Fig. 17-3 Sign conventions: (a) Q is positive (b) Q is negative (c) W is positive (d) W is negative In a steam engine (e) the heat in and the work out (done) are both defined to be positive.
Fig. 17-4 (a) Molecule hits a piston moving away from it. Fig. 17-4 (b) Molecule hits a piston moving toward it.
Fig. 17-4 (a) When a molecule hits a wall moving away from it, the molecule does work on the wall; the molecule’s speed and kinetic energy decrease. The gas does positive work on the piston. See Fig. 17-3 (c) the gas is the “system.” Fig. 17-4 (b) Molecule hits a wall moving toward it, the wall does work on the molecule; the molecule’s speed and kinetic energy increase. The gas does negative work on the piston. (Or, the piston does positive work on the gas.) See Fig. 17-3 (d) the gas is the “system.” WORK-ENERGY THEOREM