Electrical capacitance is an electrostatic phenomenon and is concerned with the storage of electrical charge. Devices that use this phenomenon are called capacitors (formerly known as condensers) and are widely used in electronic equipment. They consist essentially of two parallel plates, separated by an insulating material, which possess a certain "capacity" to store electric charge.
The capacitance of a device is determined by dividing the charge stored on the plates by the voltage the charge creates across the plates. By increasing the capacitance, more charge can be stored at the same voltage.
Capacitance can be increased in three ways: by increasing the area of the plates, by reducing the separation distance between the plates, and by using a better insulating medium, or dielectric, than air between the plates.
Dielectric materials
If an insulating material, or dielectric, is introduced between the plates and a voltage is applied across the plates, polarization will occur in the material. Polarization is the rearrangement of electric charges in the material to form dipoles, which are opposite charges very close together lined up in the material in an orderly fashion. These dipoles produce an internal field that cancels part of the field created by the applied voltage, enabling more charge to be stored for a given applied voltage.
Introducing a dielectric therefore, increases the capacitance, and the ratio of this new capacitance to that with an air gap is called the relative permittivity, or dielectric constant, of the material (symbol ε—the Greek letter epsilon). This constant varies for different materials.
Some typical materials with their dielectric constants are a vacuum, ε = 1; air, ε = 1.00059 (normally assumed to be one); polytetrafluorethylene (PTFE), ε = 2; polystyrene, ε = 2.6; barium titanate, ε = 6000. Using barium titanate as the dielectric means that, for a given capacitor size and shape, the capacitance can be increased 6,000 times.
Capacitors in electrical circuits
If a battery, switch, resistor, and capacitor are connected in series in a simple electrical circuit, the following voltage–current relationships can be observed depending on the switch position.
When the switch is closed, there is an initial current, or flow of charge, equal to the battery voltage divided by the resistance of the resistor, according to Ohm’s Law. Charge soon builds up on the capacitor plates, increasing the voltage between them. Like charges repel, and so the buildup of charge eventually slows. The current in the circuit dwindles to zero, leaving the voltage across the capacitor equal to the battery voltage. This is the charging process.
If the switch is now opened, the capacitor will retain its stored charge. In theory, it will do this indefinitely, but in practice there will always be some leakage or loss, and the charge stored and the capacitor voltage will slowly decrease.
If the battery is removed and replaced by a piece of wire there will be an initial current, in the reverse direction, equal to the voltage across the capacitor divided by the resistance of the resistor. The current dwindles as the capacitor voltage drops to zero. This is the discharge process.
When an alternating (AC) voltage source is used instead of a battery in the above circuit, the capacitor goes through repeated charge– discharge cycles. As the frequency of the alteating voltage is increased, the capacitor has little time to accumulate charge before the second half of the cycle—the discharge cycle—begins, and the maximum voltage across the capacitor gets smaller and smaller. Thus, with increasing frequency, the capacitor has less and less effect in the circuit and the current approaches the source voltage divided by the resistance of the resistor.
The behavior of a capacitor in an alternating current circuit will be different from that in a direct current circuit. When a capacitor is charged by a direct, or DC, voltage, the accumulation of charge on the plates provides a means of storing energy that can be made available on demand. An example is the capacitor used in a camera flashgun.
The frequency dependent properties of a capacitor in an AC circuit lead to several important applications, including pulse shaping and smoothing and radinterference suppression (as in the ignition system of cars). The capacitor allows higrequency currents to pass through it but at the same time blocks the low frequency and direct currents.
So far, only ideal capacitance has been discussed, whereas in practice all dielectric materials have a certain electrical resistance. For example, when a DC voltage is applied across a capacitor, a small leakage current will flow continuously through it. In actual circuits, allowances have to be made for the capacitors being less than perfect.
There are two major types of capacitors—fixed and variable—both of which are used in a wide range of electronic devices. Fixed capacitors can be further subdivided into electrolytic and nonelectrolytic types and represent the largest proportion of the market. Capacitors are sometimes still referred to as condensers, their original name.
