Environmental Engineering Reference
In-Depth Information
APPENDIX
A
Measuring Energy
Energy in materials, such as fossil or nuclear fuels, and electrical energy delivered by power lines are
commodities in industrialized economies, being traded in the marketplace. There must be standards
for measurement of the energy content and other pertinent properties of these commodities for this
market to operate efficiently. These standards of measurement are derived from the development
of modern science and technology, where agreement among researchers about how to quantify the
results of their experiments is essential to continued scientific progress.
By common agreement among scientists of all nations, a system of units of measurement has
been selected: the International System of Units, or SI for short. The SI system defines seven base
units of measurement which are mutually independent of each other, it being impossible to measure
one unit in terms of any other. Furthermore, all other physical quantities may be measured in terms
of one or more of these units. The magnitudes of the base units have been arbitrarily chosen, but are
clearly defined by agreement. The defined base units are the meter (length), kilogram (mass), second
(time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance),
and candela (luminous intensity). The first six of these are used in this text. They are listed in the
first section of Table A.1 of this Appendix, “Base units,” together with their abbreviated symbols. 1
There are many physical quantities that arise in scientific studies for which it is useful to
define a unit of measurement that is derived from the base units by a well-known physical law.
For example, the SI unit of force, the newton, is defined as the magnitude of force which, when
applied to a 1-kg mass, will cause the mass to experience an acceleration of 1 m/s 2 . By Newton's
law of motion (force
acceleration), 1 newton must equal 1 kg m/s 2 . For the physical and
chemical quantities of interest in this topic, Table A.1 lists these derived units in its second section,
“Derived units.”
Among the derived SI units, the unit of energy is the joule (J) and that of power, the time rate
of energy use, is the watt (W), which equals one joule per sec (J/s). In terms of mechanical units,
a joule equals one newton meter (Nm), or unit force times unit distance, and a watt is one newton
meter per second (Nm/s), or unit force times unit velocity. In terms of common electrical units, a
joule equals one volt ampere second (VAs), or unit charge times unit electric potential, and a watt
equals one volt ampere (VA), or unit current times unit electric potential.
For various practical reasons, including the needs of commerce and historical usage that
preceded modern science, additional SI units have been defined. The ones pertinent to this text are
=
mass
×
1 Units named after a scientist (newton, ampere, kelvin, etc.) are not capitalized when spelled out but their
abbreviations (N, A, K, etc.) are capitalized.
303
 
 
Search WWH ::




Custom Search