Atmosphere Bar

Introduction

What does atmospheric pressure—14.7 pounds per square inch— really feel like?

Concepts

• Force
• Pressure

Background

Firemen use high-pressure hoses. Scuba divers must not dive too deeply because of water pressure. Mountain climbers must not climb too high because of low air pressure. What is pressure?

Pressure is not the same as force. Force is a push or pull one body exerts on another. It is what causes the interaction between two objects. Use the eraser end of a pencil and apply a specific force (push) onto the palm of your hand. You can feel the pressure. Now, turn the pencil around and place the sharpened end into the palm of your hand and apply the same force (push). Caution: Do not push too hard as to pierce the skin. Same force—different pressure! The same force on a smaller area produces greater pressure.

Pressure (P) is the amount of force (F) exerted per unit area (A). It is often expressed as:

P =F/A

The Pascal (Pa) is the SI unit for pressure. One Pascal of pressure is defined as the force of one Newton per square meter (1 Pa = 1 N/m²).

The Earth’s atmosphere exerts a pressure on everything within it. At sea level, atmospheric pressure is equal to one atmosphere (1 atm) or 101,325 Pa. Equivalent units for 1 atm pressure are 760 mm Hg or 14.7 lbs/in². This means that at the Earth’s surface, the atmosphere exerts a force of about 100,000 Newtons on every square meter it “touches.” This amount of force (100,000 Newtons) can be compared to the weight of a large truck!

Materials

Safety Precautions

Obviously, a 15-lb steel bar can be a dangerous item if used inappropriately. Do not leave the bar unattended and supervise all demonstrations with the bar. Be careful when using the bar in a vertical position and practice holding the bar to avoid any accidental slippage.

Procedure

1. Use a student volunteer. Have the student place his hand palm up on a table top. Carefully place the “atmosphere bar” flat (horizontally) into the palm of the student’s hand. Since the force is spread over a fairly large surface, the pressure shouldn’t be too great.
2. Carefully lift the bar into a vertical position and slowly let the full mass of the bar be focused on 1 in² of the palm of the student’s hand. Support the bar carefully and only allow as much pressure as the student can withstand.
3. Have students discuss the pressure exerted by a 14.7-lb bar per square inch of a student’s hand. Students will likely remember what one atmosphere of pressure really means for a long time.

Teacher Tips

• The concept of pressure being related to the area of the applied force is often difficult for students to understand. Have students try the “pencil” activity described in the background to start a discussion of pressure and area. Other pointed and blunted objects can be used to avoid potential punctures with pencil lead. High heel and flat heel shoes might provide an interesting discussion topic and/or demonstration.
• Introduce the idea of atmospheric pressure being equal to 14.7 lbs/in². Contrast that pressure with tire pressures of 30 lbs/in² or 90 lbs/in².
• Use the Atmosphere Bar to dramatically illustrate what 14.7 lbs/in² is really like. Pass the bar around the class and let each student lift it. Most will be quite surprised at its great mass.
• Use this demonstration bar to introduce other examples of air pressure as appropriate to your teaching unit. What role does air pressure play in our lungs and respiratory system? Why don’t living organisms collapse under the pressure? What makes for good tire traction—large surface area or small surface area? How do we use air pressure to our advantage? When is air pressure a problem?
• The atmosphere bar may develop rust if you live in a humid area. Simply use steel wool to clean the bar.

References

Special thanks to Walter Rohr (retired) of Eastchester High School, Eastchester, NY, for bringing this idea to the attention of Flinn Scientific.

Meloan, C. E. J. Chem. Ed. 1988, 65, 69.