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## Fun with Magdeburg Hemispheres and Air Pressure

by | Apr 13, 2018

Illustration of Magdeburg Hemispheres Experiment

I found the facts in this IFOD mindblowing when I first learned them. I now think about breathing, straws and vacuum cleaners, among other things, differently. Hope you enjoy these facts as well.

We live in a layer of high-pressure gas which we commonly call air. The air around us is composed of billions and trillions of molecules of various types of gas. The general composition is: 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases and some water vapor.

These molecules of gas are constantly in motion, colliding with each other and bouncing off everything.  Air pressure is the measure of the force of all those air molecule collisions on a surface.  Anything that increases the speed or intensity of the air molecule collisions increases air pressure. Due to gravity, air pressure is higher closer to the earth and decreases with altitude. Warmer air has more air pressure than cooler air because heat causes air molecules to move more (also the inverse holds, higher air pressure creates higher temperatures).

At sea level, at 59 degrees Fahrenheit, standard air pressure is 14.7 pounds per square inch. This means a one inch square drawn on the ground will have around 14.7 pounds of force pushing on it.  We don’t notice all this pressure because the air inside us and our fluid and tissues are pushing outward with the same pressure* (see below on an interesting point on breathing, straws and vacuum cleaners). If you hold a piece of paper vertically, it doesn’t move because there is 14.7 psi pushing on both sides of it.

Now, if you draw a 20 inch diameter circle on the ground, the air pressure on that circle is about 4,600 pounds! (Don’t believe it, do the math, the area of a circle is Pi x radius^2 – so 10 inch radius squared is 100 inches and that multiplied by 3.14 is 314 square inches and that multiplied by 14.7 is 4,615 pounds). A grown rhino weighs about that much!

Around 1650 Otto von Guericke invented the vacuum pump and with that invention a determination of the force of air pressure could be determined.  Otto announced his invention and estimate of air pressure to the world in an extraordinary fashion.  He invented so-called Magdeburg Hemispheres which were two copper half-spheres (“hemispheres”) about 20 inches in diameter that fit together with a smooth surface.  There were loops on each side for ropes to be connected to each half. It looked like this:

Original Magdeburg Hemispheres in a Museum

Otto then used the vacuum pump to remove all the air from the inside of the sphere created by the two hemisphere halves.  He then had a team of horses attached to each of the hemispheres and had them try to pull the halves apart.  They could not! After the horse teams’ failed attempts, he opened the hose into the sphere allowing air in and the two hemispheres fell apart on their own!

The concept is that these 20 inch diameter hemispheres had 4,600 pounds of pressure pushing on their outside with no pressure coming from the inside.  Here’s a diagram of the concept:

Here’s a video of two dudes demonstrating the Magdeburg Hemispheres experiment.  Super fun!

Want to try a version of this experiment yourself (or better yet, with your kids), here’s a fun one from the book “Storm in a Teacup: The Physics of Everyday Life” by Helen Czerski: “Find a square of thick, flat cardboard that’s large enough to cover the mouth of a glass. It’s best to try this over a sink, just in case. Fill up the glass with water right to the rim and put the cardboard on top. Push it flat against the rim of the glass so there’s no air left between the surface of the water and the cardboard. Then turn the glass over—and remove your hand. The cardboard, supporting the entire weight of the water, will stay put. It stays there because air molecules are hitting it from the underside, pushing the cardboard upward. That push is easily enough to hold the water up.”

I performed this experiment and it worked! It was super fun. You can freak out your kid, spouse or co-worker by doing it over carpet or something valuable!

*The way our breathing works is that as we expand our lungs air pressure inside our lungs decreases because the air molecules are spread over a larger area. This imbalance allows the atmosphere to push in additional air to achieve parity with the air pressure outside our body.

Same thing when you drink out of a straw, you aren’t really “sucking” liquid through the straw, instead, your “sucking” has created an air pressure imbalance that causes the atmosphere to push the water through the straw.

These concepts apply to vacuum cleaners as wellThey don’t “suck”, instead they use a fan to evacuate a chamber of some of its air and this air pressure imbalance causes air from outside the vacuum to rush in through the bottom area where the dirt is collected.

1. A more fun example, one which made me quite a bit of money in bar bets, is to have one person push the tips of his index fingers against each other, creating 2 equal forces pushing in against each other which is what the half spheres do if a vaccuum is created in them. Then have a second person just take their wrists and gradually pull outwords to see if they can pull your fingers apart.

2. Fun stuff, John. This is why you can only lift water around 34′ at sea level. You need to push it if you want to move it higher. Check out an inventive device called a ram pump. Basically a few pieces of PVC pipe and a couple check valves that can raise water over 500′ without using any electricity or petroleum….pretty cool.

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