國中理化：單位測量

International System of Units

SI base units

Table 1. SI base units

SI base unit Base quantity Name Symbol length meter m mass kilogram kg time second s electric current ampere A thermodynamic temperature kelvin K amount of substance mole mol luminous intensity candela cd

Table 2. Examples of SI derived units
	SI derived unit
Derived quantity	Name	Symbol
area	square meter	m2
volume	cubic meter	m3
speed, velocity	meter per second	m/s
acceleration	meter per second squared	m/s2
wave number	reciprocal meter	m-1
mass density	kilogram per cubic meter	kg/m3
specific volume	cubic meter per kilogram	m3/kg
current density	ampere per square meter	A/m2
magnetic field strength	ampere per meter	A/m
amount-of-substance concentration	mole per cubic meter	mol/m3
luminance	candela per square meter	cd/m2
mass fraction	kilogram per kilogram, which may be represented by the number 1	kg/kg = 1

For ease of understanding and convenience, 22 SI derived units have been given special names and symbols, as shown in Table 3


SI prefixes

The 20 SI prefixes used to form decimal multiples and submultiples of SI units are given in Table 5.

Table 5. SI prefixes
Factor Name 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		Factor Name Symbol 10-1 deci d 10-2 centi c 10-3 milli m 10-6 micro µ 10-9 nano n 10-12 pico p 10-15 femto f 10-18 atto a 10-21 zepto z 10-24 yocto y

Units outside the SI

Certain units are not part of the International System of Units, that is, they are outside the SI, but are important and widely used. Consistent with the recommendations of the International Committee for Weights and Measures (CIPM, Comité International des Poids et Mesures), the units in this category that are accepted for use with the SI are given in Table 6.

Table 6. Units outside the SI that are accepted for use with the SI
Name	Symbol	Value in SI units
minute (time)	min	1 min = 60 s
hour	h	1 h = 60 min = 3600 s
day	d	1 d = 24 h = 86 400 s
degree (angle)	°	1° = ( /180) rad
minute (angle)		1 = (1/60)° = (/10 800) rad
second (angle)		1 = (1/60) = (/648 000) rad
liter	L	1 L = 1 dm3 = 10-3 m3
metric ton (a)	t	1 t = 103 kg
neper	Np	1 Np = 1
bel (b)	B	1 B = (1/2) ln 10 Np (c)
electronvolt (d)	eV	1 eV = 1.602 18 x 10-19 J, approximately
unified atomic mass unit (e)	u	1 u = 1.660 54 x 10-27 kg, approximately
astronomical unit (f)	ua	1 ua = 1.495 98 x 1011 m, approximately