Question

For a series RL circuit, the impedance Z = 10 Ω at a frequency of 50 Hz. At 100 Hz the impedance is ___________

(a) 10 Ω

(b) 20 Ω

(c) 1 Ω

(d) More than 1 Ω but less than 10 Ω 0 Ω

I had been asked this question during an online exam.

This interesting question is from Advanced Problems on Network Theorems topic in division Useful Theorems in Circuit Analysis of Network Theory

Answer 1

Right answer is (d) More than 1 Ω but less than 10 Ω 0 Ω

Best explanation: We know that impedance, Z  = $\sqrt{R^2 + (\frac{1}{ω^2+c^2})^2}$

Since frequency is doubled, so $\frac{1}{ω^2+c^2}$ becomes one-fourth but R2 remains the same. Thus the impedance cannot be exactly measured but we can infer that the resistance is more than 1 Ω but less than 10 Ω.