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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1answer
151 views

polarization of a transverse wave travelling in ionosphere with polarization direction perpendicular to earths magnetic field

Assume a transverse electromagnetic wave entering ionosphere such that its Electric field of wave is perpendicular to earths magnetic field. Now, i read that as it will enter plasma, the wave will ...
0
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0answers
132 views

Why is the direction of $I$ or $F$ different in case of Motors and Generators?

Consider the following diagram: In the first diagram, the wire is carrying current into the screen. Motion of the wire is downwards (Left Hand Rule) In the second diagram, the wire is moved ...
0
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1answer
107 views

Proof of equivalence of two different covariant notations of Maxwell's equations [closed]

My homework assignment is Prove that $$\partial_\beta F_{\gamma \delta} + \partial_\gamma F_{\delta \beta} + \partial_\delta F_{\beta \gamma} = 0$$ with the electromagnetic tensor $$F_{\...
1
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1answer
168 views

General solution to the wave equation of electromagnetic field

Suppose that we have the four-vector potential of the electromagnetic field, $$A^i$$ The wave equation is given by $$\Box A^i=\frac {\partial^2}{\partial x^k \partial x_k } A^i= \left(\frac {1}{c^2} \...
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0answers
22 views

If there is a violation in magnetic interaction b/w two non parellel current carrying wires

when two current carrying wires are put near, there is a magnetic interaction b/w the wires. if they are kept parellel, they'll follow newton's 3rd law. i want to ask that if they are not kept ...
0
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1answer
91 views

Calculation mistake some place in finding stress-energy tensor

If the Lagrangian in Maxwell's theory is $$L= R- \frac{1}{4}F_{\mu\nu}F^{\mu\nu}$$ I want to find $T_{\mu\nu} $ The procedure is that I vary the action: $$\delta S = -1/2 \int{d^4x \sqrt{g}(g_{\mu\...
2
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3answers
1k views

How does a single charge produce magnetic field?

I have studied in Introduction to electrodynamics (Griffiths) that magnetic field is actually due to effects of relativity unequal Lorentz contraction of the positive charge and negative lines, a ...
2
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1answer
226 views

How is the electostatic field propagated by the vacuum? How does a charge feel the presence of another charge when there is NOTHING between them?

Is the word "vacuum" equal to "nothing"? Imagine two charges of opposite sign, far from one another in vacuum, and moving with constant velocity - s.t. none radiates energy, none emits photons. At ...
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3answers
1k views

Polarization vector of dielectric electrostatics

I know two laws for Polarization vector of a dielectric material $$\vec P = N q \vec{\Delta L}$$ Where $N$ is the number of dipoles per meter cube, $\vec{\Delta L}$ is the vector that represents the ...
0
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1answer
239 views

Gravitational force and Electromagnetic force?

I found this interesting note in one of my textbooks, The enormous strength of the electromagnetic force compared to gravity is evident in our daily life. When we hold a book in our hand, we are ...
3
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2answers
1k views

Landau level degeneracy in symmetry gauge, finite system

As we know, Landau level degeneracy in a finite rectangular system is $\Phi/\Phi_0$, where $\Phi=BS$ is the total magnetic flux and $\Phi_0=h/q$ is the flux quanta. This can be easily derived using ...
4
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2answers
88 views

Charge orbiting gravitational body [duplicate]

I am currently rather uneducated on the subject, but I was thinking of a general relativity thought experiment. Say I take a charge from infinity and give it velocity to orbit a planet in a circle. ...
2
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2answers
166 views

What is the link between observation wavelength and spatial resolution of an instrument? [closed]

It seems that when capturing and emitting EM waves matter proceeds differently depending on the wavelenght. According to an answer from another question, EM energy is captured following three modes: ...
4
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4answers
768 views

What is the experimental evidence that light is an electromagnetic wave?

Do we have any experimental evidence to confirm that light is an electromagnetic wave? Or is it confirmed simply by Maxwell's equations showing a similarity in speed?
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5answers
97 views

Is anything coloured “infra-red”?

All our day-to-day objects are things we can see, and thus we can conclude that their colour lies in the visual spectrum. Is there matter that does not have a colour within the visual spectrum? What ...
0
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1answer
136 views

Can Gauss' and Ampere's Laws be written in terms of the divergence of an energy four-vector?

