TITLE: MTV no PTV
AUTHOR: David McDonald, Sidney High School
Sidney MT 59270
GRADE LEVEL/SUBJECT: 11, 12, general chemistry or physics.
OVERVIEW: This is a high interest introduction to the gas laws.
Stressing the interdependents of the three variables; pressure,
temperature, and volume.
PURPOSE: The introduction of Boyle's, Charles, and the universal gas laws.
To acquire a working knowledge of how the variables relate to each other in
the gas laws.
OBJECTIVES: As a result of this activity, the student will:
1. Known the variables used in the universal gas law with appropriate units.
2. Qualitatively understand how the variables relate in Boyle's, Charles,
and the Universal Gas Laws.
RESOURCES/MATERIALS: balloon, paper bags, pop can, hot plate, copper or
plastic pipe, wood dowel, and tape.
ACTIVITIES AND PROCEDURES:
1. At the beginning of the class, ask the students what are the call
letters for their favorite TV station. Hopefully they will say --> MTV
<-- . With this, lead them into a discussion of the PRESSURES from their
parents to watch less MTV. How many parents get hot under the collar,
a raise in body TEMPERATURE that could be measured. At what VOLUME of the
TV sound do their parents start telling them "turn down the sound, I
can't hear myself think". As you are leading this discussion write down
the following words on the black board. PRESSURE, TEMPERATURE, VOLUME
2. Handout a balloon to each student. Have them blow it up and hold the
balloon. Now ask the students what affects, conditions or quantities are
involved in keeping the balloon as it is. They should note the words that
are on the board.
As they mention each of the above variables get the students to state a
symbol to use for each and get a metric unit for each. To get the units
for pressure the students might have to go out to the parking lot and read
the metric units off of a car tire.
3. Hand out a tag board card (10 cm x 25 cm) and markers. Have each
student place the letters P T V in large letters on the card. Then put a
pencil hole through the center of each letter.
4. Show each of the following demo's and have students predict how each
variable will change. They will use the card and always place a pencil
in the variable that is constant. If the pressure of a gas is constant
then the pencil will be placed in the card through the hole in the P. Now
if the temperature of the gas goes up the card is pivoted on the pencil so
that the temperature goes up. One then sees that the volume of the gas goes
up.
4a. Potato Gun (Boyle's Law) ------- T cnst, V up, P down
A potato gun is made from a copper or plastic pipe 9/16 inch inner diameter
and 36 to 40 inches long that has had one end flared. For the copper pipe
use a metal flaring tool, for the plastic pipe file the inside of one end
with a rat tail file. A ram rod is made from a 1/2 inch wood dowel that is
about 6 inches longer that the pipe. Tape the dowel thickly about 4 inches
from the end. The tape acts as a hand guard.
To use the gun push the flared end of the pipe through a potato. Now take
the push rod and push the potato plug to the other end of the pipe. Push
the flared end of the pipe back into the potato so that you have two plugs
one on each end of the pipe. Using the ram rod push the second plug towards
the first plug.
The first plug will shoot out the end. (DO NOT AIM AT A STUDENT)
Ask the students which variable is held constant. -- temperature --
Have students place the pencil in the temperature hole. Now ask which of
the other two variables you are changing. -- volume of the gas --
The students will then take the card and move the volume down showing
that the volume is decreasing. The pressure of the gas is then going up!
4b. Collapsing Pop Can --------------- V cnst, T down, P down
Place a pop can one a hot plate with a small amount of water in the
can. When there is steam coming out of the can using gloves invert the can
into a pan of cool water. The can will collapse.
Ask the students which variable is held constant. -- volume ---
Have students place the pencil in the volume hole. Now ask which of the
other two variables you are changing. -- temperature of the gas --
The students will then take the card and move the temperature down showing
that the pressure of the gas inside of the can is decreasing.
4c. Inverted Paper Bag Balance (Charles' Law) ---------P cnst, T up, V down
A balance is made where the balance pans are inverted paper bags. A lit
candle is placed below one of the bags.
Ask students which variable is held constant. -- pressure --
Have students place the pencil in the pressure hole. Now ask which of the
other two variables you are changing. -- temperature of the gas --
The students will then use the card to find that the volume of the heated
gas is increasing.
4d. There are many gas demo's that use can use the more that you can
have students do the better.
Some are - exploding soap bubble, cartesian diver, handboiler, determination
of absolute zero, and open end thermometer.
TYING IT ALL TOGETHER: Have students (as homework) find two examples of
changing gases and using the PTV card find the variable that is constant and
determine the variables that will change.
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