PHLA10H3 Lecture 6: Week 6 Notes

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8 Feb 2011

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Predictively equivalent- two hypothesis that lead to the same conclusion science cant tell them apart by testing remember the conditions of plausibility. Priori can be known simply by use of concepts and without any experience or observations (posteriori) A priori truth and existence some a priori truth entail existence how do a priori existence claim affect the question: why is there something rather than nothing an a priori existence claims ensures that question. Two distinctions a priori / a posteriori necessary / contingent. First premises: god is possible noted this is an objective claim. We can conceive or imagine what is not possible ex. Second premise: existence is a kind of perfection. If it did not we could conceiver of one more perfect which is impossible. The fact that all things exist does not show that existence is not a property. X & y are very similar or identical. Two possible explanations for same conclusion common source.

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The moon is observed to have phases in which only a portion of themoon appears to be illuminated. The cycle of phases repeats itselfapproximately every month. Compose a scientific hypothesis toexplain this observation of the moon. Then explain what tests ofyour hypothesis could be made.

What happens when a flame is placed over a steaming teakettlepotand there is condensed water forming above it? and Why?

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The exact question from the text is

Watch thespout of a teakettle of boiling water. Notice thatyou cannot seethe steam that issues from the spout. Thecloud that you see fartheraway from the spout is not steam, butinstead condensed waterdroplets. Now hold the flame of acandle in the cloud of condensedsteam. Can you explain yourobservations?

Vapor cycles
5EM - Thermodynamics
September 2010
1 The T-S diagram
The temperature-entropy diagram is an alternative to the well knownpressure-
volume diagram. Before using it to analyze different vapor cycles,we need
to understand it properly.
1. Plot the following transformations for ideal gases in a T-Sdiagram
(a) Isothermal expansion
(b) Isobaric expansion
(c) Isometric heating
(d) Reversible adiabatic expansion
(e) Irreversible adiabatic expansion
2. Repeat the last question, but this time do it for a realsubstance,
starting in the compressed liquid1 region and ending in thesuperheated
region, when possible. Indicate the boundaries of eachregion.
3. We know that under certain circumstances the area under theplot
in a P-V diagram represents the work performed during theprocess.
What does it represent in this case? Under what assumptions isyour
answer true? Compare examples 6.1 and 6.2.
2 Thermal machines
We understand by thermal machine a device capable of transformingther-
mal energy (heat) into mechanical energy (work). We already knowthat
these machines have a limited efficiency which, in the two-sourcesmodel, is
obtained by Carnotâs cycle.
(Compressed liquid is sometimes also called subcooled liquid)
2.1 Carnot cycle
This cycle, although efficient, is impractical. To begin with, theextreme
temperatures of the cycle should be within the sources temperaturesfor a
finite heat flow to exist.
1. Given the temperature of the sources, draw the cycle accordingto the
named limitation.
2. Show that this leads to lower efficiency than the theoreticallimit.
3. Assume that heat flow from/to the sources is proportional to thetem-
perature difference between them and the working fluid, anddemon-
strate that a higher power output is related to a decreasedefficiency.
2.2 Rankine cycle
Out of the several practical possibilities, Rankine cycle and itsvariations is
widely used, especially for electric power generation.
1. Draw the basic cycle in a T-S diagram and explain it.
2. What does the efficiency of the cycle depend on? Comparewith
Carnotâs.
3. What are the differences between the ideal cycle and the realone?
4. Describe how it is modified by superheating and/or reheating.Explain
te benefits.
2.3 Problems
After answering the previous questions solve the followingexercises from the
book: 8.2, 8.4, 8.5, 8.13, 8.19, 8.20
3 Frigorific machines
3.1 Reverse Carnot cycle
We have already studied Carnot cycle in detail as the ideal thermalmachine.
Now we will take is as the starting point to understand thecharacteristics
of frigorific machines.
1. Draw the ideal cycle in a T-S diagram and explain it.
2. Show that this cycle has a theoretical efficiency Ã =Tcold/Thot-Tcold
3. What are the difficulties in implementing such cycle in actualdevices?
3.2 Actual refrigerators
1. Why is the turbine replaced by an expansion valve in mostcases?
2. Plot the actual cycle and explain the differences with the idealcase.
3. How do cascade and multistage systems improve the performanceof
the refrigerator?
4. What aspects are important in the choice of a refrigerant fluid?Justify
3.3 Problems
After answering the previous questions solve the followingexercises from the
book: 10.2 10.7, 10.8, 10.9, 10.22, 10.23 10.1D

PHLA10H3 Lecture 6: Week 6 Notes

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