Physics
DC Conductivity
DC conductivity refers to the ability of a material to conduct electric current under a direct current (DC) electrical field. It is a measure of how easily charges can move through a material in the absence of an alternating current. This property is important in understanding the behavior of materials in electronic and electrical applications.
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9 Key excerpts on "DC Conductivity"
eBook - PDFElectrical Processes in Organic Thin Film Devices
From Bulk Materials to Nanoscale Architectures
- Michael C. Petty(Author)
- 2022(Publication Date)
- Wiley(Publisher)
159 6 DC Conductivity 6.1 Introduction At a fundamental level, the presence of a DC voltage will produce a drift in free charges within a solid. The movement of the charges constitutes an electric current. If, at the same time, excess carriers are generated, e.g. by optical means, the process of diffusion will also contribute to the current (Section 2.7). For those materials with no or very few of these charges, the main effect of the applied field is to polarize the material. The result is a separation of the centres of positive and negative charge and the creation of electric dipoles (Section 7.2). The application of a step DC voltage across a low conductivity sample will produce an initial displacement current (Section 7.3.1) that can exceed the small ionic and electronic contributions over a long period. In extreme cases, a steady current reading will never be obtained, and a direct measurement of conductivity will not be possible. It can therefore be more appropriate to use AC methods to study electrical phenomena; this is discussed in Chapter 7. Some of the important DC, or low frequency, electrical conductivity processes observed in organic solids are outlined in Sections 6.2–6.9. These can occur simultaneously, and each may dominate at different values of the applied electric field and/or over different temperature ranges. It can therefore be a challenge to identify the physical mechanism(s) at work. Moreover, the equations governing the individual conductivity processes may contain many unknowns, each of which needs to be determined before definite conclusions can be drawn. John von Neumann famously said, ‘With four parameters I can fit an elephant, and with five I can make him wiggle his trunk’. By this, he meant that one should not be impressed when a complex model fits a data set well.
eBook - PDFElectronics
Fundamentals for the Water and Wastewater Maintenance Operator
- Frank R. Spellman, Joanne Drinan(Authors)
- 2000(Publication Date)
- CRC Press(Publisher)
Direct Current (D-C) Review TOPICS Current Flow Potential Difference (Voltage) Resistance Ohm's Law Series and Parallel Resistive Circuits Electric Power E-I Graph Kirchhoff's Laws Voltage and Current Dividers Switches Capacitors 3.1 INTRODUCTION Just as the foundation of a new house is usually constructed before the rest of the house is built upon it, the basics of electricity must be stud-ied first before attempting to study electronics. This chapter is a review on those basic aspects of d-c (direct current) that apply to electronics. By no means does it cover the whole d-c theory (for more in depth coverage of d-c, we recommend the first volume of this series: Electricity), but merely those topics that are essential to basic electronics. CURRENT FLOW Electron movement, or flow, in a conductor is called electric current. To produce current, the electrons must be moved by a potential difference (or voltage). The flow of water is usually measured as the number of Direct Current (D-C) Review 25 3 Direct Current (D-C) Review 3.2 Key Terms Used in This Chapter AMPERE The basic unit of electrical current. CAPACITOR CIRCUIT CONDUCTANCE CONDUCTOR DIRECT CURRENT (D-C) ELECTRON FUSE KIRCHHOFF'S LAWS LOAD OHM'S LAW POTENTIAL Two electrodes or sets of electrodes in the form of plates, separated from each other by an insulating material called the dielectric. The complete path of an electric current. The ability of a material to conduct or carry an electric current. It is the reci-procal of the resistance of the material and is expressed in mhos. Any material suitable for carrying electric current. An electric current that flows in one direction only. A negatively charged particle of matter. A protective device inserted in series with a circuit. It contains a metal that will melt or break when current is increased beyond a specific value for a definite period of time.
