Second Law of Thermodynamics

The maximum efficiency which can be achieved is the Carnot efficiency.

Second Law: Heat Engines

Second Law of Thermodynamics: It is impossible to extract an amount of heat Q_H from a hot reservoir and use it all to do work W. Some amount of heat Q_C must be exhausted to a cold reservoir. This precludes a perfect heat engine.

This is sometimes called the "first form" of the second law, and is referred to as the Kelvin-Planck statement of the second law.

Second Law: Refrigerator

Second Law of Thermodynamics: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object. This precludes a perfect refrigerator. The statements about refrigerators apply to air conditioners and heat pumps, which embody the same principles.

This is the "second form" or Clausius statement of the second law.

It is important to note that when it is stated that energy will not spontaneously flow from a cold object to a hot object, that statement is referring to net transfer of energy. Energy can transfer from the cold object to the hot object either by transfer of energetic particles or electromagnetic radiation, but the net transfer will be from the hot object to the cold object in any spontaneous process. Work is required to transfer net energy to the hot object.

Second Law: Entropy

Second Law of Thermodynamics: In any cyclic process the entropy will either increase or remain the same.

Entropy: 熵 :	a state variable whose change is defined for a reversible process at T where Q is the heat absorbed. 一个状态变量，其变化在可逆过程中定义于温度T，其中Q是吸收的热量。
Entropy: 熵：	a measure of the amount of energy which is unavailable to do work. 衡量无法用于做功的能量数量的指标。
Entropy: 熵 :	a measure of the disorder of a system. 系统无序程度的度量。
Entropy: 熵 :	a measure of the multiplicity of a system. 系统多样性的度量。

Since entropy gives information about the evolution of an isolated system with time, it is said to give us the direction of "time's arrow". If snapshots of a system at two different times shows one state which is more disordered, then it could be implied that this state came later in time. For an isolated system, the natural course of events takes the system to a more disordered (higher entropy) state.