The Si Units-The International System of Units

 

The International System of Units

In 1971, the 14th General Conference on Weights and Measures picked seven quantities as base quantities, thereby forming the basis of the International System of Units, abbreviated SI from its French name and popularly known as the metric system.
Table 1-1 shows the units for the three base quantities—length, mass, and time—that we use in the early chapters of this book. These units were defined to be on a “human scale.” Many SI derived units are defined in terms of these base units. For example, the SI unit for power, called the watt (W), is defined in terms of the base units for mass, length, and time. Thus,

1 watt =1 W = 1 kg m²/s³,                                                                                        (1)

Where the last collection of unit symbols is read as kilogram-meter squared per second cubed.

To express the very large and very small quantities we often run into in physics, we use scientific notation,which employs powers of 10. In this notation,

3 560 000 000 m = 3.56 × 109m                                                                            (2)

and

0.000 000 492 s = 4.92 × 10^-7 s.                                                                              (3)

Scientific notation on computers sometimes takes on an even briefer look, as in 3.56 E9 and 4.92 E–7, where E stands for “exponent of ten.” It is briefer still on some calculators, where E is replaced with an empty space.

As a further convenience when dealing with very large or very small measurements, we use the prefixes listed in Table 1-2. As you can see, each prefix represents a certain power of 10, to be used as a multiplication factor. Attaching a prefix to an SI unit has the effect of multiplying by the associated factor. Thus, we can express a particular electric power as

1.27 * 10⁹ watts = 1.27 gigawatts = 1.27 GW                                                         (4)

or a particular time interval as

            2.35 * 10^-9 s = 2.35 nanoseconds = 2.35 ns.                                                          (5)

Some prefixes, as used in milliliter, centimeter, kilogram, and megabyte, are probably familiar to you.

 

Table 1-2 Prefixes for SI Units

Factor            Prefixa            Symbol

10²⁴                 yotta-                 Y

10²¹                 zetta-                  Z

10¹⁸                 exa-                    E

10¹⁵                 peta-                  P

10¹²                 tera-                   T

10⁹                  giga-                    G

10⁶                 mega-                  M

10³                 kilo-                      k

10²                  hecto-                  h

10¹                  deka-                   da

10^-1                 deci-                   d

10^-2                 centi-                  c

10^-3                 milli-                   m

10^-6                 micro-                 m

10^-9                 nano-                  n

10^-12                pico-                   p

10^-15                femto-                f

10^-18                atto-                   a

10^-21                zepto-                 z

10^-24                yocto-                 y

"a" The most frequently used prefixes are shown in bold type.

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