Question

A metal disc of radius 7cm rotates at the rate of 1200 rpm with its plane perpendicular to the field. The induced e.m.f is 15.4mV. the value of B is

• A. 0.05 T
• B. 0.5 T
• C. 8 T
• D. 0.01 T

Answer 1

The correct option is A 0.05 T

$e=\frac{1}{2}B{r}^{2}w$
$15.4\times {10}^{-3}=\frac{1}{2}\times B\times 7^2\times {10}^{-4}\times 2\pi \frac{1200}{60}$
$30.8\times {10}^{-3}=B\times 7\times 7\times {10}^{-4}\times 2\times \frac{22}{7}\times 20$
${10}^{-3}\times 4.4=B\times 44\times {10}^{-4}\times 20$
$B=\frac{1}{20}=0.05T$