SP 212 Worksheet
Lesson 1: Ch. 21.1, Coulomb’s Law
1) Two charges lie on the x-axis of a coordinate system. The first charge, q, is located at x = 0.
The second charge, 2.5q, is located at x = a. Where on the x-axis should a third charge be placed
so that it remains in equilibrium (F~ = 0)? Express your answer as a numerical value times a.
Hint: it doesn’t matter if the third charge is positive or negative.
2) Two balls of mass m and charge q hang at rest from non-conducting, massless strings as shown.
If L = 1.0 m, m = 12 g, and x = 6.0 cm, what is the magnitude of q? (Assume θ is sufficiently small
so that tan θ ' sin θ = (x/2)/L.)
Hint: F~net = 0 for each ball.