本文探讨一维圆环上的经典理想气体微观状态熵，证实粒子的位置分布在圆环上逐渐趋向均匀化，位置熵随时间递增直到最大值，但包含位置不确定性和速度不确定性在内的微观状态总熵在数学意义下严格守恒。在同一时刻位置非常接近的粒子的速度逐渐呈现出量子化特征，相邻分立速度的间隔在长时间极限趋向于零。这意味着在物理测量意义下，由于观察者的速度分辨率不可能无限小，测量到的系统微观状态总熵在长时间极限必定增加。该简单系统可作为教学模型增进对热力学第二定律的理解。

The entropy of microscopic configurations of a classical ideal gas system located on a one-dimensional ring is investigated. It is verified that the distribution of particles on the ring becomes more and more uniform，and the positional entropy increases with time toward the maximum value，while the total entropy (contributed by positional uncertainty and velocity uncertainty) is conserved in the rigorous mathematical sense. The velocities of particles which are passing through the same position at the same time become more and more quantized，and the gap between adjacent discrete velocity values gradually reduces to zero as time increases. This quantization property means that，as the velocity resolution limit of an observer must be finite in the sense of physical measurement，the measured entropy of microscopic configurations must be increasing with time. This simple system is helpful for understanding the second law of thermodynamics.