EM3.7 Van de Graff Generator

Using the generator alone, connecting it to itself charges the two spheres to opposite charges. Bending the smaller sphere towards the bigger one, an arc will be seen by the class when the spheres are close enough. There is a variety of apparati that can be connected to the generator.

WITCH'S HAIR - Witch's hair is a rod with hair on the top. As the generator puts a charge on the rod, the hair stands up (because it has the same charge as the rod and each other piece of hair). If some volunteer with long hair is willing, the same demonstration can be done by having that person stand on an isolation platform (E26).

FARADAY'S ICE PAIL - Another is a metal beaker with coated balls on the side. As the beaker gains charge, the balls rise. A test ball will show that charge stays on the surface of the pail, and not on the inside. Twm .. This twirl has sharp points which give off charge. This causes the twirl to spin. This is an electrostatic Hero's engine.

ELECTRIC WHIRL - The Electric Whirl is made of a number of brass wires, with rearward-facing sharp points, jointed at a hub. This is pivoted atop an insulated shaft. One terminal of an electrostatic machine is connected to the whirl and the other is grounded. As the charge builds up on the metallic parts of the whirl, the equipotential lines are bunched together at the sharp points, creating a large electric field there. Eventually the field becomes large enough to ionize the air molecules and create a space charge that is of the same sign as the point. The mutual repulsion between the space charge and the point causes the wheel to spin.

FLUORESCENT LIGHT BULB - Using the new Van de Graff generator and a small fluorescent tube, you can qualitatively demonstrate both teh effects of a potential difference and the 1/r dependence of voltage from a point source. If you make the room dark, turn on the VDG, hold the fluorescent tube close to the VDG and orient it radially, you can see the tube glow. It even glows more brightly at the nearer end, due to the 1/r dependence. Students should be impressed, as there are no wires attached to the tube! If you then turn the tube azimuthally so it is perpendicular to the VDG's field lines, the bulb goes dark, since the tube is now approximately oriented along an equipotential surface.

Equipment Location: Shelf D5-2