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

2. Chapter 17 The First Law of Thermodynamics

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

4. 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.

5. Fig. 17-4 (a) Molecule hits a piston moving away from it. Fig. 17-4 (b) Molecule hits a piston moving toward it.

6. 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