The first law of thermodynamics states that energy is not created or destroyed, it just changes forms. The second law of thermodynamics describes how the energy behaves during transformations. 

Entropy has several related definitions:

• A quantitative measure of the amount of thermal energy not available to do work
• A measure of the disorder or randomness of a closed system
• The tendency for all matter and energy in the universe to evolve toward a state of inert uniformity

A better understanding of entropy comes from looking at examples of systems where it is increasing or decreasing. 

As a motor is converting chemical energy into mechanical energy, there is a release of heat into the atmosphere.  This heat will increase the entropy of the area around the motor, and is not available to do the work that the motor is intended to do.

If you spray air freshener into the air, the particles start off in a small space right around the can.  This is considered a situation with high order—that is all the air freshener is organized in a specific spot.  Over time, the molecules of the spray will spread out—the system is becoming more disordered and approaching more uniformity. 

If you add ice cubes to a warm drink, heat will flow from the drink to the ice.  The temperature of the drink will become lower and the ice will melt.  After the ice has melted, heat will continue to flow from warmer areas of the drink to colder areas until the drink has reached a uniform temperature. 

Let’s take a look at increasing entropy in action:

The following simulation starts with two squares next to each other, one is light blue and the other is dark blue.  When the simulation is started, a single pixel—chosen at random—will switch states (if it is light blue it will become dark blue, if it is dark blue it will become light blue).  This process will be repeated thousands of times very quickly with the number of light blue pixels being reported just below the two squares. 

The graph shows the total entropy of the system vs. time.

What happens to the color of the two squares over time?
Interactive popup. Assistance may be required.

Check Your Answer

The two squares become a uniform mix of light blue and dark blue.

What does this show about entropy?
Interactive popup. Assistance may be required.

Check Your Answer

The entropy of the system is increasing because the colors are becoming more uniformly mixed. The two sides are approaching equilibrium with one another, and the orderly division of the colors is becoming disordered.

There are three important conclusions that can be drawn from the second law:

1. There are no 100% efficient machines—there is always some energy lost.  This means that you can’t create a perpetual motion machine.
2. Heat will always flow from high temperature to low temperature
3. Systems will move toward equilibrium
   (This means that left by themselves, materials will tend to come to a uniform temperature and uniform pressure.)

It is possible for the entropy of a specific system to decrease, but to do that, you must do work on the system.  The work you do on the system will increase the entropy outside the system more than it decreases the entropy of the system.  For example:  If I put water in the freezer, it will turn to ice.  The order of the water has increased, and its entropy has decreased.  In order to make this happen, a condenser motor has done work to pump heat out of the freezer and into the atmosphere.  The Increase in entropy of the atmosphere will always be greater than the decrease of entropy of the water as it freezes.