Forces on Dielectrics

11 Key excerpts on "Forces on Dielectrics"

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  Steady Electric Fields and Currents

  Elementary Electromagnetic Theory

  • B. H. Chirgwin, C. Plumpton, C. W. Kilmister(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 4 DIELECTRICS 4.1 The effects of a dielectric In Chapter 1 we defined insulators as substances on which, or inside which, electric charge is unable to move continuously. Experiments show that when a piece of insulating material is introduced into a region where an electric field is already established the field is modified, the distributions of charge and the forces acting on the conductors alter, and the potentials of the con-ductors change. In addition, the insulator itself experiences (ponderomotive) forces ; this is illustrated experimentally when dust particles and small pieces of paper are attracted to a charged body such as a gramophone record. Faraday investigated another electrostatic property of an insulating material^ viz. its effect on the capacitance of a condenser. He found that when the region of the field inside a condenser (e.g. between the plates of a parallel plate condenser, or between the spheres of a spherical condenser) was filled with an insulating substance the capacitance of the condenser was multiplied by a factor K (>1) which he was able to measure. The factor K does not depend upon the charge or potential of the capacitor, but varies from one material to another. Faraday called this factor the specific inductive capacity of the material; the modern name is dielectric constant. More recently it has been found that, under a wider range of conditions than those of Faraday's, investigations, the dielectric constant of a given material alters. However, for the steady conditions of an electrostatic field of moderate intensity, K may be taken to be independent of the field quantities. When an insulator is situated in a field which presumably penetrates into the material of the insulator we call the substance of the insulator a dielectric.

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  Handbook of Electrostatic Processes

  • Jen-Shih Chang, Arnold J. Kelly, Joseph M. Crowley(Authors)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  Electrostriction has no “inverse,” as far as nonpolar materials are con­ cerned: a mechanical pressure does not induce any dielectric phenomena. In contrast, a compression (or a traction), applied in a specific direction, induces an extra polarization in solid polar materials (a voltage can then be generated). This so-called piezoelectric effect—observed in asymmetrical mineral crystals (quartz, tourmaline) or in semicrystalline polymers (poly-vinylidene fluoride: PVDF)—is a reversible one. 2. Dielectrophoresis This concerns the motion of matter due to the polarization force (not the Coulomb force). A demonstrative example, treated in many textbooks, is that of a plane capacitor with vertical electrodes, filled with a gas (er = 1). If a solid foil or a liquid (of higher permittivity) is allowed to enter the electrode gap, energy considerations show that it will tend to occupy the whole gap. In a nonuniform electric field , polarization effects may induce vigorous motion of particulate bodies (conducting or insulating), particularly so in fluids. Since a particle is polarized by the field, the separation into positive and negative charges produces a dipole that is subjected to a net force in a divergent field. If its permittivity is higher than that of the host medium, this force tends to pull it toward the region of the higher field (whatever the sign of the field, e.g., even in an ac field). The translation force is given by F = jxe*grad E, μ€ being the field induced dipolar moment of the particle. This moment can be calculated for objects of particular shape subjected to a field E and is maximum for conducting materials. A classical ELECTRICAL PHENOMENA OF DIELECTRIC MATERIALS 61 result is that for a sphere of radius R in a medium of permittivity €r, μ&= 4ττ€0tr R 3E. The effect only concerns macroscopic objects and does not apply on a molecular scale. The volumic force density in a homogeneous medium is written f = -( E 2/2) grad e.

