Water Pressure

Introduction: (Initial Observation):

While watering the grass with a water hose, I tried to use my finger to stop the water a few times, but it didn’t work. The water rushing out of the hose had too much pressure. My friend told me that they don’t have this much pressure at their house. They live up the hill, just a few blocks away. I tried their water hose and could stop it by my hand. That showed me that water pressure is not the same everywhere. I was thinking about this for a while. First, I thought the diameter of the pipe may be a factor, but that wasn’t the case. I checked them out and they both looked the same size. Thinking about this experience raised many questions in my mind.

Now my questions are, why does water have pressure? Do all liquids have pressure? How does the water pressure change at different depths? How can we reduce or increase water pressure? Can we use water pressure to do something, such as lift a car, break a tree, or dig into the ground?

Information Gathering:

Water pressure is a subject of physics. Therefore, we can search books and web sites related to physics to find information about the science and theory of water pressure. If you search for water pressure, be sure to add the key word “physics” to your search string so that you will be able to find more related information. One of the interesting web sites that I found is a good simulation of water pressure, and can show you why water pressure varies in different areas.

Then I searched for “Industrial applications of water pressure”. I found many good web sites including those describing that water pressure can actually be used as a cutting tool. Waterjet cutting uses a jet of water so powerful that it cuts cleanly and precisely through material in a single pass without shredding or crushing.

About the amount of water pressure! (By the way, I’d like to build a device that measures water pressure at different depths.) I found that even though we do not feel it, 14.7 pounds (lb.) of AIR pressure are pushing down on our bodies as we rest at sea level. Our body compensates for this weight by pushing out with the same force.

Since water is much heavier than air, this pressure increases as we venture into the water. For every 33 feet down we travel, one more atmosphere (14.7 lb.) pushes down on us. For example, at 66 feet, the pressure equals 44.1 lb., and at 99 feet, the pressure equals 58.8 lb.

To travel into this high-pressure environment, we have to make some adjustments. Humans can travel three or four atmospheres and be okay. To go farther, submarines are needed.

Animals that live in this watery environment undergo large pressure changes in short amounts of time. Sperm whales make hour-long dives 7,380 feet (2,250 meters) down. This is a pressure change of more than 223 atmospheres! By studying and understanding how these animals are able to withstand great pressure changes, scientists will be able to build better tools for humans to make such journeys.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Variables for question number 3 can be defined as follows:

The Independent variable (also known as manipulated variable) is the depth.

The dependent variable (also known as responding variable) is the water pressure.

Constants are the type of water, the temperature and experiment method.

Hypothesis:

Since I have so many questions, I also have so many hypothesis!, basically one hypothesis for each question. My hypotheses are as follows:

1. I think pressure of water is the result of its weight.
2. I think other liquids also must have pressure that varies depending on their specific gravity. (So in a space station there is no water pressure because there is no gravity).
3. Water pressure increases by increase in depth. I think water pressure is more at a deeper depth because of the weight of water above.
4. I think we can change the pressure by changing the water level. Like the simulation program.
5. I think if we have proper equipment, we can use water pressure to lift a car or do anything that requires lots of force.

Experiment Design:

For each hypothesis I will design an experiment to test it. But I first need a tool to measure the water pressure. The device that measures water pressure is called pressure gauge or manometer. Manometers can be purchased from plumbing suppliers, but commercial manometers sold in stores are designed for higher pressures and will not help us with our tests. As a quick solution we will build our own monometer.

How to build a manometer?

The monometer that I will make is a U shaped glass tube with some colored water in it. A flexible plastic tube will be attached to one end of the glass tube. The other end of the plastic tube will be a piece of glass tube that I use as a probe to test water pressure.
When there is no pressure, colored water on both sides of the U will be at the same level.

Any pressure will change the balance and cause the colored water to go up in one side and come down on the other side. The height of the U glass above is about 26 cm. The length of the plastic tube is about 50 cm, and the height of the glass tube is about 17cm. Instead of the glass tube, you can use a hard plastic tube or even a flexible plastic tube. It will just take a little more work to secure it and fix it in a straight line. In this manometer, the distance between the water level on the two sides of the U is the pressure. In the picture the pressure of 8 grams per square centimeter is shown.

Experiment 1:

The purpose of this experiment is to see if water weight is causing water pressure.

Water has a weight. It’s specific gravity is 1. In other words, one cubic centimeter of water weighs one gram. If the weight of water is the cause of water pressure, another liquid that is 10% heavier or 10% lighter than water should show a pressure that is 10% higher or 10% lower at the exact same conditions.

We can change the specific gravity or the weight of certain volumes of water by mixing it with a heavy salt, but for this test we will use another liquid such as Isopropyl alcohol that has a specific gravity of 0.78.

Procedure:

Fill up a large cup or a bottle with water. Use your manometer to read the pressure at the depth of 10 cm. Record the result. Next, fill up another cup with Isopropyl alcohol and again measure the pressure at the depth of 10 cm and record the results. If this is the weight of liquid that causes pressure, the pressure of Isopropyl alcohol must be 78% of water pressure at the same depth.

