Specific Resistance Resistivity

Specific resistance (resistivity) is the electrical resistance of a body of a unit cross section and of unit length and is expressed in ohm-centimeters  

The specific resistance of coals (Table 9.16) may vary from thousands of ohm-centimeters to millions of ohm-centimeters depending on the direction of measurement. 

Experimental data strongly indicates that anthracite and bituminous coal are electrically anisotropic. Higher resistivity/lower conductivity is observed for specimens oriented perpendicular to the bedding plane relative to those with parallel orientation. 

When wet coal is exposed to higher temperatures (0 to 200°C, 32 to 392°F), an increase in electrical resistivity (with a concurrent decrease of dielectric constant) is observed. This is due to moisture loss. After moisture removal, a temperature increase results in lower resistivity (and higher dielectric constant). The dependency of conductive properties on temperature is mainly exponential, as in any semiconductor. At lower temperatures, the effect of temperature on electrical properties is reversible. The onset of irreversible effects is rank dependent and starts at 200 to 400°C (392 to 752°F) for bituminous coal and at 500 to 700°C (932 to 1292°F) for anthracite. 

The effect of volatile matter content on electrical properties is not fully understood. Some data indicate that there is no particular correlation, whereas other data indicate a slight decrease of resistivity with decreasing volatile matter content. Both the type and quantity of volatile matter may affect electrical properties. However, it should be noted that even samples with equal volatile matter may show different properties due to different origins. Both mineral matter 

Agrawal, P. L. 1959. Proceedings of the Symposium on the Nature of Coal. Central Fuel 

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Examples include the work function, specific resistance (resistivity), elasticity, and thermodynamic properties (e.g. specific heat capacity, melting point). Intrinsic properties are determined by crystallographic structure and are not susceptible to significant change by modification of the microstmcture. Some commonly used synonyms for intrinsic macroscopic properties include global, bulk, and continuum-level. As we might... 

Figure 3.3 shows the specific resistance (resistivity) of H2SO4 solutions as a function of H2SO4 concentration at different temperatures [2]. The acid concentration window of lead—acid battery operation is marked in the figure. It can be seen that within this window the specific resistance has the lowest values. Both decrease of CH2SO4 below 1.10 and increase above 1.30 relative density cause the specific electrical resistance to increase. When the cell temperature falls below 0 °C, the specific resistance increases rapidly and the battery loses both power and energy. The temperature window with sufficiently low electrical resistance is between 0 and 50 °C. However, even at lower temperatures the battery is capable to deliver sufficient electric current to start the engine of an automobile. 

The fundamental features of the structural performances of composites are the high specific resistance (resistance/density ratio) and high specific stiffness (modulus E/density ratio), and the anisotropic and heterogeneous nature of the material. These last features convey the composite system several degrees of freedom allowing for an optimization of the material configuration. Yet, composites also show some limits that conventional monolithic material do not imply. 

Area specific resistance, resistance of unit area Entropy... 

ASR area specific resistance resistance OCV open circuit voltage. 

ASTMD 1169-11. Standard test method for specific resistance (resistivity) of electrical insulating liquids. JIS C 0052-95. Environmental testing procedure of electronic and electrical resistance to solvents. 

The resistance due to a circular junction is given by / = /2ak, where a is the radius of the junction and k is specific conductivity of the metal. For the case of two steel plates, the measured resistance is 5 x 10" Q for a load of 50 kg the yield pressure of steel is 60 kg/mm, and the specific resistance is 5x 10 Q/cm. Calculate the number of junctions, assuming that it is their combined resistance that is giving the measured value. 

Specific resistance Specific rotation Specific speed Specific viscosit Specimen contrast Speckle interferometry Spectazole... 

The concept of the specific resistance used in equation 4 is based on the assumptions that flow is one-dimensional, growth of cake is unrestricted, only soHd and Hquid phases are present, the feed is sufficiently dilute such that the soHds are freely suspended, the filtrate is free of soHds, pressure losses in feed and filtrate piping are negligible, and flow is laminar. Laminar flow is a vaHd assumption in most cake formation operations of practical interest. 

An additional benefit of prethickening is reduction in cake resistance. If the feed concentration is low, there is a general tendency of particles to pack together more tightly, thus leading to higher specific resistances. If, however, many particles approach the filter medium at the same time, they may bridge over the pores this reduces penetration into the cloth or the cake underneath and more permeable cakes are thus formed. 

This effect of concentration is particularly pronounced with irregularly shaped particles. A possible explanation of the variation in the specific resistance is in terms of the time available for the particles to orient themselves in the growing cake. At higher concentrations, but with the same approach velocities, less time, referred to as particle relaxation time, is available for a stable cake to form and a low resistance results. 

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

Resistivity/Conductivity. The resistivity or specific resistance of a material is the electric resistance offered by an element of the material having unit length and unit cross-sectional area. The current iatensity is proportional to the voltage across its path, and is iaversely proportional to resistance. This relationship is expressed by Ohm s law, where I = current ia amperes, E = poteatial ia volts, and R = resistance ia ohms. 