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  Electric Field

  • Mohsen Sheikholeslami Kandelousi(Author)
  • 2018(Publication Date)
  • IntechOpen

    (Publisher)

  Chapter 4 Dielectrics under Electric Field Liu Hongbo Additional information is available at the end of the chapter http://dx.doi.org/10.5772/intechopen.72231 Abstract The chapter first gives a brief introduction on conduction, polarization, dissipation, and breakdown of dielectrics under electric field. Then, two of electric field-related applica-tions, dielectrics for electrical energy storage and electrocaloric (EC) effect for refrigera-tion are discussed. Conclusion and perspectives are given at last. Keywords: dielectrics, electrical energy storage, electrocaloric refrigeration 1. Introduction Dielectrics are materials that can be polarized by an applied electric field. Polarizability is the essential property for dielectrics. The term is closely related to insulator. In electrical phenom-ena, insulator is commonly used especially in electronic engineering and electrical engineering, that is, in electronic packaging printed circuit board, electrical wire, high voltage system, and so on. It has a longer history than “ dielectrics. ” The main property of an insulator is to prevent the flow of current when it is not desired. This means that insulator must have low electrical conductivity and can resist breakdown under high electric field. Physically, insulator is a subgroup of dielectrics because of the existence of polarization. And dielectrics can include insulator, semiconductor, and other materials with polarizability. Nevertheless, poor insula-tion could screen the polarizability of dielectrics under electric field, which makes the polari-zation hard to be “ seen ” by electrical measurement. And in most cases, poor insulation makes dielectrics useless. Thus, insulating property is commonly expected for dielectrics.

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  Electromagnetism

  Maxwell Equations, Wave Propagation and Emission

  • Tamer Becherrawy(Author)
  • 2013(Publication Date)
  • Wiley-ISTE

    (Publisher)

  Chapter 4

  Dielectrics

  Insulators or dielectrics are mediums that contain no free charges. If a dielectric is placed in an electric field, it becomes polarized. We may consider the electronic polarization as due to the displacement of the electrons within the atoms and molecules and the orientational polarization due to the alignment of the polar molecules more or less in the direction of the electric field. The polarization of dielectrics explains some of their properties, particularly the propagation properties of electromagnetic waves (reflection, refraction, dispersion, etc.). Usually, the polarization disappears if the external field is removed, but some materials, called electrets, retain their polarization. These materials, (generally organic polymers, waxes, etc.) are the electrical analogs of permanent magnets. They are currently used in electrostatic microphones for modern phones. The purpose of this chapter is to study the polarization of dielectrics, the effects of dielectrics on the electric field, the field equations, and the energy.

  4.1. Effects of dielectric on capacitors

  In 1837, Faraday observed that, if a capacitor is maintained under a constant potential (by keeping it connected to a battery) and is filled with a dielectric, its charge is multiplied by a factor εr , which is a characteristic quantity of the dielectric called relative electric permittivity. Thus, the capacitance is multiplied by εr . On the other hand, if an empty capacitor is charged under a potential Vo ( Figure 4.1a ), the plates acquire charge densities ±qs and Gauss’s law gives the field Eo = qs /εo between the plates. If one disconnects the battery and fills the capacitor with the dielectric ( Figure 4.1b ), the charges of the isolated plates remains evidently the same, but the potential difference and hence the field are divided by εr to become

  [4.1]

  ε is the absolute permittivity (or the dielectric constant) of the dielectric. The capacitance becomes

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  Introduction to the Physics and Chemistry of Materials

  • Robert J. Naumann(Author)
  • 2008(Publication Date)
  • CRC Press

    (Publisher)

