--- type: "Chapter" knowledge_type: "chapter" entity_type: "chapter" id: "66dfe0763f8b4e9e69bf80fa" title: "ALTERNATING CURRENT" board: "CBSE" curriculum: "CBSE" class: "Class 12" subject: "Physics" book: "Physics Part - I" chapter: "ALTERNATING CURRENT" chapter_slug: "alternating-current" canonical_url: "https://www.edzy.ai/cbse-class-12-physics-physics-part-i-alternating-current" markdown_url: "https://www.edzy.ai/okf/chapter/cbse-class-12-physics-physics-part-i-alternating-current.md" source_type: "examSubjectBookChapter" source_id: "66dfe0763f8b4e9e69bf80fa" source_pdf: "https://edzy-ai.s3.ap-south-1.amazonaws.com/edzy-express-ts/55ee4eb6-79d1-4321-958d-e9de3ae7af1a.pdf" source: "Edzy" version: 1 last_updated: "2026-06-20" --- # ALTERNATING CURRENT The chapter discusses the essential concepts of alternating current (ac) and alternating voltage (ac voltage), which are prevalent in everyday electrical systems. It examines the operational principles of ac circuits, explaining how electrical energy is transmitted and utilized in practical applications. --- ## Knowledge Snapshot | Field | Details | | :--- | :--- | | Class | Class 12 | | Subject | Physics | | Book | Physics Part - I | | Chapter | ALTERNATING CURRENT | | Pages | 177-200 | --- ## Chapter Summary ### Short Summary This chapter introduces alternating currents, focusing on their properties and relevance in modern electrical systems, the influence of inductors and capacitors in ac circuits, and the use of transformers. ### Detailed Summary The chapter elaborates on various topics related to alternating current, including its generation, waveform, the behavior of ac through resistive, inductive, and capacitive components, as well as power considerations and operational principles of transformers. --- ## Topic-Wise Explanation ### INTRODUCTION The introduction explains alternating current (ac) and its prevalence over direct current (dc) due to its efficiency in transmission and usage in power systems. ### AC VOLTAGE APPLIED TO A RESISTOR This section analyzes how ac voltage is applied to a resistor using Ohm's law, illustrating the relationship between voltage and current in ac circuits. ### REPRESENTATION OF AC CURRENT AND VOLTAGE BY ROTATING VECTORS — PHASORS Phasors are introduced to represent ac voltage and current, demonstrating their phase relationships in various circuit components. ### AC VOLTAGE APPLIED TO AN INDUCTOR The chapter describes how current lags behind voltage in an inductive circuit and introduces inductive reactance. ### AC VOLTAGE APPLIED TO A CAPACITOR Capacitors are discussed with emphasis on how current leads voltage in capacitive circuits, along with the concept of capacitive reactance. ### AC VOLTAGE APPLIED TO A SERIES LCR CIRCUIT The series LCR circuit is examined in detail, illustrating the fundamental impedance and phase relationships within the circuit components. ### POWER IN AC CIRCUIT: THE POWER FACTOR This section addresses power consumption in an ac circuit, introducing the concept of power factor and its importance in determining efficiency. ### TRANSFORMERS The functioning of transformers is elaborated, highlighting how they change alternating voltage levels through mutual induction. --- ## Core Ideas | Idea | Explanation | | :--- | :--- | | Alternating Current | A current that varies over time, typically sinusoidal, allowing for efficient energy transfer and use in electrical devices. | --- ## Key Concepts | Concept | Meaning | | :--- | :--- | | Root Mean Square (RMS) | The effective value of ac current or voltage that results in the same power dissipation as an equivalent dc system. | | Inductive Reactance | The opposition that an inductor presents to the change in current, measured in ohms. | | Capacitive Reactance | The opposition that a capacitor presents to the change in voltage, measured in ohms. | | Impedance | The total opposition to ac current flow in a circuit, incorporating resistance and reactance. | | Power Factor | The ratio of the actual power consumed in a circuit to the power that could be consumed; indicates efficiency. | --- ## Important Points for Revision * AC voltage varies over time and can be represented as a sine function. * The current through a pure resistor is in phase with the voltage. * Inductive circuits cause current to lag voltage by π/2. * Capacitive circuits cause current to lead voltage by π/2. * In LCR circuits, resonant frequency occurs when inductive and capacitive reactances are equal. * Transformers step up or step down voltages while conserving power. --- ## Vocabulary and Glossary | Word / Phrase | Meaning | | :--- | :--- | | Phasor | A complex number representing the magnitude and phase of a sinusoidal function. | | Resonance | A condition that occurs in a system driven by a periodic force at its natural frequency, resulting in a large increase in amplitude. | --- ## Practice Questions ### Short Answer Questions 1. Define alternating current and list its advantages over direct current. 2. What role does a transformer play in an electrical circuit? 3. Describe the phase relationship between voltage and current in a purely inductive circuit. 4. Explain what is meant by root mean square (RMS) value in ac circuits. 5. How does capacitive reactance affect current in a circuit? ### Long Answer Questions 1. Discuss the operation of a transformer, including the relationship between primary and secondary voltages and currents. 2. Explain the importance of power factor in ac circuits and how it affects energy efficiency. 3. Analyze the behavior of a series LCR circuit at resonance and its practical applications in everyday electronics. --- ## Related Concepts * AC Circuits * Direct Current * Electromagnetic Induction --- ## Source Attribution | Field | Value | | :--- | :--- | | Source | Edzy | | Reference Type | examSubjectBookChapter | | Reference ID | 66dfe0763f8b4e9e69bf80fa | | Canonical URL | https://www.edzy.ai/cbse-class-12-physics-physics-part-i-alternating-current | | Markdown URL | https://www.edzy.ai/okf/chapter/cbse-class-12-physics-physics-part-i-alternating-current.md |