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Q & A: Usefulness of capacitors?

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Most recent answer: 06/07/2010
What is the actual use of having a capacitor in a circuit (other than blocking DC)..I mean many say that a capacitor stores charge..If it just stores charge how would it be useful in the circuit..Does it supply that stored charge later or does it perform some other activity..I really didn't actually understand the use of it in a circuit..Kindly give me a detailed answer plz....
- Prudhvi Raj Borra (age 17)
Machilipatnam,Andhra Pradesh,India
In order to understand capacitors you really need to think of them in terms of the complete frequency domain. In the DC or zero frequency sector the two most useful purposes of capacitors are just what you described:  blocking of DC current and storage of energy.   The energy stored in a capacitor is E = CV2/2.  This is usually small and so is  not economically important. 
The main use of capacitors is in the frequency domain greater than zero.   The defining equation for the current flow is IC  = C dV/dt.  In a circuit with resistors one can make either hi-pass or Lo-pass circuits.   In conjunction with inductors, having the relationship V = L dI/dt,  one can make band selective filters, resonant circuits, etc.    As usual, I recommend a nice Wiki-article: 

(published on 06/07/2010)

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