Modern Engineering Thermodynamics

42 CHAPTER 2: Thermodynamic Concepts
HOW DO I KNOW WHEN I HAVE TO USE ABSOLUTE PRESSURE
OR TEMPERATURE?
If pressure or temperature stands by itself or is raised to some power in an equation, then the values assigned to it must be
in an absolute scale of units.
and when it is changed to state 2, we can write
Now, if we subtract Eq. (2.6) from (2.7), we get
p 2 V = mRT 2 (2.7)
ðp 2 − p 1 ÞV = mRðT 2 − T 1 Þ (2.8)
Absolute pressure and temperature scales must be used in making calculations with Eqs. (2.6) and (2.7), but
either absolute or relative scales may be used in Eq. (2.8). Relative scales can be used whenever the additive
term that converts a relative scale to an absolute scale cancels out within the formula, as it does when a simple
difference is taken.
Using relative scale values where absolute scale values shouldbeusedclearlyleadsto enormous calculational
errors. If in doubt, use values in the absolute scale units.
2.8 THE ZEROTH LAW OF THERMODYNAMICS
As previously mentioned, the zeroth law was one of the last thermodynamic laws to be developed. It was
introduced by R. H. Fowler and E. A. Guggenheim in 1939. 6
This may seem trivial at first reading, but consider: If man A loves woman C and man B loves woman C, does it
follow that man A loves man B? One of the major values of the zeroth law is that it forms the theoretical basis
for temperature measurement technology. Consider the mercury in glass thermometer shown in Figure 2.6. The
zeroth law tells us that if the glass is at the same temperature as (i.e., is in thermal equilibrium with)
the surrounding fluid, and if the mercury is at the same temperature as the glass, then the mercury is at the
same temperature as the surrounding fluid. Thus, the thermometer can be graduated to show the mercury
temperature, and this temperature is automatically (via the zeroth law) equal to the temperature of its
surroundings.
ZEROTH LAW OF THERMODYNAMICS
Consider three thermodynamic systems, A, B, and C. If system A is in thermal equilibrium with (i.e., is the same temperature
as) system C and system B is in thermal equilibrium with system C, then system A is in thermal equilibrium
with system B.
CRITICAL THINKING
The text describes a love triangle that would not necessarily satisfy the zeroth law of thermodynamics. Can you think of
other human characteristics (e.g., hate) that might not satisfy this law? In the zeroth law, thermal equilibrium is the same
as temperature equilibrium, so that if T A = T C and T B = T C , then the zeroth law requires that T A = T B . Can we create yet
another thermodynamic law based on requiring a different physical property of systems A, B, and C to be in another
(say, mechanical rather than thermal) type of equilibrium? What thermodynamic value would this new “law” have?
6 Fowler, R. H., Guggenheim, E. A., 1939. Statistical Thermodynamics. Cambridge University Press, Cambridge, MA.