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A capacitor (also called condenser, which is the older term) is an electronic device that stores electric energy. It is similar to a battery, but is smaller, lightweight and charges up much quicker. Capacitors are used in many electronic devices today, and can be made out of many different types of material. The Leyden jar was one of the first capacitors invented.
Capacitors are usually made with two metal plates that are on top of each other and near each other, but that do not actually touch. When powered, they allow energy to be stored inside an electrical field. Because the plates need a lot of area to store even a small amount of charge, the plates are usually rolled up into some other shape, such as a cylinder. Sometimes, other shapes of capacitors are used for special purposes. A capacitor-like effect can also result just from two conductors being close to each other, whether you want it to exist or not.
All capacitors have two connections, or leads. Most kinds of capacitors can be changed around easily by someone who has basic skills in electronics. However, one of the more powerful types - the electrolytic capacitor - must be used the correct way, or they can explode violently.
While capacitors do have similarities to batteries in that they can store energy - as mentioned earlier - capacitors can also release all their stored energy very quickly, even faster than a second.
A charging and discharging defibrillator for heart attacks is a good example of how a capacitor gradually charges up, until it can not be filled any more, and then quickly discharging its stored power to another area that needs it to function.
Supercapacitors hold more of a charge than regular capacitors and will discharge faster. They won't be found in a TV or computer.
Polystyrene film capacitors
This type of capacitor is not for use in high frequency circuits, being made with a coil inside. They are used in filter circuits or timing circuits which run at several hundred KHz or less.
Electrolytic capacitors use a conducting surface inside a liquid electrolyte. They have polarity and so they have to be attached correctly. There are two leads; one will have a + and the other a -. This means one lead is positive and one is negative. There are two different styles: axial, where the leads are connected to each end, and radial, where the leads are connected to one end. The rating of an electrolytic capacitor is easy to find because they are printed with capacitance and voltage rating.
Since the voltage rating can be low, it is important to check that the electrolytic capacitor isn't overcharged. Capacitors can be separated from a battery, then connected in series. Because the capacitor is polarized, the positive terminal must be connected to a negative terminal. This creates correct polarity through the electrical circuit and prevents breakdown. 
- Ulaby, Fawwaz T. (1999). Fundamentals of Applied Electromagnetics (1999 ed.). Upper Saddle River, New Jersey: Prentice-Hall. p. 168. .
- Jennifer Marcus (March 15, 2012). "Researchers develop graphene supercapacitor holding promise for portable electronics". http://newsroom.ucla.edu/portal/ucla/ucla-researchers-develop-new-graphene-230478.aspx. Retrieved 29 May 2012.
- "CAPACITORS AND DIELECTRICS". http://teacher.nsrl.rochester.edu:8080/phy122/lecture_notes/chapter27/Chapter27.html. Retrieved 31 May 2012.
- Introduction To Capacitors Capsite 2009