Teaching Physics with the Physics Suite

Edward F. Redish

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Forces between three charges: Dependences

Three charges are lined up in a row as shown in the figure at the right.   For each of the items below, complete the statements given with all the possible correct answers in the box at the right.   If none are correct, write N.

A.   All the charges have the same magnitude. If we compare the force charge q1 exerts on charge q3 ( F1→3) to the force q2 exerts on charge q3 ( F2→3)

  1. F1→3 is twice as big as F2→3 .

  2. F1→3 is half as big as F2→3 .

  3. F1→3 is more than twice as big as F2→3 .

  4. F1→3 is less than half as big as F2→3 .

  5. F1→3 doesn't affect q3 at all since q2 is in the way.

B. If the amount of charge in q3 is doubled, then

  1. F1→3 stays the same and F2→3 doubles.

  2. F1→3 and F2→3 both double.

  3. F1→3 and F2→3 both stay the same.

  4. F1→3 is now the same as F1→2 .

  5. F1→3 is now half as big as F1→2.

C. With the amount of charge in q3 doubled to twice the size of q1 and q2

  1. F1→3 = 2 F3→1

  2. 2 F1→3 = F3→1

  3. F1→3 =   F3→1

  4. F3→1 =   F1→2 + F1→3

  5. F1→3 =   F2→3

D. Restore all the charges to their original (equal) values.   Now double the distance between charge 2 and charge 3.   The net electric force felt by charge 3

  1. stays the same

  2. reduces by a factor of 2

  3. reduces by a factor of 4

  4. reduces by more than 4

  5. reduces by a factor between 2 and 4.

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Page last modified February 29, 2008: F46