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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Changing the refractive index of biological tissue using Radio Frequency (RF)

I have been looking into changing the refractive index of a specific medium initially using Ultrasound in a known location via a mechanical Doppler shift of the emitter (Mahan et al., 1998) which has ...
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Current-carrying wire in a magnetic field. Cross product, vectors and scalars

We have a wire with cross-sectional area $A$, length $L$ and current $I$. If the wire is in a magnetic field $\vec B$, the magnetic force on each charge is $\vec F =q\vec v_d \times \vec B$. $\vec ...
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Permeability of the human skull using Radio Frequency (RF) and Ultrasound waves

I have been looking into the permeability of the human skull using RF and ultrasound waves, and had a few questions. Which frequencies of Ultrasound and Radio Frequency (RF) waves can pass through ...
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Why define $D_\mu = \partial_\mu -ieA_\mu$ with the electric charge $e$?

If $D_\mu = \partial_\mu - ieA_\mu$ then the QED Lagrangian is invariant under $$A_\mu \to A_\mu + \frac{1}{e}\partial_\mu\alpha(x)$$ $$\psi \to e^{i\alpha(x)}\psi$$ However if $D_\mu = \partial_\mu -...
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Changing the refractive index of a specific medium using radio frequency (RF) waves

Using Ultrasound one can change the refractive index of a specific medium such as biological tissue, I was wondering if it is possible to change the refractive index of a medium such as biological ...
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26 views

Hysteresis in ferromagnets

In my text it is given that the magnetic susceptibility X = I/H but in another it was dI/dH. But second can be used to draw the hysterisis graph so I think the second must be correct. X is dependent ...
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Symmetry reason why magnetic dipole transitions are suppressed

In the theory of light-matter interaction, electric dipole transitions between two atomic states of same parity are forbidden. This is because the Hamiltonian conserves parity. Is there a symmetry ...
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The origin of normal reaction

I want to know what actually is the origin of normal reaction between 2 objects. i have read about normal reaction from several books which generally mention it as a component of contact force which ...
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How to calculate energy loss by eddy current?

I was reading this post: Method of calculating eddy currents of a conductor, is this correct? and I found it very interesting. They came to the conclusion that the power lost due to eddy currents was: ...
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51 views

Force on a Magnetized Body in a Non-Uniform Magnetic Field

In Griffith's Electrodynamics, it is shown that when a magnetic moment, $\mathbf{m}$, is subjected to a non-uniform magnetic field, then it experiences a net force given by $\mathbf{F}=\mathbf{\nabla}...
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IN 3 phase transformer the winding are 180 degree out of phase and where it is used as application

application of 180 phase shift transformer am a novice in electrical engineering. I notice that in transformer the secondary side current & current referred to as primary are 180 degree out of ...
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Does magnetic intensity ($H$-field) 'conduct' through a material

We've been taught by our lecturer that magnetic intensity propagates through a material. But I have a doubt because I feel that it is not consistent with Ampere's law. My reasoning is explained below. ...
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Should the energy-momentum tensor be invariant under gauge transformations?

For example, consider the electromagnetic theory given by \begin{align} I=-\frac{1}{4}\int d^4x\, F_{\mu\nu}F^{\mu\nu}, \end{align} where $F_{\mu\nu}=\partial_\mu A_\nu-\partial_\nu A_\mu$. The action ...
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Preparing For Jackson Electrodynamics [duplicate]

Hypothetical Scenario: You are an undergraduate physics student who has never taken a course on E&M and you are enrolled in a graduate Jackson electrodynamics course. In general you are good at ...
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1answer
36 views

Some confusion in Drude theory of metals

Discussion on the drude theory of metal usually begin with the case of zero magnetic field so that the force acting on the electrons is just the one from the electric field. But then, this electric ...
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Magnet oscillation with coils

I am trying to conduct an experiment in which I will try to find the natural frequency of a cantilever. To oscillate the cantilever, I thought I could use a magnet and a current-carrying coil wire ...
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Moving Coil Galvanometer' s radial field

How do we produce radial and constant magnetic field for a moving coil galvanometer and also prove that is radial and constant . (i m not asking why do we need it to be radial and its benefits rather ...
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Can we apply ampere's law for a current carrying circular loop

They used the ampere law to calculate magnetic field by a toroid ( assuming perfectly circular coils provinding symmetry and neglecting effects due to helical nature) whis is $μ_0ni$(where $n$ is no ...
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1answer
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Which form of Lagrangian should I choose for uniform magnetic field?

