The Bernoulli Effect - Conceptually

by Earl Zwicker

This lesson was created as a part of the SMART website and is hosted by the Illinois Institute of Technology



This is a lesson plan for teachers.

 

Objectives:

Simply doing and/or observing the experiments (below) should excite the intellectual curiosity for people of all ages. Here is a rough guideline for what might be expected.

Grades K-5: Students should be able to describe at least two of the experiments orally and –- subject to your judgment –- in writing.

Grades 6 –9: Expect students to describe two or more experiments both orally and in writing, and to explain their observations in terms of simply    stated Bernoulli Effect:

Where the velocity is high, the pressure is low, and 

where the velocity is low, the pressure is high.

Grades 10-12: Students should explain observations conceptually and mathematically beginning with an algebraic statement of the Bernoulli Effect.

Materials:

Students:
Experiment 1
These items are usually available at grocery stores. Experiment 2 Teachers:
Experiment 3 Experiment 4 Experiment 5

Strategy:


The main idea is to do several different experiments, each time connecting specific observations with the simply stated Bernoulli Effect. This enables students to make the connection in their minds from a variety of concrete, specific observations to the abstract generalization known as our simply stated Bernoulli Effect.

Begin with Experiment 1 materials at hand placed on or near a table at the front of the room.  Hold up one of the small cups and tell your students that you want to show them something, and you would like them try it. Fill the cup nearly full of water by dipping it into the pail. Add several drops of vegetable dye (red, blue, green work well) and stir with a half straw. Hold the cup level with one hand, and - with the fingers of the same hand - hold a half straw vertically so that its bottom end is immersed in the water and its top end is about 1.5 inches above the water’s surface. Using your other hand, hold another half straw to your lips so that you can blow through it. Hold the cup so that you can blow a stream of air perpendicular to and across the top of the immersed half-straw. Stand sideways to your student audience and then blow strongly across the straw.

Water will rise up to the top of the immersed straw where it will be broken up into a spray of droplets by the air stream. You have made an “atomizer”, somewhat like the rubber bulb operated perfume bottles of years ago. You probably will have to practice a bit to make it work and you should practice in private before you show your students. (You need to have confidence in what you are doing.)

Now invite the students up to each get a cup, etc. so they can make their own atomizers. You probably will want to have the sponge and paper towels handy. Small cups limit the amount of water to sponge up when accidents occur.

After most students have made their atomizers work, get their attention, and ask, “What happens to the water in the immersed straw when you blow across the top?

Almost certainly a student will respond that the water rises up in the straw. For those who are not sure, repeat the experiment with your atomizer, and point out to them how the water in your straw rises up as you blow across its top. (If you have dyed the water in your cup dark enough, it is easy for students to see.)

Now you should ask them why this happens. After getting their input and discussion (which should be given your rapt attention), print on the board the simply stated Bernoulli Effect. Point out that it applies to fluids such as air and water, and then connect any of their statements that may be pertinent to the simply stated Bernoulli Effect. When you do this, illustrate the places of lower and higher (atmospheric) pressure with a simple sketch on the board that includes the air stream. Use a large arrow or two pushing down on the surface of the water in the cup, and a smaller arrow pushing down at the top of the straw. The speed of the air over the water in the cup is low (zero), so the pressure is high (atmospheric), while the speed of the air over the top of the straw is high, so the pressure there is low (less than atmospheric).

It’s a good idea to ask your students to come forward to dump the water from their cups into the pail. You probably will want them to keep their cups and straws to take home to show to parents and siblings.

You might ask if they can think of any practical applications of the atomizer besides perfume sprayers. After their input it may be good for you to point out that the carburetors in gasoline engines work exactly the same way to atomize liquid gasoline and mix it with air to form the vapor that is taken into the cylinders and ignited. The expanding gases then act on the pistons to make the car go. Can you think of others?

Experiment 3 probably is a good one to do next, so clear any clutter from the table at the front of the room and have the materials for this experiment on the table.

With your student audience facing the front of a table, stand at one side so they will be able to see clearly what you are doing.

Place a dime in the clear glass container. Hold it up and shake it around so everyone can hear the clinking of the dime against the glass. Then dump the dime out into your hand, hold it up for all to see, and place it on the table in front of you. Usually 6 to 10 cm from the edge of the table works well.