  The very thin dielectric layer that separates the gate from the channel in a FET is very susceptible to breakdown from static charge before the chip is installed in the device. For this reason such chips are shipped in conductive static-proof packages and extreme care in handling such components must be exercised. Unlike the Zener effect in which the breakdown voltage is engineered into the design of the dielectric, many of the above mechanisms are unpredictable and time-dependent, so the chances of breakdown increases with age. 438 Introduction to the Physics and Chemistry of Materials 23.2 Polarization in Dielectrics As stated previously, the electrical and optical properties are primarily determined by the dielectric material ’ s ability to form electric dipoles in the presence of an electric fi eld. An electric dipole can be thought of as a positive and a negative charge q separated by some distance d and the dipole moment is de fi ned as p ¼ q d . Macroscopically, the resulting electrical displacement, D ¼ « E ¼ « 0 E þ P , where « is the permittivity, P is the polarization, which is de fi ned as the dipole moment per unit volume, and « 0 is the permittivity of a vacuum ¼ 8.85 10 12 farads = m (Coulomb = V-m). For an anisotropic crystal, « is a tensor of rank two, but here we consider only isotropic materials, so « will be a scalar. It is convenient to de fi ne a relative dielectric constant « r ¼ « = « 0 . Then « r E ¼ E þ P = « 0 , P ¼ « 0 ( « r 1) E ¼ « 0 x E , where the electric susceptibility x ¼ « r 1. The susceptibility relates the amount of polarization to the applied fi eld, or as the name suggests, tells how susceptible a material is to being polarized by an electric fi eld. 23.2.1 Capacitors To illustrate how dielectrics interact electrically, we fi rst consider the capacitor, a device for storing electric charge. Basically, a capacitor is simply a sandwich consisting of a dielectric surrounded by two conductive surfaces.

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  Introduction to Electromagnetism

  • M. Sibley(Author)
  • 1995(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  If the dielectric is the insulating materia l in a capacitor , this causes a small, but very dramatic , explosion ! So, it is importan t to know the field at which the dielectric breaks down. This is the subjec t of Section 6.4. Some material s exhibit dipole moment s on a molecula r level, or macroscopi c scale . Figure 6.2 shows a charge embedde d in some insulating material . As this charge is negative , it tends to polarize the surrounding molecules producing molecular dipole moments . The net effect is that these molecula r dipoles are able to exert a force on each other, and on any externa l charges . Now, what happens if we introduce a test charge into this region? Logically each dipole momen t will exert a force on the test charge . However , the net force will be less than if the materia l was not there . This is becaus e the force acting on the test charge is due to the vector sum of the dipole moments . As the distribution of these dipoles is random throughou t the material , the net resul t is a smaller force than if the materia l was not present . An alternative explanation is to conside r the direction of the electric field produced by each dipole. As Fig. 6.2 shows , these fields act in the opposit e Electron cloud Electron cloud -■ V External E field External E field (6.2) 152 Dielectrics --~e Efield ' / o Testcharge / ~ Fig. 6.2 Effect of charge on molecular electric dipoles directionto thepoint charge field, soreducingthe field from thepoint charge. We can check the validity of this modelby noting D E=-So, if thechargeis in air, the force field is D E = ---Eo (6.3) (6.4) However,if the charge is inmaterialwith a relativepermittivity of En the force field is D E=--· which is lowerthanfor air. This impliesthatit is the polarizationof the material that causes apermittivity greaterthan air.

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  Electronic, Magnetic, and Optical Materials

  • Pradeep Fulay, Jung-Kun Lee(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  241 7 Linear Dielectric Materials 7.1 DIELECTRIC MATERIALS A dielectric material typically is a large-bandgap semiconductor ( E g ~ >4 eV) that exhibits high resistivity ( ρ ). The prefix dia means through in the Greek language. The word dielectric refers to a material that normally does not allow electricity (electrons, ions, and so on) to pass through it. There are special situations (for example, exposure to very high electric fields or changes in the composi-tion or microstructure) that may lead to a dielectric material exhibiting semiconducting or metallic behavior. However, when the term dielectric material is used, it generally is understood that the material essentially is a nonconductor of electricity. An electrical insulator is a dielectric material that exhibits a high breakdown field. 7.1.1 E LECTROSTATIC I NDUCTION To better understand the behavior of nonconducting materials, let us first examine the concept of electrostatic induction and what is meant by the terms free charge and bound charge . First, con-sider a dielectric such as a typical ceramic or a plastic that has a net positive charge on its surface. Now, assume that we bring a Conductor B near this charged Insulator A (Figure 7.1a). The electric field associated with the positively charged Insulator A pulls the electrons toward it from Conductor B. This is also described as the atoms in Conductor B being polarized or affected by the presence of an electric field. The process of the development of a negative charge on Conductor B is known as electrostatic induction. In this case, the negative charge developed on Conductor B is the bound charge because it is bound by the electric field caused by the presence of the charged Insulator A next to it. The creation of a bound negative charge on Conductor B, in turn, creates a net positive charge on the other side of the conductor because the conductor itself cannot have any net electric field within it.

