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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Can we say $i=i_{0}\lvert\sin(\omega t+ \phi) \rvert$ is alternating current?

$$i=i_{0}\lvert\sin(\omega t+ \phi) \rvert$$ $i_{0}>0$. Can we say that the current described by the above equation is an alternating current? $$i=i_{0}\sin(\omega t+ \phi)$$ We are mostly ...
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58 views

Which Maxwell equation explains how electric engines work

While I understand that electric generators (magnetic field $\to$ electric current) work according to Maxwell-Faraday law, some people say that electric engines (electric current $\to$ magnetic field) ...
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36 views

Are the boundary conditions purely a consequence of Maxwell's equations?

The boundary conditions, namely were all these, realized only by looking at Maxwell's equations? Or is there a physical reasoning behind them? For example, Why does the component of the electric ...
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1answer
20 views

moire of lines radiating from a point an a mobile display: electric charge to magnetic fields

This image has 300 lines radiating from a point, like an electric field. if you view this page on a mobile, and move it, in many cases you'll see a moire pattern associated with magnetic fields: to ...
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1answer
27 views

Do magnetic field slows in a medium?

As far as I know either you get a magnetic field to go through the material or you don't, suppose instead of say molten iron I am using drinking water maybe contains some traces of mineral such as ...
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Different wavelengths effect on matter

The figure covers xrays and up, UV, Visible, Infrared and Microwave. But what about lower wavelengths like ELF and radiowaves in the 20-900 MHz range? There are conspiracies regarding ELF and HAARP ...
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43 views

Lorentz Invariance of Scalar QED

We know from Weinberg Ch. 5.9 that a massless vector field transforms as a vector up to a gauge transformation, \begin{equation} U^{-1}(\Lambda)A_\mu(x)U(\Lambda)=\Lambda_{\;\;\mu}^{\nu}(A_\nu(\...
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28 views

Elasticity derivations from electromagnetism

I know that all elastic forces are indeed eletrcomagnetic. Is there a theory (or a study) which shows in detail the connection between these two branches of physics? For example: a good starting ...
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Is “gauge” another way of saying “choosing a coordinate system”?

So far, when I find the term "gauge" it means to choose a convenient coordinate system so a certain condition is satysfied. Is this the general meaning of "gauge"? Or is there something else to it.
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Reducing the switching force in a swichable magnet

I have been playing around with switchable magnets. Such as the Magswitch MS-400. I am faced with a burning question. Is it possible to reduce the force required to turn the magnet from off to on ...
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Charge Distribution on a Strip

let's consider a microstrip line connected to a voltage source V0 (applied between the strip and GND plane). According to foundation for microwave engineering (collin) book, the surface charge density ...
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1answer
49 views

Permittivity. What does it permit?

From the name of the concept, you would expect the permittivity of a medium to be the ability of the medium to permit something. One may expect that, permittivity would be the ability of the medium ...
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Can we change the trajectory of a body in space using external means?

Suppose a block of metal is floating around in space....can we change its trajectory by external fields?Is it possible if yes then by what type of waves?
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Checking Gauge invariance of Scalar Electrodynamic Lagrangian

I wish to show that the following Lagrangian is gauge invariant. $$L=-\frac{1}{4}F_{\mu \nu }F^{\mu\nu}+(D_\mu\phi)^*(D^\mu\phi)-m^2\phi^*\phi-V(\phi^*\phi)$$ with $V(x)$ a polynomial of second ...
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1answer
36 views

Why is the electric field induced due to a Time varying magnetic field non-conservative?

As the title states, Why is the Electric Field which is induced due to a time varying Magnetic Field Non Conservative in nature ? Everywhere I read the answer that the line integral over a closed ...
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3answers
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What determines whether we use a vector or scalar potential?

I understand that electrostatic potential is scalar because the curl of the field is zero, and this implies the electrostatic field is the gradient of the scalar potential to satisfy this. Similarly ...
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1answer
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If the $E$ component of an EM wave interacts with another $B$ field (from other source), can they generate Poyting vector (or photon flux)?

