1. Thermodynamics Chapter 24

2. Topics Thermodynamics –First law –Second law Adiabatic Processes Heat Engines Carnot Efficiency Entropy

3. Thermodynamics Is the study of heat and its transformation into mechanical energy It is the study of heat and work

4. Absolute Zero Is the lowest possible temperature that a substance may have –0 Kelvin Absolute zero is where molecules of a substance have minimum kinetic energy

5. Question Which is larger, a Celsius degree or a Kelvin? Neither. They are equal

6. First Law of Thermodynamics The heat added to a system equals the sum of the increase in internal energy plus the external work done by the system This is a restatement of the energy conservation applied to heat –Energy can neither be created nor destroyed

7. Whenever heat is added to a system, it transforms to an equal amount of some other form of energy ∆Heat =∆internal energy + work –Adding heat, increases internal energy –If heat is zero, changes in internal energy = the work done on or by the system –If work is done on system, the internal energy increases –If work is done by the system, the internal energy decreases

8. Questions If 10 J of energy is added to a system that does no external work, by how much will the internal energy of that system be raised? 10 J If 10 J of energy is added to a system that does 4 J of external work, by how much will the internal energy of that system be raised? 6J. 10J-4J=6J

9. Adiabatic Processes Is one usually of expansion or compression, wherein no heat enters or leaves a system Example: compression and expansion of gases in the cylinders of an engine

10. When work is done on a gas by adiabatically compressing it, the gas gains internal energy and becomes warmer When a gas adiabatically expands, it does work on its surroundings and gives up internal energy and becomes cooler

11. Second Law of Thermodynamics Heat does not spontaneously flow from a cold object to a hot object No machine can be completely efficient in converting energy to work; some input energy is dissipated as heat All systems tend to become more and more disordered as time goes by

12. Heat Engines and the Second Law A heat engine is any device that changes internal energy into mechanical work When work is done by a heat engine running between two temperatures, T hot and T cold, only some of the input heat at T hot can be converted to work, and the rest is expelled as heat at T cold

13. Carnot Efficiency Ideal efficiency = T hot – T cold T hot Only in perfect conditions will the ideal efficiency by 100% for a machine –friction is always present and efficiency is always less than ideal

14. Questions What is the ideal efficiency of an engine if both its hot reservoir and exhaust are the same temperature, 400K? 0; (400-400)/400=0 What is the ideal efficiency of a machine having a hot reservoir at 400 K and a cold reservoir at absolute zero, 0K? 1; (400-0)/400=1

15. Order Tends to Disorder Natural systems tend to proceed toward a state of greater disorder Example: –Consider a stack of pennies on a table, all heads up. Suppose somebody knocks them off and they topple to the floor. The pennies will not land all heads up. Order becomes disordered.

16. Entropy Is a measure of the disorder of a system Whenever energy freely transforms from one form to another, the direction of transformation is toward a state of greater disorder (greater entropy) Disorder increases; entropy increases; available energy of the system doing work decreases

17. Example A new deck of cards comes out of its box in ordered suits. Shuffle the deck once and you have disorder. Shuffle is again and you have more disorder. Think of the probability of shuffling the deck enough times to get some degree of order from the disorder