edited bv GEORGEL. GILBERT
Omison University Granville. Ohio 43023
Maximum Work SUBMITTED BY
John C. Salzsieder Phillips University Enid. OK 73702 CHECKED sv
Michael Fuson Oenison University Granville. OH 43023 When the second law of thermodynamics is discussed, students are told t h a t t h e maximum work is done when a Drocess is done reversihlv. This can he demonstrated simply and convincingly by lifting masses to various heights using rubher bands asasource of energy. Removing mass in smaller increments delivers more total work.
Materials six 100-g masses mass hanger seven small iron rings ring stand rubber bands-narrow, latex Procedure The initial state is the six 1M)-gmasses suspended by the rubber bands just above the desk top. The final state is the hanger with all 600 g removed (65 cm higher in the example of the table). Four separate demonstrations are done. First, all six masses are removed at once, and no work is done. Second, three masses are removed at a time. Third, shown in the figure, two masses are removed at a time. Fourth, one mass is removed at a time. In every step the masses are placed upon an iron ring at the height at which they are removed. Upon completion of each demonstration the students can see how much mass has been lifted and how high. The work for each step is calculated as mass X height X gravity. The total work for each demonstration is the sum of its steps. The table gives a representative set of data. The data clearly show the trend of getting more work as the increment of mass removed becomes smaller, i.e., the process beReprM.nlallve Data
Masses
Height
Removed
Mass (kg)
~lfted(m)
8 ata time 3 8ta time
0.800 0.300 0.300 0.200 0.200
0.00 0.00 0.25 0.00 0.12 0.40
2 ata time
0.200
962
Journal of Chemical Education
Total Work (J)
0.00 0.74 1.02
comes more nearly reversible. In order to find the maximum work available one would have to express height (HIas a function of mass removed (M,IIn . the ease of a rubber band. the function would be a ~~~. logistrc (s-shaped)curve. If the function is roughly apprvximated by a straight line, however, the data from the tahle giver eq I . The maximum work available is then estimated hy integration of cq 2. ~~~~~
