Atoms and Molecules Have Energies

For simple systems of independent particles such as ideal gases, we can express the total internal energy of a thermodynamic system as the sum of the particle energies  

In Example 2.2 we found that gases expand because the multiplicity IV(V) increases with volume V. The dependence of IV on V defines the force called pressure. In Example 2.3 we found that particles mix because the multiplicity W(N) increases as particle segregation decreases. This tendency defines the chemical potential. These are manifestations of the principle that systems tend toward their states of maximum multiplicity, also known as the Second Law of thermodynamics. 

The following two examples illustrate that the flow of heat from hot to cold objects is also driven by a tendency to maximize multiplicity. First, Example 3.3 shows a model for how the multiplicity IV(17) of an object depends on its energy. Then Example 3.4 shows that heat flow maximizes multiplicity. 

EXAMPLE 3.3 How does multiplicity depend ran energy Here is a miniaturized model of a material it has just three distinguishable particles labeled x, 

Increasing the energy of a system increases its multiplicity of states. It follows that a tendency toward high multiplicity is a tendency for a system to take up heat from the surroundings. Now why does heat flow from hot objects to cold ones Example 3.4 addresses this question. 

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Atoms and molecules have energies due to several types of motion. In part, the Born-Oppenheimer approximation from Chapter 12 will be applied We will treat various motions of atoms and molecules, and the energies of those motions, separately. Electronic, nuclear, rotational, and vibrational energy levels can and will be treated separately for the most part, although in some cases we will have types of spectroscopy that involve combinations of these energy levels. 

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