Physics 52 - Heat and Optics Dr. Joseph F. Becker Physics Department San Jose State University © 2003 J. F. Becker

Chapter 17 The First Law of Thermodynamics

Insert scan of fig 17-1 here Popcorn in kettle…

Fig 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 (a) Molecule hits a piston moving away from it. Fig (b) Molecule hits a piston moving toward it.

Fig (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 (c) the gas is the “system.” Fig (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 (d) the gas is the “system.” WORK-ENERGY THEOREM