Capacitors, Capacitance and Capacitive Reactance

A capacitor is an electrical device that stores electrical charge in it. It consists of two conductive plates separated by a non-conductive material called the dielectric. When a voltage is applied across the plates of a capacitor, the electric field which is created by the voltage causes the separation of charge between the plates, resulting in the build-up of electrical charge on the plates. This stored electrical charge is known as capacitance. 

Types Of Capacitors

There are so many different types of capacitors available, each with its own unique characteristics and applications. Some common types of capacitors include:

Electrolytic capacitors: 

Electrolytic capacitors make use of an electrolyte solution such as the dielectric material, and they are also used for high-voltage and high-capacity applications. They have polarized designs, meaning that they have positive and negative terminals, and they must be connected in the correct orientation..

Ceramic capacitors: 

Ceramic capacitors make use of a ceramic material as the dielectric, and they are commonly used in high-frequency circuits due to their low inductance and high stability. They also are small and lightweight, making them well-suited for use in portable devices..

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Film capacitors: 

Film capacitors make use of a thin film of plastic or metal as the dielectric material, and they are known for their high stability and low noise. They are commonly used in power supplies and other high-voltage applications.

Tantalum capacitors: 

Tantalum capacitors make  metal as the dielectric material, and they are known for their high capacity and low leakage current. They are often used in portable electronic devices and other applications where a small size is important.


Supercapacitors, also known as ultracapacitors or electrochemical double-layer capacitors (EDLCs), have an extremely high capacity and can store a large amount of energy. They are used in a variety of applications, including energy storage and power delivery.

Uses of Capacitors

Capacitors are widely used in electronic circuits for a variety of purposes. Some common uses of capacitors include:

Storing electrical charge: 

Capacitors are able to store electrical charge and release it when needed. They are often used to smooth out voltage fluctuations in power supplies or to provide a temporary power source for circuits that require a brief burst of power.

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They are used in circuits to filter out unwanted frequencies or to block DC signals while allowing AC signals to pass through.


They are used to couple two circuit stages together, allowing AC signals to pass through while blocking DC signals. This is commonly used in audio circuits to separate the DC bias voltage from the AC audio signal.


They are used in timing circuits to determine the length of time that a particular circuit function is active.


They are used to suppress voltage transients or “spikes” in a circuit. This is often done in power supply circuits to protect sensitive components from damage.

Motor starting: 

They are used in motor starting circuits to provide the extra torque needed to get a motor up to speed.

Energy storage: 

They are used in renewable energy systems to store excess energy generated by solar panels or wind turbines for use during periods when the renewable energy source is unavailable.


The capacitance of a capacitor is simply defined as the measure of the amount of electrical charge that can be stored on the plates for a given voltage applied across the plates. The unit of capacitance is the farad (F), which is defined as the amount of charge stored on the plates of a capacitor per unit of voltage applied across the plates.

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Capacitive Reactance

Capacitive reactance is defined as  the measure of the opposition to the flow of alternating current (AC) in a circuit containing a capacitor. It is similar to resistance, which is the opposition to the flow of direct current (DC) in a circuit. The unit of capacitive reactance is the ohm (Ω).

Capacitive reactance is inversely proportional to the frequency of the AC signal and the value of the capacitor. This means that at high frequencies, the capacitive reactance of a capacitor is low, and at low frequencies, the capacitive reactance is high. This property of capacitors is useful in circuits that are designed to pass or block signals of specific frequencies, such as filters and oscillators.