- Introduction to Basic Hydrostatic
Concepts
- Linking Hydrostatics and Air Pressure:
Siphons
- The Idea of Suction and the Existence
of a Vacuum
- Closure and Applications
Click for teacher notes and lesson plans.
BASIC HYDROSTATIC CONCEPTS
Historically, the comparison of the atmosphere to the ocean is
key to the understanding of air pressure. Many of the stumbling
blocks to making the analogy of air to water is that air was
not conceived to have mass or weight. A volume of air or water
in its natural state being surrounded by itself was not thought
to have weight or be able to exert pressure. People could NOT
SENSE the weight or pressure, so it was not thought to exist.
Fluids exert pressure uniformly on all surfaces at the same depth.
You don't feel the pressure because it is pushing on you equally
on all sides. It was also hard to see that the air was different
than water in that it could be compressed, whereas water basically
is NOT compressible. Hence, the density of water is constant
as depth changes. This is NOT true for air. Pressure is equal
to force divided by surface area: P = F/sa. In a liquid, the
pressure is equal to the height of the column times the density:
P = h x d
Observations of water flowing out of holes in plastic containers
Students to observe the flow of water out of various plastic
containers that have been punctured near the bottom. (use a hot
nail) The water spouts are different lengths and decrease in
length as water flows out of container. Use student observations
to discuss the cause of different spout lengths. (Ideas such
as pressure, weight, density, and force should be discussed)
This exploratory activity is ideal for allowing small group interaction
and discussion. Students should be encouraged to draw diagrams
or models and describe the forces involved in causing the observations
that they make. They should also be making hypotheses and inferences
to find help find FOCUS QUESTIONS for the next step. The Teacher
can help by recording observations and explanations on the board
or large pieces of paper to be saved and referred to later.
The two basic ideas that this activity should bring to the
surface is the relationship between depth and pressure in a
fluid. The volume of water is NOT a factor. It should also
focus on the idea of pressure being equal in all directions.
The students should be encouraged to work in groups to select
a focus question and method of gathering information to provide
support for their ideas. Groups should be asked to present
their ideas to the class. The Teacher again serves as a supervisor
in helping students design experiments and moderator in discussions.
Introduction to basic science concepts and or historical ideas
can be related to the findings of the class after THEY have
expressed THEIR ideas.
Based on their input, students can investigate a variety
of things, such as:
- The relationship between the length of the spouts and the
height of the column of water to the hole in the side. (It
should be noted that the water will not flow at a constant
rate unless you continually add water to the system to replace
the water going out the hole)
- Determine if pressure depends on depth of the hole or the
total volume of the liquid. (You will need different shaped
containers with holes at the same height. Have students bring
in various types of bottles before this unit starts and try
to make the holes all the same diameter)
- Determine if the pressure in a fluid is equal on all sides
of a fluid at the same depth. Will water volume or rate drained
out of a hole on the bottom will be the same as a hole out
the side next to the bottom or on the opposite side.
Further demonstrations with drops of food coloring in oil or
the round shape of balloon may serve as a way to visualize the
equal pressure being exerted on the surface. You might also try
watching bubbles form in soda pop in tall bottles or graduates
and see how the size changes from small to large as they rise
to the top. This will help them to see how pressure decreases
as the bubble rises and allows it to expand. Try blowing up a
balloon that is sealed to a tube at various depths in the swimming
pool. Will the balloon be harder to inflate in deeper water?
LINKING HYDROSTATICS AND AIR PRESSURE
The concept of pressure at equilibrium states for fluids was
worked out by Archimedes (250 B.C.) No significant work was done
until Simon Stevin (1575) published his hydrostatics treatises.
During this same period the work of Hero of Alexandria (100 A.D.)
was being republished and played apart in linking pressure in
water to pressure in air. These were used by Blaise Pascal and
others to develop an understanding of air pressure and the functioning
of the Torricellian Tube.
The basic explanation of the functioning of a siphon is that
the shortest arm of the siphon exerts the least force downwards.
The weight of the water in the arm is a force in opposition
to the force of air pressure pushing the water up. So, the
net force up is greater in the shorter arm of the tube. This
unbalanced force causes the water to flow up the shorter arm
and out the longer arm.