eBook - PDFElectrotechnology NQF2 SB
TVET FIRST
- Jowaheer Consulting and Technologies R Van Heerden(Author)
- 2013(Publication Date)
- Macmillan(Publisher)
When this happens, there is a flow of electric charge. This flow of electrons is known as electric current . Materials that let electric current pass through them are called conductors (see Fig. 1.10). These materials have a large number of free electrons and a high electrical conductivity . Conductivity is a measure of a material’s ability to carry electric current. Aluminium, gold, silver and copper are examples of conductors. Most metals 10 Module 1: Principles and applications of direct current technology (a) (b) Shell number n Maximum number of electrons 1 18 3 are good conductors. Carbon is the only non-metal that is a good conductor. Silver is the best conductor. Copper wires are widely used as conducting cables for electrical connections. Table 1.1 lists some metals in order of conductivity. Metal Conductivity (%) Silver 100 Copper 98 Gold 78 Aluminium 61 Zinc 30 Iron 16 Lead 15 Tin 9 Nickel 7 Table 1.1 Metals in order of conductivity Insulators Materials that block the flow of electricity are called insulators (see Fig. 1.11). These materials have very few free electrons and have a low conductivity. In other words, they do not have the ability to allow current to pass through them. Plastic, wood, rubber and porcelain are examples of insulators. Pure water is also an insulator. However, it becomes a conductor if it has some minerals or salt in it. Insulator materials prevent an electric charge from flowing into unwanted areas. For example, the insulation on an electric lamp cord prevents the bare conductors from touching one another. Fig. 1.11 Insulators Fig. 1.10 Conductors Did you know? Normally, conductors have three or less valence electrons, insulators have five or more valence electrons and semiconductors usually have four valence electrons. Silver has a higher conductivity than copper while the conductivity of aluminium is lower than copper. A larger relative conductivity means high conductivity.
eBook - PDF- SA Chuturgoon(Author)
- 2021(Publication Date)
- Troupant(Publisher)
45 Direct current (DC) circuit theory TVET FIRST switch (closed) light bulb (glowing) battery PD + – I ● The switch is closed. ● Current is now flowing, so the light bulb glows. ● The potential difference is measured across the energy source (the battery in this case). Figure 4.4: Potential difference in an electric circuit Figure 4.5 shows what actually happens in any electric circuit. Energy source • cells or batteries • generators • solar energy • heat • friction emf Effects • lighting • heating • chemical • magnetic PD I Figure 4.5: Stages in an electric circuit 4.1.4 Resistance All materials have resistance. Resistance opposes current flow. Definition of resistance Resistance is the opposition that a substance offers to the flow of electric current. In this process heat is produced. Resistance is represented by the symbol R and is measured in ohms ( Ω ). Classification of materials Materials used in the electrical industry are classified as shown in Table 4.1: Table 4.1: Classification of materials Type of material Description Examples Conductors Substances that allow current to flow through them. These substances may be good or poor conductors: • Good conductors have very low resistance values. • Poor conductors have high resistance values. Good conductors Pure metals such as gold, silver, copper and aluminium Poor conductors Steel, lead and tungsten Note Tungsten is a very poor conductor of electricity. It is highly resistent to the flow of current. It is used to manufacture the filaments of incandescent light bulbs. filament: a conducting wire with a high melting point, forming part of an electric bulb that is made incandescent by passing an electric current through it conductor: any substance that allows current to flow through it tungsten: a dense, greyish white metal with a high melting point; also called wolfram 46 Module 4 TVET FIRST Type of material Description Examples Insulators Substances that do not allow current to pass through them.
