Demos: 5A-03 Two-by-Four Electroscope

A four-foot long two-by-four, resting on a low friction pivot, is made to rotate by holding a charged rod nearby. This is a dramatic demonstration of the strength of the electric force.

Directions: Place the board on the lecture desk top, pivot side down, and allow it to come to equilibrium. Charge an ebonite rod vigorously and bring it close to the board near its end. As the board begins to move, pull the rod slowly away. The board will continue to follow.

Suggestions for Presentation: Most students think of the gravitational force as being very large and will often express surprise that it is much weaker than the electrical force. This demonstration cannot begin to show the relative sizes of the two forces (10³⁹), but if you place the uncharged rod close to the board, there will be no evidence at all of the gravitational force between them. However, when the rod is charged, the electrical force is quite evident.

Applications: Refer to the many instances in which the electrostatic forces are evident (static cling, sparks from scuffing feet, etc) but the gravitational force between the same objects is not evident at all.

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