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

Emf induced is given by, `e=|-(dphi)/(dt)|implies(dphi)/(dt)`
`implies 0.72xx10^(-3)=(4xx10^(-4)-4xx10^(-5))/(t)`
`implies t=(4xx10^(-5)xx9)/(72xx10^(-5))=(1)/(2)=0.5s`