The current in a coil decreases from 30 amp to 10 amp in 0.2 sec if the self inductance of coil is 4 Henry. What is the induced emf in volts?

To find the induced emf, we can use Faraday's law of electromagnetic induction, which states that the induced emf is equal to the rate of change of magnetic flux. In mathematical terms, it is given by:

E = -L * (ΔI/Δt)

where: E is the induced emf, L is the self-inductance of the coil, ΔI is the change in current, and Δt is the change in time.

Given in the problem: L = 4 Henry, ΔI = Final current - Initial current = 10 Amp - 30 Amp = -20 Amp (The negative sign indicates that the current is decreasing), Δt = 0.2 sec.

Substituting these values into the formula, we get:

E = -4 Henry * (-20 Amp / 0.2 sec) = 400 Volts.

So, the induced emf is 400 Volts.