The second law of thermodynamics
states that in any spontaneous process the entropy of the universe will increase. What
does this mean? It means that all spontaneous processes must increase the number of
available micro-states. The number of available micro-states after any event will always
be greater than the number of available micro-states before the event. Because atoms form
large objects like boulders, large objects must also obey the second law.
The second law can be stated in a very intuitive way. The uncertainty
of the universe increases with time. This is why it is more difficult to predict what will
happen far in the future. Weather is a great example. The weatherman may be able to
forecast rain tomorrow, but he cannot forecast rain a month in advance.
Consider a bicycle that is turned upside down. The back wheel is spun
until it is moving very fast. The second law explains why the wheel will not turn for
long. The atoms that make up the wheel are moving very fast, so these atoms have lots of
energy. As the wheel spins some of this energy is transferred as heat to the air around
the wheel and to the frame that holds the wheel. This increases the air temperature which
in turn increases the number of micro-states available to the air molecules. The entropy
of the air molecules increases. Since the frame also heats up, its atoms are free to
occupy more micro-states. Eventually all of the energy in the wheel will be dissipated as
heat. The wheel’s entropy decreases as it slows. The entropy of its surroundings
increases. The increase is more than enough to offset the decrease. Thus, the entropy of
the universe as a whole increases.
Figure 6.8: System vs. Surroundings
The equation that must always be satisfied is as follows:
entropy change of the system + entropy change of the surroundings =
entropy change of the universe > 0.
The entropy of any system can decrease as long as the entropy of the surroundings
increases, and the increase is greater than the decrease.
next: heat flow
home: Intelligent Design and the origin of life
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