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Four collinear arrangements of three fixed point charges (each of magnitude Q, signs as shown) are given, with a point P on the same line. Adjacent items (charge-charge or charge-P) are all separated by the same distance. Rank the arrangements in decreasing order of the magnitude of the net electrostatic force a test charge +q at P would experience. I. (+) (+) (+) P II. (+) (+) P (-) III. (+) (+) (-) P IV. (+) (-) (+) P

1. II > I > III > IV
2. I > II > III > IV
3. II > I > IV > III
4. III > IV > I > II

Correct answer: II > I > IV > III

Solution

Computing net forces (in units of kQq/d²): II = 1 + 1/4 + 1 = 2.25; I = 1 + 1/4 + 1/9 = 1.36; IV = 1 - 1/4 + 1/9 = 0.86; III = -1 + 1/4 + 1/9 = -0.64 (magnitude 0.64). Hence II > I > IV > III.

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