capacitors
Why is it that capacitors have one plate chargee to -q and one charged to +q?
It seems a bit extraordinary that we can _know_ that, for example, a capacitor holds 5948487574820 electrons and the other plate has lost exactly that amount from its surface. Is it just a simplification that's used for a level or something?
It seems a bit extraordinary that we can _know_ that, for example, a capacitor holds 5948487574820 electrons and the other plate has lost exactly that amount from its surface. Is it just a simplification that's used for a level or something?
(edited 10 years ago)
Original post by dire wolf
Why is it that capacitors have one plate chargee to -q and one charged to +q?
The answer lies in an understanding of what charge really is.
i.e. a fundamental force carried by some sub-atomic particles, the most familiar of these are the electron and proton.
Unlike charges attract and like charge repel. The charge force is measured to a high degree of accuracy as is the mass of the particles that carry it. So far so good.
All materials acquire their physical properties from the interaction of these charge carrying particles.
In a rigid material, protons (+ve charge) form part of an atoms nucleus, are fixed in relation to other nuclei and not free to move.
Electrons orbit the nucleus in specific shells. However, some atoms have loosely bound electrons in their outer shells which are free to be dislodged and can move through the bulk material accordingly. Most notably, these are conductors and semi-conductors and the plates of a capacitor are constructed from conductors.
When a build up of electrons occurs on one plate of a capacitor, the electrons on the other plate will exerience a replusive force (like charges repel) and will try and migrate as far as possible from the source of the repulsive force. As they do so, the fixed position atoms they leave behind will have a net +ve charge because the -ve charge electrons have migrated away leaving an excess of +ve charge protons.
Hence, the plate with the excess electrons will be net -ve charge and the plate with the excess protons will be net +ve charge.
The balance comes from the fact that the charge force carried by both protons and electrons are identical in magnitude but opposite in polarity (force vector) and therefore reach a state of equilibrium.
Original post by dire wolf
It seems a bit extraordinary that we can _know_ that, for example, a capacitor holds 5948487574820 electrons and the other plate has lost exactly that amount from its surface. Is it just a simplification that's used for a level or something?
If you can say how many atoms exactly there are in each plate, moreover, that BOTH plates have the exact same number of atoms, you have your answer.
Spoiler
(edited 10 years ago)
Original post by uberteknik
The answer lies in an understanding of what charge really is.
i.e. a fundamental force carried by some sub-atomic particles, the most familiar of these are the electron and proton.
Unlike charges attract and like charge repel. The charge force is measured to a high degree of accuracy as is the mass of the particles that carry it. So far so good.
All materials acquire their physical properties from the interaction of these charge carrying particles.
In a rigid material, protons (+ve charge) form part of an atoms nucleus, are fixed in relation to other nuclei and not free to move.
Electrons orbit the nucleus in specific shells. However, some atoms have loosely bound electrons in their outer shells which are free to be dislodged and can move through the bulk material accordingly. Most notably, these are conductors and semi-conductors and the plates of a capacitor are constructed from conductors.
When a build up of electrons occurs on one plate of a capacitor, the electrons on the other plate will exerience a replusive force (like charges repel) and will try and migrate as far as possible from the source of the repulsive force. As they do so, the fixed position atoms they leave behind will have a net +ve charge because the -ve charge electrons have migrated away leaving an excess of +ve charge protons.
Hence, the plate with the excess electrons will be net -ve charge and the plate with the excess protons will be net +ve charge.
The balance comes from the fact that the charge force carried by both protons and electrons are identical in magnitude but opposite in polarity (force vector) and therefore reach a state of equilibrium.
If you can say how many atoms exactly there are in each plate, moreover, that BOTH plates have the exact same number of atoms, you have your answer.
i.e. a fundamental force carried by some sub-atomic particles, the most familiar of these are the electron and proton.
Unlike charges attract and like charge repel. The charge force is measured to a high degree of accuracy as is the mass of the particles that carry it. So far so good.
All materials acquire their physical properties from the interaction of these charge carrying particles.
In a rigid material, protons (+ve charge) form part of an atoms nucleus, are fixed in relation to other nuclei and not free to move.
Electrons orbit the nucleus in specific shells. However, some atoms have loosely bound electrons in their outer shells which are free to be dislodged and can move through the bulk material accordingly. Most notably, these are conductors and semi-conductors and the plates of a capacitor are constructed from conductors.
When a build up of electrons occurs on one plate of a capacitor, the electrons on the other plate will exerience a replusive force (like charges repel) and will try and migrate as far as possible from the source of the repulsive force. As they do so, the fixed position atoms they leave behind will have a net +ve charge because the -ve charge electrons have migrated away leaving an excess of +ve charge protons.
Hence, the plate with the excess electrons will be net -ve charge and the plate with the excess protons will be net +ve charge.
The balance comes from the fact that the charge force carried by both protons and electrons are identical in magnitude but opposite in polarity (force vector) and therefore reach a state of equilibrium.
If you can say how many atoms exactly there are in each plate, moreover, that BOTH plates have the exact same number of atoms, you have your answer.
Spoiler
first of all, thanks for replying.
I don't think I understand what you mean by 'equilibrium' though...
Original post by dire wolf
first of all, thanks for replying.
I don't think I understand what you mean by 'equilibrium' though...
I don't think I understand what you mean by 'equilibrium' though...
The voltage potential of the power supply source will create an excess of electrons on one plate creating a net -ve charge on that plate.
This bulk charge will repel the electrons on the opposite plate which will then migrate away. This creates a net bulk positive charge on that opposite plate.
This results in an electrostatic voltage 'pressure' set up between the capacitor plates which increases until no more electrons can be pushed onto the first plate.
It's at this point, when no more electrons can be pushed onto the -ve charge plate, where the charge forces between the plates are in equilibrium.
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