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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79 views

Why do we restrict to electric field when describing light?

Why do we restrict only to electric field when describing light as electromagnetic wave? I mean from Maxwell equations we can derive wave equation for electric field and also for magnetic field but ...
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1answer
93 views

Is the Potential Energy just a bookkeeping device?

It is said that if the space is homogeneous then momentum is conserved. But I've been thinking in the following situation: Consider a parallel plates capacitor. In between the plates there is a ...
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2answers
161 views

Gauge transformation of vector potential multiplies wavefunction by phase

Consider an electron in an electromagnetic field with scalar and vector potentials $\phi, \mathbf{A}$. Suppose for simplicity that $\mathbf{A}$ is time independent. Suppose also that we know the ...
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36 views

definition of left and right circular polarization?

Which is the left and right circular polarization ? page 1 Wiki says this is right circular polarization ...
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4answers
73 views

Does momentum increase with out of phase photons?

This paper speculates that the EM drive produces thrust with out of phase photons: http://scitation.aip.org/content/aip/journal/adva/6/6/10.1063/1.4953807 My question is this, do out of phase ...
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47 views

From gauge invariance to charge conservation in covariant electrodynamics

I tried to solve the equations of motion using the action for the electromagnetic field interacting with a current, like $$ L = F_{\mu\nu}F^{\mu\nu} + A_{\nu}j^{\nu} $$ getting the right Maxwell's ...
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11 views

Reflection coefficient of transition coaxial and wave guide

Suppose you have a 1 GHz generator connected to a wave guide with a coaxial cable with a characteristic impedance of $Z_c =45 \Omega$. The wave guide is connected to the system which has a reflection ...
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2answers
94 views

Why does a electric Potential have to be real, but not a Potential in quantum mechanics?

So I had this Problem when I had to learn about classical electromagnetism: Why is it, that we use complex numbers when calculating stuff, but in the end only the real part is important (for example ...
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1answer
49 views

The form of the metric after a dimension is compactified

Upon the compactifiation of one spatial dimension, it is said (as though an axiom) that the 5 dimensional spacetime metric separates into a 4 dimensional metric, a vector, and a scalar, (4D gravity, ...
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1answer
99 views

Does Maxwells equations imply $R=const*\rho$?

Suppose we have a resistor in a strange shape, filled with a medium of resistivity $\rho$, assuming only maxwells equations apply, is it true that R is proportional to rho, even for very low ...
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2answers
34 views

Transmission Lines and Antennas

In Transmission Lines, does the electric field exist between the two conducting wires or does it exist inside a wire, pushing the flow of electrons? And do all antennas radiate energy by spark?
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2answers
33 views

Why does current follow a conductor above a ground plane

Suppose there is a conductor above a ground plane. Current flows from a source through the conductor to a load on the other side. Depending on the frequency of the current the return path through the ...
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1answer
43 views

Different voltage drops depending on the path

I was working through a problem in electromagnetic induction when I discovered that I have a fundamental ignorance of some sort that is puzzling me. Consider a circular wire, at rest, perpendicular ...
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1answer
93 views

Is there an easy way to see that $\phi =\frac{\mu_0}{4\pi} \frac{\vec m \cdot \vec r}{r^3}$?

The scalar potential of an electric dipole is given by: $$\phi =\frac{1}{4\pi\epsilon_0} \frac{\vec p \cdot \vec r}{r^3}$$ The derivation of this is quick, however, is there an easy way to see that ...
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1answer
32 views

Is it possible to measure the resistivity of something using A.C. Current and a capacitor?

I've read that A.C. current "passes" through a capacitor. Is it possible to use this and measure the resistivity of the medium between the plates? Any sources or links would be appreciated.
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16 views

Comparison of electron deflection tube and Hall effect

Consider an electron deflection tube (something like this: http://www.ld-didactic.de/documents/en-US/GA/GA/5/555/555624e.pdf). Suppose you apply a magnetic field perpendicular to the screen. Then ...
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1answer
42 views

Does magnetic fields exert forces only on moving charges?

I am confused here a bit,i read that magnetic fields exert force only on charges moving but when we take magnet and keep it near a stationary metal (assume light in weight), the magnet attracts it? ...
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0answers
8 views

What kind of flux-pinning effect will occur if a type two superconductor is subjected to an AC electromagnet?

When a supercooled type two superconductor is subjected to a static magnetic field, the superconductor pins to the flux of the field (the mixed-state meissner effect is apparent). What happens if it ...
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1answer
65 views

Specific heat of the classical ferromagnetic Heisenberg model

I have simulated the classical ferromagnetic Heisenberg model on a cubic lattice using Monte Carlo and I get a finite specific heat near zero temperature. My understanding is that from the magnon ...
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1answer
19 views

Armature of electric bell [closed]

I want to know that why is the armature of the electric bell made up of soft iron ? I googled this out and everywhere it was written that it is because of its propert to aquire electromagnetism but I ...
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0answers
43 views

Defining electromagnetic stress tensor for non-linear media

In textbooks, the electromagnetic stress tensor (in vacuum also called Maxwell stress tensor) is usually derived for linear media, implying that $$ \vec D = \epsilon_0 \epsilon_r \vec E$$ My question ...
2
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2answers
62 views

A general complex electric field

When dealing with a plane wave solution to the electric field such as $$\vec{E}(r,t)=E_{0}\cos(kz-\omega t+\phi)$$ we usually introduce a complex electric field $\tilde{E}(r,t)$ such that $\vec{E}(r,t)...
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22 views

What is the physical significance of Curie-Weiss temperature?

From the Curie-Weiss law, the CW temperature is negative for antiferromagnet (afm). what is the significance of the negative value of CW temperature of a afm?
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1answer
64 views

Why does the divergence of the Ponyting vector have energy flux density?

