A wheel with 20 metallic spokes each 1m long is rotated with a...

Given, the angular velocity, $\omega = \dfrac{2\pi}{60} \times 120rad{s}^{- 1}$.

Recall the formula of induced EMF in terms of angular velocity and radius of the ring, $ε = \dfrac{1}{2}B\omega{r}^{2}$, now put the proper values at proper places,

$⇒ ε = \dfrac{1}{2} \times \left(0 . 4 \times {10}^{- 4}\right) \times \left(2\pi\left[\dfrac{120}{60}\right]\right) ( 1{)}^{2}$

$⇒ ε = 0 . 8\pi \times {10}^{- 4}$$= 2 . 512 \times {10}^{- 4}\text{V}$.