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 4.10 × 10^-13 C/m². What is the magnitude of the electron's acceleration when it reaches radial distances 

(a) r = 0.480 R 

(b) 2.00R?