Perpetual Motion
Some people regard heat engines as imperfect because they can never be 100 percent efficient. Even idealized heat engines must always waste some energy. Why? Heat is disorganized energy, and it takes energy to organize things.
Let’s now consider engines with zero efficiency. They can do no useful work. We can make any engine drop to zero efficiency by cutting off its supply of fuel. If there is no fuel the fire goes out and the source of heat cools until its temperature is equal to the temperature of the environment, the temperature at which the engine discharges waste heat. That makes the highest temperature H equal to the lowest temperature L. When two numbers are the same, dividing one by the other yields 1. The ratio 1, subtracted from the 1 that indicates 100 percent efficiency, gives 0. An engine with no fuel has zero efficiency. It cannot do useful work.
What if an engine keeps on moving after its fuel is exhausted? The second law does not forbid perpetual motion. If it did it would be outlawing the stable movement of the electron in the hydrogen atom. The second law puts a limit on the useful work we can extract from heat energy. If there is no difference of temperature between the heat source and the reservoir where the engine discharges used heat energy, then no engine can extract work. That doesn’t mean that the parts of the engine can’t go around and around forever. They can do that if they have no friction. The planets approximate this condition by moving year after year around the Sun in the airless void of space. There is no such thing as friction in engines the size of atoms, either. Friction is a macroscopic phenomenon. Macroscopic means that the parts contain many atoms or molecules. In other words, the fact that objects of ordinary size don’t move forever doesn’t mean that there is no perpetual motion of very large or very small things.
Let’s now consider engines with zero efficiency. They can do no useful work. We can make any engine drop to zero efficiency by cutting off its supply of fuel. If there is no fuel the fire goes out and the source of heat cools until its temperature is equal to the temperature of the environment, the temperature at which the engine discharges waste heat. That makes the highest temperature H equal to the lowest temperature L. When two numbers are the same, dividing one by the other yields 1. The ratio 1, subtracted from the 1 that indicates 100 percent efficiency, gives 0. An engine with no fuel has zero efficiency. It cannot do useful work.
What if an engine keeps on moving after its fuel is exhausted? The second law does not forbid perpetual motion. If it did it would be outlawing the stable movement of the electron in the hydrogen atom. The second law puts a limit on the useful work we can extract from heat energy. If there is no difference of temperature between the heat source and the reservoir where the engine discharges used heat energy, then no engine can extract work. That doesn’t mean that the parts of the engine can’t go around and around forever. They can do that if they have no friction. The planets approximate this condition by moving year after year around the Sun in the airless void of space. There is no such thing as friction in engines the size of atoms, either. Friction is a macroscopic phenomenon. Macroscopic means that the parts contain many atoms or molecules. In other words, the fact that objects of ordinary size don’t move forever doesn’t mean that there is no perpetual motion of very large or very small things.
