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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121 views

Derivation of the speed of light using the integral forms of Maxwell's Equations

Having just finished physics 2, I've been (slightly) exposed to showing that light is a wave with speed $1/\sqrt{\mu _0 \epsilon _0 }$ using the differential forms of Maxwell's equations, though this ...
1
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0answers
53 views

Getting the electric field using Green's function [closed]

Let the Green's function for the gauge field be given (after gauge fixing) as $$G_{\mu \nu}(x,y) = \delta_{\mu \nu}G(x-y) \tag{1}$$ where $$G(x-y)= \int \frac{d^dk}{(2\pi)^d} \frac{e^{ik \cdot ...
1
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3answers
93 views

Plane waves - EM wave

An accelerating electric charge will emit transverse electromagnetic waves. These waves are propagating away in wave fronts that become flatter and flatter as getting further from the source. So they ...
0
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0answers
22 views

Free vs Bound Charges, especially in metals

Are conduction band electrons in metals generally considered free charges or bound charges for E&M purposes? I'm not asking about freedom in the sense of being able to escape the potential well ...
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0answers
17 views

How the magnet moves without pushing? [duplicate]

I video on Youtube represents that it can move through a coil without providing any external force and it seems that the force arises from the lines of forces generated in the coil. How can we ...
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0answers
48 views

Could the explanation for this simple electric train be attributed to Lenz's Law?

There is a discussion of a Youtube video where a battery with magnets attached at either end moves through a copper coil. It has been commented on in the Phys.SE post How does this "simple" ...
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27 views

Why spin-$\frac{1}{2}$ nuclei have zero electric quadrupole moment?

Why spin-$\frac{1}{2}$ nuclei have zero electric quadrupole moment? How to calculate in general?
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0answers
32 views

Parity of magnetic susceptibility $\chi(\omega)$

It is well known that real and imaginary parts of magnetic susceptbility, defined as $\chi=\chi'(\omega)-\mathrm{i}\chi''(\omega)$, ought to be even and odd to frequency $\omega$ respectively, ...
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1answer
55 views

Does reflected light change LED power consumption?

If I enclose an LED in "tin foil" so that the photons it gives off are reflected back to it, would the power consumed by the LED go down?
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3answers
38 views

Currents induced by rotation of a coil about magnetic field lines

If the coil on the left is rotated about the magnetic field lines, will there be an induced current? I know that the flux concerns only the component of the magnetic field perpendicular to the ...
2
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4answers
106 views

How to find the magnetic field around a wire with the full time dependent Maxwell Equations? [closed]

In all textbooks the magnetic field around a wire carrying a current is found from Ampere's Law. However, I would like to know what the expression for the magnetic field is, using the full ...
1
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1answer
46 views

How to get second order equation for spinor (derivation from Dirac equation)?

Dirac equation with an Abelian symmetry can be written as $$(\gamma^{\mu}D_{\mu} - m)\psi = 0$$ where $$D_{\mu}\psi = (\partial_{\mu} - iqA_{\mu})\psi$$ Then how do we get this second order equation ...
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0answers
30 views

Insignificant Potential difference?

Is it possible, that a conductor with an extremely short length, say $1\text{mm}$ or less passing a magnetic field of $1T$ with a reasonable velocity of $20\text{m}/\text{s}$ would induce little to no ...
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0answers
50 views

Why is electric charge the conserved quantity corresponding to global $U(1)$ symmetry? [duplicate]

An example of a symmetry transformation for certain Lagrangians (notably the canonical complex scalar field Lagrangian) is multiplication of the fields by a complex phase. When we multiply the fields ...
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0answers
50 views

Electromagnetic mass of a continuous charge distribution [closed]

I am trying to understand if it is possible to have a consistent solution to Maxwell's equations and Lorentz force equation simultaneously with no other forces present. The Lorentz force density is ...
2
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0answers
46 views

What is “above” and what is “below” the surface of a sphere?

When studying Electromagnetism using D.J. Griffith's Introduction to Electrodynamics, the boundary conditions for the electric potential across a surface charge density are expressed using the normal ...
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3answers
42 views

Does there exist a hyperbolic relationship between frequency $\omega$ and wavenumber $k$?

