Magdeburg Hemispheres

Air

• Magdeburg Hemispheres
• Vacuum pump

Key Points:

• In 1654, Otto von Guericke (inventor of the air pump) placed two empty copper hemispheres (called Magdeburg hemispheres after the name of the town in which the demonstration took place) together and removed the air from between them. Two teams of horses could not separate them.
• Before the experiment there is air on every side of the hemispheres.
• The air molecules bump into the sides of the hemishperes.
• There are about the same number of molecules hitting the inside and the outside (because the inside is the same size as the outside).
• If air weighs something then the molecules will push on the hemispheres when they bump into them.
• The vacuum pump pulls the air out of the inside.
• Air molecules are still hitting the outside of the hemispheres, but not the inside anymore.
• The air pushes in with a force of about 15 pounds per square inch (14.7 exactly).
• It would take over 300 pounds to pull the hemispheres apart.

Warnings:

• If you are one of the people pulling on a hemisphere when the valve is opened, make sure the hemisphere doesn’t hit the ground if/when you fall. They dent VERY easily.

Things to talk about:

• This demo comes right after states of matter stuff. Ask the audience what state of matter air is.
• Ask them if they think air weighs something.
• Tell them you’re going to do an experiment to find out.
• Show them the Magdeburg hemispheres and mention they were invented to find out whether air weighs something.
• Hold up one half and explain that there are air molecules all around it, bumping into both sides. Tell them there are about the same number hitting the inside and outside of the hemisphere.
• If air weighs something, then the molecules push on both sides of the hemispheres. If not, then air isn’t pushing on the hemispheres at all.
• In order to find out, we’re going to pull all the air out from inside the hemispheres with an air pump.
• Connect the air pump and turn it on. Make sure the valve is open (pointing toward the hose). You may have to press the hemispheres together to seal them. Wait a few seconds until the pump motor changes pitch. Close the valve and remove the hose.
• Explain that there are no air molecules bumping into the insides of the hemispheres anymore, but there is still the same number on the outside.
• Now let go of one handle and show that they are stuck together. Tell them the air molecules must be pushing them together or else they would just fall apart like before.
• Get two volunteers from the audience to try and separate the hemispheres.
• Have two Physics Van members pull on them, and explain that it would take about 300 pounds to pull the hemispheres apart.
• Ask the audience what will happen if you let the air back in. Go ahead and let the air rush in, and watch the volunteers fall over.
• Explain that once there was air on the inside again, it started pushing back like before. Then there was nothing holding the hemispheres together anymore, so they fell apart easily.

Click here for the Adobe PDF version of this demo (text only), for printing purposes.