A capacitor which used to be called a condenser, is a passive electrical component that is used to “store electricity” in the form of an electrical charge homepage. There are many different kinds of capacitors available from very small capacitor beads used in resonance circuits to large power factor correction capacitors, but they all do the same thing, they store charge.
The simplest kind of capacitor has two parallel conductive plates separated by a good insulating material called the dielectric. Due to this insulating layer, DC current can not flow through the capacitor as it blocks it allowing instead a voltage to be present across the plates in the form of an electrical charge. These conductive plates can be either circular, rectangular or cylindrical in shape with the dielectric insulating layer being air, waxed paper, plastic or some form of a liquid gel as used in electrolytic capacitors.
There are two types of electrical charge, positive charge in the form of Protons and negative charge in the form of Electrons. When a voltage is placed across a capacitor the positive (+ve) charge quickly accumulates on one plate while a corresponding negative (-ve) charge accumulates on the other plate and for every particle of +ve charge that arrives at one plate a charge of the same sign will depart from the -ve plate. Then the plates remain charge neutral as a potential difference due to this charge is established between the two plates. The amount of potential difference present across the capacitor depends upon how much charge was deposited onto the plates by the work being done by the source voltage and also by how much capacitance the capacitor has.
Capacitance is the electrical property of a capacitor and is the measure of a capacitors ability to store an electrical charge onto its two plates. If a voltage of (V) volts is connected across the capacitors two plates a positive electrical charge (Q) in coulombs will be present on one plate a negative electrical charge on the other. Then the capacitor will have a capacitance value equal to the amount of charge divided by the voltage across it giving us the equation for capacitance of: (C = QV) with the value of the capacitance in Farads, (F). However, the Farad on its own is an extremely large unit so sub-units of the Farad are commonly used such as micro-farads (uF), nano-farads (nF) and pico-farads (pF) to denote a capacitors value.
Although the capacitance, (C) of a capacitor is equal to the ratio of charge per plate to the applied voltage, it also depends on the physical size and distance between the two conductive plates. For example, if the two plates where larger or multiple plates where used then there would be more surface area for the charge to accumulate on giving a higher value of capacitance. Likewise, if the distance, (d) between the two plates is closer or a different type of dielectric is used, again more charge resulting in a higher capacitance. Then the capacitance of a capacitor can also be expressed in terms of its physical size, distance between the two plates (spacing) and type of dielectric used.
An ideal capacitor would have an extremely high dielectric resistance and zero plate resistance. This would result in the charge across the plates remaining constant indefinitely once the source voltage was removed. However, real capacitors have some leakage current which pass through the dielectric between the two plates. The amount of leakage current that a capacitor has depends upon the leakage resistance of the dielectric medium being used. Also an ideal capacitor does not lose any of the energy supplied by the source voltage as it is stored in the form of an electric field between the two plates but in real capacitors power is lost due to this leakage current and the resistance value of the plates.