Heat Engine/Gas Law Apparatus 012-06014C
14
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f. Observed transition c
➔
d: Remove the added mass and describe what actually happens. Is
this what you predicted?
g. Predicted transition d
➔
a: What do you predict will happen if you now place the can back in
the cold reservoir? Explain the reasons for your prediction.
h. Observed transition d
➔
a: Now it's time to complete the cycle by cooling the system down to
its original temperature for a minute or two before placing a new mass to be lifted on it. Place
the can in the cold reservoir and describe what actually happens to the volume of the trapped
air. In particular, how does the volume of the gas actually compare to the original volume of
the trapped air at point a at the beginning of the cycle? Is it the same or has some of the air
leaked out?
i. Theoretically, the pressure of the gas should be the same once you cool the system back to its
original temperature. Why?
Determining Pressures and Volumes for a Cycle
In order to calculate the thermodynamic work done during a cycle of this engine, you will
need to be able to plot a P-V diagram for the engine based on determinations of the volumes
and pressures of the trapped air in the cylinder, tubing, and can at the points a, b, c, and d in
the cycle.
5.2 Activity: Volume and Pressure Equations
a. What is the equation for the volume of a cylinder that has an inner diameter of d and a length
L?
b. Use the definition of pressure to derive the equation for the pressure on a gas being contained
by a vertical piston of diameter d if the total mass on the piston including its own mass and
any added mass is denoted as M. Hints: (1) What is the definition of pressure? (2) What is
the equation needed to calculate the gravitational force on a mass, M, close to the surface of
the Earth? (3) Don't forget to add in the atmospheric pressure, P
atm
, acting on the piston and
hence the gas at sea level.
Now that you have derived the basic equations you need, you should be able to take your
engine through another cycle and make the measurements necessary for calculating both the
volume and the pressure of the air and determining a P-V diagram for your heat engine.
Instead of calculating the pressures, if you have the optional equipment available, you might
want to measure the pressures with a barometer or a barometer sensor attached to a computer-
based laboratory system.
(7 pages)
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