Magnetic flux passing through a coil is initially `4xx 10^-4` Wb. It reduces to 10% of its original value in t second. If the emf induced is 0.72 mV then t in second is

Options

A. 0.3
B. 0.4
C. 0.5
D. 0.6

Answer

C

Explanation

`e = -(d phi)/(dt) = (phi_(2)-phi_(1))/(dt) and phi_(2) = (phi_(1))/(10)`
`:. phi_(1) - phi_(2) = 9/10 phi_(1)`
`:. E = (phi_(1)-phi_(2))/(dt) = 9/10 (phi_(1))/(dt)`
`:. Dt = (phi_(1) - phi_(2))/(e) = 9/10 xx (4 xx 10^(-4))/(72 xx 10^(-5))`
`= 1/2 = 0.5s`.