It is always good to save for a rainy day: be it money, water (although you don’t need water on a rainy day) or even electricity. But how you do store electricity? Well it can be stored in batteries but I am talking about a very small storage device, which is known as a capacitor.
The capacitor is an electronic device which has the capacity to store charge. Basically, a capacitor is nothing more than two conducting surfaces at a distance from each other with a dielectric material (or insulating material) in between. The charge is stored in the form of electron deficiency on one surface as compared to the excess of electrons on the other, thus this imbalanced electron creates a potential difference across the surfaces.
The characteristic of electrical energy storage in a capacitor is known as capacitance and is measured in terms of Farads (named after the British chemist and physicist, Michael Faraday). One farad of capacitance is defined as that amount of capacitance which causes one coulomb of charge to create a potential difference of one volt across the conducting surfaces of the capacitor.
For a given capacitor, the ratio of charge on the plates to the potential difference developed across them always remains constant. Whenever a capacitor is connected across a source of emf, its conducting surfaces develop a positive charge on one plate and a negative charge on the other and this in turn creates an emf in proportion to that charge. There is no flow of current through a capacitor for the reason that it does not have a conductor connecting both plates; there is an insulator or a dielectric between them.
If you charge a capacitor and leave it like that, theoretically, it will never discharge since there is no way for charge to move between the plates. But practically speaking this is not so and there is always a small leakage current due to imperfect dielectric material properties. Still, a good quality capacitor can hold the charge for several weeks if not months before losing its charge.
There are several types of capacitors depending on the material from which they are manufactured and we will study this in a different article. Capacitors can also be connected in series and parallel just like resistors but their combination follows different rules for total capacitance which will also be studied separately.
Since current does not flow through a capacitor you might think that it offers infinite resistance to current. Well this may be true of direct current but for alternating current the resistance offered is finite and is known as capacitive reactance, though its units are same as resistance - Ohms.
Capacitive reactance is inversely proportional to the frequency of the alternating current, as well as capacitance and the mathematical relation governing this is as follows:
Capacitive reactance: X =1/(2πfC)
Where f is the frequency and C is capacitance.