A small, $2.00-\mathrm{g}$ plastic ball is suspended by a $20.0-\mathrm{cm}$ long string in a uniform electric field, as shown in Figure $\mathrm{P} 15.52$. If the ball is in equilibrium when the string makes a $15.0^{\circ}$ angle with the vertical as indicated, what is the net charge on the ball?