Thermodynamics and Statistical Mechanics

1. Thermodynamics and Statistical Mechanics Spring 2006

2. Thermodynamics • Thermodynamics is an old science, formulated empirically without reference to atoms or molecules. It deals with energy, as does mechanics, but adds some new quantities, besides work, potential energy and kinetic energy. Thermo & Stat Mech - Spring 2006 Class 1

3. Thermodynamic quantities • Internal energy (U): the energy of atoms or molecules that does not give macroscopic motion. • Temperature (T): a measure of the internal energy of a system. • Heat (Q): a way to change internal energy, besides work. (Energy in transit.) Thermo & Stat Mech - Spring 2006 Class 1

4. Thermodynamic quantities • Entropy (S): a measure of the disorder of the system. The change of entropy is equal to the heat added or removed divided by the temperature. Thermo & Stat Mech - Spring 2006 Class 1

5. Laws of Thermodynamics • First law: đQ – đW = dU • Energy is conserved Thermo & Stat Mech - Spring 2006 Class 1

6. Laws of Thermodynamics • Second Law: The entropy of an isolated system increases in any irreversible process and is unaltered in any reversible process. This is the principle of increasing entropy. • DS³ 0 Thermo & Stat Mech - Spring 2006 Class 1

7. Laws of Thermodynamics • Third Law: The entropy of a true equilibrium state of a system at a temperature of absolute zero is zero. • Equivalent to: It is impossible to reduce the temperature of a system to absolute zero using a finite number of processes. Thermo & Stat Mech - Spring 2006 Class 1

8. History • Much of early thermodynamics development was driven by practical considerations. For example, building heat engines and refrigerators. The original statements of the second law were different. Thermo & Stat Mech - Spring 2006 Class 1

9. Second Law Variations • No series of processes is possible whose sole result is the absorption of heat from a thermal reservoir and the complete conversion of this energy to work. • There are no perfect engines! Thermo & Stat Mech - Spring 2006 Class 1

10. Second Law Variations • No series of processes is possible whose sole result is the transfer of heat from a reservoir at a given temperature to a reservoir at a higher temperature. • There are no perfect refrigerators! Thermo & Stat Mech - Spring 2006 Class 1

11. Third Law • It is impossible to reach a temperature of absolute zero. Thermo & Stat Mech - Spring 2006 Class 1

12. Popular versions • 1st Law: You can’t win. • 2nd Law: You can’t break even. • 3rd Law: There’s no point in trying. Thermo & Stat Mech - Spring 2006 Class 1

13. Statistical Mechanics • In the late 19th and early 20th century, with the discovery of atoms, thermodynamics was treated in terms of atoms and molecules, on a statistical basis, and the subject of statistical mechanics came about. Thermo & Stat Mech - Spring 2006 Class 1

14. Zeroth Law • If two systems are separately in thermal equilibrium with a third system, they are in thermal equilibrium with each other. Thermo & Stat Mech - Spring 2006 Class 1

15. Zeroth Law Thermo & Stat Mech - Spring 2006 Class 1

16. Constant Volume Gas Thermometer Thermo & Stat Mech - Spring 2006 Class 1

17. Constant Volume Gas Thermometer • It is calibrated at the triple point of water, which is 273.16 K on the Kelvin scale. Even better: Thermo & Stat Mech - Spring 2006 Class 1

18. Boiling Point of Water Thermo & Stat Mech - Spring 2006 Class 1