35 votes

What is it about the "conduction band" of a material that is distinct from the valence band?

A band is essentially a (near) continuous collection of momentum eigenstates. Within the band the electrons can be treated as free to a reasonable approximation, so their eigenstates are just plane ...
John Rennie's user avatar
26 votes

Why does the wave velocity of EM signals in a wire depend on the insulator's permittivity?

As it turns out, the currents and charge density oscillations in the metal wire are not the only parts of the wave. There are also the fields! And where are the fields? Well, if the wire is a good ...
Gilbert's user avatar
  • 11.9k
19 votes

What is it about the "conduction band" of a material that is distinct from the valence band?

No band is special. A partially full valence band does conduct, just like a partially full conduction band. On the other hand, a perfectly full band conducts just as well as a perfectly empty band: ...
Nanite's user avatar
  • 3,400
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
  • 100k
14 votes
Accepted

Why are ceramics good electric insulators?

For electricity to flow, electrons need to be moving. So in a conductor, there need to be free or loose electrons so they can carry the flow of electricity. Most metals fulfill this requirement, which ...
auden's user avatar
  • 7,037
14 votes
Accepted

What is the difference of the gap between superconductor and insulator?

The difference is that in a normal conductor the current is carried by fermions (i.e. electrons) while in a superconductor the current is carried by bosons (i.e. Cooper pairs). Have a read through my ...
John Rennie's user avatar
12 votes
Accepted

What is it about the "conduction band" of a material that is distinct from the valence band?

An example of a doped semiconductor might give an intuitive picture of some aspects of this topic: Consider a material like Germanium. Atoms are structured in a lattice. All valance electrons are "...
Steeven's user avatar
  • 51k
11 votes

Why are ceramics good electric insulators?

Most ceramics are ionic compounds, in which electrons are immobile. This is different to metal, in which the atoms are in a "sea of electrons" that are free to move. Note that ceramics have some kind ...
Gimelist's user avatar
  • 424
11 votes

Why are ceramics good electric insulators?

Solids can be classified according to their band gaps. The band gap determines how much energy you need to supply in order to free or promote an electron from the valence band to the conduction band. ...
hat's user avatar
  • 366
9 votes

Why does the wave velocity of EM signals in a wire depend on the insulator's permittivity?

In a transmission line (and in fact in all circuits) electric energy is actually transmitted as electromagnetic waves whose energy is concentrated in the space between the wires. In a transmission ...
Jagerber48's user avatar
  • 13.9k
7 votes
Accepted

Why is electric field in an insulator non zero?

This is a great question! The simplest way in which materials respond to the external field is via dipoles. There may already pre-existing dipoles in the bulk of the insulator that point in random ...
Superfast Jellyfish's user avatar
6 votes

What is the difference of the gap between superconductor and insulator?

Existence of a gap means that ground state and excited states are well separated and a transition from ground state and excited states requires some energy. Existence of a gap does not determine ...
atbug's user avatar
  • 1,431
6 votes

What is the difference of the gap between superconductor and insulator?

In a superconductor, there is a one-particle gap but no two-particle gap. In a true insulator, all n-particle gaps are finite. With a small electric field you can still create gapless two-particle ...
Tamaghna's user avatar
6 votes

What good reason to hang long cable carrying electric current high above the ground?

Going underground is more expensive then suspending in air in at least two ways. Initially having to dig a trench or use a "mole" (which is less disruptive) and then when there is a need for ...
Farcher's user avatar
  • 96.2k
5 votes

If vacuum is an insulator, then why do charges flow inside a vacuum tube?

Good question! The answer is that the vacuum is an insulator simply because it does not contain any charges that can move to carry current. A vacuum tube really doesn't contain a vacuum. It ...
S. McGrew's user avatar
  • 24.8k
5 votes

What does the subindex $x$ in $\text{Bi}_{1-x}\text{Sb}_{x}$ mean?

