Redistribution of charge in a capacitor

[Ratch]

ehild,

Electric charges can be separated, and it is possible to give one plate of a capacitor some charge Q while the other plate stays neutral or has some charge different from -Q, although capacitors in electric circuits usually have equal and opposite charges on the plates.

How would you put different amounts of charge of the plates of a cap? Inductive methods, perhaps?

Ratch

 

[MalachiK]

... it is possible to give one plate of a capacitor some charge Q while the other plate stays neutral or has some charge different from -Q, although capacitors in electric circuits usually have equal and opposite charges on the plates.
ehild

That's what's been bothering me. I mean, a capacitor doesn't have to be a sealed unit. I can just get two plates that I've independantly charged up with different amounts of +ve and -ve charge (opposite charges in each metal plate) and put them next to each other. BAM! I've got a capacitor with a different charge on each plate. That works, right?

This whole infinite current in infintesimal time is all too clever for me. I'm quite simple and infinites make my head hurt!

 

[ehild]

That's what's been bothering me. I mean, a capacitor doesn't have to be a sealed unit. I can just get two plates that I've independantly charged up with different amounts of +ve and -ve charge (opposite charges in each metal plate) and put them next to each other. BAM! I've got a capacitor with a different charge on each plate. That works, right?

It is right. Even a single metal sphere is a capacitor and you can give charge to a metal sphere... Just rub a comb and touch to it.

This whole infinite current in infintesimal time is all too clever for me. I'm quite simple and infinites make my head hurt!

It makes my head hurt, too. That is why I would include the resistance of the connecting wire. With that, everything is simple and all right.

ehild

 

[Ratch]

MalachiK,

That's what's been bothering me. I mean, a capacitor doesn't have to be a sealed unit. I can just get two plates that I've independantly charged up with different amounts of +ve and -ve charge (opposite charges in each metal plate) and put them next to each other. BAM! I've got a capacitor with a different charge on each plate. That works, right?

What are you going to use for positive charges? Protons?

Ratch

 

[ehild]

You can make a glass rod positively charged by rubbing it with cats fur. You touch it to a metal plate: That will be positively charged, too, as some electrons of the metal go over to the glass.

ehild

 

[Ratch]

ehild,

You can make a glass rod positively charged by rubbing it with cats fur.

Correct, by the absence of electrons, not by any positive charges.

Ratch

 

[ehild]

Absence of electrons leaves positively charged particles behind. Yes, because of the excess protons.
Positively charged particles are made in ion guns. Positively charged alpha particles can arise in radioactive decay. There is a material testing method using positrons, also positively charged particles.
There are both positively and negatively charged particles in the world, and they can be isolated.

ehild

 

[Ratch]

ehild,

Absence of electrons leaves positively charged particles behind. Yes, because of the excess protons.

Yes, but the protons are locked into the ionic core of the molecule and cannot move like the electrons can. They certainly do not move in capacitors.

Positively charged alpha particles can arise in radioactive decay.

Yes.

There is a material testing method using positrons, also positively charged particles

A positron is a antimater particle. How are they made and controlled?

There are both positively and negatively charged particles in the world, and they can be isolated.

Yes, ions in electrochemistry and holes in p-type semiconductor. But not in capacitors.

Ratch

 

[ehild]

See http://en.wikipedia.org/wiki/Positron_emission .

Well, what do you think, can an isolated capacitor have net positive charge? You have two metal plates and remove electrons from both of them. Is it possible?

I think you mix "charge" with charged particle. Charge is a property. Anything can have some charge, positive or negative.

The charged particles - electron, proton, positron, ions, - all carry some charge, integer multiple of the elementary charge. In different materials, different particles are the main charge carriers. In metals or semiconductors the main charge carriers are free electrons. In fluids and gases, the charge carriers are mainly ions. But there is ionic conductions also in solids, and ionic conduction is the main conduction process in insulators like glass or in ionic crystals.

ehild

 

[Ratch]

ehild,

Well, what do you think, can an isolated capacitor have net positive charge? You have two metal plates and remove electrons from both of them. Is it possible?

According to the link you gave, it would have to be a radioactive capacitor. But even if you could do it by some electrostatic induction method of taking away electrons, the sea of electrons present in metallic conductors would rush in and neutralize your efforts. That is why holes only exist in semiconductors. It is certainly not practical for a working circuit.

Ratch

 

[MalachiK]

ehild,
According to the link you gave, it would have to be a radioactive capacitor.
Ratch

So? Why not paint one of the plates with potassium-40? Everyone should have a hobby.