Stuck Like Glue (Air Pressure Trick)
Simple air pressure difference is all the glue you need to lift a plate with a jar.
This activity is a great way to reveal how air pressure differences cooperate. An airplane flies because of these differences cooperating but you can’t see them at all. They’re out there working under and over the wings somewhere. Here’s a way to hold the results of this cooperation in your hands, up close, and get personal with it. A flame and some wet paper make fast friends of a jar and a dinner plate. Science is so cool!
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- Smooth, heavy plastic plate
- Paper towel or napkin
- Water in a dish
- Wide mouth jar or glass (no chips in the rim)
- Lighter or matches
- Small piece of paper
- Adult Supervision
Trim or tear a paper towel (or napkin) into a shape that’s bigger than the mouth of the jar or glass you’re using. Dip the single layer of towel into the water to get it soaked. Shake off the excess water and smooth the wet towel onto the plate.
Tear off a piece of the paper roughly the size of a sticky note. Fold it, light it, and drop it into the jar while it’s burning.
Make sure the paper is burning nicely in the jar. Then, grab the jar, flip it over, and place it upside down in the center of the soaked towel on the plate. Allow the paper to burn inside the jar until the flame goes out. Observe what happens where the rim touches the wet towel.
When the flame is gone, lift the jar. Holy cow… the plate comes with it!
How Does It Work
The burning paper inside the jar is the cause of all this excitement. As it burns it uses a little oxygen, of course, but, more importantly, the heat generated by the flame causes the air inside the jar to start moving very fast, to quickly expand, and to leave the jar. With less air inside it, the pressure in the jar drops. With the jar upside down on the wet towel, you probably saw bubbles at places around the rim on the outside. That was expanding, heated air escaping the jar.
When the burning paper goes out, the air inside the jar cools and contracts quickly. Normally, air outside an open jar would enter and the pressure would be equalized instantly. This time, however, there’s a wet towel in the way that blocks the returning outside air. This keeps the lower pressure air inside the jar and the higher pressure air outside the jar. You may have seen a few bubbles around the rim on the inside this time. Some outside air managed to get inside before the seal was tight. The difference between the two pressures is enough that the higher pressure air outside pushes the plate against the lower pressure air inside jar as long as the towel stays wet. As soon as it dries a little, the two pressures will be equalized and the plate will fall off.
When you think of a pressure difference like this, you may think of the word “suction.” Scientists avoid the term and instead describe it as a push–pull. Air from outside the jar wants to get inside to make things equal. Were it not for the wet towel, the outside air would get in and the vacuum would be lost. However, the water in the paper towel fibers effectively keeps higher pressure air from rushing back into the jar.
There’s a simple explanation for this. Water (as well as other substances) has two molecular properties called adhesion and cohesion. Adhesion is a water molecule’s ability to stick to other molecules. Cohesion is, as you’d expect, a water molecule’s ability to stick to other water molecules. As long as it’s wet, adhesion and cohesion of water molecules create an airtight seal between the plate, the paper towel, and the jar. Once the towel dries enough, however, all bets are off.
Science Fair Connection
Gluing a plate to a jar using air pressure differences is a pretty cool activity but it’s not a science fair project. You create a science fair project by identifying and testing variables based on this activity. A variable is something that might change the outcome. Let’s take a look at some of the variable options you might test and write up for a science fair.
- Test what happens if you change the size of the burning piece of paper. What if you use different kinds of paper to burn? What if the paper burns longer? shorter? hotter?
- How do different styles, materials, and sizes of the glasses or jars you use change the outcome?
- Test different brands of paper towels. What if you used cloth instead? What other materials will work to make the seal?
- How does using hot or cold water change the outcome? What other liquids can form a longer lasting seal?
These are just a couple of ideas and you certainly aren’t limited to them! Come up with your own variable to test. Remember, you can change only one variable for each test while making sure that all the other factors in your test remain the same!