Materials Included In Kit

Additional Materials Required

Prelab Preparation

Experiment 1: Mini Magdeburg Hemisphere 

1. Set up a sample experimental apparatus as per the student instructions.
2. Perform a sample demonstration to show students how to pull on the spring scales (slowly) and approximately how much force will be required. Use the demonstration to prepare students for the loud “pop” that will result when the Magdeburg hemispheres separate.

Experiment 2: Pressure Paradox 

1. Place the steel ball bearings into one of the weighing trays at the lab station so the ball bearings will not roll onto the floor.
2. Place the other weighing tray on the balance and tare the balance.

Safety Precautions

Safety glasses should be worn for the Mini Magdeburg Hemispheres experiment, as there is the potential for the suction cup hemisphere to launch off the support stand clamp. The pressure bottle is safe if used properly. The bottle should not be inflated above 100 pounds per square inch (psi). Even if the bottle should “explode,” the plastic construction will only result in a quick release of air, an accompanying loud noise, and a hole in the bottle. The bottle will split but will not shatter. Teachers should inspect the pressure bottle for cracks before setting them out for student use. In the Pascal’s Law experiment, remind students to press on the input plunger with only a thumb and to hold the output syringe with their other hand to prevent the syringes from separating. Students should not use the syringes as “squirt guns.” Students should wear safety glasses while working through all these experiments. Remind students to wash their hands with soap and water after completing each laboratory activity. Students should follow all normal laboratory safety guidelines.

Disposal

The materials from each lab should be saved and stored in their original containers for future use.

Lab Hints

Experiment 1: Mini Magdeburg Hemisphere

• Enough materials are provided in this kit for three groups of students to work at the same lab station. This laboratory activity can reasonably be completed in 15 minutes. All materials are reusable.
• Remind students to pull down on the spring scales slowly and carefully.
• The Magdeburg hemispheres will not require 15 psi to separate because the plastic material is pliable and is only as strong as the weakest seal point. Also, only a partial vacuum exists inside the Magdeburg hemispheres. Students can experiment with wetting the inside of the suction cups with water before sticking them together to help form a more airtight seal. Caution: Students should not lick the suction cups. The better seal may require a spring scale that can measure up to 4000-g/40-N, however.
• Hanging masses can be used in place of spring scales. However, students need to hang the masses carefully and to keep one hand on the masses at all times to be ready to catch them when the seal breaks. Students should hang the masses as close to the tabletop as possible and place a towel under the masses to prevent them from damaging the tabletop. Slotted masses work best for this method.

Experiment 2: Pressure Paradox

• Enough materials are provided in this kit for three groups of students to work at the same lab station. This laboratory activity can reasonably be completed in 15 minutes. All materials are reusable.
• If students do not feel a weight difference, try this variation: Wrap the Styrofoam ball in aluminum foil (to give it a little extra weight). Then, have students determine which is heavier—two steel balls, or one aluminum-covered foam ball. Pre-weigh the aluminum-covered ball to make sure it is slightly heavier than the two steel balls. Students almost always observe that the two steel balls feel heavier than the foam ball.

