June 2020 Test

Three charges lie along the x-axis.
One positive. charge q1=15micro coulomb is at x =2.0m and another positive charge q2 =6.0 micro coulomb is at the origin. At what point on the x-axis must a negative charge q3 be placed so that the resultant force on it be zero?

1 answer

electrostatic force is an inverse-square relation

6.0 / d^2 = 15 /(2.0 - d)^2

solve for d

15 d^2 = 24 - 24 d + 6 d^2

9 d^2 + 24 d - 24 = 0 ... 3 d^2 + 8 d - 8 = 0