Concepts

Atmospheric pressure

Overview

When two hemispheres are sealed by vacuum grease and evacuated, it is practically impossible to pull them apart by hand. Atmospheric pressure keeps them sealed and pushed together.

Details

Equipment

• [1] Magdeburg hemispheres
  
• [1] Vacuum pump with hose
  
• [1] Power bar
  

Classroom Assembly

1. Plug in the power bar and turn it off.
2. Plug in the pump.
3. Open the valve on the hemispheres.
   

Important Notes

• Make sure people using the demo are in a clear space so if they lose grip on the hemispheres or they trip, they don't hurt themselves or anyone else.
  

Script

1. Show how easy it is to pull apart the hemispheres when the apparatus is full of air.
2. Seal the hemispheres together, rotating the seals so that the grease on them spreads evenly.
3. Connect the hemispheres to the pump and turn on the power bar.
4. When the pressure reaches about −25 inches of mercury (could go a bit lower), close the valve on the hemispheres and turn off the power bar.
5. Get volunteers to try (and fail) to pull the hemispheres apart.
6. Open the valve on the hemispheres and ask the volunteers to try again.
   

Additional Resources