Experiment 3: Boyle’s Law

• This kit contains enough materials for three groups of students to work at the same lab station. All materials are reusable. This laboratory activity can reasonably be completed in 15 minutes. It will take about 3–5 minutes to pressurize the pressure bottle to 50–60 psi. Therefore, students can share one bicycle pump at the lab station. However, separate tire gauges will be needed in order for three groups to complete the activity in 15 minutes.
• The “pressure bottle” is a 1-L PETE (polyethylene terephthalate) soda bottle. The bottle cap has been fitted with a tire valve to give an airtight seal (see Figure 11). Pumping air into the bottle using an ordinary bicycle pump makes it possible to pressurize the bottle above atmospheric pressure. The bottle retains its volume when it is pressurized—any expansion is negligible. The plastic used to make these bottles will withstand pressures up to about 100 psi.
  {12658_Hints_Figure_11}
• If the bottle leaks air around the cap when pressurized, remove the cap assembly and put additional petroleum jelly around the inside seal of the cap. Vaseline^® may also be used in place of the petroleum jelly.
• Best results are obtained using a bicycle pump with an attached pressure gauge.
• Tire gauges come in many shapes and sizes. The best gauges for this experiment are those with an attached pressure dial or digital readout rather than a “pop-out” sliding scale.
• Students may need to be instructed in the use of a tire gauge. Students usually are too cautious—they tend to press the tire gauge softly against the valve, which allows air to escape and does not give an accurate pressure reading. The best advice is to work quickly and deliberately. There is an obvious value for students in learning to use a tire gauge so that they can inflate their automobile tires to the proper pressure. Maintaining proper tire pressure improves safety, tire wear and gas mileage.
• Since the scale maximum on many tire gauges is 50 psi, students may have to bleed off enough air initially to get the pressure below 50 psi. The units shown on some pressure gauges may be psig (gauge pressure per square inch). The correct units for the “total pressures” calculated in the Data and Results Table are psia (pounds per square inch absolute) rather than psi.|For best results, use a barometer to measure the local barometric pressure. The national weather service site reports corrected sea-level air pressures. Note that these are not actual barometric pressure readings. Meteorologists convert station pressure values to what they would be if they had been taken at sea level. The following equation can be used to recalculate the barometric pressure (in inches Hg) from the reported sea-level pressure (in inches Hg). Elevation must be in meters.

  barometric pressure = sea-level pressure – (elevation/312 m)

• On many syringes, the black rubber seals have two “seal lines.” Make sure students are consistent in where they measure the volume! A small amount of pressure is released in step 9 before volume measurements are made—this is to overcome the friction between the rubber seal and the syringe barrel.

Experiment 4: Pascal’s Law

• Enough materials are provided in this kit for three groups of students at the same lab station. All materials are reusable. This laboratory activity can reasonably be completed in 15 minutes.
• The Preparation section may be performed by the teacher before class to save time for the first group of students.
• The activity is designed as a qualitative introduction to hydraulics.
• The hydraulic system will not work with air; water must be used. Air will be compressed in the input syringe before it begins to move the output plunger. Liquids are incompressible and therefore pressure is distributed equally to all surfaces inside the system.

Teacher Tips

• This activity–stations lab kit consists of four student lab activities. Enough materials are provided in the kit for three lab stations for each activity. The activities may be completed in any order. Each lab activity will require about 15 minutes for students to complete.
• Set up each lab station accordingly before class. Instruct students that they should leave the stations as they find them before they move on to the next lab station.
• Before class, prepare copies of the procedures and student worksheets for each student. The Background information for each experiment may also be copied and provided to the student at the instructor’s discretion.

Sample Data

Experiment 1: Mini Magdeburg Hemisphere

Diameter of Magdeburg hemisphere: ___2.0 cm___
Force required to separate the hemispheres, Trial 1: ___19 N___
Force required to separate the hemispheres, Trial 2: ___18 N___
Force required to separate the hemispheres, Trial 3: ___19 N___

Experiment 2: Pressure Paradox

Weight of Foam Sphere versus Steel Sphere when held in palms

The steel sphere feels heavier than the foam sphere.

Weight of Foam Sphere versus Steel Sphere when hanging between thumb and index finger

The two spheres appear to weigh about the same.

Mass of Foam Sphere: ___15.0 g___
Mass of Steel Sphere: ___8.4 g___

Experiment 3: Boyle’s Law Experiment 4: Pascal’s Law

Data Table 1 Data Table 2

Answers to Questions

Experiment 1: Mini Magdeburg Hemisphere

1. Calculate the average force required to separate the hemispheres.

   Sample calculation: (19 N + 18 N + 19 N)/3 = 19 N

2. Calculate the inside area bounded by the circumference of the Magdeburg hemispheres. Hint: The area of a circle is πr².

   Sample calculation: π(2.0 cm/2)² = 3.1 cm²

3. Calculate the pressure holding the Magdeburg hemispheres together just before they separate. Convert the pressure to pounds per square inch (psi).
   {12658_Answers_Equation_3}
4. Just before the Magdeburg hemispheres separate, what “occupies” the space inside the stretched hemispheres?

   The space is “occupied” by a partial vacuum. There is very little air inside the Magdeburg hemispheres when they are fully stretched.

5. What causes the loud “pop” when the two hemispheres separate?

   The “pop” is caused by the air rushing in to fill the partial vacuum (empty space) inside the Magdeburg hemispheres when the seal is finally broken.

