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There must be reasons that real macroscopic processes cannot be reversible. We can imagine them going in reverse. For example, heat transfer occurs spontaneously from hot to cold and never spontaneously the reverse. Yet it would not violate the first law of thermodynamics for this to happen. In fact, all spontaneous processes, such as bubbles bursting, never go in reverse. There is a second thermodynamic law that forbids them from going in reverse. When we study this law, we will learn something about nature and also find that such a law limits the efficiency of heat engines. We will find that heat engines with the greatest possible theoretical efficiency would have to use reversible processes, and even they cannot convert all heat transfer into doing work. [link] summarizes the simpler thermodynamic processes and their definitions.
Isobaric | Constant pressure |
Isochoric | Constant volume |
Isothermal | Constant temperature |
Adiabatic | No heat transfer |
Watch different types of molecules form a solid, liquid, or gas. Add or remove heat and watch the phase change. Change the temperature or volume of a container and see a pressure-temperature diagram respond in real time. Relate the interaction potential to the forces between molecules.
In [link] , how much work is done by the system in process AB?
(c)
Consider process CD in [link] . Does this represent work done by or on the system, and how much?
A thermodynamic process begins at 1.2 × 10 6 N/m 2 and 5 L. The state then changes to 1.2 × 10 6 N/m 2 and 2 L. Next it becomes 2.2 × 10 6 N/m 2 and 2 L. The next change is 2.2 × 10 6 N/m 2 and 5 L. Finally, the system ends at 1.0 × 10 6 N/m 2 and 5 L.
On [link] , this process is best described by
(d)
The first step of a thermodynamic cycle is an isobaric process with increasing volume. The second is an isochoric process, with decreasing pressure. The last step may be either an isothermal or adiabatic process, ending at the starting point of the isobaric process. Sketch a graph of these two possibilities, and comment on which will have greater net work per cycle.
In [link] , which of the following cycles has the greatest net work output?
(a)
Look at [link] , and assign values to the three pressures and two volumes given in the graph. Then calculate the net work for the cycle ABCFEDCFA using those values. How does this work compare to the heat output or input of the system? Which value(s) would you change to maximize the net work per cycle?
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