Class 12th Physics Alternating Current Chapter 7 is important as it helps students understand phasors, impedance, resonance, and transformer efficiency in a structured way.
As the Class 12 Physics board exam on February 20, 2026 approaches, this is the time to focus on precision practice rather than passive reading.
Students who practice Alternating Current Important Questions for Class 12 Physics can score better as they understand the formula application and diagram-based explanations.
Alternating Current Class 12 Important Questions
The Alternating Current chapter includes AC circuits, reactance, impedance, resonance, and transformers. Students should thoroughly practice all commonly asked Alternating Current Class 12 Important Questions and carefully review Alternating Current 12 Solutions to ensure stepwise accuracy.
Writing well-structured Alternating Current Class 12 Question Answers with correct formulas and units is important in the board exam.
1. The peak voltage of an AC is 440 volt. Its virtual voltage is [OD 2012]
2. In an AC circuit, the potential difference across an inductance and resistance joined in series are 15 V and 25 V respectively. [Foreign 2005]
(A) 15 V
(B) 20 V
(C) 25 V
(D) 50 V
3. A step-up transformer operates on a 230 V line supplies a current of 2A. The ratio of primary and secondary winding is 1: 25. The current in the primary is [Foreign 2004, Delhi 2015]
(A) 15 A
(B) 50 A
(C) 75 A
(D) 125 A
4. In any AC circuit, is the applied instantaneous voltage equal to the algebraic sum of the instantaneous voltages across the series elements of the circuit? Is the same true for rms voltage? [Comp 2013]
Sol. Yes, in any AC circuit, the applied instantaneous voltage is equal to the algebraic sum of instantaneous voltage across the series elements, because instantaneous voltages are in the same phase.
This is not true in the case of rms voltages, because they are usually not in the same phase.
5. Name the energy losses in a transformer. [OD 2013]
Sol. Energy loss in transformers are:
1. Flux leakage
2. Resistance to the windings
3. Eddy currents
4. Hysteresis.
6. Define Capacitor reactance. Write its SI units. [OD 2010]
Sol. Capacitor reactance is the resistance offered by a capacitor, when it is connected to an electric circuit. It is given by,
7. An applied voltage signal consists of a superposition of a DC voltage of high frequency. The circuit consists of an inductor and a capacitor in series. Show the DC signal will appear across C and the AC voltage across L. [Foreign 2011]
Thus, the inductive reactance has zero value for DC and very high value for high frequency AC. On the other hand capacitance offers infinite reactance to DC and very low reactance to high frequency AC.
So, when inductor and capacitor are connected in series, the capacitor gives easy path to AC and does not allow DC while inductor gives easy path to DC and offers very high opposition to AC.
Hence, DC signal will appear across C and AC voltage across L.
Sol. The instantaneous voltage,
9. Radio-frequency choke is air cored, whereas audio frequency choke is iron cored. Explain. [Delhi 2018]
Sol. Radio Frequency Choke
The inductive reactance of choke is given by,
10. Show diagrammatically two different arrangements used for winding the primary and secondary coils in a transformer. Assuming the transformer to be an ideal one, write the expression for the ration of its:
1. Output voltage to input voltage.
2. Output current to input current.
Mention two reasons for energy losses in an actual transformer. [OD 2019, Comp 2015]
Sol. Arrangement of winding of primary and secondary coil in a transformer are shown in fig (a) and (b).
Reasons for energy losses in a transformer
1. Joule Heating: Energy is lost due to heating of primary and secondary windings as heat (I2Rt).
2. Flux Leakage: Energy is lost due to coupling of primary and secondary coils not being perfect, i.e., whole of magnetic flux generated in primary coil is not linked with the secondary coil.
High-Probability Questions for 2026 Board Exam
Focus on these repeated Alternating Current Class 12 Important Questions:
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Derive impedance of series LCR circuit.
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Define RMS value and derive its expression.
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Explain resonance in AC circuit.
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Derive expression for average power in AC circuit.
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Explain working principle of transformer.
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Draw phasor diagrams for R, L, and C circuits.