What materials are called dielectrics. Dielectric materials
In order to determine: what are dielectrics in physics, we recall that the most important characteristic of a dielectric is polarization. In any substance, free charges move under the influence of electric field, an electric current appears, and the bound charges are polarized. Substances are divided into conductors and dielectrics, depending on which charges predominate (free or bound). In dielectrics, polarization occurs mainly under the influence of an external electric field. If you cut a conductor in an electric field, you can separate the charges of different signs. This cannot be done with the polarization charges of a dielectric. In metallic conductors, free charges can move over long distances, while in dielectrics, positive and negative charges move within a single molecule. In dielectrics, the energy band is completely filled.
If there is no external field, then charges having different signs are uniformly distributed throughout the volume of the dielectric. In the presence of an external electric field, the charges entering the molecule are displaced in opposite directions. This displacement manifests itself as the appearance of a charge on the surface of the dielectric, when it is placed in an external electric field - this is the phenomenon of polarization.
The polarization depends on the type in the dielectric. So, in ionic crystals, polarization occurs mainly due to the shift of ions in an electric field and only slightly due to deformation of the electron atomic shells. Whereas in diamond, which has a covalent chemical bond, polarization occurs due to the deformation of the electron atomic shells in an electric field.
A dielectric is called polar if its molecules have their own electric dipole moment. In such dielectrics, in the presence of an external electric field, the electric dipole moments are oriented along the field.
The polarization of a dielectric is determined using the polarization vector. This value is equal to the sum of the electric dipole moments of all molecules in a unit volume of the substance. If the dielectric is isotropic, then the equality holds:
where is the electrical constant; is the dielectric susceptibility of the substance. The dielectric susceptibility of a substance is related to the permittivity as:
where - characterizes the weakening of the external electric field in the dielectric due to the presence of polarization charges. Polar dielectrics have the largest values. So, for water = 81.
In some dielectrics, polarization occurs not only in an external electric field, but also under mechanical stresses. These dielectrics are called piezoelectrics.
Dielectrics have a much higher electrical resistivity than conductors. It lies in the interval: Ohm / cm. Therefore, dielectrics are used for the manufacture of insulation of electrical devices. An important application of dielectrics is their use in electrical capacitors.
All liquid and solid substances, according to the nature of the action of an electrostatic field on them, are divided into conductors, semiconductors and dielectrics.
Dielectrics (insulators) Substances that conduct electricity poorly or not at all. Dielectrics include air, some gases, glass, plastics, various resins, and many types of rubber.
If neutral bodies made of materials such as glass, ebonite are placed in an electric field, one can observe their attraction to both positively charged and negatively charged bodies, but much weaker. However, when such bodies are separated in an electric field, their parts turn out to be neutral, like the whole body as a whole.
Consequently, in such bodies there are no free electrically charged particles, able to move in the body under the influence of an external electric field. Substances that do not contain free electrically charged particles are called dielectrics or insulators.
The attraction of uncharged dielectric bodies to charged bodies is explained by their ability to polarization.
Polarization- the phenomenon of displacement of bound electric charges inside atoms, molecules or inside crystals under the action of an external electric field. Simplest polarization example is the action of an external electric field on a neutral atom. In an external electric field, the force acting on the negatively charged shell is directed opposite to the force acting on the positive nucleus. Under the influence of these forces, the electron shell is somewhat displaced relative to the nucleus and deformed. The atom remains generally neutral, but the centers of positive and negative charge in it no longer coincide. Such an atom can be considered as a system of two point charges equal in absolute value of the opposite sign, which is called a dipole. 
If a dielectric plate is placed between two metal plates with opposite charges, all dipoles in the dielectric under the action of an external electric field turn out to be positively charged to the negative plate and negatively charged to the positively charged plate. The dielectric plate remains generally neutral, but its surfaces are covered with bound charges opposite in sign.
In an electric field, polarization charges on the dielectric surface create an electric field opposite to the external electric field. As a result, the electric field strength in the dielectric decreases, but does not become equal to zero.
The ratio of the strength modulus E 0 of the electric field in vacuum to the strength modulus E of the electric field in a homogeneous dielectric is called permittivity ɛ of a substance:
ɛ \u003d E 0 / E
When two point electric charges interact in a medium with permittivity ɛ, as a result of a decrease in the field strength by ɛ times, the Coulomb force also decreases by ɛ times:
F e \u003d k (q 1 q 2 / ɛr 2)
Dielectrics are capable of weakening an external electric field. This property is used in capacitors.
