The magnetic flux through a coil is `4 xx 10^(-4) Wb//m^(2)` at time `t=0`. It reduces to `10%` of its original value in 't' seconds. If the induceds e.m.f. Is 0.72 mV, then the time t is

Options

A. 0.25 s
B. 0.5 s
C. 0.75 s
D. 1 s

Answer

B

Explanation

`e = - (d phi)/(dt)=-(phi_(2)-phi_(1))/(dt) and phi_(2)=1/10 phi_(1)`
`:. phi_(1)-phi_(2) = 9/10 phi_(1)`
`:. dt = (phi_(1)-phi_(2))/(e)=9/10 xx (4xx10^(-4))/(72xx10^(-5))=1/2=0.5 s`.