Lc Oscillations
A charged capacitor of 2 microfarads is connected across an ideal inductor of 8 millihenry to form a closed loop with negligible resistance. What is the frequency of the electrical oscillations set up in this LC circuit?
Select the correct option:
Solution
1.26 kHz
NCERT Class 12, Chapter 7 (Alternating Current) describes how a charged capacitor connected to an inductor produces electrical oscillations in which energy shuttles between the capacitor's electric field and the inductor's magnetic field, at frequency f=2πLC1. Substituting L=8×10−3 H and C=2×10−6 F: LC=16×10−9, so LC=4×10−4.5=1.265×10−4 s. Then f=2π×1.265×10−41=7.95×10−41≈1258 Hz≈1.26 kHz. The option 2.52 kHz doubles the correct frequency. The option 0.63 kHz halves it. The option 7.96 kHz forgets the factor of 2π, giving angular rather than ordinary frequency divided incorrectly. A plausibility check confirms the analogy with a mechanical oscillator, where the LC circuit behaves like a mass-spring system and its natural frequency depends only on L and C, giving a physically reasonable kilohertz-scale oscillation. In this analogy the inductance plays the role of the mass, representing electrical inertia that resists changes in current, while the reciprocal of the capacitance behaves like the spring stiffness. At the moment the capacitor is fully charged, all the energy resides in its electric field and the current is momentarily zero; a quarter-period later the capacitor is fully discharged, the current is maximum, and all the energy has transferred to the inductor's magnetic field. In a truly ideal loop with zero resistance these oscillations would persist forever, but any real resistance would gradually dissipate the energy as heat, damping the oscillation exactly as friction damps a mechanical oscillator.
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About This Question
- Subject
- physics
- Chapter
- electromagnetic induction and alternating currents
- Topic
- lc oscillations
- Difficulty
- Hard
- Year
- 2025
Solution
Correct Answer:
1.26 kHz
NCERT Class 12, Chapter 7 (Alternating Current) describes how a charged capacitor connected to an inductor produces electrical oscillations in which energy shuttles between the capacitor's electric field and the inductor's magnetic field, at frequency f=2πLC1. Substituting L=8×10−3 H and C=2×10−6 F: LC=16×10−9, so LC=4×10−4.5=1.265×10−4 s. Then f=2π×1.265×10−41=7.95×10−41≈1258 Hz≈1.26 kHz. The option 2.52 kHz doubles the correct frequency. The option 0.63 kHz halves it. The option 7.96 kHz forgets the factor of 2π, giving angular rather than ordinary frequency divided incorrectly. A plausibility check confirms the analogy with a mechanical oscillator, where the LC circuit behaves like a mass-spring system and its natural frequency depends only on L and C, giving a physically reasonable kilohertz-scale oscillation. In this analogy the inductance plays the role of the mass, representing electrical inertia that resists changes in current, while the reciprocal of the capacitance behaves like the spring stiffness. At the moment the capacitor is fully charged, all the energy resides in its electric field and the current is momentarily zero; a quarter-period later the capacitor is fully discharged, the current is maximum, and all the energy has transferred to the inductor's magnetic field. In a truly ideal loop with zero resistance these oscillations would persist forever, but any real resistance would gradually dissipate the energy as heat, damping the oscillation exactly as friction damps a mechanical oscillator.
This hard difficulty physics question is from the chapter electromagnetic induction and alternating currents, covering the topic of lc oscillations. It appeared in the 2025 exam.
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