13) 83.7 N
14) 6.19 x 107 N 45° above the x-axis
15) 2.96 x 107 N toward the center of the square
16a) 10.1(kQ2 / l 2) at 61°
16b) 14.8(kQ2 / l 2) at 332°
17) 0.30 N at 365°; 0.26 N at 139°; 0.26 N at 30°
18) 5.54 x 10-4 C and 5.54 x 10-6 C
19) magnitude = 0.40Q0 ; distance = 0.37l from –Q0 towards -3Q0
20) 116 cm = 1.16 m
21a) 69.9 x 10-6 C and 22.1 x 10-6 C
21b) 104.4 x 10-6 C and -14.4 x 10-6 C
22) 1.10 x 1012 electrons