
13. We choose the coordinate axes as shown on the diagram below. At the center of the square, the electric
fields produced by the charges at the lower left and upper right corners are both along the xaxis and
each points away from the center and toward the charge that produces it. Since each charge is a distance
d=√2a/2=a/√2 away from the center, the net field due to these two charges is
Ex=1
4πε02q
a2/2−q
a2/2=1
4πε0
q
a2/2
=8.99 ×109N·m2/C21.0×10−8C
(0.050 m)2/2=7.19 ×104N/C.
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d
d
a
a
x
y
−q
q−2q
2q
At the center of the square, the field produced by the charges at the upper left and lower right corners
are both along the yaxis and each points away from the charge that produces it. The net field produced
at the center by these charges is
Ey=1
4πε02q
a2/2−q
a2/2=1
4πε0
q
a2/2=7.19 ×104N/C.
The magnitude of the field is
E=E2
x+E2
y=2(7.19 ×104N/C)2=1.02 ×105N/C
and the angle it makes with the xaxis is
θ=tan
−1Ey
Ex
=tan
−1(1) = 45◦.
It is upward in the diagram, from the center of the square toward the center of the upper side.