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A parallel circuit has R = 1kΩ, C= 50µF and L = 10mH. The quality factor at resonance is ________.

  • A.100
  • B.90.86
  • C.70.70
  • D.78.76

Answer: C

The quality factor is about 70.70.

For a parallel resonant circuit with resistance R in parallel with L and C, the quality factor is \(Q = R\sqrt{C/L}\).

\[ \begin{aligned} Q &= 1000\sqrt{\frac{50 \times 10^{-6}}{10 \times 10^{-3}}} \\ &= 1000 \times \sqrt{5 \times 10^{-3}} \\ &= 1000 \times 0.0707 = 70.7 \end{aligned} \]

Using the series formula \(\frac{1}{R}\sqrt{L/C}\) would give a tiny value, and 100, 90.86 and 78.76 do not follow from these component values.

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