Experiment 2: Pressure Paradox

1. Define pressure.

   Pressure is the result of a force applied to a surface. It is a force per unit area.

2. Which sphere has the greater mass? How does the actual mass compare to the sphere that appeared to weigh more while held in the palm of the hand? How does it compare to sphere that appeared to weigh more while held by the thumb and index finger?

   The Styrofoam sphere had the greater mass. However, the steel sphere appeared to weigh more when held in the palm of the hand. The steel sphere and foam sphere appeared to weigh about the same when held by the thumb and index finger.

3. Which ball has greater surface area contacting the palm of the hand?

   The foam sphere has greater surface area in contact with the palm of the hand compared to the steel sphere.

4. When the spheres are held in the palm of the hand, what physical property is actually felt?

   The feeling of weight is actually a feeling of pressure. The smaller surface area of the steel sphere produces more pressure on the skin so it “feels” heavier than the foam sphere.

5. Is it better to compare the weight of two objects by holding them in the palms of your hand or by hanging the objects between your fingertips? Explain.

   Hanging the objects by the fingertips helps to actually feel the weight of the object compared to the pressure it exerts when held in the palm. It is better to hold the objects between the fingers and hang them instead of having them rest in the palm of your hand.

Experiment 3: Boyle’s Law

1. Convert the local barometric pressure to psi units and enter the value to the nearest psi in the data and results table. Some appropriate conversion factors are as follows: 1 atm = 760 mm Hg = 29.92 in Hg = 14.7 psi.
   {12658_Answers_Equation_4}
2. The pressure gauge measures the relative pressure in psi above atmospheric pressure. For each pressure reading in the data and results table, add the local barometric pressure to the gauge pressure to determine the total pressure of air inside the pressure bottle. Enter the total pressure to the nearest psi in the data table.

   Sample calculation for Trial 1: Total pressure = 42 psi + 15 psi = 57 psi

3. Plot a graph of volume on the y-axis versus total pressure on the x-axis. Note: The origin of the graph should be (0,0). Choose a suitable scale for each axis so that the data points fill the graph as completely as possible. Remember to label each axis and give the graph a title.
   {12658_Answers_Figure_7}
4. Describe the shape of the graph. Draw a best-fit straight or curved line, whichever seems appropriate, to illustrate how the volume of a gas changes as the pressure changes.

   The graph is curved. The volume decreases as the pressure increases. At first, there is a sharp reduction in the volume as the pressure increases. The decrease in volume then becomes more gradual and the volume appears to level off as the pressure increases further. Mathematically, the shape of the curve is described as hyperbolic. A hyperbolic curve of this type is obtained when there is an inverse relationship between two variables (y ∝ 1/x).

5. The relationship between pressure and volume is called an “inverse” relationship—as the pressure increases the volume of air trapped in the syringe decreases. This inverse relationship may be expressed mathematically as P ∝ 1/V. Another way of expressing an inverse relationship between two variables is to say that the product of the two variables is a constant (P × V = constant). Multiply the total pressure (P) times the volume (V) for each set of data points. Enter the results in the data table.

   Sample calculation for Trial 1: 2.0 mL × 57 psi = 114 psi•mL

6. How constant is the P x V “constant?” Explain any possible sources of error.

   The P x V “constant” was consistent. The range of values for P x V may be attributed to experimental error. As the syringe compressed, the gas trapped inside the syringe becomes more compressed and acts less and less like an ideal gas. Also, the grease helped to limit the friction of the syringe plunger and tube, but the friction was not eliminated completely. So, the syringe may not have expanded as much when the pressure was released.

Experiment 4: Pascal’s Law

1. Review your observations. Was it easier to move the 20-mL plunger by pressing the 1-mL plunger, or to move the 1-mL plunger by pressing the 20-mL plunger? Which plunger has the larger surface area? Use Pascal’s law to explain the result.

   It was easy to press on the 1-mL plunger to move the 20-mL plunger. It was very difficult to move the 1-mL plunger by pressing on the 20-mL plunger. The 20-mL plunger has the larger surface area compared to the 1-mL plunger. In terms of Pascal’s law, the 20-mL plunger was harder to move compared to the 1-mL plunger because the surface area of the plunger is larger, meaning a larger force is needed to move it, since the pressure in the syringe system is constant.

