Capacitors

Type of physical science: Classical physics

Field of study: Electromagnetism

Capacitors are among the most basic electrical circuit devices. They are used to store charge and energy and, by extension of these functions, to detect or alter electrical signals in a wide variety of ways.

Overview

A capacitor is a device used in electrical circuits that consists of two electrically charged objects. The charges are the same in amount but of opposite sign, one negative and the other positive. A difference of electrical potential, a voltage difference, appears between the objects as a result of the charges. In practice, the objects are usually parallel, metal plates because the objects need to be electrical conductors in order to ease the addition and removal of charges.

A voltage applied between the plates (by connecting each plate to a terminal of a battery or other such device) supplies the equal and opposite charges. Since a capacitor is primarily a device for storing charge, its capacitance is defined as the charge (in units of coulombs) stored on the positive plate divided by the voltage between the plates. This ratio is constant for any capacitor.

The capacitance of a parallel-plate capacitor is proportional to the area of each plate and inversely proportional to the distance between the plates. Thus, increasing capacitance requires either making a bigger capacitor with larger plates or putting the plates closer together. Most capacitors have their capacitance printed on them with a tolerance (percentage of uncertainty) also included in the markings.

The space between the objects is also part of the capacitor. It can be filled with any electrical insulator without disturbing the basic character of the device. Filling it with an electrical conductor, however, would form a short circuit and prevent any charge from being stored on the plates.

The incentive for putting a dielectric (electrical insulator) between the plates is twofold.

A relatively solid dielectric will help keep the plates apart, preventing short-circuiting. More important, however, the dielectric increases the capacitance. This increase occurs because, as soon as opposite charges begin to accumulate on the plates, the molecules in the dielectric polarize. The electrons in the molecules are attracted to the positive plate and move toward it, but still remain in the molecule. The molecules of the dielectric then appear to have a negative side (toward the positive plate) and a positive side (toward the negative plate). In some dielectrics, the molecules may even rotate to allow the charges to get closer to the plates attracting them. The result of the polarization of these molecules is that the entire dielectric appears polarized, with the face toward the positive plate being negative and vice versa. The faces of the dielectric are in contact with the metal plates but, since the dielectric is an insulator, the polarization charges cannot move onto the metal plates. Thus, the plates of the capacitor have their effective total charge reduced by the amount of the polarization charge. Therefore, more actual charge can be stored on the plates than would be possible without the presence of the dielectric; the capacitance is increased.

A large number of dielectric materials (electrical insulators) are available to put between the plates. The two choices of air or a vacuum share with gases and dielectric liquids the important characteristic that they are all "healing dielectrics." This term refers to the ability of the dielectric to return to its original state after a dielectric breakdown, which occurs when too great a voltage is applied to the plates. Most capacitors have a maximum safe voltage of operation stamped on them to avoid this problem. Too high a voltage across the dielectric will strip electrons from the molecules of the dielectric, creating an ionized path for charge on one plate to travel to and cancel the charge on the other plate. Dielectric breakdown is what occurs to the air of the atmosphere when a lightning bolt strikes. Most dielectrics are destroyed by a dielectric breakdown, and glass will be shattered, paper burned, and plastics burned and shattered. Fluids such as air will heal after a dielectric breakdown because they can flow. A vacuum heals because there are no molecules in it to ionize.

The voltage at which breakdown occurs differs among dielectrics because atoms and molecules differ in their ability to hold onto electrons. In addition, the amount of increase in capacitance is different for different dielectrics because of different abilities to polarize. The amount by which a dielectric increases capacitance is known as the dielectric constant of the material. For example, Teflon, with a dielectric constant of two, doubles the capacitance of a capacitor above what it would be with a vacuum or air between the plates.

Dielectric constants can be as high as six thousand in certain barium titanate ceramic materials. Such dielectrics form the basis for one type of capacitor design, the multilayer ceramic capacitor (MLCC). These capacitors consist of combinations of many small parallel plate capacitors with high dielectric constant ceramics between the plates. MLCCs are made with techniques founded in integrated chip technology, and they have very large capacitances for their size.

Capacitors can be either variable or fixed. In a variable capacitor, the effective area of the plates or the spacing between them can be changed. A fixed capacitor has neither capability.

Fixed capacitors are generally cylindrical with a wire protruding from each end. A dielectric is sandwiched between the two plates, and the entire sheet is then rolled into a cylinder with a wire attached to each plate. In this process, one plate necessarily ends up on the outside. The packaging of the cylinder often is marked to indicate which wire is connected to the outer plate.

This wire should be grounded in the circuit so that the capacitor does not become an antenna for stray electrical signals.

