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