Capacitors are passive components that can store energy in an electrical field. They are one of the most basic electrical components and can be found in almost all electrical devices. Capacitance is measured in Farads (F), but we mostly deal with smaller values such as 1 mF (millifarad) or 1 μF (microfarad).
When we have a voltage difference across a capacitor, electrons will gather and build up a negative charge in the capacitor. When the voltage difference disappears, these electrons will seep out again flow into the circuit until the capacitor is at equilibrium again. To understand how much power might be stored in a capacitor we first need a unit of charge. One Coulomb is a unit of a charge equivalent to that transported by 1 ampere in 1 second.
One Farad is equal to one coulomb when we have an applied voltage drop of 1 volt. This is expressed in the following formula:
F = C/V
In other words, a one-farad capacitor will hold 1 amp-second of electrons at 1 volt, or 0.2 amp-seconds at 5 volts.
Capacitors are often used to provide a momentary supply of energy during a brief power shortage. They can also be used to smooth out high-frequency noise in a power supply, to protect sensitive components. Many Integrated Circuit datasheets will suggest one or more capacitors of a particular value be connected across the negative and positive terminals for these reasons.
A common situation for Physical Computing is an Arduino going haywire when switching high loads, such as a motor. Since the motor draws such a high current, the power supply may momentarily drop its voltage level. This can be remedied by placing a capacitor in series with the Arduino.
Ceramic Capacitors are cheap, compact but generally come in smaller capacities (normally less than 10 uF). Ceramic capacitors have not polarity, so they can be placed without concern of direction.
Aluminium and Tantalum Electrolytic Capacitors generally provide larger capacitance, and with it larger physical packages sizes. Electrolytic Capacitors do have a polarity, so they must are placed as indicated by the markings on the package. If placed incorrectly, they will fail and sometimes even explode!