The first law of thermodynamics
is the law of conservation of energy. The second
law of thermodynamics states that some of the heat input to a heat engine
must be wasted in order for the engine to operate.
Entropy is a measure of the
disorder or randomness of the particles that make up a body of matter. In a
system of any kind isolated from the rest of the universe, entropy cannot
decrease.
Imagine making a salad. In a
large bowl you put the lettuce, cucumbers, slices tomatoes and carrots and then
you mix them up by tossing the salad. You
have created a less ordered system. The
lettuce, cucumbers, tomatoes and carrots will not suddenly jump back to their
individual bowls. This is an
example of entropy or randomness not being reversible.
If you were to separate them out yourself you would decrease the entropy
of the salad (make it more ordered) but in doing so your body would convert
sugars to carbon dioxide and water and this process would more than counteract
the entropy loss of the salad. The
total entropy of the universe increases with every natural process.
Another example of entropy we can see when we drop a small amount of food
coloring in to a glass of water. The color spreads out. The
food coloring is less ordered when it is spread out throughout the water.
What drives this dilution? The
answer is the drive for greater randomness or entropy.
Here is another implication of the second law of thermodynamics: Heat naturally flows form a region of higher temperature to a region of lower temperature. In terms of “order”, heat energy is more “ordered” when it is concentrated. When transferred to a region of lower temperature, it is “spread out “ or more “disordered”, and the entropy increases. Hence the universe eventually should cool down to a final common temperature when the entropy of the universe has reached a maximum. This possible fate is sometimes referred to as the “heat death” of the universe.