\(\tau = 4\,{\text{Nm}},l = 2\,{\text{cm}} = 0.02\,{\text{m}},q = ?\)
\(\tau = pE\sin \theta = (ql)E\sin \theta \)
\(\therefore q =\frac{\tau}{E l \sin \theta}=\frac{4}{2 \times 10^{5} \times 0.02 \times \frac{1}{2}}\)
\( = \frac{4}{{2 \times {{10}^3}}} = 2 \times {10^{ - 3}}{\text{C}} = 2\,{\text{mC}}\)