Alternating Current | Class 12 Physics Notes
By ConceptScroll Team · Published on 17 July 2026 · 3 min read
Alternating Current – this guide gives you a concise, exam-ready overview of Alternating Current from Class 12 Physics, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
SUMMARY
This section summarizes the key concepts covered in the chapter on Alternating Current. It reiterates that AC voltage and current vary sinusoidally with time and introduces their mathematical forms. In a resistor, voltage and current are in phase; in an inductor, current lags voltage by π/2; in a capacitor, current leads voltage by π/2. Root mean square (rms) values of voltage and current are defined to express power dissipation analogous to DC circuits. The concept of reactance (inductive X_L and capacitive X_C) and impedance Z in AC circuits is explained, along with phase angle φ between voltage and current. Resonance in series LCR circuits occurs when X_L = X_C, minimizing impedance and maximizing current. Power factor cos φ quantifies the effective power usage, with pure inductive or capacitive circuits having zero power factor. Phasors are introduced as rotating vectors representing AC quantities and their phase relationships. Transformers are described as devices changing voltage and current levels using mutual induction, essential for efficient power transmission. The summary also includes a table of physical quantities with symbols, units, and remarks, and points to ponder highlighting practical considerations in AC circuits.
📊 Diagram: See table_1 and table_2: Physical quantities, symbols, units, and remarks; figures from previous sections illustrating phasors, impedance, resonance, and transformers.
🔗 Connection: Prepares students for exercises applying the concepts learned in the chapter.
Table on page 22 (4×5)
| Physical quantity | Symbol | Dimensions | Unit | Remarks |
|---|---|---|---|---|
| rms voltage | V | [M L²T⁻³A⁻¹] | V | $$V = \frac{v_m}{\sqrt{2}} \quad , \quad v_m \text{ is the amplitude of the ac voltage.}$$ |
| rms current | I | [A] | A | $$I = \frac{i_m}{\sqrt{2}} \quad , \quad i_m \text{ is the amplitude of the ac current.}$$ |
| Reactance:
Table on page 22 (4×5)
| X_{C} | [M L²T⁻³A⁻²] | X_{C} = 1/ω C | ||
|---|---|---|---|---|
| Impedance | Z | [M L²T⁻³A⁻²] | Depends on elements present in the circuit. | |
| Resonant frequency | ω_{r} or ω_{0} | [T⁻¹] | Hz | ω_{0} = $\frac{1}{\sqrt{LC}}$ for a series RLC circuit |
| Quality factor | Q | Dimensionless | $$Q = \frac{\omega_0 L}{R} = \frac{1}{\omega_0 C R} \quad \text{for a series RLC circuit.}$$ | |
| Power factor | Dimensionless | = cosφ, φ is the phase difference between voltage applied and current in the circuit. |
Frequently asked questions
What is the resistance offered by a pure inductor for DC?
zero
If the power factor of LCR circuit is one then the circuit is purely
resistive
A long wire carries a steady current. It is bent into a circle of one turn and the magnetic field at the centre of the coil is B. It is then bent into a circular loop of smaller radius, having ‘n’ turns. The magnetic field at the centre of the coil will be
n 2 B
When an AC is connected to a resistor what is the phase difference between the current and voltage?
0 0
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