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  General Physics Electromagnetism Optics

  • Pierluigi Zotto, Sergio Lo Russo, Paolo Sartori(Authors)
  • 2022(Publication Date)
  • Società Editrice Esculapio

    (Publisher)

  Dielectrics 6.1 Introduction Consider a parallel plate capacitor featuring plates of area A, placed at distance d from each other, charged at a voltage V0. Its capacitance is C 0 = ε 0 A d . Once charged, the capacitor is isolated, so the free charge q on the plates keeps constant, and a further conductive plate of thickness h < d is inserted in the middle of the capacitor. Electrostatic induction causes some free charge inside this plate to move to the plate faces in order to restore electrostatic equilibrium. Hence, the system becomes equivalent to a series connection of two identical capacitors, C 1 = C 2 , featuring plates of area A and dis- tance between the plates d − h ( ) 2 , so its capacitance is C = C 1 C 2 C 1 + C 2 = C 1 2 = 1 2 ε 0 A d − h 2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = ε 0 A d − h > C 0 . The capacitance increases and, being the free excess charge on the plates constant, a lower voltage across the capacitor V = q C < V 0 = q C 0 is measured. Since electrostatic induction is negligible in a dielectric, being free charges a very low number, it is expected that, if such an experiment is repeated by inserting a dielectric, noth- ing occurs, but, on the contrary, the voltage reduces anyway, even if by a smaller amount and with a magnitude depending on the material of the inserted plate. Therefore a dielectric reacts to the action of an external electric field, but no charge dis- placement occurs: this phenomenon is called electric polarisation of a dielectric. 6.2 Electric Polarisation Mechanisms The polarisation of a dielectric occurs by means of two possible mechanisms: orienta- tion polarisation and electronic polarisation. The molecules in a dielectric can be polar, that is, the atoms which constitute them can be arranged in an asymmetric way. In this case they have a small electric dipole moment of the order of p ~10 –30 Cm.

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  Engineering Electromagnetics

  Pergamon Unified Engineering Series

  • David T. Thomas, Thomas F. Irvine, James P. Hartnett, William F. Hughes(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  The basic happening in dielectric materials is polarization, or separation of charge centers of orbital electrons and nuclei, of positive and negative ions with-out physically breaking their bonds and freeing them. EXERCISES 4-1. The plane z = 0 is the boundary between dielectric and free space. Near z = 0 the polarization, P, varies as shown in Fig. E.4-1. Estimate p P everywhere, including surface polarization charge, p SP , on the dielectric surface. 4-2. A solid dielectric cylinder of length L and radius a is uniformly polarized with Fig. E.4-1. Polarization near a boundary. 138 Dielectric Materials axially directed polarization, P. Determine the electric field along the axis, both within and outside the cylinder. 4-3. Show from the definition of P that dP/dt has dimensions of a current density, J P . Show }p is consistent with the expression, where p, v are charge density and velocity, respectively. 4-4. An electric field of 10,000 V/m is applied to a helium gas. For atom density n = 5 x 10 25 atoms/m 3 and an average electron cloud shift of 10 18 m find the polarization, P, polarization charge, p P , and permittivity of the helium. 4-5. A coaxial capacitor is partially filled with dielectric as shown. The inner and outer conductor radii are a, c respectively. The dielectric fills the coaxial to radius b. Find the fields. How does the presence of dielectric affect possible breakdown of the air? 4-6. A long coaxial capacitor is immersed in water (e=8l€ 0 ) (see Fig. E.4-6). If a voltage V 0 is applied, how far will the liquid rise in the capacitor? 4-7. The plasma frequency is the natural oscillatory frequency of an ionized gas. Fig.E.4-5. Dielectric loaded coaxial capacitor. Assuming 100% ionization, what is the plasma frequency of a hydrogen gas (density 10-3 mmHg)? HINT: The force on an electron in an electric field is eE, which causes motion predicted by Newton's Law, 4-8.