In the first 20 minutes of this video, Susskind derives the continuity equation for charge conservation: $$\dot{\rho}+\nabla\cdot\vec{J}=0$$ (Where $\vec{J}=\frac{\partial\dot{q}^m}{\partial A^m} \:;...
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0answers
74 views

Electric Field Inside Dielectric Filled Cavity

I want to determine the electric and magnetic fields inside the following cavity: perfect conducting walls at $z = 0, z=d, x=0, x=b, y=t, -t^\prime$, with a dielectric $\epsilon$ filling $-t^\prime<...
3
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2answers
230 views

Electromagnetic reaction force?

The classical (retarded) Lienard-Wiechert scalar and vector potentials describe the electromagnetic field due to an arbitrarily moving electric point charge. Thus given the motion of electron $A$ one ...
2
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4answers
610 views

How any material can provide electrostatic and magnetic shielding

Several metallic objects like Iron, Copper etc can provide electrostatic shielding (one should remain inside the car during thunderstorm) and several superconductors like HTS (High Temperature ...
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0answers
26 views

Assumption of Positive Dielectric Value in Derivation of Dipole Field Around a Spherical Dielectric? Implications for Plasmonics

I am going through the solutions to Laplace's equation in spherical coordinates. In particular the boundary value problem of a dielectric sphere of radius $a$ and permittivity $\epsilon$ which has a ...
0
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1answer
57 views

Tranverse electromagnetic waves

TEM waves do not exist in waveguide. Is this the correct explanation- Both curl and divergence of TEM are zero inside the waveguide and because of the boundary conditions (electric field zero at every ...
2
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1answer
53 views

momentum and energy of the rods

I have two electromagnets and I attach them to two rods(same mass) such that, first one will be attached to centre of first rod and the second one will be to the end of second rod. Now I bring them ...
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1answer
199 views

Physical meaning of magnetic vector potential [duplicate]

Can anyone give me an intuition of why the magnetic vector potential $A$ is sometimes interpreted as the electromagnetic momentum ? I don't know analytical mechanics, just classical electromagnetism
2
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0answers
112 views

A-level electromagnetism experiments? [closed]

Does anybody know any A-level experiments investigating electromagnetism that can produce good data relatively simply?
2
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2answers
159 views

Divergence of Magnetic Field in an Electromagnetic Wave

In many text books of physics (especially those coping with Electromagnetic Theory), we see that an electromagnetic wave (EMW) is drawn as an electric field varying with a sinusoidal form and exactly ...
0
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1answer
270 views

Is the physics of $j = \rho v$ questionable? What are the consequences? [closed]

In electrodynamics you have the quantity $\mathbf{j}$, which represents density of current per surface area. It is often said that $\mathbf{j} = \rho\mathbf{v}$, for geometric reasons. When I make ...
1
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1answer
181 views

Euler-Lagrange equations of a current-loop pendulum in a magnetic field

I am reading "Nonlinear Electromechanics", by Dmitry Skubov and Kamil S. Khodzhaev, Springer 2008. Here is the relevant and freely available chapter. Essentially, a loop of area $S$, mass $m$, moment ...
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0answers
63 views

Magnetic field derived from a scalar function

Question: Show that $\mathbf B = \hat r \times \nabla g(\mathbf r)$, where $g$ is an arbitrary scalar function, is a plausible magnetic field. What current density $\mathbf J $ can produce ...
2
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2answers
550 views

Use of Amperian loop to calculate the $B$-field inside a co-axial wire

The problem asks to calculate the magnetic field at point $a$ of a co-axial cable with a wire at the center surrounded by a rubber layer, and then another hollowed cylindrical wire followed by a ...
2
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2answers
400 views

Why does the existence of an aether go against Maxwell's equations

I am trying to determine why there is a conflict between Maxwell's equations and Galilean relativity. The standard way I have seen it explained is: The Galilean world model says velocities transform ...
2
votes
1answer
108 views

Introduction of the vector potential $A_{\mu}$ for the local gauge invariance of the complex scalar field lagrangian [duplicate]

In Ryder, when trying to restore the local $U(1)$ gauge symmetry of the complex scalar field $\phi=\phi_1+i\phi_2$, the final Lagrangian consists of the following four parts: $$L_0=(\partial_{\mu}\phi)...
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3answers
2k views

Gravity and Magnetism… One and the same? [duplicate]

I asked this in another question as if it was an answer so as to use a bit of that questions popularity and possibly get some clarifications on this but I also prefer to make a question of my own, so ...
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2answers
69 views

What should be the direction of motion of a conductor in a magnetic field to produce motional emf?