Another type of fixed capacitor with a very large capacitance for its size is the electrolytic capacitor. In this type of capacitor, one of the metal plates is replaced with an electrically conducting liquid (an electrolyte). The metal plate is rolled into a cylinder and placed in the electrolyte, and a current is passed through the combination. The current causes a chemical reaction which deposits a very thin layer of insulator (usually the oxide of the metal of the plate) over the surface of the plate. Once the plate is completely coated, the device becomes a capacitor with a very small plate separation and, consequently, a very large capacitance. This type of capacitor can be "unmade" (sometimes explosively) if the current flow is reversed or a voltage is placed across the device which exceeds the voltage used to create the coating. Hence, electrolytic capacitors always have markings that indicate which end should be positive and which should be negative in a circuit. These capacitors are always voltage rated.

A clever and interesting variation on the electrolytic capacitor is the tantalum capacitor.

Tantalum metal in powder form is pressed into a porous solid. The surface area of the pores is much greater than that of the outer surface of the metal. As in an electrolytic capacitor, an electric current is used to oxidize the exposed metal surfaces. Then, a solution of manganese nitrate is used to deposit manganese dioxide over the tantalum oxide. Manganese dioxide, a semiconductor, constitutes the second object of the capacitor. The huge surface-to-volume ratio of the porous tantalum gives these capacitors relatively high capacitance for their size.

Applications

Although capacitors are essentially very simple devices, they can be used for many purposes and made from a great variety of materials. Those devices with large capacitances are used to store charge and energy for use in power supplies and other applications where quick energy delivery is needed. An extreme example of such systems is the capacitor banks that are used to drive projectiles in a nail gun.

The distributor of an automobile uses capacitors to another purpose. A capacitor (often identified as a "condenser") is connected across the spark gap of the distributor. It suppresses the arcing of the high voltage from the coil by slowing the buildup of voltage. Without the condenser, the premature firing of cylinders could occur. A similar use of capacitors appears in circuits where capacitors are connected across the ends of switches to prevent voltage surges as the switch is opened or closed.

Capacitors are also used to separate slowly varying electrical signals from rapidly varying ones. It takes time to build up charge (and voltage) on the plates of a capacitor.

Therefore, if a capacitor is in a circuit with a resistor and the driving voltage across the combination changes in time, then the voltage across the capacitor changes in time in a way which is different from the voltage across the resistor. By focusing voltage across the capacitor, the lower frequencies in the driving voltage are selected, while the voltage across the resistor selects for the higher frequencies in the signal. This simple combination is the high pass/low pass filter that is basic to sound systems.

Capacitors are also used to separate variable from constant voltages when the two serve different purposes such as in the biasing (application of a slight voltage) of a transistor. To operate consistently, a transistor requires certain constant voltages. Yet, it is usually used to amplify a variable voltage signal. Hence, variable and constant voltages are always present in such circuits, and capacitors are necessary to force the two types of signals to appear only in the appropriate parts of the circuit.

In combination with resistors and diodes, capacitors can be used to affect the timing of electrical signals and circuits and to shape and alter the signals in a variety of ways: The top and/or bottom of a wave signal can be cut off, parts of the signal can be inverted or delayed in arriving, or all or part of a signal can be distorted. Voltages can be saved in capacitors and recombined to double, triple, or quadruple the original voltage values.

Combined with inductors, capacitors can create oscillating electrical signals or be used to tune circuits to respond only to certain frequencies of oscillatory signals. Such circuits are basic to radio and television tuning devices. The oscillations arise because the behavior of inductors is complementary to that of capacitors. Whereas capacitors store energy by storing charge, inductors store energy as charge flows through them. A highly charged capacitor wants to discharge and reduce the energy that is stored in it. If the discharge is forced to flow through an inductor, then the level of stored energy increases in the inductor and decreases in the capacitor. The inductor then has excess energy, which it uses to increase the charge on and energy stored in the capacitor. This trading of energy back and forth can be endless, and it constitutes an oscillation of charge between the two devices.

Capacitance can be measured in several ways, but the capacitance bridge is probably the best. With this method, the measurement itself does not disturb the currents in the circuit and, hence, the resulting capacitance value. The capacitor to be measured is compared with a known, standard capacitor using variable resistors to make necessary adjustments in the circuit. A capacitor which is placed permanently in the bridge can then detect any effect that could vary the capacitance. For example, accelerometers can be made that measure the change of capacitance of tiny beams of silicon dioxide anchored at one end over a pit in a silicon chip. The other end of the beam has a small button of gold deposited on it to add inertia to the beam. The top of the beam has a strip of metal deposited on it, and another strip is deposited onto the bottom of the pit. The metal strips become the two plates of a capacitor with air as the dielectric between them.

As the chip is accelerated, the beam bends, altering the distance between plates and, therefore, the capacitance. An amplifier and bridge circuit etched into the chip when the beams and pits were etched on gives a readout of the capacitance changes, which correlate with the acceleration experienced by the device.

