Work that is done on a conservative force, returns all the energy from that amount of work. For example, when tossing a ball upwards, it will return to the same height with the same speed. The force of gravity is a conservative force. This process is reversible because all of the energy is returned.

When running on a grass field, a player slides into a ball and comes to a rest. The initial kinetic energy of the player is lost to friction and eventually to heating up the ground and then the air. The initial kinetic energy is lost to the universe and is not recoverable. This is irreversible.

Entropy can be described as the amount of disorder. The 2nd law of thermodynamics says the entropy of a CLOSED system will never decrease. In other words, the randomness of the molecules that make up the system, will tend to remain unchanged or if left on their own, will move to a state of higher disorder. The variable for entropy is the letter "S." "S" is chosen to honor Sadi Carnot. This is why it is capital "S." The higher the "S" value, the more spread out the and unorganized the molecules are. This has huge implications.

High temperatures contain molecules that move with more kinetic energy. Lower temperatures contain molecules that move at lower kinetic energy. Take a metal rod where the end of it is heated up over a candle flame and the other end is at room temperature. After a few moments, the end that is over the candle is removed from the flame. When left on its own the warm end of the rod will expand towards the room temperature end. The lower temperature end of the rod has a lower entropy than the hot end. The 2nd law says that the energy will flow to the cooler end to randomize it. The the hotter end is becoming more organized, the system containing the entire rod has a rising average temperature. If left long enough, the rod will cool down. That's lowering the entropy of the rod. But that can't happen. What must be redefined to make sure the 2nd law is still true.

Entropy applies to irreversible processes. A glass falls off the table and shatters when it hits the flow. The glass molecules are no longer organized. The glasses pieces are made more random on the floor. We all expect to get a broom and clean up the glass pieces. But there is probability that the glass will reconstitute itself to maintain the same level of disorganization -or entropy. But this is to highly unlikely that it will not happen our first response when the glass breaks is to get a broom.

Entropy also gives time a direction. The 2nd law says that over time, entropy increases. We know time is passing because items break and decay. This give us a sense of history.

Entropy can be measured by the equation below.

"E" is the change in energy for an irreversible process. For example looking at the change in kinetic energy for a motion. "T" is the temperature the action occurred at in Kelvins. The units of entropy are "J/K." The amount of entropy of a body does not mean anything. It is useful for comparing two states to see which has the higher entropy.