Physics
Unit Conversion
Unit conversion in physics refers to the process of changing a quantity from one unit to another. This is often necessary when working with different measurement systems or when comparing data from different sources. The conversion is typically done using conversion factors or equations that relate the original unit to the desired unit.
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4 Key excerpts on "Unit Conversion"
eBook - PDF- Robert Resnick, David Halliday, Kenneth S. Krane(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
1 MEASUREMENT D espite the mathematical beauty of some of its most complex and abstract theories, physics is above all an experimental science. It is therefore critical that those who make precise measurements be able to agree on standards in which to express the results of those measurements, so that they can be communicated from one laboratory to another and verified. In this chapter we begin our study of physics by introducing some of the basic units of physical quanti- ties and the standards that have been accepted for their measurement. We consider the proper way to ex- press the results of calculations and measurements, including the appropriate dimensions and number of significant figures. We discuss and illustrate the importance of paying attention to the dimensions of the quantities that appear in our equations. Later in the text, other basic units and many derived units are in- troduced as they are needed. 1-1 PHYSICAL QUANTITIES, STANDARDS, AND UNITS The laws of physics are expressed in terms of many differ- ent quantities: mass, length, time, force, speed, density, re- sistance, temperature, luminous intensity, magnetic field strength, and many more. Each of these terms has a precise meaning, and they form part of the common language that physicists and other scientists use to communicate with one another — when a physicist uses a term such as “kinetic en- ergy,” all other physicists will immediately understand what is meant. Each of these terms also represents a quantity that can be measured in the laboratory, and just as there must be agreement on the meaning of these terms, there must also be agreement about the units used to express their values. Without such agreement, it would not be possible for scien- tists to communicate their results to one another or to com- pare the results of experiments from different laboratories. Such comparisons require the development and accep- tance of a set of standards for units of measurement.
eBook - PDF- Paul Regtien, F. van der Heijden, M. J. Korsten, W Otthius(Authors)
- 2004(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 2 Basics of Measurement In this chapter we discuss various basic aspects of measurement science. First, the system of units is presented, and how this system has been developed from its introduction in 1795 to the present time with basically seven standard units. The materialization of a unit quantity is the next issue to be discussed. It will be shown that at present all standards (except for the kilogram) are related to funda- mental physical constants. Obviously, there are many more quantities than those seven basic quantities. In the third section we give an overview of the most import- ant quantities and properties, used in various physical domains: the geometric, electrical, thermal, mechanical, and optical domain. Relations between quantities from different domains that are fundamental to the measurement of non-electrical quantities and parameters are also given. Finally, some general aspects of sensors, the devices that convert information from one domain to another, are discussed. 2.1. System of units A unit is a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value. The value of a physical quantity is the quantitative expression of a particular physical quantity as the product of a number and a unit, the number being its numerical value. For example, the circumference of the earth around the equator is given by: Ce = 40,074.103 m (2.1) where Ce is the physical quantity and the number 40,074. 103 is the numerical value of this quantity expressed in the unit meter. Obviously, the numerical value of a particular physical quantity depends on the unit in which it is expressed. Therefore, to any measurement result the unit in which it is expressed should be added explicitly. To avoid misinterpretations the use of the international system of units (Systrme International d'unitrs, SI) is highly recommended when presenting measurement results.
eBook - PDFIntroduction to Chemical Engineering
Tools for Today and Tomorrow
- Kenneth A. Solen, John N. Harb(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
Each of the basic dimensions can be expressed in a variety of units (e.g., time can 43 44 Chapter 4 Describing Physical Quantities be represented in seconds, minutes, hours, days, and so forth). However, one can define subsystems wherein one unit of measurement is defined as the “base” unit of measurement with other units as multiples of that base unit. For example, the SI system mentioned pre- viously is a subsystem of the metric system and was defined with the base unit of length being the meter (m), the base unit of mass being the kilogram (kg), and the base unit of time being the second (s). This is in contrast with another subset of the metric system, the cgs system, that uses the centimeter (cm), gram (g), and second (s) as the base units for length, mass, and time, respectively. Table 4.1 summarizes the base units of mass, length, time, and temperature for the cgs and SI subsystems. While the American engineering system is a general system of units (like the metric system) and no base units are strictly defined, sample units for each category have been included in Table 4.1 for comparison. Table 4.1 Base or Sample Units for Three Measurement Systems System Mass Length Time Temperature cgs g cm s Celsius SI kg m s Kelvin American lb m ft s Fahrenheit 4.1.1 Conversion Factors It is frequently necessary to convert from one type of unit to another – for example, from inches to feet, from grams to pounds-mass (lb m ), or from seconds to hours. The key to such conversions is the use of conversion factors: Definition of a Conversion Factor A conversion factor is a relationship expressed by an equation where the entries on both sides of the equation are the same quantity but expressed in different units. The following are a few common conversion factors: 12 in = 1 ft 1000 g = 1 kg 60 s = 1 min A more complete list of conversion factors is found in the beginning of this book.
- Cohen, Douglas L.(Authors)
- 2001(Publication Date)
Fortunately, there do exist procedures using elementary mechanical units that are analogous to what goes on when we change the kinds of units used in electromagnetic theory; we can use this analogy to in-troduce the appropriate perspective for understanding electromagnetic units. This chapter begins by presenting material with which the reader is probably already familiar—what a unit is, what a dimension is, the rules for manipulating units in-side equations—and then moves on to describe the procedures applied to standard mechanical units in quantum mechanics and relativistic physics to simplify the forms of complicated equations. It should be emphasized that all these equations are presented as “given,” with no expectation that the reader will gain or have any particular knowledge of how the equation is derived; we just show how to simplify the equation by changing the units in which it is expressed. By the end of the chap-ter the reader will have acquired the rules and terminology needed to describe how and why the equations of classical electromagnetism change form when moving from one system of units to another. 1 2 C HAPTER 1 1.1 T HE BASIC IDEA OF A UNIT Measurements create numbers, and units give meaning to numbers by connect-ing them to measurements. For example, to call a length “8.5” is completely am-biguous; but to call it “8.5 centimeters,” or “8.5 feet,” or “8.5 miles,” does have meaning—and the meaning changes when the attached unit changes. In equations, a physical quantity, such as a length L , is treated as if it were the product of a numeric or pure number, like the 8.5 in “8.5 centimeters,” and a unit, like the cen-timeters in “8.5 centimeters.” Length L is specified in centimeters (cm) using the equation L = 8 .
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