The Second Law of Thermodynamics
Entropy Increases and Efficiency of Heat Engines Is Less Than 100%
The second law of thermodynamics states that the efficiency of a heat engine is always less than 100% and that the entropy of a closed system must always increase.
Thermodynamics
There are three laws of thermodynamics. They originated in the early days of the industrial revolution. Engineers and physicists studied heat engines, engines that use heat energy to perform work, to improve their efficiency.
The laws of thermodynamics place fundamental limits on the efficiencies of heat engines. Good thermodynamic engineering can in principle produce heat engines that perform near these limits. However even the very best engineers cannot design engines that exceed these limits without violating fundamental physical laws.
The First Law of Thermodynamics
The first law of thermodynamics states that the total energy input for a heat engine equals the work output plus the change in internal heat energy of the system. This internal heat energy is the waste heat energy from whatever process operates the engine. If the internal heat energy does not change, then the energy going into a heat engine, or other system, equals the useful work output. In this case the efficiency of the engine is 100%. The first law allows an engine with 100% efficiency, but the second law of thermodynamics does not.
The Second Law of Thermodynamics and Efficiency
One statement of the second law of thermodynamics is that the efficiency of any heat engine or other thermodynamic process is always less that 100%. There will always be some type of friction or other inefficiency that will generate waste heat. The useful work that a heat engine can perform will therefore always be less than the energy put into the system. Engines must be cooled, as a radiator cools a car engine, because they generate waste heat.
Entropy and the Second Law of Thermodynamics
Entropy is a measure of the disorder or randomness of a system. A highly random disordered system has more entropy. To mathematically compute the change in the entropy of a system, divide the change in heat energy by the Kelvin temperature. Heat or thermal energy causes individual atoms or molecules to increase their random motions. Increasing the internal heat energy of a system therefore increases its randomness, disorder, and entropy.
If a heat engine, or other thermodynamic process, has an efficiency less than 100%, it generates waste heat and increases its internal heat energy. This increased internal thermal energy increases the random motions at the atomic or molecular level. Hence any process with less than 100% efficiency will increase the entropy. The second law of thermodynamics states that any process must always have less than 100% efficiency. Therefore any process must increase the total entropy of the universe.
A logically equivalent alternate form of the second law states that any thermodynamic process in a closed system will increase the entropy of the system. If the system is not closed, the process will increase the entropy of the universe.
The total entropy of the universe can never decrease. The universe is becoming more disordered.
Carnot Cycle
The French engineer, Sadi Carnot (1796 - 1832), did the pioneering analysis on the efficiency of heat engines. He found, as another statement of the second law, that the maximum possible efficiency for a heat engine occurred when it used a specific cycle, now called the Carnot cycle. The Carnot cycle allows 100% efficiency, if the waste heat is dumped into a heat reservoir at a temperature of absolute zero. The third law of thermodynamics closes this loophole.
The second law of thermodynamics states that the efficiency of any process must be less than 100% or in alternate form that any process must increase the total entropy of the universe.
Further Reading:
Zemansky, M. and Dittman, H. Heat and Thermodynamics 7th ed., McGraw Hill, 1996.
