Resistance, of a conductor

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

Electrical resistance monitors use the fact that the resistance of a conductor varies inversely as its cross-sectional area. In principle, then, a wire or strip of the metal of interest is exposed to the corrodent and its resistance is measured at regular intervals. In practice, since the resistance also varies with temperature, the resistance of the exposed element is compared in a Wheatstone bridge circuit to that of a similar element which is protected from the corrodent but which experiences the same temperature. 

As schematized in Figure 2, the decrease in resistance of a conductor with temperature is a general law of physics. The vibrations of the metal... 

OHM (ft). A unit of resistance (and of impedance). The resistance of a conductor such that a constant cunent of 1 ampere m it produces a voltage diffei tnces of 1 volt between its ends. (The SI unit of resistance.)... 

The principle of the resistance strain gauge 2 0 is that the electrical resistance of a conductor will change when it is stretched or compressed due to the consequent variation in its physical dimensions. There is an additional effect called the piezoresistance which is the relation between the resistivity p of the material and the mechanical strain. The resistance R of a conductor of area of cross-section A and length x is given by ... 

As explained in Section 3.5, the electrical resistance of a conductor depends on the conductor characteristics (particularly, conductivity and geometry) and on the path of current that passes through it. When a PEN structure is considered for modelling the fuel cell, Ohm s law is used in a finite form, i.e. expression (3.74). This expression assumes that the resistance of the PEN is known. 

The resistivity (or specific resistance) of a conductor may be thought of as the resistance of a cube of 1-cm edge, with the current assumed to be uniform and perpendicular to two opposite faces of the cube. The resistance of a conductor of uniform cross section is given by... 

The specific resistance of a conductor is the resistance across two opposite faces of a 1 cm cube of the conductor material. 

The e.m. and e.s. units described above are not all of a convenient magnitude for experimental purposes, and so a set of practical units have been defined. The practical unit of current, the ampere, often abbreviated to amp., is one-tenth the e.m. (c.g.s.) unit, and the corresponding unit of charge or quantity of electricity is the coulomb the latter is the quantity of electricity passing when one ampere flows for one second. The practical unit of potential or e.m.f. is the volt, defined as 10 e.m. units. Corresponding to these practical units of current and e.m.f. there is a unit of electrical resistance this is called the ohm, and it is the resistance of a conductor through which a current of one ampere passes when the potential difference between the ends is one volt. With these units of current, e.m.f. and resistance it is possible to write Ohm s law in the form... 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

Electrical methods. The electrical methods of measuring temperature are based on two facts, firstly, that the resistance of a conductor varies with the temperature, and secondly, that the electromotive force which is produced at a point of contact between two different metals or alloys is hkewise a function of the temperature. If, therefore, we close a circuit consisting of two wires of different metals, so that there are two joints in the circuit where two metals meet, a current will flow in general so long as these joints are not at the same temperature. If the temperature of the one joint is known, a measurement of the electromotive force enables us to determine the temperature of the other. On account of the great sensibility of electrical measurements, it is possible to measure very small differences of temperature by either of these methods. They have the further advantage over the first and second methods, that we are enabled by their means to measure very high and very low temperatures in a most convenient manner. The small bulk occupied by a thermocouple is often important from an experimental point of view, and for this reason thermocouples are preferable in some cases to all other forms of thermometer. 

An int. ampere is the current which flawing for 1 sec. gives 1 int. coulomb of electricity, and an int. ohm is the resistance of a conductor through which passes 1 int. ampere when 1 int. volt is applied. 

The resistance of a conductor is a characteristic property by virtue of which the energy of an electric current passing through it is converted into heat The heat, H, in joules, developed in die conductor may be computed by the formula... 

Electrical conductivity measures the ease with which electrons pass through a substance. It is measured in the unit S nr1. S is the symbol for the unit siemens, which is equivalent to ohm-1. An ohm (Q) is a measure of electrical resistance, which tells us at what rate electrons will move through a substance for a given applied voltage. An ohm is the resistance of a conductor, which, when a potential difference of 1 volt is applied across it, passes a current of 1 ampere that is, 1 ohm = 1 voll/1 ampere (in absolute terms defined as 1 m2kgs 3A 2). 

The resistance of a conductor can be calculated from its physical dimensions by... 

Thermal effects linked with lightning current are due to the resistive heating caused by the circulation of an electric current flowing through the resistance of a conductor as a metallic storage tank. Thermal effects are also relevant to the heat generated in the root of the arcs at the attachment point and in all the isolated parts involved in arc development (e.g. spark gaps). 

The electrical resistance pyrometer depends on the principle that the electrical resistance of a conductor changes with temperature the instrument is designed to measure these changes and is calibrated in... 

Resistance of a conductor n. At 0°C, of length /, cross-section s and specific resistance p,... 

At high frequencies, the surface of the insulator may have a different resistivity from the bulk of the material owing to impurities absorbed on the surface, external contamination, or water moisture hence, electric current is conducted chiefly near the surface of the conductor (i.e., skin effect). The depth, S, at which the current density falls to 1/e of its value at the surface is called the skin depth. The skin depth and the surface resistance are dependent upon the AC frequency. The surface resistivity, R, expressed in 2, is the DC sheet resistivity of a conductor having a thickness of one skin depth ... 

Effective resistance The increased resistance of a conductor to an alternating current resulting from the skin effect, relative to the direct-current resistance of the conductor. Higher fi equencies tend to travel only on the outer skin of the conductor, whereas dc flows uniformly through the entire area. 

Resistance of a conductor. (From Krum, A., Course Notes, UCLA Extension, Engineering 881.152, Power Hybrids Design and Processing, April 1995. With permission.)... 

In the equivalent circuit analog, resistors represent conductive pathways for ion and electron transfer. As such, they represent the bulk resistance of a material to charge transport such as the resistance of the electrolyte to ion transport or the resistance of a conductor to electron transport. Resistors are also used to represent the resistance to the charge-transfer process at the electrode surface. Capacitors and inductors are associated with space-charge polarization regions, such as the electrochemical double layer, and adsorption/ desorption processes at an electrode, respectively. 

Ohm found by painstaking experiments with homemade equipment that the resistance of a conductor of uniform cross section is proportional to its length and inversely proportional to its cross-sectional area. We define the resistivity r of a conducting object shaped as in Figure 10.9 by... 

Power transmission. Given the respective electrical resistivities of aluminium and copper, for the same electrical resistance of a conductor or an equivalent drop in voltage, the 1370 conductor is half the weight of the copper conductor. 

A change on the resistance of a conductor in a magnetic field was first noticed by Lord Kelvin in 1856. The effect is caused by the Lorentz force which deflects the trajectory of the current carriers as in the Hall effect (see Section 18.5) and is maximized when the current flow is perpendicular to the magnetic field. The effect is called ordinary magnetoresistance (OMR) or ordinary magnetoresistance. 

Parameter inference. The forward model or theory relates the input properties and geometry to the instrument response. In the simplest case the theory will predict the response as a simple function of the input, such that the inverse formula, stating the input parameter as a function of the response can be obtained analytically. An example is calculating the resistance of a conductor as the ratio of potential gradient to electric current, or permeability as the ratio of fluid flux to pressure gradient. In a more complicated case there may be several parameters such as the radii of the different zones of mud invasion. When there are just a few parameters it may be possible to infer the parameters from one or more instrument responses using a least squares procedure. This is an application of the usual methods of the theory of measurement as reviewed in Section 5. 

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