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Three charges are placed at the vertices of an equilateral triangle of side $a$ as shown in the figure. The force experienced by the charge placed at vertex A in a direction normal to BC is:

1. $\dfrac{4}{2\pi\varepsilon_0 a^2}$
2. $-\dfrac{4}{2\pi\varepsilon_0 a^2}$
3. Zero
4. $\dfrac{2}{2\pi\varepsilon_0 a^2}$

Correct answer: Zero

Solution

In an equilateral triangle, the forces on the charge at A due to the other two charges are symmetric about the median from A to BC. Their components normal to BC are equal and opposite, so the net force in that direction is zero.

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