Question

Magnetic flux passing through a coil is initially $4\times {10}^{-4}$Wb. It reduces to 10% of its original value in 't' second. If the e.m.f. induced is 0.72 mV then 't' in second is:

• A. 0.3
• B. 0.4
• C. 0.5
• D. 0.6

Answer 1

The correct option is C 0.5

Magnetic flux gets reduced to 10% of its original value in time $t$.

Change in magnetic flux $|\Delta \varphi |=0.9{\varphi }_{initial}$

$\therefore$ $|\Delta \varphi |=0.9\times 4\times {10}^{-4}=0.36\times {10}^{-3}$ Wb

Emf induced in the coil $E=0.72\times {10}^{-3}V$

Using $E=\frac{|\Delta \varphi |}{t}$

$\therefore$ $0.72\times {10}^{-3}=\frac{0.36\times {10}^{-3}}{t}$

$⟹$ $t=0.5$ s