A capacitor stores electricity in a circuit and discharges it when needed.  The main parts of a capacitor are two metal plates and an insulator called dielectric between them.

 Capacitance :

 Capacitance is the ability of a capacitor to store electric charges.  It is the amount of charge required to create a unit potential difference between the plates.

Series and parallel connection of capacitors

 Series Combination :
 Three capacitors with capacitances C₁, C2 and C3 are connected in series one behind the other.

 Let V₁, V2, and V3 be the potential drop across the three capacitors, V be the total applied voltage, and C be the total capacitance.

 When connected in this way the charge Q flowing through them will be the same

 In series connection,
V = V1 + V2 + V3

 Otherwise

 Q/C = Q/C₁ + Q/C₂ + Q/C3

 (V = Q/c in capacitor)

 Taking Q as a constant,

 1/C = 1/C₁ + 1/C2 + 1/C3



Parallel Combination:
If three capacitors of capacitances C₁, C2 and C3 are connected in parallel with voltage V then V will be constant across all capacitors.  But the charge Q is divided across each of the capacitors Q1, Q₂ and Q3.

 Q = Q1+Q2+Q3

 = C1V + C2V+ C3V

 =V(C1 +C₂ + C3)

 If the total capacitance is V,

 Q1+Q2+Q3 = V × C

 Total capacitance = sum of capacitances.

 that is,
 C = C + C₂ + C


 Note :

In a series combination of capacitors, the charge across all the capacitors is the same and the potential difference is different.  In parallel combination the potential difference across each capacitor is the same and the charge is different.