As the title states, is it possible to derive a hyperbolic relationship in the form of $\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$ between frequency $\omega$ and wavenumber $k$ I have tried to start this ...
3
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2answers
107 views

Do the fields exist without electric charges? [closed]

I read in an old book on electrodynamics by Pauli that theoretically there does not exist any need of charges to be there. Fields can even exist without the charges but still independent fields ...
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0answers
16 views

Fresnel equations of reflection applied to microparticles

I wanted to know if the Fresnel equations of reflection, as described here, can be used to calculate the reflectance on the interface of particles which have a diameter near the size of the wavelength ...
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0answers
51 views

Basic question in electromagnetic duality

In electromagnetic duality, where does this relation come from? $$\overrightarrow{E} \rightarrow \cos \alpha\overrightarrow{E} - \sin \alpha\overrightarrow{ B}$$ $$\overrightarrow{B} \rightarrow \cos ...
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0answers
33 views

Electromagnetic duality and (electric induction D and magnetic intensity H)

We originally have $$\overrightarrow{\nabla}\cdot\overrightarrow{B} = 0$$ $$\overrightarrow{\nabla}\times\overrightarrow{E} = -\frac{\partial \overrightarrow B}{\partial t}$$ When electromagnetic ...
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2answers
65 views

Why is electricity 'the screwing force'?

In one of his lectures Richard Feynman, a great genius and a 'naughty Bronx kid', refers to to the Coulomb force, electricity as "the screwing force". Apart from the obvious joke and double entendre, ...
1
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1answer
53 views

What are the physical interpretations of $\nabla \times \textbf{P}$ and $\nabla \cdot \textbf{M}$?

In the text I am reading, it is made clear that it is incorrect to assume that the electric displacement $\textbf{D}$ is identical to the electric field with the exception that it is raised from the ...
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0answers
34 views

Vector potential in presence of monopole [duplicate]

In this paper http://www.hcs.harvard.edu/~jus/0302/song.pdf when Song was explaining dirac string. He said "In the presence of a magnetic monopole, the vector potential cannot be defined everywhere. ...
0
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1answer
46 views

I wnt to know HOW to create TM, TE, and TEM waves; and is it being used in industry? [closed]

Transverse Magnetic (TM) and Transverse Electric (TE) modes So I'm wholly ignorant of any knowledge surrounding these modes, so bear with me here.. One component is infinitesimally small in the ...
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1answer
36 views

What's the relationship between velocity factor and refractive index?

Wikipedia on velocity factor explains: For optical signals, the velocity factor is the reciprocal of the refractive index. On refractive index, Wikipedia also has an explanation of refractive ...
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2answers
52 views

Attributes of a polarized light

Which of the following is true for a polarized light? The electric vector is constant in its module and direction The electric vector oscillates in every plane that passes through the ...
3
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3answers
535 views

Why do we use the Coulomb potential for the hydrogen atom?

When solving the Schrodinger equation for the hydrogen atom, the Coulomb potential $V = \frac{e^2}{4 \pi \epsilon_0 r}$ is used. The Coulomb potential comes from classical electrodynamics, so why ...
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4answers
65 views

Does a magnetic field do work on a moving rod?

In a case such as this one, my textbook says that I might be tempted to think the magnetic force is doing work (against the moving rod). It then says this isnt the case because "charges move ...
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0answers
58 views

Spontaneous de-excitation of an atom

Consider the Hamiltonian for the process of spontaneous de-excitation on an excited atom, (an electron jumped to a higher energy level while a lower energy level is not complete). This Hamiltonian ...
0
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1answer
34 views

Speed of an electromagnetic soliton in free space

What is the speed of an electromagnetic soliton in free space? Is it equal to 'c' ? P.S. My understanding of the Fourier transform says it's not.
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0answers
24 views

Charge on a fork in a microwave

I left a fork in the microwave and when the turntable pushed it against the interior wall, there were sparks and black arc welder-like burns on the wall. I think this means that the microwaves induce ...
3
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2answers
80 views

Is Magnetism and Electromagnetism the Same Thing?

I keep hearing everywhere that magnetism and electromagnetism are different but is seems to me that when a current is moving and it creates a "magnetic field", it is just electrons repulsing other ...
0
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1answer
33 views

Too many antennas around a single energy transmitting antenna?

