Two point charges +q and −4q are fixed in air, 0.40m apart.
a. (a) Locate the point(s) on the line joining the charges where the net electrostatic force on a small positive test charge would be zero. Express the position(s) as the distance from the +q charge.
b. (b) If the test charge is slightly displaced towards the +q charge from the nearer null point, will it return to the null point or move further away? State the type of equilibrium at that null point with a brief reason.