Alternating Current
Alternating Current is a significant chapter of the Class 12 Physics syllabus that is concerned with electric current with changing magnitude and direction with time. This chapter gives the students the idea of how alternating current is produced, how it acts in the various elements of a circuit and why it is extensively utilised in power transmission systems. These concepts can be discussed in physics based on mathematical analysis, representation of waveforms, and practical circuit modelling, both of which are important to the study of AC systems used in real life.
In order to investigate alternating current correctly, one has to establish a clear definition of such notions as instantaneous current, root mean square (rms) value, reactance, impedance, and phase difference and learn the ways in which they are measured. It is not enough to say that a current is alternating without defining its amplitude, frequency and its relationship with the phase. Likewise, the action of AC in resistors, capacitors and inductors cannot be described without introducing relations, relating voltage, current and change in time. Analysis and comparison of AC behaviour can be done with precision by using standard units and mathematical expressions that have clear definitions. Physics uses universally standardised units, conventions and mathematical representations towards uniform scientific knowledge throughout the world. All the AC quantities are not only represented by a numerical value but also by a standard unit, making the calculation and communication of AC quantities accurate and understandable. In this way, the chapter Alternating Current provides a good groundwork in knowing about AC circuits, power transmission, transformers, and high-tech applications of electrical and electronic technology.
Important Topics of Alternating Current
The chapter Alternating Current describes how an electric current and voltage change periodically with time. It is concerned with the study of AC waveforms, analysis of circuits with resistors, inductors and capacitors, and the usefulness of AC power systems. The topics are vital in creating transparency in the analysis of AC circuits that are critical in Class 12 board exams, JEE, and NEET
1. Alternating Current and Alternating Voltage
Alternating current and alternating voltage are electric current and electric potential difference which change their magnitude and direction periodically with time. In this topic, the current gives the basic understanding of what AC is and its difference from direct current. It adds the most prevalent form of AC, which is sinuoidal variation.
2. Instantaneous, Peak, and RMS Values of AC
This topic explains how the value of alternating current or voltage changes at every instant of time. It brings the notions of instantaneous value, peak value, and root mean square (rms) value. The rms value is elaborated as the effective value that causes heating effects. Rms values are obtained mathematically. This is used in the calculation of power in AC circuits.
3. Average Value of Alternating Current
This topic discusses the average value of alternating current. It justifies the reason why the mean value of a whole cycle of AC is zero. The average value within half a cycle is obtained and derived mathematically. This concept helps in understanding the measurement aspects of AC. It is mainly theoretical but important for conceptual clarity.
4. AC Voltage Applied to a Resistor
This topic explains the behaviour of alternating current when applied across a purely resistive circuit. It reveals that both current and voltage are in one phase. Ohm's law is used to get the expression of the instantaneous current. Dissipation of power in a resistive circuit using AC is described. The analysis of AC circuits is the simplest case based on this topic.
5. AC Voltage Applied to a Capacitor
6. AC Voltage Applied to an Inductor
This topic studies the behaviour of alternating current in a purely inductive circuit. It presents the idea of inductive reactance. The phase difference between current and voltage is explained, showing that current lags voltage by 90 degrees. Energy storage in the magnetic field of the inductor is discussed. This topic is important for understanding inductive effects in AC circuits.
7. AC Voltage Applied to Series LCR Circuit
This topic explains the behaviour of alternating current in a circuit containing a resistor, inductor, and capacitor connected in series. It brings in the principle of impedance as effective resistance to AC. The relative phase between current and voltage is addressed. This topic forms the core of AC circuit analysis. Numerical problems are frequently asked in this section.
8. Resonance in AC Circuits
The resonance in the AC circuits is created when the capacitive reactance is equal to the inductive reactance. The condition of resonance is described, as well as the concept of resonant frequency. It demonstrates that at resonance current is maximum and the impedance is minimum. Resonance applications are discussed in practice. It is a highly important topic for the board and competitive exams.
9. Power in AC Circuits
This topic explains how electrical power is consumed in AC circuits. It presents the average power and the power factor concept. The phase difference on which power relies between current and voltage is discussed
10. Transformers
Transformers are electrical devices used to change the magnitude of alternating voltage without changing its frequency. This topic explains the principle of mutual induction, along with the construction and working of step-up and step-down transformers. It also discusses how transformers enable efficient long-distance power transmission by reducing energy losses. Understanding this topic is essential for learning practical applications of alternating current in electrical power systems.
