Thermodynamics: Reversibility and Carnot
From Warmup It seems like this reading (for Friday) explained the homework assigned for Wednesday's lecture. Is homework based on the previous lecture, or the current reading assignment? HW on today s reading will be due on Monday. There is a lot of overlap between the today s lecture and Wednesday s lecture. Today s reading probably will help make homework based on Wednesday s reading (due today) more clear, but in principle all the information to do the HW due today was covered in Wednesday s reading and lecture.
Finishing up from last time Demos Sample engine problem
From Warmup We seem to be spending a lot of time discussing engines. Do these principles have more broad applications? Yes! The question we really want to answer is: what types of processes are possible? We use engines as a context for exploring that question. I'm not entirely sure on my answer as to why the Carnot Engine matters. Maybe we can discuss that a little further detail? The Carnot engine will help us find the dividing line between the possible and the impossible. Are reversible process only relevant for Carnot engines? No. Reversible processes are the dividing line between possible and impossible processes, even outside the context of engines.
Reversible Processes What's the difference between a reversible and an irreversible process? Reversible processes are at equilibrium at all points on the pv diagram. A reversible processes is impossible to attain and irreversible processes occur in nature.
Why do we care? The Carnot engine is completely impractical---because it has to operate reversibly, it would take it an infinite time to complete a cycle. (Even operating *almost* reversibly, it will take a long time to complete a cycle---it would still be impractical.) Why then do we bother? What is important about this engine? It sets a standard to base the efficiency of real engines on. It gives us an upper limit on engine efficiency.
The limit of what is possible All processes you could ever dream of. Things that could actually happen Things that are impossible Reversible Processes
Second Law of Thermodynamics Kelvin-Planck It is impossible to construct a heat engine that, operating in a cycle, produces no effect other than the input of energy by heat from a reservoir and the performance of an equal amount of work Work Heat in (Higher T) Engine
Second Law of Thermodynamics Classius Statement It is impossible to construct a cyclical machine whose sole effect is to transfer energy continuously by heat from one object to another object at a higher temperature without the input of work. Engine Exhaust out (Higher T) Heat in (Lower T)
Carnot Cycle All heat added/subtracted reversibly During constant temperature processes Isothermal processes are typically slow
Warmup review Why doesn t the Carnot engine have perfect efficiency? Who cares? It's not real anyway. Because the lowest temperature cannot be zero kelvin. Because it has to have a point in the cycle at 0 K, which we can't attain. There is always energy lost due to friction or the dissipation of heat. The correct answer: The second law of thermodynamics. Because entropy. No real process is truly reversible, and heat energy will dissipate randomly in such a manner that it cannot be controlled to perfection with work.
Carnot Theorem & the 2 nd Law Second law of thermodynamics: You can t fully convert heat into work Heat does not spontaneously flow from cold to hot You can t have an efficiency of 100% Carnot Theorem You can t even have that!
Laws of Thermodynamics 0: There is a game. 1: You can t win. 2: You can t break even, except on a very cold day. 3: It doesn t get that cold.
Hot Reservoir Cold Reservoir Carnot Theorem Heat Out (Higher T) Carnot Engine (Reversed) Heat In (Lower T) Work Heat in (Higher T) Super Engine Exhaust Out (Lower T)
Hot Reservoir Cold Reservoir What s wrong with this situation? Heat Out (Higher T) Reversed inefficient engine Heat In (Lower T) Work Heat in (Higher T) Carnot Engine Exhaust Out (Lower T)