Physics I Basic Concepts of Thermodynamics Prof WAN

Physics I Basic Concepts of Thermodynamics Prof. WAN, Xin xinwan@zju. edu. cn http: //zimp. zju. edu. cn/~xinwan/

Temperature F [Operational definition] Temperature is what you measure with a thermometer.

Temperature F F [Operational definition] Temperature is what you measure with a thermometer. [Let’s go to a hospital] The steps of the measurement are: – Contact of object and thermometer. What kind of contact? – Wait for some time. How long shall we wait? – Read the temperature. How do we quantify?

Temperature F Contact of object and thermometer. What kind of contact? – Allowing spontaneous energy transfer, i. e. , heat. No particle transfer! F Wait for some time. How long shall we wait? – Relaxation time. Should not be long. F Read the temperature. How do we quantify? – We need a law, or an equation.

Thermal Equilibrium F F After two objects have been in contact long enough (such that their macroscopic properties no longer change), we say that they are in thermal equilibrium (microscopic properties still change). The zeroth law of thermodynamics: If two systems are in thermal equilibrium with a third system, then they must be in thermal equilibrium with each other.

Temperature, Again F [Definition] Temperature is a measure of the tendency of an object to spontaneously give up energy to its surroundings. When two objects are in thermal contact, the one that tends to spontaneously lose energy is at the higher temperature. TA TB Q TA< T B T A Q TB A B

Thermometers F Physical properties that change with temperature: – volume of a liquid – length of a solid – pressure of a gas at constant volume – volume of a gas at constant pressure – electric resistance of a conductor – color of an object

Thermometer I thermoresistance R G

Thermometer II F Constant-volume gas thermometer

Absolute Zero Why universal?

The Third Law of Thermodynamics F The third law of thermodynamics: "It is impossible for any procedure to lead to the isotherm T = 0 in a finite number of steps. " -- Walther Nernst

Temperature Scales F F F Celsius scale Fahrenheit scale Absolute temperature (Kelvin) scale: – 1 / 273. 16 of the difference between absolute zero and the temperature Water Ice of the triple point Vapor of water at a pressure of 4. 58 mm of mercury

Temperature Scales Boiling of H 2 O Freezing of H 2 O Boiling of N 2 Absolute zero Kelvin Celsius Fahrenheit

Thermometer III F Thermal Expansion – Linear expansion T L 0 T+ T L L = a T + b ( T)2 + c ( T)3 + ··· for sufficiently small T

Thermometer III F Thermal Expansion – Linear expansion T L 0 T+ T L L = a T What should a depend on?

Thermometer III F Thermal Expansion – Linear expansion T L 0 T+ T L L = a L 0 T , or L = L 0 (1 + a T )

Linear vs Volume Expansion F Thermal Expansion – Linear expansion – Volume expansion L = a L 0 T V = b V 0 T Are a and b independent coefficients?

Linear vs Volume Expansion F Thermal Expansion – Linear expansion – Volume expansion L = a L 0 T V = b V 0 T L 3 = (L 0+ L)3 = L 0 (1 + a T)3 = L 03 [1 + 3 a T+O( T)2] V = V 0 (1 + b T) b = 3 a

Near Room Temperature What should a or b depend on? • Average distance between molecules • Size of molecules • Range of interaction • ……

Railroad Track F A steel railroad track has a length of 30 m when the temperature is 0. 0 o. C. (a) What is its length when the temperature is 40 o. C?

Railroad Track F (b) Suppose that the ends of the rail are rigidly clamped at 0. 0 o. C so that expansion is prevented. What is thermal stress set up in the rail if its temperature is raised to 40. 0 o. C?

CRH Railroad Track F The force of compression in the rail can be as large as 105 N! (Assume a cross-sectional area of ~ 10 cm 2)

Bimetallic Strip

Unusual Behavior of Water As the water freezes, the ice remains on the surface because ice is less dense than water. The ice continues to build up at the surface, while water near the bottom remains at 4 o. C.

Low-Density Gases F Equation of state for an ideal gas (ideal gas law) Experiment observation Boltzmann’s constant

Thermal Expansion for Gases b For an ideal gas at 0 o. C, b = 1 / 273. 15 = 0. 00366 What about air? Let’s go back and check.

Real Gases F The van der Waals equation of state F At large molecular/atomic spacing F b: volume of a molecule a: due to potential energy

Q&A: Four Real Gases Why a varies significantly, while b not so much? Which is which? D: H 2 O A: He C: CO 2 B: O 2

Heat F F Heat is defined as the transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings. Heat transfer – Conduction, convection, radiation

Homework CHAP. 21 Exercises 1, 4 (P 493) 8, 10, 11 (P 494)