02.04 Lost Work Versus Reversibility


The reversible process is a common conception throughout discussions of thermodynamics. One very common illustration has to do with grains of sand being removed from a piston (KhanAcademy, 15min). It is also helpful to put the relation of reversibility into context with the work as a path function. (YouTube, 1.5min).

Comprehension Questions
1. Describe what happens when you knock a complete block off a piston under pressure. Assume the piston has mass roughly equal to the block, the cylinder is infinitely tall and everything is adiabatic, the gas in the cylinder is ideal, and the open side of the cylinder is at atmospheric pressure. In particular, does the piston go monotonically to its equilibrium position or does it do something else? If not, then what causes it to approach in a different way and why does it eventually reach its equilibrium state?
2. Consider the same piston/cylinder as above but compare it to a piston/cylinder with grains of sand. Is the final height of the piston the same, lower, or higher when you remove the weight one grain at a time vs. knocking the block off all at once? Hint: write the energy balance and carefully consider the work accomplished in each case.