[sdiy] stupid cap question

[Simon Brouwer]

Hi Antti,

At 19:31 6-10-2005, you wrote:
>Hi!
>
>This isn't exactly on topic since it's a question about basic electronics 
>and not synth DIY, but I hope you guys don't mind I ask this here.
>
>So: Lately I've really tried to learn about electronics, so I could learn 
>not only to build stuff but also to understand how they work. I understand 
>all the basic stuff, like Ohm's law, Kirchoff stuff, and I also understand 
>some stuff about transistors and tubes, but there's one very basic thing 
>that I don't exactly understand why it happens. It's about capacitors. I 
>understand that they block direct current because of the very high 
>resistance between the plates (or block after being fully charged!). I 
>also understand what happens when u put them in series, parallel etc. I 
>also understand why there can be accent current in a circuit with a 
>capacitor, if the other plate is connected to AC source and the other one 
>to ground -  the cap is consantly charged and discharged (or I can 
>understand why it happens on the plate and part of the circuit that is 
>connected to the AC source). But why does accent voltage, well I mean 
>signal, actually pass the capacitor? What happens there? All the books 
>I've read simply say that it happens but they don't say why. There are all 
>the theories about coils and transistors etc., and I understand them, but 
>why, oh why does AC, or maybe it would be more correct to ask, why does 
>accent voltage, pass a capacitor since no electrons can actually go trough 
>it? What's the theory behind this?
>
>I know all I would really need to know is simply that accent voltage 
>passes a cap, but I'm one of those people who want to know more :) I hope 
>my question made any sence!
>
>Please help me so I can sleep ;)

Capacitors work because of the electrical field that exists around an 
object when it is charged. An object with a positive charge will attract 
electrons, and one with a negative charge will repel them. The effect is 
larger at close distance. Note that the gap between the plates in 
capacitors is very small indeed.

Now if you charge one plate of a capacitor to a positive voltage, its 
electrical field will attract electrons in the other plate, so that they 
move to the part of the plate that is closest to the gap. But that means 
that the rest of the plate (including the terminal) now has a shortage of 
electrons. In other words, it becomes positively charged as well!

So, if you charge one side to a certain voltage, the second side will tend 
to follow that voltage.

However, if you allow this shortage of electrons to be replenished (for 
instance via a resistor connected to ground) then after a while the second 
side will be discharged to zero volts again. The voltage on the second 
plate, therefore, will only be nonzero for a short time after the voltage 
on the first plate has changed.

So you have to keep changing the voltage (AC) on one side of a capacitor if 
you want to see something happening on the other side. A DC voltage (or the 
DC component of a voltage) *doesn't* change, so it will be "blocked" by the 
capacitor.

I hope this explanation will help you to sleep better :)


Vriendelijke groet,
Simon Brouwer.

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