How does the drinking bird work

Drinking bird

A Drinking bird (also known as "Wippvogel", "Pickvogel", "Schluckspecht", "Trinkstorch", "Nickente", "Nickvogel" or "Glass piston duck") is a physical toy.

construction

File: Drinking bird 01 ies.webm The drinking bird is usually constructed as a hollow body made of glass, consisting of a neck that merges into a felt-covered head with a beak and protrudes below into a (closed) belly, which without air with a liquid with a low boiling point close to room temperature (e.g. ether, not permitted in the EU). The bird lies on a frame and can tip forward about an axis of rotation above the belly. If it tips forward far enough, its beak protrudes into a glass of water in front of it.

functionality

The water on the damp beak evaporates into the outside air. The heat required for this is withdrawn from the glass head. As a result of the evaporation cooling, not only the glass head but also the ether vapor in the head cools and condenses. This reduces the vapor pressure here. The vapor in the lower part of the body can now expand and push the colored liquid upwards in the glass tube. This raises the center of gravity, the bird becomes unstable and tilts forward into an inclined position. At the same time, due to the inclination, the lower end of the tube moves out of the liquid in the abdomen, so that pressure equalization can take place. As a result, colder liquid flows back into the stomach and warmer steam rises into the head, the bird tilts back to its starting position. He oscillates around the starting position and the process starts all over again. If you provide an automatic water replenishment, the bird can nod "forever". It is therefore a heat engine, or more precisely a material engine, with a cycle. The selected liquid must be easily evaporable at room temperature. Their saturation vapor pressure must also increase sensitively with temperature. Ether would be suitable, but it is flammable, dichloromethane is harmful to the environment, so secret colored mixtures are used.

If the drinking glass is filled with alcohol, the bird drinks more often because it evaporates more easily. The tilting movement can also be triggered by targeted heating of the lower body with a hair dryer or by thermal radiation.

Physical terms and laws that are addressed with the toy: ideal gas, saturation vapor pressure, vapor-liquid balance, Clausius-Clapeyron equation, heat of vaporization, condensation, center of gravity, torque, 2nd law of thermodynamics.

The drinking bird is seemingly a perpetual motion machine of the second type, as it gets its drive energy from the ambient heat and can thus generate kinetic energy by lowering the outside temperature. In reality, however, it fulfills the second law of thermodynamics, since an increase in entropy does take place when the water evaporates from the glass at the beak, turns into the gaseous state and thus the entire system water glass + air closer to its equilibrium state (100% relative humidity) brings up.

In a closed system, the relative humidity soon reaches 100% and the cooling evaporation process comes to a standstill. This can easily be shown by putting a glass bell over the bird: after a few minutes the movement comes to a standstill. If you remove the bell, the bird starts moving again.

literature

  • Wolfgang Bürger: The paradoxical egg cup. Physical gadgets from Professor Bürger's cabinet. Birkhäuser, Basel et al. 1995, ISBN 3-7643-5105-5, pp. 154-161.
  • J. Güémez, R. Valiente, C. Fiolhais, M. Fiolhais: Experiments with the drinking bird. In: American Journal of Physics. Vol. 71, No. 12, 2003, pp. 1257-1267, doi: 10.1119 / 1.1603272.

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