Insulators And Dielectrics

Type of physical science: Classical physics

Field of study: Electromagnetism

Some form of insulator or dielectric, a substance that is a poor conductor of electricity, is incorporated into every electrical device. The study of dielectrics was a major driving force in the creation of modern science.

Overview

Electric insulators are very poor electrical conductors. They can conduct some current, but for most purposes, the amount is truly insignificant. Following the path of least resistance, it is easier for electric current to travel 100 kilometers in the copper wire from a power plant to a house, turn the motor in a refrigerator, and then travel back to the power plant than it is for the same current to bridge the fraction-of-a-millimeter-thick coating of varnish that separates the windings of copper wire in the refrigerator motor.

An electric current in a wire may be defined as a flow of electrons through the wire.

Good conductors, such as metals, have an ample supply of electrons free to flow. They are called conduction electrons, and they were originally the outermost electrons of each metal atom. What determines how much current will flow? Consider the simple experiment of taking various lengths and thicknesses of copper wire and connecting them one at a time across the terminals of a battery. Current will flow in the wire; less current flows if the wire is longer, while more current flows if the wire is thicker. The resistance of the wire is the voltage across the wire (between its ends) divided by the current flowing through the wire. Resistance is measured in ohms, named for Georg Simon Ohm. The resistivity of a substance is defined as the measured resistance of the substance multiplied by its cross-sectional area and divided by its length. With this definition, resistivity depends only on the substance, since the geometric factors of length and cross-sectional area have been canceled out. Resistivity is measured in ohm meters. A good insulator, such as amber, has a resistivity of 5 x 10¹⁴ ohm meters. Quartz is even better as an insulator with a resistivity of 5 x 10¹⁶ ohm meters. A typical resistivity for the semiconductor silicon is 640 ohm meters, while that of silver is 1.6 x 10^-8 ohm meters. The resistivity of a very good insulator is more than 10²⁴ times that of a good conductor, which is a truly phenomenal range for a single characteristic.

What causes electric current to flow in a conductor? If the electrons inside a wire could be seen, they would be a blur of motion, colliding with atoms and with one another, often changing directions, but seldom changing speeds. Normally, their motions are random, with as many going one direction as any other. With no net flow of charge, there is no current. Yet, if the wire is connected to the terminals of a battery, a current flows. Electrons may be pictured in the wire as being attracted to the positive battery terminal and repelled by the negative terminal.

While this seems to be a simple case of positive charge attracting electrons and negative charge repelling them, one may wonder how the electrons "know" that those positive and negative battery terminals are even there. One very fruitful explanation is to say that the presence of electric charge modifies the space around it by establishing an electric field. One may think of the electric field of a charged particle, a proton, for example, as a continuous series of ripples spreading outward through space much as ripples move outward from a pebble dropped into still water.

Suppose the electric field of the proton encounters another charged particle such as an electron. Interacting with the field, the electron will experience a force toward the proton. When a conducting wire is connected across the terminals of a battery, an electric field propagates down the wire. This field exerts a force on the electrons, causing enough of them to move down the wire so that now there is a current.

If the charge causing the electric field is changed--that is, made smaller or larger, or moved to one side--then a change in the electric field must propagate outward from the charge.

James Clerk Maxwell showed that electric field changes travel outward at the speed of light.

Electric fields may exist in "empty" space or inside materials. The speed of light (the speed at which changes in the electric field travel) is always lower inside a material than it is in free space.

When an electric field is established in a conductor, conduction electrons experience a force and current flows. What happens when an electric field is applied to a dielectric? A dielectric is an electrical insulator and the word "dielectric" is generally used to emphasize the electrical properties that such a material has in addition to its insulating properties. The first two letters of the word, "di," come from the latin dia, meaning "through or across." When an electric field is applied throughout a dielectric, there are no free electrons to form an electric current, as there are in a conductor. The electrons bound to atoms try to move as directed by the electric field; however, the best they can manage is to spend a little more time on the near side of the atom than on the far side. Each atom is now slightly negative on one side and slightly positive on the other. The atoms and the dielectric are said to be "polarized." Polarized atoms set up their own electric field in such a direction as to reduce the field that caused the polarization.

