Magnetic Atmosphere Model

Science Snack
Magnetic Atmosphere Model
Levitating magnets show how Earth’s atmosphere stacks up.
Magnetic Atmosphere Model
Levitating magnets show how Earth’s atmosphere stacks up.

Ring magnets stacked on a pencil aren’t just fun to play with—they also serve as a model of our atmosphere, demonstrating how both density and pressure decrease with height.

Tools and Materials
• Five ring magnets (aka donut magnets) sized to fit around a pencil, about 1 1/8 inches (27 mm) in diameter and 1/4 inch (6 mm) thick, with a center hole at least 1/4 inch (6 mm) across
• Pencil
• Optional: balloon, plastic syringe
Assembly
1. Holding the pencil upright on a table, slide one magnet down the length of the pencil so it rests on the tabletop.
2. Now add a second magnet, making sure you’ve got it oriented so the two magnets repel. You’ll know you have it right if the second magnet floats above the first; you’ll know you have it backwards if the two magnets stick together. If this happens, remove the magnet, turn it over, and try again. (The magnets will only levitate if they are oriented opposite with respect to each other, so that either their north poles face each other or their south poles face each other. Remember: Like poles repel.)
To Do and Notice

Push one magnet toward the other and feel how the magnetic force of repulsion increases as the two magnets are pushed closer together. You can see the force between the two magnets in the form of the gap between them—almost as if there were an invisible spring between the two magnets.

Now add a third magnet so it floats above the second magnet. If it doesn’t float, flip it over (see Assembly, Step 2).

Add a fourth and then a fifth magnet so that they also float. What do you notice about the size of the gaps between the magnets?

What's Going On?

Your stacked magnets aren’t just a fine demonstration of magnetic levitation, they also serve as a model for Earth’s atmosphere—especially in the spacing between the magnets.

The gaps between the magnets are visible evidence of the forces at work here. The magnetic force of the bottom magnet has to hold up the weight of four magnets overhead, so the gap between the bottom two magnets is small. Near the top of the stack, the fourth magnet only needs to hold up one overlying magnet, so the gap between them is wide.

In the atmosphere, the pushback against gravity comes from air pressure, not magnetism, but the result is much the same: The spacing between air molecules increases with altitude, so the density and pressure of the air are higher below, lower above.

Air is compressible, which means it behaves a lot like a spring. Near the surface of the earth, air is squashed by the weight of the air above. The result is relatively high density and pressure. As you rise higher and higher in the atmosphere, there’s less overlying air to support, so pressure and density decrease.

You’ve probably experienced this change in air pressure and density personally—while traveling, for example, when your ears “pop” in an airplane, or when an unopened bag of potato chips explodes as you drive over a mountain pass.

At the summit of Mt. Kilimanjaro (18,000 feet, or 5.6 km above sea level) you’re above half the earth’s atmosphere. In other words, the atmospheric pressure and density there is half that at sea level.

Going Further

Convince yourself that air behaves like a spring by inflating a balloon and squeezing it between your hands. Notice that the harder you squeeze the balloon, the smaller it gets, and the harder it pushes back. Or, instead of squeezing a balloon (which can be tricky), push on a syringe full of air that’s closed at the end. Feel how the force you exert increases as you squeeze the plunger further. While the magnetic force between magnets does not follow the same exact law as the force between molecules of air, this model is still qualitatively correct.

Teaching Tips

In classroom discussions, if you consider the force of the magnets as being distributed over the area of each magnet, you can talk about the pressure (defined as force per area) exerted by the magnetic force, and thereby make a connection between this magnet model and pressure in the atmosphere.