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# Why does having more "free" electrons in a superconductor reduce its resistance? watch

1. So I know that in a superconductor, a heat increase can result in there being more "free" electrons/charge carriers, so there is more current.

However, I don't get how there being more charge carriers to carry current would cause the resistance to decrease. Can you explain that to me please? I know that resistance is caused by collisions between electrons/charge carriers and lattice ions, but I don't get why having more charge carriers would cause the resistance to decrease.

Thank you!
2. (Original post by blobbybill)
So I know that in a superconductor, a heat increase can result in there being more "free" electrons/charge carriers, so there is more current.

However, I don't get how there being more charge carriers to carry current would cause the resistance to decrease. Can you explain that to me please? I know that resistance is caused by collisions between electrons/charge carriers and lattice ions, but I don't get why having more charge carriers would cause the resistance to decrease.

Thank you!
Do you mean a semiconductor? In a superconductor you don't want temperature rises to occur, since you run the risk of going above the critical temperature, and there is no resistance anyway below the critical temperature.
3. (Original post by an_atheist)
Do you mean a semiconductor? In a superconductor you don't want temperature rises to occur, since you run the risk of going above the critical temperature, and there is no resistance anyway below the critical temperature.
Ah, yes. I do.
4. Resistance can also be considered as how difficult it is to push a charge through (hence you need a higher electromotive force to move the electrons at higher resistances). As electrons are charge carriers, more electrons makes it easier for charge to travel through the material
5. (Original post by blobbybill)
So I know that in a superconductor, a heat increase can result in there being more "free" electrons/charge carriers, so there is more current.

However, I don't get how there being more charge carriers to carry current would cause the resistance to decrease. Can you explain that to me please? I know that resistance is caused by collisions between electrons/charge carriers and lattice ions, but I don't get why having more charge carriers would cause the resistance to decrease.

Thank you!
Pretty sure you're thinking about semiconductors not superconductors.

assuming semiconductors...

in metals you have an atomic lattice in a 'sea' of electrons which are able to conduct readily.
semiconductors are different - in a pure semiconductor you don't have the same sea of electrons, the electrons pretty much stay with their own atom unless they're dislodged, e.g. by collision between atoms in a hot piece of semiconductor.
compared to metals semiconductors are poor conductors of electricity.
6. Due to the I = V/R inverse relationship between current and resistance. Under the same potential difference applied between two points of the semiconductor, as the charge carriers movement increase, the current increases, so the resistance has to decrease.
7. (Original post by Joinedup)
Pretty sure you're thinking about semiconductors not superconductors.

assuming semiconductors...

in metals you have an atomic lattice in a 'sea' of electrons which are able to conduct readily.
semiconductors are different - in a pure semiconductor you don't have the same sea of electrons, the electrons pretty much stay with their own atom unless they're dislodged, e.g. by collision between atoms in a hot piece of semiconductor.
compared to metals semiconductors are poor conductors of electricity.
Thanks.

In a metal, I know you have the sea of delocalised electrons, where one (or a couple) of electrons are lost from the outer shell of each nucleus, what is the reason as to why the electrons come away from the nucleus and become free electrons?

I know there is a weak force holding the outer layer of electrons to the actual electron shell, but why do some of those electrons escape the shell to become delocalised (free) electrons? What attracts them to leave the electron shell?
8. (Original post by blobbybill)
Thanks.

In a metal, I know you have the sea of delocalised electrons, where one (or a couple) of electrons are lost from the outer shell of each nucleus, what is the reason as to why the electrons come away from the nucleus and become free electrons?

I know there is a weak force holding the outer layer of electrons to the actual electron shell, but why do some of those electrons escape the shell to become delocalised (free) electrons? What attracts them to leave the electron shell?
It's more to do with the repulsion of the negatively charged electron against the positively charged nucleus.
9. (Original post by blobbybill)
Thanks.

In a metal, I know you have the sea of delocalised electrons, where one (or a couple) of electrons are lost from the outer shell of each nucleus, what is the reason as to why the electrons come away from the nucleus and become free electrons?

I know there is a weak force holding the outer layer of electrons to the actual electron shell, but why do some of those electrons escape the shell to become delocalised (free) electrons? What attracts them to leave the electron shell?
The number of free charge carriers in a semiconductor is temperature dependent.

At low temperatures, electrons are bound to the lattice and all of the covalent bonds are complete. At higher temperatures, a small number of valence electrons attain sufficient thermal energy to break away from their bonds and are then able to move through the lattice and available for conduction. This leaves a 'hole' in the atom from where the electron was dislodged and consequently that atom gains a net +ve charge and able to attract a free electron.

In other words, the conductivity of a semiconductor increases with (and is induced by) increasing temperature.
10. (Original post by uberteknik)
The number of free charge carriers in a semiconductor is temperature dependent.

At low temperatures, electrons are bound to the lattice and all of the covalent bonds are complete. At higher temperatures, a small number of valence electrons attain sufficient thermal energy to break away from their bonds and are then able to move through the lattice and available for conduction. This leaves a 'hole' in the atom from where the electron was dislodged and consequently that atom gains a net +ve charge and able to attract a free electron.

In other words, the conductivity of a semiconductor increases with (and is induced by) increasing temperature.
When the atom loses an alectron and then the atom gains a net positive charge and can attract a free electron, why doesn't it attract the electron back again?

If it loses one electron to become positive, you said it then had a positive charge (which I get), but what do you mean by then the atom is able to attract a free electron?

If it just lost an electron, becomes positive and can then attract a free electron, whats to stop it just attracting back the electron it lost?
11. (Original post by blobbybill)
When the atom loses an alectron and then the atom gains a net positive charge and can attract a free electron, why doesn't it attract the electron back again?
It is possible but statistically unlikely. Because the thermal energy required to break free will propel the electron away from the parent atom, it can be attracted to other atoms with a free space in the valence shell, or be swept away by the e.m.f. of the power supply placed across the material. The escape and recombination of electrons is random, but with a supply e.m.f. across the ends of the semiconductor bulk material, the free electrons 'drift' towards the anode end.

If it loses one electron to become positive, you said it then had a positive charge (which I get), but what do you mean by then the atom is able to attract a free electron?
Electrons are -ve charge. Opposite charges attract. A free electron moving on a trajectory which puts it in the vicinity of an atom with a free space in its' valence shell, can then 'capture' that electron where the net charge of the combination becomes neutral once again.

If it just lost an electron, becomes positive and can then attract a free electron, whats to stop it just attracting back the electron it lost?

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