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The Dipping Bird
The dipping bird seems to go forever but it's not perpetual motion!
A dipping bird is an example of a heat engine. It converts a difference in temperature (between the head cooled by evaporation and the bottom at room temperature) into cyclical motion.
- Dipping bird (can be obtained from Edmund Scientific, catalog number 53617, $7.95 for a package of 2 birds. You can also try novelty or magic shops.)
- Cup or glass
- Water
Wet the bird's head thoroughly with water. Allow enough time for the fuzzy material on the head to absorb water (a few seconds should do it).
Fill a cup or glass with water and place it so that the bird's beak will dip into the water each time the bird tips. You may have to place pieces of wood or cardboard under the cup or glass if it's too short, or get a smaller glass if it's too tall.
Watch the bird go through its cycle. Notice what happens to the liquid inside the bird at different positions in the cycle.
When the bird is manufactured, most of the air is removed from the inside. The gas that remains is largely the vapor from the red liquid, which vaporizes very easily. When the fuzzy coating on the bird's head is wet, water evaporates and cools the vapor inside the bird's head. This condenses the vapor back to red liquid and reduces the pressure in the bird's head. When the fuzzy coating on the bird's head is wet, water evaporates and cools the vapor inside the bird's head. This condenses the vapor back to liquid and reduces the pressure in the bird's head. The bird's head keeps moving.
Since the pressure of the vapor in the bird's body is now higher than the pressure in its head, liquid is forced from the bottom up the tube toward the head. As the liquid moves up the tube, the center of gravity of the bird is raised, and the bird begins to tip around its fulcrum. When the bird finally dips into the water, a clear passage is opened between the head and the body, allowing the pressures to equalize and the liquid to fall back down to the body. The bird returns to the upright position and the whole process repeats.
Each time the bird's beak dips into the water, the fuzzy material absorbs a little water to replace any that has evaporated. This prevents the bird's head from drying out. The bird will continue its cycle until the head dries out, and evaporation can no longer cool it.
In summary, the steps in the cycle are as follows:
- The bird's head dips and gets wet.
- Water evaporates from the fuzzy head.
- The vapor in the bird's head condenses into liquid.
- Pressure in the bird's head is reduced because the liquid takes up less space than the vapor.
- Liquid moves up the tube into the low- pressure area in the head; the cycle repeats.
An interesting extension is to paint the bottom chamber of the bird black. An essential requirement to make the bird dip is to get the head cooler than the body. Normally this is accomplished by evaporation of water from the head. By painting the body black and exposing the bird to a hot lamp or to sunlight, the body will become warmer than the head. In this way, you can either enhance the normal operation of the duck, or get it to operate without wetting the head at all.
Don rathjen has measured the power output of a dipping bird by attaching it to a windlass and using it to raise paper clips. He managed to extract a nanohorsepower of work from his dipping duck. (A nano-horsepower is about a microwatt.)
References
1. Mentzer, Robert, "The Drinking Bird - The Little Heat Engine That Could," The Physics Teacher, February 1993.
2. Bent, Harry, and Harold Teague, "The Hydro-Thermal-Dynamical Duck; A Sketch of His Uses in the Classroom and the Laboratory," Journal of College Science Teaching, September 1978.
3. Rathjen, Don, "Duckpower," Exploring, Winter 1994, pp. 7 - 8. ("Duckpower" shows how to use the dipping bird as a heat engine to lift a weight, and discusses the work and the horsepower involved. Exploring is the quarterly magazine of the Exploratorium.)
Posted in: Science Snacks
