Page 34 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

18 Chapter 2

INTERNAL ENERGY, U
Energy is the ability or capacity to do work. The unit of energy is the
joule, J. The internal energy, U, is defined as the total energy of all
forms within a system. Energy is transferred into or out of a system in
two ways: (a) by heat transfer, q, and/or (b) by work, w, as shown in
Figure 2.2.

SURROUNDINGS SURROUNDINGS
IN: q +ve OUT: w -ve
Meat (9) SYSTEM Work (w)
OUT q -ve IN: w +ve
SURROUNDINGS SURROUNDINGS

Figure 2.2 Transfer of energy into and out of a system
If the work is done by the system on the surroundings, w is -ve,
whereas if work is done by the surroundings on the system, w is + ve.
Hence, by definition, -1

The change in internal energy is defined as A U = Ufinal - Uinitial-

ENTHALPY, H
Consider the expansion of a gas, of initial volume, Vinitial, to a final
volume, Vfinal, as shown in Figure 2.3, at constant pressure. In this
situation, the gas expands, and hence work is done by the system on
its surroundings, i.e. w is -ve.

U
Expansion

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Figure 2.3 Expansion of a gas. Since the gas expands, the system does work on
the surroundings, therefore, w is - ve