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I. What do you mean… (Class 12 Physics SQP 2026-27, Q33B)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 5 marks · Long answer

CBSE Class 12 Physics sample paper 2026-27: question 33(B)

From the official CBSE sample question paper for 2026-27, section E, question 33(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(B) · 5 marks · Long answerAlternating Current

I. What do you mean by the resonance condition of a series LCR–circuit?
II. Write an expression for the resonant frequency.
Describe the use of a series resonant circuit in the tuning of a radio receiver.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Compare the inductive and capacitive reactances; resonance happens when they cancel each other.

Hint 2

Set and solve for .

Hint 3

In a radio, which component is varied by the tuning knob, and what happens to the current at the matching frequency?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. I. Resonance condition. A series LCR circuit driven by an a.c. source is at resonance when the inductive reactance equals the capacitive reactance:

    1 mark

  2. At resonance the net reactance is , so the impedance , which is its minimum value. The circuit then behaves as purely resistive, and the voltage and current are in phase.

    ½ mark

  3. The current is therefore maximum at resonance.

    ½ mark

  4. II. Resonant frequency:

    , or

    1 mark

  5. Use in tuning a radio. Many stations broadcast at different frequencies, and all of their signals reach the antenna.

    The receiver's input stage has a resonant circuit with a fixed inductor and a variable capacitor . Turning the tuning knob changes , and hence .

    1 mark

  6. When is made equal to the frequency of the desired station:

    • the impedance is minimum for that frequency;
    • its current, and hence the signal, is maximum.

    Other stations are off resonance, so they give very small currents and are rejected. In this way one station is selected.

    1 mark

Answer: Resonance: , so (minimum) and the current is maximum; ; tuning is done by varying until equals the station frequency.

Where the marks are

  • States the resonance condition , i.e. (1 mark)
  • At resonance , the minimum value (½ mark)
  • At resonance the current is maximum (½ mark)
  • Resonant frequency (1 mark)
  • Variable capacitor changes in the receiver (1 mark)
  • Matching to the station gives maximum current for that station and rejects others (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Alternating Current, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.