A capacitor is a simple passive component that can store electric charge on its plates when connected to a voltage source.

　　A capacitor is a component that possesses the ability to store energy, or “capacitance.” By accumulating electric charge on its plates, it generates a potential difference (a static voltage), much like a small rechargeable battery.

　　There are many different types of capacitors, ranging from very small ones used in resonant circuits to large power-factor-correction capacitors; however, they all perform the same function: storing electrical charge.

　　In its simplest form, a capacitor consists of two or more parallel conductive (metallic) plates that are not connected to each other or in direct contact; instead, they are electrically insulated from one another by air or by some high‑quality insulating material—such as waxed paper, mica, ceramic, plastic, or a liquid gel—similar to the insulator used in electrolytic capacitors. The insulating layer between the capacitor plates is commonly referred to as the dielectric.

　　Typical capacitor

　　Due to this insulating layer, direct current cannot pass through the capacitor—it is blocked—while allowing voltage to exist on the circuit board in the form of electric charge.

　　Conductive metal‑plate capacitors may be square, circular, or rectangular, and can also be cylindrical or spherical, exhibiting the typical shape, dimensions, and structure of parallel‑plate capacitors, depending on their application and rated voltage.

　　In a DC circuit, a capacitor charges to the supply voltage but blocks current flow because its dielectric is nonconductive and essentially an insulator. However, when a capacitor is connected to an AC circuit, current appears to pass directly through it with little or no resistance.

　　There are two types of electric charge: positive charge, carried by protons, and negative charge, carried by electrons. When a DC voltage is applied across a capacitor, positive (+ve) charge rapidly accumulates on one plate, while the corresponding opposite negative (–ve) charge accumulates on the other plate. For every +ve charge particle that arrives at one plate, an equal amount of like‑sign charge departs from the –ve plate.

　　Then the plates remain electrically neutral, and a potential difference arises between the charges on the two plates. Once the capacitor reaches its steady-state condition, no current can flow through the capacitor itself or through the surrounding circuit, owing to the insulating properties of the dielectric separating the plates.

　　The flow of electrons onto the capacitor’s upper plate is referred to as the capacitor charging current; this current continues to flow until the voltage across the two plates—and thus across the capacitor—equals the applied voltage Vc. At this point, the capacitor is said to be “fully charged.”

　　When the battery plate is fully discharged (initial condition), the charging current reaches its maximum magnitude, and it drops to zero when the capacitor plate is charged to a potential difference equal to the source voltage.

　　The potential difference across a capacitor depends on the amount of charge stored on it. This is determined by the supply voltage and the capacitor’s capacitance, as will be explained below.

　　A parallel-plate capacitor is the simplest type of capacitor. It can be constructed by arranging two metal plates or metal foils at a fixed, parallel separation; its capacitance, measured in farads, is determined by the surface area of the conductive plates and the distance between them. Altering either of these parameters changes the capacitance, which forms the basis for the operation of variable capacitors.

　　Additionally, since a capacitor stores electrical energy in the form of electric charge, the larger the plates and/or the smaller the separation between them, the greater the charge the capacitor can hold for a given voltage across its plates. In other words, larger plates and a smaller gap result in higher capacitance.

　　By applying a voltage across the capacitor and measuring the charge on its plates, the ratio of the charge Q to the voltage V yields the capacitor’s capacitance, giving: C = Q / V. This equation can also be rearranged to produce the familiar formula for the charge on the plates: Q = C × V.

　　Although we have stated that charge is stored on the capacitor’s plates, more precisely, the energy associated with the charge is stored in the “electrostatic field” between the two plates. When current flows into the capacitor, it charges, and the electrostatic field strengthens as more energy is stored between the plates.

　　Similarly, when current flows out of the capacitor during discharge, as energy is removed from the plates, the potential difference between the two plates decreases, and the electrostatic field weakens.

　　The property of a capacitor to store electric charge on its plates in the form of an electrostatic field is called its capacitance. Moreover, capacitance is a characteristic of the capacitor that enables it to resist changes in voltage.

　　Capacitance of a parallel-plate capacitor

　　The capacitance of a parallel-plate capacitor is directly proportional to the plate area, A, expressed in square meters, and inversely proportional to the distance, or separation, d between the two plates, which is the thickness of the dielectric material separating the conductive plates, also expressed in meters.

　　The generalized formula for the capacitance of a parallel-plate capacitor is: C = ε(A / d), where ε denotes the absolute permittivity of the dielectric material used. The permittivity of free space, ε₀—also referred to as the “permittivity of free space”—has a value of 8.84 × 10⁻¹² farads per meter.

　　To simplify the mathematics, the permittivity of free space, ε₀, can be expressed as 1/(4π × 9 × 10⁹), or alternatively given in units of picofarads per meter (pF/m): 8.84 represents the value for the available space. Note that the resulting capacitance will be in picofarads rather than farads.

　　Typically, the conductive plates of a capacitor are separated by some insulating material or gel, rather than a perfect vacuum. When calculating a capacitor’s capacitance, we can approximate the dielectric constant of air—especially that of dry air—since it is very close to the value for a vacuum.