eBook - ePubElectromagnetic Fields
Theory and Applications
- Ahmad Shahid Khan, Saurabh Kumar Mukerji(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
8 Electrical Field in Materials 8.1 Introduction In Chapter 7, a new quantity referred to as potential was introduced and its relations to different types of charge distributions and the electric field intensity were discussed. This quantity is frequently used even though the user may not be aware of its technical aspects and real meaning. Similarly, there is another quantity that is also known to the masses is the current. This chapter describes the meaning, types, and relations associated with the current and the current density. This chapter also includes a brief description of the nature of conducting, semiconducting, dielectric, and superconducting materials. This chapter also describes the conditions satisfied by different field quantities at the boundaries between different types of materials. 8.2 Electrical Current and Current Density This section describes two commonly used terms, current and current density. 8.2.1 Electrical Current The flow of electric charges is referred to as an electric current. This charge is carried by moving electrons in wires, by ions in electrolytes and by both (ions and electrons) in plasma. In electronic devices, the electric current may be caused due to the flow of electrons through resistors or vacuum in vacuum tubes, flow of ions in a battery or a neuron, and the flow of holes within a semiconductor. The current is usually denoted by the symbol I and is measured in ampères (abbreviated by A) through a device called ammeter. The one ampere current is the flow of electric charge across a surface at the rate of one coulomb per second. The electric currents can have many effects including generation of heat and creation of magnetic field used in inductors, motors, and generators. In wires and conductors made of metals, positive charges are immobile and charge carriers are electrons
eBook - PDF- Edward Purcell(Author)
- 2011(Publication Date)
- Cambridge University Press(Publisher)
That constitutes a perfectly good current. In the atmosphere, charged water droplets fall- ing because of their weight form a component of the electric current system of the earth. In this section we shall be interested in a more common agent of charge transport, the force exerted on a charge car- rier by an electric field. An electric field E pushes positive charge car- riers in one direction, negative charge carriers in the opposite direc- tion. lf either or both can move, the result is an electric current in the direction of E. In most substances, and over a wide range of electric field strengths, we find that the current density is proportional to the strength of the electric field that causes it. The linear relation between current density and field is expressed by J = O"E (10) The factor 0" is called the conductivity of the material. Its value depends on the material in question; it is very large for metallic con- ductors, extremely small for good insulators. It may depend too on the physical state of the material-on its temperature, for instance. But with such conditions given, it does not depend on the magnitude of E. lf you double the field strength, holding everything else constant, you get twice the current density. In Eq. 100" may be considered a scalar quantity, implying that the direction of J is always the same as the direction of E. That is surely what we would expect within a material whose structure has no "built-in" preferred direction. Materials do exist in which the electri- cal conductivity itself depends on the angle the applied field E makes with some intrinsic axis in the material. One example is a single crys- tal of graphite which has a layered structure on an atomic scale. For another example, see Problem 4.7. In such cases J may not have the direction of E. But there still are linear relations between the compo- nents of J and the components of E, relations expressed by Eq.
- Wei Gao, Nigel M Sammes;;;(Authors)
- 1999(Publication Date)
- WSPC(Publisher)
Two Classical Theory of Electrical Conduction and Conducting Materials In this chapter, the free electron conduction theory is described. This description is then used to explain the conduction properties of materials. Finally, materials which are used for electrical conduction in electrical and electronic industries are introduced. In classical electron conduction theory, an electron is treated as a very small particle with certain mass and electric charge: Electron mass m e = 9.1 x 10~ 31 kg Electron charge e = -1.6 x 10 19 C. Because electrons behave like particles in this theory, they obey Newton's Laws of motion. In Sections 2.3 to 2.5, we will apply this theory to describe the electron conduction behaviour in conductors. 2.1. Resistivity and Temperature Coefficient of Resistivity (TCR), Matthiessen's Rule 2.1.1. Resistivity The electrical resistance, R, of a material is defined as R oc l/A (2.1) 8 Classical Theory of Electrical Conduction and Conducting Materials 9 or R = pi IA , (2.2) where I is the length, A is the area of the cross-section of the conductor and p is called electrical resistivity. Equation (2.2) indicates that p is the resistance of a material in unit length and unit cross-section area. 2.1.2. Matthiessen's Rule and TCR For pure metals, resistivity p is the sum of two items: a residual part, p r , and a thermal part, p t (see Fig. 2.1). This is called Matthiessen's rule: P(total) = Pr + Pt P = Pr(l+Pt/Pr) Pt/Pr = f(T) P = /V[1 + /(T)]. (2.3) (2.4) (2.5) (2.6) For most metals and alloys, p is approximately proportional to temperature T and can be written as (see Fig. 2.2) p = p 0 (l + aAT), (2.7) where a is called the temperature coefficient of resistivity (TCR). Temperature, K Fig. 2.1. Resistivity versus temperature for a typical metal, Matthiessen's Rule. 10 An Introduction to Electronic and Ionic Materials -273 -200 0 100 200 Temperature. C Fig.2.2. The effect of temperature on the resistivity of selected metals.
