Capacitor Basics: What It Is, Types & How It Works

What is a capacitor? In electrical engineering, a capacitor is a two-terminal electrical device that stores electrical energy by collecting electric charge on two closely spaced, insulated surfaces. The area between the conductors can be filled with either a vacuum or an insulating material called a dielectric. Capacitors were initially called condensers, a name still found in some compound terms, like the condenser microphone.

Capacitors, alongside resistors and inductors, are some of the most fundamental passive components used in electronics — it would be hard to find a circuit without one. In this guide, we’ll cover how capacitors work, the main types you’ll encounter, and why they’re essential in electronic circuits.

What Is a Capacitor?

A capacitor is an electronic component that stores electrostatic energy within an electric field, similar in some ways to a battery but operating quite differently. While a battery converts chemical energy into electrical energy, a capacitor stores and releases energy instantly, without a chemical reaction — think of it as a small electronic storage tank for electrical charge.

Inside a capacitor are two conducting metal plates separated by an insulating material called a dielectric. The plates can be made of different metal alloys, such as aluminum or tantalum, depending on the capacitor type. The dielectric keeps the plates from touching while maintaining the separation needed to store charge.

A Brief History of the Capacitor

In 1745, Ewald Georg von Kleist discovered that charge could be stored by connecting a high-voltage generator to water in a glass jar. This setup, acting as a primitive capacitor, produced a powerful spark when touched. The following year, Pieter van Musschenbroek created a similar device known as the Leyden jar, named after the University of Leiden. Both scientists were amazed by the shock produced by these early capacitors.

In 1957, H. Becker invented the electric double-layer capacitor — now known as the supercapacitor — patenting a “low voltage electrolytic capacitor with porous carbon electrodes.” Becker believed the energy was stored as charge in the carbon pores of his capacitor, similar to the pores of etched foils in electrolytic capacitors. He acknowledged in the patent that the exact storage mechanism was unknown, but noted the component’s extremely high capacity. You can read more about how that technology has evolved in our supercapacitor guide.

The metal–oxide–semiconductor (MOS) capacitor evolved from the metal–oxide–semiconductor field-effect transistor (MOSFET) structure, invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959. The MOS capacitor became widely used as a storage capacitor in memory chips and as the fundamental component of charge-coupled devices (CCDs) in image sensors. In dynamic random-access memory (DRAM), each memory cell typically includes a MOSFET and a MOS capacitor.

How Capacitors Work

When a voltage is applied to a capacitor, it starts charging, storing electrical energy as electrons on one plate while the other plate becomes positively charged to balance things out. This charge separation creates a voltage potential and an electric field between the plates, which is where the energy is actually stored.

When the voltage source is removed, the capacitor retains its charge and can release that stored energy when needed. This charge-and-discharge behavior is what makes capacitors so versatile and valuable across electronic circuits.

What Does Capacitance Tell Us?

Capacitance tells us how much electrical charge a capacitor can store per unit of voltage — it quantifies a capacitor’s ability to hold and release energy. In simpler terms, it measures the “size” of a capacitor’s storage tank for electrical charge.

Capacitance is measured in a unit called the farad. A farad is a fairly large unit, so capacitors used in everyday electronics are usually measured in microfarads (µF), nanofarads (nF), or picofarads (pF) — smaller fractions of a farad, depending on the capacitor’s size.

How to Calculate the Capacitance of a Parallel-Plate Capacitor

The capacitance of a parallel-plate capacitor is determined by the area of the plates, the distance between them, and the permittivity of the dielectric material between the plates, shown in the equation below. For a deeper dive into the dielectric constant and how different dielectric materials affect capacitance, see our guide to capacitance and dielectrics.

where:

• C is the capacitance in farads,
• A is the area of overlap of the two plates in square meters,
• ε_r is the relative static permittivity (dielectric constant) of the material between the plates,
• ε₀ is the electric constant (ε₀≈8.854×10⁻¹² F m⁻¹),
• d is the separation between the plates in meters.

This formula is fundamental to understanding the capacitance of a simple capacitor, and is derived from the geometry and physical properties of the materials involved.

Energy Storage in Capacitors

The energy stored in a capacitor is proportional to the square of the voltage across it and the capacitance of the capacitor — a crucial concept for understanding how capacitors store and release energy in electronic circuits.

E = 0.5CV²

Where:

• E is the energy stored in joules,
• C is the capacitance in farads,
• V is the voltage across the capacitor in volts.

Geometric Ways to Increase Capacitance

Several geometric changes can increase a capacitor’s capacitance. Decreasing the separation between the plates strengthens the electric field between them, which increases capacitance. Increasing the area of the facing plates adds more surface for charge storage, which also boosts capacitance.

Connecting capacitors in parallel increases total capacitance by effectively adding their individual capacitances together. Using a material with a higher dielectric constant between the plates also increases capacitance by strengthening the electric field and improving charge storage.

RC Time Constant: How Fast a Capacitor Charges and Discharges

In a real circuit, a capacitor doesn’t charge or discharge instantly — it charges through whatever resistance is in the circuit, and that rate is described by the RC time constant, τ (tau):

τ = RC

where R is the circuit resistance in ohms and C is the capacitance in farads. One time constant is the time it takes the capacitor to charge to about 63% of the supply voltage (or discharge to about 37% of its initial voltage); after roughly five time constants, the capacitor is considered fully charged or discharged. This relationship is the basis for timing circuits, filters, and debounce circuits throughout electronics.

