Physics
Null Resistivity
Null resistivity refers to the absence of electrical resistance in a material. This means that the material can conduct electricity without any loss of energy. This phenomenon is observed in superconductors at very low temperatures.
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5 Key excerpts on "Null Resistivity"
- Tuchin, Valery V.(Authors)
- 2012(Publication Date)
For anisotropic materials the conductivity is a 3 × 3 matrix (a rank-2 tensor), which is generally symmetric. The conductivity is the inverse of 127 electrical resistance electrical resistivity ρ and has the SI units of siemens per meter (S/m): ∑ = 1 / ρ , where ρ is in ohm/m. electrical resistance: A measure of opposition of an object to the passage of a steady electric current through it. An object of uniform cross section A will have an electrical resistance R proportional to its length l , inversely proportional to its cross-sectional area, and proportional to the resistivity of the material: R = ρ l / A (measured in SI units as ohms, Ω ), where ρ is the static electrical resistivity [measured in ohm × meters ( Ω × m ) ]. R is the inverse of electrical conductance measured in siemens . For a wide variety of materials and conditions, R does not depend on the amount of current through or the potential difference (voltage) across the object, meaning that the resistance R is constant for the given temperature and material. Therefore, the resistance of an object can be defined as the ratio of voltage ( V ) to current ( I ) , in accordance with Ohm’s law: R = V / I . electrical resistivity: Specific electrical resistance or volumetric resistivity. It is a measure of how strongly a material opposes the flow of electric current. Low resistivity indicates a material that readily allows the movement of electrical charge. Electrical resistivity is defined as ρ = E / j , where ρ is the static resistivity (in SI units measured in volt × meters per ampere, V × m / A) . E is the magnitude of the electric field (volts per meter, V/m) and j is the magnitude of the current density (amperes per square meter, A / m 2 ). The electrical resistivity ρ can also be given as ρ = R ( A / l ) , where ρ is the static resistivity [ohm × meters ( Ω × m) ]. R is the electrical resistance of a uniform specimen of the material (ohms, Ω ) and l is the length of the piece of material (meters, m).
- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
The other electron in the pair may then be attracted toward this positive charge. According to the theory, such coordination between electrons would prevent them from colliding with the molecules of the material and thus would eliminate electrical resistance. The theory worked well to explain the pre-1986, lower temperature superconductors, but new theories appear to be needed for the newer, higher temperature superconductors. 0.16 0.08 0 0 2 4 6 Temperature (K) Resistance ( ) Ω FIGURE 26.5.2 The resistance of mer- cury drops to zero at a temperature of about 4 K. A disk-shaped magnet is levitated above a superconducting material that has been cooled by liquid nitrogen. The goldfish is along for the ride. Courtesy of Shoji Tonaka/International Super- conductivity Technology Center, Tokyo, Japan Current An electric current i in a conductor is defined by i = dq ___ dt . (26.1.1) Here dq is the amount of (positive) charge that passes in time dt through a hypothetical surface that cuts across the conductor. By convention, the direction of electric current is taken as the direction in which positive charge carriers would move. The SI unit of electric current is the ampere (A): 1 A = 1 C/s. Current Density Current (a scalar) is related to current density J → (a vector) by i = J → ⋅ dA → , (26.2.1) REVIEW & SUMMARY where dA → is a vector perpendicular to a surface element of area dA and the integral is taken over any surface cutting across the conductor. J → has the same direction as the velocity of the mov- ing charges if they are positive and the opposite direction if they are negative. Drift Speed of the Charge Carriers When an electric field E → is established in a conductor, the charge carriers (assumed posi- tive) acquire a drift speed v d in the direction of E → ; the velocity v → d is related to the current density by J → = (ne) v → d , (26.2.4) where ne is the carrier charge density.