Here's the problem and corresponding question. Let's consider a uniform magnetic field $\vec{B}=B_0\hat{z}$. Looking for the solution of vector potential satisfying $\vec{B}=\vec{\nabla}\times \vec{A}...
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3answers
134 views

How long does it take for an electron to reach equilibrium with blackbody radiation?

While teaching a course on electrodynamics, I thought of an interesting question that I think deserves some attention. Consider an ensemble of electrons all with momentum $\hbar \mathbf{k}$ ...
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How polar researchers were able to pinpoint the south magnetic pole in 1900s?

I am reading a few books on polar explorations done in 1900s and one of the aim of these explorers was to locate the south magnetic pole. I wonder how able to pinpoint the location of south magnetic ...
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1answer
46 views

If electric field is conservative then line integral along closed path is zero, then why is potential not zero?

Here in the first image it is said $\displaystyle \int_a ^b E\cdot \mathrm dl\,$ is $0 $: Here in the second image it looks like $\displaystyle \int_a ^b E\cdot \mathrm dl\,$ is not $0$: If field ...
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Electric Field in conductor near Solenoid

My question is essentially a follow up to this question. It's answer says, the mobile charges in the conductor distribute themselves to make E = 0 inside the conductor. But, I don't understand how a ...
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Stability of a dipole magnet undergoing eddy current braking in a cylindrical tube

Consider a cylindrical dipole magnet with strength $B$ undergoing eddy current braking in a conductive cylindrical tube with the axis of the magnet aligned with the axis of the tube. The magnet ...
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20 views

How to analyse dielectric discontinuity in Waveguide?

I came across this question the solution given was to use the formulae for reflection coefficient with characteristic impedance of wave guide replacing the ones in boundless medium. What I am ...
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1answer
42 views

Longest distance travelled by an electron in E&M field

Let's assume we have a cylindrical wire where the currents are following. Since this is not a static case, we would have magnetic and electric field both from the current carrying wire. Here I have ...
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1answer
63 views

Covariant form of Green's function for wave equation

In J. D. Jackson's "Classical Electrodynamics", page 614 in the 3rd edition, he states that you can write the Green's functions for the wave equation in covariant form using the fact that \begin{align*...
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Faraday disk - Electric fields

I was thinking about an experiment when you let a homogenous metallic disc spin in a homogeneous magnetic field, that is parallel with the axis of the disk. If you connect the center of the disk to ...
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How much energy, momentum, and/or angular momentum are lost to radiation in Rutherford scattering?

Inspired by this question: Consider two charged particles, of masses $m_1$ & $m_2$ and charges $q_1$ and $q_2$. They approach each other from a great distance, interact via their electromagnetic ...
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Why is there a negative sign in the (non-relativistic) bivector formulation of the Lorentz force?

I'm currently trying to update my understanding of basic (Newtonian, non-relativistic) physics to use bivectors and Clifford products instead of pseudovectors and cross products. And I've come up ...
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The physics of microwave vacuum tubes

I've searched this extensively, but I couldn't find anything (the only thing I found was this one). I know this question seems to be more of engineering than physics, but I've found some engineering ...
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1answer
50 views

What exactly happens when a charged conductor comes into contact with an electric insulator?

