1. Lesson 5 Introduction to Entropy Liceo Da Procida

2. Reminder Last time: First law of thermodynamics, PV diagrams ΔU = Q – W PV Processes: Isothermic Adiabatic Isobaric Isochoric

3. Practice Problems Sketch a PV diagram of this cycle: 2L of an ideal gas at 10 5 Pa are cooled at constant pressure to a volume of 1L, and then expanded isothermally back to 2L, then the pressure is increased at constant volume until 10 5 Pa. 1L of air at 10 5 Pa is heated at constant pressure until its volume is 2L, then it is compressed isothermally back to 1L, then the pressure is decreased at constant volume back to 10 5 Pa. Sketch the PV diagram.

4. Practice Problems The internal energy at point A is 800J. The work transfer from B to C is 300J, and from C to A is 100J. What is the total work done by the gas in this cycle? What is the final internal energy?

5. What is Entropy? Entropy has to do with how “spread out” energy is. We will define it in a bit Energy always wants to spread out if possible We will look at some physical processes to see how this happens

6. Physical Processes A ball is lifted in the air. What type of energy does it mainly have? Where is this energy? When there is nothing blocking it, which way will it want to move? Its potential energy turns into what kind of energy? The kinetic energy is used to push aside air, to give off sound, to heat up the ball, etc. Now where is the energy? The energy spreads out in the process

7. Physical Processes Now let’s think about a campfire. What form of energy does it give off (heat or work)? Where does the heat begin? Where does the heat end? The energy spreads out in the process

8. Physical Processes Now let’s think about an ideal gas entering a vacuum. Where does the energy begin? After it enters the vacuum, do the particles stay together or separate? Where is the energy now? The energy spreads out in the process

9. Physical Processes Now let’s think about a balloon. If I puncture it with a needle, what happens? Where does the energy start/end? If you add a drop of red dye to a glass of water, what happens? Where does the energy start/end? If I throw a pack of playing cards in the air, what happens? Where does the energy start/end? We now know that energy always wants to spread out

10. Now, what is entropy? Entropy is a measure of how spread out the energy is within a system We generally talk about entropy change, just like we talk about change in internal energy Entropy change is a measure of how much energy has been spread out in a system at a specific temperature Entropy always wants to increase in natural processes

11. Videos 3 entropy examples Marbles

12. Concept Question In natural processes, entropy always wants to 1) Increase 2) Decrease

13. Concept Question In natural processes, entropy always wants to 1) Increase 2) Decrease

14. Concept Question Would the following examples have a positive or negative entropy change? Gasoline vaporizes in the carburetor of an automobile engine Air is pumped into a tire Raindrops form in a cloud Sugar dissolves in coffee A refrigerator takes in warm air and makes it cooler

15. Concept Question It is impossible for a substance to decrease in entropy 1) True 2) False

16. Concept Question It is impossible for a substance to decrease in entropy 1) True 2) False