physics i basic concepts of thermodynamics prof. wan, xin xinwan@zju.edu.cn xinwan

Physics IPhysics I

Basic Concepts of Basic Concepts of ThermodynamicsThermodynamics

Prof. WAN, Xin

xinwan@zju.edu.cnhttp://zimp.zju.edu.cn/~xinwan/

TemperatureTemperature

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

TemperatureTemperature

[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?

TemperatureTemperature

Contact of object and thermometer. What kind of contact?

– Allowing spontaneous energy transfer, i.e., heat. No particle transfer!

Wait for some time. How long shall we wait?

– Relaxation time. Should not be long. Read the temperature. How do we

quantify? – We need a law, or an equation.

Thermal EquilibriumThermal Equilibrium

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, AgainTemperature, Again

[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

A

TB

BQTA TB T T

Q

TA< TB

ThermometersThermometers

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 IThermometer I

XT R P, V, L ,

quantity physical

R

G

T

T

RRT

thermoresistance

Thermometer IIThermometer II

Constant-volume gas thermometer

Absolute ZeroAbsolute Zero

Why universal?

The Third Law of ThermodynamicsThe Third Law of Thermodynamics

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 ScalesTemperature Scales

Celsius scale Fahrenheit scale Absolute temperature (Kelvin) scale:

– 1 / 273.16 of the difference between absolute zero and the temperature of the triple point of water

Ice Water

Vapor at a pressure of 4.58 mm of mercury

Absolute zeroCelsius FahrenheitKelvin

K0 C015.273 F067.459

Boiling of N2K77 C1960 F3210

Boiling of H2OK125.373 C0100 F2120

Freezing of H2O K15.273 C00 F320

Temperature ScalesTemperature Scales

Thermometer IIIThermometer III

Thermal Expansion– Linear expansion

T

T+T

L0

L

L = a T + b (T)2 + c (T)3 + ···for sufficiently small T

Thermometer IIIThermometer III

Thermal Expansion– Linear expansion

T

T+T

L0

L

L = a T What should a depend on?

Thermometer IIIThermometer III

Thermal Expansion– Linear expansion

T

T+T

L0

L

L = L0 T , or L = L0 (1 + T )

Linear vs Volume ExpansionLinear vs Volume Expansion

Thermal Expansion– Linear expansion

– Volume expansion

L = L0 T

V = V0 T

Are and independent coefficients?

Linear vs Volume ExpansionLinear vs Volume Expansion

Thermal Expansion– Linear expansion

– Volume expansion

L = L0 T

V = V0 T

L3 = (L0+L)3 = L0 (1 + T)3

= L03 [1 + 3T+(T)2]

V = V0 (1 + T) = 3

• Average distance between molecules

• Size of molecules

• Range of interaction

• ……

What should or depend on?

Near Room TemperatureNear Room Temperature

Railroad TrackRailroad Track

A steel railroad track has a length of 30 m when the temperature is 0.0oC. (a) What is its length when the temperature is 40oC?

Railroad TrackRailroad Track

(b) Suppose that the ends of the rail are rigidly clamped at 0.0oC so that expansion is prevented. What is the thermal stress set up in the rail if its temperature is raised to 40.0oC?

CRH Railroad TrackCRH Railroad Track

The force of compression in the rail can be as large as 105 N! (Assume a cross-sectional area of ~ 10 cm2)

Bimetallic StripBimetallic Strip

Unusual Behavior of WaterUnusual 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 4oC.

Low-Density GasesLow-Density Gases

Equation of state for an ideal gas

nRTPV (ideal gas law)

TNkRTN

NnRTPV B

A

Boltzmann’s constant KJN

Rk

AB /1038.1 23

Experiment observation

Thermal Expansion for GasesThermal Expansion for Gases

For an ideal gas at 0oC, = 1 / 273.15 = 0.00366

What about air? Let’s go back and check.

Real GasesReal Gases

The van der Waals equation of state

At large molecular/atomic spacing

b: volume of a molecule

a: due to potential energy

TNkPV B

TNkbVV

aNP B

2

2

bVT ,0

2

2

V

aNPP

Q&A: Four Real GasesQ&A: Four Real Gases

H2OHe

O2

CO2

Which is which?

Why a varies significantly, while b not so much?

A:

B:

C:D:

HeatHeat

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

HomeworkHomework

CHAP. 21 Exercises1, 4 (P493) 8, 10, 11 (P494)