Record your results in a table like this:

Experiment 2:

The purpose of this experiment is to see if all liquids have pressure.

Procedure:

Collect samples of different liquids such as water, Isopropyl alcohol, mineral oil, glycerin, Ethanol (Ethyl alcohol) and test their pressure at a certain depth such as 10 cm. Notice that you don’t have to have large samples of these liquids for testing. Instead of cups or bottles, you can have your samples in test tubes.

Record the results in a table like this:

Experiment 3:

The purpose of this experiment is to see how the pressure of water changes at different depths.

Procedure : (the effect of depth)

1. Insert the probe of your nanometer in a glass of water.
2. Move it to the left and right. Does the nanometer show any change in pressure?
3. Move the probe up and down. Does the manometer show any change in pressure?
4. Hold the manometer at 10 different depths and record the pressure for each depth. Record your observations on a table like this:

Simple Experiment:

The purpose of this experiment is to see if water pressure varies in different depths.

Procedure:

Attach a longer flexible tube (about 1 meter) to the manometer so that you could test the pressure at different depths in a bucket of water or in a pool. Keep the manometer probe or tube faced down and take it to different depths while reading the pressure.

Is water pressure different in different depths?

A simpler procedure for this experiment is this one:

Put the end of the straw just below the surface of the water in the jar and blow. Now put the end of the straw near the bottom of the jar and blow again.

Which way is harder to blow bubbles?

In this picture we are using an aneroid manometer to test the water pressure at different depths. Water is not used in the construction of aneroid manometers, so they can be used at almost any condition to test the water pressure. They will even work in low gravity conditions.

Experiment 4: (Similar to the experiment 3)

The purpose of this experiment is to see how can we increase or decrease the water pressure at a certain location?

Variables:

Independent variable (manipulated variable) is the water level above any point. (The point where we measure water pressure).

Dependent variable (responding variable) is the water pressure at that point.

Materials:

1. A manometer
2. A graduated cylinder or any other cylindrical container
3. A metric ruler stick that measures centimeters
4. Water

Procedure:

Place the sensor or probe of the manometer at the bottom of the cylindrical container. Use a piece of tape, a clip or a heavy object to hold the probe at the bottom of the container. (Note that in a home made manometer, the open end of the tube is the sensor or the probe of the manometer.

Insert the ruler in the cylindrical container so you can measure the height of the water. At this time the probe of the manometer (end of the tube) must be at the same level as 0 in your ruler.

Start to add water to the container to the height of 1 cm as shown by the ruler. Read and record the pressure.
Add more water so the water level will be at 2, 3, 4,…. 10 centimeters. At each water level observe and record the pressure.

Does increasing the water level increase the water pressure?

Report the results.

Experiment 5: (Try this, you will like it.)

The purpose of this project is to use water pressure to do something useful. Something more than taking a shower or washing the dishes. While searching about water pressure, I read somewhere that the normal water pressure in a town is 45 lbs. That means 45 pounds per square inch. A simple calculation shows that this pressure equals to 6480 lbs per square foot. That is double the weight of a mini van! So in theory, I should be able to lift a minivan by a bag of water about 1 cubic foot attached to a water hose. At this time I will do this experiment in a smaller scale to see how it works. Creating a pressure of 100 grams per square centimeter (100 g/s2) is easy. This is the pressure of a column of water, 100 centimeter tall. 100 g/s2 is about 1.4 lbs per square inch. So it is almost 30 times less than water pressure in our homes. If I be able to lift 100 lbs per square inch with this pressure, that means that lifting a car with the pressure of a water hose should not be a problem.

Variables for this experiment are as follows:

• Independent variable is the water pressure. Water pressure will be calculated based on the level of water in the input tube.
• Dependent variable is the weight lifted,
• Constants are the the pressure area and experiment method.

Procedure:

For this experiment you will need a roll of polyethylene tube. Polyethylene tubes come in roll and that is what companies use to make plastic bags. The tube that you get can be as low as 4 inches up to 20 inches wide. See what you can find. Local hardware store may be a good place to start. You will need about 2 to 5 meters (6 to 15 feet).

Seal one end of your tube and leave the other end open. If you don’t have access to a plastic sealer, just make a big knot to close one end. Or you can cover the end of your plastic tube with aluminum foil and use a hot iron to seal it. (Danger, Danger, Danger…. Don’t burn yourself, get help from adults. Adult supervision is a must).

The first simple test that you can do is to hang your plastic tube from a height of about 2 to 3 feet in a way that about one foot of the tube stays on the ground. Then start filling up the tube with water. When the water goes up the tube about 2 feet, touch the part of the inflated tube that is on the floor with the palm of your hand. Try to push it down. How much pressure do you need to push it down and make it flat? How much weight can this tube carry?

The plastic tube that you see in this picture is a 3MIL polyethylene tube, 4 inches wide.

For second test, put your plastic bag in a box and leave the open end out. Put another piece of cardboard or wood over the plastic bag and place a heavy object on the top of that. Start to add water from the open end. Soon your heavy object should start to rise while the plastic bag gets filled up with water. In this picture shows that we are using a metal can instead of the box, but the results are the same.

