5.1 Introduction from Office of Academic Technologies on Vimeo.

**5.01 Introduction**

We’re going to introduce an important concept associated with the electrical interactions, which is called capacitance. Before we go into the details of this concept, let’s consider a couple of mechanical systems. A spring-mass system, for example, if we expand a spring-mass system some certain distance from its equilibrium, let’s say by a distance of x, then we store a certain potential energy into the spring which we call that potential energy as elastic potential energy. And that is equal to one-half times the spring constant times the square of the distance from the equilibrium position of the spring.

In a similar way, if we just lift an object against the gravity of Earth, we know that the gravitational field of the Earth always points in downward direction. Or, from the global point of view, it always points towards the center of the Earth.

If you take an object, move against this gravity, or give a certain height by lifting it up relative to the Earth’s surface, then we end up by storing gravitational potential energy into the system. And the gravitational potential energy in this case, if this height is h, the gravitational potential energy we store into the system becomes mg times the height that we move, relative to the surface of the earth.

From a similar point of view, one can store potential energy in an electrostatic field. As you can see, the forces associated with these cases should be conservative forces because, at the end, we end up by having explicit potential energy functions.

We have seen earlier that the Coulomb force is also a conservative force. In other words, work done by this force is independent of the path, which allows us to come out with an explicit potential energy function.

One can store potential energy in an electrostatic field. From this point of view, a capacitor is a device that generates small electric field packages for the purpose of storing electrical potential energy into those fields.

We can state, therefore, by saying that from this point of view, a capacitor is a device that generates small electric field packages, or package, for the purpose of storing electrical potential energy.

Today, capacitors are vital components of modern electric circuitry, namely radio, TV, memory banks of computers, et cetera.

A capacitor is a device that consists of two conducting plates separated by an insulating medium.