Table of Contents
What is Capacitance?
Capacitance is the ability of a system to store electric charge. A capacitor consists of two conductors separated by an insulating material (dielectric). When a voltage is applied, charge accumulates on the conductors, creating an electric field in the dielectric. The capacitance is defined as the ratio of stored charge to the applied voltage, measured in farads (F).
The capacitance depends entirely on the geometry of the conductors and the properties of the dielectric material between them. Larger conductor areas, smaller separation distances, and higher dielectric constants all increase capacitance. Understanding these relationships is essential for designing capacitors, transmission lines, sensors, and many other electrical components.
Capacitor Formulas
Where ε0 = 8.854 x 10-12 F/m is the permittivity of free space, A is plate area, d is separation, L is length, a is inner radius, and b is outer radius.
Dielectric Materials
| Material | εr | Breakdown (kV/mm) |
|---|---|---|
| Vacuum/Air | 1.0 | 3 |
| Paper | 3.5 | 14 |
| Glass | 4-10 | 10-40 |
| Ceramic (X7R) | 2000-4000 | 10-25 |
| Barium Titanate | 1200-10000 | 5 |
Frequently Asked Questions
Why is a farad such a large unit?
One farad stores one coulomb of charge at one volt, which is an enormous amount of charge storage. A parallel plate capacitor with 1F capacitance using air dielectric would need plates separated by 1mm with an area of about 113 million square meters (roughly the area of Paris). Practical capacitors range from picofarads (pF) to millifarads (mF). Supercapacitors achieve farads by using nanoscale electrode separations and enormous surface areas.
How does the dielectric increase capacitance?
A dielectric material between the plates reduces the effective electric field by aligning its molecular dipoles opposite to the applied field. This reduces the voltage for a given charge, thereby increasing the charge-to-voltage ratio (capacitance) by the factor epsilon_r. Additionally, dielectrics allow closer plate spacing before breakdown occurs, further increasing capacitance. Modern ceramic capacitors use materials with epsilon_r of several thousand.
What limits maximum capacitance?
The maximum capacitance is limited by dielectric breakdown voltage, the available area, and the minimum achievable dielectric thickness. Making the dielectric thinner increases capacitance but reduces the maximum voltage. For energy storage applications, the energy stored equals 1/2 CV squared, so the practical limit involves balancing capacitance and voltage rating. Supercapacitors achieve extremely high capacitance using nanoporous carbon electrodes with molecular-scale gaps.