student conceptions of the bernoulli principle: a particle...
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Student Conceptions of the Bernoulli Principle: A Particle Approach
Katherine Misaiko and James Vesenka Department of Chemistry and Physics
University of New England, Biddeford, Maine 04005
Objec&ves We seek to iden+fy life science students misconcep+ons of the Bernoulli principle. Results are used to inform instruc+on and develop lab interven+ons.
Hypothesis A par+cle model, based on simple kine+c theory, is an important tool for conceptual understanding of fluid dynamics and Bernoulli’s principle.
Interview Process Students are asked to: -‐ Make a predic+on about an experiment, observe
then explain. -‐ Two experiments involving pressure change,
either as a func+on of temperature or speed, are examined
Interviews: Typical Groups During the pre-‐interview: -‐ Mathema+cally weak students could not draw
the graph of an inverse rela+onship
-‐ Incorrectly used the ideal gas law -‐ Made no men+on of a vacuum By the post interview:
-‐ Some students transi+oned to using a par+cle model
-‐ Changed explana+on from pulling the water up the tube to pushing the water from the other side
-‐ Stated that the air molecules “move around the vacuum”
Weak Groups During the pre-‐interview:
-‐ Areas of low concentra+on move to areas of higher concentra+on
-‐ Vacuum “sucks” the air up as opposed to pushing from the other side
-‐ Pressure = Density
By the post interview: -‐ Required promp+ng to use a par+cle model
-‐ Clueless about inverse rela+onship
Most Common Misconcep&ons 1. Lack of ability to employ par+cle models 2. Par+cles move from areas of low concentra+on to
areas of high concentra+on 3. Behavior of molecules verses the number of
molecules 4. Concept of a vacuum 5. Pulling versus pushing
Sta&s&cs Pressure vs. Temperature
Decreased temperature reduces molecular ac+vity, therefore decreasing the pressure on the leV side. The right side is s+ll at atmospheric pressure and has greater molecular ac+vity. The higher pressure will push the water up the leV side of the tube in order to equilibrate.
Pressure vs Velocity
A stream of air is blown across the opening of the end of the tube crea+ng a par+al vacuum, decreasing the pressure in the leV side of the tube. The right side is s+ll at atmospheric pressure and will push the water up the leV side of the tube in order to equilibrate.
The Venturi Tube
-‐ Conserva+on of mass -‐ Greater area à slower speed à higher pressure -‐ Smaller area à faster speed à lower pressure
-‐ The faster the speed the lower the pressure and therefore the lower the fluid will be pushed up into the ports. There is less chance for water molecules to interact with the opening of the ports.
Related Research Recktenwald et al. iden+fied two common misconcep+ons with engineering students associated with the Bernoulli equa+on: “fluid pressure must always decrease in the direc+on of flow,” and “the Bernoulli equa+on can always be applied.”1
Conclusions -‐ A par+cle approach improves predic+ons and
understanding of Bernoulli Principle
-‐ When par+cle models are not men+oned, students ability to predict and explain Bernoulli Principle decreased.
Acknowledgements This research has been supported by NSF DUE 0737458 and 1044154
grants to JV. Reference
Recktenwald et al., “A Simple Experiment to Expose Misconcep+ons about the Bernoulli Equa+on”, Proceedings of IMECE 2009, November 13-‐19, 2009 IMECE2009-‐10964
Interven&ons -‐ Quan+ta+ve lab ac+vity. Plot P vs. v to show that
pressure was dependent on the square of speed with slope of half the density of air.
-‐ Conceptual hard sphere par+cle model or beach ball model.
P vs. T
P vs. v
Unequal Sides (Pressure)
Equal Sides (Pressure)
Hard Sphere Model
Radius “r”
Barometer pressure
“P” vout = 0
vin
!
Strong Groups -‐ Correct predic+ons of all experiments -‐ Unprompted use of par+cle model both pre and
post
-‐ Good understanding of how par+cles interact with each other and at the water level
-‐ Strong mathema+cal skills
-‐ Proper use of Ideal Gas Law
ΔP =−12ρΔ(v2)