Technology & Engineering
Conductors
Conductors are materials that allow the flow of electric current. They typically have a high density of free electrons, which can move easily in response to an electric field. Common examples of conductors include metals like copper and aluminum. In electrical and electronic systems, conductors are essential for transmitting and distributing electricity.
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4 Key excerpts on "Conductors"
eBook - PDF- Patrick M. Woodward, Pavel Karen, John S. O. Evans, Thomas Vogt(Authors)
- 2021(Publication Date)
- Cambridge University Press(Publisher)
10 Conducting Materials 10.1 Conducting Materials An electrical current is a flow of charged particles, and electrical conductivity is a measure of how easily a current can pass through a material. Materials where the current is carried by electrons are called electronic Conductors, while those where the current is carried by ions are known as ionic Conductors. In this chapter we take close look at the origins of electronic conductivity in a wide variety of materials. Ionic Conductors are covered in Chapter 13. It is difficult to imagine a material property that spans a greater range than electrical conductivity, nearly 30 orders of magnitude (Table 10.1). To put the conductivities of various materials in context, we need to define the units associated with conductivity. We begin with Ohm’s law: I ¼ V R (10.1) where I is the current in amperes, V is the potential difference in volts, and R is the resistance in ohms. Resistance is the material property of interest to us, but, as an extrinsic property, its value depends on the size and shape of the sample. To express Ohm’s law in a form that is independent of the sample dimensions, we replace current in Equation (10.1) with current density, J = I/A (current per area), 1 potential difference with electric-field intensity, E = V/L (voltage per length), and resistance with resistivity, ρ = RA/L (resistance per length and per inverse area). In doing so, Equation (10.1) becomes: J ¼ V AR ¼ EL AR ¼ EL AðρL=AÞ ¼ E ρ ¼ σE (10.2) 1 Because the electric current (in amperes) is the number of flowing charges (in coulombs) per second, the current density J (in A/m 2 ) is the same as the charge flux J [in [C/(m 2 s)]. We’ve encountered the flux J already in Chapter 3. 396 where σ is conductivity, the inverse of resistivity, σ = 1/ρ. Resistivity has units of ohms meter [Ω m = (V/A) m] and conductivity has units of siemens per meter [S/m = (A/V)/m], where 1 S = 1 Ω −1 . Conductivity and resistivity are intrinsic properties of materials.
eBook - PDF- George Murray, Charles V. White, Wolfgang Weise(Authors)
- 2007(Publication Date)
- CRC Press(Publisher)
309 10 Electrical Properties of Materials 10.1 INTRODUCTION The electron conductivity of solid materials gives an almost unambiguous way to classify them. Simply put, on the basis of electrical conductivity, materials are either insulators, semiConductors, Conductors, or superConductors. Superconduc-tors are a special class of materials that exhibit zero resistance below a certain temperature. They will not be considered here. The conductivity of all of the more common and widely used materials is shown in Figure 10.1. The range of conductivities is quite large. Where we draw the lines for these materials appears to be somewhat arbitrary, but we can define these three categories fairly precisely in terms of the number of electrons available for conduction. This number can be computed using the energy band structure for the valence electrons, a subject covered in the following section. Insulators and most polymers have a low conductivity because of their strong covalent bonds and the absence of free electrons, but in some polymers a conducting powder is mixed with the polymer to form a conducting composite. In a few others of the so-called conducting polymers, there exist some free electrons within the polymer structure, creating conductivity on the order of that found in crystalline semiConductors, and in some conducting polymers the conductivity approaches that of metals. There is a tremendously large variation in the conductivity of solids, being about a factor of 10 25 from Conductors to insulators. Ohm’s law can be used to express conductivity and its reciprocal, resistivity, which are not functions of specimen dimensions, and the conductance and resis-tance, which are functions of specimen dimensions. Resistance is related to resistivity by: R = ρ l /A where R = resistance ρ = resistivity (usually expressed in Ω · m) l = specimen length A = specimen area and in terms of conductivity, σ , σ = l /RA units are ( Ω · m) − 1
eBook - PDF- David W. Tyler(Author)
- 2013(Publication Date)
- Newnes(Publisher)
4 Materials and their applications in the electrical industry Aims: At the end of this chapter you should be able to: Compare aluminium and copper as conductor materials. Explain the effect on cable rating of ambient temperature and apply grouping factors. Use tables to determine cable sizes for specified loadings and situ-ations. Describe the construction of printed circuit boards and explain the need to limit the current density in the Conductors. Explain why heat sinks are necessary for many electronic devices. Explain why dielectrics fail in service. Describe the constructional details of a range of cables. Explain why core losses occur in iron-cored coils and transformers. Compare the properties of permanent magnet steels with those of soft ferro-magnetic materials. Describe the required properties for core materials for use with a range of frequencies. The best electrical conductor known is silver but this is far too expensive and rare to provide all the conductor material required by the electrical industry. Next in order of conductivity come copper and aluminium and these are the most important current carrying materials used in cable and line manufacture. Other materials used are cadmium-copper, phosphor-bronze and for some high-voltage low-power links, galvanised steel. The conductivity of both copper and aluminium falls very rapidly with very small additions of alloying elements so that they are gener-ally used pure. In the case of aluminium the mechanical strength is improved by using a stranded conductor with steel strands at the centre. Figure 4.1 shows a stranded conductor and the make up will be 1 strand of steel plus 6 strands of aluminium; 7 strands of steel plus 12 strands of aluminium, or by adding a further layer of aluminium strands, 7 of steel plus 30 of aluminium. The conductivity of the whole is taken as that of the aluminium alone since steel has a very high impedance and the current flows almost exclusively in the aluminium.
eBook - PDF- Paul Rosenberg, Robert Gordon Middleton(Authors)
- 2004(Publication Date)
- Audel(Publisher)
Chapter 2 Conductors and Insulators A conductor is a substance that carries electric current. An insula-tor is a substance that does not carry electric current. Because no conductor is perfect, and because any conductor has at least a small amount of resistance, it is better to define a conductor as a substance with a very low resistance. We will also find that no insulator is per-fect, and because no insulator has an infinite resistance, it is better to define an insulator as a substance with a very high resistance. Therefore, Conductors, resistors, and insulators are all basically re-sistive substances. However, they are classified into different groups because a practical conductor has extremely low resistance, a load resistor has moderate resistance, and a good insulator has extremely high resistance. Classes of Conductors The substances listed in Table 2-1 have different conductivities. The best Conductors are listed in the first column in the order of decreas-ing conductivity. For example, silver is the best conductor, lead has less conductivity, carbon has still less conductivity, moist earth is a poorer conductor than carbon, and slate has such a high resis-tance that it is called an insulator. Of the insulators listed, dry air is the best. A high vacuum is a better insulator than dry air; how-ever, a vacuum can be used only in special devices such as rectifier tubes. Therefore, we will be concerned in this chapter only with Table 2-1 Conductors and Insulators Good Conductors Fair Conductors Insulators Silver Charcoal and coke Slate Copper Carbon Oils Aluminum Acid solutions Porcelain Zinc Sea water Dry paper Brass Saline solutions Silk Platinum Metallic ores Sealing wax Iron Living vegetable Ebonite Nickel substances Mica Tin Moist earth Glass Lead Dry air Note: In each column, the best conductor is at the top, the best insulator at the bottom. 35 36 Chapter 2 the more common insulators used in electrical work.
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