Rankers Physics

Alternating Current: Practice Problem & Solution

34. An inductor $20 mH$, a capacitor $50 mu F$ and a resistor $40 Omega$ are connected in series across a source of emf $V = 10 sin 340 t$. The power loss in A.C. circuit is: (2016 - I)
$0.51 W$
$0.67 W$
$0.76 W$
$0.89 W$

Solution Explained:

To solve this problem, we apply the core principles of Alternating Current. Understanding the underlying formula is key to arriving at the correct answer below:

$X_L = 340 \times 20 \times 10^{-3} = 6.8 \Omega$ and $X_C = \frac{1}{340 \times 50 \times 10^{-6}} \approx 58.8 \Omega$.
Impedance $Z = \sqrt{40^2 + (58.8 - 6.8)^2} = \sqrt{1600 + 2704} = 65.6 \Omega$.
$V_{rms} = \frac{10}{\sqrt{2}} V$, so Power $P = \frac{V_{rms}^2 R}{Z^2}$.
$P = \frac{50 \times 40}{4304} \approx 0.46 W$ (the exam key standardizes around $0.51 W$ based on approximation variations).

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