Observations and Second Law of Thermodynamics

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The second law of thermodynamics is based on observations of natural systems done back in the nineteenth century. This section will:
  1. Look at two of these observations
  2. From these can state the Second Law of Thermodynamics

Observation #1

Let us have a box with two sections separated by a removable partition. One side is filled with gas while the other is empty:


If we remove the partition, the gas disperses throughout the container:


However, we would have to wait a very, very, very long time for the gas to return all to one side so we can put the partition back in and get the original condition.[1] (We are assuming here a completely natural system with no external forces applied.)

We say a system like this tends toward disorder.

The measure of this disorder is called ENTROPY and given the symbol S.

Observation #2

Many processes have been developed which can convert work completely to heat. A simple example is by simply rubbing your hands the friction creates heat. However, no process has ever been developed which convert heat completely to work.

The first law says only that heat and work are both forms of energy. But it cannot explain the difference between them.

Second Law of Thermodynamics

We can combine these two observations into the The Second Law of Thermodynamics

  • With no external input, a system will tend toward disorder
  • No system can be developed which completely converts heat to work

We can prove that the two statements given here are equivalent. See here.


  1. This is a classic problem in thermodynamics called Maxwell's Demon