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 $c_m$: $$ \overbrace{\varepsilon}^\mathrm{matrix} \underbrace{\mu}_\mathrm{matrix}=c_m^{-2}\ \leftarrow\ \text{matrix as well} $$ ...
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20 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 ...
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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 ...
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  • 10.5k
16 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 ...
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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 ...
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  • 64.1k
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 ...
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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 ...
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11 votes
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What is the dielectric constant of a pure conductor?

The permittivity of a conductor is infinite. Let the value of an external electric field in free space (relative permittivity = 1) be $E$. If this is applied to a material of relative ...
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  • 76.5k
10 votes
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What is the force between two charged objects when the space between them is partially filled by a dielectric medium?

A dielectric effectively behaves as if it was thicker than it is. If the dielectric constant is $K$ and the thickness of the dielectric is $t$, then for calculating the force it behaves as if the ...
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10 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 "...
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9 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 ...
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8 votes

Difference between electric field $\mathbf E$ and electric displacement field $\mathbf D$

$D$ is the electric displacement field or commonly the flux density and $E$ is the field intensity. There is a fundamental difference between them which will be understood to certain extent as you go ...
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  • 81
7 votes

How is bound charge and free charge possible?

Imagine a blob of liquid water. Each molecule is polar because the electrons are closer to the oxygen than the hydrogens. Without a large external electric field, the water is moving around bumping ...
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  • 24.6k
7 votes
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What would be electric potential due to induced charge sphere?

Short answer: yes, the surface charges are taken into account; in fact, they're what ensures that $\vec{E} = 0$ inside the conductor. The electric field at any point in space can be viewed as the ...
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7 votes

How is complex permittivity measured?

First make a parallel plate capacitor with plates of area A and spacing d. Fill the space between the plates with the dielectric whose complex permittivity $\epsilon(\omega)$ you wish to measure. ...
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  • 2,783
7 votes

Inserting dielectric slab into a capacitor

how the direction of the force on slab in both situation differs? Recall that the energy stored in a capacitor is given by $$W = \frac{1}{2}CV^2 = \frac{1}{2}\frac{Q^2}{C}$$ where $V$ is the ...
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6 votes
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Refractive index of dielectric in different frames of reference

The problem here isn't a simple algebra error, but rather an issue with the physics. A medium which at rest is isotropic no longer behaves as an isotropic medium when it is moving relativistically. ...
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  • 7,481
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 ...
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  • 116k
6 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 ...
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  • 2,498
5 votes

Basic of Capacitor

The only property of metals used in deriving $C=\varepsilon A/d$ is that they are perfect conductors. Ideally, all metals have this property. So even if you change the metal, it should not matter. ...
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  • 326
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 ...
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5 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 ...
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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 ...
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  • 116k
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 ...
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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 ...
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  • 15.3k
5 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.
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  • 21.7k
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 ...
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  • 4,659
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 ...
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  • 76.5k
5 votes

What prevents ice from being an electret?

In the structure of ordinary hexagonal ice at low pressure, each molecule is held in a sort of tetrahedral cage by its four nearest neighbors. It acts as donor to two hydrogen bonds, and acceptor to ...
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  • 3,013
5 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 ...
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