capacitor charging and discharging

I understand how capacitpr block dc but how capacitor let go ac??? I understand by mathematical expression but what actually happen there??? Can anybody explain briefly???

See a capacitor as a battery.
Even during dc there is a one time charge as a result there is a current pulse for a moment.
During AC the positive and negative is changing all the time as a result there is a voltage building up and down fast and as a result there is a current.

It is called equilibrium. When there is a changing electric current we call that AC. A discharged cap charges up from zero, then it reaches equilibrium or full then it stops, that is a changing current and it is referred to as AC, it's really that simple. Reverse for when the cap discharges.

The AC current doesn't actually flow _through_ the capacitor. That's an illusion.

Imagine a jar whose interior is divided by a rubber membrane -- divided all the way to the jar lid (which is screwed on to the top of the jar). Now, imagine that there are two holes in the jar lid, such that one hole is on one side of the membrane and the other hole is on the other side. If you suck the air out of one side of the jar (via the hole on that side of the jar), air will flow into the other hole and cause the membrane to extend into the side of the jar where the air is being removed. The appearance is that air is flowing into one hole and out the other, but in reality, the air coming into the jar never reaches the other hole, but is, instead, _stored_ in the divided space of the jar.

A capacitor is composed of two "plates" separated by an insulator. The insulator is like the membrane and the plates are like the two sides of the jar interior that are separated by the membrane. The air is like the electrons that flow on and off the plates when an AC voltage is applied to the capacitor. AC is like someone trying to breath through the jar. When they inhale, air goes into the other hole (the hole that doesn't have this sorry person's lips wrapped around it), and when they exhale, air flows out of the other hole. Soon, the person turns blue -- that's know as "skin effect"

Hi Steve , yes i agree that the current does not flow through the capacitor. But it really helps in order to explain the process to some body that does have problems with the basic.

My son i explained by use of a plasma boll , the glass is insulating and still the spark is discharging.

hi Dixit ,
capacitor has two conductive plates when we apply dc voltage whatever +ve charges of dielectric material attract to plate connected to -ve terminal & -ve charge of dielectric attract to plate connected to +ve terminal.And as a dc supply it has zero frequency it blocks DC.So plates potential not going to change.
If suppose now you are increased frequency it will become ac supply then continiously plates are changing their potential connections now at positive cycle -ve charges attract & at -ve cycle +ve charge attract, so alternately charge will flow or transfers from one plate to other plate.

Hi Abhijeet,
Very good ,easily understandable theoratical explanation. Keep up good work.

Thanks

Hello Vinod,
It feels good to me ,that you understood my explanation.

Yes but technically misleading. If you apply DC to the plates and the plates are not, "Charged Up," current will flow in the external circuit until the cap is charged. The current will start at max then go to zero. That is the definition of AC, while one should not assume AC means an alternating sinusoidal. Also by external I mean no current flows in the space between the plates, although under different circumstances that could happen.

So you can take the meaning that you understand and that with an applied DC, current can flow, even constant current can flow ( with an applied ramp (AC) voltage).

With AC voltage, the current will always flow because the voltage is changing and therefore the current must change according. The cap has an AC impedance, it has a infinite impedance at zero hertz which is DC, which is why when DC is applied the current will eventually go to zero.

That is the nice thing about it all. If you do not understand a single theory you can try to explain by use of others but you are never correct.

By the way a other way in order to explain:
See it as water and you have a vessel filled with water and in the middle there is a rubber membrane. If you put pressure on one side there is water coming out on the other side but there is no flow.

Yes, these are models. A model is a tool used in science to visualize or conceptualize a concept to make it more accessible to our feeble minds ;)

A model is always doomed to be an imprecise analogue of the real thing. But, if it overlaps reality just enough, then it is useful in that it facilitates either application or iteration to a more accurate model which, then, may facilitate application.

BTW: there _is_ "flow" but not "flow through". Water does flow in and out of the vessel, but it doesn't flow _through_ the vessel.