Magic capacitors!

Sorry but I'm afraid you've fallen into the 'capacitor stores charge' trap John. It doesn't, as I said in another post, capacitors don't store charge, the plates are equal and opposite in the excess or depletion of electrons (they can't store electrical charge, since we all know the current going in and the current coming out of a capacitor is equal, and current is Coulombs per second). The total stored electrical charge in a capacitor is zero. That negates the whole premise of this analysis.

Mark.

Charge 1uF to 50V and put your tongue across the terminals, now tell me there's nothing there ;)

Grant.

I didn't say a capacitor can't store energy, it certainly can. When you say 'charge 1uF to 50V' the 'charge' bit refers to putting energy into the capacitor, not electrical charge. When you put your tongue across the terminals you form an electrical circuit, the plate with excess electrons pushes electrons through the tongue and the plate with a depletion of electrons sucks exactly the same number of electrons back in. The capacitor at the end of that 'discharge' cycle has the same number of electrons in it as it had when it was 'charged'.

Here's a good explanation of capacitors:

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Idiot! His point was/is that it stores ENERGY, NOT charge. His detail of the process was concise enough for a monkey to understand it. The fact that you did not is quite a tell.

The correct term would be "fill" a 1=B5F cap to 50V.

Another proof is that it takes POWER to "charge" a battery. It does NOT take any power to "fill" a cap. It is merely a temporary transference point for an energy "packet". If there is a load on the cap, THEN it will take power to TRANSFER energy THROUGH the cap.

Practical users of caps use the terms charge and discharge without qualification. Same as they accept electricity flows from positive to negative ;) Convention, or convenient lies? Does it matter?

Grant.

Charge has many meanings:

Note definition no. 11: "to supply with a quantity of electric charge or electrical energy: to charge a storage battery".

It can mean a transfer of electric charge *or* energy. However, for capacitors it means energy because a capacitor can't store net electrical charge.

Mark.

Right. And battery chargers are an elaborate public fraud, since batteries don't store charge.

And power mosfet data sheets are all wrong.

John

Maybe where you work.

Cool. You can somehow put 600 volts across (or into?) a 10,000 uF cap without using any power to do it? I'm impressed.

John

That's not what they taught us in college, and that's not the way we do engineering. We say that a capacitor stores charge, the amount being C*V in coulombs, and it works. My whole point, which has evoked such ranting, is that when you use this convention, be careful about designing using the concept that (this kind of) charge is always conserved.

John

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OK has been fun reading all this but I give up! I will let you know a secret. We know nothing! The more we learn, the less we know. Why? Let's look at this logically. The universe is infinite and hence its laws and the achievable knowledge is infinite. At this stage our knowledge is insignificant. Welcome to the classroom kiddies!

With all due respect, we don't, and shouldn't, say a capacitor 'stores charge'. The misconception comes from the use of the word charge when talking about putting energy into a capacitor, and more explictly the significant lack of clarification given on this when being taught. This is compounded by a confusion of the q=C*V equation which actually relates to the charge on the plates, but one of the plates is of the same value but opposite in polarity, so the sum of those is zero. This is an extremely popular misunderstanding unfortunately, and leads to conclusions that electrical charge is not conserved. In fact, in a closed system where no electrical charge can get in or out, within that system electrical charge

The same current flows in and out of a capacitor when it is being 'charged' (I assume you are not going to deny that). Note I said the same current, but they are not made of the same electrons because those can't cross the plate barrier. The same amount of electrical charge that goes in comes right out again. How can the capacitor possibly end up with a net charge in it? If it can, where has the electrical charge come from? Have electrons just been conjured up out of nowhere?

Mark.

What a hoot !-) ...Jim Thompson

I don't see how this applies to my flux capacitor though...

Isopropyl alcohol can fix that.

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You really don't know why those shorting wires get put onto caps, do ya?

It all depends on how fast you want it to get filled. In the long run, it does require the same power in both cases. So, yes, the internal resistance of the cap would determine the power required to fill it quickly. You missed the point.

What do you mean by "we"? Electronic design engineers do this all the time, with reference to capacitors and batteries. You pump X coulombs into a cap; it becomes stored, coincidentally as C*V. You can extract those coulombs later, and the accounting is correct. The math works. The gear works.

The misconception comes from the use of the word charge when

It's a different convention. Words. But the units work and the numbers work, so we use it. Call our kind of charge "charge separation" or "plate charge differential" if it makes you happier.

Do you design electronics? Do mosfet data sheets refer to stored gate energy, or stored gate charge?

John

That's wronger than Always Wrong! That's Olympic class wrongness!

John

What??!!

Would you really expect a janitor to discuss the finer points of capacitor behaviour?

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