10. For the magnetic circuit of Fig. 10, the current \( i \) flows through the coil of \( N \) turns. The mean core length is \( \ell_c \), and the air gap length is \( \ell_g \). Assume no magnetic leakage. The flux crossing the cross-sectional area \( A \). The relative permeability of core is \( \mu_r \). The permeability of air is \( \mu_0 \). Find the flux in the air gap. (10%)