Electric Field and Flux

  • EM4.1 Grass Seeds and Electric Fields

    An impressive visualization of electric fields. Sprinkle grass seeds on the top of a glass plate, and use the Van de Graff generator to charge up various metal configurations glued to the underside of the glass plate: 2 point charges (dipole field), parallel plate capacitor, concentric circular capacitor, and Faraday's ice pail. Watch the induced dipoles within the grass seeds line up with the electric fields. Can be visualized using the overhead projector.

  • EM4.2 Electric Flux

    This uses a vu graph and a plexiglass contraption. The apparatus has a piece of plexiglass in which holes have been drilled that rotates about an axis on its side. On the vu graph, when the plate is parallel to the light source, the class sees all the dots perfectly. An the angel between the light source and the plate is increased, the image of the dots changes. They get weaker with the increase of angle. This shows the cosine dependency of flux.

  • EM4.3 Faraday Ice Pail Experiment

    This consists of a metal beaker with coated balls on its side. The beaker is charged with the Van de Graaff generator. As the beaker gains charge, the balls rise. This is observed by transferring charge to the projection electroscope with a test ball on a string. Charge remains on the surface and not inside.

  • EM4.4 Plotting of Equipotential Lines

    This is a complicated set up to hear field strength. Two pieces of metal are placed in a glass tray of water on the vu graph. They are connected to opposite ends of an audio oscillator. Two probes are attached to an amplifier, which is in turn connected to a speaker. The probes are placed in the water, and the class hears the oscillator. The intensity depends on the difference in potential between probes. If the straight pieces of metal are used, sound increase from one to the other, but not parallel to the pieces. The kit comes with circular pieces to demonstrate circular fields, and other figures. For plotting purposes and for better view, a piece of transparent graph paper is placed underneath the glass tray.

  • EM4.5 Faraday Cage

    Very short and sweet demo that demonstrates electrostatic shielding, a topic associated with Gauss's law. Tune a small hand-held radio to some FM radio station, and then place the radio inside a small aluminum cage. The signal completely disappears. (At this stage in the course, they haven't encountered EM waves, but most are satisfied with a simple explanation, such as "associated with radio waves are electric fields that cannot penetrate the conducting shell".) Should make clear why a radio antenna must be placed outside of the frame of a car.