Second Law of Thermodynamics Revisited

Second Law of Thermodynamics Revisited

Chapter 3 described the second law of thermodynamics in terms of heat and work. This is perfectly correct, but there is another way of formulating the second law: in an isolated system, entro- py will stay the same or increase over time, it will not decrease. In other words, in an object or system that is not infl uenced by any outside activity, order will tend to turn into disorder.

But if everything tends to get more disorganized over time, how does anything ever get ordered in the fi rst place? What the second law says is that entropy generally increases in isolated systems and for spontaneous processes. A refrigerator pumps heat from the cold interior to the warm room outside, so the interior of the re- frigerator gets cooler and, according to the simple equation given above, the entropy inside decreases—the refrigerator removes heat, and the change in heat is therefore negative. But this is not

a spontaneous process because the refrigerator had to do work, and it would not function if someone pulled the plug. If the whole system of objects is taken together, then the second law says that the entropy increases—the entropy is reduced inside the refrigera- tor (because of the work done by electricity), but the refrigerator pumps heat into the room, and the room’s entropy increases. In any process, the sum of the entropy of all affected objects either stays the same or, more likely, rises. That is the second law.

There is one more important consideration of the second law of thermodynamics—it is a statistical law, not an absolute one. This means that the law does not guarantee the predicted out- come—a rise in entropy—although it is extremely likely to occur. The reason for this is clear from considering the card experiment discussed earlier. If someone throws an out-of-order pack of cards onto the fl oor and picks up the cards at random, it is possible, although highly unlikely, that the resulting sequence of cards will be ordered. There is also a small chance that the resulting sequence could be slightly more ordered than before, so that although it is not perfectly ordered it is better than it was before. In this case, entropy would have increased slightly. But because there are so many more states of a system that are disordered than ordered, almost always a system that evolves in time (with- out any help from the outside) will end up in a more disordered state. The pack of cards, for example, has only one perfectly or- dered sequence and a huge number of other possible sequences, many of which are quite jumbled. In systems with more elements than a pack of cards—and most systems have a huge number of elements—an increase in entropy is overwhelmingly probable.

118 Time and Thermodynamics

so that the entropy of the whole system (the heat engine and the surroundings) increases, or at the very least, stays the same. Oth- erwise the heat engine would not work. Some engineers consider the heat lost in the exhaust of heat engines a “fee” or a “tribute” that must be paid to the second law of thermodynamics.

Thermodynamics also explains the failure of a bouncing ball to continue bouncing. The loss of energy is converted into heat— the ball and the object on which it is bouncing both get slightly warmer. Heat is disordered motion, as discussed in chapter 1— heat is the random motion of an object’s atoms and molecules. The second law of thermodynamics says that spontaneous processes get more disordered, and this includes motion. The neat, orderly up- and-down motion of the ball slowly gets transformed into random motion of atoms and molecules, and temperature rises. Basketball players must keep bouncing the ball with their hands in order to dribble toward the hoop, otherwise the second law of thermody- namics would leave the ball on the court behind them.

Entropy is what provides physics with a direction of time. The flow of time is in the direction of increasing entropy, or disorder. While watching the movie scene with the car accident, as men- tioned earlier, viewers realize something is wrong when the film is run backward and the crumpled car straightens itself out. What is wrong is that the second law makes it extremely unlikely that objects and processes spontaneously go from a disordered state to an ordered one.