The poynting vector is defined as $\vec{S}=\mu_{0}^{-1}\vec{E}\times \vec{B}$ Taking the divergence of the poynting vector, one arrives at $\vec{\nabla} \cdot \vec{S}=-\frac{\partial u}{\...
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1answer
40 views

Why metal sphere (conductor) is used in Van de graff as the output terminal, not any other material , specifically any insulator?

Why metal sphere (conductor) is used in Van De Graff as the output terminal, not any other material, specifically any insulator?
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35 views

optical properties of metal-dielectric-metal microcavities

I would like to figure out the optical properties of metal-dielectric-metal microcavities, used in the fabrication of micro patch antenna. The paper that I'm reading is this one. Now the specific ...
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23 views

Intrinsic and normal B-H curves in magnets

In a magnet, what is the difference between the intrinsic $B-H$ demagnetization curve going from $B_r$ to $H_C$ (which has a knee) and the normal $B-H$ curve going from $B_r$ to $H_{ci}$ (which is ...
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1answer
40 views

what is the physical significance of dielectric constant and loss tangent?

I want to know that the significance of dielectric constant and loss tangent behaviour. How it characterises the materials. and possible relation between the dielectric constant and polarisation ?
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1answer
47 views

Would moving quarks constitute a current?

I know that it would take an almost infinite amount of energy to isolate quarks, but let's say we've gotten past that and we have isolated quarks and put them in motion. Would that be a current since ...
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0answers
24 views

Double Fraunhofer , Fourier transform to the image plane?

Consider the diagram below: I know that from Franhofer diffraction we have: $$u_f(x_f)\propto \int u_o(x_o) e^{-k\frac{x_f}{f}x_o} dx_o$$ Assuming small angles. I also know that $u_i(x_i)$ is ...
5
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2answers
169 views

According to relativity theory, what is the force that two electrons moving radially apart exert on each other?

According to relativity theory, what is the most general expression for the force that two electrons moving radially apart exert on each other? I am looking for a function $$F(r(t)),$$ where $F$ is ...
2
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1answer
42 views

Problem with magnetic field due to relative motion

We know that, moving charge produces magnetic field in the surrounding space. Consider this scenario : A charge 'q' is moving with a constant speed 'v' in the direction of positive x axis of a ...
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26 views

Scanning Electron Microscopy on a smaller scale

Is it possible to sufficiently focus a ~1nm beam of electrons into a sample chamber in less than 10cm of vertical space, therefore reducing the volume of the entire vacuum chamber, and subsequently ...
2
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2answers
100 views

Why doesn't sunlight ALWAYS get split (into monochromatic) when going through the clouds?

It is my understanding that clouds are largely made up of water, which is known to split white light into its frequency components, and that's why we see rainbows sometimes. My question is, with ...
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0answers
41 views

Mass term in Maxwell's Lagragian for Electromagnetism

In the scalar field Lagrangian the mass term is given by $$m^2 \phi^2.$$ But the equivalent term in Maxwell's Lagrangian for electromagnetism is $$m^2A_{\mu}A^{\mu}.$$ But I don't know why the ...
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1answer
46 views

Electromagnetic waves of accelerating charges

Is it true that more accelerating a charge will cause electromagnetic waves of more energy than the energy of the electromagnetic waves cause by lesser accelerated charge ?
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34 views

northern hemisphere compass points where in southern hemisphere?

If Northern Hemisphere compass points to magnetic north and this same compass is brought to the Southern Hemisphere, will the 'painted red north' compass needle be drawn to the magnetic south pole?
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1answer
39 views

Amplitude of electromagnetic wave

For an EM wave propagating in +z direction, it is described by $ \underline{E} = \underline{E_0}(x, y)e^{i(kz - wt)}$ I know that the imaginary term in the expression describes phase and $|\...
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0answers
19 views

Constant scalar magnetic potential in ferrofluid

I have read in an article that a ferrofluid with very large magnetic permeability (I gess it means that the magnetization of the fluid is the saturation magnetization) under a constant external ...
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0answers
30 views

Charge that moves inside a conducting sphere

I have a conducting sphere of charge Q and inside it we set a charge -q from the centre with initial velocity uo and i would like to know with what speed it reaches the surface of the sphere and what ...
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0answers
15 views

Sources for the magnetic exchange integral in group d transition metals

Does anyone know where I can find values for the magnetic exchange energy, J for elements with the atomic numbers 20-30? I've found values of 0.015meV, 0.03meV and 0.02meV for iron, cobalt and Nickel ...
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0answers
17 views

Boundary conditions while finding electric potential inside some specific volume

Please see the figure. Continuous charge distributions are in blue color. Inside cavity there is some charge distribution. Also On surfaces S1 and S2 there are some charge distributions (not shown)....
3
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2answers
123 views

Proton and electron joining

If I have a proton and an electron at rest at some distance apart. Will they form an hydrogen atom when released or they will join together? My intuition says it will form H atom. But I cannot ...
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0answers
32 views

Maxwells equations at a point

Why is it that whenever a problem is posed for maxwells equations -say the electric field at some area- that it is only requested for a fixed/given point? It would seem like you would want the field ...
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0answers
28 views

Self propagating EM waves mechanism of propagation

Electric fields generate magnetic fields and visa/versa,and this is attributed to be how a light ray self propagates. Can anyone further clarify how exactly light rays self propagate? If the mechanism ...
4
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1answer
96 views

Lagrangian density for Lorentz force of continuous charge distribution in external field?

It's frequently an exercise to derive the Lorentz force law for a particle with charge $q$ in an external electromagnetic field given by the following Lagrangian: $$L = -mc^2\sqrt{1-\frac{\dot{r}^2}{...