This week we will build our own capacitor and investigate the relationship between charge and voltage. The capacitor is based upon the primitive ones built in the 1700s known as Leyden Jars. Electrical pioneers like Benjamin Franklin bult Leyden Jars large enough to contain a lethal amount of charge. While the ones we build today are small enough to be safe, students should know that scaling up these capacitors can be dangerous.
To build the capacitor for today's lab, we will make use of a relic, a film can. We have a small supply for this lab. As you may recall, a capacitor is made of two conductors separated by a dielectric material. The smaller the distance between the two conductors, the stronger the capacitor. The plastic body of the film can will act as our dielectric. We will coat the outside with aluminum foil to act as one conductor, and will fill the can with water to act as the other conductor. Step-by-step instructions follow:
Get your film can, some aluminum foil, and some glue. The foil should be long enough to wrap the can, and wide enough to cover most of the can and a little bit of the bottom. Since you want to make sure that the conductor has good contact with the can, eliminate wrinkles in the foil. The easiest way to do this is to place the foil on the table and rub the bottom of the can on the foil. Apply glue to the foil and then wrap the can, folding over some foil onto the bottom of the can. While you are waiting for the glue to dry, skip down a few paragraphs and build your testing rig.
Fill the can with water to about 2/3 full. The capacitor will need to have a current path to the inner conductor. To do this, puncture the cap of the film can with a bent paperclip. Put the top on the can and you are ready to go.
If you are waiting for the glue to dry, you should be reading this paragraph. In today's lab you will charge up your capacitor and then discharge it. Since the length of the resulting spark is of interest, we'll have to build a device where we can make careful measurements. Use the rods and clamps to build a rig that will support a piece of foamboard. Puncture the foamboard with a nail. This nail is what we will use to discharge the capactior. See the photo below to see how to set up your experiment:
Once you have built your capacitor and rig, tape a wire to your capacitor to act as a discharge path. If the contacts look dirty, get a piece of sand paper to clean the metal. The other end of the wire goes to an alligator clip that attaches to the nail.
Use a piece of fur to put a charge on a PVC pipe. Do this in a repeatable fashion. If you do it with 15 swipes the first time, do it with 15 swipes every time. After you are done charging your pipe, move the paperclip along the length of the pipe to collect the charge onto the plates of the capacitor. Place the capacitor under the nail, connect the capacitor to the nail using the alligator clip, and slowly push the nail through the foamboard until you see the spark. Measure the position of the nail, and then push the nail down until it touches the paperclip. Measure again to see the distance of the gap that the spark jumped across. Do this three times. Then do another three trials, this time collecting charge from the rod twice before discharging. Take similar data for 3, 4 and 5 collections. Note that it is very helpful to place tape upon the table so that you can put the capacitor back in the same position every time.
Air breaks down and becomes a conductor when it is exposed to an electric field of roughly 3300V/mm. In other words, if your spark jumped 2mm, there was a potential difference of 6600 volts between the two conductors. Graph your spark distance on the y axis and the number of "collections" on the x axis. These substitute for voltage on the y axis and charge on the x axis. What do you expect your graph to look like? What do you actually find?