Teaching Physics with the Physics Suite

Edward F. Redish

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A positive and negative test

In the figure below are shown four situations in which charges have been placed.   Each of the positive and negative charges has the same magnitude.   The positive and negative charges are fixed in their position.   Each situation is independent and you do not need to be concerned about any charges that might be outside the boxes.

A. If in each situation, we put a small positive test charge, q, at the indicated positions. Rank the magnitude of the force that the test charge would feel. Use a ranking such as E>F=G>H=0, that is, only use greater than signs, indicate if any two situations produce equal forces, and indicate if any of the forces are zero.

B. Instead of using a positive test charge, we use a negative test charge of the same magnitude.   How would the forces felt by the test charge in the four situations change?

C. Instead of using our original positive test charge, q, we use a smaller negative test charge, -q/2. How would the electric field measured by this test charge differ from the one measured by our original test charge?

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Page last modified February 10, 2008: E41