Environmental Engineering Reference
In-Depth Information
TABLE A.3
Measured Quantities
Quantity
Symbol
SI Unit Value
Charge of electron
1.6030E(
−
19) C
Electron volt
eV
1.6030E(
−
19) J
Faraday
F
9.6485E(4) C/mol
Unified atomic mass unit
amu
1.66054E(
−
27) kg
Avogadro's number
N
0
6.0221E(23)/mol
Universal gas constant
R
8.3143E(3) J/kg K
9.80665 m/s
2
Standard gravitational acceleration
g
Standard atmospheric pressure
1.01325E(5) Pa
Melting point of ice (0
◦
C
=
32
◦
F)
273.15 K
5.6704E(
−
8) W/m
2
Stefan-Boltzmann constant
σ
TABLE A.4
SI Unit Prefixes
Factor
Prefix
Symbol
U.S. Word Modifier
1E(18)
exa
E
1E(15)
peta
P
quadrillion
1E(12)
tera
T
trillion
1E(9)
giga
G
billion
1E(6)
mega
M
million
1E(3)
kilo
k
thousand
1E(2)
hecto
h
hundred
1E(
−
1)
deci
d
1E(
−
2)
centi
c
percent
1E(
−
3)
milli
m
1E(
−
6)
micro
µ
1E(
−
9)
nano
n
1E(
−
12)
pico
p
physical constants. Some of these that we use are listed in Table A.3. Among these are the electron
volt (eV), the Faraday (
), Avogadro's number (
N
0
), and the universal gas constant (
R
), used in
analyzing electrochemical processes.
The SI units of Table A.1 are often of inconvenient size. Just as paper currency comes in
different denominations, physical quantities need to have different sizes to accommodate different
uses. The SI system includes the use of prefixes to change the size of units by factors of 10, up
or down—for example, kilometer (km), centimeter (cm), micrometer (
F
m). Table A.4 lists the SI
unit prefixes that cover a range of 30 orders of magnitude, enough for most practical purposes.
Occasionally we use practical, if not always logical, units such as the unit of electrical energy,
the kilowatt hour (kWh), which equals 3.6 megajoules (MJ). The kilowatt hour, an amount of
energy that will light a kilowatt bulb for one hour, is a better unit for commercial use than the
megajoule.
µ
