Instructor Outline:
Pressure and Fluid Flow:
Pascal and Bernoulli
UM Physics Demo Lab 07/2013
Lab length: 70 minutes
Lab objective: To demonstrate the concepts of pressure, compressible and
incompressible fluids, hydrostatic pressure of a fluid column, continuity (mass
conservation) in fluid flow, Pascal’s Principle, Pascal’s Law, gauge pressure, work and
energy in a flowing fluid and Bernoulli effects due to the inverse relationship between
flow speed and pressure in a moving fluid.
Materials
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Bernoulli Tanks/Tubs
aluminum rulers for tubs
nylon plug (tank valves)
Plastic Tubs – about 20” x 15” x 6”
plastic storage box ramp supports (to aid in draining tubs)
Homer buckets (to drain tubs)
large Tygon drain tubes
6’ Tygon Siphon tubes
2’ Tygon tubes
squeeze bulbs
veterinary syringes
plastic drinking cups
plastic soda straws
plastic coke bottles
eye-droppers
½” cut steel washers
hair dryers
Shared Components:
table tennis balls
4 power strips
clear plastic rulers
scrap paper
calculators
2 sponge mops
sponges
rags
Optional demonstrations at instructor’s request:
2B30.30 Magdeburg Hemispheres
2B30.15 Pop Can Collapse
2C20.15-1 Venturi Tubes
2C20.z -Fluid Dynamics Kit ("Bernoulli")
 Bottle of soda to simulate the rapid decompression of a scuba diver
Introduction: 5 minutes – Lecture
Pressure is defined.
Exploration Stage: 15 minutes – Group Lab Work
The students explore atmospheric pressure and study how pressure differences can
support a fluid column and drive fluid flow. Next they explore the difference between
compressible air and incompressible water. Finally they demonstrate continuity (mass
conservation) for water flowing out of the tank.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
Analysis Stage: 5 minutes – lecture
Atmospheric pressure is defined and quantified. Large forces due to atmospheric pressure
are demonstrated by passing around the evacuated Magdeburg spheres. Pascal’s Principle
and Pascal’s Law are introduced. Continuity is formally defined.
Application Stage: 10 minutes – group lab-work
The students construct a Cartesian diver and explain its operation using the concepts of
pressure, density, compressibility and Pascal’s Principle.
Analysis Stage: 5 minutes – Lecture
The concepts of work and energy introduced in the context of a flowing fluid, stressing
that the application of the Work-Energy Theorem to a drop of flowing fluid results in
Bernoulli’s equation. The inverse relationship between fluid speed and pressure dictated
by Bernoulli’s equation is introduced and demonstrated.
Exploration Stage: 20 minutes – Group Lab Work
The students next explore the consequences of energy conservation in a flowing fluid, first
by comparing the motion of a horizontal stream of fluid to the motion of a horizontally
launched projectile and then by considering energy conservation to relate the exit speed
of the stream to the column height in the tank. The exit speed for fluid in a siphon is
correlated with the column height driving the siphon flow. They discover that the depth
the siphon inlet is submerged below the water surface does not affect the exit speed for
the siphon.
Application Stage: 5 minutes – Group Lab-Work
The students trap a table tennis ball in the flow column from a hair drier and explain the
effect using the inverse pressure-speed result from Bernoulli’s equation and the concept of
force as the product of a pressure difference and the cross-sectional area of an object.
Analysis and Summary Stage: 5 minutes – Lecture
The results from the siphon experiment are reviewed and the work-energy relationships
determining the observed flow behavior are discussed.
Concepts developed:
1. Definition of Pressure
2. Compressible and incompressible fluids
3. Mass conservation (continuity) during incompressible fluid flow.
4. Pascal’s Principle: Pressure is transmitted uniformly throughout a fluid and to the
vessel walls.
5. Pascal’s Law: The additional pressure due to a fluid column of height h is given by
P   gh .
6. Gauge pressure is the excess pressure relative to atmospheric pressure. The
gauge pressure at a depth h in a fluid is therefore Pgauge   gh
7. Application of the Work-Energy Theorem to a moving fluid yields Bernoulli’s
equation.
8. The exit speed for fluid from a hole or siphon is dictated by energy conservation
and depends on the height of the fluid level above the exit hole.
9. For a fluid flowing in a level pipe, Bernoulli’s equation, and hence energy
conservation, dictates that pressure decreases as speed increases.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109