Chapter 18

Temperature, Heat, and the First Law

of Thermodynamics

Things to learnWe will learn concepts useful in thermodynamics (temperature, heat, work, and heat capacity) and relations among them.

We will learn laws of thermodynamics and their applications:

-Zeroth law: T equal at thermal equilibrium

-First law: energy conservation- Adiabatic, constant-volume, cyclical, free-

expansion processes.

We will learn effects of heat:

-Change in T

-Change of phase (heat of transformation)

-Thermal expansion

We will learn modes of heat transfer.

18-2 Temperature

Thermodynamics=Thermal physics

TemperatureHot? Cold?

Scale?Kelvin (K) in SI unit

[273 + Celsius]

Universe was very hot right after Big Bang(1039 K)

Now about (3K)

18-3 The zeroth Law of Thermodynamics

If bodies A and B are each in thermal equilibrium with a

third body T, then they are in thermal equilibrium with each

other.

18-4 Measuring temperature

Practical standard0 degrees Celsius : freezing point of water100 degrees Celsius : boiling point of water

Easily reproducible; these may vary!

Scientific reference scaleT of Triple-point of water is defined by

273.16 K

International agreementreproducible

Calibrating thermometers

Triple-point of water = vapor, water, and

ice are in equilibrium(273.16 K)

Make a gas thermometer in

equilibrium with a triple-point water

cell which has a definite temperature

Constant-volume gas thermometer

Use the fact: T= C pT: temperature

C: constant if the volume of a gas is fixed

P: pressure

Constant-volume gas thermometergives correct value as p3 0

18-5 Celsius & Fahrenheit

18-6 Thermal expansion

Thermal expansions

-Expansion slots for sections of the bridge

-Dental cavity filling: thermal expansion property should be matched.

-Bimetal strip

Linear expansion

Volume expansion

SP 18-2

A oil trucker loaded 37,000 L of diesel fuel in Las Vegas and delivered his entire load in Payson, UT (23 oC lower).

β of diesel fuel = 9.5 × 10-4 /oC

α of steel truck tank = 11 × 10-6 /oC

How many liters did he deliver?

18-7 Temperature and Heat

Heat is the energy transferred between system and environment

because of the temperature difference

a) System is hotter than environmentSystem loses heat (Q<0)

b) Same temperature (in equilibrium)No heat flows (Q=0)

c) System is colder than environmentSystem gains heat (Q>0)

18-8 The absorption of heat by solids and liquids

Heat Capacity C

Specific Heat (heat capacity per unit mass)

Molar Specific Heat (heat capacity per one mole)

• 1 mol = 6.02 X 1023

An important point:

• constant pressure vs. constant volume

Heat of Transformation

-Heat of vaporization

- Lv=539 cal/g = 40.7 kJ/mol = 2256 kJ/kg (water)

-Heat of fusion

- LF=79.5 cal/g = 6.01 kJ/mol = 333 kJ/kg (water)

Molar Specific Heat

Problem 23

SP 18-3

18-9 A closer look at Heat and Work

P-V plot

Problem 43

18-10 The first law of Thermodynamics18-11 Special cases

Adiabatic process: no heat transfer

Constant volume process: no work

Cyclical process = [Q=W]

Returns to the same state:If the system received a

certain amount of heat, the same amount of work

has been done by the system.

Internal energy remains the same after a cycle.

Problem 48 cyclic process

Free expansion = [pext=0:W=0]

No heat transfer.No external pressure.

No work in expansion.No equilibrium during

the expansion

SP 18-5Boiling water steam

Constant pressure (1atm)Work done by the gas?

18-12 Heat Transfer Mechanisms: conduction

Problem 61

Conduction through a composite slabP must be invariant

(energy conservation)

Parallel combination:P=P1+P2 energy is transferred through two channels

Convection

Difference in temperature

Difference in pressure

Fluid can flow(from high pressure to low pressure)

Heat is transferred

Radiation

Electromagnetic wavee.g. radiation from the Sun

Any object with T > 0 K radiates!

SP 18-7

Ts = -23 oC; Ta = + 6.0 oC

Water: m = 4.5 g, A = 9.0 cm2, d = 5.0 mm

ε = 0.90

Find the time required for the water to freeze via radiation. Can the freezing be accomplished during one night?