Capacitance and Capacitors: Farads, Dielectrics & Current

What Is a Capacitor

A capacitor is composed of two insulated conductors that carry equal and opposite charges. It stores electric energy in the electrostatic field between its conductors.

Capacitance and Storage Units

Capacitance (C) is the property that relates the charge stored (Q) to the potential difference (V) between the plates: Q = C · V. Different capacitors can acquire different charges under the same potential difference because they have different capacitances. Capacitance determines how much electric charge a device can store for a given voltage.

Farad and Its Multiples

A farad (F) is the SI unit of capacitance: a capacitor has a capacitance of one farad if it stores one coulomb (1 C) of charge when the potential difference is one volt (1 V). Common multiples and submultiples include microfarad (µF), nanofarad (nF), picofarad (pF), and millifarad (mF).

Capacitors in Circuits

When a switch is closed and current begins to flow into a circuit containing a capacitor, the capacitor charges. As the capacitor charges the charging current decreases; once the capacitor reaches its maximum (steady-state) charge, the current in the circuit ceases (neglecting leakage or other circuit elements).

Parallel-Plate Capacitor with Dielectric

A parallel-plate capacitor consists of two parallel plates of equal area separated by a distance. One plate carries charge +Q and the other -Q, with electric potentials V1 and V2 respectively. Placing a dielectric material between the plates reduces the effective electric field and increases the capacitance.

Capacitor Construction and Dielectrics

Capacitors are commonly constructed from flat metal strips or sheets separated by a dielectric material. Typical dielectrics include waxed paper, plastic film, ceramic, or other insulating materials. The dielectric serves to increase capacitance and prevent electrical contact between plates.

Capacitors in Series and Parallel

Capacitors can be connected in series or in parallel; their equivalent capacitance depends on the configuration:

• Series (two capacitors): C_eq = (C1 × C2) / (C1 + C2)
• Series (n equal capacitors): C_eq = C / n
• Parallel: C_eq = C1 + C2 + ... + Cn — the total capacitance equals the sum of individual capacitances because the effective plate area adds.

Electric Current and Its Types

Electric current is the flow of electric charge (often a stream of electrons) through a material. Some materials, called conductors, have free electrons that move easily from one atom to another.

Types of current that commonly exist include:

• DC (Direct Current): Current that flows in one direction.
• AC (Alternating Current): Current that periodically reverses its direction with a certain frequency. See related movement description here.