Chapter 18

Temperature, Heat, and the First Law of Thermodynamics

Temperature

• Thermodynamics – branch of physics studying thermal energy of systems

• Temperature (T), a scalar – measure of the thermal (internal) energy of a system

• SI unit: K (Kelvin)

• Kelvin scale has a lower limit (absolute zero) and has no upper limit

William Thomson(Lord Kelvin)

(1824 - 1907)

Kelvin scale

• Kelvin scale is defined by the temperature of the triple point of pure water

• Triple point – set of pressure and temperature values at which solid, liquid, and gas phases can coexist

• International convention:T of the triple point of water is

KT 16.2733

The zeroth law of thermodynamics

• If two (or more) bodies in contact don’t change their internal energy with time, they are in thermal equilibrium

• 0th law of thermodynamics: if bodies are in thermal equilibrium, their temperatures are equal

Measuring temperature

• Temperature measurement principle: if bodies A and B are each in thermal equilibrium with a third body T, then A and B are in thermal equilibrium with each other (and their temperatures are equal)

• The standard temperature for the Kelvin scale is measured by the constant-volume gas thermometer

Constant-volume gas thermometer

ghpp 0

CpT

33 CpT

33 p

pTT

3

16.273p

pK

Celsius and Fahrenheit scales

• Celsius scale:

• Fahrenheit scale:

Anders Cornelius Celsius

(1701 - 1744)

Gabriel DanielFahrenheit

(1686 - 1736)

15.273TTC

325

9CF TT

Thermal expansion

• Thermal expansion: increase in size with an increase of a temperature

• Linear expansion:

• Volume expansion:

TL

L

3

TV

V

Chapter 18Problem 19

Temperature and heat

• Heat (Q): energy transferred between a system and its environment because of a temperature difference that exists between them

• SI Unit: Joule

• Alternative unit: calorie (cal): Jcal 1868.4 1

Absorption of heat

• Specific heat (c): heat capacity per unit mass

• Common states (phases) of matter: solid, liquid, gas

• Heat of transformation (L): the amount of energy per unit mass transferred during a phase change (boiling, condensation, melting, freezing, etc.)

)( if TTcmTcmQ

LmQ

Q Q

Chapter 18Problem 37

)( if TTcmTcmQ LmQ

Heat and work

Thermodynamic cycle

sdFdW

dspA )( )(Adsp pdV

f

i

V

VpdVW

The first law of thermodynamics

• Work and heat are path-dependent quantities

• Quantity Q – W = Eint (internal energy) is path-

independent

• 1st law of thermodynamics: the internal energy of a system increases if heat is added to the system and decreases if work is done by the system

WQEEE if int,int,int

The first law of thermodynamics

• Adiabatic process: no heat transfer between the system and the environment

• Isochoric (constant volume) process

• Free expansion:

• Cyclical process:

WWE 0int

QQE 0int

000int E

0int WQE

WQ

Chapter 18Problem 48

Heat transfer mechanisms

• Thermal conduction

• Conduction rate:

• Thermal resistance:

• Conduction through a composite slab:

L

TTkA

t

QP CH

cond

2211 // kLkL

TTAP CH

cond

k

LR Thermal conductivity

21 RR

TTA CH

Heat transfer mechanisms

• Thermal radiation

• Radiation rate:

• Stefan-Boltzmann constant:

• Absorption rate:

4ATPrad

4envabs ATP

428 /1067.5 KmW

)( 44 TTA env Ludwig Eduard

Boltzmann(1844-1906)

Josef Stefan(1835-1893)

radabsnet PPP

Emissivity

Chapter 18Problem 52

Heat transfer mechanisms

• Convection

Heat transfer mechanisms

Answers to the even-numbered problems

Chapter 18:

Problem 4

(a) 320º F; (b) -12.3º F

Answers to the even-numbered problems

Chapter 18:

Problem 10

1.1 cm

Answers to the even-numbered problems

Chapter 18:

Problem 22

109 g

Answers to the even-numbered problems

Chapter 18:

Problem 44

(a) − 200 J; (b) − 293 J; (c) − 93 J

Answers to the even-numbered problems

Chapter 18:

Problem 50

(a) 0.42 m; (b) 7.4 km