Apparatus Competition

2007 AAPT Summer Meeting

Greensburo, NC

Improved Air Capacitor

Brett Carroll

Green River Community College

12401 SE 320^th St

Auburn, WA 98092

253-833-9111

bcarroll@greenriver.edu

Abstract

“Air Capacitors” are an analog of electrical capacitors that are used in many physics education programs, including the CASTLE (Capacitor-Aided System for Teaching and Learning Electricity) materials. While a very useful device in introductory electricity, previous sealed versions often invite student complaints about the foul-tasting air that builds up inside. They are also bulky and difficult to store. The improved air capacitor comes apart for cleaning and storage, and presents a much more attractive and inviting appearance to the student.

Construction of Apparatus:

These improved air capacitors are made from disposable storage containers that can be found in most grocery stores. Unlike the peanut butter jars of past versions, these require no cleaning (or massive peanut-butter consumption) before use. They are widely available and inexpensive – the jars for a set of ten capacitors can be purchased for less than $40.

It is important that the storage containers have a screw-on (not snap-on) lid to hold them on against student-supplied air pressure. A powered hand tool such as a Dremel™ is the easiest way to prepare the lids for use, but other methods could be improvised.

With a drill-cut bit in the tool (the kind that can drill in and then cut sideways), remove the inner portion of each lid, cutting out everything in the center and leaving only the outer raised rim that contains the threads. Remove any leftover sharp edges with a sanding tool until the inside of the rim is circular and reasonably smooth. Prepare two of the lids and place them top-to-top, then tape all around the outside perimeter with a piece of duct tape as wide as the two lids are high.

Take a large balloon (I use 17 inch balloons, but you can experiment) and cut off the neck, leaving about ¾ of the balloon in one piece. Insert the double lid into the dome of the balloon so that it is perpendicular to the neck (or what’s left of it) and about halfway in. The open part of the balloon will spring back around the lids when you release it, and the cut-away opening that was the mouth should be centered over the top lid with at least 1” of rubber coming over all sides. If it is not even just pull and strect the rubber until you have a good wide lip on all sides. Trim away any excess rubber to reduce the lip to about ¾” wide inside the upper lid.

The containers should be prepared to accept a straw, either directly though a small hole or through a grommet or stopper if you prefer. I drill a small hole (about 1/8 inch) in the side near the bottom of the container, the enlarge it with a ¼ inch bit to accept a straw. Drilling the ¼ inch hole directly will often crack the plastic, so use a pilot hole first.

Now screw the jars into the lid/balloon combo, beginning with the side that is completely covered with rubber. That is the harder side to screw in, as it must stretch the rubber into the threads of the lid. Try to avoid wrinkles in the rubber as you tighten it, but make sure it is tightly screwed on or the lid can pop off under pressure. Then screw the second container into the other side, again checking for a tight seal. It sometimes helps to retighten each side again as the rubber settles into the threads.

Your air capacitor is now ready for use. Slice a straw off at an angle to insert it into the hole, which should be just big enough to fit the straw, and blow to fill the capacitor with air.

Use of Apparatus:

Air capacitors are an excellent analog for the operation of electrical capacitors. An air flow (electrical analog: current) enters on one side, leading to a buildup of air (charge) in one chamber (capacitor plate). At the same time an air flow (current) out of the other chamber (plate) occurs, leading to a depletion of air (charge) in that chamber (plate).

Even though air is flowing in to one side of the air capacitor and out the other, no air moves through the rubber diaphragm (dielectric), exactly as in an electrical capacitor. The air (charge) buildup in one chamber (plate) leads to a pressure (voltage) that the student can feel, and the air flow (current) can be felt with the lips and fingers at the inlet and outlet.

If the small hole on the other side of the air capacitor is covered with a finger after filling (charging), the pressure (voltage) will be maintained until the finger is released, analogous to an electrical capacitor holding charge until a circuit is provided for the charge imbalance to cancel out (there is usually a slow leak and the air capacitor will discharge slowly even when plugged, and that is also true of most real-life capacitors, particularly if there is a bleed-off circuit in place).

When the finger is released, the stored air rushes out of the filled side and into the depleted side just as charges flow into and out of capacitor plates when a circuit is provided. The stored energy of the air due to the increased pressure in one chamber can be felt and heard when the air re-equalizes upon release. Similarly, the charge stored in a capacitor holds energy as a voltage (pressure) differential and can do work upon its release.

The air capacitor can be “charged” in the opposite sense by sucking the air out of one chamber with the straw, analogous to a charge of opposite polarity being placed upon an electrical capacitor.

To get even more mileage out of the analogy, drill another set of holes in the containers just large enough to fit a coffee straw, and compare the filling rate with a smaller straw (larger resistor) with that of the larger straw. The unused holes can be covered with a piece of duct tape. Then compare the rate at which the air capacitor empties with the two different sizes of straw. That gives you a simple analogy for the charging rate and time constant of an RC circuit with different resistors.

When you are finished with the air capacitors, the containers can be unscrewed from the lids and cleaned if desired. Even if they are not cleaned, opening the chambers lets the bad-tasting chemicals dissipate during storage and vastly improves the flavor of the air.

They containers can now be stacked for easy storage, taking up much less room than the older version that was usually stored assembled because of the difficulty of re-assembly. If you purchase thick-skin balloons (Most 17 inch balloons are thick-skinned, and are available online from many party suppliers) they should last for many re-uses even after repeated assembly and reassembly. If a balloon should burst during use it takes only a minute to replace. For longest storage life, keep the lids and balloons in a light-tight and reasonably air-tight box until use.

Parts List

Equipment and costs required to construct apparatus:

Item	Source	Part number	Cost
Ziplock disposable container (2)	Grocery store		3.79
Balloon (17”)	mylarballoonfactory.com	17” latex model	0.20
Straw	Grocery store		0.02
Duct tape	Hardware store		0.10
Total Cost			4.11