If you ever want to use this method to lift a car or any similar heavy object, you need to cover your plastic bag with a strong fabric bag. A plastic bag by itself does not resist high pressure and will burst. By covering it with a strong fabric, you can give it more strength.

The picture on the left is a simple drawing of using a larger plastic bag and inflating it with water pressure.
On the right, we filled up a one gallon plastic container and used it as a heavy object. Water pressure raised it 3 inches.

Control experiment can be a similar setup, but you do nothing with that. In other words you don’t add water, so it will not lift any weight. The purpose of having a control experiment is to show that no other external factor is causing our observation.

Other experiments related to water pressure: (For Display)

Make a Cartesian Diver:

Fill a two liter soda bottle with water. Take a medicine dropper and fill it partially, so that the dropper just sinks in the water inside the bottle. It will take some experimentation to find the right amount of water in the dropper. Then put the lid tightly onto the bottle. Squeeze the sides of the bottle. Notice that, as the water pressure inside the bottle is increased thereby, the air in the dropper is compressed, and the dropper falls. You might enjoy repeating the experiment with an oval-shaped bottle. The device is called a Cartesian Diver.

Make an atomizer:

Fill a bottle completely with water, and insert a plastic straw. Hold an inch or two of the straw out of the bottle. By tape or any method of your liking, secure the straw onto the bottle. Blow gently over the straw, either directly or by means of another straw. If possible, use an air pump to force air over the top of the straw. You will notice that water comes out of the bottle. The water is forced up and out of the straw because of the Bernoulli Principle. This mechanism is used in aerosol cans, in that a liquid is ejected from the reservoir by passing a gas over a tube at high speed.

Optional Experiment: Effect of gravity on water pressure

To test the effect of gravity, we need to take our test instruments to a place where there is no gravity or little gravity. In other words we should go to outer space. Since we cannot go to outer space, we came up with an alternate solution that will simulate a low gravity situation.

For example, we can do our test on a roller coaster. When a roller coaster is going down and we are going down with it, that is a simulation of near zero gravity. (Actually, gravity is everywhere. The fact that astronauts feel zero gravity is that they are constantly falling just like a roller coaster.) Also, when the roller coaster is going up, that simulates extra gravity. Your setup must be very small so you can take it with you on a roller coaster. If the gravity be a factor, when the roller coaster is coming down, the manometer must show less pressure than when it is going up.

I did this experiment and made a small setup, but instead of testing it on a roller coaster, I decided to hold it in my hands and quickly sit or quickly stand and see the results. Surprisingly, there was no change of pressure shown in my manometer while sitting or standing. So what do you think? What was the problem?

By looking at my manometer setup, I noticed that my manometer by itself is working based on the pressure of the liquid inside of it. Therefore, when the pressure drops, it drops both in my manometer and in my test liquid. That’s why they stay in balance. So for this test we need a manometer that does not work based on liquid pressure. We will not do this at this time because we do not have a manometer that does not use water pressure, and because the answer to this question can be concluded from our first experiment.

Materials and Equipment:

List of material can be extracted from the experiment section. The following is a partial list:

• A quart jar full of water
• Several two liter “pop bottles” and assorted plastic bottles
• Several “push pins” for making holes
• A medicine dropper for the cartesian diver
• A large clear funnel with a stand for holding it in place
• Several plastic straws and a supply of strong plastic tape or Duck Tape

Results of Experiment (Observation):

Write the results of your experiments as you perform the tests.

Summary of Results:

Water has weight, as anyone who has lifted a bucket of water knows. The pressure in the ocean, or any body of water, is proportionate to the depth below the surface. At the great depths of underwater exploration the pressure may be tremendous and any underwater vehicle, like a submarine, must be very strong in order to withstand the weight of the water.

Write more, this is just a start…

Conclusion:

What did you learn from your experiments? What are the answers to all of your questions about water pressure?

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

References:

List of References

http://www.seagrant.wisc.edu/madisonjason11/experiment_pressure.html

http://www.wwp.org/dockom/experiments.asp

http://www.mste.uiuc.edu/murphy/WaterTower/default.html

DOES WATER SEEK ITS OWN LEVEL?

PURPOSE: A trick to challenge the students.

DESCRIPTION:

The liquid level in the left side of the U-tube is higher than that in the right side of the U-tube. How does one explain this?

Two immiscible fluids of different densities, which are identical in physical appearance, are in the two ends of the U-tube. The point where they meet (which could be easily seen) is covered by the clamp which holds the U-tube.

PASCAL’S LAW – COILED TUBE PARADOX

PURPOSE: To illustrate Pascal’s law in a dramatic way.

DESCRIPTION:

Referring to the photograph on the right, pouring water (colored green) into the tube at the left in the photograph causes the asymmetric configuration shown due to the equalization of pressure in the central air bubble. Similarly, if one end of a three-turn loop of tubing is raised vertically, as in the photograph at the left above, water poured into the high end will never come out the bottom end, even when the bottom end is lying flat on the table, as seen in the center photograph.