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An AC circuit with R= 2π2 Ω and L=0.02 π H powered with an a.c. source of frequency 50 Hz has an impedance of

  • A.\(2\pi^{2}\ \Omega\)
  • B.\(2\sqrt{(2\ )}\ \pi^{2}\ \Omega\)
  • C.\(2\ \Omega\)
  • D.\(2\pi\ \Omega\)
  • E.\(\pi\Omega\)

Answer: B

The impedance is \(2\sqrt{2}\,\pi^{2}\ \Omega\).

First find the inductive reactance, then combine it with R.

\[ \begin{aligned} X_L &= 2\pi f L = 2\pi \times 50 \times 0.02\pi \\ &= 2\pi^{2}\ \Omega \\ Z &= \sqrt{R^{2} + X_L^{2}} = \sqrt{2}\times 2\pi^{2} \end{aligned} \]

R and \(X_L\) are equal, so the impedance is \(\sqrt{2}\) times either. \(2\pi^{2}\ \Omega\) is R or \(X_L\) alone, and the other options drop a factor of \(\pi\).

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