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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21 views

How does a general electromagnetic wave equation looks like?

I know that electromagnetic wave consists of electric field and magnetic field, say: $ \overrightarrow{E}\left(x,t\right)=E_{0}\cos\left(kx-\omega t\right) $ $ \overrightarrow{B}\left(z,t\right)=B_{o}\...
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Determine initial permeability from experimental data

How to determine initial permeability of the sample having only its dimensions (ring with $R$ - external radius, $r$ - internal radius, and thickness $h$), the number of coils around it $N$, and only ...
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Rate of absorption, how is it used in laser theory?

Let's consider an atom with two non degenerate energy levels, $E_1$ and $E_2$, that is interacting with an monochromatic EM wave. The wave function can be expressed as $\psi=a_1(t)\psi_1+a_2(t)\psi_2$ ...
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Will electromagnetism separate into electricity and magnetism as the universe cools down?

The electroweak force separated into the weak force and electromagnetism. So, will the electromagnetic force eventually separate into electricity and magnetism?
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Multipole terms of Electrostatic energy

If we consider the Potential and electric field of a Dipole, could we infer that for a dipole : $V \propto 1/r^2$, $E \propto 1/r^3$ , $da \propto r^2$, then Electrostatic potential energy should ...
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Induced Current Density of Circular Disk

Hello I'm having trouble setting up the correct form of the Maxwell-Faraday law to help me find the induced current density on a disk in an alternating magnetic field. The problem is set out as ...
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What is the magnetic field of a uniform steady volume current?

Consider a slab of a material of finite, non-zero conductivity, infinite in the $y$ and $z$ directions but sandwiched by two metal plates at $x=x_{1}$ and $x=x_{2}$, say i.e. we are essentially ...
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If a cosine wave satisfies Maxwell's equations then how does a sine satisfy the equations as well?

Say the real part of $$\tilde{\mathbf{E}}(z, t)=\tilde{\mathbf{E}}_{0} e^{i(k z-\omega t)}$$ satisfies all Maxwell's equations. Then how can we say the imaginary part satisfies the equations as well? ...
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Analyze Coil - Iron system Problem

I'm working on a coil - iron system analyze project A problem that I faced with is effect of movements of the iron on the coil current & force I want to analyze force of the coil on iron block in ...
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1answer
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Power and power loss during transmission of power

So let's say power to be delivered to homes is 80kW($P_3$=80kW) and the houses have to receive it at 220V($V_3$=220V). The substation is a 4000V to 220V step down transformer and $R_2=15 \Omega$ It ...
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Charge piled up at the junction of two different materials

In this question, I am unable to understand why charge would pile up at the junction in the first place. This is current electricity, not electrostatics, and the current is same through both the ...
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Which of the four fundamental forces is responsible for covalent bonds? [duplicate]

Molecules are the result of covalent bonds forming between atoms. Which of the four fundamental forces is responsible for covalent bonding? (Sorry if this is basic -- I'm guessing electromagnetic, but ...
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Can the Peak intensity of a Gaussian Beam, be fully defined from the wavelength and the beam waist width?

So I've been asked to plot an intensity graph along the z-axis for a gaussian beam knowing it's wavelength and waist width. The gaussian beam propogates though free space. $$n = 1$$ $$\eta = 377 \...
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How can we use the electrostatic boundary conditions in electromagnetic waves?

The boundary conditions for an electromagnetic wave passing from one linear dielectric media to the other (both having no free charges or current) are taken as: $$B_{\perp_1} -B_{\perp_2} =0$$ $${\...
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Does conservation of energy across “worlds” explain probabilistic filtering of photons?

We can set up a light filter and a light source such that angling the filter may block out some/all of the light. For example, with a certain filter angle we might see that $30\%$ of the original ...
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1answer
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What are the superscript negative signs after the $E$ and $H$ in the equation for 'Wave impedance'?

