Day 60: Inconsistent Thermodynamics Problem?

Today in AP Physics B, we whiteboarded problems in preparation for tomorrow’s thermodynamics exam. One problem was from Giancoli (5th Edition), Chapter 15, problem 53 which reads: When 5.30e4 J of heat are added to a bass enclosed in a cylinder fitting with a light frictionless piston maintained at atmospheric pressure, the volume is observed to increase from 1.9 m3 to 4.1 m3. Calculate (a) the work done by the bass, and (b) the change in internal energy of the bass. (c) Graph this process on a PV diagram. Here’s their solution:


The answers matched the back of the book and make sense in terms of the calculations, but something didn’t sit right with me. I hoped one of the students would ask about it, but since no one did, I asked if we would expect the internal energy of the gas to increase or decrease based on the PV diagram. Since the pressure is constant and the volume is increasing, the internal energy must increase. However, based on the values supplied and the first law of thermodynamics, the change in internal energy is negative. After discussing, we decided that the values provided in the problem were just inconsistent and that the heat added to the gas should have been much larger. Are we right? Did I overlook something?

  ##whiteboarding ##mistakes  

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