Introduction/ General Description:
Welcome to the wonderful world of electricity. The study of electricity combines the sciences of chemistry and physics, to give us an understanding of how everyday things, like the household light bulb, work. Electricity can be described as the flow of electrons. Electrons are the negatively charged particles that make up the fundamental unit of matter, the atom. To understand how electrons move, we must first understand how charged particles around one another. Particles with similar charges (two positive or two negative particles) experience repulsion forces. So they tend to move away from each other. Opposite charges (positive and negative) are attracted to each other, and thus move towards each other. It is expected, according to this law, that electrons (being negatively charged) will flow from areas of negative charge to areas of positive charge. However, when electricity was first studied, scientists thought that the positive atomic particles were moving, thus creating the convention that electric current actually flows from areas of positive charge to areas of negative charge.
The flow of electrons can not be seen, but the effects of the energy it gives off can. This energy is seen in the form of light, and felt in the form of heat. (What happens when you turn on a light switch? What happens when you touch a bulb that has been on for a long time?)
1997 Science Connector Kent Stevens demonstrates the effects of static electricity produced with a Van de Graaf generator.
Static electricity is one form of electricity that utilizes the motion of two objects to create the flow of charge. Have you ever rubbed your feet along the carpet as you crossed the room, and then touched a light switch? What happened? You probably received a shock. You were creating static electricity by rubbing your feet along the carpet fibers.
A device designed to generate large sparks of static electricity was produced in the 1930s by Robert van de Graaf, an American physicist. The Van De Graaf generator (see figure) has a belt that moves inside a column to create a charge. The charge then builds up on the dome and the electricity looks for a way to ground itself, because it is collecting a lot of negative charges that want to get away from each other (it will jump to any conducting object close to it). Think of what lightning does when it "strikes" - it goes directly for the ground from the air where it has built up a net negative charge.
Primary Activities:
- Disruptive Arc: This demonstration uses the smaller discharge electrode. The electrode is held 3 to 4 inches from the dome of the generator (it is important to remember to ground the electrode to the binding post on the base of the generator to avoid unnecessary shocks!). Once the generator is turned on, a bright bluish-white arc will jump from the dome to the electrode. You should also hear a sharp crack. This is the electricity moving to another object in search of a way to ground itself.
- Charging Person's Hair: In this demonstration, the human body is used as a conductor for the static electricity created from the generator. A volunteer is first insulated from the ground (so electricity will have no way of grounding itself) by having them stand on a plastic stool or crate or by making them kneel on a wooden stool. He/she then places both his/her hands on the generator before the generator is turned on. Once it is turned on, the volunteer must NOT remove his/her hand from the generator until it is turned off (once again, to avoid receiving a shock). If the demonstration is a success, the subject's hair should stand up (best results come from clean, straight, dry hair that is 3 to 4 inches long). From this demonstration, we can see how important it is to keep the subject insulated from the ground, so as to limit the number of ways he/she can "lose" charge. If too much is lost to the ground, not enough electricity will go through the hair. Be sure to use the discharge electrode to remove the charge on the Van de Graaf before the person releases his/her hands.
Optional Activities:
- Electric Wind: This activity demonstrates how the air molecules around the generator are indeed charged because of the static electricity. Bring the small electrode near the dome, and place a lit candle in between the two objects. The flame should be bent side ways. This happens because the charged air particles are trying to get away from each other (like charges repel each other).
- Gas Tube: Hold a tube filled with neon gas (or another of the various gas tubes) near the generator, and it will glow. The basic explanation for this is that the electricity created by the generator is flowing through the tube and exciting the gas particles. These gas particles then give off (emit) light once excited. Each gas will give off a different color glow depending on its identity.
Conclusion:
These activities use various techniques to allow participants to view the effects of electricity. It is important that participants are made to understand why each effect is seen. Otherwise the activities will just carry the merit of a magic show, entertainment rather than education. To do this, the instructors need ask the students: "Why is this effect being seen?" Instructors should allow open discussion, but should make sure that observations are being attributed to the properties of electricity at the level of electron behavior. Discussion should also lead to an awareness of : insulators, conductors, and resistance. An understanding of such related subjects should be encouraged, and are a good place to explore follow up activities. Most importantly, try to get students to use their new found understanding of electricity to explain phenomena more common to their everyday life.
