Part 3 Thermodynamics
Chapter 19: Temperature and the Ideal Gas Law
Reading assignment: Chapter 20.1-20.5
Homework : (due Wednesday, Nov. 30, 2005):
Problems:
Chapter 19: 1, 14, 17, 28, 30
Chapter 20: 3, 5, 13, 20, 28, 32
Chapter 19
•Temperature scales: Fahrenheit, Celsius (centigrade), Kelvin
• Thermal expansion
• The ideal gas law
Temperature
Scales:
Reliable and
quantifiable
way of
measuring
how “hot” an
object is.
Fahrenheit (1686-1736) established three fixed points on his
thermometer.
0 degrees: temperature of an ice, water, and salt mixture.
32 degrees: water-ice combination stabilized at "the thirty-second degree."
98 degrees: “when the thermometer is held in the mouth or under the armpit of a
living man in good health."
Celsius (1701-1744) established two fixed points on his
thermometer.
0°C: representing the freezing point of water.
100°C: the boiling point of water.
Kelvin (1834-1907) used the same gradation as Celsius but set his
zero point at absolute zero (All molecular motion ceases at absolute
zero).
How to convert from one temperature scale to another:
TC  T  273.15
TF
TF
9

 TC  32 F
5
9

 T  459.67 F
5
• Where TC, TF, T is the temperature in Celsius
(centigrade), Fahrenheit and Kelvin, respectively
• Kelvin is the proper SI unit
Black board example 19.1
“When the thermometer is held in the mouth or under the armpit of
a living man in good health” it indicates 98 F (Can fluctuate
between 97.5F and 98.9F).
a) What is the temperature in
Celsius (centigrade)?
b) What is the temperature in
Kelvin?
Heat
• is the energy that flows between objects
because of their difference in temperatures
• Heat is thermal energy on the move
• Technically: object’s don’t contain heat
Thermal equilibrium
• Two objects are in thermal equilibrium
when they have the same temperature.
 They cease to exchange heat.
• Zeroth law of thermodynamics: If object A and B are separately in
thermal equilibrium with a third object C (thermometer), then objects
A and B are in thermal equilibrium.
Heat and Temperature
• Touching objects exchange thermal energy
– Microscopically, energy flows both ways
– On average, energy flows one way
• Temperature predicts energy flow direction
– Energy flows from hotter to colder
– No flow  thermal equilibrium  same temp
• Temperature turns out to be
– average thermal kinetic energy per particle
• More on heat in Chapter 20
Thermal expansion of solids and liquids
When objects are heated up they typically expand. This is due
to the increased motion of molecules at elevated temperatures.
Change in Length:
L    Li  T
 … coefficient of linear expansion
Li … initial length
L … change in length
T …change in temperature (in centigrade or Kelvin)
Application: Bimetallic strip
Black board example 19.2
Problem 19. 8
The New River Gorge Bridge in West Virginia is a steel arch
bridge 518 m in length. (alpha=11 x 10-6/C)
How much does its length change between temperature extremes
of -20.0°C and +35.0°C
Thermal expansion of solids and liquids
When objects are heated up they typically expand. This is due
to the increased motion of molecules at elevated temperatures.
Change in Volume:
V   Vi  T
 … coefficient of volume expansion
Vi … initial volume
V … change in volume,
T …change in temperature (in centigrade or Kelvin)
The Unusual behavior of water.
Or: What is the temperature at the bottom of a deep lake?
Or: Why does a lake freeze from the top and not the bottom?
The ideal gas law
Idealized assumptions (work well for many gases at medium
temperatures and low pressures):
- Gas molecules don’t interact upon collision
- Molecular volume of gas molecules is negligible compared with
volume of container
P V  n  R  T
P … Pressure of gas
V … Volume of gas
n … number of moles of gas
R … Universal gas constant, R = 8.315 J/mol·K
T … absolute Temperature (in Kelvin)!!
Black board example 19.3
Problem 19.28
Nine grams of water are
placed in a 2.00 L pressure
cooker and heated to
500°C.
What is the pressure inside the container if no
gas escapes?