Electrostatic Potential and Capacitance - Result Question 8

9. Four electric charges $+q,+q,-q$ and $-q$ are placed at the corners of a square of side 2L (see figure). The electric potential at point A, midway between the two charges $+q$ and $+q$, is [2011]

(a) $\frac{1}{4 \pi \epsilon_0} \frac{2 q}{L}(1+\sqrt{5})$

(b) $\frac{1}{4 \pi \in_0} \frac{2 q}{L}(1+\frac{1}{\sqrt{5}})$

(c) $\frac{1}{4 \pi \in_0} \frac{2 q}{L}(1-\frac{1}{\sqrt{5}})$

(d) zero

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Answer:

Correct Answer: 9. (c)

Solution:

  1. (c) Distance of point A from the two $+q$ charges $=L$.

Distance of point A from the two $-q$ charges $=\sqrt{L^{2}+(2 L)^{2}}=\sqrt{5} L$.

$\therefore V_A=(\frac{Kq}{L} \times 2)-(\frac{Kq}{\sqrt{5} L} \times 2)$

$=\frac{2 Kq}{L}[1-\frac{1}{\sqrt{5}}]$

$=\frac{1}{4 \pi \epsilon_0} \cdot \frac{2 q}{L}(1-\frac{1}{\sqrt{5}})$