As shown in the figure, a planar EM wave propagates in the $z$-direction, and its electric component is in the $x$-direction. Meanwhile, in the whole region, there is a static magnetic field B ...
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Interface Condition between Two Coaxial Cables with Different Dielectrics

let's consider this situation: In practice, there are two pieces of coaxial cables (region 1 from z = -l to z = 0, and region 2 from z = 0 to z = inf) with different dielectrics. This exercise asks ...
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Mathematica script to calculate the Stark effect [on hold]

I am trying to start a script for Mathematica to solve and plot splitting for the Wannier-Mott exciton. I have the wavefunctions for both the electron and hole, but I am new to Mathematica. Does ...
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4 point charges q,2q-2q,-qhow would you place 2q that flux would 0 [on hold]

Four point charges 2q, q, -2q and –q. how wpould you place the surface which enclose the charge 2q and through which the net electric flux is (a) zero (b) 3q/Є̥ (c) -2q/ Є̥
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Time it takes a coin to fall in a magnetic field

This was a homework problem of mine was: "A coin is placed on its edge in a vertical magnetic field of $B=20$ T. The initial angle between the coin and vertical is $\theta_0=0.1$ rad. Estimate how ...
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Rotating two plate capacitor with radially separated areas [on hold]

If I assemble a two circular plates capacitor where one plate has multiple areas separated radially (split lines going from axis to edge but with common connected axis being the source path/wire for ...
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2answers
75 views

Vector Laplacian in electromagnetic wave theory

I am a bit confused as to how we obtain regular wave equations from Maxwell's equations when the vector Laplacian is defined the way it is. We have the differential equation for waves in the form: $$ \...
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1answer
28 views

Physical significance of Lenz law [on hold]

When flux associated with a current carrying coil changes an emf is induced in the coil. The resulting feild from induced current tries to prevent magnetic flux within the coil from changing. If flux ...
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Properties of the electromagnetic field tensor $F^{\mu\nu}$

From the euler lagrange equations of electromagnetism in vacuum we derive the equation of motion that reads $$ \nabla_\mu F^{\mu\nu} = 0 $$ Now if we multiply both sides with a double metric we have $...
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A question on Ampere Circuital Law

If we consider an infinitely long wire carrying current and apply the law by considering a circular loop at a distance $r$ we get, $$B=\frac{\mu i}{2r\pi}$$ All good. But If I apply the same law for ...
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1answer
52 views

Resistor and capacitor in series

If you put a resistor and a capacitor in series with a 9V battery so that the resistor is in the wire going out from the positive terminal of the battery to a plate of the capacitor. In my opinion ...
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1answer
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Confirming $\nabla_\mu T^{\mu\nu} = 0$ for the electromagnetic field tensor in curved spacetime

I'm trying to show that $\nabla_\mu T^{\mu\nu} = 0$ for the electromagnetic stress energy tensor which I derived to be, $$ T^{\mu\nu} = F^{\mu\beta}F^{\nu}_{\beta}-\frac{1}{4}g^{\mu\nu}F_{\alpha\beta}...
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How exchanging virtual particles works as a fundamental force (other than gravity)?

Force is rate of change of momentum. But is my following description of how exchanging virtual particles can mediate a fundamental force (other than gravity) correct? If there are $2$ subatomic ...
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1answer
25 views

Magnetic pole in a wire

When you pass a current through a wire you create magnetic fields around it. In this case where is the the North Pole and the South Pole?
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Modified Wireless Charging Tech for Cellphones: Enable EMF Induction - Household Appliances and Wiring [closed]

1) How to measure Household EMF: The magnetic field you are trying to detect is varying at 50Hz50Hz. -The variation of magnetic field may not show @50Hz50Hz because the sample rate is not great ...
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A vector potential of a magnetic field and the gauge freedom of the Lagrangian

I'm currently studying a course of mechanical analytics. we used some terms that I just can't wrap my head around. I was wandering if anyone could explain\refer me to a good place to read about it: ...
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2answers
51 views

Can a magnet deflect a static charge? If not, how do photons work?

A moving magnet is supposed to generate an electric field. That implies that a moving magnet should be able to deflect a static charge. Will the charge be deflected? If it does not deflect a static ...
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Magnetic field due to moving point charges with biot-savart law [closed]

I used the biot-savart law to solve but it's still wrong?? B=(μ0/4π)(q2vsin(θ)/r^2)) I used tan-1(.16/.25) for the angle using trig
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1answer
65 views

Euler-Lagrange equation in curved spacetime: how to calculate $\partial \mathcal{L}/\partial(\nabla_\nu A_\mu)$?

