Question

A charge Q1=45.5nC is located at the origin. A negative charge Q2=-7.5nC is located on the positive x-axis p=5cm from the origin. Calculate the location , x, on the x-axis, in centimeters, where the electric field is equal to zero.

Answer 1

electric field will be zero to the right of Q2

let the point be x cm to the right of Q2

kQ1 / r1^2 = kQ2 / r2^2

45.5 / (5 +x)^2 = 7.5 / x^2

45.5x^2 = 7.5(25 + x^2 + 10x)

45.5x^2 = 187.5 + 7.5x^2 + 75x

38x^2 -75x - 187.5 = 0

x = 75 + sqrt(75^2 + 4*187.5*38) / 76

= 3.42

so EF will be zero at (5 + 3.42) = 8.42 cm