Unit-1.B_Measurement_physical-quantities-units-and-conversion.pdf

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Unit 1.2. Physical quantities, units, and conversion
Textbook Readings:
Topic References
Units of measurement Section 1.5 of Brown et al. 2018
Contents
1.2.1. Physical Quantities and Units of Measurement ........................................................................................ 1
1.2.2. Unit prefixes and Scientific Notation ......................................................................................................... 3
1.2.3. Conversion of Units and Quantity Calculus ............................................................................................... 4
1.2.1. Physical Quantities and Units of Measurement
• Many properties of matter are quantitative , that is, they are associated with numbers
which in turn indicate
o multitude (many or few), or
o magnitud e (big or small.
A property of this kind is known as a quantity .
• A quantity is specified by performing a measurement , which is a process of assigning of a
number to a characteristic or property of an object or a phenomenon.
• In the process of measurement, the units of the quantity must also be specified.
o The unit of measurement is a definite multitude or magnitude of a quantity defined
by law (of man and the universe) or convention.
o Examples are the ones defined by the Système International d’Unités (French name)
or SI Units, which we will use generally in the course.
• We also often use the term physical quantity to describe a property of matter, energy or
change that can be specified by measurement.
• In the SI units, there are seven base units for the most fundamental physical quantities,
which are used (first by physicists) to quantitatively describe everything in the universe
(that is known to human beings). All other units can be derived from these base units.
Table 1. SI Base Units
Physical
quantity
Name of
unit
Symbol Definition
Time second S The duration of 9,192,631,770 cycles of the
radiation of a certain emission of the
cesium atom.
Length meter M Distance travelled by light in a vacuum
during a time interval of 1/299,792,458 s

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Mass kilogram Kg Defined by taking the fixed numerical value
of the Planck constant, h, to be 6.626 070
15 × 10 − 34 when expressed in
the unit J-s, which is equal to kg m 2 s − 1 . The
previous definition of kg is it is the mass of
a prototype made of platinum-iridium kept
at the International Bureau of Weights and
Measures in France.