Modern Thermodynamics
- Chapter 1
11
Box 1.1 Basic Definitions
Pressure is defined as the force per unit area. The Pascal is the SI unit of pressure:
Pascal (Pa) = 1 N
m–2.
The pressure due to a column of fluid of uniform density ρ and height h equals hρg where g is the
acceleration due to gravity (9.806 m/s2). The pressure due to the Earth's atmosphere changes with
location and time, but it is often close to
105Pa
at the ocean level. For this reason a unit called the bar is
defined:
1 bar =
105Pa
= 100 kPa
The atmospheric pressure at the Earth's surface is also nearly equal to the pressure due to a 760 mm
column of mercury. For this reason, the following units are defined:
torr = pressure due to 1.00 mm column of mercury
1 atmosphere (atm) = 760 torr = 101.325 kPa
1 atm approximately equals 1 kg/cm2 or 15 lb/inch2. The atmospheric pressure decreases exponentially
with altitude (see Box 1.2)
Temperature is usually measured in Kelvin (K), Celsius (˚C) or Fahrenheit (˚F). The Celsius and the
Fahrenheit scales are empirical while (as we shall see in chapter 3) the Kelvin scale is the absolute scale
based on the Second Law of thermodynamics. Zero Kelvin is the absolute zero, the lowest possible
temperature. Temperatures measured in these scales are related as follows:
(T/˚C) = (5/9)[(T/˚F)–32] (T/K) = (T/˚C) + 273.15
On the Earth, the highest recorded temperature is 57.8˚C or 136˚F; it was recorded in El Azizia, Libiya, in
the year 1922. The lowest recorded temperature is –88.3˚C or –129˚F; it was recorded in Vostok,
Antarctica. In the laboratory, sodium gas has been cooled to temperatures as low as 10-9K and, in nuclear
fusion reactors, temperatures as high as 108K have been reached.
Heat was initially thought to be an indestructible substance called the caloric. According to this view,
caloric, a fluid without mass, passed from one body to another causing changes in temperature. However,
in the 19th century it was established that heat was not an indestructible caloric but a form of energy that
can convert to other forms of energy(see chapter 2). Hence heat is measured in the units of energy. In
this text we shall mostly use the SI units in which heat is measured in Joules, though the calorie is an
often used unit of heat. A calorie was originally defined as the amount of heat required to increase the
temperature of one gram of water from 14.5˚C to 15.5˚C. The current practice is to define a thermo-
chemical calorie as 4.184 Joules.
The Gas Constant R appears in the ideal gas law, pV=NRT. Its numerical values are:
R = 8.314 J K mol-1 (or Pa m3 K-1 mol-1) = 0.08314 bar L K-1 mol-1 = 0.0821 atm L K-1 mol-1.
The Avogadro Number NA = 6.023x1023 mol-1. The Boltzmann constant kB=R/NA= 1.3807 J K-1.
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