b
\( 2 \mathrm{~T} \sin \frac{\mathrm{d} \theta}{2}=\frac{\mathrm{kq}_0}{\mathrm{R}^2} \cdot \lambda \mathrm{Rd} \theta \)

\( {\left[\lambda=\frac{\mathrm{Q}}{2 \pi \mathrm{R}}\right]}\)

\( \Rightarrow \mathrm{T}=\frac{\mathrm{Kq} \mathrm{q}_0 \mathrm{Q}}{\left(\mathrm{R}^2\right) \times 2 \pi} \)

\( =\frac{\left(9 \times 10^9\right)\left(2 \pi \times 30 \times 10^{-12}\right)}{(0.30)^2 \times 2 \pi} \)

\( =\frac{9 \times 10^{-3} \times 30}{9 \times 10^{-2}}=3 \mathrm{~N}\)