We're All Under Pressure
Four investigations help students understand air pressure and the laws governing it.
By Erin Bailey
Have you ever wondered why the shower curtain is sucked inward when you turn on the water? Or marveled as a jumbo jet defies gravity at liftoff?
The answer to both of these phenomena is air pressure. Although it is all around us and affects us every day, air pressure is tricky to teach. It is an abstract concept that takes repeated exposure for children to grasp its meaning and implications for everyday life. Daniel Bernoulli figured out the principles of how air pressure behaves under various conditions. Below are a few ways to engage your young scientists with these principles. Before we begin, however, let’s take a look at the scientists and the laws that laid the groundwork.
In 1662, Robert Boyle published a paper that led to a better understanding of the relationship between pressure and the volume of a gas. Boyle’s Law stated that “the pressure exerted by a gas held at a constant temperature varies inversely with the volume of the gas.” In other words, as a gas is compressed into a container, the volume is decreased but the pressure inside the container increases. His work is helpful in understanding steam engines as well as the pop we feel in our ears when changing altitude rapidly.
In the 1800s, Jacques Charles used Boyle’s Law to improve the sport of hot air ballooning, which was popular at the time. Eventually, his research led him to a law stating that the volume of a gas increases as the temperature increases and the volume decreases as the temperature decreases. More heat made the balloon rise higher while less heat meant the balloon would descend.
Daniel Bernoulli worked with another state of matter – liquid. He concluded that when gas or liquid particles speed up, they actually push less, lowering the air pressure. This explains why windows explode during a hurricane rather than being blown inward. The air outside the house is moving very fast which causes a drop in pressure. Because the air pressure inside the house is higher, the windows are pushed out.
For students to experiment with these principles, set up the following investigations. Each station should have a copy of the above laws, and each participant should have a recording sheet to keep track of the following:
- Predicting what will happen at each station
- Observations of what occurs
- Conclusions about why the event happened
Other than reading through the principles with learners, don’t explain what is happening at each station until the end.
Materials: ping pong ball, kitchen funnel
- Have participants place the ball into the wide part of the funnel. Instruct them to blow hard into the narrow end. No matter how hard they blow, the ball will not come out of the funnel. Why?
As air is blown into the funnel, the air moves faster, which lowers the pressure beneath the ball. Since the air pressure above the ball is higher, it pushes the ball down into the funnel.
Materials: two text books of equal thickness, a sheet of printer paper, a drinking straw
- Place the books about four centimeters apart and stretch the paper over the top of them, creating a covered tunnel between the books. Learners should blow through the straw between the books. Have them observe what happens to the paper.
As the air speeds up, the air pressure between the books decreases. The paper will dip as a result.
Materials: balloon, flask*, pan of ice water, pan of hot water
- Stretch the deflated balloon over the mouth of the flask. Instruct learners to place the flask into the hot water and observe what happens to the balloon. Then move the flask to the cold water and observe what happens.
As the air in the flask heats up, it expands and moves into the balloon, inflating it a bit. The inverse occurs when the flask is placed in the cold water.
*A shorter flask will produce quicker results.
Materials: empty soda can, empty pan, heat source (hot plate or stove), shallow dish, ice water, tongs
Because this station requires a heat source, the teacher should perform the experiment once for everyone.
- Place about two inches of water in the soda can. Put the soda can into the pan and heat the water in the can. When it boils, pour about an inch of very cold ice water into the shallow dish. As steam begins to escape the can, use the tongs to invert the can quickly into the shallow dish. Crunch! The sides of the can will cave inward.
The water and air molecules inside the can speed up as they heat. Inverting the can into the ice water causes them to contract rapidly. Since the pressure outside the can is now higher, the sides of the can collapse.
Wrap up with a discussion of what conclusions your learners have made. Help them make connections between their observations and the meaning of the laws.