Capacitors are electrical devices for the accumulation of electric charges. The simplest capacitor consists of two parallel metal plates separated by a dielectric layer. When communicating to the plates equal in magnitude and opposite in sign charges +q and -q between the plates creates an electric field with intensity E. Outside the plates, the action of electric fields directed by oppositely charged plates is mutually compensated, the field strength is zero. Voltage U between the plates is directly proportional to the charge on one plate, so the charge ratio q to voltage U
C=q/U
is a constant value for the capacitor for any charge values q. This attitude FROM is called the capacitance of the capacitor.
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The permittivity may have dispersion.
A number of dielectrics exhibit interesting physical properties.
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See what "Dielectrics" are in other dictionaries:
DIELECTRIC, substances that conduct electricity poorly (resistivity of the order of 1010 ohm?m). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of the dielectric. In some hard ... ... Modern Encyclopedia
Dielectrics- DIELECTRIC, substances that conduct electricity poorly (resistivity of the order of 1010 Ohm'm). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of the dielectric. In some hard ... ... Illustrated Encyclopedic Dictionary
Substances that conduct electricity poorly (electrical resistivity 108 1012 ohm cm). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of dielectrics. In some solid dielectrics ... ... Big encyclopedic Dictionary
- (English dielectric, from Greek dia through, through and English electric electric), substances that conduct electricity poorly. current. The term "D." introduced by Faraday to designate in in, in which electric penetrates. field. D. yavl. all gases (non-ionized), some ... Physical Encyclopedia
DIELECTRIC- DIELECTRIC, non-conductors, or insulators of the body that conduct electricity poorly or not at all. Such bodies are eg. glass, mica, sulfur, paraffin, ebonite, porcelain, etc. For a long time in the study of electricity ... ... Big Medical Encyclopedia
- (insulators) substances that do not conduct electricity. Examples of dielectrics: mica, amber, rubber, sulfur, glass, porcelain, various types of oils, etc. Samoilov K.I. Marine Dictionary. M. L .: State Naval Publishing House of the NKVMF of the Union ... Marine Dictionary
The name given by Michael Faraday to bodies that are non-conductive or, otherwise, badly conduct electricity, such as air, glass, various resins, sulfur, etc. Such bodies are also called insulators. Before Faraday's research, carried out in the 30s ... ... Encyclopedia of Brockhaus and Efron
DIELECTRIC- Substances that practically do not conduct electric current; are solid, liquid and gaseous. D. are polarized in an external electric field. They are used to isolate electrical devices, in electrical capacitors, in quantum ... ... Great Polytechnic Encyclopedia
Substances that conduct electricity poorly. The term "D." (from Greek diá through and English electric electric) was introduced by M. Faraday (See Faraday) to designate substances through which electric fields penetrate. In any substance... Great Soviet Encyclopedia
Substances that conduct electricity poorly (electrical conductivity of a dielectric is 10 8 10 17 Ohm 1 cm 1). There are solid, liquid and gaseous dielectrics. An external electric field causes polarization of dielectrics. In some hard ... ... encyclopedic Dictionary
Books
- Dielectrics and Waves, A. R. Hippel. The author of the monograph brought to the attention of readers, a well-known researcher in the field of dielectrics, the American scientist A. Hippel has repeatedly spoken in the periodical press and in ...
- Action of laser radiation on polymeric materials. Scientific foundations and applied problems. In 2 books. Book 1. Polymeric materials. Scientific foundations of laser action on polymeric dielectrics, B. A. Vinogradov, K. E. Perepelkin, G. P. Meshcheryakova. The proposed book contains information about the structure and basic thermal and optical properties of polymeric materials, the mechanism of action of laser radiation on them in infrared, visible ...
A dielectric is a substance that does not or does not conduct electricity well. Charge carriers in a dielectric have a density of no more than 108 pieces per cubic centimeter. One of the main properties of such materials is the ability to polarize in an electric field.
The parameter that characterizes dielectrics is called the permittivity, which can have dispersion. Dielectrics include chemically pure water, air, plastics, resins, glass, and various gases.
