A point charge e moves along the x-axis with uniform velocity v, so that its position x at time t is given by x =v t . Derive an explicit expression for the potential φ(t ) at the point (0, b, 0). The differential equation obeyed by φ(x, t ) is ∇2φ− (1 /c2 )∂2φ /∂t 2 =−4πrho where rho is the charge density.