Magic capacitors!

Yes, I'm actually an electronics design consultant. Take a look at this:

Note the equivalent circuits, which show capacitance between the gate, source and drain. They talk about 'gate charge' as being a conveient way of relating the capacitance charging and discharging (energy, not electrical charge) with current, and hence time. Again, confusion can arrise because they use the word 'charge' in two contexts.

The fact is that since current flows in and out of these capacitors in equal amounts the net stored electrical charge on each one is zero. However the Q=CV equation relates to the magnitude of charge that each of the plates of these capacitances carries, but for each capacitor there is another plate with equal and opposite charge.

Here is a good derivation of the elecrostatic forces between the plates of a parallel plate capacitor. Note that the electrical charge on each plate has the same magnitude Q, but one is positive and the other negative. If you think this is not correct maybe you should contact the University of Pennsylvania and tell them :)

Mark.

Perhaps he was confused with "filling" his mop bucket?

I was making Leyden Jars at 7 in the late 60s. You are an idiot. You'd be better off FOAD. Go find a bridge and jump off it.

Do you know how many electrons can sit on the surface of a 32" CRT screen? Do you know how they get there? Do you know how they get removed? Do you know why? I doubt that you have any grasp of any of it at all.

The guy is right. The net 'charge' is zero. John is also right. We use the term 'charge' and describe the action of a capacitor as a charge, discharge cycle. They are very precisely sized, temporary storage 'spaces' for 'some' electrons. We have become very good at knowing how much the gozinta and gozoutta sputters at.

They use "charge", in many places, the same way most electronics engineers use the word, namely C*V.

Exactly. We say a cap is "charged" if C*V 0. In fact, C*V is the exact charge. We say that a cap integrates charge into voltage, and that it can return that same charge as we drain it down to zero volts. So it's handy to think that a cap can store that charge for us, which it actually does.

The plates need not gave the same absolute Q, because the overall cap can have a net charge, what we electronic guys would call an electrostatic charge. That ususlly doesn't matter to us, so we use the conviently short word "charge" to mean C*V, where V is the sort of potential difference we measure with a 2-terminal voltmeter. When we rarely refer to physics-type net charge, we say "electrostatic charge."

To express the concept of "charge on a capacitor" any way other than the way we use it would be grammatically and numerically very messy.

But when we use the term this way, we have to be careful to remember what it means to us, and we can't blindly say stuff like "charge is conserved" without thinking carefully. It's safe to say "energy is conserved."

John

Shame you learned nothing from it, you pig-ignorant sack of shit.

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Well, theoretically, if the cap was say at ground potential on both plates before being added to the circuit, but the circuit is designed to run at some value above ground potential when running, it COULD get a few extra electrons added, giving a net increase of charge... ;-)

Charlie

For there to be a net increase in charge one plate would have more electrical charge than the other (either positive or nagative). This would require more current going in than coming out (or vice versa). That doesn't happen in electrical circuits. I'm ignoring static electricity which is a different subject.

Mark.

I'm giving up. Your concept of charge is obviously not Coulombs!

To be honest I'm staggered of how many people think like this, what they don't get is that a capacitor is *by definition* two electrodes separated by an insulator, it necessarily has two electrical charges, one positive and one negative if it is 'charged' by passing a current through it. Since the same current goes in and out of the capacitor, and there is an insulating barrier, electrons can't pass the barrier so they build up on one electrode, giving it a negative electrical charge (which by the way is where the voltage comes from. It's like pushing against an ever stronger spring, to get more electrons on that plate require more force, and that gets harder and harder and the plate becomes more and more negative). Since exactly the same number of electrons are coming out the other terminal, this means there must be an equal depletion of electrons on that electrode, yielding an equal and opposite electrical charge on that electrode. The sum of the amount of electrical charge therefore is a big fat zero.

You can talk of a single capacitor electrode having a stored charge, and it does. But the definition of a capacitor is one that has two electrodes, the one has negative charge and the other equal, and opposite, charge. So a capacitor that is charged up this way cannot have a net storage of electrical charge. I'm ignoring static electricty, this situation is normal 'dynamic' electricty stuff.

Mark.

Larkin will further bloviate, but the _science_ says, "Within a closed Universe". ...Jim Thompson

Cool. I assume you do all RC circuits, and opamp integrators, and active filter designs, and sample/hold circuits, in terms of energy stored in caps in joules.

That must be fun, doing capacitor+current source timing ramp calculations in terms of energy, not charge.

John

The equation Q=CV works because it describes the the charge going into a capacitor one end *and coming out the other*, and relates that to voltage across the capacitor and to the current, which in an electrical circuit is the same throughout that circuit. But in no way does that mean or imply that net electrical charge is stored in a capacitor.

Imagine you have a closed system (an infinitely big box say), where no electrical charge can enter or escape, with a 'charged' capacitor of capacity C at voltage V and a parallel resistor with a switch. You're saying that capacitor has stored electrical charge, C*V Coulombs. The switch is then closed, the capacitor 'discharges' through the resistor. After a very long time the voltage would practically zero, and you would say the stored electrical charge would be practically zero. This has violated the Law of Conservation of Charge, which is a fundamental law of physics:

If you think this hasn't please explain why not?

Mark.

Well, you don't get the joke...

Think BOTH plates get a net negative (or positve) charge...

Charlie

"markp" is one of Larkin's little-boy-toys... a first-class fairy who's going to "complain" about _me_ :-)

Bwahahahahahaha! ...Jim Thompson

Q=C*V is coulombs. The units work. If a cap stores 5 coulombs, I can load it at 2.5 amps for 2 seconds, and then the charge is gone. It works.

John

Why? are there two different kinds of electricity?

John

Yep. That with charge conservation and that without :-) ...Jim Thompson

How many angels can dance on the top of your head?

Actually the capacitor stores + 5 Coulombs on one plate, and -5 Coulombs on the other plate. When you 'load it at 2.5 Amps for 2.5 seconds' the negatively charged electrons move from the negatively charged plate, through the external circuit, and into the positively charged plate. At the end there are zero Coulombs stored on both plates. This is why the maths works, at no point does the capacitor as a whole ever have net stored charge, the sum of the charges of both plates is always zero. In your system of stored charge you cannot explain why, if the same current is flowing in and out of the capacitor, it doesn't violate the Law of Conservation of Charge.

Mark.