Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

11.1 Temperature as a Fundamental Dimension 307

Material properties are a function of temperature. Physical and thermal properties of

solids, liquids, and gases vary with temperature. For example, as you know, cold air is denser

than warm air. The air resistance to your car’s motion is greater in winter than it is in sum-

mer, provided the car is moving at the same speed. Most of you who live in a cold climate

know it is harder to start your car in the morning in the winter. As you may know, starting

difficulty is the result of the viscosity of oil increasing as the temperature decreases. The vari-

ation in density of air with temperature is shown in Figure 11.2. The temperature dependence

of SAE 10W, SAE 10W-30, and SAE 30 oil is shown in Figure 11.3. These are but a few

examples of why as engineers you need to have a good understanding of temperature and its

role in design.

Let us now examine more closely what we mean by temperature. Temperature provides a

measure of molecular activity and the internal energy of an object. Recall that all objects and

living things are made of matter, and matter itself is made up of atoms or chemical elements.

Moreover, atoms are combined naturally, or in a laboratory setting, to create molecules. For

1.35

1.30

1.25

1.20

1.15

1.10

1.05

015510 202530

Temperature (°C)

Density of air as a function

of temperature

5550454035

Density (kg/m

3 )

■Figure 11.2

Density of air at atmospheric pressure as a function of

temperature.

1.0

0.8

0.6

0.4

0.2

0.1

0.08

0.06

0.04

0.02

0.01

0.008

0.006

0.004

0.002

0.001

 20 0 20 40 60

Temperature (°C)

12010080

Viscosity (N· s/m

2 )

SAE 10W-30

SAE 30

SAE 10W

■Figure 11.3

Viscosity of SAE 10W, SAE 10W-30, and SAE 30 oil as a function of

temperature.

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