A tiny ball (mass = 0.012 kg) carries a charge of -15C. What is the electric field magnitude that would cause the ball to float above the ground? (chp18,p39)

An air-filled capacitor is formed from two long conducting cylindrical shells that are coaxial and have radii of 26 mm and 103 mm. the electric potential of the inner conductor with respect to the outer conductor is - 700 v (k = 1/4πε 0 = 8.99 × 109 n · m2/c2) the energy stored in a 1.0-m length of this capacitor is closest to