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21 votes
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Why aren't all dielectrics transparent?

Just because the material doesn't conduct currents on a macroscopic scale, does not mean it doesn't contain any movable charges at all. In fact, as the very name “dielectric” suggests, such a material ...
leftaroundabout's user avatar
20 votes
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How does $\epsilon \mu = 1/c_m^2$ change when $\mu$ or $\epsilon$ (permeability or permittivity) is a tensor?

If $\varepsilon$ or $\mu$ are tensors (read, matrices), then so is $c_m$: $$ \overbrace{\varepsilon}^\mathrm{matrix} \underbrace{\mu}_\mathrm{matrix}=c_m^{-2}\ \leftarrow\ \text{matrix as well} $$ ...
AccidentalFourierTransform's user avatar
19 votes

Why aren't all dielectrics transparent?

You cannot totally avoid quantum mechanics, but it may suffice to say that reflections by free electrons are not the only way to prevent transmission. Any situation where light can promote an electron ...
Gilbert's user avatar
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18 votes
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How can the refractive index be below 1 in a dielectric?

What you are describing is anomalous dispersion. This happens when a material becomes strongly absorbing, typically near an absorption line, and the refractive index becomes complex. In these ...
John Rennie's user avatar
16 votes

Why is wood opaque?

Wood is actually not opaque. Here is an image showing that light passes through wood. Wood is highly scattering, meaning that light goes a very short distance through wood before abruptly changing ...
Dale's user avatar
  • 103k
16 votes
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Why do oxides form amorphous films instead of crystalline films?

This is because the volume occupied by an Al2O3 molecule is different from that occupied by two Al's. This volumetric mismatch means that you can't retain the crystalline lattice of the Al across the ...
niels nielsen's user avatar
15 votes
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How London dispersion forces come into existence out of nowhere?

The explanation of the force as due to oscillating dipoles is only an approximate description and should be regard only as a guide for students. The actual explanation is that the wavefunctions of the ...
John Rennie's user avatar
13 votes

How does current flow in a circuit with a capacitor?

how is it possible that current flows in a circuit with capacitor since according to Ohm's law current is inversely proportional to resistance and insulator by definition has a big resistance, so we ...
Bob D's user avatar
  • 73.6k
13 votes

How does current flow in a circuit with a capacitor?

Since this is a physics q and a, a physics explanation is in order. There are two kinds of current. Conduction current is a net flow of charges. It is was people usually think of when the word "...
Math Keeps Me Busy's user avatar
10 votes
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What is the intuition behind Kramers-Kronig relations?

The Kramers-Kronig relations are the expression, in the Fourier frequency domain, of the fact that the linear susceptibility $\chi(\tau)$ is a causal function, i.e. that the dielectric response of the ...
Emilio Pisanty's user avatar
8 votes
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Main cause of self-charging of unshorted capacitors?

The phenomenon you are talking about is called dielectric absorption. The way it works is this: Let's say you've just discharged a capacitor. An ideal capacitor would remain at zero volts after this. ...
auden's user avatar
  • 7,057
7 votes

Why is water not used as a dielectric in the condenser?

"Water capacitors", where water is the dielectric, are commonly used in very high voltage pulse systems. For example, high-power nitrogen lasers commonly use water capacitors as their energy storage ...
Prutchi's user avatar
  • 71
7 votes
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Griffiths on Averaging and the Macroscopic Electric Field

Since Griffiths' argument relies heavily on two results that are left as an exercise in the book (Exercise 3.47), it's worth proving them first. Previous results The first result is to show that the ...
Urb's user avatar
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7 votes
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Does the Coulomb's force depend on the configuration of the medium?

Assuming that the rectangle represents the region occupied by the dielectric, the answer is yes; the force depends not only on the distance between the two charges but also on their distance from the ...
GiorgioP-DoomsdayClockIsAt-90's user avatar
7 votes
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Where did this extra energy in the capacitor come from?

