Electricity & Magnetism

Charge and Carry

When a Styrofoam plate is rubbed with a wool cloth, it becomes charged negatively. That's because the Styrofoam attracts electrons from the cloth. Often, a plate fresh from the package will start with a positive charge. If it does, it will have to be rubbed long enough to cancel this initial charge before it can begin building a sizable negative charge. When the pie pan is placed on the Styrofoam, the electrons on the Styrofoam repel the electrons on the pan. The electrons can't leave the pie pan because it is completely surrounded by insulating air and Styrofoam, so the pan retains its neutral charge. If a visitor touches the pie pan while it is near the Styrofoam, the mobile electrons will be pushed off the pan and onto the person’s body. The electrons make a spark as they jump a few millimeters through the air to reach the finger. The air in the spark is ionized as the moving electrons knock other electrons off air molecules. The ionized air emits light and sound. After the electrons leap to the user’s body, the pan has a positive charge. Physicists say the pan has been charged by induction. The pan can be carried around by its handle, bringing the positive charge to other objects. If the positive pan is brought near any object that can be a source of electrons, the pan will attract electrons, creating a second spark.