Normally the index at an element tells me "how many atoms of that element there are", but in this case we are discussing values of $x = 0.04$ etc. so this does not make sense. That is still what it ...
Emilio Pisanty's user avatar
5 votes

Wouldn't every material act as an insulator on 0°K?

In metals, electrons need not jump. They are always present in the conduction band. Even for metals that aren't superconductors, resistivity decreases with decreasing temperature, as scattering of ...
John Doty's user avatar
  • 21k
4 votes

What is the 'Drude Weight' and why is it important?

Thanks to the feedback of @guangcun I have changed parts of this answer. As I understand it now, the Drude weight distinguishes between an insulator and a metal in a clean model system. This means, ...
RGWinston's user avatar
  • 301
3 votes
Accepted

Charging by friction

Let's consider the following case: an object made of ebonite and some cat fur. Electron's from cat fur will move to the ebonite. The friction energy $E$ is transferred to the electrons on the outside ...
Mihai B.'s user avatar
  • 582
3 votes

How can a metal and an insulator both have high dielectric constants, yet one is conducting while the other is insulating?

Hi. I'm first going to explain what the dielectric constant is, since I think you may be a bit confused. Then I'll answer the question as to why we say metals have infinite dielectric constants. I ...
joshuaronis's user avatar
  • 3,055
3 votes

Are materials which are bad at conducting heat always bad at conducting electricity also?

Diamond is a good thermal conductor but a poor electrical conductor. Diamond at Wikipedia
badjohn's user avatar
  • 2,073
3 votes
Accepted

Is diamond able to conduct electricity given 5.5 eV of energy? How do we then identify insulators and semiconductors?

There isn't an intrinsic difference between a band insulator and a semiconductor - the difference between the two is one of degree, not of kind. Generally, we use the term 'semiconductor' to refer to ...
Emilio Pisanty's user avatar
3 votes
Accepted

Why don't insulators conduct electricity?

If you have a chunk of insulator sitting on your workbench at a temperature $T$ then the energy available for exciting electrons is of order $kT$, where $k$ is Boltzmann's constant. If we take room ...
John Rennie's user avatar
3 votes

When you introduce excess charge into an insulator the charge stays still. Why is this?

It is also the electric force, but caused by atoms/molecules of the insulating solid. Such an electric force is also found in conductors; the difference between conductors and insulators is in its ...
dominecf's user avatar
  • 1,340
3 votes

Why ferroelectric material needs to be insulating?

Ferroelectric materials are polar, meaning that they exhibit a spontaneous dipole moment (i.e. local electric field) within the unit cell. Metals cannot (typically) be polar, as conductive materials ...
koysean's user avatar
  • 105
3 votes

How to derive the Clausius–Mossotti relation using Feynman diagrams?

After several months I finally find out how. The C-M relation can't be obtained by integrating out electron propagators; what we need is actually integrating out microscopic electromagnetic fields. ...
jywu's user avatar
  • 351
3 votes

Why can't an insulator show self-capacitance?

The self-capacitance of a conductor is defined by the ratio of charge and electric potential. The operation of a capacitor relies on the fact that we can change charge by applying voltage. Otherwise, ...
freude's user avatar
  • 1,725
2 votes

Why do we get electric shocks when most structures are insulators?

No, there is no such thing as perfect insulation. All insulation is merely a very very high resistance with a high breakdown voltage (Insulation is the ability of a material to block the flow of ...
आर्यभट्ट's user avatar
2 votes

What is the difference of the gap between superconductor and insulator?

Normally Fermi energy is temperature dependent. We define the Fermi level at absolute zero. An insulator at temperatures near absolute zero have very less energy compared to the conduction energies. A ...
UKH's user avatar
  • 4,881
2 votes
Accepted

Is an EMF more/same/less in an insulator than in a conductor?

emf exists in a closed path in space irrespective of the presence of a material medium. The presence of a conductor thus allows a path for electrons to flow.
Lelouch's user avatar
  • 3,616

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