Related Topics,
Important Formulas of Alternating Current
- Instantaneous value of AC current
$
i=i_0 \sin (\omega t)
$
- Instantaneous value of AC voltage
$
v=v_0 \sin (\omega t)
$
- Angular frequency
$
\omega=2 \pi f
$
2. RMS and Average Values:
- RMS value of current
$
i_{\mathrm{rms}}=\frac{i_0}{\sqrt{2}}
$
- RMS value of voltage
$
v_{\mathrm{rms}}=\frac{v_0}{\sqrt{2}}
$
- Average value of current (over half cycle)
$
i_{\text {avg }}=\frac{2 i_0}{\pi}
$
3. AC Through Circuit Elements:
- Pure Resistor
$
i=\frac{v}{R}
$
(Current and voltage are in phase)
- Pure Inductor
Inductive reactance
$
X_L=\omega L
$
(Current lags voltage by $90^{\circ}$ )
- Pure Capacitor
Capacitive reactance
$
X_C=\frac{1}{\omega C}
$
(Current leads voltage by $90^{\circ}$ )
4. Series LCR Circuit:
- Impedance
$
Z=\sqrt{R^2+\left(X_L-X_C\right)^2}
$
- Current
$
I=\frac{V}{Z}
$
- Phase angle
$
\tan \phi=\frac{X_L-X_C}{R}
$
5. Resonance:
- Condition of resonance
$
X_L=X_C
$
- Resonant frequency
$
f_0=\frac{1}{2 \pi \sqrt{L C}}
$
- Impedance at resonance
$
Z=R
$
6. Power in AC Circuits:
- Average power
$
P=V_{\mathrm{rms}} I_{\mathrm{rms}} \cos \phi
$
- Power factor
$
\cos \phi=\frac{R}{Z}
$
7. Transformers:
- Voltage ratio
$
\frac{V_s}{V_p}=\frac{N_s}{N_p}
$
- Current ratio
$
\frac{I_s}{I_p}=\frac{N_p}{N_s}
$
- Efficiency
$
\eta=\frac{\text { Output Power }}{\text { Input Power }} \times 100 \%
$
Alternating Current: Previous Year Questions
When $L$ is removed,
$
\begin{aligned}
& \tan \phi=\frac{\left|X_C\right|}{R} \\
& \Rightarrow \tan \frac{\pi}{3}=\frac{X_c}{R} .
\end{aligned}
$
When C is removed,
$
\begin{aligned}
& \tan \phi=\frac{\left|X_L\right|}{R} \\
& \Rightarrow \tan \frac{\pi}{3}=\frac{X_L}{R} \cdots
\end{aligned}
$
From (i) and (ii),
Since $X_L=X_C$, the circuit is in resonance
$
Z=R
$
Power factor $=\cos \phi=\frac{R}{Z}=1$
Question 2:
The primary of a transformer, when connected to a dc battery of 10 volt draws a current of 1mA. The number of turns of the primary and secondary winding is 50 and 10,0 respectively. The voltage in the secondary and the current drawn by the circuit in the secondary are respectively
Solution:
A transformer cannot work on $d c$.
$
\therefore V_s=0 \text { and } I_s=0
$
Question 3:
An ac voltage is applied to a resistance $R$ and an inductor $L$ in series. If $R$ and the inductive reactance are both equal to $3 \Omega$, the phase difference between the applied voltage and the current in the circuit is:
Solution:
Here, $R=3 \Omega$
Inductive reactance, $X_L=3 \Omega$
Thephase difference between the applied voltage and the current in the circuit is
$
\begin{aligned}
& \tan \phi=\frac{X_L}{R}=\frac{3 \Omega}{3 \Omega}=1 \\
& \phi=\tan ^{-1}(1) \text { or } \phi=\frac{\pi}{4}
\end{aligned}
$
Alternating Current in Different Exams
Alternating Current (AC) is also an important chapter in Class 12 Physics, and it has strong conceptual and numerical significance. This chapter has questions that check the knowledge of students concerning the AC waveforms, LCR circuit, power, resonance and transformers, which are high in usage of electrical systems in real life environment. The understanding of exam-wise focus and weightage also allows students to be better prepared for board and competitive exams.
| Exam | Focus Areas | Common Questions Asked | Marks / Questions Weightage | Preparation Tips |
|---|---|---|---|---|
| JEE Main | RMS values, LCR circuits, resonance, power factor | Numerical problems on impedance, current, and resonance | 1–2 questions (4–8 marks) | Practice standard numericals and formula application |
| JEE Advanced | LCR circuits, resonance, transformers | Multi-step conceptual and numerical problems | 1 question (6–12 marks, sometimes linked) | Focus on derivations, phasor diagrams, and concepts |
| NEET | RMS values, power in AC, transformers | Direct formula-based MCQs | 1–2 questions (4–8 marks) | Memorise formulas and practice NCERT-based questions |
| UPSC CDS / NDA | Basic AC concepts, transformers | Conceptual and simple numerical questions | 1 question (2–4 marks) | Revise definitions and basic relations |
| State-Level Exams (WBJEE, MHT CET, etc.) | LCR circuits, resonance, power | Conceptual + numerical questions | 1–2 questions (4–8 marks) | Practice previous year questions and numericals |
| GATE | AC circuit analysis, power, resonance | Conceptual and numerical engineering problems | 1 question (2–5 marks) | Focus on phasor analysis and circuit theory |
| School-Level (CBSE, ICSE, State Boards) | Entire chapter (theory + numericals) | Short answers, derivations, and numericals | 6–10 marks | Learn derivations, practice NCERT examples |
| CUET | Conceptual understanding of AC | MCQs, assertion–reason questions | 1–2 questions (4–6 marks) | Strengthen basics and revise key formulas |
| SSC & Banking Exams | Basic AC and transformer concepts | One-liners, matching | 1 question (1–2 marks) | Revise basic terms and SI units |
Important Books and Resources for Class 12 Alternating Current
In order to understand the chapter Alternating Current, students need to see good textbooks, reference guides, and practice materials which cover both the theory and the numerical methods. These books assist in the study of the AC waveforms, LCR circuits, resonance, power factor and transformers, which are significant to the exams (both board and competitive) such as JEE Main, JEE Advanced and NEET.
