Heat Engine/Gas Law Apparatus 012-06014C
18
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b. What is the equation you need to use to calculate the useful mechanical work done in lifting
the mass from one level to another?
c. Use the result for the height that the mass is lifted in the power stroke of the engine to
calculate the useful mechanical work performed by the heat engine.
d. How does the thermodynamic work compare to the useful mechanical work? Please use the
correct number of significant figures in your comparison (as you have been doing all along,
right?)
The Incredible Mass Lifter Engine Is Not So Simple
Understanding the stages of the engine cycle on a P-V diagram is reasonably straightfor-
ward. However, it is difficult to use equations for adiabatic expansion and compression and
the ideal gas law to determine the temperature (and hence the internal energy of the air
throughout the cycle. There are several reasons for this. First, air is not an ideal gas.
Second, the mass lifter engine is not well insulated and so the air that is warmed in the hot
reservoir transfers heat energy through the cylinder walls. Thus, the air in the can and in the
cylinder are probably not at the same temperature. Third, air does leak out around the
piston, especially when larger masses are added to the platform. This means that the
number of moles of air decreases over time. You can observe this by noting that in the
transition from point d to point a, the piston can actually end up in a lower position than it
had at the beginning of the previous cycle. However, the Incredible Mass Lifter Engine
does help us understand typical stages of operation of a real heat engine.
➤ Note: The previous experiment was intended to help students consolidate the concepts
of pressure and volume by taking their own data for height and mass in each part of the
cycle and then calculating the pressures using the basic definition of pressure vs. force
per unit area. An alternate method for doing this experiment is to use the Science
Workshop computer interface with the Pressure Sensor (CI-6532) in conjunction with
either a Motion Sensor (CI-6529) or Rotary Motion Sensor (CI-6538) to detect pressure,
volume, and height automatically with a computer.
(7 pages)
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