Apparatus Competition

2007 AAPT Summer Meeting

Greensboro, NC

Hero’s Engine LN₂

Doug Johnson, Hector Maciel, Maria Salazar, Anatol Hoemke

California State Polytechnic University, Pomona

3801 W. Temple Ave

Pomona, CA 91768

909-869-4033

dcjohnson@csupomona.edu, hmmaciel@csupomona.edu, mariasalazar@csupomona.edu, ashoemke@csupomona.edu

Abstract

This device is an inexpensive project (not including the liquid nitrogen) that demonstrates Newton’s 3rd law and the conversion of heat energy into mechanical energy. As the nitrogen absorbs heat energy from the water, it vaporizes. The expanding volume of gas exits the bottle and is directed in such a way that the force it imparts pushes the tabs in the direction opposite the exiting gas, causing the spin.

Construction of Apparatus:

Materials Needed:

3/4" (I.D.) PVC pipe

~.5 L (16.9 fl. oz.) Plastic bottle w/ cap

1 Hex head cap screw, 1/4" diameter, 3" length, 1" of threads

2 Washers and nuts for the cap screw

Liquid Nitrogen

Bucket of room temperature water

Tools Needed:

Saw

Drill press or power drill with various drill bits

Vise

Pliers

Exacto-knife

1. Cut the pipe to 18".

2. Drill a hole slightly larger than the cap screw 1" in from one end of the PVC pipe.

3. Drill a 1/4" hole through the center of the bottle cap.

4. Pull the cap screw through the hole in the pipe with washers on both sides of the pipe. Thread one of the nuts to the end of the threads on the cap screw. Twist the cap up onto the threads so that it sits against the nut. Tighten the second nut under the cap to fasten the cap between the two nuts.

5. Cut 2 tabs into the bottle and bend them slightly outward as shown on the schematic so that the exiting vapor causes the bottle to turn counter-clockwise, or into the cap threads.

Use of Apparatus:

** Wear eye protection! Due to the possibility of defects in the plastic bottles, the temperature change and pressure build up could cause the bottle to explode.**

Pour about a 1/4 - 1/3 cup (2 – 3 oz) of liquid nitrogen into the plastic bottle. Tightly fasten the plastic bottle to the cap. Touch the bottom of the plastic bottle to the room temperature water and hold it in contact. The liquid nitrogen will quickly vaporize and the pressure build-up will cause the nitrogen gas to exit the bottle through the small tabs. The gas hitting the tabs will force the bottle to spin rapidly. Once it has been spinning for a few seconds, you can raise the bottle up and away from the water and it should still spin for a short while. The nitrogen exiting the bottle will generate a nice cloud of vapor around the bottle.

Fig. 1 – Design Schematic.

Fig. 2 – Engine Usage (from left to right): First contact with water; sustained contact (bottle spinning and producing vapor cloud); bottle removed from water while still spinning.

Equipment and costs required to construct apparatus:

Item	Source	Part number	Cost
¾” PVC pipe, 18” length	Home Depot		$0.80
~.5L plastic bottle w/ cap	Water or soda bottle		$1.00
3” Hex head cap screw	Home Depot		$0.61
2 Washers	Home Depot		$0.48
2 Hex nuts	Home Depot		$0.30
Total Cost			$3.19