Let us say we have a negatively charged conducting sphere: If we put an insulator into contact with the sphere: Would the negative charges located in the contact region transfer from the surface of ...
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1answer
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Potential Drop across Inductor VS Potential drop across Rotating coils in $B$-Field

I was trying to understand the difference between the Back emf generated across Inductor due to change in current and Back Emf Generated across a coil that is rotating in presence of B Field. ...
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Energy stored in a sphere of variable charge density

I have seen a derivation of the energy stored in a uniformly charged sphere. However, I would like to generalize this derivation to include a variable charge density (varying with radius). The ...
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RF Transmission Loss

I am aware that an ideal 3rd order circuit with R=0 will oscillate forever, but even this this is not true. Power is dissipated through the em waves. How does one calculate the losses due to these ...
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Charge distribution for three connected conductor spheres

In the auxiliary material of the physics textbook of Halliday, first chapter about electrostatics, there is an example that has the following statement and solution: basically there are three ...
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Forces between particles rotating and how to represent their effect on them mathematically in 3D

If 4 atoms are represented in three dimensions, with each particle having coordinates $(x,y,z)$, one can compute the rotations around their nucleus with the Rodrigues Rotation formula. I’m now left ...
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1answer
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About fixing the potential on the surface of a conductor

In Purcell's Electricity and Magnetism, p.116 section 3.3, the author spoke about Laplace's equation and said that the boundary conditions for the potential$\,\phi$ on the surface of the conductor may ...
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128 views

Are photons emitted by a magnet?

If you put a photon detector near a magnet (with the magnetic field static in time), is there some probability that the photon detector will detect a photon? Does QFT not predict that a photon could ...
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1answer
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Is the passage of lightning on a transmission line? [closed]

If so, then shouldn't lightning be reflected back into the sky everytime it hits the earth, due to an impedance change?
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1answer
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Electromagnetic wave and skin depth, skin effect

In EM theory concept of skin depth is induced which is a measure of how much a EM wave can penetrate the medium, from it arises the concept of skin effect which is for EM wave but we use this to ...
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3answers
127 views

Are the electrons' orbitals the same for all atoms?

Are the electronic orbitals of an atom always quantified in the same way (i.e. the same energy required to reach the next level), or does each atom have its own values for each level? If the ...
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Conventions for direction of current flow and forming a charge difference

I have explored many videos, postings, and lectures regarding the interaction of electricity, magnetism, and motion, including Fleming's Right Hand Rule and Left Hand Rule. While the explanations of ...
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1answer
60 views

Why can't we have an $E$-field like this one inside a conductor?

From what I understand, there can be no $E$-field inside a conductor in an electrostatic situation, because had there been any field within it, the charges on the surface would start to move, thus ...
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Physical degree of freedom and gauge fixing?

I'm confused with the gauge fixing in the Higgs mechanism. So if we have an action like $$S=\int |D\phi|-\frac{1}{4}F^2 -V(\phi) ~ ,\tag{1}$$ then expand around some non-trivial vacuum, then we have ...
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1answer
58 views

How do the spins of electrons line up in magnets?

I understand that a magnetic effect is produced when the spins of electrons in an object line up. But why does this phenomenon occur? Are there some materials that cause this to occur?
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“Conductors having excess charge can attain static equilibrium”, is this an empirical law?

Is the redistribution of excess charge in a conductor onto its surface, thereby reaching static equilibrium (a steady state), only an empirical observation? Or, equivalently, is it guaranteed, i.e. ...
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Regarding charge densities and the amount of charge located on a point for continuous charge distributions

Suppose we have a volume $V$ containing a charge distribution defined by $\rho (\textbf{x})$. The amount of charge $q~(P)$ located at an arbitrary point $P(x_{0},y_{0},z_{0})$ is : $$q(P)=\int_{x_{0}...
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How current induced when magnet moves in coil? [duplicate]

How is current induced when a magnet moves through a coil? How does it work? I would like to know how it works and how to explain it in a somewhat simple language.
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Magnet moving between $2$ separate coils, connected to a circuit

I was thinking about a question regarding a magnet entering a coil when the following thought came: what if we unravelled the middle of the coil such that it splits into $2$ equal parts, and connected ...