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  Electromagnetism for Engineers

  • Andrew J. Flewitt(Author)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  In this case, heavily doped p-type silicon (p + Si) and n-type silicon (n Si) are indicated. Electric Fields in Materials 21 a voltage is applied between these two terminals. It is important that the current flow in the device is dominated by that between the drain and source and is not significantly due to any current across the dielectric from the gate. Therefore, it is vital that the gate does not break down. However, to increase processing power in a microprocessor or the capacity of a memory, it is necessary to decrease the physical size of each of the transistors. As the gate dielectric is made thinner, so the voltages applied to the device have to be reduced to avoid breakdown of the dielectric. In practice, dielectrics will conduct at much lower fields than the breakdown field either due to quantum mechanical tunnelling through the dielectric, tunnelling of electrons between localized states in the dielectric or even an ohmic conduc- tion due to a small number of free charge carriers that are present. There is therefore interest in finding materials with high relative permittivity, as increasing the permittivity allows a lower voltage to be used to induce the same charge in the semiconductor and therefore the same switching effect in the transistor, but with lower electric fields (Lo et al. 1997). While we have considered polarization in a dielectric to be due to the displacement of the negative and positive charge on an atomic scale, other processes can also take place, such as deformation of an ionic lattice or rotation of molecules with an inherent dipole moment. Liquid crystals are materials in that consists of long chain molecules which, although in a liquid state, are ordered with the same orientation within a macroscopic region of space called a domain. A consequence of this is that the relative permittivity is not isotropic, but is dependent on the direction in which an electric field is acting relative to the direction of orientation.

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  Principles of Engineering Physics 2

  • Md Nazoor Khan, Simanchala Panigrahi(Authors)
  • 2017(Publication Date)
  • Cambridge University Press

    (Publisher)

  Electronic, ionic and orientation polarization contributions to the dielectric constant have been shown electricity. This phenomenon in which a dielectric material conducts electricity under a certain high potential difference is called dielectric breakdown and the corresponding high potential difference is called breakdown voltage. The minimum electric field at which a dielectric conducts electricity is called dielectric strength and is measured in volt/meter. The following three types of mechanism play a role in the dielectric breakdown phenomenon. i. Avalanche breakdown/intrinsic breakdown. ii. Thermal breakdown. iii. Electrochemical breakdown. iv. Defect breakdown. 9.8.1 Avalanche breakdown Dielectrics are perfect insulators. They have a completely filled valence band and a completely empty conduction band. The band gap between the conduction band and the valence band is more than 5 eV. When a dielectric is subjected to high electric field, the electron in the valence band gains sufficient energy to be excited to the conduction band above the valence band. These electrons are accelerated highly by the high electric field and by collisions, they excite more electrons to the conduction band. Thus, more and more electrons are released to the conduction band resulting in an avalanche of conduction electrons. Ultimately, the dielectric becomes highly conducting and conducts large currents. 336 Principles of Engineering Physics 2 Characteristics i. It can occur at any temperature, even at low temperatures. ii. High voltage or large electric field is essential for its occurrence. iii. Avalanche breakdown/intrinsic breakdown occurs generally in thin samples. iv. It does not depend upon the electrode configuration or shape of the samples. 9.8.2 Thermal breakdown When a high frequency electric field is applied to a dielectric material, due to energy loss, heat is produced in it. This heat from the dielectric has to be dissipated out from the material.

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