In a spherical metal shell of radius R, an electron is shot from the center directly toward a tiny hole in the shell, through which it escapes. The shell is negatively charged with a surface charge density (charge per unit area, ?) of 6.90 × 10-13 C/m2. What is the magnitude of the electron's acceleration when it reaches radial distances
(a) r = 0.500R and
(b) 2.00R?.