CAPACITOR

INTRODUCTION-      

                                                    A    capacitor is a passive element designed to store energy in its electric field.

There are many different kinds of capacitors available from very small capacitor beads used in the resonance circuit to large power factor correction capacitors, but they all do the same things, they store charge.

 In its basic form-

                                   A capacitor consists of two or more parallel conductive (metal) plates separated by an insulator(or dielectric).
In many practical applications, the plates may be aluminium foil while the dielectric may be air, ceramic, paper,  or mica.

                                                                  fig -1 

                                 A CAPACITOR WITH  APPLIED VOLTAGE  V

                                         

In fig- 1, when a voltage source V   is connected to the capacitor, the source on the deposits a positive charge Q on one plate and a negative charge -Q on the other. the capacitor is said to store the electric charge. the amount of charge stored, represented by Q, is directly proportional to applied voltage V so that 

                                                                       Q  =      CV                                                                                                                         

where C, the constant of proportional, is known as the capacitance of the capacitor.

Capacitance-

                         capacitance is the electrical property of a capacitor and is the measure of capacitors ability to store an electrical charge onto its two plates. 

 Unit of capacitance - Farad (abbreviated to F ) named after the British physicist Michael Faraday.

capacitance is defined as being that a capacitor has the capacitance of one Farad

when a charge of one Coulomb is stored on the plates by a voltage of one volt.

1 farad = 1 coulomb/volts

Note that capacitance, C is always positive in value and has no negative units. However, the farad is a large unit of measurement to use on its own so sub-multiples of the farad are generally used such as micro-farad, nano-farad and pico- farad. 

for example -

                               Standard Units of capacitance

Microfarad = 1/10^6 F

Nano farad =  1/10^9 F

Pico farad   =   1/10^12 F                                           

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.
The conductive metal plates of a capacitor can be either square, circular or rectangular, or they can be of a cylindrical or spherical shape with the general shape, size and construction of a parallel plate capacitor depending on its application and voltage rating.                                                                 
When used in a direct current or DC circuit, a capacitor charges up to its supply voltage but blocks the flow of current through it because the dielectric of a capacitor is non-conductive and basically an insulator. However, when a capacitor is connected to an alternating current or AC circuit, the flow of the current appears to pass straight through the capacitor with little or no resistance.                                                                     
There are two types of electrical charge, a positive charge in the form of Protons and a negative charge in the form of Electrons. When a DC voltage is placed across a capacitor, the positive (+ve) charge quickly accumulates on one plate while a corresponding and opposite negative (-ve) charge accumulates on the other plate. 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 and a potential difference due to this charge is established between the two plates. Once the capacitor reaches its steady state condition an electrical current is unable to flow through the capacitor itself and around the circuit due to the insulating properties of the dielectric used to separate the plates.                                                           
The flow of electrons onto the plates is known as the capacitors Charging Current which continues to flow until the voltage across both plates (and hence the capacitor) is equal to the applied voltage Vc. At this point the capacitor is said to be “fully charged” with electrons.                               
The strength or rate of this charging current is at its maximum value when the plates are fully discharged (initial condition) and slowly reduces in value to zero as the plates charge up to a potential difference across the capacitors plates equal to the source voltage.                                         
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 and this is illustrated below.

Types of capacitor-

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