 |
| Pressure and volume changes during the Stirling Cycle. |
First entropy can show the disorder of particles in a
system. This means that when a gas is heated and gains energy the particles are
moving faster, there is more chaos. This is interpreted as more disorder which
equates to more entropy. Likewise if particles where slowed down to 0 kelvin
they stop moving. This is interpreted as no disorder or no entropy. Another way
to show disorder is through volume changes. When gases have larger volumes they
have more room to move. This gives the particles more ways to “bounce” than
when in a smaller volume. As a result larger volumes can be interpreted as
having more disorder in the gas or more entropy.
The second definition of Entropy states that entropy is a
measure of energy in a system or process that is unable to do work. This is
applicable to the Stirling Engine when analyzing the graphs shown to the right.
 |
| This graph shows changes in temperature and entropy during the Stirling Cycle. |
At points 1 and 2 the working piston is at the top of its
cycle. The piston has already preformed its power stroke and cannot do more
work. As a result the energy in the system is unable to do work and there is
high entropy. The graph shown to the right is an idealized case. Realistically,
it should look more “banana-shaped” because point 1 should have more entropy
than point 2. This explained from the previous definition of entropy where a
lower temperature has less entropy than a higher temperature. Since at point 1
the particles are at a higher temperature, point one should have more entropy
than the entropy at point 2.
The opposite process occurs at points 3 and 4. At both of
these points the piston is at the bottom of its cycle. It is preparing for its
power stroke and can do work. As a result the energy in the system is able to
do work, which means less energy is unable to work. According to the second
definition of entropy this means there is less entropy in the system.
Furthermore, like points 1 and 2, points 3 and 4 do not have the same amount of
entropy. At point 3 the gas is cooler than at point 4 where the gas has been
heated. Since point 3 is cooler, its particles have less disorder and have less
entropy.
A final way to explain the entropy in the engine is to look
at the changes in volume. As discussed earlier the volume of the gas changes
with the motion of the working piston. The piston is pushed upwards from points
4 to 1. This increases the volume and also increases the entropy. Similarly,
from points 2 to 3 the piston is falling, decreasing the volume. As a result
there is less entropy.
The multiple definitions of entropy give many ways to
explain the changes in entropy in the Stirling Engine. This also made it very
difficult to understand what entropy was. I learn best when I am
able to quantify things I am learning. When reading about entropy everything was very qualitative. As a result I didn’t understand what I was reading. After finding the Temperature vs Entropy graph of the engine I began to look at what was changing between each point on the graph. This meant creating diagrams of the position of the displacer and working piston for these points and slowly rotating my engine. Eventually, I understood what entropy was and how it was occurring. The paragraphs above show the different ways I was able to describe changes in Entropy. Like other parts of my project, I found the textbooks and many internet sources expected a higher knowledge of thermodynamics and general physics when describing entropy. As a result I had to try to explain what entropy was through the simplest definitions and what I already knew about thermodynamics.
Sources:
Hooper, C & Reader, T. G. (1983). STIRLING ENGINES. New York, NY: E. & F. N. Spon.
Rutgers. (n.d.). Lecture 11. Retrieved June 2, 2014 from “physics.rutgers”: www.physics.rutgers.edu
Serway, R. (1982). Physics:
For scientists and Engineers. New York, NY: CBS College Publishing
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