Charges $+q$ and $-q$ are placed at points $\mathrm{A}$ and $\mathrm{B}$ respectively which are a distance…

Charges $+q$ and $-q$ are placed at points $\mathrm{A}$ and $\mathrm{B}$ respectively which are a distance $2 L$ apart, $\mathrm{C}$ is the midpoint between $\mathrm{A}$ and $\mathrm{B}$. The work done in moving a charge $+\mathrm{Q}$ along the semicircle CRD is
  1. $\frac{q Q}{2 \pi \varepsilon_0 L}$
  2. $-\frac{q Q}{6 \pi \varepsilon_0 L}$
  3. $\frac{q Q}{3 \pi \varepsilon_0 L}$
  4. $\frac{q Q}{4 \pi \varepsilon_0 L}$

Solution

Potential at \(\mathrm{C}=\mathrm{V}_{\mathrm{C}}=0\) Potential at \(\mathrm{D}=\mathrm{V}_{\mathrm{D}}\) \(=k\left(\frac{-q}{L}\right)+\frac{k q}{3 L}=-\frac{2}{3} \frac{k q}{L}\) Potential difference \(\mathrm{V}_{\mathrm{D}}-\mathrm{V}_{\mathrm{C}}=-\frac{2}{3} \frac{k q}{L}=\frac{1}{4 \pi \varepsilon_0}\left(-\frac{2}{3} \cdot \frac{q}{L}\right)\) \(\Rightarrow\) Work done \(=\mathrm{Q}\left(\mathrm{V}_{\mathrm{D}}-\mathrm{V}_{\mathrm{C}}\right)\) \(=-\frac{2}{3} \times \frac{1}{4 \pi \varepsilon_0} \frac{q Q}{L}=\frac{-q Q}{6 \pi \varepsilon_0 L}\)

Asked in: NEET 2007

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