Students will have problems visualizing the air as being
a force pushing on the surface of the water. They will have
difficulty in thinking about the ability of water to transmit
force throughout a container. The goal in this section is to
let them "mess around" with this simple device and
make observations and conclusions based on the information
THEY collect. The telling and modeling of "proper" science
ideas should wait until they have some experiences to relate
it to.
Practical experiments using a siphon
- Give students task of seeing which team can siphon water
from one container to another the fastest or slowest. Ask
them to describe the conditions and limits. When does it
not work? How many different ways can a siphon be started?
What factors control the rate of flow? Develop an explanation
that includes a diagram or model of the forces you perceive
acting on the water, tube, and containers. Compare student
explanations of how siphons work. Record theories and inferences
about the observations that have been made. Define controversial
ideas and models of how a siphon works.
- Ask students to predict what will happen if you try to
siphon out of a sealed container? Using the information generated
previously, students should work with a group/partner to
compare ideas and record a reason/hypothesis to explain their
prediction. Using two liter pop bottles with tubes sealed
through the top try getting the siphon to work. (use clay
to make a seal) Record observations and rework hypothesis.
- Assign an out of class activity of designing and building
a device that uses a siphon or principles of a siphon. Copies
of Hero's practical devices could serve as an idea starter.
Give the groups the task of describing how they think it works
or why it doesn't work and analyze each others systems and
explanations. This could be used as an assessment activity
where by students have to show general knowledge of the topic
to supply an analysis.
THE IDEA OF SUCTION AND THE EXISTENCE OF
A VACUUM
Students are to investigate the phenomenon of "suction," compressibility
and pressure of air in various settings. Up to now the idea that
air exerts pressure by the fact that it has mass and we are at
the bottom of an "ocean" of it, probably has only been
hinted at by your students. In trying to explain the siphon some
of them probably used air pressure as an idea to help explain
the observations they made. You've probably had to bite your
tongue to keep from just telling them about it. Try to hold on
if its not too sore, we are getting close to using this in our
explanation.
Experimenting with tubes, bottles and balloons
- The U Tube.
Using a U shaped flexible tube 150 centimeters long
and partially filled with water and colored with food
coloring: Students are asked to explore all the various
way to get the liquid in the tube to move, stay in balance
or be unbalanced.
Numerous varieties exist... Blow in one end and close
the other. Blow in both ends. Suck in both ends. Suck
in one end and close the other. Have contests of who
can exert the most pressure by sucking or blowing. Students
should be given the task record the set ups and supply
an explanation of the motion of liquid in the tube.
The point of this activity is to get students to think
in terms of the motion of the fluid being related to
the existence of balanced and unbalanced pressure in
the tube. Also, they will experience the creation of
a vacuum or "empty space". Students should
be encouraged to have FUN! while also being responsible
for recording observations and reflecting on finding
patterns or relationships between the movement of liquid
in the tube and forces being applied.
Things to observe in this activity...
- The HEIGHT of water in each arm of the tube is EQUAL
if:
- No pressure is applied to both ends.
- If equal pressure is applied to both ends.
- If equal suction is applied to both ends.
- Also...The HEIGHT of the water in each arm of the
tube is UNEQUAL if:
- Pressure is applied only to one side.
- If only one side is "sucked" on.
- Also...The column of water can defy gravity if you
blow it to one end and seal the arm of the tube and
stop blowing. The column will fill back down if you
break the seal of the arm with the water in it.
- Also...A vacuum or "empty space" can be
created if you blow the water into one arm, seal it
with your finger, and then suck on the other end. Try
pinching off the tube with a clamp to keep the vacuum
from collapsing. What's inside the empty space? What
causes the water to stay up? What causes the water
to rush out when you let go of the seal?
- Revisit the siphon and use air pressure to help
explain its function
Use the results and ideas of this activity to reflect
on the explanations for the functioning of the siphon.
Student should discuss the following questions and
rework their ideas about how the siphon works. Students
might publish a treatise on the function of siphons
recording and documenting their ideas and have a debate
about how they work.
Why doesn't a siphon work in a closed container?
