Warning: Be careful not to shock yourself with the van de Graaff. Always keep the grounding wand connected to ground, and closer to the generator than you are.
Theory: Objects with the same sign charge will repel each other. The electric force experienced by a charged object is along the direction of the electric field lines.
Description: Before doing anything else, connect the grounding rod to the ground (middle) connection of a plugged-in extension cord. Pour enough styrofoam peanuts into the pan to cover the bottom, and then add a few more. Balance the pan on top of the generator, and turn it on to a moderate level. A setting of 30 - 50 is about right. After a few seconds, peanuts should start flying out of the pan. Because they are so light, close to the generator the electric force dominates, and the peanuts travel along field lines.
This demonstration works best in dry weather. In humid weather, the strings that show field lines should be taped down to the inside of the pan; otherwise they bleed the charge away too fast.
Setup time: 15 minutes.
References: (available in the demo room)