Materials Included In Kit

Additional Materials Required

Prelab Preparation

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.

Safety Precautions

Safety glasses should be worn for this experiment because there is the potential for the suction cup hemisphere to launch off the support stand clamp. Students should follow all normal laboratory safety guidelines.

Lab Hints

• Enough materials are provided in this kit for 15 groups of students to working in groups of two. This laboratory activity can reasonably be completed in 30 minutes.
• 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 herispheres. 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. However, 2000 to 3000 g of masses will be required for each lab station.

Sample Data

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___

Answers to Questions

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 r².

   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).
   {12673_Answers_Equation_1}
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.

Introduction

Experiment with mini Magdeburg hemispheres to feel the strength of atmospheric pressure.

Concepts

• Atmospheric pressure
• Magdeburg hemispheres

Background

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 of the hemispheres. It requires a pulling force equal to the force acting on the hemispheres from the unbalanced atmospheric pressure to separate the hemispheres.

Materials

Safety Precautions

Wear safety glasses for this experiment—the suction cups may come flying off the support stand clamp. Please follow all normal laboratory safety guidelines.

Procedure

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 1).
   {12673_Procedure_Figure_1}
5. Tie a large looping knot at the other end of the string (to fit over the support stand clamp screw). See Figure 2.
   {12673_Procedure_Figure_2}
6. Repeats steps 2–5 for the other mini Magdeburg hemisphere.
7. Set up the support stand with support stand clamp (see Figure 3).
   {12673_Procedure_Figure_3}

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 3).
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.

Student Worksheet PDF

12673_Student1.pdf