A constant voltage of 60 V is applied at t = 0 across a series R-L circuit as shown in the figure. Determine the current (in A) in the circuit at t = 0 +

Concept:

Transients are present in the network when the network is having any energy storage elements.

1. Inductor does not allow sudden change of current and it stores energy in the form of the magnetic field.

(Inductor allows sudden change of voltage).

2. Capacitor does not allow sudden change of voltage and it stores energy in the form of the electric field.

(Capacitor allows sudden change of current).

Now as,

\(V=L\frac{di}{dt}\)

For a sudden change of current, we require dt = 0,

then V = ∞, but practically this much voltage not possible.

Hence inductor does not allow sudden change of current.

Analysis:

Initially the switch is open, hence the current flowing through the circuit is zero.

\(I\left( {{0^ - }} \right) = 0\;A\)

After switch closed, \(I\left( {{0^ + }} \right) = I\left( {{0^ - }} \right) = 0\;A\)

Important Points

Before Switching             After Switching