Making Two Types of Barometers

Objective:

Show that over a time period, the levels of water change. This can be shown to correlate with changes in weather. This experiment uses simple materials and can be used over a long duration.

First Type of Barometer

List of Materials:

• Small Coffee Can
• Plastic Wrap
• Scissors
• Straw
• Index Card
• Rubber Band

Procedure:

First, tightly cover the top of the coffee can with plastic wrap, using a rubber band to hold the plastic wrap in place. The cover should be taut making the can airtight. Next, place the straw horizontally on the plastic wrap so that two-thirds of the straw is on the can. Tape the straw to the middle of the plastic wrap so that it will not fall off. Tape an index card to the can behind the straw. The straw will act as a pointer on the card. Carefully record the location of the straw on the index card. If desired, hatchmarks can be drawn on the index card to make observing the changes easier. After 15 minutes, record the new location of the straw on the index card. Continue checking and recording the straw location as often as desired. It will change day to day, and if looking closely, even hour to hour. Be careful not to place your barometer near a window, as the barometer is sensitive to temperature as well as air pressure.

Just as you can feel pressure from the water at the bottom of a swimming pool, there is also air pressure from the weight of air in the atmosphere. In this experiment, high pressure will make the plastic cave in, and the straw go up. Low pressure will make the plastic puff up, and the straw go down. If possible, check your measurements with a real barometer. Notice what happens to the barometer when a big storm comes. Can you use your barometer to predict a storm? Older kids, what IS the relationship between air pressure and temperature? (Hint - ask your teacher about the Ideal Gas Law)

The barometer was first invented by an Italian man named Torricelli. The barometer consists of a tall narrow glass tube that has been filled with mercury and inverted in a pool of mercury. The force of air on the pool of mercury supports the column of mercury and varies from day to day and with altitude. This is much the same as the barometer we make in the next experiment.

Air pressure is measured in units including Pascals (kg m-1 s-2), Millibars (10 -2 Pa), and Atmospheres. 1 atm = 760 mm (29.92 in) Hg (mercury). The unit of 1.00 mm of Hg is sometimes called a torr in honor of Torricelli. Pressure at the earth's surface (depending on altitude) averages 1013Mb.

The following internet site has a very good (and very detailed) discussion on a variety of topics dealing with fluid . It would be appropriate for a Senior-level discussion.

References:

http://nish.mit.edu/~2006/Textbook/Nodes/chap02/node10.html

Second Type of Barometer

List of Materials:

• Measuring Cup (preferably the glass kind with a handle and spout)
• Water
• Food Coloring
• Marker
• Plastic Soda Bottle (it should fit snugly in the measuring cup, so that the mouth of the bottle does not touch the bottom of the measuring cup)

Procedure:

Fill the measuring cup with water and add some food coloring. On a rainy or stormy day, flip the empty soda bottle upside-down into the glass measuring cup.

Assure that you use a bottle that is just the right size. Make sure that the level of the water extends into the neck of the bottle. Make a mark on the cup to indicate the water level within the bottle. When the weather changes / storms move away, examine the water level in the bottle.

The amount of air within the bottle is fixed at whatever the atmospheric pressure was on the day you turned the bottle upside down. The pressure on the surface of the water depends on the current air pressure. As the weather becomes drier (high pressure replaces lower pressure), the air pressure increases, forcing the water to rise in the bottle. Note: if you leave this set up for a long time, you will notice changes due to evaporation.

The atmosphere pressure can be calculated by the equation p=F/A Where F is the weight of the air, and A is the area.

References:

http://www.miamisci.org/hurricane/barometer.html