field according to Planck's law $$ B_\nu = \...
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1answer
27 views

Confusion with representing the negative rate of change of magnetic flux in Faraday's law

So Faraday's law states in differential form that $$ \nabla \times \vec{E} = -\frac{\delta H}{\delta t} $$ Using Stoke's theorem, the right hand side (the magnetic flux rate of change) is expressed as ...
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1answer
62 views

An absurd consequence of magnetic field as an axial vector

One of the definitions of magnetic field (in free space): Force on magnetic north pole per unit pole. Magnetic field is an axial vector. So it changes its sign when we use left handed coordinate ...
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What is the exact mechanism of flow of electricity?

When a steady current flows through a conductor, the electrons in it move with a certain average ‘drift speed’. One can calculate this drift speed of electrons for a typical copper wire carrying a ...
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1answer
40 views

Modifying the Hamiltonian when there is a presence of the Coulomb interaction

Referring to the Hamiltonian of a system of free electrons, $$ H_0= \sum_{\sigma} \int d^3rd^3r' \psi_{\sigma}^{\dagger}(\mathbf{r})\left(- \frac{\hbar^2}{2m}\nabla^2\right)\delta(\mathbf{r}-\mathbf{...
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1answer
40 views

Different formulas for magnetic flux

I am familiar with the formula $\phi=BA\cos(\theta)$ fo magnetic flux, where I know $\theta$ is the angle between $B$ and $A$. I came across another formula ; $\phi=Ba\cos(\omega t)$. Wanted to ask ...
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Metal sphere in uniform Electric Field—doesn’t behave like dipole at large r?

I’m going over Griffiths EM example 3.9 (different question on this example is here Metal Sphere in a Uniform Electric field). I’m wondering where the logic for $$V\rightarrow -E_0z= -E_0r\cos(\theta)...
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1answer
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Do potentials make sense in relativistic quantum theory?

In Peskin & Schröder QFT, just before equation 7.93, he writes in passing, Next let us examine how $\Pi _2 (q^2)$ modifies the electromagnetic interaction, as determined by Eq. (7.77). In the ...
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1answer
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Magnetic Field Manipulation

For a reference of my level of knowledge, I have taken an undergrad electromagnetism class, and it wasn't pretty but I passed. I have a real interest in this so I don't mind doing more research. If ...