Properties of dielectrics
If substances had their own heraldry, then the coat of arms of Rochelle salt would certainly be decorated with vines, a hysteresis loop, and the symbolism of many branches of modern science and technology.
The pedigree of Rochelle salt begins in 1672. When the French pharmacist Pierre Segnet first obtained colorless crystals from vines and used them for medicinal purposes.
Then it was still impossible to assume that these crystals have amazing properties. These properties gave us the right to distinguish special groups from a huge number of dielectrics:
- Piezoelectrics.
- Pyroelectrics.
- Ferroelectrics.
It has been known since the time of Faraday that dielectric materials are polarized in an external electric field. In this case, each elementary cell has an electric moment similar to an electric dipole. And the total dipole moment per unit volume determines the polarization vector.
In conventional dielectrics, the polarization uniquely and linearly depends on the magnitude of the external electric field. Therefore, the dielectric susceptibility of almost all dielectrics is constant.
P/E=X=const
The crystal lattices of most dielectrics are built from positive and negative ions. Of the crystalline substances, crystals with a cubic lattice have the highest symmetry. Under the action of an external electric field, the crystal is polarized, and its symmetry decreases. When the external field disappears, the crystal restores its symmetry.
In some crystals, electric polarization can occur spontaneously even in the absence of an external field. This is what a crystal of gadolinium molybdenate looks like in polarized light. Usually spontaneous polarization is non-uniform. The crystal is divided into domains - regions with uniform polarization. The development of a multidomain structure reduces the total polarization.
Pyroelectrics
In pyroelectrics, spontaneous polarization shields with free charges that cancel out bound charges. Heating a pyroelectric changes its polarization. At the melting temperature, pyroelectric properties disappear altogether.
Some pyroelectrics are classified as ferroelectrics. Their direction of polarization can be changed by an external electric field.
There is a hysteresis dependence between the polarization orientation of a ferroelectric and the magnitude of the external field.
In sufficiently weak fields, the polarization depends linearly on the field strength. With its further increase, all domains are oriented along the direction of the field, passing into the saturation mode. When the field is reduced to zero, the crystal remains polarized. The segment CO is called the residual polarization.
The field at which the direction of polarization changes, the segment DO is called the coercive force.
Finally, the crystal completely reverses the direction of polarization. With the next change in the field, the polarization curve closes.
However, the ferroelectric state of a crystal exists only in a certain temperature range. In particular, Rochelle salt has two Curie points: -18 and +24 degrees, at which second-order phase transitions occur.
Groups of ferroelectrics
The microscopic theory of phase transitions divides ferroelectrics into two groups.
First group
Barium titanate belongs to the first group, and as it is also called, the group of ferroelectrics of the displacement type. In the non-polar state, barium titanate has cubic symmetry.
During the phase transition to the polar state, the ionic sublattices are displaced, and the symmetry of the crystal structure decreases.
Second group
The second group includes crystals of the sodium nitrate type, which have a disordered sublattice of structural elements in the nonpolar phase. Here, the phase transition to the polar state is associated with the ordering of the crystal structure.
Moreover, in different crystals there can be two or more probable positions of equilibrium. There are crystals in which the dipole chains have antiparallel orientations. The total dipole moment of such crystals is zero. Such crystals are called antiferroelectrics.
In them, the polarization dependence is linear, up to the critical value of the field.
A further increase in the field strength is accompanied by a transition to the ferroelectric phase.
Third group
There is another group of crystals - ferroelectrics.
The orientation of their dipole moments is such that in one direction they have the properties of antiferroelectrics, and in another direction they have the properties of ferroelectrics. Phase transitions in ferroelectrics are of two kinds.
During a second-order phase transition at the Curie point, the spontaneous polarization gradually decreases to zero, while the dielectric susceptibility, changing sharply, reaches enormous values.
In a first-order phase transition, the polarization disappears abruptly. The electrical susceptibility also changes abruptly.
The large value of the dielectric constant, the electropolarization of ferroelectrics, makes them promising materials. modern technology. For example, the nonlinear properties of transparent ferroelectric ceramics are already widely used. The brighter the light, the more it is absorbed by the special glasses.
This is an effective eye protection for workers in some industries where sudden and intense flashes of light are involved. To transmit information using a laser beam, ferroelectric crystals with an electro-optical effect are used. Within the line of sight, the laser beam is simulated in the crystal. Then the beam enters the complex of receiving equipment, where the information is extracted and reproduced.
