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In a critically damped series LCR circuit :

  • A.\(R^{2} = 4L/C\)
  • B.\(R^{2} = 2L/C\)
  • C.\(R = \sqrt{L/C}\)
  • D.\(R^{2} = 4L^{2}C^{2}\)

Answer: A

For critical damping \(R^{2} = 4L/C\).

The charge in a series LCR circuit obeys \(L\ddot{q} + R\dot{q} + q/C = 0\), with characteristic equation \(Ls^{2} + Rs + 1/C = 0\). Critical damping is the boundary between oscillation and slow decay, where the discriminant is zero:

\[ \begin{aligned} R^{2} - \frac{4L}{C} &= 0 \\ R &= 2\sqrt{\frac{L}{C}} \end{aligned} \]

If \(R^{2}\) is smaller, the circuit oscillates (underdamped), and if larger, it is overdamped. \(R = \sqrt{L/C}\) is half the critical value, and the other forms do not even have the dimensions of resistance squared.

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