How can we do work with heat? How can we convert some disordered energy back into ordered energy?
In this exercise you will analyze data obtained with is a "real" thermal
engine that can be taken through a four-stage expansion and compression cycle
and that can do useful mechanical work by lifting small masses from one height
to another. You will determine the useful mechanical work done by the
engine by measuring the vertical distance y a mass m is lifted. You will
compare this mechanical work Wmech = mgy to the net thermodynamic
work done during a cycle. The pressure as a function of volume is recorded
for one cycle, and the net thermodynamic work done by the engine equals the
enclosed area on the PV diagram,
Wnet = ∫closed path PdV.
The PASCO TD-8572 Heat Engine/Gas Law Apparatus is used to obtain the data. The heart of this apparatus is a nearly friction-free piston-cylinder system. The graphite piston fits snugly into a precision-ground Pyrex cylinder so that the system produces almost friction-free motion and negligible leakage. The Heat Engine/Gas Law Apparatus is designed with two pressure ports with quick-connect fittings for connecting to an air chamber and a pressure sensor with tubing.
With no mass on the platform, the piston is raised ~4 cm, and the air chamber with tubing is connected to the engine. The piston stays at a height of ~4 cm.
The computer is set up to record the pressure (in units of kPa), the volume of the gas in the cylinder (in units of cm3), and the position of the cylinder above its starting position (in units of m) as a function of time and to produce plots of pressure versus volume and position versus time.
(a) The air chamber is placed into ice water.
(b) A 100 g mass is placed on the platform. The weight of the mass increases the pressure at constant temperature, and the volume and therefore the height of the piston decrease by some amount.
(c) The air chamber is placed into hot water. The temperature rises and the gas expands. The volume and therefore the height of the piston increase.
(d) When the volume is no longer increasing, the mass is removed from the platform. Removing the weight decreases the pressure at constant temperature, and the volume and therefore the height of the piston increase by some amount.
(e) The air chamber is placed back into the ice water. The temperature decreases and the gas contracts. The volume and therefore the height of the piston decrease.
Plot of pressure versus volume:
Plot of piston position versus time:
||Work done by heat engine
from P-V diagram (J)
|Change in potential
energy of mass (J)
Paste your table into your Word document and answer the following questions.
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contains your data tables, graphs, comments and answers.
Save your Word document (your name_exm3.docx), go to Canvas, Assignments, Extra Credit 3, and submit your document.