The wave impedance is given by ${\displaystyle Z={E_{0}^{-}(x) \over H_{0}^{-}(x)}}$ where ${\displaystyle E_{0}^{-}(x)}$ is the electric field and ${\displaystyle H_{0}^{-}(x)} $ is the magnetic ...
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PMDC Motor Characteristic Graph for Given Parameters

TL;DR Need to make a graph which showcases all parameters of a PMDC motor against the torque input. I am trying create a graph similar to as shown below. However, I am confused as to how the torque is ...
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Is the Green function of electromagnetism a scalar or a tensor?

When I check classical electromagnetism books (such as Jackson), Maxwell equations \begin{equation} \Box A^\nu (x)=\frac{4\pi}{c}j^\nu (x) \end{equation} can be solved using a scalar Green function $G(...
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Microscopic explanation of the Joule effect and dielectric on capacitors

I understand intuitively that the resistor heats up when electrical energy travels through it and mathematically that the capacitor stores more charge when we fill the space between the conductive ...
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1answer
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Changes of electric potential energy in a split liquid drop

A liquid drop with radius $R$, with a uniform charge $Q$, divides in two, with equal radius and charges, that move away until they are very far from each other. What is the variation of electric ...
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Induced EMF in open loop

Take a wire that doesn't form a closed loop, moving at velocity $v$ perpendicular to a constant magnetic field. In the frame of reference to a stationary person the induced emf caused by the magnetic ...
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Can an induced EMF be generated from a loop kept in an constant electric field?

I am wondering whether the following set-up can be used to generate an induced EMF. Since the electric field between the parallel-plates is in such a direction that the positive charge accumulate at ...
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1answer
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What is the physical meaning of the complex field amplitude?

I see that the harmonic field is sometimes written in exponential form. But sometimes the complex amplitude of this form is just a constant and in others (like when talking about modes) is dependent ...
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How is Louis de Broglie's theory of matter waves incorporated into Bohr's model of the atom?

I would like to know how Louis de Broglie's theory of matter waves is incorporated into Bohr's model of the atom and how the angular momentum is quantized in this model.
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Calculate the Electric field (Numerically) at point given a point in space for a uniformly charged cuboid of dimension 20 x 20 x 1 [closed]

Given a point in space r(30,30,5) and the cuboid has charge density of p = 0.1. Take the origin at one of the corners of the cuboid on the bottom edge. Electric Field generated by continuous charge ...
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How is Fourier analysis related to this statement?

The following statement is made when separating the spatial and time components of the field. But I do not see how Fourier is related.
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1answer
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Why can the electric and magnetic field be separated in spatial and time components?

I was reading the Jackson and suddenly in one line they wrote the electric field as: $$E(x,t)=E(x)e^{-iwt}$$ why is this valid?
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Literature Recommendation - Classical Electromagnetic Scattering Theory

I'd like to ask for your recommendation on literature regarding classical electromagnetic scattering at an introductory level. The aim is understanding the mathematics behind simple scattering ...
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74 views

Where does the energy go when I cut a current carrying loop?

The energy in a current carrying loop is given as $E_{\mathrm{m}}=\frac{1}{2 \mu_{0}} \int B^{2} d \tau$ If I cut the loop abruptly the current and hence the magnetic field quickly fall to zero and ...
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1answer
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Poynting vector on the surface of an isolated system

We've an isolated system inside which currents and charges reside. If the Poynting vector on the surface of the system is non zero, then there is energy exchange between the system and the ...
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1answer
29 views

Curl of an Electrostatic Field

According to Maxwell's equations: $$ \vec{\nabla}\times\vec{E} = -\frac{\partial B}{\partial t}$$ But if we have an electric field that is dependent upon both the radial distance and angles, we can ...
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How to draw all the Electric field vectors associated with an em wave?

This is a single ray of an electromagnetic wave propagating in one dimension. But we know an electromagnetic wave travels in three dimensions. So, I wish to draw the Electric field and magnetic field ...
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1answer
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Ampere's law: the position of amperian loop

Can Amperian loop be taken parallel to current. I.e suppose current is in a circular loop in xy plane. And let the screen you are watching is xy plane . Can we take amperian loop in the same plane. ...
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1answer
45 views

Gauss law for moving charges

since gauss law for electricity is derived from the divergence theorem and couloumbs law. why can it be assumed it is valid for moving charges as coulombs law is only valid for static charges since it ...
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2answers
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Does a charge accelerated by a magnetic field emit radiation?