An atypical capacitor--one not used in an electric circuit--is formed between the earth and a cloud. These two objects can develop opposite charges, especially in a rainstorm. They then become a capacitor with the air between them as the separating dielectric. Despite the great size of the cloud, the capacitance of such a system is not high because of the enormous distance between the cloud and the earth.

Context

A glass jar lined inside and outside with metal foil was the first type of capacitor.

Called a Leyden jar after the home of Pieter van Musschenbroek, a claimant to its invention in 1746, this device had an insulating stopper at the mouth of the jar through which was pushed a metal rod to communicate electric charge to the inner foil. The Leyden jar was a major advance in the study of electricity, which until that time had been greatly hampered by the difficulty of storing and collecting electric charge. Within a few years, a number of investigators, including Benjamin Franklin, were using the Leyden jar in experiments which led eventually to key developments in the understanding of electricity. Franklin's work especially was crucial in showing that lightning is a form of electrical discharge which can be controlled. He also constructed a working theory of electricity called Franklin's single fluid theory. Along the way, Franklin made a key analysis of the action and behavior of the Leyden jar. Thus, the discovery and use of capacitors have been critical to a proper understanding of the nature of electricity.

The invention of the battery (voltaic cell) eliminated the need for capacitors as collectors of electricity. Hence, from 1800 to the 1840's, efforts were concentrated on resistance and current in electric circuits and little was done with capacitors. With the experiments of Rudolph Kohlrausch, however, the situation changed. Kohlrausch used Leyden jars in circuits, and this work led Gustav Kirchhoff to recognize the equivalence of tension (voltage) with electrical potential. Kirchhoff was able to begin to construct a more coherent, mathematical theory of electricity which enabled him to realize by 1857 that oscillatory electrical currents must propagate at the speed of light in ideal conductors. Thus, work with capacitors led to the first hint of a connection between light and electricity.

Very shortly thereafter, in 1862, James Clerk Maxwell made the connection between light and electricity much firmer with his celebrated mathematical theory of electricity and magnetism. Maxwell found it necessary to invent the idea of displacement current in capacitors in order to formulate a fully consisent theory. Since that time, capacitors have continued to be essential devices in electronics. They have shown themselves to be an indispensable part of applied science.

Principal terms

CAPACITANCE: the value of the charge on the positively charged side of a capacitor divided by the voltage between the two sides

CAPACITOR: a device which consists of a pair of objects with opposite but equal charges

DIELECTRIC: an electrical insulator, which is a substance that will not allow electric charge to flow

DIELECTRIC BREAKDOWN: the result that occurs when the charges on the opposite sides of a capacitor recombine by flowing through the dielectric separating them

DIELECTRIC CONSTANT: the factor by which replacing a vacuum between the plates of a capacitor with a particular dielectric will increase the capacitance

FREQUENCY: the number of cycles, oscillations, or waves that occur per unit of time

POLARIZATION OF A DIELECTRIC: the situation that occurs when the average centers of negative and positive charges in a dielectric are separated, so that the dielectric appears positive at one end and negative at the other

Bibliography

Asimov, Isaac. THE HISTORY OF PHYSICS. New York: Walker, 1966. The section on condensers contains historical details about capacitors and a qualitative description of their basic functioning.

Benrey, Ronald. ELECTRONICS FOR EVERYBODY. New York: Harper & Row, 1970. A very practical book on basic electronics. The discussion on capacitors focuses on the uses and types of capacitors.

Cohen, I. Bernard. BENJAMIN FRANKLIN'S SCIENCE. Cambridge, Mass.: Harvard University Press, 1990. Contains a section on Franklin's analysis of the Leyden jar. A very readable book with interesting material on the work of one of the major early scientists in the field of electricity.

Georgiev, Alexander M. THE ELECTROLYTIC CAPACITOR. New York: Murray Hill Books, 1945. This is a rarity: an entire book devoted to the study of capacitors. The style is thorough but nonmathematical. Gives a wealth of details about the manufacture and behavior of electrolytic capacitors.

Lewellen, John. UNDERSTANDING ELECTRONICS. New York: Thomas Y. Crowell, 1957. This book presents a good qualitative discussion of the action of capacitors in both direct and alternating current situations. Aimed at young readers and is likely to be found in the juvenile section of a library.

Trotter, Donald M., Jr. "Capacitors." SCIENTIFIC AMERICAN 259 (July, 1988): 86-90B. An excellent article on the methods of manufacturing capacitors, with an emphasis on multilayer ceramic and tantalum capacitors.

Charges and Currents

Conductors and Resistors

Generating and Detecting Electromagnetic Waves

Forces on Charges and Currents

Insulators and Dielectrics