Applications

Consider the device that was formerly called a condenser but is now called a capacitor, since it has the capacity to store electric charge. Suppose that two conducting plates are placed one over the other with a gap between them so that they make an air sandwich. If the plates are connected to opposite poles of a battery, current will flow momentarily. One plate will become positive and the other negative. When the voltage across the plates is equal to the voltage of the battery, no more current will flow.

It is useful to think of the charges on the plates as establishing an electric field between the plates. If an insulator or dielectric is placed between the plates of the capacitor, then the electric field polarizes the dielectric in such a direction as to reduce the electric field. More charge must be placed on the capacitor in order to return the field to its original value; that is, the charge that can be stored on the capacitor is increased by some ratio called K, which is known as the dielectric coefficient. Values of K range from two or three for oil to ten or twenty thousand for a compound called barium titanate. Placing a solid dielectric between the plates of the capacitor actually accomplishes three things: First, it provides a mechanical means to hold the plates apart; second, it increases the capacitance by a factor of K; third, it increases the breakdown voltage of the capacitor.

At the breakdown voltage, the electric field is strong enough to excite electrons into a conducting state or even to pull the molecules of the dielectric apart into ions. In either case, the dielectric will then conduct electricity between the plates and thereby discharge the electrons.

The breakdown voltage for dry air occurs with 3 million volts on plates that are 1 meter apart.

Certain types of glass can withstand one hundred times that voltage.

The coefficient K is also related to the speed of light in a dielectric. Light is an electromagnetic wave, and the electric field of the wave interacts with the electrons of the dielectric. The interaction is such that the average speed of light in the dielectric is less than it is in free space. The ratio of the speed of light in free space to that in the dielectric is called the index of refraction (n). For nonmagnetic dielectrics (the usual case), n is simply the square root of K.

The construction of antireflection coatings and the construction of dielectric mirrors are two very important applications of the ability to select a dielectric with the appropriate coefficient K. In order to see this, some properties of light as an electromagnetic wave must be considered. The electric field of a light wave oscillates: It rises to a maximum positive value, sinks to zero, sinks further to its most negative value, rises to zero, rises to the maximum positive value, and repeats the process. It is well represented by regularly spaced water waves, whose crests represent the maximums and whose troughs are the minimums.

Normally, when light strikes a dielectric such as glass, at least several percent of the intensity is reflected. The light lost from the transmitted beam reaches serious proportions in a lens system with many surfaces. The reflected light intensity may be greatly reduced by depositing a quarter-wavelength-thick transparent coating on the glass. Magnesium fluoride is generally used for the coating because it is durable and has nearly the required index of refraction.

Imagine light coming from the left and shining on a coated lens. When light strikes the first surface, that of the magnesium fluoride, most of it will be transmitted, but some will be reflected back to the left. Upon reaching the second surface, the magnesium fluoride glass interface, again, most of the light will be transmitted on toward the right, while a small amount will again be reflected to the left. If the coating is one-fourth of a wavelength thick, the second reflected ray (having traversed the coating twice) will be one-half wavelength further along in its cycle than is the beam reflected from the first surface when the two beams meet at the first surface. Therefore, crests of the first wave will overlap with troughs of the second wave. They will cancel to produce a condition of no reflected light. (In reality, the wave model allows one to predict the outcome, but not the intermediate steps. With the coating in place, neither reflected beam existed.) In order for this to work as described, the dielectric coefficient of the coating must be equal to the square root of that for the glass from which the lens is fashioned.

Obviously, the conditions for no reflected beam can be satisfied for only a single wavelength, although it will also be approximately satisfied for nearby wavelengths. The wavelength usually chosen is that of yellow light in the middle of the visible spectrum. Light from the two ends of the spectrum, violet and red, will still be reflected. This is why a coated lens has a purplish cast when viewed by reflected light. Adding quarter-wavelength coatings of zirconium dioxide and cerium fluoride allows antireflectance to be achieved more uniformly across the spectrum.

