Geology Reference
In-Depth Information
Table A2
Derived SI units
(a)
2
Quantity
Unit
Abbreviation
In terms of basic SI
units
q
Δ
y
Force
newton
N
kg m s
−
2
= J m
−
1
p
1
Energy, work
joule
J
kg m
2
s
−
2
= N m
Δ
x
Power
watt
W
kg m
2
s
−
3
= J s
−
1
Pressure
pascal
Pa
kg m
−
1
s
−
2
= N m
−
2
= J m
−
3
Electric charge
coulomb
c
A s
Intercept
c
Electrical potential
difference
volt
v
kg m
2
s
−
3
A
−
1
= W A
−
1
0
1
2
3
x
(b)
Table A3
SI prefixes and examples of use
y
y
petametre
Pm
= 10
15
m = 10
12
km
terabyte
Tb
= 10
12
bytes
gigapascal
GPa
= 10
9
Pa
x
x
megajoule
MJ
= 10
6
J
Positive
m
Positive
c
Negative
m
Positive
c
kilometre
km
= 10
3
m
millisecond
ms
= 0.001 s = 10
−
3
s
micrometre
µm
= 10
−
6
m
nanogram
ng
= 10
−
9
g = 10
−
12
kg
picometre
pm
= 10
−
12
m
y
y
Positive
m
Negative
c
(c) The units of heat are the same as those of mechanical
energy and work, avoiding the need for the 'mechan-
ical equivalent of heat' constant (converting calories
to joules) required by earlier systems of units.
(d) The system recognizes prefixes that multiply or
divide units by factors of 10
3
(see Table A3).
Note that all prefixes that represent multipliers
greater than
1 (with the notable historical exception
of the k in km) are abbreviated using
capital
letters,
whereas prefixes for multipliers
less than
1 use
lower-case
letters. Popular publications often fail to
adhere to this helpful distinction.
Some sciences have been slow to adopt the SI
system. For example, units called
bars
(=10
5
Pa) and
kilobars
(kb = 10
8
Pa) continued to be used as units
of pressure in geological phase diagrams long after
formal adoption of the SI system.
x
x
Negative
m
Negative
c
Figure A1
(a) Parameters of a straight line in
x-y
space.
Note that both axes are plotted at the same scale here,
although this is not the case for all graphs; this plot also
shows the origin (where
x
= 0,
y
= 0), although not all graphs
do so. (b) How
m
and
c
reflect location and orientation.
corresponding increase in the value of
y
. The rate at
which
y
rises with increasing
x
is measured by the
gradient
or
slope
of the line,
m
:
=
∆
∆
y
x
m
(A1)
Here Δ
y
= 0.5 and Δ
x
= 2.0, so
m
= 0.5/2.0 = 0.25.
A steep line would have a large value of
m
, whereas a
shallow line would have a low value of
m
. Note that
the value of
y
at which the line crosses the
y
axis (where
x
= 0) is called the
intercept
, usually symbolized as c. If
x
and
y
had units attached,
m
and
c
would need to be
given consistent units: for example, if y were measured
Equation of a straight line
Figure A1(a) shows a straight line plotted in the con-
ventional way, against perpendicular
x
and
y
axes. For
a line with this orientation, increasing the value of
x
(e.g. from point p to point q) brings about a
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