Question

Fig. 4.39 illustrates a segment of a double line carrying direct current whose direction is indicated by the arrows. Taking into account that the potential φ₂ > φ₁ and making use of the Poynting vector, establish on which side (left or right) the source of the current is located.

Answer 1

Given φ₂ > φ₁ 

The electric field is as shown by the dashed lines (— →). The magnetic field is as shown  emerging out of the paper.  is parallel to the wires and towards right.

Hence source must be on the left.