Let us assume a charged particle moving in a circle due to a constant magnetic field. Does this particle emit radiation? If so, where this energy comes from?
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Dielectric constant or permitivity of copper

I'm trying to simulate a capacitor made of copper plates in comsol multiphysics. What is the value that I should use for the dielectric constant or permittivity of the copper plates? I've read that ...
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2answers
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What determines picking a gaussian surface?

I feel like I am missing something simple here, but how exactly are gaussian surface determined? Looking at the case of a charge outside a sphere, why don't we pick a gaussian surface not including ...
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1answer
38 views

How is potential difference defined across a resistor with time varying current

From this discussion How can we define a potential for a moving charge? we know that we cannot define a scalar potential (as in electrostatics) in the case of moving charges as described by G. Smith: ...
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Abraham-Lorentz Force

Can anyone clear this up for me? when a charged particle accelerates it emits an electromagnetic wave carrying energy away from it, the wave being emitted exerts a force called the abraham-lorentz ...
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When do travelling (accelerating) charged particles emit radiation in their direction of travel and when perpendicular to their velocity? [closed]

I have read numerous things saying that a travelling charged particle emits radiation primarily in a cone in the direction of travel, and other places say the radiation is emitted and polarized in an ...
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1answer
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How do I compute $v$ from angular speed cross radius when using Lorentz force equation?

I'm working on a problem in Griffiths E&M and I can't seem to figure out to get $$\vec{v}=R\omega\hat{\phi}+\dot{z}\hat{z}$$ from just $R$ which is the radius of a disk centered at the origin in ...
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4answers
109 views

How can classical physics explain what happens when a charge fixed by two other charges experiences a magnetic force?

I have invented this little problem to help me understand the magnetic force better. Imagine 3 positive charges all on a line. The first charge is separated by distance D from the second charge, and ...
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Moving charge thought experiment paradox

Let’s say you have a moving charge, relative to a stationary observer. They measure that it has an electric field $E$ and a magnetic field $B$. Because the observer is stationary, they experience no ...
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1answer
109 views
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Why curl of Dirac string attached to Dirac monopole is zero?

So let we have a magnetic field which is $$B_\mu=\frac{q}{2}\frac{x_\mu}{|x|^3}-2\pi q\delta_{3\mu}\theta(x_3)\delta{(x_1)}\delta{(x_2)},\tag{4.65}$$ where $\theta$ is step function and $\delta{(x_\mu)...
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92 views

How can we define a potential for a moving charge?

Say a charge is moving in space. Ignoring relativistic effects, how can we define a scalar potential for its electric field ? My thoughts are that we can define the potential in exactly the same way ...
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1answer
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If an event horizon never forms for an outside observer, then what do (or don't) we see in the middle area on this real image of an actual black hole?

There are a lot of questions about the m87 image on this site, non of them actually answer my question. I have read this question: Does an expanding event horizon "swallow" nearby objects? ...
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Conservation of energy in superconducting transitions with Meissner effect

In Type I superconductors the transition from normal to superconducting state occurs exactly at its transition temperature. Does this mean that we could theoretically use only a little bit of energy ...
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How fast in mph would wind have to be coming towards a 175 pound air foil curved at about 45 degrees in order to lift it? [closed]

This is at sea level. Slightly curved air foil, you can come up with your own CL as long as its pointing near 45 degrees. If initial friction before lift is needed choose any number you think is ...
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2answers
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What does the work to induce an electric field?

When a conducting rod moves orthogonally through a magnetic field, charges in that conductor experience a force that is perpendicular to both the magnetic field and their velocity. Let's say that ...
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Why are small planar current loops of sufficiently small size interesting? (magnetic moments)

In the book Basic Laws of Electromagnetism by Igor Irodov, He introduces force on a closed current-carrying loop as; $$ F = \oint I \vec{dl} \times \vec{B}$$ And then he states the most interesting ...

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