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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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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19 views

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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22 views

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
41 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
30 views

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? [on hold]

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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16 views

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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112 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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Is the passage of lightning on a transmission line? [on hold]

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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Pronunciations of Law of Biot & Savart [on hold]

I was studying the Chapter in Magnetic Field due to Current from the Book Principles of Physics by Walker, Hallisay and Resnick. It had the following line- The vector equation and it's scalar form, ...
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1answer
37 views

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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Finding magnetic flux density of a point a certain distance away from the curved face of a disc magnet [on hold]

I have used this website here because I am trying to find a formula that will tell me the magnetic field strength (B) of a point a certain distance away from the curved part/round face of a circular/...
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I got the answer but I suppose I did not understand the question very well [duplicate]

See as per the question, The Inductor pretty much will induce emf due to magnetic field and not due to other coil/current nearby. Thus I used $E= - d(phi(t))/dt$ as the induced emf. Then using (...
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3answers
104 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
56 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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41 views

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
56 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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1answer
35 views

Electron in crossed fields [closed]

A homogenous magnetic field $ \overrightarrow{\boldsymbol{B}} $ is perpendicular to the electric field $ \overrightarrow{\boldsymbol{E}} $ in a plate capacitor with the plates $d$ distance apart. From ...
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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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Show that $c^2 B^2=E^2$ is invariant under Lorentz transformations [closed]

Hello to every one please help me for this question, Show that $c^2 B^2=E^2$ is invariant under Lorentz transformations I try but can't.
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2answers
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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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Would quantum mechanics help me understand what's happening when the electrostatic models break down? [closed]

Could quantum mechanics help me clear my misunderstandings in electrostatics? especially when some equations seem contradictory, for instance, the $E$-field going like $r$ inside a solid uniformly ...
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4answers
83 views

Why does inductor current lag the applied voltage at its terminal by 90 degrees?

I studied electromagnetism and I am currently working with Inductors. I could not figure out the physical reason based on electromagnetics on why inductor current lags the applied voltage by 90 ...
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1answer
25 views

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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1answer
30 views

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 ...
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1answer
34 views

Which is induced first EMF or Electric field?

I am quite amazed by the reasoning given by my teacher about induced electric field and induced EMF.Well,the case was of a closed conducting loop placed in a time varying but uniform(in space) ...
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What is electric field and potential at any point on the right bisector of the dipole? [closed]

At centre of dipole line joining two charges and on equatorial?
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How can I calculate magnetic field strength of a variable gap magnet?

I am currently doing an experiment where an aluminium pendulum is passed through a variable gap magnet (creating eddy currents). The variable I am changing is distance between the two grade N35 ...
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How is the 4th option related to the question? [closed]

I cant seem to understand the 4th option at all. How is it related to the question? Can you please help?
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31 views

Lorentz Oscillator and dielectric function: Incorrect formulation in textbook?

According to what I was taught in school the relative dielectric function for a Lorentz-Oscillator can be formulated as: $ \epsilon(\omega)=\epsilon_\infty+\frac{\omega_p^2}{\omega_0^2-\omega^2+j\...
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30 views

My electromagnetic field and voltage related questions [closed]

I am curious about how voltages effects the electromagnetic field. Also about how to calculate a voltage from a electromagnetic field with knowledge of the following objects effecting the ...
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0answers
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Concept of Flux Linkage in solving Faraday's Law Problems

I understand that the definition of flux linkage as kind of the sum of all the fluxes going through the surfaces bound by all the turns. That is, $\Lambda = N \Phi $. However what I don't understand ...
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1answer
75 views

Simple question on quantization of EM field

This is part of the lecture note of QFT by Tong: I have a question on the last part of (6.27). Why does it hold? Actually this question is not about QFT and is about just integral. The typical way to ...
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0answers
38 views

Trajectory of two charges in free space

I am talking about the "two-body problem" for charges. Apparently, for two identical particles $\mathbf{x,y}$, each with charge $q$ and mass $m$, ignoring the gravitational force, the equations of ...
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1answer
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Two explanations of how electric motor works: are they correct?

Let's consider the simplest model for a DC electric motor: For simplicity, we assume that over a short period of time, the current through the motor is constant. All the discussion below refers to ...
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Constructing an equation for the induced EMF, in terms of the radius of a circular loop

In order to calculate for the curl of the induced electric field for a loop moving in a uniform magnetic field, and using the cylindrical coordinate system for a curl, it's my understanding that since ...
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2answers
57 views

Null electric field at infinity? How? [closed]

Suppose there are two charges (4uC each) fixed in the horizontal axis. One is in x=0 and the other in x=8m. I've obtained the electric field: $E=-k\cdot4\mu C \cdot [\frac{1}{x^2}+\frac{1}{(x-8m)^2}]...
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2answers
122 views

Derivation of Ampere's Law from Biot-Savart

Our aim is to derive $\nabla\times \mathbf B=\mu_0(\mathbf J+\epsilon_0\frac{\partial E}{\partial t})$. To begin with, let $ \mathbf A=\frac{\mu_0}{4\pi}\int_{\mathbb R^3}\frac{\mathbf J(\mathbf r')}{...
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1answer
20 views

Calculating the accurate emf of an electric generator

Let's consider the following simple model of an electric generator: Many textbook say the following: The magnetic flux through the coil is $\Phi=BA\sin \theta$, where $\theta=\omega t+\phi$ is the ...
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2answers
83 views

Derivation: (lossless case) Plane waves have no electric field component in the direction of propagation

Set up Assume that a wave is propagating in the $\hat{z}$ direction with the $E$-field polarized along the $\hat{x}$ direction. In the lossless case, $$0 = \nabla \cdot E = \frac{\partial E_x}{\...
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1answer
14 views

How to use the definition of volume current density?

The volume current density $ J$ is defined as $ \frac {dI}{da_{\perp}}$ where $dI$ is a small current segment flowing in the volume with cross section $da_{\perp}$ where $da_{\perp}$ is a small area ...
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1answer
42 views

If an electric circuit creates a magnetic field then why won't it attract magnets or iron filings?

In an electric circuit, a thick copper wire is placed. If we place a small compass near to this copper wire it deflects, thus it creates a magnetic field. If it creates a magnetic field then why can't ...
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How to replicate the effects of a geomagnetic storm (solar storm)? That one from October 13, 2017 is my goal [closed]

When a geomagnetic storm, or solar storm, impacts our planet, what are the effects and changes on our magnetic field? Particularly, I need to know to what intensity and frequency of magnetism we were ...
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1answer
38 views

How electric field and magnetic field propagate through vacuum where there's no charge present? [duplicate]

As we know light travels as an electromagnetic wave...but electric fields and magnetic fields are interactions between charges...they are the forces acting on stationary and moving charges ...