ABSTRACT
The isothermal expansion of an ideal gas (see Topic A1) against an external pressure is usually given to illustrate the difference between these two conditions. The work, w, done by the gas is given by:
Against a constant pressure (i.e. non-reversible conditions) this integrates to w=p(V1−V2). Under reversible conditions against an infinitesimally smaller pressure, p may be re-written as (nRT/V), and the expression integrates to nRTln(V1/V2). The difference is illustrated graphically for one mole of perfect gas expanding from a pressure of 3 bar down to 1 bar in Fig. 1. The total amount of work done in each case is equal to the area under the line.