2. Use the measurements from the data table to determine the ideal mechanical advantage of the following syringe systems.

   Input: 1-mL, Output: 20-mL; Ideal Mechanical Advantage: 5.7 cm/0.3 cm = 19
   Input: 3-mL, Output: 20-mL; Ideal Mechanical Advantage: 5.4
   Input: 3-mL, Output: 3-mL; Ideal Mechanical Advantage: 1
   Input: 20-mL, Output: 3-mL; Ideal Mechanical Advantage: 0.19
   Input: 20-mL, Output: 1-mL; Ideal Mechanical Advantage: 0.05

3. Compare the mechanical advantage calculations from Question 2 to how hard or easy it was to move the respective input plunger. Is it better to have a mechanical advantage greater than one or less than one?

   The easiest system to move has the highest mechanical advantage (19). A mechanical advantage less than one corresponded to a very difficult system to push.

4. Refer to Figure 10. Who would Pascal predict as the winner in a “thumb wars” match between A and B? Why?

   Pascal would predict that syringe A would win because the smaller syringe supplies more force on the large syringe. The mechanical advantage of the large syringe is very small, so even a large force supplied to the large plunger is still a very weak force on the small plunger.

Introduction

This all-in-one Investigating Pressure Kit is designed to provide students the opportunity to explore the fundamental principles relating to pressure. Four hands-on lab stations can be arranged so student groups can experiment with atmospheric pressure, the sensation of pressure, Boyle’s law and Pascal’s law. Students experiment with mini Magdeburg hemispheres to feel how strong atmospheric pressure is. In the Pressure Paradox experiment, students determine which object weighs more by the “weight” they feel. With the Pressure Bottle experiment, students learn about the pressure-volume relationship, or Boyle’s law. And finally, students use simple hydraulic systems to study Pascal’s law.

Concepts

• Atmospheric pressure
• Magdeburg hemispheres
• Force
• Area
• Pressure
• Gas properties
• Boyle’s law
• Pascal’s law
• Mechanical advantage
• Hydraulics

Background

Experiment 1: Mini Magdeburg Hemisphere
Magdeburg hemispheres were developed by the German scientist Otto von Guericke (1602–1686) in 1650. The Magdeburg hemispheres received their name from Guericke’s hometown of Magdeburg, Germany, where he was also the mayor. On May 8, 1654, Guericke performed a famous experiment in front of Emperor Ferdinand III in which he connected two large copper hemispheres together and used his newly invented vacuum pump to evacuate the air from inside the hemispheres. He then attached a team of horses to both sides of the hemispheres and had each team pull in opposite directions in an effort to separate the hemispheres. No matter how hard they pulled, the horses could not pull the hemispheres apart. They were held together by the surrounding atmospheric pressure. This experiment later piqued the curiosity of Robert Boyle (1627–1691), who performed further research that lead him to determine the volume–pressure relationship of a gas (pressure multiplied by volume equals a constant value), later known as Boyle’s law.

Earth’s atmosphere exerts a pressure on everything within it (due to the force from the rapidly moving air molecules colliding with objects within the atmosphere). At sea level, atmospheric pressure is 101,325 pascal (the SI unit of pressure), where 1 pa is equal to 1 N/m². This is equal to 1 atmosphere, 760 mm of mercury (Hg), or 14.7 pounds per square inch (lb/in²). For every square inch of surface at sea level, the atmospheric molecules exert a force of 14.7 pounds. When the air is extracted or squeezed out from the inside of the Magdeburg hemispheres, air pressure inside the attached hemispheres is much lower than the air surrounding the outside to the hemispheres. It requires a pulling force equal to the force acting on the hemispheres from the unbalanced atmospheric pressure to separate the hemispheres.

Experiment 2: Pressure Paradox
Pressure (P) is defined as the amount of force (F) exerted on a given area (A), or a force per unit area (P = F/A). Some common units of pressure are pounds per square inch (lb/in²), newtons per square meter (N/m²) [also known as a pascal (pa)], atmospheres (atm), and millimeters of mercury (mm Hg) (“millimeters of mercury” corresponds to the height of a small column of mercury that is held up by a pressure). A pressure exists whenever two objects are in contact with each other.