The energy come from the work you must do to pull the dielectric out of the capacitor. The dielectric is atracted to the charges because it is polarizable -- jut like a pice of paper is attracted to ...
mike stone's user avatar
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7 votes

Permittivity of free space

$\epsilon_0 \rightarrow 0$ would cause the Coulomb force: $$ F = \frac 1 {4\pi\epsilon_0} \frac{q_1q_2}{R^2} $$ to diverge because: $$ \nabla\cdot\vec E = \rho/\epsilon_0 $$ would lead to an infinite ...
JEB's user avatar
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7 votes

Permittivity of free space

I think the essential point here is the difference between a multiplicative constant and an additive constant. The permittivity is a proportionality constant appearing in a formula such as $$ F = \...
Andrew Steane's user avatar
7 votes
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Dielectric and dipole

An atom may be neutral as a whole but its parts aren't. The electric field applies opposite forces to the negatively-charged electron cloud and the positively-charged nucleus, resulting in a net ...
Vincent Thacker's user avatar
6 votes
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Why does the polarization vector P depend on the net electric field and not the original one?

These kinds of problems are called self consistent. You say, How can the polarization density depend on a field that itself depends on it? Shouldn't it depend on the original field between the ...
Parabola's user avatar
  • 102
6 votes

Will a circular Gaussian beam reflected at a dielectric interface become elliptical?

I believe you are talking aboug the Goos-Hänchen shift which is described in this paper. From there, the following diagram: That link also gives a detailed mathematical description. The original ...
Floris's user avatar
  • 119k
6 votes

Capacitor in a dielectric fluid

That there is a fringe electric field outside a parallel plate capacitor can be seen from the result of a computation done using the finite difference method. The electric field strength is ...
Farcher's user avatar
  • 97.9k
6 votes

Can relative permittivity be less than 1?

Yes this is quite common in metals at optical frequencies. For silver at 600 nm the real part of epsilon is -16.
my2cts's user avatar
  • 25.3k
6 votes

How to measure vacuum permittivity?

In this question, the first answer states that $ϵ_0$ is the proportionality constant in Gauss' law. If that's the case why isn't it assumed to be just “$1$“. The constant $\epsilon_0$ can indeed be ...
Dale's user avatar
  • 103k
5 votes

What is the force between two charged objects when the space between them is partially filled by a dielectric medium?

Boundary conditions on a dielectric are generally very complicated. While general ideas about how polarization works and affects the overall field can be talked about, the specifics of even simple ...
napstablook's user avatar
5 votes

What exactly is breakdown voltage of air?

It is correct that there is no such thing as a breakdown voltage, as stated earlier. This becomes obvious since the units are given as V/m, which is represents Volts per meter, or Electric Field ...
Greg Herman's user avatar
5 votes

Electric field lines in a uniformly charged dielectric solid sphere

If you have a sphere with uniformly distributed charge, the solution must be spherically symmetrical. Specifically, we know that the field intensity at a radius $r$ is proportional to the charge ...
Floris's user avatar
  • 119k
5 votes
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Breaking up a capacitor

When you have a dividing line which is between two dielectrics parallel to the plates you have to ask yourself; is the dividing line an equipotential? That is relatively easy for diagram B as there ...
Farcher's user avatar
  • 97.9k
5 votes
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What happens when a dielectric is inserted in a capacitor connected to a battery?

As you correctly observed, the electric field stays the same in the capacitor after insertion of the dielectric because the applied voltage is constant. This is accomplished by the increase in ...
freecharly's user avatar
  • 16.9k
5 votes

Contradiction on the behavior of refractive index

Permittivity and permeability are not just constants, but instead are complex functions that depend on a number of other quantities, including the wavelength of the light. In fact, the refractive ...
DK2AX's user avatar
  • 4,788
5 votes
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Why do we use dielectric materials in a capacitor?

. . . . why don't we just put these metal plates as close as possible without touching each other . . . . How is this going to be done? One of the functions of a solid dielectric is to keep the ...
Farcher's user avatar
  • 97.9k

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