Standard Unit

5 Key excerpts on "Standard Unit"

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  eBook - PDF

  Introduction to Physics

  Mechanics, Hydrodynamics Thermodynamics

  • P. Frauenfelder, P. Huber(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  Hence a definition is the precise formulation of a scientific concept. Quantities which can vary in space or time (most quantities can) must be defined in differential form. To correlate data obtained in the laboratory with the results of applica-tions of the basic principles and empirical laws to real processes, we must consider the theory of errors (see Section 7). 2. T H E THREE STANDARDS OF MEASUREMENT Space, time, and matter (mass) are the independent elements of mechanics. The units of space, time, and mass are then the independent basic units of measurement and thus have a fundamental significance. From them, we ι 2 I N T R O D U C T I O N T O P H Y S I C S can derive all the other units of mechanics by using the basic principles and empirical laws. Physical phenomena can be understood either on a qualitative level, by careful description without the use of mathematics, or on a quantitative level, by means of numerical verification of the relationships in question. To measure a quantity means to compare it with a Standard Unit of that quantity. The fundamental units are established by means of definitions; that is, they are completely arbitrary. However, the definitions should be formulated so that the units are as exact as possible, can be easily reproduced, and lead to simple numerical values for the most commonly occurring measurements. 2-1 The Standard of Length In the classical physics of Newton, the concept of space is understood to be intuitively obvious. It exists independently of time and of any objects embedded in it. A further postulate of classical physics, and one which is broadly supported by experience, is that the geometry of space is Eucli-dean.f In Euclidean geometry, the measurement of space is based on a unit of length. A definition of the standard of length determines the units of area and volume as well as the unit of length.

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  eBook - PDF

  Fundamentals of Physics, Extended

  • David Halliday, Robert Resnick, Jearl Walker(Authors)
  • 2018(Publication Date)
  • Wiley

    (Publisher)

  As you will see, the standard for length, which corresponds Key Ideas ● Physics is based on measurement of physical quanti- ties. Certain physical quantities have been chosen as base quantities (such as length, time, and mass); each has been defined in terms of a standard and given a unit of measure (such as meter, second, and kilogram). Other physical quantities are defined in terms of the base quantities and their standards and units. ● The unit system emphasized in this book is the Interna- tional System of Units (SI). The three physical quantities displayed in Table 1-1 are used in the early chapters. Standards, which must be both accessible and invari- able, have been established for these base quantities by international agreement. These standards are used in all physical measurement, for both the base quantities and the quantities derived from them. Scientific notation and the prefixes of Table 1-2 are used to simplify measure- ment notation. ● Conversion of units may be performed by using chain- link conversions in which the original data are multiplied successively by conversion factors written as unity and the units are manipulated like algebraic quantities until only the desired units remain. ● The meter is defined as the distance traveled by light during a precisely specified time interval. to exactly 1.0 m, is the distance traveled by light in a vacuum during a certain fraction of a second. We can define a unit and its standard in any way we care to. However, the important thing is to do so in such a way that scientists around the world will agree that our definitions are both sensible and practical. Once we have set up a standard—say, for length—we must work out pro- cedures by which any length whatever, be it the radius of a hydrogen atom, the wheelbase of a skateboard, or the distance to a star, can be expressed in terms of the standard. Rulers, which approximate our length standard, give us one such procedure for measuring length.

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  eBook - ePub

  The Reform of the International System of Units (SI)

  Philosophical, Historical and Sociological Issues

  • Nadine de Courtenay, Olivier Darrigol, Oliver Schlaudt(Authors)
  • 2019(Publication Date)
  • Routledge

    (Publisher)

  The definition of a unit as in equation (4) or equation (5), that does not refer to the relations in the system of quantities, is independent, and dependent is instead a definition, as in equation (6), which does refer to such relations. For example, a definition of length unit based on the relation of length with time and velocity is a dependent definition.

  2.4 Concepts related to the realization of the definition of quantity units

  The main concepts assumed related to the realization of the definition of quantity units are:

  1. quantity realizing the definition of a unit;
  2. object realizing the definition of a unit;
  3. calibration;
  4. metrological traceability chain;
  5. calibration uncertainty.