Common mirrors are made by depositing a shiny metal, such as aluminum, onto the back side of a piece of glass. The glass protects the metal coating. Mirrors used in sciences are often front surface mirrors. Since the light does not enter the glass, it is not distorted by it.

Dielectric mirrors are better yet. Coatings similar to antireflective coatings are used, but this time their thicknesses and dielectric coefficients are chosen so that light waves reflected by the various layers all add up to give a strong reflected beam. Generally, eight or ten layers, which are alternatively zinc sulfide or magnesium fluoride, are used.

Not only does a dielectric mirror reflect nearly 10 percent more light than a metalized mirror but also it can be designed to reflect different wavelengths selectively. For example, a bright tungsten filament emits 95 percent of its energy as heat. In a film projector, one wishes to focus only the visible light on the film. A curved dielectric mirror placed behind the light bulb can be designed to focus only the visible light while allowing the infrared (radiant heat) to pass through. Another dielectric mirror between the bulb and the film can be made to reflect infrared back toward the bulb while passing visible light onto the film.

Dielectric mirrors may be found in many places. Most lasers depend upon the high reflectivity of dielectric mirrors for their operation. The iridescence of peacocks' tail feathers and of mother-of-pearl is caused by nature's version of the dielectric mirror. One can tell that one is not simply seeing colored feathers or colored shell because the colors will change as they are observed from different angles. (This is best done using the illumination of a single light bulb.)

Many insects' wings have this same property.

One aspect of dielectrics many find amusing uses rubber balloons. If an inflated balloon is rubbed against a shirt or other piece of fabric, it generally acquires a static charge.

When brought near a wall, the charge on the balloon polarizes molecules in the wall. The direction of polarization will always be such that the balloon is attracted to the wall. In fact, it will stick to the wall until its acquired charge leaks away.

Synthetic fabrics are often good insulators and may become charged by rubbing against other things. Once charged, these fabrics are very slow to lose their charge because almost no current can flow in an insulator. The result is known as "static cling," which causes clothes to hold tightly to the body and produces a crackling sound. Since moist air is somewhat conducting, static cling is a problem only when humidity is low. Products that reduce static cling contain complex molecules called humectants that bond with water molecules. These water molecules are eventually released and help to carry away the static charge from the fabric.

Many smoke detectors use the insulating properties of air. When voltage is placed across an air gap, normally no current flows. A fire produces many ions, however, and these ions cause the air gap to conduct, causing the smoke detector to sound an alarm. This sensitive device can detect a fire before smoke is visible to the eye.

Dielectrics are used in capacitors to increase both their capacitance and their breakdown voltage. In a somewhat similar fashion, solid insulators are used to support large transformer coils, which are then surrounded by oil. Oil has a higher breakdown voltage than air, and convection currents in the oil help control the temperature of the coils.

In summary, it should be obvious that insulators are essential for the functioning of every electrical device. They are just as essential as conductors for the operation of motors, generators, radios, electric lights, power lines, and so forth. The existence of those partial insulators, the semiconductors, made the transistor and integrated circuits possible, which in turn led to the computer revolution. Finally, it should be noted that even nature uses a system of ionic conductors and dielectrics in human nerve and muscle cells.

Context

It was the study of insulators, and not the study of conductors, that led to the discovery of electricity. Furthermore, these studies provided an important impetus to the development of science.

Thousands of years ago, petrified tree resin was discovered on the shores of the Baltic Sea. It was the golden transparent stone called "amber," and it was highly prized by many peoples. The Greeks used it in jewelry and as a palace decoration. Amber is easily charged by rubbing, and being an excellent insulator, it can hold a charge for many minutes. The Greek name for amber is elektron, which provides the modern term "electron."

The ancient Roman author Pliny relates that Syrian women who spun thread called amber "the clutcher," evidently a reference to its ability to attract small objects when charged. In spinning by hand, thread is attached to a spindle that is periodically dropped to the floor while still spinning in order to twist the thread. In this manner, a spindle made of amber becomes charged. The negative charge on the amber polarizes the atoms of a piece of fluff, leaving the side nearest the amber positively charged. Positive being attracted to negative, the fluff flies to the amber. Such experiments with static electricity were easy to perform and were often done for entertainment. The experiments were entrancing because an inanimate stone was showing a characteristic of life: The stone could pull things to it. This property of amber inspired many scientific studies.

