Major Concepts in Physics Lecture 15.

1. Major Concepts in Physics Lecture 15. Prof Simon Catterall Office 309 Physics, x 5978 smc@physics.syr.edu http://physics/courses/PHY102.08Spring PHY102

2. Plan for today … quick tour of someconcepts of thermodynamics … • Exam 2 • Recap last lecture: microscopic origins of temperature, pressure and internal energy. Work and heat and 1st law of thermodynamics • Entropy and 2nd law of thermodynamics. Disorder and heat engines • NOTE: no workshops this week! PHY102

3. Exam 2 • Mean score: 37/43=86% (B-) • 40/43 (A-) • 32/43 (C-) • Generally very good… PHY102

4. Temperature, Pressure • Temperature is a measure of the mean kinetic energy of the atoms/molecules that comprise body • K=1/2m<v2> = 3/2 kT • Pressure is a measure of the (average) force exerted on walls of container as a consequence of molecular collisions • P=2/3 (N/V) K PV=NkT (ideal gas) PHY102

5. Thermal equilibrium • Place two bodies in thermal contact: molecules of 1 body can collide with the other  net transfer of energy. • After a while mean kinetic energies will be equal (temperatures same) • We often say that heat Q flows between them until their temperatures are equalHeat is thus energy in transit PHY102

6. Internal energy U • The internal energy of a gas/body is the sum of all molecular energies • For an ideal gas: just kinetic energy • For real gas: • Potential energy associated with intermolecular forces (electrical in origin) • Energy of vibration and rotation for molecules PHY102

7. Work • Two ways to increase U for a body. Add heat Q or do work W on body eg compress it • Work is a macroscopic way to add energy to a body. Heat is a microscopic process. • Work is the concept encountered in mechanics (force)x(distance). Compressing the body requires a force and leads to an increase in kinetic and/or stored molecular potential energies … PHY102

8. 1st law of thermodynamics • Generalizes conservation of (mechanical+electrical) energy learnt in PHY101 to all types of energy • Specifically including heat Q and internal energy U. • Write DU=Q+W PHY102

9. Heat is taken from a system. Does the internal energy increase, decrease or stay the same ? • A: increase • B decrease • C: stays the same PHY102

10. A gas expands in a cylinder contained by a piston. No heat enters or leaves the system. Does the internal energy increase, decrease or stay the same ? • A. increase • B: decrease • C: stays the same PHY102

11. Direction of heat flow • We observe that heat always flows from a hot body to a cold body • This seemingly trivial observation is one statement of the 2nd law of thermodynamics. • Why is it true ? It reflects the fact that when one set of highly energetic molecules is mixed with another less energetic set it is overwhelming more likely that the less energetic set will pick up energy and the more energetic set will lose some PHY102

12. Notice • The other possibility is possible from the point of view of the 1st law … • i.e the slower molecules couldgive up some of their energy to allow the (initially faster) guys to go even faster while conserving total energy but its not what you would expect or observe • So 2nd law is needed to fully predict/understand thermodynamic processes PHY102

13. Further thoughts … • Notice that for just two molecules this opposite scenario is quite possible – in fact it will happen 50% time – the slower guy will for time give up some of its energy to the faster guy … but not for long … • Once we have a lot of molecules and we average over timescales large compared to molecular collision times we will always find the 2nd law to hold … PHY102

14. Direction of time … after before 2nd law determines our sense of direction of time. We would never Expect to see the bold arrow reverse …`before’ swapping with `after’ PHY102

15. Reversible and irreversible • Previous picture shows that most processes in the world are irreversible in the sense that the time reversed process would never be seen. • A reversible process can be time reversed – eg elastic collision of two billiard balls … • Notice it is not just the energy that is mixed by such an irreversible process – the green/red balls also get mixed up … PHY102

16. Heat engines • Idealization of real world engines. Easy to analyse. Governed by laws of thermodynamics • Heat engine is some device designed to convert disordered energy eg heat into ordered form of energy – work • Eg steam engine uses hot steam to drive piston which may be used eg to turn a wheel • Nuclear power stations uses energy released by radioactive decay to produce steam etc PHY102

17. Cyclic nature of engine All work in a cycle: engine returns to some initial state after a time – DU=0 Internal combustion engine PHY102

18. Work done by heat engine • Since DU=0 using 1st law: Wnet=Qnet • Thus engine can do work because there is net flow of heat during the cycle. Simple engine think of this as a flow of heat from a hot reservoir to a cold one. Eg the temperature of the exploding fuel/air mixture and the temperature of the exhaust gas. PHY102

19. Heat engine demos • Cool flask down. Connect to piston. Dump in hot water – net heat flow expands gas in piston – does work against atmospheric pressure • Engine. Cold reservoir ice temp. Hot is steam temp. Again net cyclic heat flow can be converted to macroscopic work – the propeller rotates. PHY102

20. Entropy • Seen that flow of heat is an irreversible process • Two gas example: after mixing we know less about the system than before. The system is more disordered • Irreversible processes always lead to an increase in disorder. • Thus 2nd law restated: disorder can only increase in time PHY102

21. Entropy • Quantitative measure of disorder: entropy S • If heat Q is added to some system at a fixed temperature T its entropy S will increase as DS=Q/T • Engines obey 2nd law: entropy change of hot reservoir DSH=-Q/TH entropy change of cold reservoir DSC=Q/TC Total entropy increases DS= DSH+DSC>0 PHY102

22. Summary • Observe that systems evolve so as to increase their disorder or entropy DS=Q/T • Called 2nd law of thermodynamics • Governs operation of idealized engines (heat engines) – use heat flow between two bodies at different temperatures to do work. • A reversible process can be time reversed – hence in such case entropy does not change. DS >= 0 PHY102