If you're wondering what capacitors are actually used for in real-life electronic circuits, the following describes the most common ways that capacitors are used:
Storing energy: Once charged, a capacitor has the ability to store a lot of energy and discharge it when needed. One of the most familiar uses of this ability is in a camera flash. Other examples are in devices such as alarm clocks that need to stay powered up even when household power is disrupted for a few moments.
Timing circuits: Many circuits depend on capacitors along with resistors to provide timing intervals. For example, you may use a resistor and capacitor together along with a transistor to create a circuit that flashes an LED on and off.
Stabilizing shaky direct current: Many electronic devices run on direct current but derive their power from household AC. These devices use a power-supply circuit that converts the AC to DC. An important part of this circuit is a capacitor that creates steady DC voltage from the constantly changing voltage of an AC circuit.
Blocking DC while passing AC: Sometimes you need to prevent direct current from flowing while allowing alternating current to pass. A capacitor can do this. The basic trick is to choose a capacitor that will fully charge and discharge within the alternating current cycle.
Remember that as the capacitor is charging and discharging, it allows current to flow. So as long as you can keep the capacitor charging and discharging, it will pass the current.
Filtering certain frequencies: Capacitors are often used in audio or radio circuits to select certain frequencies. For example, a variable capacitor is often used in a radio circuit to tune the circuit to a certain frequency.
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Source:http://www.dummies.com/how-to/content/electronics-components-put-capacitors-to-work.html
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