Heat & thermodynamics › Maxwell’s demon
Maxwell’s demon and entropy laboratory
Let an information-processing “demon” sort gas particles—and account for the memory and erasure costs that preserve the second law.
Why the second law survives
The demon can apparently lower the gas entropy by measuring and sorting particles. But its records occupy physical memory. Resetting that memory is a logically irreversible operation with a minimum thermodynamic cost. Measurement can in principle be reversible; erasure is the key step in Landauer’s principle.
S = kB ln Ω · H = −Σpᵢlog₂pᵢ · Eerase,min = kBT ln 2 per bit
Illustrative two-dimensional entropy accounting. The laboratory does not generate free energy or provide a complete microscopic thermodynamic calculation.