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Question

An ac circuit contains a resistance of $1 \text{ k}\Omega$, a capacitor of $0.1 \ \mu\text{F}$ and an inductor of $1 \text{ mH}$ connected in series. The resonance frequency of the circuit is approximately :

This question was previously asked in
NEET UG Re-Exam 2026 Question Paper (21-Jun-2026)
The correct answer is
$15.9 \text{ kHz}$

The question asks for the resonance frequency ($f_0$) of a series AC circuit containing a resistor (R), capacitor (C), and inductor (L).

Resonance Frequency Formula

The resonance frequency in an RLC circuit is the frequency at which the inductive reactance equals the capacitive reactance. It is independent of the resistance and is calculated using the formula:

$f_0 = \frac{1}{2\pi\sqrt{LC}}$

Circuit Component Values

Given values are:

  • Inductance, $L = 1 \text{ mH} = 1 \times 10^{-3} \text{ H}$
  • Capacitance, $C = 0.1 \ \mu\text{F} = 0.1 \times 10^{-6} \text{ F} = 1 \times 10^{-7} \text{ F}$
  • Resistance, $R = 1 \text{ k}\Omega = 1000 \ \Omega$ (Note: Resistance does not affect the resonance frequency itself).

Calculation Steps

  1. Substitute the values of L and C into the resonance frequency formula:

    $f_0 = \frac{1}{2\pi\sqrt{(1 \times 10^{-3} \text{ H}) \times (1 \times 10^{-7} \text{ F})}}$.

  2. Simplify the term under the square root:

    $LC = 1 \times 10^{-10} \text{ H}\cdot\text{F}$

    $\sqrt{LC} = \sqrt{1 \times 10^{-10}} = 1 \times 10^{-5} \text{ s}$

  3. Calculate the resonance frequency in Hertz (Hz):

    $f_0 = \frac{1}{2\pi \times (1 \times 10^{-5})} = \frac{10^5}{2\pi} \text{ Hz}$

    Using $\pi \approx 3.14159$,

    $f_0 \approx \frac{100000}{2 \times 3.14159} \approx \frac{100000}{6.28318} \approx 15915.5 \text{ Hz}$

  4. Convert the frequency to kilohertz (kHz):

    $f_0 \approx 15.9155 \text{ kHz}$

Conclusion

The calculated resonance frequency is approximately $15.9 \text{ kHz}$.

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Important Questions from Electricity and Magnetism

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  3. The electric field of an electromagnetic wave travelling through a medium is given by $\vec{E}(x,t) = 25 \sin(2.0 \times 10^{15} t - 10^7 x) \hat{n}$
    then the refractive index of the medium is ________.
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  4. Three long straight wires carrying current are arranged mutually parallel as shown in the figure. The force experienced by $15 \text{ cm}$ length of wire $Q$ is________.

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  5. For the two cells having same EMF $E$ and internal resistance $r$, the current passing through the external resistor $6\text{ }\Omega$ is same when both the cells are connected either in parallel or in series. The value of internal resistance $r$ is ________$\Omega$.
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