Fascinated by displays of static electricity, Stephen Gray, in 1729, procured a glass tube with which to experiment that was about 1 meter long and about 3 centimeters in diameter.

In order to keep dust out, he corked the ends. Charging the glass by rubbing it, he noted the usual effect of small bits of feathers and foil being attracted to the charged glass. To his surprise, however, he found that they were also attracted to the corks in the ends of the tube. At once, he set about attaching other objects to the end of the glass tube. He soon showed that electric "virtue" (electric charge) can flow along wooden sticks, bricks, chalk, vegetables, twine, and a teakettle (full or empty). Eventually, Gray discovered that he could electrify a length of twine nearly 270 meters long, but only if it were suspended on wooden posts or by silk thread. Metal wires did not work, and Gray correctly supposed that metal allowed charge to flow out of the twine. Thus, Gray had discovered the distinction between conductors and insulators.

Principal terms

CAPACITOR: an electrical device that stores charge when voltage is applied to it

DIELECTRIC: an insulator

INSULATOR: a substance that is an extremely poor electrical conductor

POLARIZED: the state in which an atom has slightly more negative charge on one end and slightly more positive charge on the opposite end

RESISTANCE: the hindrance to the flow of electricity through a material, which depends upon the material, its size, and its shape

RESISTIVITY: the intrinsic resistance of a substance independent of the size and shape of the sample

Bibliography

Baumeister, Philip, and Gerald Pincus. "Optical Interference Coatings." SCIENTIFIC AMERICAN 223 (December, 1970): 59-75. This is a fascinating article about dielectric mirrors, dielectric filters, antireflective coatings, and their applications. Suitable for the general reader.

Benjamin, Park. A HISTORY OF ELECTRICITY. New York: John Wiley & Sons, 1898. Reprint. New York: Arno Press, 1975. This book is a marvelous source of information and is easily read. It traces electricity from antiquity to the days of Benjamin Franklin. Benjamin traces the growth of science and shows that even incorrect ideas can lead to useful results.

Friedel, Robert, and Paul Israel. EDISON'S ELECTRIC LIGHT: BIOGRAPHY OF AN INVENTION. New Brunswick, N.J.: Rutgers University Press, 1986. The electric light filament has to be a good enough conductor to carry current, but it must also have enough resistance to be heated to incandescence by the current. The development of the electric light was a symphony of conductors and insulators. As is often the case, the search for new technology also drove science forward.

Hewitt, Paul G. CONCEPTUAL PHYSICS: A NEW INTRODUCTION TO YOUR ENVIRONMENT. 2d ed. Boston: Little, Brown, 1971. The reader who has little background in electricity would do well to consult an excellent physical science textbook such as this one. Its purpose is to explain the concepts of physics to liberal arts students. Topics covered include conductors, insulators, semiconducters, electric fields, currents, resistance, the index of refraction, and the quantum nature of the atom.

Moore, A. P., ed. ELECTROSTATICS AND ITS APPLICATIONS. New York: John Wiley & Sons, 1973. While the book is largely descriptive, it does use some mathematics. Anyone should find the book to be helpful, but those who have had a good elementary physics course will get the most out of it. Topics covered include insulators, dielectrics, breakdown voltage, capacitors, conduction bands, fermi levels, and the conductivity of air. Applications include ink-jet printing and electrostatic precipitation.

Trotter, Donald M., Jr. "Capacitors." SCIENTIFIC AMERICAN 259 (July, 1988): 86-90B. An elementary and easily read article containing no mathematics. Begins with the Leyden jar, the capacitor of the early eighteenth century, and proceeds to the multilayer ceramic capacitor and the electrolytic capacitor. Discusses the role of the dielectric including those with dielectric constants as high as twenty thousand.

Conductors and Resistors

Electrical Properties of Solids