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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

Asked in CBSE AIPMT 2011 · Potential of point charges

• (1) 1/(4πε₀)(2q)/L(1+√5)
• (2) 1/(4πε₀)(2q)/L(1+1/(√5))
• (3) 1/(4πε₀)(2q)/L(1-1/(√5))
• (4) zero

Step-by-step solution

Why the other options are wrong

• (1) Two slips together: the sign of the -q charges dropped and the distance √5 L multiplied instead of divided.
• (2) Drops the minus sign of the two -q charges, so their contributions add instead of subtract.
• (4) Assumes that zero net charge gives zero potential. The +q charges are nearer to A (L) than the -q charges (√5 L), so they do not cancel.

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