  A measurement process is based on a measurement procedure that specifies a way for comparing the measurand and the unit. This comparison implies the availability of a quantity that realizes the definition of the unit through the realization of a support of that quantity. The object realizing the definition of a unit is called a measurement standard. The comparison with a unit is then generally obtained by realizing a chain of measurement standards, from a primary one to the working standard used in measurement and calibrated against the primary standard through a metrological traceability chain. In the processes of calibration that yields the chain, a calibration uncertainty is produced that generally increases in each step of the chain from the primary standard to the standard involved in the measuring system.

  3 Basic criteria

  The main operative target of the definition of a system of units is to provide a basis for measurement, for which quantity units are indeed essential.10

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  The Uncertainty of Measurements

  Physical and Chemical Metrology: Impact and Analysis

  • Shri Krishna Kimothi(Author)
  • 2001(Publication Date)
  • ASQ Quality Press

    (Publisher)

  For acceptability, it is also essential that the measurement results for the same parameter measured at dif- ferent places and by different people be in agreement. This implies that mea- surement results should be correlated. To achieve this agreement in measurement results, it is essential that everybody draw their measurement units from a common acceptable standard. Thus, a standard is a physical object or a characteristic of a physical apparatus that represents the conceptual 112 Chapter Five unit chosen to represent a particular measurable attribute. Like the need for measurement, the need for measurement standards has been felt by human beings from time immemorial. It is believed that the foot ruler as a measure- ment standard might have been started with the declaration by some king that the standard of length should be equal to the length of his foot. There is his- torical evidence of artifacts’ being used as standards of measurement. It is believed that around 3000 B.C. Egyptians used the royal cubit as a standard of length. The royal cubit was considered a master standard and was made of granite to endure for all time. Craftspeople used cubits made of wood for mea- surement purposes while engaged in the construction of the pyramids. All the craftspeople were required to bring their cubits for comparison with the royal cubit on the day of each full moon. This might be an early example of the con- cepts of calibration and measurement traceability. Through this process of measurement, the Egyptians achieved a measurement accuracy of 0.05% in measuring the dimensions of the great pyramids. The development of physical standards and their units has been a long- drawn process. At various times throughout history rulers have realized the need for legal metrology. Today, the whole task of evolving a measurement system is quite complex. It starts with the development and maintenance of primary physical standards, which is a specialized area of metrology.

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  eBook - PDF

  Physics, Volume 1

  • Robert Resnick, David Halliday, Kenneth S. Krane(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  CGPM) beginning in 1889; the twenty-first meeting was held in 1999.* Fortunately, it is not necessary to establish a measure- ment standard for every physical quantity — some quanti- ties can be regarded as fundamental, and the standards for other quantities can be derived from the fundamental ones. For example, length and time were once regarded as funda- mental quantities with their individual established standards (respectively the meter and the second); the measurement standard for speed ( length/time) could then be derived in terms of those standards. However, in more recent years the speed of light has been measured to a precision exceeding that of the former standard meter; as a result, today we still use a fundamental standard for the second, but we define the standard for length (the meter) in terms of the speed of light and the second (see Section 1-4). This case illustrates how measurements of increasing precision can change the established standards and how rapidly such standards evolve. Since the publication of the first edition of this text- book, the precision of the Standard Unit for time (the sec- ond) has improved by more than a factor of 1000. The basic problem therefore is to choose a system in- volving the smallest number of physical quantities as fun- damental and to agree on accessible and invariable stan- dards for their measurement. In the next sections of this chapter we discuss the internationally accepted system and some of its fundamental quantities. 1-2 THE INTERNATIONAL SYSTEM OF UNITS** At its various meetings, the General Conference on Weights and Measures selected as base units the seven quantities displayed in Table 1-1. This is the basis of the International System of Units, abbreviated SI from the French Le Sys- tème International d’Unités. SI is the modern form of what is known generally as the metric system.

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