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What is capacitance?

23 Oct 2020 | Electrical
What is capacitance?

Capacitance is the ability of a component or circuit to collect and store energy in the form of an electrical charge.

Capacitors are energy-storing devices available in many sizes and shapes. They consist of two plates of conducting material (usually a thin metal) sandwiched between an insulator made of ceramic, film, glass or other materials, even air.

The insulator is also known as a dielectric, and it boosts a capacitor's charging capacity. Capacitors are sometimes called condensers in the automotive, marine and aviation industries.

The internal plates are wired to two external terminals, which sometimes are long and thin and can resemble tiny metallic antennae or legs. These terminals can be plugged into a circuit.

Capacitors and batteries both store energy. While batteries release energy gradually, capacitors discharge it quickly.

How does a capacitor work?

A capacitor collects energy (voltage) as current flows through an electrical circuit. Both plates hold equal charges, and as the positive plate collects a charge, an equal charge flows off the negative plate.

When the circuit is switched off, a capacitor retains the energy it has gathered, though slight leakage usually occurs.

A variety of capacitors (shown in color) in circuit board.
A variety of capacitors (shown in color) in circuit board.

Capacitance is expressed as the ratio of the electric charge on each conductor to the potential difference (i.e., voltage) between them.

The capacitance value of a capacitor is measured in farads (F), units named for English physicist Michael Faraday (1791–1867).

A farad is a large quantity of capacitance. Most household electrical devices include capacitors that produce only a fraction of a farad, often a thousandth of a farad (or microfarad, µF) or as small as a picofarad (a trillionth, pF).

Supercapacitors, meanwhile, can store very large electrical charges of thousands of farads.

How to increase capacitance

Capacitance can be increased when:

  • A capacitor's plates (conductors) are positioned closer together.
  • Larger plates offer more surface area.
  • The dielectric is the best possible insulator for the application.
Capacitors come in various shapes.
Capacitors come in various shapes.

In electrical circuits, capacitors are frequently used to block direct current (dc) while permitting alternating current (ac) to flow.

Some digital multimeters offer a capacitance measurement function so technicians can:

  • Identify an unknown or unlabeled capacitor.
  • Detect open or shorted capacitors.
  • Measure capacitors directly and display their value.

Reference: Digital Multimeter Principles by Glen A. Mazur, American Technical Publishers.