Piezoelectric effect
In 1880, the Curie brothers discovered that during the deformation of Rochelle salt, polarization charges arise on its surface. This phenomenon has been called the direct piezoelectric effect.
If the crystal is exposed to an external electric field, it begins to deform, that is, an inverse piezoelectric effect occurs.
However, these changes are not observed in crystals having a center of symmetry, for example, in lead sulfide.
If such a crystal is exposed to an external electric field, the sublattices of negative and positive ions will shift in opposite directions. This leads to the polarization of the crystals.
In this case, we observe electrostriction, in which the deformation is proportional to the square of the electric field. Therefore, electrostriction is referred to the class of even effects.
∆X1=∆X2
If such a crystal is stretched or compressed, then the electric moments of the positive dipoles will be equal in magnitude to the electric moments of the negative dipoles. That is, there is no change in the polarization of the dielectric, and the piezoelectric effect does not occur.
In crystals with low symmetry, additional forces of the inverse piezoelectric effect appear during deformation, counteracting external influences.
Thus, in a crystal without a center of symmetry in the charge distribution, the magnitude and direction of the displacement vector depend on the magnitude and direction of the external field.
Due to this, it is possible to carry out various types of deformation of piezocrystals. By gluing piezoelectric plates, you can get a compression element.
In this design, the piezoelectric plate works in bending.
Piezoceramic
If an alternating field is applied to such a piezoelectric element, elastic oscillations will be excited in it and acoustic waves will arise. Piezoceramics are used to make piezoelectric products. It represents polycrystals of ferroelectric compounds or solid solutions based on them. By changing the composition of the components and the geometric shapes of ceramics, it is possible to control its piezoelectric parameters.
Direct and inverse piezoelectric effects are used in a variety of electronic equipment. Many components of electro-acoustic, radio-electronic and measuring equipment: waveguides, resonators, frequency multipliers, microcircuits, filters work using the properties of piezoceramics.
Piezoelectric motors
The active element of the piezoelectric motor is the piezoelectric element.
During one period of oscillation of the source of an alternating electric field, it stretches and interacts with the rotor, and in the other it returns to its original position.
Excellent electrical and mechanical characteristics allow the piezo motor to compete successfully with conventional electric micromachines.
Piezoelectric transformers
The principle of their operation is also based on the use of the properties of piezoceramics. Under the action of the input voltage in the exciter, an inverse piezoelectric effect occurs.
The deformation wave is transmitted to the generator section, where, due to the direct piezoelectric effect, the polarization of the dielectric changes, which leads to a change in the output voltage.
Since the input and output of a piezotransformer are galvanically isolated, the functionality of converting the input signal by voltage and current, matching it with the load by input and output, is better than that of conventional transformers.
Research into various phenomena of ferroelectricity and piezoelectricity continues. There is no doubt that devices based on new and surprising physical effects in solids will appear in the future.
Classification of dielectrics
Depending on various factors, they show their insulation properties in different ways, which determine their scope of use. The diagram below shows the classification structure of dielectrics.
In the national economy, dielectrics consisting of inorganic and organic elements have become popular.
Inorganic materials are compounds of carbon with various elements. Carbon has a high capacity for chemical compounds.
Mineral dielectrics
This type of dielectric appeared with the development of the electrical industry. The production technology of mineral dielectrics and their types has been significantly improved. Therefore, such materials are already replacing chemical and natural dielectrics.
Mineral dielectric materials include:
- Glass(capacitors, lamps) - an amorphous material, consists of a system of complex oxides: silicon, calcium, aluminum. They improve the dielectric properties of the material.
- glass enamel- applied to a metal surface.
- Fiberglass- glass filaments from which fiberglass fabrics are obtained.
- Light guides- light-conducting glass fiber, a bundle of fibers.
- Sitally- crystalline silicates.
- Ceramics- porcelain, steatite.
- Mica- mikalex, mica, micanite.
- Asbestos- minerals with a fibrous structure.
Various dielectrics do not always replace each other. Their scope depends on the cost, ease of use, properties. In addition to insulating properties, thermal and mechanical requirements are imposed on dielectrics.
Liquid dielectrics
Petroleum oils
transformer oil poured into . It is most popular in electrical engineering.