Types of Capacitors

Capacitors come in several common types, each with its own characteristics and typical applications:

Type	Polarized?	Typical Capacitance Range	Common Uses
Electrolytic	Yes	1 µF – tens of thousands of µF	Power supply filtering, audio systems
Ceramic	No	pF – low µF	Decoupling, general-purpose circuits
Tantalum	Yes	0.1 µF – hundreds of µF	Medical devices, automotive electronics
Film	No	pF – tens of µF	Audio systems, electronic filters

1. Electrolytic capacitors: Polarized capacitors commonly used for high capacitance values, often found in power supply circuits and audio systems. Connecting them with reversed polarity can cause them to fail — sometimes dramatically — so correct orientation matters.
2. Ceramic capacitors: Small, inexpensive, and non-polarized, used in a wide range of applications. They come in various sizes and capacitance values, making them versatile for different circuit needs — you’ll find them in everything from computers to radios.
3. Tantalum capacitors: Similar to electrolytic capacitors, tantalum capacitors are polarized and offer high capacitance values. They’re known for stability and reliability, making them suitable for applications where accuracy matters, like medical devices and automotive electronics.
4. Film capacitors: Made from a thin film of metal or metalized film. They come in several types — polyester, polypropylene, polystyrene — each with specific characteristics, and are commonly used in audio systems and electronic filters.

Some capacitors are polarized, meaning they can only be connected one way around — electrolytic and tantalum capacitors are polarized, while ceramic and film capacitors typically are not. On a schematic, a polarized capacitor is usually drawn with one straight (+) plate and one curved (–) plate, while a non-polarized capacitor is drawn with two straight parallel lines.

What Is a Practical Capacitor?

A practical capacitor consists of two sets of semicircular aluminum or brass plates separated by a dielectric material. Practical capacitors can be constructed by interleaving the plates with two dielectric layers and rolling them up. By staggering the plates, connections can be made to one plate at each end of the roll.

Capacitors in Electronic Circuits

Capacitors play several key roles in electronic circuits, including:

1. Power supply filtering: Capacitors smooth out the voltage provided by power supplies, reducing ripples or fluctuations and acting as a buffer to ensure a stable, reliable power source for the rest of the circuit.
2. Timing circuits: Capacitors, paired with resistors (see the RC time constant above), can create precise time delays or oscillations — useful for generating clock signals, timing events, or frequency references.
3. Coupling and decoupling: Capacitors allow AC signals to pass through while blocking DC components. They’re used to couple different amplifier stages, letting the AC audio signal flow while blocking DC bias, and decoupling capacitors provide a local reserve of charge to prevent voltage drops and minimize noise in sensitive circuits.
4. Motor starting and suppression: Capacitors give electric motors an initial boost during startup, helping overcome high starting current and improving efficiency. They’re also used to suppress electromagnetic interference (EMI) generated by motors and other high-power devices.

Capacitors also play a central role alongside resistors and inductors in 2nd order RLC circuits, where their interaction with inductance produces the damped and oscillatory responses common in filter and timing circuit design.

Capacitors in Parallel: How Total Capacitance Changes

When capacitors are connected in parallel, their total capacitance increases, since the combined capacitors can store more electrical charge. Connecting capacitors in parallel is like adding more tanks to hold the charge — the equivalent capacitance is the sum of the individual capacitances of all capacitors involved.

Capacitors in Series: How Total Capacitance Changes

When capacitors are connected in series, their total capacitance is less than that of any individual capacitor in the string. The voltage across each capacitor in a series circuit differs, but the total charge stored is the same across all of them. The capacitance of two or more capacitors in series is found from the sum of the reciprocals of the individual capacitances.

How a Capacitive Acceleration Sensor Works

A capacitive acceleration sensor works by using the principle of capacitance change due to acceleration. It typically consists of a movable mass attached to one of the capacitor plates. When the sensor experiences acceleration, the mass moves, altering the distance between the plates, which changes the capacitance and allows the acceleration to be measured.

Frequently Asked Questions About Capacitors

What does a capacitor do in simple terms?

A capacitor stores electrical energy in an electric field between two plates, then releases that energy when needed. It works like a small, fast-acting storage tank for electrical charge.

What is the difference between a capacitor and a battery?

A battery stores energy chemically and releases it gradually through a chemical reaction. A capacitor stores energy electrostatically in an electric field and can charge or discharge almost instantly, but typically holds far less total energy than a battery of similar size.

What is the unit of capacitance?

Capacitance is measured in farads (F). Since a farad is a very large unit, most everyday capacitors are rated in microfarads (µF), nanofarads (nF), or picofarads (pF).

What happens if you connect an electrolytic capacitor backward?

Electrolytic and tantalum capacitors are polarized, so connecting them with reversed polarity can damage the dielectric and cause the capacitor to overheat or fail, sometimes dramatically. Always match the polarity markings when installing a polarized capacitor.

What is the RC time constant?

The RC time constant (τ = RC) describes how quickly a capacitor charges or discharges through a resistor. After one time constant, a capacitor reaches about 63% of its final charge; after about five time constants, it’s considered fully charged or discharged.

Key Takeaways

Capacitors may seem like small, simple components, but they play a vital role in nearly every electronic device — filtering power supply voltage, providing precise timing, coupling signals, and starting motors. Understanding how they charge, discharge, and combine in series and parallel is foundational to working with any circuit.

To go deeper on the concepts introduced here, see our guides to capacitance and dielectrics, supercapacitors, and how capacitors compare to other passive and semiconductor components like inductors and diodes.