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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
Concepts grouped the way the chapter is taught — open the bucket that matches what you want to revise.
Core Concepts
high priorityOpen the chapter concepts in a clean revision order.
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 pi/2 and opposition to AC is inductive reactance XL = omega L. In a pure capacitor, current leads voltage by pi/2 and opposition to AC is capacitive reactance XC = 1/(omega 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 = sqrt(R^2 + (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
Use this section to decide what deserves the most revision time.
High Probability Topics
- AC Voltage Across a Pure Resistor
- AC Through Pure Inductor and Pure Capacitor
- Series LCR Circuit: Impedance and Phasor Relation
- Resonance in a Series LCR Circuit
- Power Factor and Average Power in AC Circuits
- Transformer: Step-Up, Step-Down, Efficiency and Losses
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 omega0 = sqrt(LC) instead of omega0 = 1/sqrt(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.
Likely Question Types
- MCQ: concept checks, applications, and common mistakes
- Very short answer: definitions, formulas, conditions, or terms
- Short answer: process, diagram, reasoning, or worked method
- Case-based: chapter scenario with linked subparts
Quick Revision
Concept, formula or equation to remember, and the trap that loses marks — in one scannable view.
- Pure resistor: V and I are in phase, I0 = V0/R, and average power is maximum for given RMS values.
- Pure inductor: XL = omega L and current lags voltage by pi/2.
- Pure capacitor: XC = 1/(omega C) and current leads voltage by pi/2.
- Series LCR: Z = sqrt(R^2 + (XL - XC)^2) and tan phi = (XL - XC)/R.
- Resonance: XL = XC, omega0 = 1/sqrt(LC), current maximum, circuit purely resistive.
- AC power: P = Vrms Irms cos phi; wattless current occurs when phi = pi/2.
- Transformer: Vs/Vp = Ns/Np; step-up has Ns > Np and step-down has Ns < Np.
- 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 voltag…
Practice
Use short concept checks first, then move into the full chapter test.
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