A device consisting of two electrical conductors that are separated by a distance and used to store electrical charges is called a capacitor. The space between the conductors is either a vacuum or an insulating material, called a dielectric. Capacitors have many applications, ranging from filtering static from radio reception to energy storage in heart defibrillators.

When the conductors are two identical parallel plates, it is called a parallel plate capacitor. When battery terminals are connected to an initially uncharged capacitor, the battery potential moves a charge of magnitude Q from the positive plate to the negative plate. The capacitor remains neutral overall, but charges +Q and −Q reside on opposite plates. The magnitude of the electrical field in the space between the parallel plates is directly proportional to the surface charge density on one plate. Since the surface charge density is the charge per unit surface area, the magnitude of the electric field is directly proportional to the charge.

The capacitance of a capacitor is defined as the ratio of the maximum charge that can be stored in a capacitor to the applied voltage across its plates. The SI unit of capacitance is the farad (F), named after Michael Faraday. Since capacitance is the charge per unit voltage, one farad is one coulomb per one volt.

For a parallel plate capacitor, increasing the area of the plates increases the amount of charge that can be stored, thereby increasing the capacitance for a larger plate area. Similarly, the closer the plates are together, the greater the attraction of the opposite charges on them. Therefore, capacitance is greater for a smaller distance between the plates.