Hurricanes > Lab 5: All About Air Pressure

# All About Air Pressure

For this lab, you and your team will be moving through a series of stations completing hands-on experiments. These experiments will help you understand the basic principles that describe how air pressure responds to and is responsible for various phenomena. As you work through the activities, keep in mind that your eventual goal will be to understand the connection between air pressure and hurricanes.

At each station, read the procedure and use the materials to perform the experiment or demonstration.

## Station 1: Air pressure on a pop can

#### Materials

• Aluminum pop/soda can
• Hot plate or Bunsen burner and striker
• Tongs or claw holder (the kind that can be attached to ring stands)
• Bucket of cold water
• Oven mitt (or equivalent) and safety glasses
• Metric rulers

#### Procedure

1. Measure and record the height and diameter of the can.
2. Put a small amount of water in the bottom of the can, just enough to cover the bottom.
3. Wearing goggles and an oven mitt, place the can on the hot plate or use the tongs to hold the can over the heat source. Do so until there is a good chimney of steam coming out the opening in the can. This might take a couple of minutes.
4. Quickly invert the can into the bucket of cold water and watch the results.
5. Complete the Stop and Think questions.

## Stop and Think

1. Follow these three steps to calculate the total force that air pressure is exerting on the can:
• Use the chart to find the average barometric pressure at your altitude. This value is the pressure.
• Use the dimensions of the can to calculate its approximate surface area.
Surface Area = 2π (radius) (height) + π (radius)2
• Multiply the pressure by the can's surface area.
2. If all that force is being exerted on the outside of the can all the time, why didn't it collapse before you put it in the cold water?

## Station 2: Balloon in a Bell Jar

#### Materials

• Bell jar
• Vacuum pump
• 2 Small balloons, partially inflated to the same size.

#### Procedure

1. Tape one of the balloons to the top inside of the bell jar. Leave the other on the outside for comparison. (This step won't be necessary for subsequent groups.)
2. Connect the vacuum pump and evacuate some air out of the bell jar.
3. Complete questions on activity sheet.
4. Release the vacuum so that the apparatus is ready for the next group.

## Stop and Think

3. What happened to the balloon as you pumped air out of the jar?
4. Brainstorm an explanation for why this happened.

## Station 3: Ruler and Newspaper

#### Materials

• Sheets of newspaper
• Wooden rulers or flat pieces of wood

#### Procedure

Part A
1. Place the ruler on a bench top with about a quarter of its length hanging over the edge.
2. Make sure the area around you is clear of people, then give the overhanging piece a quick "karate chop" with your hand.
3. Retrieve the ruler and replace it in the same position on the bench.

4. Part B
5. Lay one full sheet of newspaper over the part of the ruler that is on the bench.
6. Repeat your chop to the overhanging part of the ruler.
7. Record your observations and answer the questions on the activity sheet.

## Stop and Think

5. What did you expect to happen during each part of the activity?
6. Is it the weight of the newspaper that causes the difference? Back up your answer with your observations of the experiment and materials.

## Station 4: Egg in a Bottle

#### Materials

• Hard boiled eggs (with shells removed).
• A bottle or flask with an opening that is just small enough to prevent the egg from entering the bottle. Each group should have their own bottle as one of the students will have to put it to their mouth.
• Matches

#### Procedure

Part A
1. Drop a burning match into the bottom of the bottle.
2. After a few seconds, place the egg onto the mouth of the bottle.
3. Compete the relevant areas on the activity sheet.
4. Part B
5. Now that the egg is in the bottle, turn the bottle upside down so that the egg is resting in the neck of the bottle.
6. Tip back your head, place your mouth over the bottle opening and blow vigorously into the bottle.
7. Quickly remove your lips from the bottle hold it over the bench.
8. Complete this section of the activity sheet.

9. Cleanup
10. Wash the bottle with soap and hot water so that it's ready for the next class.

## Stop and Think

7. What causes the egg to be pushed into the bottle? What does the match have to do with it?
8. Why does blowing into the bottle cause the egg to pop back out?

