Ratchet and Pawl: Feynman's Physics Explained
Imagine a tiny paddle wheel in a box of gas, attached to a ratchet that lets it turn only one way. Won't random molecular hits be rectified into useful rotation, lifting a weight from pure heat? No — and the reason is subtle. The ratchet's pawl is also being bombarded and jiggling, and at the same temperature it lets the wheel slip backward exactly as often as it is kicked forward. The Second Law holds even for the tiniest machines.
The big idea
You can't cheat the Second Law, not even with a microscopic machine.
Think about it
What goes wrong with the dream of a machine that turns the random heat of a room directly into useful work?
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