Peoples Physics Book Version-2

20.1. Thermodynamics http://www.ck12.org

air pressure. To measure the difference between the air pressure and the gas pressure, we can use a simple gauge:
a piston connected to a spring. If we connect it to the inside of the balloon (without any gas leaks), the piston will
contract the spring until the spring force is equal to the pressure of the gas times the area of the piston.

FIGURE 20.2

An illustration of gas pressure and its measurement

Temperature

Defining temperature rigorously is a difficult task and we will attempt to refine our understanding through the
following chapters. As a first approximation, temperature can be defined as a quantifiable measure of the ’hotness’
of an object. It’s important to note that temperature is rigorously defined only for systems inthermodynamic
equilibrium:

Thermodynamic Equilibrium

A closed system is in thermodynamic equilibrium if the macroscopic parameters associated with it (such as

pressure, temperature, and volume) will remain constant indefinitely.

In mechanics, an object was in equilibrium if the forces on it were balanced; when this was true the quantities
associated with the object (velocity, position, acceleration) would not change. Likewise, a system in thermodynamic