Thermodynamics and the Atomic and Molecular Structure of Matter 95
Another example of the natural increase entropy can be seen by
considering a box with two partitions and a wall between them. In the
left hand side of the box we place 2 grams of hydrogen gas or 6 × 10^23
hydrogen molecules, while we leave the other side empty. Once we
remove the wall between the two partitions there will soon be
approximately 3 × 10^23 hydrogen molecules on each side of the box.
Although it is physically possible for all of the molecules to move back
to one side of the box the probability of this is so small, 1 part
8 100,000,000,000,000,000,000,000 that it can be effectively ruled out. Life would be
fairly dangerous if this were possible; imagine choking to death as all the
air molecules on your side of the room suddenly moved over to the other
side of the room.
An increase of entropy occurs whenever two bodies of different
temperature are placed in thermal contact. Invariably the warmer body
loses heat to the cooler until the two of them are at the same temperature.
Although conservation of energy does not forbid two bodies of the same
temperature transferring heat between them so that one becomes warmer
and the other cooler, this process does not occur naturally. When two
bodies are at different temperatures more of the molecular motion is with
the warmer body. When these two bodies are placed in thermal contact
so that motion becomes more evenly distributed between the two bodies,
resulting in a more random distribution of the motion. If one wants to
distribute heat from one body to another when both are initially at the
same temperature one needs a heat pump, a refrigerator, or an air
conditioner and the energy it takes to run these devices. So it takes work
to create a temperature difference. On the other hand if one has a
temperature difference between two gases there will be a flow of
molecules between them, which can be harnessed to do work, which is
basically how a steam engine or a gasoline internal combustion engines
works.