Two equal positive point charges +q are on the y axis; one is at y = +a and the other is at y = -a. The electric field at the origin is zero. A test charge q0 placed at the origin will therefore be in equilibrium. Discuss the stability of the equilibrium for a positive test charge by considering small displacements from equilibrium along the x-axis and small displacements along the y-axis. Repeat Part the discussion for a negative test charge and Find the magnitude and sign of a charge q0 that when placed at the origin results in a net force of zero on each of the three charges.