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D4.7. A portion of a two-dimensional (Ez = 0) potential field is shown in Figure 4.7. The grid lines are 1 mm apart in the actual field. Determine approx- imate values for E in rectangular coordinates at: (a) a; (b) b; (c) c. 112 V 110 V 108 V 106 V 104 V b 102 V