Now challenge your students. Ask if anyone can move the dime into the beaker without touching the dime with any object or moving the table. After getting their input, you are ready to show them how to do it.

With the beaker on the table behind the dime (from your perspective), tip the beaker so that the bottom edge of its opening is 1 to 2 cm above the table surface and about 6 to 10 cm behind the dime.

Place your lips just above the edge of the table, in front of the dime (you probably will have to kneel on the floor to do this) and then blow a strong puff of air across the top of the dime. It will jump up into the air stream and travel with it into the beaker with a clinking sound. (Practice until you can do it! If it is difficult to kneel, consider using any kind of elevated surface on the table top, like a cardboard box that will be a convenient height for you. When you do this in front of your students, if it doesn’t work the first try to two, point out to them that it is tricky to do, and continue until it works. If your students get restless, invite a volunteer up to try.)

Hold the beaker up for everyone to see and shake it so that they can hear the dime clinking on the glass.

Repeat the experiment once or twice more. This is always important; observers need to confirm their first observation and make certain of what they observed. If time permits you may wish to let them try with their own cups and dimes or you might suggest they try this at home with the cups you have given them.

As you did with the atomizer experiment, it is important to help students connect their concrete observations with the abstract, simply stated Bernoulli Effect. Always use a simple sketch at the board. Show the dime on the table (leave a narrow space between dime and table), the puff of air over its top and the tipped beaker behind it. Ask “Where is the pressure low?” Perhaps the students will identify the region over the top of the dime. But the answer to “Where is the pressure high?” (under the dime) probably will require some explanation from you. You may explain that layers of air, even though only hundreds of molecules thick, exist on all surfaces that we can see and they form a thin layer on the bottom of the dime and the table. They are part of the atmosphere and are at atmospheric pressure. Since the speed of the air under the dime is low (zero), the pressure there is high (atmospheric). But the speed of air at the top of the dime is high, so the pressure is low (less than atmospheric). Drawing a large arrow acting up on the dime and a small arrow acting down will help to show that the dime will be raised up by the pressure difference. This sketch forms a picture in the student’s mind to help make connections between the concrete and the abstract.

Next, clear the table and place the materials for Experiment 4 near the rear edge of the table.

Hold up the various materials and name them so the students know what they are. Place the paper clip flat on the table and straighten one segment so that it points straight up. Then use it to poke a hole through the center of the 2 in. ´ 2 in. card. With the straightened segment sticking up through the card, apply a short piece of sticky tape to attach the paper clip to the card from below.

With the card lying flat on the table, place the thread spool on top of the card with the hole along its vertical axis enclosing the vertical part of the paper clip. Now blow strongly and steadily through the hole at the top of the spool, and raise the spool up off the table while doing so. The card will stick to its bottom end until you stop blowing, and then it will fall off! Do this several times. Ask the students why this happens. (You may want to explain that the purpose of the paper clip is to keep the card from slipping sideways out of the air stream at the bottom of the spool.) In order to convince them that you are not sucking air up through the hole instead of blowing air down through it, you can puff your cheeks out as you blow. Then point out that you could not do this if you were sucking air up. Ask them to try sucking air into their lungs with their cheeks puffed out, and they will understand.

In order to dispel any remaining skepticism, holding a piece of yarn in the air stream may be convincing.

Ask them to explain why the card sticks when you blow into the spool. After listening carefully, make a sketch on the board, showing the spool with the card below and a narrow space between. Draw arrows showing the path of the air from the top of the spool to the card below. When the air strikes the card below, it changes its direction and moves radially outward (horizontally) in the space between the spool and the card. The pressure is lower (below atmospheric - a small arrow pushing down) where the speed of the air over the top of the card is high, and pressure is higher (atmospheric - a large arrow pushing up) where the speed of the air is low (zero). The pressure difference supports the card against its weight, which pulls down, so the card “sticks” to the spool.

If you are able to provide enough spools, etc., and if time allows, would it be a good idea to ask your students to do this experiment? Would it be a good idea to let them discover for themselves that the card sticks when they blow air through their spool?

