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Charges +q and -q are placed at points A and B respectively which are a distance 2L apart, C is the midpoint between A and B. The work done in moving a charge +Q along the semicircle CRD is

Asked in CBSE AIPMT 2007 · Work done in moving a charge

• (1) (qQ)/(2πε₀ L)
• (2) (qQ)/(6πε₀ L)
• (3) -(qQ)/(6πε₀ L)
• (4) (qQ)/(4πε₀ L)

Step-by-step solution

Why the other options are wrong

• (1) Three times the correct magnitude and with the wrong sign; no single slip in V_D-V_C reproduces it.
• (2) Right size, wrong sign: +Q moves closer to -q, so the potential falls and the work by the external agent is negative.
• (4) Keeps only the -q charge at D (distance L) and also drops the sign; the +q charge at distance 3L has been left out.

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