Students Guide to the Capacitor
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 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).
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.
The symbolic representation of a capacitor in an electrical circuit is that of two parallel lines separated by a small gap with a positive plus (+) sign above the top plate if the capacitor is of a polarised type. Like resistors, capacitors can be connected together in several ways either in a series, parallel or a combination of the two. In a parallel combination the potential difference across each capacitor is the same and equal to the source voltage, V and each capacitor stores a charge. The total stored charge, (QT) will be equal to the sum of all the individual charges. As charge Q = CV (from above) and the voltage across a parallel combination is the same the total capacitance will be the sum of the individual capacitances so C total = C1 + C2 + C3 + C4 etc. By connecting together capacitors in parallel a much high capacitance value can be obtained from small individual capacitors.
For a series combination of capacitors, the charging current flowing through the capacitors is the same so the magnitude of the charge is the same on all the plates. Knowing that V = Q/C dividing through by Q will give the total capacitance as the reciprocal of all the individual capacitances added together so 1/CT = 1/C1 + 1/C2 + 1/C + 1/C4 etc. By connecting together capacitors in series the equivalent capacitance is less than that of the smallest value capacitor.
I hope that this short students/beginners guide to the capacitor has been helpful to anyone who is new to the world of electronics. To learn more about capacitors, the movement of charge and how to connect capacitors together in a circuit, you can visit my Capacitors section to read more.