University College - Chemistry Chem 112A Lecture Notes - Lecture 14: Horse Length, Thermodynamics, Ideal Gas
13 Dec 2016
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A is the area of the piston and d is the distance the piston travels: as the gas expands it does pvwork = -pext* v. Pext is the external pressure, which we assume is constant here. This expression is negative indicating the gas system is doing work on the surroundings. If w = f*d, f/a = p, w = p*a*d = p v. From our example, the heat is being converted to work: q = heat, w = mechanical work. If q > 0, then heat is being added to the system. If w > 0, then work is done to the system by the surroundings (compression) If q < 0, then heat is removed from the system. This means that there are no bonds/interactions (a source of potential energy), so the total energy will just be equal to the kinetic energy: u = q + w.
