Question:
One end of a $10 \mathrm{~cm}$ long silk thread is fixed to a large vertical surface of a charged nonconducting plate and the other end is fastened to a small ball having a mass of $10 \mathrm{~g}$ and a charge of $4.0 \times 10^{-6} \mathrm{C}$. In equilibrium, the thread makes an angle of $60^{\circ}$ with the vertical. Find the surface charge density on the plate.
Solution:
