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

According to question,
Initial flux `(phi_1) = 4 xx10^-4` Wb
Final flux `(phi_2)=(4xx10^-4xx10)/100`
=`4xx10^-5` Wb
Emf induced is given by
`e=|-(dphi)/dt| Rightarrow(dphi)/dt`
Rightarrow 0.72xx10^-3=(4xx10^-4xx10^-5)/t`
Rightarrow `t=(4xx10^-5xx9)/(72xx10^-5)=1/2=0.5`s