Capacitance is defined as the ratio of the change in the electric charge of a system to the corresponding change in its electric potential. This term is used for capacitors, electrical components designed to store electric charge. In this article, we will investigate the concept of capacitance and how it applies to everyday electronics. 

History

Leyden Jar – Source

The concept of capacitance was introduced with the creation of an electric component, the capacitor. It was discovered in 1975 by Ewald Georg von Kleist. He discovered that electric charges coming from an electrostatic machine could be stored and released after a short period of time. The device that he used to do this became known as a Leyden Jar.

Capacitance is measured in Farads and is expressed in terms of the ratio of the change in the electric charge of a system to the corresponding change in its electric potential.

Energy Stored in a Capacitor

The energy stored in a capacitor is equivalent to the area under the graph of the voltage measured across a capacitor against the total charge of the capacitor. This can be expressed as:

Which is equivalent to:

Capacitors in Series & Parallel


You may remember these circuit configurations from our article on resistance. However, the equations we use to describe the total capacitance in a series and parallel circuits are flipped. For capacitors in series, we use the following equation to calculate the total capacitance. 

Series Circuit Configuration:

For capacitors in a parallel configuration, we using the following equation to calculate the total capacitance. 

Parallel Circuit Configuration:

Charging & Discharging Capacitors

Time Constant

The concept of time constant is essential as it determines how long a capacitor takes to charge and discharge. The time constant of a capacitor can be calculated using T = R x C.

In one time constant, the capacitor reaches 63% of the total voltage when charging. In 5 time constants, a capacitor can be said to be fully charged or discharged. 

A capacitor can only be charged or discharged when paired with a load (a resistor)—which is the value of R in the time constant equation. At the same time, C is just the capacitance of the capacitor. 

These are the graphs used to model the voltage across the capacitor with respect to the time constant. 

Equation used to model a charging capacitor: 

Equation used to model a discharging capacitor: 

AC Current

Throughout this article, we refer to the interaction between DC and capacitors, where a charge will continue to flow until a capacitor is fully charged. At this point, no current will flow. This inadvertently blocks DC currents as it only flows in one direction. On the other hand, AC oscillates in different directions, resulting in a continuous switch between charging and discharging a capacitor: a widely used phenomenon in the communications industry. 

That’s it for the basics of capacitance! Please feel free to ask any questions in the comments section below.