When does a siphon not work? What causes the rates
or direction of flow to change? How does blowing and
sucking in a tube (inverted siphon) relate to the siphon.
In the experiments on hydrostatics we showed that water
exerts pressure relative to its depth; does air?
- The idea of suction.
The notion that suction is a force PULLING the
water UP will come out of this activity if it hasn't
already. Challenge students to find proof for this
idea. Many activities can be used to investigate
the idea of suction and air pressure. Here are
a few I like:
- Challenge students to a drinking race using
pop bottles with tops sealed with clay and straws
inserted. Also, use unsealed bottles, and straws
with pinholes in them.
- Show them a balloon that is inflated in a pop
bottle and supply them with a balloon, penny,
paperclip, straw, string, and bottle. Have groups
of students work to inflate the balloon in the
bottle.
- Use 2 liter pop bottles that have an extra
screw top opening glue at the bottom. Have students
explore ways to inflate a balloon inside the
pop bottle and record explanations of their findings.
This double or triple screw top system allows
for further combinations of blowing, sucking
and the inflation or deflation of balloons. You
are only limited by your students imagination.
- Demonstrate suction cups or dent pullers. What
creates the force that hold them down? How much
force/weight can the dent puller/ suction cup
hold? Is there a relationship to the size or
surface area?
- Try pulling a plastic/playtex glove out of
a gallon mason jar that has been sealed to the
opening with duct tape. Reverse the situation
using a glove that has been inflated outside
of the jar and sealed to the opening. Can't pull
it out and you can't push it in. Why????
- Crush popcans with air pressure by heating
up water in the can and inverting them in cold
water or make water go up an inverted testube
that has been heated. Ask students to describe
the forces involved in crushing the can or moving
the water. What does heating have to do with
it? Attach a balloon to the top of a pop bottle
that has been heated in a pan of boiling water
and allow to cool. Observe your results and provide
an explanation.
CREATING CLOSURE AND APPLYING CONCEPTS
Comparing a straw to a suction pump: What's
the limit?
By now you've pretty much exhausted the ideas necessary
to explain the motion of liquids in tubes and the
phenomenon related to suction and vacuums.
This would be a good point to start tying up
the loose ends and relate it to the historical
mystery of suction pumps and the debate about the
existence of vacuums. The question The Duke of
Tuscany posed to Galileo about the limit of a suction
pump could serve as point of departure to discuss
ideas about matter and the existence of vacuums
and the role of air in explaining the phenomenon.
Basically, a suction pump work just like a straw.
If you have access to a stairwell or vertical space
of 35 feet or more. Students can experiment with
the limits of a straw. Using a flexible tube students
can try "lifting" water to the highest
height. Is there a limit? What causes the limit?
If more suction force could be applied could we
get the water higher? Is it a problem with the
materials or does it violate some kind of Natural
Law? Does "Nature Abhor a Vacuum?" Is
there a subtle matter that is able to move through
the pores in the tube to fill the "empty space?"
You could also create a Torricellian Tube using
water instead of mercury. The limit to the suction
pump and the height to which water will extend
in a closed vertical tube is about 34 feet. This
being due to the force of the weight air on the
surface of the liquid is equal to the force of
the weight of the column of water in the tube.
All you have to do is find a way to fill the tube.
Seal one end and lift the sealed end to the top.
Meanwhile, keep the other end in the water. An
empty space or vacuum should form at the top. Students
could be asked to calculate the volume and mass
of water in the tube and create a measuring scale
to observe the changes in air pressure. Using the
idea of balanced forces you should be able to estimate
the force or weight of the column of air above
the surface of the container of water (14.7 lbs/sq.in.).
If you can leave the tube up, you could use it
in Earth Science class to measure changes in air
pressure as long as it is in a place where temperature
is constant.
The mystery of the snorkel
As a cumulative activity you can have your students
wrestle with this Question??? Skin divers snorkel
with a twelve inch Snorkel to breath through. Why
aren't snorkels longer? How long can a snorkel be?
If there is a limit..What limits the length? Experiment
with students trying to breath through a snorkel
made of plastic tubing of varying lengths in the
swimming pool. Supply an explanation for your observations.
Try it!
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