The points of pressure only occur at the points of contact. The amount of pressure depends on the force and the surface area that the force acts over. For a given amount of force, a larger surface area will decrease the amount of pressure on the surface compared to a smaller surface area. A 10-lb ball and a 10-lb wood block both exert the same force due to gravity. A 10-lb ball, however, will exert more pressure on a tabletop than a 10-lb block of the same material because the round surface of the ball has a very small contact area with the tabletop compared to the larger rectangular surface of the 10-lb block. The force “feels” stronger when it acts on a smaller area because the force is more concentrated. The same amount of force acting over a larger surface area is spread out and “diluted” over the entire area so it “feels” weaker, even though it is the same force (weight).

The steel sphere in this experiment feels heavier than the foam sphere when the two spheres are held side-by-side in each palm, but the foam sphere has more mass and therefore actually weighs more. The foam sphere has a larger curvature and makes contact with more of the palm’s surface. This lowers the pressure the foam sphere exerts on the hand. The weight (force) of the foam sphere is spread out over a larger region of the palm. The smaller contact area of the steel sphere “concentrates” the weight and produces a higher pressure on the palm, even though it has a smaller mass. Our brain interprets the larger pressure as a sensation of weight, or force, and the steel sphere is predicted to weigh more than the foam sphere.

Experiment 3: Boyle’s Law
In 1642, Evangelista Torricelli (1608–1647), who had worked as an assistant to Galileo (1564–1642), conducted a famous experiment demonstrating that the weight of air would support a column of mercury about 30 inches high in an inverted tube. Torricelli’s experiment provided the first measurement of the invisible pressure of air. Robert Boyle (1627–1691), a “skeptical chemist” working in England, was inspired by Torricelli’s experiment to measure the pressure of air when it was compressed or expanded. The results of Boyle’s experiments were published in 1662 and became essentially the first gas law—a mathematical equation describing the relationship between the volume and pressure of air.

Robert Boyle built a simple apparatus to measure the relationship between the pressure and volume of air. The apparatus consisted of a J-shaped glass tube that was sealed at one end and open to the atmosphere at the other end. A sample of air was trapped in the sealed end by pouring mercury into the tube (see Figure 1).

In the beginning of the experiment, the height of the mercury column was equal in the two sides of the tube. The pressure of the air trapped in the sealed end was equal to that of the surrounding air, equivalent to 29.9 inches (760 mm) of mercury. When Boyle added more mercury to the open end of the tube, the air trapped in the sealed end was compressed into a smaller volume (see Figure 2). The difference in height of the two columns of mercury (Δh in Figure 2) was due to the additional pressure exerted by the compressed air compared to the surrounding air. Boyle found that when the volume of trapped air was reduced to one-half its original volume, the additional height of the column of mercury in the open end of the tube measured 29.9 inches. The pressure exerted by the compressed air was twice as great as atmospheric pressure. The mathematical relationship between the volume of the air and the pressure it exerts was confirmed through a series of measurements. Experiment 4: Pascal’s Law
Blaise Pascal (1623–1662) is well known as a mathematician. Pascal also had a strong interest in physical events and spent much of his time trying to explain the phenomena he witnessed in his experiments. He performed many experiments involving pressure in fluids. One of the most important principles he discovered became known as Pascal’s law or Pascal’s rule. Pascal’s law states that pressure applied anywhere to a incompressible fluid causes a pressure to be transmitted equally in all directions. A force applied at one end is transmitted throughout the entire fluid system. Pressure is equal to a force per unit area (P = F/A). Therefore, if the pressure in a fluid is constant, then the larger the surface area the pressure is in contact with, the larger the force, and the smaller the surface area, the smaller the force. By arranging liquid columns of different sizes Pascal discovered that a relatively small force could lift a very heavy load. Figure 3 shows a force of 20 newtons pushing a fluid through a 1-cm² opening. Therefore, according to Pascal’s law, every square centimeter of the entire system is under a force of 20 N. The piston in the larger container has a surface area of 100 cm², making the amount of force lifting up the piston in the larger container equal to 2000 N (20 N/cm² x 100 cm²). The total force increases as the surface area increases. Pascal’s law serves as the basis for the development of much of what is now known as hydraulics. Mechanical advantage (MA) is a ratio of the output force (F_o) compared to the input force (F_i). The higher the mechanical advantage of a system, the higher the output force is compared to the input force. The higher the mechanical advantage, the easier it is to do the work. However, mechanical advantage does not give something for nothing. With a large mechanical advantage, it is easy to move a heavy load with a relatively smaller force. The trade-off is that the smaller applied force must be carried over a longer distance compared to the distance the heavy load is moved. This is a result of the conservation of energy principle, in which the total energy in equals the total energy out. A small force will move a large distance while the large load moves a small distance. The ideal mechanical advantage of a hydraulic system can also be determined by comparing the distance the input force moves compared to the output force.

