Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

We emphasize that the magnitudes of each division of 1 K and 1°C are
identical (Fig. 1–35). Therefore, when we are dealing with temperature dif-
ferences T, the temperature interval on both scales is the same. Raising the
temperature of a substance by 10°C is the same as raising it by 10 K. That is,

(1–13)

(1–14)

Some thermodynamic relations involve the temperature Tand often the
question arises of whether it is in K or °C. If the relation involves tempera-
ture differences (such as abT), it makes no difference and either can be
used. However, if the relation involves temperatures only instead of temper-
ature differences (such as abT) then K must be used. When in doubt, it is
always safe to use K because there are virtually no situations in which the
use of K is incorrect, but there are many thermodynamic relations that will
yield an erroneous result if °C is used.

¢T 1 R 2 ¢T 1 °F 2

¢T 1 K 2 ¢T 1 °C 2

Chapter 1 | 21

1 K 1 °C 1.8 R 1.8°F

FIGURE 1–35

Comparison of magnitudes of various

temperature units.

EXAMPLE 1–4 Expressing Temperature Rise in Different Units

During a heating process, the temperature of a system rises by 10°C. Express

this rise in temperature in K, °F, and R.

Solution The temperature rise of a system is to be expressed in different

units.

Analysis This problem deals with temperature changes, which are identical

in Kelvin and Celsius scales. Then,

The temperature changes in Fahrenheit and Rankine scales are also identical

and are related to the changes in Celsius and Kelvin scales through Eqs.

1–11 and 1–14:

and

Discussion Note that the units °C and K are interchangeable when dealing

with temperature differences.

¢T 1 °F 2 ¢T 1 R 2  18 °F

¢T 1 R 2 1.8 ¢T 1 K 2  1 1.8 21102 18 R

¢T 1 K 2 ¢T 1 °C 2 10 K

1–9 ■ PRESSURE

Pressureis defined as a normal force exerted by a fluid per unit area. We
speak of pressure only when we deal with a gas or a liquid. The counterpart
of pressure in solids is normal stress. Since pressure is defined as force per
unit area, it has the unit of newtons per square meter (N/m^2 ), which is called
a pascal(Pa). That is,

1 Pa1 N>m^2

SEE TUTORIAL CH. 1, SEC. 9 ON THE DVD.

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