Alternating Current (AC) is one of the most important chapters in Class 12 Physics. This chapter plays a crucial role in board exams, JEE, NEET, and other competitive examinations. Alternating current is widely used in our daily life because it can be transmitted over long distances with very less energy loss.
In this chapter, students learn about AC voltage, AC current, AC circuits, reactance, impedance, power factor, transformers, and LCR circuits. These concepts help us understand how electrical appliances, power stations, and transmission lines work.
These Alternating Current Class 12 Notes are written in a very simple and detailed manner so that students can easily understand each concept and score well in exams.
Alternating Current Class 12 Notes PDF | AC Physics Chapter Explanation & Formulas
What is Alternating Current?
Alternating current is the type of current whose magnitude and direction change continuously with time.
- In AC, electrons move back and forth.
- The current reverses its direction periodically.
- AC is generated by AC generators.
Example
The electricity supplied to our homes is alternating current.
Alternating Voltage
An alternating voltage varies with time and is represented as:
V = V_0 \sin(\omega t)
Where:
- = Peak voltage
- = Angular frequency
- = Time
The voltage continuously changes between positive and negative values.
AC Current
The alternating current is expressed as:
I = I_0 \sin(\omega t)
Where:
- = Peak current
AC current follows the same sinusoidal pattern as AC voltage.
Peak Value and RMS Value
Peak Value
- Maximum value of current or voltage.
- Denoted by or .
RMS Value (Root Mean Square)
RMS value is the effective value of AC.
I_{rms} = \frac{I_0}{\sqrt{2}}, \quad V_{rms} = \frac{V_0}{\sqrt{2}}
RMS value gives the same heating effect as DC.
AC Through a Resistor
When AC flows through a resistor:
- Current and voltage are in the same phase.
- Ohm’s law is valid.
V = IR
Power consumed:
P = I_{rms}^2 R
AC Through an Inductor
Inductive Reactance
Inductor opposes the change in current.
X_L = \omega L
- Current lags behind voltage by 90°.
- Energy is stored in the magnetic field.
AC Through a Capacitor
Capacitive Reactance
Capacitor opposes the change in voltage.
X_C = \frac{1}{\omega C}
- Current leads voltage by 90°.
- Energy is stored in the electric field.
LCR Series Circuit
An LCR circuit consists of:
- Inductor (L)
- Capacitor (C)
- Resistor (R)
Impedance
Impedance is the total opposition to AC.
Z = \sqrt{R^2 + (X_L - X_C)^2}
Current in LCR Circuit
I = \frac{V}{Z}
Phase Angle
Phase difference between voltage and current is given by:
\tan \phi = \frac{X_L - X_C}{R}
- If : Current lags
- If : Current leads
Resonance in LCR Circuit
Resonance occurs when:
X_L = X_C
At resonance:
- Impedance is minimum
- Current is maximum
- Circuit behaves like a pure resistor
Resonant Frequency
f_0 = \frac{1}{2\pi\sqrt{LC}}
Power in AC Circuit
Average Power
P = V_{rms} I_{rms} \cos \phi
Where is called Power Factor.
Power Factor
Power factor indicates how efficiently electrical power is used.
\text{Power Factor} = \cos \phi
- Maximum power when power factor = 1
- Low power factor causes energy loss
Transformer
What is a Transformer?
A transformer is a device used to increase or decrease AC voltage.
Working Principle
Transformer works on Mutual Induction.
Types of Transformers
- Step-up Transformer
- Step-down Transformer
Transformer Formula
\frac{V_s}{V_p} = \frac{N_s}{N_p}
Where:
- = Secondary and Primary voltage
- = Number of turns
Energy Losses in Transformer
- Copper loss
- Eddy current loss
- Hysteresis loss
- Flux leakage
Advantages of AC Over DC
- Easy transmission over long distances
- Less power loss
- Voltage can be easily increased or decreased
Repeated & Important Questions (Exam Oriented)
- What is alternating current?
- Define RMS value of AC.
- What is impedance?
- Explain resonance in LCR circuit.
- Define power factor.
- Write working principle of transformer.
- Why AC is preferred over DC?
- Derive expression for resonant frequency.
- Explain energy losses in transformer.
- What is capacitive reactance?
Conclusion-Alternating Current Class 12 Notes PDF
Alternating Current is a very scoring chapter if concepts are clear and formulas are properly understood. These Alternating Current Class 12 Notes are designed to help students in board exams and competitive exams. Regular practice of numericals and revision of formulas will ensure excellent performance.
🔁 Repeated Questions – Alternating Current Class 12 Physics
1 Mark Repeated Questions
- What is alternating current (AC)?
- Write the SI unit of frequency of AC.
- What is the RMS value of AC current?
- Define peak value of alternating current.
- What is power factor?
- What is impedance?
- Name the device used to change AC voltage.
- What is the phase difference between current and voltage in a pure resistor?
- Write the formula of inductive reactance.
- Write the formula of capacitive reactance.
2 Marks Repeated Questions
- Define RMS value of alternating current and write its significance.
- Why AC is preferred over DC for long-distance transmission?
- What is the phase relation between current and voltage in an inductor?
- What happens to capacitive reactance when frequency increases?
- Define resonance in an LCR circuit.
- What is the role of power factor in AC circuits?
- Write any two advantages of alternating current.
- Distinguish between peak value and RMS value.
- What is the working principle of a transformer?
- Why is no power loss in a pure capacitor?
3 Marks Repeated Questions
- Explain AC through a resistor with the help of a phasor diagram.
- Define impedance and write its expression.
- Explain inductive reactance and its dependence on frequency.
- Explain capacitive reactance and its dependence on frequency.
- What is resonance frequency? Write its formula.
- Explain why current is maximum at resonance.
- Write three losses in a transformer.
- What is average power in an AC circuit?
- Explain the significance of power factor.
- Draw voltage-current graph for AC.
4 Marks Repeated / Long Answer Questions
- Explain AC through an LCR series circuit with a neat phasor diagram.
- Derive the expression for impedance of an LCR circuit.
- Derive the condition of resonance in an LCR circuit.
- Explain the working of a transformer with diagram.
- Derive the expression for average power in AC circuit.
- Explain energy losses in a transformer and methods to reduce them.
- Why is AC transmission more economical than DC transmission?
- Explain the concept of power factor and its importance.
- Derive the expression for resonant frequency of LCR circuit.
- Explain current and voltage relations in resistor, inductor, and capacitor.
Numerical-Based Repeated Questions
- Find RMS value when peak current is given.
- Calculate inductive reactance at a given frequency.
- Find impedance of an LCR circuit.
- Calculate resonant frequency using given L and C.
- Find average power consumed in AC circuit.
- Calculate power factor of an AC circuit.
- Determine current at resonance.
- Find voltage ratio in transformer.
- Calculate secondary voltage of a transformer.
- Find frequency when capacitive reactance is given.
Most Important Board Exam Tip ⭐
👉 RMS value, LCR circuit, resonance, transformer, power factor.