A transformer with toroidal core of permeability $\mu$ is shown in the figure. Assuming uniform flux density across the circular core cross-section of radius $r < < R$, and neglecting any leakage flux, the best estimate for the mean radius $R$ is
- $\frac{\mu V r^2 N_P^2 \omega}{I} \\ $
- $\frac{\mu I r^2 N_P N_S \omega}{V} \\ $
- $\frac{\mu V r^2 N_P^2 \omega}{2I} \\ $
- $\frac{\mu I r^2 N_P^2 \omega}{2V} $