AP Physics C E and M

Question

A charge of magnitude 2 nC is placed at the origin. An identical charge is placed 2 millimeters from the origin on the x axis, and a third identical charge is placed 2 millimeters from the origin on the y axis. What is the magnitude of the force on the charge at the origin?

Answer 1

All charges are identical, therefore, the force is repelling force

F12 = k Q1 Q2/r² (-y)

F23 = k Q2 Q3/r² (-x)

|F12| = |F23| = kQ²/r²

|F| = SQRT (F²12 + F²23)

|F| = kQ²/r² (sqrt(2))

plug in numbers: k, Q and r