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A point P is 40 m away from a 20 μC point charge and 20 m from a 4 μC point charge. The electric potential at P is _______ V. [K = 9×10⁹ N m² C⁻²]

Asked in GSEB Board July 2018 · Potential of point charges

• (1) 1300
• (2) 6300
• (3) 2700
• (4) 4500

Step-by-step solution

Why the other options are wrong

• (1) No single slip gives 1300 V. Sanity check: the 20 μC charge alone contributes 4500 V at P, and both charges are positive, so the total must be more than 4500 V.
• (3) Subtracts instead of adding, as if the 4 μC charge were negative: 4500 - 1800 = 2700 V. Potential is a scalar and both charges are positive, so the contributions add.
• (4) Counts only the 20 μC charge, (9×10⁹×20×10⁻⁶)/(40) = 4500 V, and leaves out the 1800 V from the 4 μC charge.

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