Cable oils used in manufacturing. They impregnate the paper insulation of cables. This increases the electrical strength and removes heat.
Synthetic liquid dielectrics
To impregnate capacitors, a liquid dielectric is needed to increase the capacitance. Such substances are synthetic-based liquid dielectrics, which are superior to petroleum oils.
Chlorinated hydrocarbons are formed from hydrocarbons by replacing molecules of hydrogen atoms in them with chlorine atoms. The polar products of diphenyl, which include C 12 H 10 -nC Ln, are very popular.
Their advantage is resistance to burning. Among the shortcomings can be noted their toxicity. The viscosity of chlorinated biphenyls is high, so they have to be diluted with less viscous hydrocarbons.
Silicone fluids have low hygroscopicity and high temperature resistance. Their viscosity depends very little on temperature. Such liquids are expensive.
Organofluorine liquids have similar properties. Some liquid samples can work at 2000 degrees for a long time. Such liquids in the form of octol consist of a mixture of isobutylene polymers obtained from petroleum cracking gas products and are of low cost.
natural resins
Rosin- This is a resin with increased fragility, and obtained from resin (pine resin). Rosin consists of organic acids, easily soluble in petroleum oils when heated, as well as in other hydrocarbons, alcohol and turpentine.
The softening point of rosin is 50-700 degrees. In the open air, rosin oxidizes, softens faster, and dissolves worse. Dissolved rosin in petroleum oil is used to impregnate cables.
Vegetable oils
These oils are viscous liquids that are obtained from various plant seeds. Most important are drying oils, which can solidify when heated. A thin layer of oil on the surface of the material, when dried, forms a solid, durable electrical insulating film.
The drying rate of the oil increases with increasing temperature, lighting, when using catalysts - driers (compounds of cobalt, calcium, lead).
Linseed oil has a golden yellow color. It is obtained from flax seeds. The pour point of linseed oil is -200 degrees.
Tung oil made from the seeds of the tung tree. This tree grows on Far East and also in the Caucasus. This oil is non-toxic, but not edible. Tung oil hardens at a temperature of 0-50 degrees. Such oils are used in electrical engineering for the production of varnishes, varnished fabrics, wood impregnation, and also as liquid dielectrics.
Castor oil is used to impregnate paper-filled capacitors. This oil is obtained from castor bean seeds. It freezes at a temperature of -10 -180 degrees. Castor oil is easily soluble in ethyl alcohol, but insoluble in gasoline.
Dielectric materials in electronic equipment are electrically separated, and solid materials are mechanically combined and combined by conductors under different electrical potentials. They are used for electrical insulation of equipment elements, for the accumulation of electric field energy (capacitors), for the manufacture of structural parts, as well as in the form of coatings on the surface of parts, for gluing parts.
Dielectric properties of materials
The main property of a dielectric is not to conduct electric current. SPECIFIC VOLUME RESISTANCE of dielectrics is high: from 108 to 1018 ohm, since there are almost no free charge carriers in them. Some conduction is caused by impurities and structural defects.
There are always more impurities and defects on the surface of any body, therefore, for dielectrics, the concept of surface conductivity and the SPECIFIC SURFACE RESISTANCE s parameter are introduced, defined as the resistance measured between two linear conductors 1 m long each, located parallel to each other at a distance of 1 m on the surface of the dielectric . The value of s strongly depends on the method of obtaining (processing) the surface and its condition (dust content, moisture, etc.). Since the surface conductivity is usually much greater than the bulk conductivity, measures are taken to reduce it.
A dielectric is an insulator only with respect to direct voltage. In an alternating electric field, a current flows through the dielectric due to its polarization.
POLARIZATION is the process of displacement of bound charges over a limited distance under the action of an external electric field.
The electrons of atoms are shifted towards the positive pole, the nuclei of atoms - towards the negative. The same thing happens with ions in ionic crystals, with molecules or regions of molecules with an uneven distribution of charged particles in the volume they occupy. As a result of polarization in the dielectric, its own internal field is formed, its vector is smaller in magnitude and opposite in direction to the external field vector. The electric capacitance between electrodes with a dielectric is greater than between the same electrodes without a dielectric by a factor of where is the RELATIVE DIELECTRIC PERMITTIVATION OF THE DIELECTRIC.
