The Celsius and Fahrenheit Scales
So far, we have discussed only the Kelvin scale, used in basic scientific work. In
nearly all countries of the world, the Celsius scale (formerly called the centigrade
scale) is the scale of choice for popular and commercial use and much scientific
use. Celsius temperatures are measured in degrees, and the Celsius degree has
the same size as the kelvin. However, the zero of the Celsius scale is shifted to a
more convenient value than absolute zero. If T C represents a Celsius temperature
and T a Kelvin temperature, then
T C ϭ T Ϫ 273.15Њ.
( 1 8 - 7 )
In expressing temperatures on the Celsius scale, the degree symbol is commonly
used. Thus, we write 20.00ЊC for a Celsius reading but 293.15 K for a Kelvin
reading.
The Fahrenheit scale, used in the United States, employs a smaller degree than
the Celsius scale and a different zero of temperature. You can easily verify both
these differences by examining an ordinary room thermometer on which both scales
are marked.The relation between the Celsius and Fahrenheit scales is
(18-8)
where T F is Fahrenheit temperature. Converting between these two scales can be
done easily by remembering a few corresponding points, such as the freezing and
boiling points of water (Table 18-1). Figure 18-7 compares the Kelvin, Celsius,
and Fahrenheit scales.
T F ϭ
9
5 T C ϩ 32Њ,
518
CHAPTE R 18 TE M PE RATU R E, H EAT, AN D TH E FI RST L AW OF TH E R M ODYNAM ICS
18-2 THE CELSIUS AND FAHRENHEIT SCALES
After reading this module, you should be able to . . .
18.06 Convert a temperature between any two (linear)
temperature scales, including the Celsius, Fahrenheit,
and Kelvin scales.
18.07 Identify that a change of one degree is the same on the
Celsius and Kelvin scales.
Learning Objectives
● The Celsius temperature scale is defined by
T C ϭ T Ϫ 273.15Њ,
Key Idea
Figure 18-7 The Kelvin, Celsius, and
Fahrenheit temperature scales compared.
Triple
point of
water
Absolute
zero
273.16 K
0.01°C
32.02°F
0 K
–273.15°C
–459.67°F
Table 18-1 Some Corresponding Temperatures
Temperature
C
F
Boiling point of water
a
100
212
Normal body temperature
37.0
98.6
Accepted comfort level
20
68
Freezing point of water
a
0
32
Zero of Fahrenheit scale
Ϸ Ϫ18
0
Scales coincide
Ϫ40
Ϫ40
a
Strictly, the boiling point of water on the Celsius scale is 99.975 C,
and the freezing point is 0.00 C.Thus, there is slightly less than 100 C
between those two points.
Њ
Њ
Њ
Њ
Њ
with T in kelvins. The Fahrenheit temperature scale is defined by
T F ϭ
9
5 T C ϩ 32Њ.
So far, we have discussed only the Kelvin scale, used in basic scientific work. In
nearly all countries of the world, the Celsius scale (formerly called the centigrade
scale) is the scale of choice for popular and commercial use and much scientific
use. Celsius temperatures are measured in degrees, and the Celsius degree has
the same size as the kelvin. However, the zero of the Celsius scale is shifted to a
more convenient value than absolute zero. If T C represents a Celsius temperature
and T a Kelvin temperature, then
T C ϭ T Ϫ 273.15Њ.
( 1 8 - 7 )
In expressing temperatures on the Celsius scale, the degree symbol is commonly
used. Thus, we write 20.00ЊC for a Celsius reading but 293.15 K for a Kelvin
reading.
The Fahrenheit scale, used in the United States, employs a smaller degree than
the Celsius scale and a different zero of temperature. You can easily verify both
these differences by examining an ordinary room thermometer on which both scales
are marked.The relation between the Celsius and Fahrenheit scales is
(18-8)
where T F is Fahrenheit temperature. Converting between these two scales can be
done easily by remembering a few corresponding points, such as the freezing and
boiling points of water (Table 18-1). Figure 18-7 compares the Kelvin, Celsius,
and Fahrenheit scales.
T F ϭ
9
5 T C ϩ 32Њ,
518
CHAPTE R 18 TE M PE RATU R E, H EAT, AN D TH E FI RST L AW OF TH E R M ODYNAM ICS
18-2 THE CELSIUS AND FAHRENHEIT SCALES
After reading this module, you should be able to . . .
18.06 Convert a temperature between any two (linear)
temperature scales, including the Celsius, Fahrenheit,
and Kelvin scales.
18.07 Identify that a change of one degree is the same on the
Celsius and Kelvin scales.
Learning Objectives
● The Celsius temperature scale is defined by
T C ϭ T Ϫ 273.15Њ,
Key Idea
Figure 18-7 The Kelvin, Celsius, and
Fahrenheit temperature scales compared.
Triple
point of
water
Absolute
zero
273.16 K
0.01°C
32.02°F
0 K
–273.15°C
–459.67°F
Table 18-1 Some Corresponding Temperatures
Temperature
C
F
Boiling point of water
a
100
212
Normal body temperature
37.0
98.6
Accepted comfort level
20
68
Freezing point of water
a
0
32
Zero of Fahrenheit scale
Ϸ Ϫ18
0
Scales coincide
Ϫ40
Ϫ40
a
Strictly, the boiling point of water on the Celsius scale is 99.975 C,
and the freezing point is 0.00 C.Thus, there is slightly less than 100 C
between those two points.
Њ
Њ
Њ
Њ
Њ
with T in kelvins. The Fahrenheit temperature scale is defined by
T F ϭ
9
5 T C ϩ 32Њ.