- Capacitors are charge storing devices.
- Capacitance of a capacitor is the ratio of charge(Q) given and the potential (V) to which it is raised.
- The SI unit of capacitance is Farad and its CGS unit is Stat farad.
- 1 stat farad = farad
- The dimensional formula of electrical capacitance is 
- The capacitance of spherical conductor is : where R is the radius of the conductor.
- The capacitance of concentric spherical capacitor is: where 'b' is the radius of outer sphere and 'a' is the radius of inner sphere.
- The capacitance of the earth is farad = 712F
Parallel plate capacitor:
- It is the simplest capacitor where two conductors are separated by insulator/dielectric.
- The capacitance is given by: where A is the area of each plate, d is distance between them and K is the dielectric constant.
- The capacitance does not depend on material of plates, charge given to plates, potential difference between the plates.
- The value of C depends on size, shape and relative position of the two coatings of the capacitor. It also depends on the medium separating the two coatings.
- When the space between the plates is partially filled with medium of dielectric constant K and thickness t then capacitance becomes:
Combination of capacitors:
- Series combination:
- Parallel combination:
Dielectric in series:
- The capacitance of a parallel plate capacitor having a number of slabs of thickness and dielectric constants respectively in between is:
Dielectric in parallel:
- When a number of dielectric slabs of same thickness(d) and different areas of cross section having dielectric constants respectively are placed between the plates of a parallel plate capacitor, its capacitance is given by:
Energy stored in a capacitor:
Energy density: where E is the electric field intensity
Some important relations:
- Regrouping of capacitors:
- When like plates are joined:
Loss in energy =
- When unlike plates are joined:
Loss in energy =
- For charging a capacitor:
- For discharging a capacitor: