An insulated box containing only air. The box is divided into halves and there is a trap door in the middle. When it opens, it allow molecules to move from one part of the box to the other. The 2 halves started off which each air pressure and there is no temperature difference between them.
The movement of individual molecule is random. Some molecules moves faster than the other. When the trap door is opened, there are on average same number of molecules moving to the other side.
Maxwell's demon controls the trap door and he only allows molecules to go from left compartment to the right. In time, there will be more molecules in the right compartment which means higher pressure and becomes hotter. This seems to violate the Second Law of Thermodynamics and what is needed is just information. However, to obtain the information and making the decision will also requires energy out of the system. Thus, the law was not broken.
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