PURPOSE: To stimulate discussion of Bernoulli's principle and common misconceptions surrounding its application.

DESCRIPTION:A water reservoir has four chimneys, each topped with a different surface. A stream of air moving over the concave flange (at left, above and below) causes the water level to go down. This is the result of the centripital force required to make the air move in a curved path along the flange. Air moving over the convex flange (left center) causes the water level to rise. This is a result of the Coanda effect, wherein the airflow "sticks" to the surface with the corresponding reaction force causing a decrease in the air pressure in the tube. Moving air flowing over the flat flange (at right) has no noticeable effect. Moving air flowing over a naked end of the the tube results in a lower pressure, causing the water to rise in the tube.

Click your mouse on the appropriate photograph to see a video of the effect.

We would like to thank John Welch of Cabrillo College (Aptos, CA) for creating this demonstration and encouraging us to duplicate it. Click for photograph of apparatus by John Welch. Further information about the apparatus built by John Welch will be found on his web page, Bernoulli Effect myths.

SUGGESTIONS:See Question of the Week #231 for suggestions on using this demonstration to enhance class involvement.

REFERENCES: None

EQUIPMENT: "Bernoulli" demonstrator and hair dryer.

SETUP TIME: None.