d_i = input distance
d_o = output distance

Experiment Overview

Experiment 1: Mini Magdeburg Hemisphere
Experiment with mini Magdeburg hemispheres to feel the strength of atmospheric pressure.

Experiment 2: Pressure Paradox
Pressure is the result of a force acting on a specific surface area. An equal force acting on two different surface areas will result in different pressure values. Use your sense of touch to compare the weights of two different size spheres.

Experiment 3: Boyle’s Law
The purpose of this experiment is to perform a modern version of Boyle’s classic experiment to determine the volume–pressure relationship of a gas. The experiment will be carried out using air trapped inside a sealed syringe within a “pressure bottle.” The bottle will be pressurized by pumping in air to obtain a pressure several times greater than that of the surrounding air. As some of the excess pressure within the bottle is then released, the volume of the trapped air inside the syringe will change. Volume measurements will be made at several different pressures and the results will be analyzed by graphing to derive the mathematical relationship between pressure and volume.

Experiment 4: Pascal’s Law
Pascal’s law applies to many aspects of our lives. An extremely important application of Pascal’s law occurs when a driver presses on a brake pedal to stop a car. Pascal’s law is also at work when a mechanic easily lifts a car using a hydraulic jack. In this activity, “syringe hydraulics” will be studied to gain a better understanding of the principles of Pascal’s law.

Materials

Safety Precautions

Wear safety glasses for this experiment—the suction cups may come flying off the support stand clamp. The pressure bottle is safe if used properly. The bottle should not be inflated about 100 pounds per square inch (psi). Even if the bottle should “pop,” the plastic construction will only result in a quick release of air, an accompanying loud noise, and a hole in the bottle. The bottle will split but will not shatter. Wear safety glasses when working with the pressure bottle. When pressing on the syringe systems, be sure to press only the input plunger with your thumb and hold the output syringe with your other hand to prevent the syringes from separating. Do not use the syringe as a “squirt gun.” Please follow all normal laboratory safety guidelines. Wash your hands with soap and water after completing this laboratory activity.

Procedure

Experiment 1: Mini Magdeburg Hemispheres

Preparation

1. Obtain two mini Magdeburg hemispheres (suction cups), string and scissors.
2. If necessary, carefully twist and pull off the metal hook attached to the grooved top of the Magdeburg hemisphere.
3. Cut a length of string approximately 20 cm.
4. Securely tie one end of the string around the grooved top of one of the hemispheres (see Figure 4).
   {12658_Procedure_Figure_4}
5. Tie a large looping knot at the other end of the string (to fit over the support stand clamp screw) (see Figure 5).
   {12658_Procedure_Figure_5}
6. Repeats steps 2–5 for the other mini Magdeburg hemisphere.
7. Set up the support stand with support stand clamp (see Figure 6).
   {12658_Procedure_Figure_6}

Procedure

1. Measure the diameter in centimeters of one of the mini Magdeburg hemispheres and record the value on the Magdeburg Hemispheres Worksheet.
2. Press the two Magdeburg hemispheres together so they “stick” to each other.
3. Hang the Magdeburg hemispheres on the screw of a support stand clamp by one of the looping knots in the string (see Figure 6).
4. Wrap the string around the screw until the Magdeburg hemispheres hang about 2 cm below the support stand clamp.
5. Raise the support stand clamp to within 5 cm from the top of the support stand rod.
6. Hang the spring scale hook on the free looping knot.
7. Slowly pull down on the spring scale until the Magdeburg hemispheres are pulled apart. Do not pull on the spring scale quickly. Record the force, in newtons, necessary to break the seal under Trial 1 on the Magdeburg Hemispheres Worksheet.
8. Repeat steps 2–7 two more times for Trials 2 and 3.

