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NEET Physics · Electromagnetic Induction and Alternating Currents

Reactance, impedance and the LCR series circuit

Taught in NCERT Class 12 Physics, Chapter 7: Alternating Current.

3 verified questions: Easy 0 · Medium 2 · NEET level 1

Sample questions

Sample 1Medium

A series circuit has a resistor of 30 Ω, an inductor of reactance 80 Ω and a capacitor of reactance 40 Ω, connected to an AC source of 100 V rms. What is the rms current?

  1. A

    0.67 A

  2. B

    2.0 A

  3. C

    1.33 A

  4. D

    3.33 A

Show the answer
The answer is B.

Z = √[R² + (X_L − X_C)²] = √(30² + 40²) = 50 Ω, so I = 100/50 = 2 A.

  • A.

    0.67 A divides by R + X_L + X_C = 150 Ω, adding the impedances arithmetically.

  • C.

    1.33 A uses √(R² + X_L²) and ignores the capacitor's reactance.

  • D.

    3.33 A divides by R alone, which is correct only at resonance.

NCERT: Class 12 Physics, Chapter 7

Sample 2NEET level

A series LCR circuit has resistance 30 Ω, inductive reactance 80 Ω and capacitive reactance 40 Ω. It is connected to an ac source of rms voltage 100 V. The rms current in the circuit is

  1. A

    0.81 A

  2. B

    1.43 A

  3. C

    2 A

  4. D

    3.33 A

Show the answer
The answer is C.

The impedance is Z = √(R² + (X_L − X_C)²) = √(30² + 40²) = 50 Ω, so I_rms = V_rms/Z = 100/50 = 2 A. The two reactances oppose each other, so their difference enters Z.

  • A.

    Adds the reactances: Z = √(30² + 120²) ≈ 124 Ω.

  • B.

    Adds R and (X_L − X_C) directly: Z = 70 Ω, ignoring the phase difference.

  • D.

    Uses only the resistance, Z = R = 30 Ω, ignoring the reactances.

NCERT: Class 12 Physics, Chapter 7 (Alternating Current), Series LCR circuit

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