Capacitance

1.Capacitance : Capacitor is a passive element that can store electric charges.The capacitor is made of two close conductors ( usually plates ) that are separated by dielectric materials.The plates accumulate electric charge when connected to power source.The capacitor sometimes referred to as a condenser.The capacitor is a component which has the ability or "capacity" to store energy in the form of an electrical charge producing a potential difference (static Voltage) across it's plates.

                The property of capacitor to store charge on it's plates in the form of an electrostatic field is called the capacitance of the capacitor. Not only that ,but capacitance is also the property of a capacitor which Capacitance of a capacitor is given by ;

                  C= Q/V

Where Q = charge stored 

V= Potential Difference between two plates.

Standard unit of capacitance is F (Farad ). Smaller units of capacitance may be  uF,nF,pF etc. A capacitor consists of  two or more parallel conductive (metal) plates which are not connected or touching each other . The plates are electrically separated either by air or by some form of a good insulating material such as waxed paper,mica , ceramic, plastic or some form of a liquid gel as used in electrolytic capacitor. The insulating layer between capacitors plates is commonly called the dielectric.Due to this insulating layer ,DC current cannot 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 conducting Metal plates of a capacitor can be 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.

The parallel plate capacitor is the simplest form of capacitor.It can be constructed using two metal or metalized foil plates at a distance parallel to each other Capacitance of parallel plate capacitor is given by ;

           C=eA/d

 Where e = permittivity of the medium

               A= Overlapping area between two plates

               d= Separation between two plates.

Since capacitors store the energy of the electrons in the form of an electrical charge on the plates,the larger the plates and / or smaller their separation the greater will be the charge that the Capacitor holds for any given voltage across its plates . In other words, larger plates and smaller distance gives more Capacitance.

Role of Capacitor in Circuits: 

i. Capacitors are used to block while allowing AC to pass because in a dc circuit, the Capacitor once charged with the applied voltage acts as a open switch.

ii. In power supply, it acts to filter the output so there is little ripple. Moreover, it acts as a high pass filter, blocking lower frequencies and passing higher ones ( high reactance at low frequency and low reactance at high frequency)

iii. In combination with inductors , Capacitor creates oscillating circuit,used for purposes including tone generation and radio tuning. In power circuit it ca correct the power factor so that the current and voltage are in phase because it draws leading current to compensate the lagging current drawn by inductive load .

iv. Capacitors are also used in power systems are reactive compensators to improve Voltage in the system in case of heavy load in the transmission lines . These compensators are either connected in series or parallel.

v. In single phase induction motors capacitor creates phase shift so that they can be started. Capacitors are also used to store electricity to fill in for momentary fluctuations in a DC supply.