Experiment 2: Pressure Paradox

1. Pick up the steel and foam spheres and hold the steel sphere in the palm of one hand and the foam sphere in the palm of the other hand.
2. Compare the apparent weight of the two spheres. Toss the spheres into the air a few centimeters and catch them to get a better “feel.” Which sphere feels heavier? Record your observations on the Pressure Paradox Worksheet.
3. Hang the two spheres, one in each hand, between your thumb and index finger (see Figure 7).
   {12658_Procedure_Figure_7}
4. Compare the apparent weight of the two spheres again. Record your observations on the Pressure Paradox Worksheet.
5. Place a weighing tray on a balance and tare the balance.
6. After all the students in the group have held the spheres and recorded their observations, weigh the foam sphere using a balance (place it onto the weighing tray). Record the mass of the foam sphere on the Pressure Paradox Worksheet.
7. Remove the foam sphere and weigh the steel sphere. Record the mass of the steel sphere on the Pressure Paradox Worksheet.
8. Answers the questions on the Pressure Paradox Worksheet.

Experiment 3: Boyle’s Law

1. Using a barometer, measure the value of the local air pressure. Note: If a barometer is not available, consult an Internet site such as the national weather service site (http://weather.gov/) to obtain a current pressure reading for your area. Record the barometric pressure in the data table on the Boyle’s Law Worksheet.
2. Obtain a 1-L pressure bottle and a 10-mL syringe with a rubber syringe tip cap.
3. Remove the tip cap from the syringe and pull on the plunger to draw about 9 mL of air into the syringe. Replace the tip cap to seal the air inside the syringe.
4. Place the sealed syringe inside the 1-L pressure bottle.
5. Run a small bead of petroleum jelly around the rim of the bottle.
6. Close the bottle with the special cap fitted with a tire valve. Tighten the cap securely.
7. Connect the tire valve to a bicycle pump.
8. Pump air into the pressure bottle to obtain a pressure reading of 50–60 psi on the tire gauge. Do NOT exceed 100 psi. Note: Using a manual pump provides its own safety feature—it is very difficult to pump more than about 70 psi into the pressure bottle by hand.
9. If the bicycle pump has an attached pressure gauge, loosen the connection between the pressure bottle–tire valve and the pump to release a small amount of pressure. As soon as you see the syringe plunger start to move, immediately retighten the tire valve to the pump.
10. If the bicycle pump does not have an attached pressure gauge, stop pumping air into the bottle when it becomes very difficult. Remove the bicycle pump from the tire valve and use a tire gauge to measure the pressure inside the pressure bottle.
11. Using the pressure gauge (attached or unattached), measure and record the pressure to within ±1 psi.
12. Measure and record the volume of air trapped in the syringe at this bottle pressure. Note: Measure the volume at the black rubber seal, not at the inverted V-shaped projection (see Figure 8). The syringe barrel has major scale divisions marked every milliliter, and minor scale divisions every 0.2 mL. The volume should be estimated to within ±0.1 mL.
    {12658_Procedure_Figure_8}
13. If the bicycle pump has an attached pressure gauge, loosen the connection between the pressure bottle–tire valve and the bicycle pump to release a small amount of pressure from the pressure bottle. Try to reduce the pressure by no more than about 10 psi. Immediately retighten the tire valve to the pump.
14. If the bicycle pump does not have an attached pressure gauge, use a pen to press in the pin of the tire valve for 1–2 seconds to release air from inside the pressure bottle. Try to reduce the pressure by no more than about 10 psi.
15. Measure both the new pressure on the pressure gauge and the new volume of the air trapped inside the syringe. Record all data in the data table on the Boyle’s Law Worksheet.
16. Repeat steps 13–15 to measure the volume of gas trapped in the syringe at several different pressures down to about 5 psi. It should be possible to obtain at least 5–6 pressure and volume measurements in this range.
17. When the pressure on the tire gauge measures close to zero, remove the tire valve from the pump. Press down on the pin inside the tire valve to release all of the excess pressure within the pressure bottle. Record the final volume of air in the syringe at atmospheric pressure.
18. If time permits, repeat steps 7–17 to obtain a second set of pressure–volume data. Record this data as Trial 2 in the data table on the Boyle’s Law Worksheet.

