1 / 12

Understanding Thermodynamics: Reversible and Irreversible Processes

Learn about the classification of processes as reversible and irreversible in thermodynamics, and the consequences of the second law of thermodynamics. Discover the limitations of achieving 100% efficiency in real-world heat engines and the impact of wasted energy. Explore the factors that affect the efficiency of power plants and the challenges in harnessing energy from fossil fuels. Find out why perpetual motion machines are not possible and the motivation behind claims of their invention.

tgenevieve
Download Presentation

Understanding Thermodynamics: Reversible and Irreversible Processes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. http://www.nearingzero.net (nz396.jpe)

  2. Today’s Agenda Thermodynamics, part 3. Fossil fuels—whatever I have time for. Next Monday’s Agenda Times Beach video

  3. Oral report grading plan. Was the topic related to the environment? (0-3) _____ Did the talk include evidence of “science?” (0-5) _____ Was there an effort by the presenter to evaluate the evidence/situation? (0-4) _____ Was the talk organized? Did it flow logically? (0-5) _____ Was there evidence of thought on the part of the presenter? (0-5) _____ Was the presenter organized and “enthusiastic?” (0-3) _____ Total (0-25) _____ Now back to Thermodynamics…

  4. Physicists classify processes as reversible and irreversible. • Things that make processes irreversible: •  friction •  heat transfer •  mass transfer •  mixing. An irreversible process would be burning a piece of paper, blowing up a stick of dynamite, or frying an egg. There aren't a lot of truly reversible processes around. Letting a gas expand v e r y s l o w l y in a chamber, slowly enough that its temperature doesn't change, is an example of a reversible process. Here’s a “movie” of an (almost) reversible process, and an irreversible process. Here’s a web link to the movie. Can you tell which is “forward” and which is “reverse?”

  5. The second law of thermodynamics says that entropy increases in all irreversible processes, and remains constant in all reversible processes, so that the total entropy of a system never decreases. Equivalently, it says that the entropy of the universe is always increasing towards a maximum. That sounds very abstract, but it has a number of enormously significant consequences.

  6. “Wasted” energy must be nonzero. You can’t even get out work equal to the energy you spent. Sorry.

  7. I'm going to hit you with an equation here, but it won't hurt too bad. Starting from the second law of thermodynamics, you can show that the efficiency of an ideal heat engine is

  8. Notice that you can have an efficiency of 1 (100% efficient) only if the temperature of the heat sink (the river, in the case of the power plant) is at absolute zero, or if the heat source (the steam) is at an infinite temperature. Neither of these ideal cases is even remotely practical. Worse yet, any real engine will have other places where energy is lost (e.g. friction) so real engines have efficiencies lower than the theoretical best. heat in (TH) work done “waste” heat (TC)

  9. Let's suppose our power plant uses steam (under pressure) at 700 C, which is equal to 973 K. (The equation I wrote down before requires absolute temperature in degrees Kelvin.) That's pretty hot steam. Let's suppose the power plant dumps waste heat into the river at a temperature of 27 C (or 300 K). That's actually hotter than a free-flowing natural river, but not by a whole lot. Our equation says If this power plant burns a ton of coal, about 30% is “lost” due to Physics. Another 15% or so is lost due to “friction.” About 7% is lost transmitting the electricity over power lines. We’re lucky if even half of the coal was put to “good” use.

  10. This inefficiency is not due to poor engineering or lack of cleverness; it is a limitation built into nature which we cannot overcome. In fact, any power plant uses mechanical parts to turn the energy of the steam into work, and all of these mechanical parts are subject to inefficiencies such as friction. This is where clever engineering can help; it can help us approach the theoretical maximum efficiency. According to the textbook I looked in, a power plant operating at the above temperatures might have an actual efficiency of about 47%. Over half the energy it uses is wasted before it “leaves” the plant!

  11. Let me close by reminding you of the second law of thermodynamics, which says no process is possible whose sole result is the extraction of heat from a reservoir and the conversion of it into useful work. A machine purported to do just that is called a perpetual motion machine of the second kind; no such machine can be made. The equation for efficiency tells you how much of the energy must be wasted. A question for you psych. people in this class: what (big, general) things motivate people?

  12. Maybe not the most effective motivator, but rather important. Those who claim to have invented perpetual motion machines usually claim a conspiracy of scientists or utility companies prevents them from getting their machine accepted. According to US law, all they have to do is plug their machine into the power grid (at the proper phase) and start billing the utility companies. If billions of dollars were to be made, would you neglect to plug your perpetual motion machine into the power grid?

More Related