eBook - PDF- Tommy Ferreira, Trevor Adams, Tommy Ferreira, Trevor Adams(Authors)
- 2021(Publication Date)
- Future Managers(Publisher)
Definition Resistivity – the resisting power of a specified material 59 N1 Electrical Trade Theory 4.3.2 Resistivity Resistivity, or specific resistance, refers to the resisting power of a specific material. Some materials are better conductors than others. Alternatively, it could be said that: • a good conductor has a low resistivity or a low resistance; and • a poor conductor has a high resistivity or a high resistance. To be able to compare the resistance of the different materials, a constant (ρ) has to be introduced, the value of which depends on the specific material. To ensure that the sample of material is standardised, the unit is made of length and unit area. This means that the constant ρ is the resistance of a unit cube of material; it is expressed in ohms per metre cubed (Ω/m 3 ). eLink Visit this link to learn more about resistivity: bit.ly/ResistivityExplained The table below indicates the resistivity of different materials: Material Resistivity (Ω.m) Silver 1,62 × 10 −8 Copper 1,72 × 10 −8 Gold 2,45 × 10 −8 Aluminium 2,83 × 10 −8 Tungsten 5,50 × 10 −8 Nickel 7,80 × 10 −8 In formulating the equation to calculate the resistivity of a conductor, the following have to be taken into account: • length (ℓ) of the conductor in metres (m) • cross-sectional area (A) in square metres (m²) • resistivity (Rho) (ρ) (see table above). Taking these factors into consideration, the formula to calculate resistivity is: R = ρ × ℓ _____ A . 60 Module 4 • Direct current (DC) circuit theory 4.3.3 Resistance of a conductor at constant temperature The calculation of the resistance of a conductor at constant temperature is shown by the examples below. Example 4.24 Calculate the resistance of a copper conductor that has a cross-sectional area of 28,27 mm 2 and a length of 500 m. Take the resistivity of copper as 0,0172 micro ohm metre. Solution Given ℓ = 500 m, A = 28,27 × 10 –6 m 2 and ρ = 0,0172 × 10 –6 ohm metre.
eBook - PDF- Jia-Ming Liu, I-Tan Lin(Authors)
- 2018(Publication Date)
- Cambridge University Press(Publisher)
2 Electronic Properties 2.1 Current and Conductivity of a Two-Dimensional Material The flow of free charge carriers, i.e., electrons in the conduction band or holes in the valence band, in a semiconductor that is subject to an electric field is accelerated by the electric field but is hindered by scattering events. In a semiconductor, free carriers accelerate in the presence of an electric field. The randomly distributed scattering centers, such as impurities and defects, act as a counter force that decelerates and deflects the carriers. When the steady state is eventually reached under a constant electric field, a constant flow of carriers is achieved. In graphene, by contrast, charge carriers on the Dirac cone have a constant speed and do not accelerate or decelerate in response to the electric field or the scattering centers; instead, the effect of the electric field is rather to align the motion of carriers to the direction of the electric field, and the scattering centers act as a source to disturb this alignment process. This process is well captured by the Boltzmann transport equation, which has successfully described many statistical behaviors of carriers in metals and semiconductors. The purpose of this chapter is to describe the electronic properties of graphene, starting from the Boltzmann transport equation. In Section 2.5, the theoretically derived electronic properties are then com- pared with experimental observations to show the agreement between the theory and the experimental data. In this chapter, only the DC electronic properties are considered. Therefore, the electric field and the electric current density considered in this chapter are both time- independent real quantities, though both of them can vary with space to account for their spatial distributions.
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