丁香园AV

Pie statics ~30 Minutes

NOTE: This experiment will not work well in humid conditions. Wait for a dry spell, or try it during the winter term.

Objectives: To produce a portable electric charge.

BC Science Content:
-electricity (grade 7)

Materials : Aluminum pie plate, flat piece(enough surface area for the pie plate to rest on) of Styrofoam(the blue insulating Styrofoam works best) , piece of fur or wool sock/sweater, sheet of tissue paper, tape.

Doing it:
A1. Place the pie plate on the table upside down. Tear very small pieces of tissue paper and place about 10 on top of the pie plate.
A2. Rub the Styrofoam with the fur/wool to charge it, this may take several minutes if this is the first time trying with this piece of Styrofoam.
A3. hold the charged Styrofoam above the pie plate and slowly bring it closer. When it is about 5-10cm away, the pieces of tissue paper should jump back and forth between the pie plate and the Styrofoam.

B1. Cut a shaft of Styrofoam from the large piece (about 2cm x 2cm x 10cm) and tape it sticking out of the inside of the pie plate. You will use this as an insulating handle to move the pie plate while it is charged.
B2. Charge the Styrofoam again and place it on the table. Slowly lower the pie plate down onto the Styrofoam. (with the lights off?) Slowly bring your finger close to the edge of the pie plate. At about 1cm from the edge there will be an electrostatic spark from the plate to your finger.

B3. Now lift the plate up using the handle and move it away from the Styrofoam. It is now charged. You can discharge it by again bringing your finger close and creating another(but smaller) spark.

C1. Follow the steps in B to charge the pie plate.
C2. Put an empty pop can on its side and move the plate close to one side.

Explanation:
When you rub the Styrofoam with a wool cloth, you charge it negatively. That's because the Styrofoam attracts electrons from the cloth. Styrofoam is an insulator; it will hold its charge until it is discharged by electrons jumping into the air.

When you bring the Styrofoam close to the pie pan with the tissue bits, the bits are attracted to the charge of the Styrofoam and jump up to it. Once they touch the Styrofoam they pick up their own charge and are repelled bouncing back to the pie plate. Once they touch the pie plate they lose their charge and bounce back up.

When you place the pie pan on the Styrofoam, the electrons on the Styrofoam repel the electrons on the pan. Since the electrons can't leave the pie pan because it is completely surrounded by insulating air and Styrofoam, the pan retains its neutral charge. The electrons move as far away from the Styrofoam as possible, to the rim. If you put your finger near the rim, the electrons make a spark as they jump a few millimeters through the air to reach your finger. The air is ionized as the moving electrons knock other electrons off air molecules. The ionized air emits light and sound. You can also feel the flow of electrons though your finger.

After the electrons leap to your finger, the pan has a positive charge. Physicists say the pan has been charged by induction. You can carry the positively charged pan around by its handle and carry the positive charge to other objects. If you bring the positive pan near your finger again, or near any object that can be a source of electrons, the pan will attract electrons, creating a second spark. The Pan will slowly lose its charge by attracting stray electrons in the air.