Insulators, static and electron mobility....

[TooLateForAName]

Question from my 16yr old daughter:


An insulator has tightly bound electrons in its normal state.

You can generate static on an insulator by adding electrons.

Why is it that these extra electrons are also to be tighly bound? What is it about the insulator that prevents the extra electrons flowing across the surface of the insulator? and turning the insulator into what would be a structural carrier for electrons? ie a conductor.

She used the analogy of a conductor being a pipe with water flowing through it and an insulator being a strand of wire that water can run along.

[FarmyardPants]

TooLateForAName said:

What is it about the insulator that prevents the extra electrons flowing across the surface of the insulator?

There’s nothing special about the surface, it’s made of the same stuff as the rest of the insulator, which doesn’t allow electrons to migrate through it from atom to atom. For the placed electrons to spread across the surface, they need somewhere to go - a vacated space (lack of electron/net positive charge) in the adjacent atoms, but there is no such vacant “charge space” for them to move to. Even though they are repelling one another and would like to move.

In a conductor, the placed electrons quickly redistribute themselves because they repel (electrostatically push on) the nearby electrons of the conductor material, which are able to move through it.

[TooLateForAName]

FarmyardPants said:

There’s nothing special about the surface, it’s made of the same stuff as the rest of the insulator, which doesn’t allow electrons to migrate through it from atom to atom. For the placed electrons to spread across the surface, they need somewhere to go - a vacated space (lack of electron/net positive charge) in the adjacent atoms, but there is no such vacant “charge space” for them to move to. Even though they are repelling one another and would like to move.

In a conductor, the placed electrons quickly redistribute themselves because they repel (electrostatically push on) the nearby electrons of the conductor material, which are able to move through it.

Ok thanks.

So what is it about the structure of conductors and insulators that allows one to have free flowing electrons and the other to hold them in place - is there something about electron energy levels?

[FarmyardPants]

Conductors usually (always?) have their atoms arranged in a lattice where a particular electron is not associated with any given atom - the lattice as a whole shares its ‘sea’ of electrons (the atoms jiggle about according to temperature and this wobbling slightly obstructs the passage of travelling electrons).

Insulators tend to be made of molecules, each of which is either stable on its own or covalently shares one or more electrons with a neighbouring molecule at a specific site.

Edited by FarmyardPants on Sunday 13th September 10:00

[TooLateForAName]

So the static electricity electrons are probably taking up the positions of covalent electrons - that makes sense.

thanks