A highly conducting ring of radius R is perpendicular to and concentric with the axis of a long solenoid as shown in Fig. 3.182. The ring has a narrow gap of width δ in its circumference. The cross-sectional area of the solenoid is a. The solenoid has a unifoem unitial field of magnitude B(t)=B0+βt, where β>0. Assuming that no charge can flow across the gap, the face (s) accumulating an excess of positive charge is /are