Two-dimensional dipole
Two parallel wires with uniform linear charge densities λ and −λ are separated by a distance ℓ. Consider the electric field at points in a given plane perpendicular to the wires. The fields from the wires fall off like 1/r. So at points far from the wires, we effectively have a 2D version of a dipole, where the wires act like point charges with 1/r fields instead of the usual 1/r² Coulomb fields.
Repeat the process in Section 2.7 for this 2D dipole. That is, find φ(r,θ) and E(r,θ), and also find the shapes of the field-line and constant-potential curves. (The individual potentials relative to infinity diverge, so you will want to pick a local point as the reference point. Any choice will work, but you may as well pick the point midway between the wires.)