Capacitors
A capacitor is a device that can store electrical energy by storing charge. In a capacitor equal amounts of positive and negative charges are stored on two separate, closely spaced conductors. One arrangement, called the parallel plate capacitor consists of two conducting plates separated by a gap. Electrolytic capacitors must be connected in a specific polarity ( + of capacitor to + of battery) or they will be ruined.
The presence of the charges produces a voltage across the capacitor. The greater the charge Q, the greater the voltage V; in other words, the charge is proportional to the voltage. This relationship can be expressed by
Q = CV
The constant of proportionality is C, the capacitance of the capacitor. The capacitance C depends only on the physical attributes of the capacitor, such as the area of the plates, the distance separating them, the shape of the capacitor, and the characteristics of the material that fills the gap. This material is nonconducting and could be air or a material that enhances the capacitance, called a dielectric.
Units
Conversion table:
1 millifarad (mF) = 0.001 F or (10-3 F)
1 microfarad (µF) = 0.000001 F or (10-6 F)
1 nanofarad (nF) = 0.000000001 F or (10-9 F)
1 picofarad (pF) = 0.000000000001 F or (10-12 F)The unit of capacitance is a farad (F). One farad is a pretty large capacitance: the expression Q = CV means that a voltage of one volt can produce a charge of one coulomb (6.25 x 1018 or 6250 million billion electrons) in a 1F capacitor! Capacitors commonly used in electronic circuits have capacitances ranging from a few millifarads to micro- or pico farads.
Capacitors and Batteries
A capacitor is different from a battery. In a battery (NL), a chemical reaction generates charge. A capacitor does not have a chemical reaction occuring inside it that generates charge. It merely stores charge. One has to charge it using a source of voltage such as a battery. When a capacitor is connected to a battery, positive and negative charges accumulate on the plates. It can be discharged by connecting it to a circuit with a resistor, such as a light bulb. The charge flows through the bulb, lighting it up for a while. When all the charge on the capacitor is depleted, no current flows, and the capacitor has to be recharged again.
Uses of capacitors
Capacitors can be used to store charge. A more frequent modern use is as vital elements in microelectronic circuitry. Capacitors are used to tune radio and TV transmitters and receivers. They are also used in the memory banks of computers and other digital circuitry, where they provide on-off information. If they are charged, a voltage is present and they are counted as "on". If they are discharged, the voltage is zero and they are "off".
