Electric Force: What is the net Force on the middle charge?
+75 mC
1.5 m
+45 mC
1.5 m
-90 mC
Fon middle due to L = (9 * 109 N m2/C2) * (75 * 10-3 C * 45 * 10-3 C) / (1.5 m)2
Fon middle due to L = 1.35 * 107 N, positive, pointing to the R
Fon middle due to R = (9 * 109 N m2/C2) * (-90 * 10-3 C * 45 * 10-3 C) / (1.5 m)2
Fon middle due to R = -1.62 * 107 N, negative, pointing to the L
Net Force = Fon middle due to L + Fon middle due to R
Net Force = 1.35 * 107 N - 1.62 * 107 N
Net Force = -0.27 * 107 N, pointing left
This is the method to solve any Force or E field problem with multiple charges!
What is the magnitude and direction of the electric field
at a point midway between a -20 µC and a + 60 µC
charge 40 cm apart?
a.  9.0 * 106 J (N/C)
-20 µC
0.4 m
+60 µC
How to solve:
Put yourself at the middle point. Look at the charge on the left. What is the E
field you see due to that charge? It’s just
E = k * (-20 µC) / (0.2)2 , pointing towards the left because E fields point in towards
negative charges
Look at the charge on the right. What is the E field you see due to that
charge? ? It’s just
E = k * 60 µC / (0.2)2 , pointing towards the left because E fields point away from
positive charges
What is the magnitude and direction of the electric field
at a point midway between a -20 µC and a + 60 µC
charge 40 cm apart?
a.  9.0 * 106 N/C
-20 µC
0.4 m
Eon middle due to L = (9 * 109 N m2/C2) * (-20 * 10-6 C) / (0.2 m)2
Eon middle due to L = -4.5 * 106 N/C, pointing left
Eon middle due to R = (9 * 109 N m2/C2) * (+60* 10-6 C) / (0.2 m)2
Eon middle due to R = 1.34 * 107 N/C,
Net Field = Eon middle due to L + Eon middle due to R
Net Field = 1.35 * 107 N – 4.5 * 106 N
Net Field = 9.0* 106 N, pointing left:
+60 µC