http://www.ck12.org Chapter 20. Gas Laws
equilibrium is in a kind of balance: if left alone, the relevant macroscopic parameters will not change.
The usefulness of the concept of temperature becomes apparent with the following experimentally verifiable fact:
when two objects or systemsat different temperaturesare brought into thermal contact, the hotter one will cool,
while the cooler one will heat up, until their temperatures are equal. As we will see more clearly later, this is a
corollary of the famousSecond Law of Thermodynamics.
FIGURE 20.3
When two objects at different temperatures are brought into contact, their temperatures will get closer until the
objects are in thermal equilibrium.With these definitions, we can refine our understanding of temperature in terms of the following property.
Temperature
Ranks substances according to their ’hotness’; systems at different temperatures that are brought into thermal
contact will eventually reach thermal equilibrium and have equal temperatures.A Note on Temperature Scales and Measurement
To measure temperature, we have to use a device called a thermometer, which can be anything that exhibitsquantifi-
able physical changes with changes in temperature. An example of this is the familiar mercury-based thermome-
ter. This type of thermometer, which resembles the model below, works because the length of the mercury strip
(in reality, its volume) increases linearly with temperature (this relationship breaks down under certain conditions,
but we can ignore this). As an aside, it should be noted that this discussion of temperature is far from complete.
However, for the sake of brevity, we leave this definition as is and will return to a more formal approach in the next
chapter (which should clear up some of the confusion that may arise).
A thermometer is brought into thermal contact with a system, and once it is in thermal equilibrium with the system
— exactly like in the illustration above — its particular physical property (for example, the length of the mercury
strip) is translated into a number that is referred to as the temperature of the system. Of course, the thermometer
alters the temperature of the system as well, but generally this change is small and can be ignored (for instance, a
human being is much much bigger than a mercury thermometer).
There are different temperature scales: defining the units of this measure is up to us. The most frequently used ones
are called Fahrenheit, Celsius, and Kelvin, after their respective popularizers. So, does any particular choice of units
for temperature matter? Not really; any consistent scale will work. Consider the SI unit of length, the meter: it was
actually originally defined as one ten-millionth of the distance from the Equator to the North Pole through Paris.