Bernoulli Effect

Author(s): Gertrude Cole, George Lyle
Discrepant Event - Teacher's Guide
SED 695B; Fall 2005
Detailed Explanation of Discrepant Event

Principles illustrated

• Bernoulli Effect
• Pressure

The Bernoulli effect is the reduction of air pressure in a moving fluid. In this demonstration, air is blown over a strip of paper held just below the lips.

The moving airstream above the paper has a slightly lower pressure than the static atmosphere. The static air below the paper has a higher pressure and tends to move toward the area of low pressure. The paper, being in the way, is moved upward by the higher pressure static air.

Standards

Physical Science Standards, 8th Grade: Forces

2. Unbalanced forces cause changes in velocity. As a basis for understanding this concept:

 a. Students know a force has both direction and magnitude. b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

Questioning Script

Prior knowledge & experience:
Student has seen paper and fabric being blown by the wind

Root question:
What will happen if air is blown over a piece of paper?

Expected Response:
The paper will be blown away from the airstream

Target response:
The paper rises toward the airstream

Common Misconceptions:
Moving air has higher pressure than stationary air because of its kinetic energy.

Prior knowledge & experience:
Student has seen steam being forced from a teakettle or pressure cooker and water being forced from a hose.

Root question:
Why do fluids move from one place to another?

Expected Response:
Fluids move from one place to another because of the momentum of the fluid

Target response:
Fluids move from areas of high pressure to areas of low pressure.

Common Misconceptions:
Many students believe that fluids move from one place to another because of the inertial and momentum of the fluid.

While this demonstration can be done with a full 8-1/2 x 11 inch sheet of paper, it is better done with a strip of paper 6-8cm wide. This paper requires less blowing effort to produce dramatic results.

The demo is best done against a dark background facing sideways to the class. Simply hold the end of the strip between your finger and the indentation between your chin and lower lip. Blow moderately straight out.

As with all demonstrations, it is a good idea to practice this before presenting it to students.