Here's the textbook question: Twelve wires of equal length 'l' are connected to form a skeleton cube which moves with constant velocity v perpendicular to the magnetic field B. What will be the ...
2
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0answers
776 views

Field at the tip of a rotating cone. Is (THIS STEP) in my solution right? [closed]

** Edited: I'm going to CLARIFY where and what my question is. Problem: A hollow cone(like a party hat) has vertex angle $2\theta$, slant height $L$, and surface charge density $\sigma$. It spins ...
1
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1answer
206 views

How does the electric field pulse propagate during a change in the magnetic vector potential?

The electric field in terms of the electric scalar potential, and the magnetic vector potential is: $E = -\nabla\phi - \frac{\partial A}{\partial t}$, where $A$ is such that $B = \nabla \times A$. ...
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0answers
69 views

faradays law in differential form [closed]

A conducting spherical balloon is charged with a charge $Q$ constant in time and uniformly distributed on its surface. If the radius of the balloon is made to vary sinusoidally in time between a ...
5
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1answer
232 views

Faraday's law: sin and cos?

I am looking at this paper (Hanna S. A., Varhue W. J. and Titcomb S., IEEE Trans. on Instrumentaion and Measurement, Vol. 58, No. 1, 2009). They claim that the voltage generated in a loop of $N$ turns ...
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0answers
77 views

Analytic (or just qualitative) solution to *any* nontrivial electrodynamic dynamic problem?

I just realized that, after a basic course in electrodynamics, I have no idea whatsoever how to solve the most trivial non-static example in an "exact" way. By this, I mean the following: I know (at ...
9
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3answers
399 views

Why are permanent magnets permanent?

Let me see if I get it right. When an iron bar is attracted by a permanent magnet it becomes a magnet itself because all of its magnetic domains start to point in the same direction. When the iron bar ...
3
votes
1answer
166 views

Is it possible to eliminate Van der Waals interactions?

I came to know that the friction force actually depends on the surface contact area due to weak interactions (adhesion due to Van der Waals forces) between the atoms of both materials increasing in ...
2
votes
1answer
228 views

Lagrangian of Non-Relativistic Charged Particle in a Magnetic Field

I'm trying to derive the Lagrangian for a non-relativistic charged particle under the influence of a magnetic potential. I'm assuming that $F=-\textrm{grad}(V)$ and so by the Lorentz force we have $-\...
0
votes
1answer
100 views

Does the medium matter when calculating Optical Period?

If I consider the optical period $T$ for a given wavelength $\lambda$ in vacuum, I can calculate it as: $$ T = \frac{1}{f} = \frac{\lambda}{c} $$ However, in a particular material/medium, I am not ...
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2answers
66 views

Using the force law to obtain total energy of an electron as a function of its radius

I am working on a problem which starts saying determine the total energy of a hydrogen atom with an electron moving with momentum $p$ at a radius $r$. For that part I got: $E = \frac{p^2}{2m_e} - \...
1
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1answer
67 views

How to model the impedance of a coil with a dielectric slab on the top

I have sketched what I meant in the image below. Were there no dielectric slab, the coil could have been simply modeled as an inductor. But I am not sure what the impedance can be if there is a ...
1
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1answer
181 views

Influence of applied voltage to an electron of a metal

I would like to ask what would happen to the potential well of an electron being trapped in a metal? If I apply a voltage trying to accelerate the electron out of the potential well. Would It make the ...
0
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1answer
82 views

Mimic Magnetic Repulsive Resistance through a spring

I have two identical magnets and I push them towards each other so that they are repelling. Is it possible to accurately mimic this repulsion effect (magnetic energy) just using a metal spring (...
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1answer
240 views

Magnetic force as a relativistic effect?

There is something I am confused about when it comes to the force between two parallel wires carrying current, specifically why when they carry current in the same direction the wires are always ...
0
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2answers
546 views

Why aren't Faraday's law of induction and Maxwell-Ampere's law (without sources) symmetric?

I was wondering why Faraday's law of induction and Maxwell-Ampere's law (without sources) are not totally symmetric in the sense that Maxwell-Ampere's law has a $\epsilon_0 \mu_0$ term on the right (...
1
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1answer
415 views

What causes the light spectrum to appear on oil surface? [duplicate]

I have observed that when petrol or oil leaks, the light spectrum appear on it. What causes light to split into its constituent colours?
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0answers
22 views

What is the rate at which the field of an electron spreads? [duplicate]

There are 2 steps in the creation of a force field: (1) first the object comes into existence (2) then the field appears. In the case when a photon splits into an electron and an antielectron, these 2 ...