Inflating a Balloon

Objective:

In this experiment, we learn about warm air expanding and cold air contracting.

Materials:

• Hot water
• Cold Water
• Balloon
• Bottle
• Bowl

Procedure:

Start by filling the bottle with warm to very warm water. You may need to experiment with bottle and balloon size. Fill a bowl with cold water. You will want a bowl that is big enough for the bottle to fit in and, with water in the bowl, be able to cover up to ½ of the bottle. Let the warm water sit in the bottle for about a minute. Then, empty the bottle. Immediately stretch a balloon over the top of the bottle and set the bottle in the bowl of cold water. You may need to wiggle the balloon a little, but it should invert back into the neck of the bottle, and start to inflate.

The warm water heated the bottle. Once the water was removed, the bottle itself was still warm, thus warming the air inside of the bottle. Gases, in this case air, expand when heated at a constant pressure, and contract when cooled. When the balloon was put on the top, it sealed the bottle, so no outside air could enter in. Once the bottle was placed in the cold water, the air inside of the bottle cooled and contracted (got smaller, took up less room) and then tried to suck in outside air. This caused the balloon to pull in and inflate inside the bottle. Heating a gas causes the material's atoms or molecules to move faster. The faster they move, the farther apart they try to move. This also creates higher pressure. While the bottle had warm air in it, the pressure in the bottle was higher than the pressure of the ambient air outside the bottle. But once the air inside the bottle began to cool, it was colder than the outside air. Air moves from high to low pressure, thus the balloon inverting and expanding inside the bottle. This works the same way in the atmosphere, with air moving from high pressure to low pressure. A rule of thumb...if the wind is at your back the low is on your left. Pressure is defined as force acting on a certain area:

Example Problem: The top of a student's head is approximately circular with a radius of 3.50 inches. What force is exerted on the top of the student's head by normal atmospheric pressure (14.7 lbs/in2)?

The area of the top of the student's head is found from:

We find the force exerted on the top of the student's head by rearranging the equation to F = p*A

so F = (14.7 lb/in2) (38.5 in2) = 566 lb of pressure! No wonder I have a headache!

References:

http://www.flash.net/~spartech/ReekoScience/ExpInflatingBalloons.htm

Extra-Credit:

This experiment can be expanded to relate pressure to temperature. The Ideal Gas Law describes this relationship:

Where P = Pressure (Newtons)

= Density

R= Universal Gas Constant ( 287 J kg-1 K-1)

and T = Temperature (degrees Kelvin)

You can use this relationship to discuss why the pressure in a closed container (like a pressure cooker) increases as the temperature increases. This relationship is useful to meteorologists to help describe rising motions in the atmosphere and how temperature changes can force the air to move.

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