A Maxwell demon is a small scale construction that supposedly violates the second law of thermodynamics by rectifying thermal fluctuations. We show that one of the best known models, the Feynman-Smoluchowski ratchet, is unnecessarily complicated. It can be replaced by a construction involving only hard core interactions. This model can be studied numerically in great detail by molecular dynamics. Furthermore a microscopically exact analytic calculation is possible in the limit of ideal gases. When the system is at equilibrium, no rectification of thermal fluctuations takes place. Outside of equilibrium, the machine functions as a Brownian motor. We briefly discuss an error in the estimation of the resulting efficiency by Feynman. We also show that the construction can be used as a Brownian refrigerator. Finally we make the connection with the Jarzynski, Crooks and Cohen-Gallavotti relations.

Refs:
– C. Van den Broeck, R. Kawai and P. Meurs, “Microscopic Analysis of a Thermal Brownian Motor”, Phys. Rev. Lett. 93, 090601 (2004).
– P. Meurs, C. Van den Broeck and A. Garcia, “Rectifying Thermal Fluctuations in Ideal Gases”, Phys. Rev. E 70, 051109 (2004).
– C. Van den Broeck, P. Meurs and R. Kawai, “From Maxwell Demon to Brownian Motor”, New J. Phys. 7, 10 (2005).
– I. Bena, C. Van den Broeck and R. Kawai, “Jarzynski Equality for the Jepsen Gas”, Europhys. Lett., to appear (2005).

Université de Hasselt