Experiment 5 is next, using the leaf blower. You will want the yarn within easy reach, perhaps in a pocket, and the ball nearby on the floor. What follows may well be done on a tabletop if you are tall enough. I have always done it on the floor in front of the table. And of course, you should practice this so you know exactly what you are going to do and that it works.

With the leaf blower plugged in and resting on the floor, hold it by its tube and point it so it will blow air vertically straight up. Then turn it on (at its lowest speed). Place the ball in the air stream and release it so that the ball “floats.” Steadily increase the speed of the air to its maximum, floating the ball to a higher altitude above the opening of the air tube. Then decrease the speed to a minimum, so the ball descends to its lowest altitude again. Turn off the blower and ask, “What held the ball up?”

Most students will give the obvious answer that the force of the air stream held it up against its weight pulling down. To which you may respond, “Let’s see.”

Again, place the ball in the vertical air stream at its lowest speed. Gradually increase the speed to its maximum. Now gradually tilt the blower from the vertical toward the horizontal. Be sure you are aiming the blower parallel to the front of the table, perpendicular view of the students. (Don’t blow air at the students.) When you have reached an angle of perhaps 60 degrees from the vertical (prior experimentation will help you determine how great you may make the angle before the ball falls to the floor), stop and allow the students a few seconds to observe that the ball is still suspended in the air stream, then move the tube with air stream and floating ball back to the vertical, lower the air speed to minimum and shut off the blower.

You may now ask, “What held the ball up? If the air stream were just pushing on the ball when it is at an angle, why wouldn’t the ball be pushed away and out of the air stream so it would fall to the floor?”

After getting student responses, make a sketch on the board showing the blower and air stream at an angle and the ball suspended in the stream. Draw an arrow to show the ball’s weight pulling down and label it with the letter W.

Next, repeat the experiment with the ball suspended at an angle in the air stream. Now hold the piece yarn (about 8 to 12 inches long) in your other hand and move your hand so that the yarn is at the top of the ball. The air will blow the yarn so that it stretches out to show the direction of the stream. Now move your hand to follow behind the ball and directly below it. The yarn will hang limp because the air stream below the ball is not strong enough to support it. Do this several times, top of ball to just under the ball. It will become evident that the speed of the air over the top of the ball is great, while its speed across the bottom of the ball is small.

Restore the system to rest, as in two paragraphs above. Make another sketch of the ball suspended in the air stream but this time locate the ball lower in the air stream, so its bottom actually lies below the line depicting the lower boundary of the stream. Ask, “Where is the air speed high?” And when the students tell you at the top of the ball, draw an arrow in the direction of airflow at the top (which is at about 60 degrees from the vertical) and label it “high speed.” Ask, “Where is the air speed low?” -- and draw a very small arrow at the bottom of the ball. Then draw a very small arrow (preferably in a different color, and pointing downward and perpendicular to the air stream) to show the force from small pressure at the top. Label it P-top. Then draw a larger arrow (same color, pointing upward and perpendicular to the air stream, labeled P-bottom) to show the larger pressure at the bottom. Point out the agreement with the simply stated Bernoulli Effect.

The directions of the force arrows, W (straight down), P-bottom and P-top should serve to satisfy students that the net force due to pressure difference serves to suspend the ball and to hold it in the air stream.

In order to further show this, suspend the ball in a vertical, maximum speed air stream again. Using three fingers, push sideways (horizontally) on the ball, displacing it from the center of the stream, but not so far that it falls. Move the ball back and forth several times this way, pointing out that the part of the ball at the edge of the air stream is subjected to higher pressure, so it is forced back toward the center of the air stream. This is the reason the ball remains suspended in the vertical air stream, despite the fact that it experiences some horizontal “jiggling” forces (due to air turbulence) that tend to force it out so it would fall.

Performance Assessment:

Give each student a sheet of 8.5 inch ´ 11 inch paper. Ask them to hold the sheet by the corners of its shorter edge and to hold the edge tautly and horizontal. Now challenge them to make the paper rise up without moving it or touching it with any object.

You probably will see a student blow across the top of his/her sheet, causing it to rise up. When most students have caught on, ask them to write an explanation on the paper, using the simply stated Bernoulli Effect.

Questions Students Ask

1. Why is pressure low where velocity is high?

2. Who was Bernoulli

3. When we talk, sing, or make sounds, is Bernoulli Effect involved?


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