1
HEAT AND THERMODYNAMICS
1. The Absolute Temperature Scale (Knight §16.3)
In the textbook you have been introduced to the concept of temperature, and to the fact
that there is a natural zero of temperature, the temperature at which the motion of mole
cules ceases. This temperature is called
absolute zero
. With a simple experiment we can
determine the location of absolute zero on the Celsius scale, by making use of the
ideal
gas law
,
PV
=
Nk
B
T
, where
P
is the pressure in N/m
2
,
N
is the number of atoms or mole
cules in the gas,
k
B
is the Boltzmann’s constant (
k
B
= 1.38 x 10
23
Joule/K), and
T
is the
temperature in Kelvins. For our purposes the formula can be rewritten
p
=
Nk
B
V
T
If we change the temperature of a fixed volume of gas, then everything inside the paren
theses in the above equation will remain constant; the pressure should therefore be di
rectly proportional to the absolute temperature.
We can use this fact to determine where absolute zero temperature must lie on the Celsius
scale.
(You likely already know, of course, that absolute zero = 273.15 Celsius, but sus
pend that knowledge for now  this lab is where you get to
prove
it.)
Simply take a fixed
volume of gas, measure its pressure at two or more different temperatures, and extrapo
late backwards to find the temperature at which
P
would fall to zero.
Experiment:
You will find two sealed, stainlesssteel bulbs, one containing helium gas and one con
taining neon gas. Each bulb has an attached pressure gauge which measures the pressure
of the gas inside the bulb.
Note that these gauges read
absolute
pressure, not “gauge
pressure”.
Read the pressure of each bulb at room temperature.
(It won’t necessarily be one atmos
phere, since the bulbs are sealed and are not in contact with the air outside.)
Immerse each bulb in a bath of ice water.
Give them time to come to equilibrium, so that
the gas inside the bulb is the same temperature as the ice water (0 degrees Celsius, of
course.)
How do you predict the pressure in the bulb will change?
(You don’t have to
make a quantitative prediction, just a general trend.)
Once the bulbs have come to equi
librium at their new temperature, record the pressure that you actually measure for each
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 Winter '07
 Graham
 Thermodynamics, Absolute Zero, Heat, degrees Celsius, Pasco, Thermodynamic temperature

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