Chapter Hub
Alternating Current
Alternating current is a current whose magnitude and direction change periodically with time. In Class 12 Physics, AC is studied mainly through sinusoidal voltage and current, RMS values, phase difference, reactance, impedance, resonance, power factor, and transformers. The chapter connects circuit behaviour with rotating phasors. A resistor, inductor, and capacitor respond differently to AC: a resistor keeps current in phase with voltage, an inductor makes current lag, and a capacitor makes current lead. These phase relations are central to numerical questions and assertion-reason questions. Series LCR circuits combine resistance, inductive reactance, and capacitive reactance. Their impedance and phase angle decide current, voltage distribution, power consumption, and resonance. Resonance explains why current becomes maximum when inductive and capacitive reactances cancel each other. Power in AC circuits depends not only on RMS voltage and current but also on the power factor. Transformers use mutual induction to change AC voltage levels and are important in power transmission because they help reduce energy loss in long-distance lines.
Difficulty
Medium
Study time
70-90 min
Plan by time
Pick the window that matches what you have right now.
If you have 15 min
Last-pass revision
Skim the Quick Revision table — definitions, formulas, and the traps board examiners reuse.
Open Quick RevisionIf you have 45 min
Targeted practice
Read the high-priority concepts, then drill the common-trap list before moving on.
Open Key ConceptsIf you have 70 min
First full pass
Walk every concept in chapter order, then revise and quiz. Best for the first time you study this chapter.
Open Key ConceptsChapter Learning Map
Start with one of the buckets below, then open the full map when you want the complete concept roadmap.
Key Concepts
Work through the concepts in order. Each one has an explanation, a worked example, a self-check and an exam answer.
All Concepts
All 6 concepts of the chapter, in the order the NCERT chapter teaches them.
AC Voltage Across a Pure Resistor
When a sinusoidal AC voltage is applied across a pure resistor, the current is also sinusoidal and remains in phase with the applied voltage.
AC Through Pure Inductor and Pure Capacitor
In a pure inductor, current lags voltage by π/2 and opposition to AC is inductive reactance XL = ω L. In a pure capacitor, current leads voltage by π/2 and opposition to AC is capacitive reactance XC = 1/(ω C).
Series LCR Circuit: Impedance and Phasor Relation
A series LCR circuit contains a resistor, inductor, and capacitor in series with an AC source; its total opposition to current is impedance Z = √(R2 + (XL - XC)2).
Resonance in a Series LCR Circuit
Resonance in a series LCR circuit occurs when inductive reactance equals capacitive reactance, making impedance minimum and current maximum.
Power Factor and Average Power in AC Circuits
Average power consumed in an AC circuit is P = Vrms Irms cos phi, where cos phi is the power factor and phi is the phase difference between voltage and current.
Transformer: Step-Up, Step-Down, Efficiency and Losses
A transformer is an AC device that changes voltage from one value to another using mutual induction between two coils wound on a common magnetic core.
Exam Intelligence
The traps that cost marks in this chapter, and how its main concepts turn up in the paper.
Common Traps
- Using peak values in place of RMS values without conversion.
- Interchanging lead-lag rules for inductor and capacitor.
- Adding XL and XC instead of taking their difference in a series LCR circuit.
- Using ω0 = √LC instead of ω0 = 1/√LC.
- Calculating AC power as VI for all circuits without power factor.
- Assuming transformers work with steady DC.
- Thinking step-up transformer increases both voltage and current in an ideal case.
How CBSE Asks This Chapter
- AC Voltage Across a Pure Resistor
Questions usually ask for the expression of current, RMS value, phase difference, average power, or a graph/phasor showing V and I in a resistor.
- AC Through Pure Inductor and Pure Capacitor
Exams ask students to calculate reactance, compare frequency dependence, draw phasors, identify leading or lagging current, or reason why ideal L and C consume no average power.
- Series LCR Circuit: Impedance and Phasor Relation
Questions often ask for impedance, phase angle, current, voltage across each element, phasor diagram, or whether the circuit is inductive, capacitive, or resistive.
- Resonance in a Series LCR Circuit
Exams ask for resonant frequency, condition for resonance, maximum current, power factor at resonance, Q-factor meaning, or interpretation of a current-frequency graph.
Quick Revision
Concept, formula or equation to remember, and the trap that loses marks — in one scannable view.
AC Voltage Across a Pure Resistor
Remember
Instantaneous voltage: V = V0 sin ω t.Trap to avoid
Students often use peak voltage as if it were RMS voltage, or say that current lags in a resistor.
AC Through Pure Inductor and Pure Capacitor
Remember
Inductor: XL = ω L, where XL is in ohm, ω is angular frequency in rad s−1, and L is inductance in henry.Trap to avoid
A common error is to interchange the phase rule: students write that current leads in an inductor or lags in a capacitor.
Series LCR Circuit: Impedance and Phasor Relation
Remember
Z = √(R2 + (XL - XC)2), tan phi = (XL - XC)/R, I = Vrms/Z.Trap to avoid
Students often add XL and XC directly in series LCR.
Resonance in a Series LCR Circuit
Remember
Condition: XL = XC.Trap to avoid
Students sometimes say resonance means voltage is maximum across the source.
Power Factor and Average Power in AC Circuits
Remember
Average power: P = Vrms Irms cos phi.Trap to avoid
Students often calculate AC power as only VI without multiplying by cos phi.
Transformer: Step-Up, Step-Down, Efficiency and Losses
Remember
Ideal transformer relation: Vs/Vp = Ns/Np.Trap to avoid
Students often say a transformer works with DC.
| Concept | Remember | Trap to avoid |
|---|---|---|
| AC Voltage Across a Pure Resistor | Instantaneous voltage: V = V0 sin ω t. | Students often use peak voltage as if it were RMS voltage, or say that current lags in a resistor. |
| AC Through Pure Inductor and Pure Capacitor | Inductor: XL = ω L, where XL is in ohm, ω is angular frequency in rad s−1, and L is inductance in henry. | A common error is to interchange the phase rule: students write that current leads in an inductor or lags in a capacitor. |
| Series LCR Circuit: Impedance and Phasor Relation | Z = √(R2 + (XL - XC)2), tan phi = (XL - XC)/R, I = Vrms/Z. | Students often add XL and XC directly in series LCR. |
| Resonance in a Series LCR Circuit | Condition: XL = XC. | Students sometimes say resonance means voltage is maximum across the source. |
| Power Factor and Average Power in AC Circuits | Average power: P = Vrms Irms cos phi. | Students often calculate AC power as only VI without multiplying by cos phi. |
| Transformer: Step-Up, Step-Down, Efficiency and Losses | Ideal transformer relation: Vs/Vp = Ns/Np. | Students often say a transformer works with DC. |
Practice
Use short concept checks first, then move into the full chapter test.
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