Teaching Physics with the Physics Suite

Edward F. Redish

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What's the charge, officer?

Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (nylon against silk, glass against polyester, etc.) and each of the pithballs is charged by touching them with one of these objects. In each case, be sure to discuss why you chose the answer you did.

A. In one set of experiments, it is found that pithballs 1 and 2 repel each other and that pithballs 2 and 3 repel each other. From this we can conclude that

  1. 1 and 3 carry charges of opposite sign.
  2. 1 and 3 carry charges of equal sign.
  3. all three carry the charges of the same sign.
  4. one of the objects carries no charge.
  5. we need to do more experiments to determine the sign of the charges.

B. In a second set of experiments, it is found that pithballs 1 and 2 attract each other and that pithballs 2 and 3 repel each other. From this we can conclude that

  1. 1 and 3 carry charges of opposite sign.
  2. 1 and 3 carry charges of equal sign.
  3. all three carry the charges of the same sign.
  4. one of the objects carries no charge.
  5. we need to do more experiments to determine the sign of the charges.

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Page last modified February 11, 2006: E28