Experiment 4: Pascal’s Law

Preparation

1. Obtain three 3-mL, one 1-mL and two 20-mL syringes, three syringe-tip connectors and a 600-mL beaker three-quarters full of water.
2. Remove the plunger from one of the 3-mL syringes. (Call this Syringe A.)
3. Connect the Luer Lock syringe-tip connector to another 3-mL syringe. (Call this Syringe B.)
4. Place the open end of the syringe-tip connector on Syringe B into the 600-mL beaker of water. Pull back on Syringe B’s plunger to draw water into the syringe until it is approximately three-quarters full.
5. Invert the syringe so that the tip is up and the plunger end is down—any air bubbles trapped in the syringe will rise to the tip end. Carefully flick the syringe body with a finger to release any air bubbles clinging to the sides of the syringe.
6. Connect Syringe A to the open end of the Luer Lock syringe-tip connector on Syringe B.
7. Hold the coupled syringes vertically so that Syringe A is above Syringe B. Then, slowly push on Syringe B’s plunger to fill Syringe A (see Figure 9a).
   {12658_Procedure_Figure_9}
8. If Syringe A does not become completely filled, use a 50-mL graduated cylinder to add enough water to Syringe A until it is just overflowing (see Figure 9b).
9. Once Syringe A is overflowing, replace the plunger into Syringe A making sure no air bubbles become trapped in the syringe body (see Figure 9c). Repeat steps 4–9 if air bubbles become trapped.
10. Repeat steps 2–9 for a 20-mL/3-mL system. Refer to the 3-mL syringe as “Syringe A” and the 20-mL syringe as “Syringe B” in the procedure.
11. Repeat steps 2–9 for a 20-mL/1-mL system. Refer to the 1-mL syringe as “Syringe A” and the 20-mL syringe as “Syringe B” in the procedure. Note: The 1-mL syringe does not have the Luer Lock tip. It will not lock into the syringe tip connector.

Observations

1. Work with each of the three syringe systems, one at a time.
2. Press down first on one plunger and watch the movement of the second plunger. Then press on the second plunger and watch the movement of the first plunger. How do the opposing plungers in each system move? Is it easier to push on the smaller or larger plunger in each system? Record all observations in Data Table 1 on the Pascal’s Law Worksheet. Note: When pressing on the plunger of a syringe, be sure to hold the syringe body of the other syringe to prevent the syringes from coming apart. This is especially important for the 1-mL/20-mL system because the 1-mL syringe does not have a Luer Lock tip. Also, for the 20-mL syringe systems do not push too hard on the 20-mL plunger since this may cause the 1-mL and 3-mL plungers to “pop out” of their respective syringes.

Ideal Mechanical Advantage

1. Obtain the 3-mL/3-mL syringe system.
2. Press one plunger all the way down until it no longer moves, thereby pushing all the water into one of the syringe bodies. The empty syringe (with plunger pushed all the way in) will be the “output” syringe. The water-filled syringe will be the “input” syringe.
3. Measure the initial distance the input plunger is from the end of the syringe body (see Figure 10). Record this distance to the nearest 0.1 cm in Data Table 2 on the Pascal’s Law Worksheet.
   {12658_Procedure_Figure_10}
4. Measure the initial distance the output plunger is from the end of the syringe body (see Figure 10). Record this distance to the nearest 0.1 cm in Data Table 2 on the Pascal’s Law Worksheet.
5. Press on the input plunger until the output plunger has moved to the end of the syringe body, or until the input syringe is completely empty and the plunger stops at the syringe body whichever comes first. Caution: Do not push the output plunger out of the syringe body.
6. Measure the final distance the input plunger is from the syringe body. Record this distance to the nearest 0.1 cm in Data Table 2.
7. Measure the final distance of the output plunger from the syringe body. Record this distance to the nearest 0.1 cm in Data Table 2.
8. Repeat Procedure steps 2–7 for the 1-mL/20-mL and 3-mL/20-mL syringe systems. The 1-mL and 3-mL syringes should be used as the input syringes for each respective system. Record all measurements in Data Table 2.
9. Answer the questions on the Pascal’s Law Worksheet.
10. Leave the syringes filled for the next group of students.

Student Worksheet PDF

12658_Student1.pdf