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Encyclopedia of Electrical Engineering
HOME Q & A TABLE OF CONTENTS
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Encyclopedia of Electrical Engineering
HOME Q & A TABLE OF CONTENTS

Capacitance

Capacitance is a measure of a capacitor's ability to store charge on its plates, in other words, its storage capacity.
The higher the capacitance of a capacitor, the greater is the amount of charge stored on the plates for the same applied voltage.
capacitor charging circuit
Fig.no.1: Capacitor charging circuit.
Thus far, we have examined only isolated positive and negative spherical charges, but the description can be extended to charged surfaces of any shape and size. In Fig.no.1 for example, two parallel plates of a material such as aluminum (the most commonly used metal in the construction of capacitors) have been connected through a switch and a resistor to a battery. If the parallel plates are initially uncharged and the switch is left open, no net positive or negative charge exists on either plate. The instant the switch is closed, however, electrons are drawn from the upper plate through the resistor to the positive terminal of the battery. There will be a surge of current at first, limited in magnitude by the resistance present. The level of flow then declines, as will be demonstrated in the sections to follow. This action creates a net positive charge on the top plate. Electrons are being repelled by the negative terminal through the lower conductor to the bottom plate at the same rate they are being drawn to the positive terminal. This transfer of electrons continues until the potential difference across the parallel plates is exactly equal to the battery voltage. The final result is a net positive charge on the top plate and a negative charge on the bottom plate, very similar in many respects to the two isolated charges in Fig.no.1. Before continuing, it is important to note that the entire flow of charge is through the battery and resistor not through the region between the plates. In every sense of the definition, there is an open circuit between the plates of the capacitor.
This element, constructed simply of two conducting surfaces separated by the air gap, is called a capacitor.
The unit of measure applied to capacitors is the farad (F), named after an English scientist, Michael Faraday.
In particular, a capacitor has a capacitance of 1 F if 1 C of charge ($6.242 \times 10^{18}$ electrons) is deposited on the plates by a potential difference of 1 V across its plates.
The farad, however, is generally too large a measure of capacitance for most practical applications, so the microfarad ($10^{-6}$) or picofarad ($10^{-12}$) are more commonly encountered. The relationship connecting the applied voltage, the charge on the plates, and the capacitance level is defined by the following equation:
$$\bbox[5px,border:1px solid red] {\color{blue}{ C = {Q \over V}}}$$Eq.(1)
$$\bbox[5px,border:1px solid red] {\color{blue}{ Q = CV}}$$Eq.(2)
where C is capacitance, Q is charges, and V is potential difference.