CHAPTER 3
3 The First Law
In science, alawis a statement or mathematical relation that concisely describes repro-
ducible experimental observations. Classical thermodynamics is built on a foundation of
three laws, none of which can be derived from principles that are any more fundamental.
This chapter discusses theoretical aspects of the first law; gives examples ofreversibleand
irreversibleprocesses and the heat and work that occur in them; and introduces the extensive
state functionheat capacity.
3.1 Heat, Work, and the First Law
The box below gives two forms of thefirst law of thermodynamics.
In a closed system:
dUD∂qC∂w ÅUDqCw
whereUis the internal energy of the system, a state function;
qis heat; and
wis thermodynamic work.The equation dUD∂qC∂wis thedifferentialform of the first law, andÅUDqCwis
theintegratedform.
The heat and work appearing in the first law are two different modes of energy transfer.
They can be defined in a general way as follows.
Heat refers to the transfer of energy across the boundary caused by a temperature gradient
at the boundary.
Work refers to the transfer of energy across the boundary caused by the displacement of a
macroscopic portion of the system on which the surroundings exert a force, or because
of other kinds of concerted, directed movement of entities (e.g., electrons) on which an
external force is exerted.
An infinitesimal quantity of energy transferred as heat at a surface element of the bound-
ary is written∂q, and a finite quantity is writtenq(Sec.2.5). To obtain the total finite heat
for a process fromqD
R
∂q(Eq.2.5.3), we must integrate over the total boundary surface
and the entire path of the process.