In a coil of self inductance 10 H, the rate of change of current is \( 4\,\mathrm{A\,s^{-1}} \). The electromotive force (emf) induced in the coil will be
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( -40\,\mathrm{V} \)
A changing current in a coil induces an emf in the same coil (self-induction). The minus sign is Lenz's law: the induced emf opposes the change of current.
\( \displaystyle \varepsilon = -L\,\frac{dI}{dt} \)
\( L = 10\,\mathrm{H} \)
\( dI/dt = 4\,\mathrm{A\,s^{-1}} \)
\( \displaystyle \varepsilon = -10 \times 4 = -40\,\mathrm{V} \)