I am not a physicist. So I am not even sure, if I am phrasing this question properly. Lets say there is a wireless transmitter transmitting energy. Let us say I place $N$ number of antennas around it ...
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0answers
38 views

How to design a Bitter electromagnet? [closed]

I am designing electromagnets at my home and want to know how can I design electromagnets at home. Bitter electromagnets are really a challenge.
0
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1answer
34 views

Ampere's loop inside capacitor

The figure is a view of one plate of a parallel-plate capacitor from within the capacitor. In the question, we are required to rank the 4 paths (a, b, c and d) according to the value of $\oint ...
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0answers
15 views

Question about calculating the torque of a magnet in an additive field

I did an experiment in the lab today where I placed a magnet in between two coiled magnets, with one coiled magnet having its south side up and north side down and the other coiled magnet with the ...
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3answers
62 views

Induced EMF of a rectangular loop should be zero?

Considering the shape of a rectangular loop in a changing magnetic field: The induced $\epsilon$ would be zero? Since a rectangular loop is a combination of wires in series to create such a shape. ...
0
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1answer
22 views

Does the Magnetic Flux change (at a point) If the wire which causes it was cutted into a half?

If the electric charges move through a wire Does the generated Magnetic flux at any certain point chanage if this wire was cut into a half
0
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1answer
38 views

What is the connection between linear, circular and elliptical polarization versus perpendicular and parallel polarization?

In the EM course, we suddenly went from linear, circular and elliptical polarization to perpendicular and parallel polarization. What is the connection between these concepts and when it is ...
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0answers
31 views

Electric field due to rotating charged sphere

Consider a spherically symmetric charged object (charge $Q$) rotating about its axis. From Gauss's law we know that all that matters for the electric field $\mathbf{E}$ is the charge $Q$ enclosed ...
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0answers
28 views

Work done by the Lorentz force?

Im trying to calculate the work done on a single wire placed inside a magnetic field. Now for the force I've used the Lorentz force formula to calculate it, for the distance should I used the width of ...
0
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2answers
71 views

Deriving Maxwell's Equations from Electromagnetic Tensor

Given $ F_{\mu\nu} = \partial_{\mu}A_{\nu} - \partial_{\nu}A_{\mu} $ It is obvious that the diagonals are zero, as $ F_{ii} =\partial_{i}A_{i} - \partial_{i}A_{i} = 0 $ And, ...
0
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0answers
38 views

How to calculate the necessary magnetic induction to magnetize a certain ferromagnetic object and hold it

Well pretty much what I want to do is to build an electromagnetic coil with a nail inside of it for it to hold a steel ball. But I've come into some trouble, because I don't quite get how to calculate ...
2
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3answers
62 views

How is energy conserved in magnet?

When an object (i.e.magnetic) is attracted towards the magnet, work is done so that the energy is used but from where this energy comes from?
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0answers
20 views

The inverse radiation problem

I posted this question on Mathoverflow.net, but it seems that no one show interest. This is the radiation problem. Suppose the current $f(x)\in L^2(\mathbb{R})$ has a compact support in $[-a,a]$. And ...
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0answers
24 views

Maxwell Lagrangian at weak fields

In http://arxiv.org/abs/hep-th/9506035 the authors said after writing this equation: $$\frac{1}{4}\eta^{\mu\nu\lambda\rho} F_{\mu\nu}F_{\lambda\rho} = \eta_{\sigma\tau\alpha\beta}\frac{\partial ...
0
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1answer
24 views

Given $\tilde {E}$, which formula should I use to find $\tilde {H}$

Given $\tilde {E}$, there exist two formulas in my book (Cheng) to compute $\tilde {H}$: Maxwell's formula: $\nabla \times \tilde {E} = -j\omega\mu \tilde {H}$ Plane wave formula: $\tilde {H} = ...
0
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1answer
29 views

Force on a loop with induced current

Consider an infinitely long ideal solenoid with current $I$, radius $a$, turns per unit length $n$. Put a closed conducting loop around it (radius $b > a$), on a common axis through their centers. ...
0
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1answer
45 views

Why do Magnetic North and South Poles never exist by themselves? [duplicate]

As the title suggest, whilst I was reading I saw written that a 'Magnetic North or South Pole has never been found by itself'. And I was wondering why this was?