NCERT Class 12 Physics Physics Part-I: Chapter 7 — 1 INTRODUCTION
This chapter introduces Alternating Current (AC) and AC voltage, contrasting them with Direct Current (DC). It explains why AC is predominantly used in power distribution due to efficient voltage conversion via transformers and economical long-distance transmission. The chapter details the application of AC voltage to a resistor, deriving Ohm's law for AC circuits and illustrating that voltage and current are in phase. It highlights that while instantaneous AC values vary, the average current over a cycle is zero. The significance of AC in modern electrical devices and technologies is emphasized, setting the stage for further exploration of AC circuit properties.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Physics |
| Book | Physics Part-I |
| Chapter | Chapter 7 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 676 |
Learning outcomes
- Understand the concept of alternating current (AC) and alternating voltage.
- Differentiate between AC and DC.
- Explain the advantages of AC over DC in power transmission and distribution.
- Analyze the behavior of a resistor in an AC circuit.
- Understand the concept of phase relationship between voltage and current in an AC circuit.
Vocabulary
| Word | Meaning |
|---|---|
| Alternating Current (AC) | Current that changes direction with time, typically varying sinusoidally. |
| Alternating Voltage (AC Voltage) | Voltage that changes direction with time, typically varying sinusoidally. |
| Direct Current (DC) | Current that does not change direction with time. |
| Amplitude | The maximum value of an oscillating quantity (voltage or current). |
| Angular Frequency | A measure of how fast something oscillates, related to frequency. |
| Ohm's Law | Relationship between voltage, current, and resistance (V=IR), applicable to resistors in AC circuits. |
| In Phase | When two oscillating quantities reach their maximum, minimum, and zero values at the same time. |
| Transformer | Device used to easily and efficiently convert AC voltages from one level to another. |
The complete chapter text is read in the official NCERT PDF viewer below (streamed from ncert.nic.in). This page provides NCERT Help study material — summary, vocabulary, practice questions, and FAQs — not a full reproduction of the textbook.
Read chapter online
This PDF is loaded from the official NCERT website (ncert.nic.in). Use the page buttons below to read — download is disabled on NCERT Help.
Read page by page below. PDF is streamed from the official NCERT website — no download button on this page.
Practice questions
- What is the main advantage of AC over DC for power transmission? Answer: AC voltages can be easily and efficiently converted to different levels using transformers, allowing for economical transmission over long distances.
- In an AC circuit with a pure resistor, what is the phase relationship between voltage and current? Answer: The voltage and current are in phase, meaning they reach their maximum, minimum, and zero values at the same time.
- What is the average value of an alternating current over one complete cycle? Answer: The average value of an alternating current over one complete cycle is zero.
Practice MCQs
Q1. Which of the following is a primary reason for the widespread use of AC in power distribution?
Explanation: Transformers allow for efficient voltage conversion, which is crucial for transmitting electricity economically over long distances.
Q2. If the AC voltage is given by v(t) = vm sin(ωt), what is the current through a pure resistor R?
Explanation: According to Ohm's law (i = v/R), for a pure resistor, the current is directly proportional to the voltage. Since v(t) = vm sin(ωt), then i(t) = vm sin(ωt) / R = (vm/R) sin(ωt).
Q3. What does it mean for voltage and current to be 'in phase' in an AC circuit?
Explanation: When voltage and current are in phase, their sinusoidal variations occur simultaneously, meaning they cross zero, reach peaks, and troughs at the same instants.
Q4. Nicola Tesla is credited with pioneering which concept crucial for AC machinery?
Explanation: Nicola Tesla's work on the rotating magnetic field was fundamental to the development of alternating current machinery and the polyphase system.
Q5. What is the SI unit named in honor of Nicola Tesla?
Explanation: The SI unit of magnetic flux density, the Tesla (T), is named in honor of Nicola Tesla for his contributions to electromagnetism.
Frequently asked questions
What is the difference between AC and DC?
DC (Direct Current) flows in one direction only, while AC (Alternating Current) periodically reverses direction.
Why is AC preferred over DC for power transmission?
AC can be easily stepped up or down using transformers, allowing for efficient and economical transmission of electricity over long distances.
What is the formula for AC voltage given in the chapter?
The AC voltage is given by v(t) = vm sin(ωt), where vm is the amplitude and ω is the angular frequency.
How does the current relate to the voltage in a pure resistor connected to an AC source?
The current through a pure resistor is given by i(t) = im sin(ωt), where im = vm/R. The voltage and current are in phase.
What is the average value of an AC current over a full cycle?
The average value of an AC current over a full cycle is zero because the positive and negative halves cancel each other out.
Who was Nicola Tesla and what is his significance in AC technology?
Nicola Tesla was a Serbian-American scientist and inventor who made significant contributions to AC power systems, including the concept of the rotating magnetic field and the polyphase system.
Related resources
Important topics
Topics covered
NCERT Class 12 Physics — Physics Part-I — Chapter 7 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.