Question

Does the Maxwell’s demon violate the 2nd law of thermodynamics? If not, why?

Answer 1

We know that second law of thermodynamics says that entropy in an isolated system should not decrease. it means disorder is always increasing.

Now, Maxwell's demon is an imaginary being. Consider a box with some molecules roaming here and there in a disordered manner. Some are hotter and some are cooler. Now, we introduce a partition with a small hole, inside the box. A demon sits near the hole. His duty is to allow only hotter molecules to remain on one side, while the cooler than average molecules are allowed into the other side of partition. The end result is that all the hot molecules end up on one side of the box and cooler ones on other side.

Our first thought would be that demon has succeeded to violate the second law but not , it is not true.

Explanation - a demon uses his brain and energy to get all the information about the molecules. and lower the entropy of the box. However, entropy in brain is increasing as the demon stores the information. As only finite information can be stored, the demon has to erase some information to get new information about the changing state of molecules. This will need some work and some heat will be created which will increase the entropy of the overall system (as the demon is a part of system, he sits near the hole), hence the second law of thermodynamics holds well.