Dippy Bird

Figure 1: General Setup of the Dippy Bird

Materials:

  1. Dippy bird and stand (blue liquid bird)
  2. Yellow felt (to put beaker to the proper height)
  3. 250 mL beaker, filled to ~1 cm below top, with water
  4. Camera for large classes
  5. Eyedropper to get head wet to begin motion
  6. Towel for cleanup

Instructions:

  1. Fill the beaker with water, douse the bird’s head with water until the red felt is wet. Water should not be dripping down onto the bottom of the bird; if it is, dry with a towel.
  2. Set the bird such that when it dips down, the beak is submerged. If the whole head submerges or it gets so wet that water drips down off of the head and onto the body, you must lower the height of the water.
  3. Leave it alone!
    1. Works best when the head is cooler than the bulb at the bottom (if it isn’t working, try blowing on the head so convection currents cool down the head).
    2. Should go indefinitely if water in the beaker is replenished.

Explanation:

The dipping bird models a heat engine, which turns a difference in temperature into a cycle. For the dipping bird, that temperature difference is measured between its head, which becomes cooled by evaporation, and the bulb at the bottom, which is maintained at room temperature.

Before the demonstration is started, the dipping bird remains still because temperature is uniform throughout the bird, so the vapor pressure at its head is equivalent to that of the bottom.

Wetting the red felt starts off the cycle, since that water then evaporates and cools the vapor inside the bird’s head. When the temperature falls, it lowers the pressure at the top. As the vapor condenses into liquid, the reduced pressure at the head becomes much lower than the vapor pressure at the bottom of the bird, which forces liquid from the bulb at the bottom to go to its head. With the rise in liquid, the bird’s center of gravity then shifts higher, where the beak then dips into the water from the beaker. This creates the dipping motion of the bird as shown in the demo. With the bird’s head absorbing more water, it replaces the water that has been evaporated previously. This starts off the cycle once again, until the head fully dries out and evaporation can no longer cool it.

Notes:

Test beforehand to get a feel for the demo! It can be finicky so make sure to practice before showing to a large lecture hall.