Capacitor with a Dielectric – en

Capacitor

A capacitor is a device that can store electric charge and normally consists of two conducting objects (usually plates or sheets) placed near each other but not touching. Basically, capacitors consist of two metal plates separated by an insulator. The insulator is called a dielectric. (e.g., polystyrene, oil, or air).  Capacitors are one of the most used and useful electronic components used in any modern electronic and electrical circuit and devices.

Their capacitance depends only on the size, shape, and relative position of the two conductors, and also on the material that separates them.

A simple capacitor consists of a pair of parallel plates of area A separated by a small distance d. Often the two plates are rolled into the form of a cylinder with plastic, paper, or other insulator separating the plates. 

In circuit diagrams, a capacitor is represented by either of these symbols:

Types of Capacitors

Capacitors are divided into three basic groups:

• Fixed capacitors. The capacitors whose value is fixed during the manufacturing process and cannot be latter altered are called fixed capacitors. Fixed capacitors are also further classified into two kinds, electrolytic and non-electrolytic capacitors.
• Polarized capacitors. Polarized Capacitors are the ones that have specific positive and negative polarities. While using these capacitors in circuits, it should always be taken care that they are connected in perfect polarities.
• Variable capacitors with a variable capacitance. They are made as trimmers that are typically adjusted only during circuit calibration, and as a device tunable during the operation of the electronic instrument.

The most common group is fixed capacitors. Many are named based on the type of dielectric. For a systematic classification, these characteristics cannot be used because one of the oldest, the electrolytic capacitor, is named instead by its cathode construction.

The most common kinds of capacitors are:

• Electrolytic capacitors
  • Tantalum capacitors
  • Niobium capacitors
  • Aluminum capacitors
• Ceramic capacitors
• Paper capacitors
• Film capacitors
• Mica capacitors
• Supercapacitors
• ….

Capacitor with a Dielectric

Dielectrics have many applications, but the most significant use is in capacitors. In many capacitors, there is an insulating material such as paper or plastic between the plates. Such a material, called a dielectric, can be used to maintain a physical separation of the plates.

Placing a solid dielectric between the plates of a capacitor serves three functions.

1. Mechanical separation
2. Electrical isolation – higher voltage possible
3. Electric field reduction – higher capacitance 

First, it solves the mechanical problem of maintaining two large metal sheets at a very small separation without actual contact. 

Second, using a dielectric increases the maximum possible potential difference between the capacitor plates. Any insulating material, when subjected to a sufficiently large electric field, experiences a partial ionization that permits conduction through it. This is called dielectric breakdown. Note that, the dielectric strength of air is approximately 3 kV/mm. Many dielectric materials can tolerate stronger electric fields without breakdown than can air. Thus using a dielectric allows a capacitor to sustain a higher potential difference and so store greater amounts of charge and energy. 

Third, experimentally it was found that capacitance C increases when the space between the conductors is filled with dielectrics. The polarisation of the dielectric by the applied electric field increases the capacitor’s surface charge for the given electric field strength. Application of an electric field between the two plates induces some opposite charge on the dielectric, which opposes the applied electric field. The net result is that the electric field inside the dielectric (which fills the area between the plates) is reduced, allowing the capacitor to store more charge.

To see how this happens, suppose a capacitor has a capacitance when there is no material between the plates. When a dielectric material is inserted to completely fill the space between the plates, the capacitance increases to:

C = κ_eC₀

where κ_e  is called the dielectric constant.

As can be seen, there are two cases. If the potential difference between the plates of a capacitor is maintained, as by battery B, the effect of a dielectric is to increase the charge on the plates. (b) If the charge on the capacitor plates is maintained, as in this case, the effect of a dielectric is to reduce the potential difference between the plates.

In the table below, we show some dielectric materials with their dielectric constant. Experiments indicate that all dielectric materials have  κ_e > 1. Note that every dielectric material has a characteristic dielectric strength which is the maximum value of the electric field before breakdown occurs and charges begin to flow.