During ELECTRONIC POLARIZATION, under the action of an external electric field, the electron shells of the atoms of a substance are deformed. It is characterized by a short (about 10–15 s) settling time and, therefore, is inertialess for radio frequencies, does not depend on frequency, weakly depends on temperature, and occurs with virtually no losses. Substances with predominantly electronic polarization (weakly polar dielectrics) have a low dielectric constant: from 1.8 to 2.5. This type of polarization is inherent in all substances.
IONIC POLARIZATION occurs in ionic solids, has a settling time of the order of 10-13 s, therefore, it practically does not depend on the field frequency, and weakly depends on temperature. The value for most materials with ionic polarization is from 5 to 10.
DIPOLE (ORIENTATIONAL) POLARIZATION manifests itself as orientation under the action of the field of polar molecules or groups of atoms. For example, water molecules are polar, in which hydrogen atoms are located asymmetrically relative to the oxygen atom, or vinyl chloride (polyvinyl chloride monomer) H2C-CHCl. To overcome the interaction of molecules and friction forces, the energy of the field is consumed, which turns into thermal energy, therefore, the dipole polarization has an inelastic, relaxation character. Due to the large sizes and masses of the dipoles involved in the dipole polarization, its inertia is significant and manifests itself in the form of a strong dependence of the permittivity and energy loss on frequency.
MIGRATION POLARIZATION is caused by inelastic displacements of weakly bound impurity ions over short distances. In terms of consequences (energy loss, frequency dependence), this polarization is similar to a dipole one.
The energy loss in the dielectric during polarization is estimated by the LOSS ANGLE tangent tg . A dielectric with losses in an electrical circuit is represented as an equivalent circuit: an ideal capacitor and a loss resistance connected in parallel to it. The angle complements up to 90o the shift angle between current and voltage on the vector diagram of such a two-terminal network. Good (weakly polar) dielectrics have tg10-3, which depends little on frequency. Bad dielectrics have tg, measured in tenths of a unit and even more, strongly dependent on frequency.
Special types form polarization under the action of mechanical stresses, observed in PIEZOELECTRIC, as well as SPONTANEOUS POLARIZATION in PYROELECTRIC and FERROELECTRIC. Such dielectrics are called ACTIVE and are used in special devices: in resonators, filters, piezoelectric generators and transformers, radiation converters, high specific capacitance capacitors, etc.
ELECTRICAL STRENGTH - the ability of a dielectric to maintain high resistivity in high voltage circuits. It is estimated by the breakdown field strength Еpr=Upr/d, where Upr is the breakdown voltage, d is the thickness of the dielectric. Dimension Epr - V / m. For different dielectrics, Epr=10...1000 MV/m, and even for the same material, this value varies widely depending on the thickness, shape of the electrodes, temperature, and a number of other factors. The reason for this is the variety of processes during breakdown. ELECTRIC BREAKDOWN is caused by the tunneling transition of electrons to the conduction band from the valence band, from impurity levels or metal electrodes, as well as by their avalanche multiplication due to impact ionization in high-intensity fields. ELECTRICAL BREAKDOWN is caused by an exponential increase in the electrical conductivity of a dielectric with an increase in its temperature. At the same time, the leakage current increases, heating the dielectric even more, a conducting channel is formed in its thickness, the resistance drops sharply, and melting, evaporation, and destruction of the material occur in the thermal impact zone. ELECTROCHEMICAL BREAKDOWN is due to the phenomena of electrolysis, ion migration and, consequently, changes in the composition of the material. IONIZATION BREAKDOWN occurs as a result of partial discharges in a dielectric with air inclusions. The electrical strength of air is lower, and the field strength in these inclusions is higher than in a dense dielectric. This type of breakdown is typical for porous materials. SURFACE BREAKDOWN (OVERLAPPING) of the dielectric occurs due to unacceptably large surface currents. With sufficient power of the current source, the surface breakdown develops through the air and turns into an arc one. Conditions contributing to this breakdown: cracks, other irregularities and contamination on the surface of the dielectric, humidity, dust, low atmospheric air pressure.
For reliable operation of any electrical device, the operating voltage of its insulation Uwork must be significantly less than the breakdown voltage Upr. The ratio Upr/Urab is called the INSULATION ELECTRICAL STRENGTH STOCK FACTOR.