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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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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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?

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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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

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

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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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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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 [on hold]

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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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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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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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 [on hold]

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 [on hold]

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 ...
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1answer
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Misunderstanding the calculation of the potential for a system of 2 concentric shells

Let's say we have 2 concentric shells. The little one has a charge $Q_1$ and a radius $R_1$ and the greater one has a charge $Q_2$ and a radius $R_2$ with $R_2$ > $R_1$. We want to calculate the ...
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Can we create artificial one-way electromagnetic attraction by just using photons?

As usual, I apologize for asking dumb questions, and I hope this isn't one of them. If we know that photons are the carriers of the electromagnetic force, could we somehow create a device that simply ...
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Is pair production a type of elastic scattering?

Pair production is the creation of a subatomic particle and its antiparticle from a boson. Pair production often refers specifically to a photon creating an electron–positron pair near a ...
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Why do tokamaks have so few toroidal field magnets?

From the ITER webpage itself, the tokamak has only eighteen toroidal field magnets. If the magnetic field is directly and linearly dependant on the number of coils around the torus, then why did the ...
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EM parameters of fiberglass

My question is very short: does anyone know the electromagnetic parameters $\epsilon$, $\mu$ and $\sigma$ of the 'fiberglass' material, in X-band? I searched on various sources but I didn't find any ...
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Leaked electric field in Transmission Line theory

In the previous image I can't understand why the leaked electric field in plate conductors is axial (On the same axis of propagation), And if it becomes axial then the wave in the conductor is not <...
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1answer
35 views

Current densities in a 3D object

What is a smart way to calculate current densities in 3D objects with unorthodox boundary conditions? For example, J(x,y,z) in a cube with constant resistivity with applied voltages Va in a vertex ...
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43 views

Lorenz Gauge Condition in Helmholtz equation

I am working with electromagnetic fields in the frequency domain. I do not understand why we can apply the Lorenz condition in Helmholtz equation. What is its physical meaning? Any help is appreciated....
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Standing Wave in a transmission line and Resonance

let's consider a transmission line of length L closed on a mismatched load. So, there will be a travelling wave and a reverse travelling wave: V(z) = V+(z) + V-(z) My question is: will there always ...
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Why should $E$ and $B$ both contribute $(1/2) kT$ in equilibrium radiation?

At high temperatures/low frequency, energy distributes itself by equipartition. There's a factor of $\frac{1}{2}kT$ for each degree of freedom that contributes quadratically to the energy. So, for ...
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Qualitative definition for potentials

I am doing an E&M course and just started with the concept of electric potential, potential difference, etc. I know how to work with them mathematically, but I don't have an intuition for their ...
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Vector components for the dipole of water

I was looking online for the components for the dipole moment vector of water in the molecular frame. Does anyone know what the values would be for $\mu_x$, $\mu_y$ and $\mu_z$ for a water molecule in ...
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Power absorption - ferromagnetic resonance

Magnetic energy is given by: $$ E(t) = \mathbf{M}(t)\cdot \mathbf{H}(t)$$ Hence, the magnetic power becomes: $$P(t) = \mathbf{M}(t) \cdot \frac{\mathbf{H}(t)}{dt} + \mathbf{H}(t) \cdot \frac{\mathbf{...
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Classical (non-quantum) explanation for the transparency of glass?

Is there a classical explanation for the transparency of glass? How did physicists explain the transparency of glass before quantum mechanics?
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How does the induced E field for a uniformly varying B field obey spatial symmetry?

Consider this answer giving the induced E field by a uniformly varying B field. In this case, if you place a point positive charge at the origin it will experience 0 force, but if you put it at some ...
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1answer
41 views

Same length insulated wire small gauge vs larger gauge

I'm doing an school experiment with my daughter and I made two alternating current generators. One is a coil of 22 gauge wire and the other is a coil of 28 gauge wire where we rub a strong block of ...
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1answer
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Is it possible to change the direction of magnetization of a magnetic sheet?

I want to try an experiment where I have to attract or repel thin sheets of material using an electromagnet. I have found lots of magnetic sheets for sale, but all of them are magnetized so that the ...
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1answer
57 views

Does a time-varying magnetic field ALWAYS produce an electric field?

Assume two similar point charges located a distance $\Delta y$ away from each other are set in motion with the same velocity $v$ in opposite directions as shown in the figure. The lab observer located ...
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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^\...
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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 ...
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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}\...
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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 ...
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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 ...
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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 ...
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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 ...