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 6.40 × 10^-13 c/m^2. what is the magnitude of the electron's acceleration when it reaches radial distances (a) r = 0.690 r and (b) 2.00r?