Capacitance

Definition – The storage capacity of charge of a capacitor is called the capacitance. It is denoted by C. The unit of capacitance is Farad, which is abbreviated by F. The value of capacitance depends on – the area of plate (A), distance (d) & dielectric strength of the insulating material between the plates.

Capacitance of a capacitor can be calculated as:    C ά A d   or   C = ε A d

where:- ε = permittivity of the dielectric, its value varies depending on the type of material,

A = area of the plates,                                          d = distance between the plates,

CapacitorA capacitor is a device that is used to store an electric charge. It consists of two plates (or conductors) separated by an insulator. Sometimes is referred to as a condenser.

Working principle of a capacitor – A capacitor consists of two conducting plates 1&2 which are separated by some an insulating material. This insulating material is called dielectric medium. We should remember that any conducting plate has both positive & negative charges & it remains in balanced condition due to equal amount of positive (protons) & negative (electrons) charges.

When voltage is applied across the capacitor, plate-2 which is connected with neutral gets negative charges first because electrons flow from neutral towards phase-2 & these electrons accumulates on plate-2. Now these excess electrons push back electrons of plate-1 due to repulsion between electrons (or two negative charge particles) hence now plate -1 has positive charge on it. After sometimes the amount of positive & negative charges becomes stable & this is the stored energy in capacitor in the form of electric field. Stored energy of capacitor is denoted by Q. The value of stored energy depends on applied voltage & capacitance of capacitor.

The stored energy is used later time when capacitor is discharged.

Stored Energy in capacitor

The stored charge can be calculated by – Q = C V

Where Q = Charge stored in capacitor, C = capacitance & V = voltage across capacitor.

(Note – The function of capacitor is to store energy in the form of electric field & it is used for various purposes at later time. Capacitor doesn’t generate energy.)

 

Charging current of capacitor

When voltage is applied across the capacitor, it gets charged. During charging of capacitor a current flows in the circuit & its value depends on capacitance & rate of change of voltage.

Charging current ——                      i = C x dV/dt

As the value of capacitance is constant, the value of current is directly proportional to the rate of change of voltage. It means, high current will flow in the circuit when rate of change of voltage is high & vice versa. Or we can say that when there is no rate of change of voltage, no current will flow through (like DC voltage) the capacitor.

(Note – Capacitor gets charged on ac voltage only & not on dc supply because of zero rate of change of voltage in dc supply.)

Material used in capacitors

 

Two types of material is used in making capacitors –

  • Dielectric material – The dielectric material is an insulating material which can be a paper, air, plastic, rubber, glass etc.
  • Plates – The plates are made of a conductive material like aluminum, tantalum, silver, or other metals.

Range of capacitors

There are many different kinds of capacitors available from very small capacitor to large capacitors & are used for various purposes. Range of capacitors is given below –

Abbreviation Name Weight Equivalent Farads
pF Pico farad 10-12 0.000000000001 F
nF Nano farad 10-9 0.000000001 F
µF Micro farad 10-6 0.000001 F
mF Mili farad 10-3 0.001 F
kF Kilo farad 103 1000 F

Applications of Capacitor

 

Capacitors are used for following purposes

  • Power Supply smoothing applications,
  • Audio frequency coupling uses in electronic circuits,
  • RF coupling applications in electronic circuits,
  • RF decoupling applications in electronic circuits,
  • Tuned circuit uses in electronic circuits.

 

Series & Parallel connection of Capacitor

Series connection of capacitors

When Capacitors are connected in series, the equivalent capacitance of capacitors in series is the inverse of the sum of all inverse capacitances.

I/Ceq = 1/C1 + 1/C2 …   or    Ceq   = C1 C2 / (C1 + C2 )

Parallel connections of capacitors

When capacitors are connected in parallel with each another, the total capacitance is the sum of all capacitances.

Ceq = C1+C2+…

 

 

COMPARISION BETWEEN CAPACITOR & BATTERY

CAPACITOR BATTERY
1 Energy is stored on the conducting plates  in the form of electric field Energy is stored in chemical form
2 Capacitor is able to charge & discharge at fast rate Battery is charged & discharged at a slower rate due to chemical properties
3 Charging & discharging of capacitor depends on the conducting properties of the plates Charging & discharging of capacitor depends on chemical properties of battery
4 Electric field has less density storage Chemical storage has more energy densities