## Station 5: Soda Bottle and Ping Pong Ball Barometer

Based on work by Patrick Cooney of Millersville University

#### Materials

• Soda bottle (Sobe 20oz bottles work well)
• 1 Ping pong ball
• Metric ruler
• Beaker to collect water
• A barometer (ideally, one that reads in kPa; if not available, convert pressure units to kPa)

#### Procedure

Image of the final state of Station 5. The ping pong ball is being held in place by the balanced forces inside and outside the bottle. Image by John McDaris
1. Obtain a reading of atmospheric pressure from the barometer at the station. Record this on the activity sheet.
2. Fill the bottle all the way to the top with water.
3. Push the ping pong ball onto the top to squeeze out a small amount of water.
4. Now, pour about one third of the water from the bottle into the graduated cylinder. Measure and record the amount of water in the graduated cylinder as V0 (pronounced "vee sub zero").
5. Hold the ping pong ball on top of the bottle and invert it over the beaker. Hold the ball loosely against the opening so that some water is allowed to leak past the ball into the beaker. Don't jiggle or rotate the ball during this process.
6. Eventually, enough water will leak out that the pressure on both sides of the ball will be the sameyou can take your hand away and the ball will stay in place. Add the water that leaked out into the graduated cylinder. Measure and record this total amount as V1 (pronounced "vee sub one") on the activity sheet.
7. Measure the distance from the mouth of the bottle to the top of the water it encloses and record this distance as D on the activity sheet.
8. Complete the calculations called for on your activity sheet to determine the atmospheric pressure in your classroom. Compare your calculated value to the reading you took off the barometer and answer the questions on the activity sheet.

When you turn the bottle upside down and allow a little bit of water to leak out, you are increasing the "empty" space inside the bottle without letting in any more air. In other words, the same amount of air that was in the bottle before now has more room to "spread out" and the pressure of the air in the bottle is lower than the pressure of the air outside the bottle.

When you reach the point where the ball stays in place without you needing to hold it, the pressure on both sides of the ball is the same. This fact allows us to do a few calculations and figure out what the atmospheric pressure is. Calculation of Atmospheric Pressure: Because the pressure is the same on both sides of the ball after the water has leaked out, that means that: P1 + ρgD = Atmospheric Pressure Where P1 ("P sub 1") is the air pressure in the bottle, ρ (rho – pronounced like "row") is the density of water, g is the acceleration due to gravity, and D is the height of water in the bottle after some has leaked out (as shown above).

The term ρgD represents the pressure that the water exerts on the ball. We also know that, originally, the pressure inside the bottle was equal to atmospheric pressure. So, now we can say that: P1 + ρgD = P0 Since air is nearly an ideal gas, we can make the approximation that P1 V1 = P0 V0 and that means that P1 = (P0 V0)/V1. If we put this result in the equation above and solve for P0, we get a relation that will allow us to calculate what the atmospheric pressure is. P0 = ρgD [V1 / (V1 – V0)]

## Stop and Think

9. Substitute your measurements into the following equation and calculate P0, the atmospheric pressure. Show your work.
P0 = ρgD [V1 / (V1 – V0)]

10. Compare your result to the atmospheric pressure you read off the barometer at the beginning. Are they the same? What are some factors that could affect how closely your result matches the measurement?

## Station 6: News Article Review

#### Materials

Copies of these news articles:

#### Procedure

1. Everyone in the group should pick an article to read. Everyone should take a different one unless there are more group members than articles.
2. Spend the first few minutes reading your article and then write a paragraph summary (on your activity sheet) of what the main points of the article were and what you learned from it.
3. When everyone is finished, each person should spend 1 minute telling the rest of the group about the article and fielding any questions their group-mates might have about the material.
4. Keep an eye on the time so that everyone gets a chance to share what they learned!
5. In your own words, write a couple of sentences based on the summary that your group-mates give of their articles.
6. Leave the articles for the other groups to use when you move on to your next lab station.

## Extending Your Learning

To learn more about the connections between air pressure and hurricanes, check out the additional resources listed below. You'll also be working with air pressure in Lab 6: Why Keep and Eye on the Barometer?

Pressure differences get the wind going
This short page from USA Today talks explicitly about how differences in atmospheric pressure between places creates wind as air moves from areas of high pressure to areas of low pressure.

Global Atmospheric Sea Level Pressure during Hurricane Frances
This visualization from the NASA Scientific Visualization Studio shows the global sea level atmospheric pressure for a period of 4 days in September 2004. This is the period when Hurricane Frances was approaching the East Coast of North America and Typhoon Songda was spinning in the western Pacific Ocean.

## Checking In Questions

• Can you pick the two storms out in the visualization? What do they look like?
Hurricane Frances and Typhoon Songda show up as dark blue circles on the general green background indicating that they have lower pressure inside them than the surrounding atmosphere.

NDBC Hurricane Information Page
This page from the National Data Buoy Center talks about hurricanes and provides data collected from a buoy during Hurricane Bertha in 1996. There is a map showing where the buoy was located and a graph of several types of data that it gathered. Pay special attention to the red line on the graph - this represents the barometric (atmospheric) pressure measured by the buoy as the storm passed over it.