Learners investigate what a vacuum actually is and how it behaves, discovering that a vacuum is not a force that pulls but simply a region of lower pressure that outside air rushes into. Students trace the same pressure rule they learned in earlier lessons into more dramatic territory: suction cups, vacuum cleaners, the barometer, and the famous Magdeburg hemispheres experiment of 1654, in which 16 horses could not pull apart two metal bowls held together by nothing but atmospheric pressure. Students also connect vacuum and pressure to the weight of the atmosphere, the decrease in pressure with elevation, and the origin of wind.

Key Ideas

• A vacuum is a space with less air pressure inside than outside. It can range from a partial vacuum to a total vacuum with no particles at all.
• A vacuum does not pull anything. It simply has lower pressure inside. Outside air, at higher pressure, pushes inward.
• Atmospheric pressure decreases with elevation because there is less air above you pressing down. At sea level it is about 14.7 psi. At the top of Mount Everest it is about 4.9 psi.
• The atmosphere is held to Earth by gravity. Without gravity it would escape into space, as it has on smaller bodies like the Moon.
• Wind is air moving from regions of high pressure to regions of low pressure. Greater pressure difference means stronger wind.

Vocabulary

• Vacuum: A space with less air pressure inside than outside. Ranges from slightly reduced pressure to completely empty of particles.
• Partial Vacuum: A space where some air particles have been removed. Pressure is lower than outside but not zero.
• Total Vacuum: A space with no particles at all. Zero pressure. Outer space is the closest natural example.
• Atmospheric Pressure: The weight of the entire atmosphere pressing down on Earth’s surface. About 14.7 psi at sea level.
• Barometer: An instrument that measures atmospheric pressure by comparing it against a total vacuum.
• Elevation: Height above sea level. As elevation increases, less air is above you, so atmospheric pressure decreases.

Discussion Questions

• In 1654, 16 horses could not pull apart two metal bowls with a vacuum inside. What force was actually holding the bowls together, and why did they fall apart the instant a valve was opened?
• Why do astronauts need pressurized suits outside the International Space Station? What would happen to the body if they did not?
• Weather maps show H for high pressure and L for low pressure. If you see a large H near one city and a large L near another, which direction would the wind blow between them, and why?

Hands-On Activity: Suction Cup Pressure Test
Students use suction cups to experience the force of atmospheric pressure directly and reason through what is actually holding the cups to the surface.

Supply List

• Two suction cups (available at most hardware or dollar stores)
• A smooth flat surface such as a window or tile
• A notebook and pencil

Instructions

• Press one suction cup firmly onto a smooth surface. Try to pull it straight off. How hard is it?
• Now slide a fingernail under the edge just slightly to let air in. What happens immediately?
• Press two suction cups together face-to-face and try to pull them apart. Describe the resistance.
• In your notebook, draw a diagram showing why the suction cup sticks. Label the pressure inside (between the cup and the surface) and outside (on the back of the cup). Draw arrows showing which direction the pressure pushes.
• Explain in writing: does the cup stick because it is pulling on the surface, or because something is pushing it toward the surface? What is doing the pushing?
• Extension: how do you think the Magdeburg hemispheres worked? The bowls were large metal cups pressed together with the air pumped out. Use your diagram to predict what would have happened and why 16 horses could not pull them apart.