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Drinking bird = heat engine

posted by Jason Kottke   Mar 09, 2009

Note: the drinking bird is not a perpetual motion machine. But it is a heat engine. Here’s how it works:

The water evaporates from the felt on the head (Maxwell-Boltzmann distribution). Evaporation lowers the temperature of the glass head (heat of vaporization). The temperature’s drop causes some of the dichloromethane vapor in the head to condense. The lower temperature and condensation together cause the pressure to drop in the head (ideal gas law). The pressure differential between the head and base causes the liquid to be pushed up from the base. As liquid flows into the head, the bird becomes top heavy and tips over during its oscillations. When the bird tips over, the bottom end of the neck tube rises above the surface of the liquid. A bubble of vapor rises up the tube through this gap, displacing liquid as it goes. Liquid flows back to the bottom bulb, and vapor pressure equalizes between the top and bottom bulbs. The weight of the liquid in the bottom bulb restores the bird to its vertical position. The liquid in the bottom bulb is heated by ambient air, which is at a temperature slightly higher than the temperature of the bird’s head.

Also, it’s drinking the water!