Chapter 11

1. Chapter 11 Thermodynamics Heat used to do work

2. Heat and Work • A system is objects or substances with a well defined border to which energy is added or taken away. • Heat is transferred from the system to its surroundings (environment) because of a temperature difference.

3. Heat and Work of a System • A Erlenmeyer flask with a balloon fastened to the top, containing a small amount of boiling water • The hot plate transfers energy as heat to a flask/balloon system • The system’s internal energy increases and steam expands the balloon. • Steam does work as a force exerted against air outside of the balloon. • Heat is transferred to the environment (air)

4. Heat and Work B A Energy transferred to heat turns water into steam which then exerts a force on a turbine and does work. Identify the system and environment.

5. Work Equation W = P x ∆V • Work = Pressure x Volume Change • Work units are in Joules (J)

6. Work done by a gas+ Work Work done on a gas - Work Decreasing distance Decreasing pressure Increasing distance Increasing pressure WORK DONE ON PISTON BY EXPANDING GAS IS POSITIVE WORK DONE BY COMPRESSING THE PISTON ON THE GAS IS NEGATIVE

7. 1st Law of Thermodynamics • Expresses Conservation of Energy • Conservation requires that the total change in internal energy of a system be equal to the transfer of energy to or from the system as heat and work.

8. 1st Law Equation • ∆U = Q – W • The Q and W can be positive or negative depending on circumstances. • If the Q is negative and the W is positive, just insert that into the above equation: • ∆U = -Q –W and you would get a negative number for your answer.

9. Signs for Q and W • Q = + if energy is added to the system as heat. • Q = - if energy is removed from the system as heat. • W = + if work is done by the gas • W = - if work is done on the gas

10. Isovolumetric SystemExample A car with closed windows and doors is parked in a garage. The internal energy of the closedsystem (inside of car) increases as energy is transferred as heat into the car from the air in the garage (surroundings). W = PΔV so for isovolumetric process if ΔV = 0 then W = 0 for + Q added to system

11. Isovolumetric System • Volume is constant • No work is done • All changes in internal energy due to transfer of heat only • Q (heat transfer) can be + or -

12. Isothermal Process Example When a large storm is approaching the air pressure drops. The air pressure inside the house slowly decreases and the balloon slowly expands doing work on the air outside the balloon.

13. Isothermal System Example • What would happen to a balloon inside a house as the storm approaches? • Air inside the balloon is the same as outside it. No temperature difference then no change in internal energy.

14. Isothermal System Example • What would be the temperature be inside the house and inside the balloon? • Energy transferred out of balloon as work is matched by energy transferred into balloon as heat.

15. Isothermal System Example • The air pressure inside the house slowly decreases and the balloon slowly expands doing work on the air outside the balloon. • Air inside the balloon is the same as outside it. No temperature difference then no change in internal energy. • Energy transferred out of balloon as work is matched by energy transferred into balloon as heat.

16. Isothermal System • Temperature remains constant • Internal energy remains unchanged • Energy slowly removed from system as work while equivalent amount of energy is added as heat • Pressure can change

17. Adiabatic Systems • Adiabatic systems occur when a gas expands, or contracts, very quickly. • Popping a balloon • Ignition of a spark plug • Firing a single shoot from a gun • There is no time for any heat to flow in or out of the gas. No energy transferred. Q = 0 Joules • In this case any work done must change the internal energy. (PV is not constant.)

18. Table of Simple Systems

19. 2nd Law of Thermodynamics • No cyclic process that converts heat entirely into work is possible. • In other words, some energy must always be transferred as heat to the environment. • Cannot be 100% efficient!

20. Heat Engines • Heat engines use heat to do work. • Equation to measure amount of work: • Work net = Heat transferred from the heat engine – Heat transferred to the cooler environment • Wnet = Qh - Qc

21. Gas Engine - Steps • Step 1: Spark plug fires. • Step 2: Gas is ignited. • Step 3: Gas creates pressure. • Step 4: Pressure moves Piston. • Step 5: Piston moves crankshaft.

22. Cyclic Process • A thermodynamic process in which a system returns to the same conditions under which it started with no loss or gain of energy is called a cyclic process. • Example: a refrigerator

23. Efficiency Equation • Eff = Wnet / Qh • Eff = (Qh – Qc)/Qh • Eff = 1 – (Qc/Qh)

24. Entropy • Entropy is the measure of a system’s disorder. • In general, it is believed that without interference, disorder is more likely than order. • The greater the system’s disorder, the greater the system’s entropy.

25. Greater disorder or entropy means there is less energy to do work. • Imagine atoms in an engine bouncing around chaotically compared to atoms all bouncing (pushing) in the same direction. The ordered atoms will accomplish more work.

26. THE END OF THE UNIVERSE! • Since everything in the world is moving towards chaos, it has been suggested that eventually the entire world will reach a maximum value of entropy (chaos). • At that time, the universe will reach a state of thermal equilibrium and the temperature will be the same everywhere. • Since there will be no temp difference, no heat can be transferred and thus no work can be done. • This is called ultimate “heat death” of the universe and is predicted to happen in 100 trillion years. So make your plans now!

27. Vocabulary • Heat • Internal Energy • Energy • Work • System • Environment