D. Tracey and K. Richard, "Mechanisms of Immuno stimulation by Pyrimidiuones," iu Immunopharmacology of Infectious Diseases Uaccine Mdjuvants and Modulators of Non-specific Resistance, Alan R. Liss, Inc., 1987, pp. 279—289. 

Sihcone fluids have good dielectric properties, loss factor, specific resistance, and dielectric strength at normal operating conditions, and the properties vary only slightly with temperature (59,328,350). The properties in combination with relatively low flammabiUty have led to the use of siUcones in transformers and other large electrical appHcations (351). The dielectric constant of a 1000-cSt oil is 2.8 at 30°C and 2.6 at 100°C. The loss factor is low,... 

Silver sulfide, when pure, conducts electricity like a metal of high specific resistance, yet it has a zero temperature coefficient. This metallic conduction is beheved to result from a few silver ions existing in the divalent state, and thus providing free electrons to transport current. The use of silver sulfide as a soHd electrolyte in batteries has been described (57). 

Fig. 3. Relationship of moisture content and relative humidity to specific resistance of polymeric, fibrous materials.

Fig. 4. (a) Yam resistance in n-cm vs amount of antistatic agent on the yam. The agent is the ethyl sulfate salt of an amine, (b) Resistance vs amount of nonionic, hygroscopic agent on the yam. Dotted lines are calculated from the specific resistance of the dry bulk solution soHd lines are experimental yam... 

The specific resistance of natural graphite crystals is ca Hem (room temperature) along the a axis parallel to the network basal plane. The resistance along the c axis (perpendicular to the basal plane) is ca 1 Q. The cja axis anisotropy ratio is, therefore, ca 10 . Screw dislocations within the crystal may short-circuit the current path parallel to the c axis and cause lower anisotropic ratios separation of planes may cause higher anisotropic ratios. 

The specific electrical conductivity of dry coals is very low, specific resistance 10 ° - ohm-cm, although it increases with rank. Coal has semiconducting properties. The conductivity tends to increase exponentially with increasing temperatures (4,6). As coals are heated to above ca 600°C the conductivity rises especially rapidly owing to rearrangements in the carbon stmcture, although thermal decomposition contributes somewhat below this temperature. Moisture increases conductivity of coal samples through the water film. 

The specific resistance of the International Annealed Copper Standard copper is reported as 1.682 x 10 H at 20°C, which is defined as a... 

If expression is effective, it reduces the permeabiHty of the cake being compacted and, as a consequence, the resistance to flow of the Hquid increases considerably (27). The effectiveness of expression is governed by cake thickness, specific resistance, consoHdation properties, and shear forces. 

The specific resistance coefficient for the dust layer Ko was originally denned by Williams et al. [Heat. Piping Air Cond., 12, 259 (1940)], who proposed estimating values of the coefficient by use of the Kozeny-Carman equation [Carman, Trans. Inst. Chem. Fng. (London), 15, 150 (1937)]. In practice, K and Ko are measured directly in filtration experiments. The K and Ko values can be corrected for temperature by multiplying by the ratio of the gas viscosity at the desired condition to the gas viscosity at the original experimental conditions. Values of Ko determined for certain dfists by Williams et al. (op. cit.) are presented in Table 17-5. 

TABLE 17-5 Specific Resistance Coefficients for Certain Dusts ... 

Darcy s law has been used to derive an expression which reflects not only the effect of a change in elevation, but also provides a means for estimating changes in air rate resulting from changes in vacuum level and cake thickness (or cake weight per unit areaj. In order for this relationship to hold for changes in vacuum and cake thickness, it must be assumed that both cakes have the same specific resistance. 

It is known that the specific resistance for centrifuge cake, especially for compressible cake, is greater than that of the pressure or vacuum filter. Therefore, the specific resistance has to be measured from centrifuge tests for different cake thicknesses so as to scale up accurately for centrifuge performance. It cannot be extrapolated from pressure and vacuum filtration data. For cake thickness that is much smaller compared to the basket radius, Eq. (18-116 7) can be approximated by... 

Possesses high specific resistance - 120 pH-cm and low temperature coefficient of 0.00022 Causes negligible... 

Volume resistivity or specific resistance this is the resistance of a conductor of unit length and unit cross-sectional area. i.e. 

The grounding or penetration depth of the electrical resistance in conductors is, according to Eq. (3-42), dependent on the specific resistance and the frequency. The penetration depth, t, is the distance at which the field strength has fallen by 1/e,- is the relative permeability [35] ... 

There is a direct and an indirect method of measuring specific soil resistivity. The direct method is carried out in the laboratoiy on a soil sample using a soil box as shown in Fig. 3-16. The resistivity of a soil specimen of cross-section, S, and length, I, is measured and the specific resistivity determined ... 

If the specific soil resistivity varies vertically with depth, t, an apparent specific resistivity can be obtained from a combination of the resistivity of the upper and... 

The following terms apply to the specific coating resistance which is related to the surface, S r is the value calculated from the specific resistance of the coating material using Eq. (5-1) ... 

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