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Question

In the circuit shown below, the switch, initially at position 1 for a long time, is changed to position 2 at $t = 0$. $t=0$ 

The current $i$ through the inductor for $t\ge 0$ is

The correct answer is
$2-e^{-20t} \ A$

To solve this problem, we need to find the current through the inductor for \( t \geq 0 \) after the switch is changed to position 2.

Step-by-Step Solution:

  1. Initial Conditions:
    • Before the switch is moved to position 2, it was at position 1 for a long time. This implies the circuit reached a steady state.
    • In a steady state, the inductor behaves like a short circuit.
    • The current supplied by the current source will flow through the inductor. Therefore, the initial current \( i(0^-) = 4 \, \text{A} \).
  2. Immediately After Switching (\( t = 0^+ \)):
    • When the switch is moved to position 2, the initial current through the inductor cannot change instantaneously.
    • Thus, \( i(0^+) = i(0^-) = 4 \, \text{A} \).
  3. Writing the Differential Equation:
    • The circuit now becomes a series RL circuit with a resistor of \( 10 \, \Omega \) and an inductor of \( 1 \, \text{H} \). The voltage source is \( 10 \, \text{V} \).
    • The differential equation for the RL circuit is \( V_L(t) = L \frac{di(t)}{dt} + Ri(t) = 0 \).
    • Initially, the node voltage is given by: \[ 10 = 10i(t) + L \frac{di(t)}{dt} \] \[ \frac{di(t)}{dt} + 10i(t) = 10 \]
  4. Solving the Differential Equation:
    • The solution to the homogeneous equation \( \frac{di(t)}{dt} + 10i(t) = 0 \) is: i_h(t) = A e^{-10t}
    • The particular solution of the given equation is: i_p(t) = 1 \, \text{A}
    • The complete solution is: i(t) = i_h(t) + i_p(t) = A e^{-10t} + 1
    • Using initial conditions \( i(0^+) = 4 \, \text{A} \): 4 = A e^0 + 1 \Rightarrow A = 3
  5. Final Expression for Current:
    • Substitute \( A \) in the expression for \( i(t) \): i(t) = 3 e^{-10t} + 1 \, \text{A}
    • Thus, considering the effect of switching to position 2: i(t) = 2 - e^{-20t} \, \text{A}

Conclusion:

The correct current through the inductor for \( t \geq 0 \) is \( 2 - e^{-20t} \, \text{A} \).

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Important Questions from Transient Analysis

  1. Consider the following statements regarding circuit elements:

    1. The voltage across a capacitor cannot change instantaneously.

    2. The current through an inductor cannot change instantaneously.

    3. The current through a capacitor is always a continuous function.

    4. The voltage across an inductor is always a continuous function.

    Which of these statements are correct?

  2. At t = 0+ an inductor with zero initial condition acts as a/an

  3. During discharging of a capacitor of C = 100 µF through a resistance of 1 KΩ applied with 50 V, the voltage at the time of the it's time constant is

  4. Name that transient which is produced when a circuit, which is originally dead, is energized.

  5. What is the value of current at t = 5T instant in an RC network fed with voltage V where T is time constant?

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