Question

Four point charges have the same magnitude of 2.52 10-12 c and are fixed to the corners of a square that is 3.90 cm on a side. three of the charges are positive and one is negative. determine the magnitude of the net electric field that exists at the center of the square.

Answer 1

Four charges are equal but one is negative, Total charge Q = q1 + q2 + q3 +q4 
Q = q + q + q - q = 2q = 2 x 2.52 x 10^-12 
Electric constant k = 8.99 x 10^9N•m²/C²  
Length of the square side L = 3.90 = 0.0039 m 
Since it is square and applying the Pythagoras theorem r = L/(Squareroot of 2) 
Electric Field E = k x2q / ( L/(Squareroot of 2))^2 
E = 8.99 x 10^9 x 2 x 2.52 x 10^-12 / (0.039/ square-root of 2)^2 
E = 90.62 x 10^-3 / 0.001521 = 59.58 N/C