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?