31(b)] What is the electrostatic
energy stored by the system FIGURE 2 31
Solution
(a) The charge on the capacitor is
      Q = CV = 900 × 10–12 F × 100 V = 9 × 10–8 C
The energy stored by the capacitor is
      = (1/2) CV 2 = (1/2) QV
= (1/2) × 9 × 10–8C × 100 V = 4