|
"graininess" or quantization of momentum and energy as well as matter becomes
important so that kinetic theory is also an idealization of nature.
Week of April 27, 1998
Lesson 15Thermodynamic Cycles, the Second Law, and Entropy
Keywords: heat and thermodynamics, the laws of thermodynamics, the second law
of thermodynamics, reversible and irreversible processes, Carnot Cycle,
efficiency, heat engines, heat pumps;
OBJECTIVES:
*Define the following terms: reversible process, irreversible process, state variable,
cycle, efficiency, Carnot cycle, heat engine, refrigerator.
* Solve problems involving the efficiency of idealized thermodynamic cycles.
Comments: Suppose you get into your car and drive to the store. During this
trip, (1) the burning of the gasoline in the engine cylinders converts chemical
energy into thermal energy of the gases, that is, they become very hot; (2) the
expansion of these hot gases turns the crankshaft, performing work; and (3) this
work, transmitted to the wheels, gives the car kinetic energy (which must be
continually replenished, because of depletion by friction and air resistance). At
the end of this trip, a quantity of gasoline has been converted into water vapor,
carbon dioxide, and sundry less desirable vapors, scattered across the intervening
countryside. In addition, the road, the air along it, and your engine have been
heated up. No energy has been lost, but energy has been converted to different
forms. So why not somehow collect all that energy again, and use it to drive
another engine (presumably of a different kind)? That would not violate the first
law of thermodynamics--but try to do it!Well, then, perhaps we should
concentrate just on the operation of the heat engine, step 2 above. Present-day
gasoline engines convert only a small fraction of the "heat energy" released in
their cylinders into useful work. Since we are being pinched by energy shortages,
why not gear up a research program to develop engines with (say) 90%
efficiency? Again, this too is impossible!These are illustrations of the fact that
energy is often unavailable (or only partially available) for conversion into work.
There is a fundamental limit to the efficiency that can be obtained in this
conversion--a limit that apparently cannot be transcended by any new
technological developments. The basis of that limit is the subject of this lesson.
PREREQUISITES: Constructing and interpreting pV graphs for adiabatic and
isothermal processes. Stating and using the first law of thermodynamics. Evaluating
definite integrals.
|
|
1
2
3
4
5
6
7
8
9
10
11
12
13
