Though Eq. ( ) is useful for the general spatial study of the layer potential in the tube, it is convenient to have an approximate form for U (r) from which other relations could be derived. One might assume that the primary contributions to U , would occur for small angles α. In this regime, cos α ≈ 1. As before, considering U along the line of θ = 0 and letting δ 1 and δ 2 be small angles and cos α = 1, Eq. ( ) can be separated into repulsive and attractive integrals as