Chapter 22

1. Chapter 22 Heat Engines, Entropy, and the Second Law of Thermodynamics (cont.) PHY 1361

2. Outline • Carnot theorem and maximum efficiency (22.3) • Entropy (22.6) PHY 1361

3. Carnot theorem and maximum efficiency • In 1824, Sadi Carnot: Under what conditions will a heat engine have maximum efficiency? • Carnot’s Theorem: • If an engine operating between two constant-temperature reservoirs is to have maximum efficiency, it must be an engine in which all processes are reversible. In addition, all reversible engines operating between the same two temperatures, Tc and Th have the same efficiency. • No real engine can ever be perfectly reversible. The concept of a reversible engine is a useful idealization. • Maximum efficiency of a heat engine: • emax = 1 – Tc/Th (Temperatures must be in Kelvin) Sadi Carnot PHY 1361

4. Examples 22.3 The steam engine • A steam engine has a boiler that operates at 500 K. The energy from the burning fuel changes water to steam, and this steam then drives a piston. The cold reservoir’s temperature is that of the outside air, approximately 300 K. • (A) What is the maximum thermal efficiency of this steam engine? • (B) Determine the maximum work that the engine can perform in each cycle if it absorbs 200 J of energy from the hot reservoir during each cycle. • Answer: 40% and 80 J. PHY 1361

5. Entropy (conceptual discussion) • Entropy: a fundamental quantity that is related to the amount of disorder in a system. • Entropy in the universe: • The total entropy of the universe increases whenever an irreversible process occurs. The total entropy of the universe is unchanged whenever a reversible process occurs. • Since all real processes are irreversible, the total entropy of the universe continually increases. • A “directionality” in nature. PHY 1361

6. Order, disorder, and entropy • Entropy can be thought of as a measure of the amount of disorder in the universe. • As the entropy of a system increases, its disorder increases as well; that is, an increase in entropy is the same as a decrease in order. PHY 1361

7. Homework • Ch. 22, P. 700, Problems: #9, 10, 12. PHY 1361

8. Chapter 23 Electric Fields PHY 1361

9. Outline • Properties of electric charges (23.1) • Charging objects by induction (23.2) PHY 1361

10. Properties of electric charges • There are two kinds of electric charges in nature, “positive” and “negative”. • Example: Electrons possess “-” charge and protons possess “+” charge. • Charges of the same sign repel one another and charges with opposite signs attract one another. • Total charge in an isolated system is always conserved. • Charge is quantized: q = Ne • N: some integer; e: a fundamental amount of charge • Charge of an electron = -e; charge of a proton: + e. PHY 1361

11. Electrical conductors, insulators and semiconductors • Electrical conductors: materials in which some of the electrons are free electrons that are not bound to atoms and can move relatively freely through the material. • Copper, aluminum, silver. • Electrical insulators: materials in which all electrons are bound to atoms and cannot move freely through the material. • Glass, rubber, wood. • Semiconductors: between insulators and conductors. • Silicon, germanium PHY 1361

12. Charging objects by induction • Charging an object by induction requires no contact with the object inducing the charge. • In contrast to charging an object by rubbing. PHY 1361

13. Charging a conductor by induction PHY 1361

14. Electrical polarization PHY 1361