My problem is mostly with notation. When we apply the Euler-Lagrange equation to the Lagrangian scalar $$\mathcal{L}= -\frac{1}{4}F^{\mu\nu}F_{\mu\nu}$$ Where $$F_{\mu\nu} = \partial_\mu A_\nu -\...
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Radiation pressure OAM

I have read this question: Where does the photon orbital angular momentum go in light-matter interactions? But the answers were not really explaining the photon level and the classical light level. ...
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2answers
21 views

How does adding a second magnet on the other side of a coil affect induced voltage?

From what I understand, if you have a magnet moving with a relative velocity towards a coil, Lenz's law states that the current flow induced in the coil will create a magnetic field that opposes the ...
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52 views

How does the concept of plane wave make sense?

The speed of light can be used by applying Maxwell's equation to plane waves and EM wave equation is found using the plane wave model and the wave equation. But if we think of what is actually ...
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1answer
32 views

Why is the wave equation applicable to the EM wave?

I was first exposed to the wave equation in which it was derived from a model of 1-D string. But afterwards the derived wave equation somehow was universal and thus applicable to EM waves. But ...
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2answers
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Electromagnetic vs magnetic resistance in exercise bike?

I'm trying to understand how an exercise bike advertised as using magnetic resistance, but not electromagnetic resistance (it doesn't require connection to a power outlet), would execute different ...
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0answers
79 views

Making sense of the Pauli equation's notation

The Pauli equation is the generalization of the Schrodinger equation to charged particles in an electromagnetic field. It is written in a very compact way with heavy use of operators, so it can be ...
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1answer
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Materials Electrical resistance and conductance

These images are right if the material is good conductor. But if this object is made by resistive material, two plates are made by resistive material, Can it possible ?
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1answer
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Faraday Law: integral and differential forms

Let's consider both the integral and differential equations which express the Faraday Law (3rd Maxwell Equation): $$\oint_{\partial \Sigma} \mathbf E \cdot d\mathbf l = -\frac{d}{dt}\iint_{\Sigma} \...
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1answer
41 views

Action of a relativistic charged particle in an electromagnetic field

In my Classical field theory lectures we have derived the lorentz invariant action for the motion of particles in EM fields given by: $S_{particle}=-\int mc^2 d \tau -\int eA_\mu dx^\mu $ Where we ...
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1answer
53 views

Does vacuum absorb electromagnetic waves?

Since the refractive index of vacuum is real, this is highly unlikely. However, it seems that it does following the absorption per unit volume defined in terms of the Poynting vector: \begin{equation}...
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Lorentz transformations of spatial functions of coordinate time

Explicitly, my question is: Suppose we have $f(x-x_p(t))$ in our unprimed frame. We now consider a primed frame moving along the x-axis, where $x' = \gamma(x-\beta ct)$ and $ct' =\gamma(ct-\beta x).$ ...
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2answers
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In what sense is $\sum_{r =1}^3 \epsilon^r_\mu \epsilon^r_\nu $ a projection operator?

It is mentioned in this answer that the completeness relation for the polarization vectors of a (massive) electromagnetic field $$ \sum_\lambda \varepsilon^\mu(\lambda,k) \varepsilon^{\nu*}(\lambda,k) ...
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Two different forms of the electromagnetic wave equation

Wikipedia says that the electromagnetic wave equation written in terms of the electric field $\mathbf{E}$ is $${\displaystyle {\begin{aligned}\left(v_{ph}^{2}\nabla ^{2}-{\frac {\partial ^{2}}{\...
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What does negative terms in parallel plate's maxwell stress tensor represent?

For a parallel plate without magnetic field, $T= \begin{pmatrix} -\frac{\sigma ^2}{2 \epsilon _0} & 0 & 0 \\ 0 & -\frac{\sigma ^2}{2 \epsilon _0} & 0 \\ 0 & 0 & \frac{\...
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Vector potential, magnetic field and dipole moment due to a rotating cylinder

I've been struggling with the following problem: Consider a cylinder with height h and radius a with a homogeneous surface charge density $\sigma$ rotating about its symmetry axis with constant ...