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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What does perpendicular to the plane of paper signify?

I am getting confused with the term.. Magnetic field acting perpendicular to the plane of paper.,.. Does it means that field is into the plane of paper or outside it.?
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Derivation of Lorentz force law from the Biot-Savart Law

How to derive the Lorentz force (F=qv X B) law from the definition of the magnetic field and the Biot-Savart Law?
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Radiation Emission in Rotating and Vibrating polar molecules

So, I am currently doing a course in Molecular Spectroscopy and I am finding some concepts very non-intuitive and very difficult to understand(maybe due to lack of my knowledge or the lack of ...
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Work on two charges in mutually retarded field

Consider two particles with charges $q$ and $Q$ and with an initial distance of $3L$. Now I move each charge a distance $L$ closer to the other, such that the final distance is $L$. The initial and ...
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Can we apply the equivalence principle in electromagnetics to multiple regions simultaneously?

Note: I posted the same question on the electrical engineering stack exchange with no luck so far, possibly because it's quite theoretical, so I'm posting it here again. I'm not sure if it's ...
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28 views

Electrical impulse through a mic

For a typical mic a singer uses to sing on the stage, how much electrical current is generated when the sound gets converted into electrical impulses? Can I get an electric shock from the energy ...
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45 views

Will copper filings react to a changing magnetic field?

I mean, what would happen if we spread out some good amount of copper filings on a paper and place a kind of hypothetical device which produces a magnetic field which changes at some regular intervals?...
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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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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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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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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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Flux crossing a portion [on hold]

Please solve ques 3.12 Ans is.5coulomb
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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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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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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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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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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
36 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
54 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
47 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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What force per unit of length does each line charge exert on the other? [closed]

What force per unit of length does each line charge exert on the other? I have Two identical uniform line charges that are $d=0.8\,\rm m$ apart. They are parallel to the z axis. They have an ...
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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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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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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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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 ...