- J.S. Sumner(Author)
- 2012(Publication Date)
- Elsevier Science(Publisher)
Resistivity is a measure of the opposition to flow of charge in a material, whereas electrical conductivity is the flow mobility of charge car-riers. Conductivity σ is derived from the relationship σ = ηβμ, where η is the density of charge carriers, e is their charge, and μ is their mobility measured by velocity in meters per second per unit electric field. The charge carriers may be ions, electrons, or holes (the absence of a charge). Although the concept of conductivity is perhaps physically more funda-mental than that of resistivity, for the sake of consistency, in this book resistivity will usually be referred to rather than conductivity because of the prior usage in the study of resistivity as a geophysical exploration method. Low-frequency IP measurements are more often related to Ohm's law and notions of resistivity. The more complex higher frequency electrical phenom-ena usually introduce the concepts of conductivity. Other factors remain-ing constant, apparent resistivity is proportional to potential difference and resistivity is directly related to voltage values measured in the field. 21 The physical units or dimensions of resistivity in the mks system are ohm-meters (Ωιη), which is resistance times length. Equations that define the resistivity of materials by measurement, such as are developed in this chapter, can be rearranged as ρ = (V/I)K, where Κ is known as a geometric factor or form factor with units of length. The geometric factor Κ is con-stant for a given array and spacing, varying with the electrode interval. Some groups doing resistivity and IP surveying have preferred to leave the 2π term from eq. 17 on the same side of the resistivity equation with p, thus creating a ρ/2π unit of resistivity, the definition for which is 2π or 6.283 times the unit resistivity of ρ. Table II gives the relationship of the various units that have been used for resistivity measurements.
eBook - PDF- A.C. Rose-Innes(Author)
- 2012(Publication Date)
- Pergamon(Publisher)
1.4. A.C . Resistivit y T h e fact that a superconducting metal has no resistance means, of course, that there is no voltage drop along the metal when a current is passed through it, and no power is generated by the passage of the current. This, however, is only strictly true for a direct current of cons-tant value. If the current is changing an electric field is developed and some power is dissipated. T o understand the reason for this we must first discuss briefly some aspects of the behaviour of conduction elec-trons in superconductors. Many of the properties of superconductors can be explained if it is supposed that below the transition temperature the conduction electrons divide into two classes, some behaving as superelectrons which can pass through the metal without resistance (i.e. suffering no collisions), the remainder behaving as normal electrons which can be scattered and so experience resistance just like conduction electrons in a normal metal. T h e fraction of superelectrons appears to decrease as the temperature is raised towards the transition temperature. At 0°K all con- ZER O RESISTANC E 13 auction electrons behave like superelectrons, but, if the temperature is raised, a few begin to behave as normal electrons, and on further heating the proportion of normal electrons increases. Eventually, at the transi-tion temperature, all the electrons have become normal electrons and the metal loses its superconductive properties. Hence a superconductor below its transition temperature appears to be permeated by two elec-tron fluids, one of normal electrons and one of superelectrons. T h e relative electron density in the two fluids depends on the temperature. This two-fluid model is suggested by thermodynamic arguments based on the results of specific heat and similar measurements on super-conductors, which will be discussed in Chapter 5. In a superconducting metal the current can in general be carried by both the normal and superelectrons.
eBook - PDF- William Moebs, Samuel J. Ling, Jeff Sanny(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
9 | CURRENT AND RESISTANCE Figure 9.1 Magnetic resonance imaging (MRI) uses superconducting magnets and produces high-resolution images without the danger of radiation. The image on the left shows the spacing of vertebrae along a human spinal column, with the circle indicating where the vertebrae are too close due to a ruptured disc. On the right is a picture of the MRI instrument, which surrounds the patient on all sides. A large amount of electrical current is required to operate the electromagnets (credit right: modification of work by “digital cat”/Flickr). Chapter Outline 9.1 Electrical Current 9.2 Model of Conduction in Metals 9.3 Resistivity and Resistance 9.4 Ohm's Law 9.5 Electrical Energy and Power 9.6 Superconductors Introduction In this chapter, we study the electrical current through a material, where the electrical current is the rate of flow of charge. We also examine a characteristic of materials known as the resistance. Resistance is a measure of how much a material impedes the flow of charge, and it will be shown that the resistance depends on temperature. In general, a good conductor, such as copper, gold, or silver, has very low resistance. Some materials, called superconductors, have zero resistance at very low temperatures. High currents are required for the operation of electromagnets. Superconductors can be used to make electromagnets that are 10 times stronger than the strongest conventional electromagnets. These superconducting magnets are used in the construction of magnetic resonance imaging (MRI) devices that can be used to make high-resolution images of the human body. The chapter-opening picture shows an MRI image of the vertebrae of a human subject and the MRI device itself. Superconducting magnets have many other uses. For example, superconducting magnets are used in the Large Hadron Collider (LHC) to curve the path of protons in the ring. Chapter 9 | Current and Resistance 385
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