| Book Title | Author / Publisher | Description |
|---|---|---|
| NCERT Class 12 Physics (Part II) | NCERT | Official textbook covering all Alternating Current concepts, definitions, derivations, and NCERT-based examples directly from the syllabus. |
| NCERT Exemplar Physics (Class 12) | NCERT | Offers higher-order and application-based questions on AC topics, including RMS, LCR circuits, and transformers for deeper practice. |
| Concepts of Physics – Volume 2 | H.C. Verma | Provides clear conceptual explanations and challenging numerical problems on alternating current and associated circuit analysis. |
| Understanding Physics: AC & Electromagnetic Theory | D.C. Pandey (Arihant) | Topic-wise detailed explanations, solved examples, and practice questions focused on AC circuits and transformers for JEE & NEET preparation. |
| Physics for Class 12 | R.D. Sharma | Step-by-step theory and a broad range of solved/unsolved problems related to alternating current and power in AC circuits. |
| Arihant All-In-One Physics (Class 12) | Arihant | Comprehensive guide with concise theory, solved examples, and practice questions on alternating current and related topics. |
| MTG Chapter-wise Previous Year Questions – Physics | MTG | Helps students analyse and practice past questions specifically from the Alternating Current chapter across board and competitive exams. |
NCERT Resources for Alternating Current
NCERT resources for Alternating Current provide clear and syllabus-aligned material to help students grasp the core concepts of AC circuits, waveforms, impedance, resonance, power factor, and transformers. The NCERT textbook, along with its exemplar problems and solutions, explains theory in a structured way and offers practice questions that are directly relevant to Class 12 board exams as well as competitive tests like JEE Main, JEE Advanced, and NEET.
-
NCERT solutions for class 12 physics chapter alternating current
-
NCERT exemplar solutions for class 12 physics chapter alternating current
- NCERT notes for class 12 physics chapter alternating current
Related Topics,
NCERT Subjectwise Resources
NCERT subject-wise materials are organised and syllabus-based learning content on various subjects, which assist students in developing a good conceptual basis. They consist of textbooks, exemplar problems, and solutions and can thus be very helpful in the preparation for the board exams and even competitive exams such as JEE and NEET.
Practice Questions based on Alternating Current
Alternating Current Practice Questions are absolutely necessary in enhancing the knowledge of AC waveforms and circuit behaviour. These questions discuss significant topics like RMS and average values, AC through resistors, inductors and capacitors, LCR circuits, resonance, power factor and transformers. Practice enhances numerical precision, understanding of concepts and numerical certainty in formula application. Solving a wide range of problems prepares students effectively for Class 12 board exams and competitive exams like JEE Main, JEE Advanced, and NEET.
Conclusion
The chapter Alternating current lays a solid ground for the understanding of electrical systems where current and voltage alternate with time. Students can acquire clear conceptual knowledge through practising the main concepts, fundamental formulae, and numerical methods of solving problems involving AC waveforms, LCR circuits, resonance, power factor, and transformers. Such a well-organised preparation not only increases confidence but is also very useful in doing well in Class 12 board exams as well as in competitive exams like JEE Main, JEE Advanced and NEET.
Frequently Asked Questions (FAQs)
An alternating current is a current whose amplitude fluctuates over time and whose direction is reversed on a regular basis.
Nikola Tesla, a Yugoslav physicist, created the alternating current
Direct current (DC) is the one-directional flow of electric charge
Examples of DC electronics include Cell phones, Flat-screen TVs (AC goes into the TV, which is converted to DC) , Flashlights, Hybrid and electric vehicles. Many equipment, particularly domestic appliances like lights, washing machines, and refrigerators, rely on AC power, which is provided straight from the power grid through power plugs.
Because of the easy ways we may use to scale up and down voltages, AC is superior to DC when transporting power over long distances. Transformers can accomplish this with AC but not with DC, or at least not in a way that is as sophisticated as DC.
The terms AC and DC are used to describe different kinds of current flow in a circuit. The electric charge (current) in direct current (DC) only flows in one direction. In alternating current (AC), on the other hand, the direction of electric charge changes on a regular basis.
